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In order to study the effect of molecular structure on intramolecular charge transfer character, we designed and synthesized three methoxyphenyl substituted 1,3,4-oxadiazole derivatives, where the D-A branches are attached to the core benzene ring in different ways. The photophysical properties of these three molecules were systematically investigated by means of experimental and theoretical studies. Different degrees of fluorescence red-shift in polar solvents were observed in these compounds. Both 1,4-substituted L-OXD and 1,3-substituted B-OXD exhibit a 30˜40 nm red-shift in the fluorescence spectra during the increase of solvent polarity, while 1,3,5-substituted T-OXD exhibits more obvious solvatochromic behavior (>100 nm) than L-OXD and B-OXD, points to a large increase in the dipole moment of the excited state of T-OXD with solvent polarity. Furthermore, theoretical analysis on the electron density map and ADCH charge indicated that the electron is transferred from side methoxyphenyl units to 1,3,4-oxadiazole ring and center benzene ring, and more transferred charge could be observed in T-OXD. These results reflect that constructing octupolar molecular structures could be an effective way to influence the ICT character.

Light-induced intersystem crossings in different Ni(II) macrocyclic ligand complexes with square-pyramidal and octahedral ligand-metal coordination have been investigated by means of static (DFT) and time-dependent density functional theory (TD-DFT) calculations, considering the MN12-SX exchange-correlation (XC) functional together with the def2-TZVP basis set. For the quantitative validation of the applied XC functional, the theoretical UV absorption spectra of azopyridine functionalized Ni-porphyrin macrocyclic ligand complex with square-pyramidal coordination have been compared with the experimental results obtained by Venkataramani et al. (Science, 331 (2011) 445). Using the TD-DFT method, the stability of the light–mediated reversible ligand coordination and the switching of magnetic properties have been characterized by identifying the active electronic excited states both in singlet and triplet spin configuration involved in the light-induced excited spin-state trapping. The location of the intersystem crossing points between different spin states has been performed and the strength of the spin-orbit coupling between them has been computed

Five new triphenylamine (TPA) based D-π-A structural motifs were strategically designed and synthesized in high purity. The photophysical, electrochemical, photovoltaic properties were studied and the optimized geometry calculations of D-π-A chromophores and dyes were ascertained by DFT and TD-DFT methods. Photoluminescence properties of all the chromophores were studied in different organic solvents as well as in aggregated states. Three chromophores emit blue fluorescence in ethanol solution. However, two chromophores TPA-PHA [(E)-4′-(4-(diphenylamino)styryl)-[1,1′-biphenyl]-4-carbaldehyde], TPA-PFA [(E)-4′-(4-(diphenylamino)styryl)-2-fluoro-[1,1′-biphenyl]-4-carbaldehyde] exhibited Aggregation-induced emission enhancement (AIEE) revealing the formation of excimer in excited state. The TPA-PFA had lesser quantum yield than TPA-PHA in an aggregated state, since the m-fluorine substitution on phenyl containing chromophore makes an “inductive effect”. Substitution of aldehyde group causes enhanced formation of ICT from donor to acceptor. From FE-SEM analysis we concluded that both AIEE chromophores exhibit two different kinds of “nano-aggregates” in the solid state. The AIEE chromophores (TPA-PHA and TPA-PFA) and its derivatives i.e., Dye-1, Dye-2 were utilized in DSSC application for the first time. The dye-1 and dye-2 exhibit power conversion efficiencies (PCE) of 1.9% (Jsc = 4.68 mA/cm2, Voc = 0.70 V and FF = 0.59) and 2.1% (Jsc = 5.05 mA/cm2, Voc = 0.68 V and FF = 0.61), respectively.

Determination of primary/intrinsic electron transfer rate (ket = 1/τet, s−1) in bimolecular photoinduced electron transfer reactions (PET) in solution phase monitoring fluorescence quenching is a controversial issue, predominantly due to constraint imposed by diffusion, and measurement of rate of formation of transient intermediates, therefore primary electron transfer rate, was suggested (J. Phys. Chem. B 2015, 119, 11253 − 11261). Herein, we report electron transfer dynamic of excited 7-methylbenzo[a]pyrene (MBP) in acetonitrile (ACN) in presence of different aromatic amine donors from very low concentration to finally in neat donors to access intrinsic, diffusion free, rate constants of bimolecular PET by using steady state and femtosecond time resolved fluorescence and femtosecond transient absorption (fsTA) spectroscopy. It appears that in lower concentration of donors, diffusion control or stationary regime of ET, observed dynamics by time resolved fluorescence as well as transient absorption spectroscopy replicate to each other leading to determine the bimolecular quenching rates (kd, M−1 s−1) instead of primary or intrinsic ET rates. In very higher donor concentration or in neat donors, non-diffusive or intrinsic regime of ET, amalgamation of femtosecond time-resolved emission and fsTA studies along with global and target analysis determines the primary/intrinsic ET rates (ket) between excited MBP and studied donors which are one to two orders of magnitude larger than corresponding bimolecular quenching rate constants (kd) in ACN. Intrinsic electron transfer occurs with very high rate of ket (∼1011 s−1) and the process lies in Marcus normal region and back electron transfer occur very slow kbet (∼108 s−1), the process lies in Marcus inverted region.

It is well known that upon electronic excitation various π-stacked dimers readily exhibit excimer formation, facilitated by a perfect sandwich-like arrangement between the chromophores. However, it is unclear whether such a dimer is also capable of electron transfer upon excitation, if a strong electron-donating group is covalently attached. In this work, we probe the nature of the excited state in a series of cofacially arrayed polyfluorene derivatives with electron-rich aromatic donor attached via a methylene linker. Our studies show that in all cases excimer formation is energetically favorable, and promotion of a charge-transfer state in such systems is possible but requires a free energy for electron transfer far exceeding 1 V. These findings shed light on important design principles for molecular scaffolds capable of stabilizing both excimeric and charge-transfer states upon their excitation.

Although excited state proton transfer (ESPT) dynamics has been extensively investigated in several aqueous-organic mixtures in the literature, but those are based on analysis of single wavelength transient representing either of the protonated or the deprotonated emission. Single wavelength decay, in principle, contains all necessary dynamic information but usually susceptible to potential error, if overlap of different excited species are not properly considered. Moreover, analysis of these decays are less intuitive and more challenging, since various dynamic parameters operate together. Recently, we proposed a simple and intuitive analysis to extract ESPT dynamics of 8-hydroxypyrene-1,3,6-trisulphonic acid (HPTS) from time-resolved area-normalised emission spectrum (TRANES). Here, we further refined the analysis by inducting contact ion-pair into the ESPT scheme and applied to elucidate ESPT dynamics of HPTS in methanol-water mixtures at different mole fractions of methanol. The time constants were extracted from the decay and rise of the overlap-corrected protonated or deprotonated intensities (or areas of individual emission bands) of TRANES by either a global fit or from the ratio of their intensities (or areas). We found an exponential correlation between the deprotonation times and the mole fraction of methanol, which leads to the conclusion that the activation energy of ESPT in the mixture increases linearly with increase of the mole fraction of methanol.

The electron transfer dynamics of tetra(4-carboxyphenyl)porphyrin with different central metal ions Fe(III), Cu(II), Co(II),Ni(II), Zn(II) adsorbed on ZnO nanorods have been investigated by using photophysical and electrochemical studies. Spectroscopic studies confirmed that metalloporphyrins anchored onto ZnO through the carboxylic group. XPS studies indicated that replacement of free base porphyrin with different metals resulted in change in symmetry with uniform distribution of electron density of nitrogen atom of pyrrole ring which gives one broad peak of N1S state. Absorption studies revealed the bathochromic shift of soret band in ZnO-TCPPM nanocomposites as compare to free base porphyrin and metalloporphyrins owing to the electronic coupling between ZnO and TCPPM which stabilize the π* orbital. The photoluminescence properties of metalloporphyrins are shown to be highly dependent on central metal atom. Steady state and PL lifetime measurements studies shows the electron transfer in ZnO-TCPPM occurred feasibly for TCPPZn sensitizer as Zn(II)porphyrin have long lived excited state as compare to other metalloporphyrins. Improved photovoltaic properties of the metalloporphyrins are originated from electron injection driving force from dye to ZnO. Introduction of metal in porphyrin cavity increase the electron transfer process. Incase of TCPPCo, TCPPNi fast recombination exists between metal cation and photogenerated electron that inhibits the photocurrent generation. J–V Characteristics revealed there is 4.5 fold increases in efficiency for ZnO-TCPPM device as compare to device composed of free base porphyrin.

We report here design and synthesis of four new D-π-A-π-D type dyes (4 and 5) with triphenylamine/N-ethyl carbazole as fixed rotor/donor and N, N-diethyl amine as a secondary donor having dicyanovinylene as a common acceptor. The Influence of π-linker (thiophene) along with an auxillary cyano group on the nonlinear and linear optical properties is investigated. Molecules with thiophene spacer (4b and 5b) exhibit progressive variation in their absorption and emission profiles reflecting the effect of π-linker, compared to those devoid of thiophene spacer (4a and 5a). The dyes display solvent dependent emission [e.g. 4b (toluene = 689 nm) and 5b (DMSO = 795 nm)] suggesting a more polarized photo-excited state arising as a result of enhanced intramolecular charge transfer in the excited state. The solvatochromic nature is supportive of a general solvent effect, which is confirmed by solvent polarity plots (Weller’s and Rettig’s plot). The trends in the optical properties were also validated by time dependent density functional theory computations which further reveals charge transfer (from secondary donor to acceptor) for the long wavelength absorption. Dye with the highest charge transfer dipole moment relatively has the maximum two-photon absorption cross-section area (4b = 246–399 GM) which was established using theoretical two-level model. Non-linear optical properties were investigated using solvatochromic two-level model and computationally using global and range separated hybrid functionals. The dyes show high first-order hyperpolarizability (β0) in the range of 191–1162 × 10−30 e.s.u. and second-order hyperpolarizability (γ0) in the range of 491–6,704 × 10−36 e.s.u. Z-scan results show 4a (3.79 × 10−13 e.s.u.) possesses a higher value of third order nonlinear susceptibility in DMSO. Thus, the dyes can be considered as promising candidates for NLO application in material science.

The spectral and thermodynamic properties of charge transfer complexes D∙A and D∙A∙D between (E)-bis(18-crown-6)stilbene (D) and 1,1′-bis(2-ammonioethyl)-4,4′-bipyridinium tetraperchlorate (A) in MeCN were studied. The complex D∙A is highly stable, while the complex D∙A∙D is weakly stable. D∙A does not fluoresce due to fast intramolecular processes of direct and reverse electron transfer. The efficiency of D∙A fluorescence ignition upon adding alkanediammonium salts depends on the length of a carbon chain. The spectral and kinetic characteristics of D∙A and D∙A∙D CT states were obtained by femtosecond transient absorption spectroscopy. It was established that the characteristic time of back electron transfer in the CT state of D∙A∙D (770 fs) is significantly higher than that in the CT state of D∙A (400 fs). In a number of charge transfer complexes, formed by derivatives of 4,4′-bipyridinium and di-(4-pyridinium)-ethylene with ammonioethyl and ammoniopropyl N-substituents, the rate of back electron transfer depends weakly on the acceptor nature, but is determined by the length of the carbon chain of terminal groups.

Photoinduced axial ligation process in the excited state of nickel(II) octabutoxyphthalocyanine (NiPcOBu8) in pyridine solvent has been investigated by ultrafast optical transient absorption and X-ray transient absorption spectroscopies. Although Ni phthalocyanine complexes have been reported to have no tendency to form six-coordinate structure, the (dz2,dx2-y2) state of NiPcOBu8 generated by photoexcitation of Q-band was discovered to axially ligate with the pyridine molecule very rapidly, on a timescale of ˜100 ps. Although the presence of an excited unligated (d,d) state cannot be confirmed due to the time resolution limit of the synchrotron X-ray probe pulse with ˜80 ps FWHM, the biexponential kinetics of the ground state recovery detected by the 1s → 4pz feature suggests the decay of the unligated (d,d) species with an 82 ps time constant, while that of the ligated NiPcOBu8 species is 3–5 ns. The photoinduced ligation dynamics of NiPcOBu8 is vastly different compared to that of Ni porphyrin complexes. This study provides a structural basis on the ligation chemistry in both the ground and excited states of nickel complexes based on their electronic and nuclear structures.

With the wide application of two-photon technology in biological field, lipid-water amphiphilic chromophores containing a large conjugation structure are required. In this study, six polyethylene glycol (PEG) modified benzylidene cycloalkanone photosensitizers (PSs) were selected as models to investigate the effect of their aggregation behaviors on their photophysical properties in deionized water. The results showed that all of these PSs could form aggregates at a suitable concentration in aqueous solution. However, the aggregation behaviors of symmetrically modified PSs (B2, X2 and M2) and asymmetrically modified ones (B3, X3 and M3) were quite different, which had significant effects on their photophysical properties, especially for two-photon excited fluorescence (2PEF). Under a high concentration (2×10−4 M), the compact aggregates of B3 and X3 exhibited enhanced 2PEF intensity versus fluorescence quenching of others. Moreover, B2, X2 and M2 presented higher polarity and lower cytotoxicity, while B2 and B3 with shorter PEG showed good performance in two-photon polymerization, indicating B2 will be a potential PS for 3D microfabrication of biosafety materials.

Novel β-fused BODIPY-Porphyrin compounds that contain free base porphyrin (TPP2BDP) and its Ni(II) complex (NiTPP2BDP) were synthesized to investigate intramolecular energy transfer mechanisms of β-fused BODIPY-porphyrin dyads and effect of the unfilled d shell metal ion on energy transfer mechanism. Fluorescence spectra of compounds exhibited that BODIPY emission was diminished upon excitation of the BODIPY unit because of the energy transfer from BODIPY to porphyrin unit. Femtosecond pump-probe spectroscopy measurements revealed that energy transfer of investigated compounds are faster than previous studies in literature. Rapid energy transfer (about 500 fs) from BODIPY to porphyrin was observed for both compounds when BODIPY unit is excited due to direct linkage of BODIPY to porphyrin unit. Intersystem crossing mechanism was also observed for the compound that contains free base porphyrin (TPP2BDP), whereas d-d transition was observed for the compound that contains metalloporphyrin (NiTPP2BDP) due to unfilled d orbital of Ni(II) ion.

Pterins are low molecular weight heterocyclic compounds that are widely distributed in living organisms, primarily in the form of reduced coenzyme forms. Oxidized pterins are present in the cells for the most part as oxidation products of tetrahydropterins and dihydropterins. Oxidized pterins are known to cause DNA photodamage under UV-irradiation. Photosensitization of oxidized pterins may cause oxidative stress in the human skin depigmentation disorder vitiligo. There have been made attempts to use oxidized pterins as sensitizers in photodynamic therapy (PDT) of cancer. The photoreactions between a set of six pterin compounds and molecular oxygen were explored using density functional theory, a time-dependent formalism for excited states (TD-DFT) and a continuum COSMO model to include the effects of H2O solvation. Both acid and base forms of pterins were taken into consideration. The computed singlet and triplet excitation energies are in agreement with the experimental data. We showed that compounds with the most electronegative lateral substituent (formyl, carboxyl) at C6 position have the highest value of both S0 and T1 state ionization potential. We conclude that pterin molecules do participate in photoreactions with molecular oxygen. Oxidized pterins are able to generate singlet oxygen and may also produce superoxide-anion radicals indirectly through autoionization reactions. Direct electron transfer reactions between pterins and oxygen were also studied.

Hemithioindigo dyes (HTIs) are a class of emerging visible light photoswitches. By using M06, M062X, B3LYP and BMK density functionals, we have investigated the local excited (LE) state and twisted intramolecular charge transfer (TICT) state of HTIs. Both the two excited states are located in HTIs Z1 and Z2 after combining the results of all functionals. The formation of either the LE or TICT state is found to be controlled by the direction of the C2 C3 single bond rotation. Different functionals are found to have intrinsic biases on LE and TICT states. Thus, a reasonable combination of various functionals is a convenient method to investigate a system possessing multiple excited-state minima. We recommend a combination of the M06 and M062X functionals.

The phenomena of ground and excited state proton transfer (GSPT and ESPT) of the synthesized Schiff base, (E)-2-((napthalen-2-ylimino)-methyl)phenol (NMP) have been unraveled in aqueous medium using various spectroscopic techniques. Changes in the absorption and emission spectra of an aqueous solution of the probe upon addition of an external base are ascribed to the proton transfer in both the ground and the excited states. Existence of equilibria between the neutral and the deprotonated species in the two electronic states is established from the observation of the isosbestic and isoemissive points in the absorption and emission spectra. Acid dissociation constants in the ground state (pKa) and photoexcited state (pKa*) of NMP in aqueous medium have been derived from the steady-state experiments using Weller’s method. The time resolved experiments clearly indicate the reversibility of the proton transfer reaction in the photo-excited state. The pKa* value determined from the kinetic measurements corroborates that obtained from the steady state methods. The higher value of pKa* than pKa is rationalized from the higher electron density on oxygen in the photoexcited state relative to the ground state, as obtained from the Mulliken charge densities through quantum chemical calculations. Extending our studies to the micellar environments, it is established that the deprotonation process is favored in cationic micelles while the same is disfavored in anionic or non-ionic micelles. This has been rationalized in terms of the surface charge characteristics of the individual micelles.

Six new highly fluorescent 5-(aryl)-1-phenyl-3-(pyren-1yl)-2-pyrazolines were synthesized by reaction of various (E)-3-aryl-1-(pyren-1-yl)prop-2-en-1-ones with phenyl hydrazine in the presence of sulfonated mesoporous silica (MCM-SO3H) as efficient and eco-friendly acidic catalyst. The chemical structures of all synthesized compounds were illustrated on the basis of spectral data (IR, 1H NMR and 13C NMR). This was followed by photophysical properties-based absorption and emission studies of the target compounds in solutions of different solvent polarities. The microenvironment-sensitive fluorescent pyrazolines labeled with pyrene exhibited valuable fluorescence properties with emission in the range of 600–700 nm with a marked response to change in the environmental polarities. A significant and pronounced red shift was observed in the emission spectrum of 5-(aryl)-1-phenyl-3-(pyren-1yl)-2-pyrazolines Δλ ∼ 50 nm compared to the absorption spectrum Δλ ∼ 10 nm upon increasing the solvent polarity. This indicated the presence of higher dipole moment in the excited state than in the ground state and the transition involved are π-π* transition through the charge transfer phenomena. The fundamental understanding of solvatochromic properties were analyzed through Lippert-Mataga and Reichardts correlations in order to estimate the change in dipole moments (Δμ) which suggested the emissive state of designed fluorescent 2-pyrazoline derivatives is of strong ICT character.

In the present work, the photophysical properties of three novel optoelectronically active thiophene substituted 1,3,4-oxadiazole derivatives, namely 2-(4-(1H-inden-2-yl)phenyl)-5-(5-(1H-inden-2- yl) thiophen-2-yl)-1,3,4-oxadiazole [TIO], 2-(4-(thiophen-3-yl)phenyl)-5-(5-(thiophen-3-yl)thiophen-2-yl)-1,3,4-oxadiazole [TTO] and 2-(4-(benzo [b] thiophen-2-yl)phenyl)-5-(5-(benzo[b]thiophen-2-yl)thiophen-2-yl)-1,3,4-oxadiazole [TPO] have been investigated. The ground and excited state dipole moments (μe) were determined experimentally by solvatochromic shift method using various solvatochromic correlations like Lippert’s, Bakhshiev’s, Kawski-Chamma-Viallet’s and solvent polarity parameter ( E T N ) equations. The ground state dipole moments (μg) were also estimated from ab initio computations using Gaussian 09 W software. It is observed that, the excited state dipole moments are higher than the ground state dipole moments for all the probe molecules. This indicates that, the probe molecules are more polar in the excited state than in the ground state. The HOMO-LUMO energy gap computed using density functional theory (DFT) and from absorption threshold wavelengths are found to be in good agreement. The chemical hardness (η) was determined for all the probes from HOMO-LUMO energies and results suggest the soft nature of the molecules. Further, the reactive centers like electrophilic site and nucleophilic site were identified with the help of molecular electrostatic potential (MESP) 3D plots using DFT computational analysis. Our preliminary investigations suggest that, the derivatives of 1,3,4-oxadiazoles namely TIO, TTO and TPO could play an important role in photonic, sensor and optoelectronic devices in future.

Photophysical properties of the ground and excited states of 1-naphthol-4-sulfonate (1NO4S), photoacid, are investigated in different solvents and cyclodextrin derivatives of different size and structure. Fluorescence decay traces of 1NO4S are well fitted to double exponential decay function in all solvents. The short-lived traces with higher amplitude were assigned to emission of the neutral form (ROH*), while the long-lived traces with low amplitude were assigned to emission of the anionic form (RO−*). The photophysical properties of 1NO4S were correlated with a number of solvent parameters such as the solvent polarity parameters E T N and the reaction field factor Δf. Three linear solvation energy relationships developed by Kamlet-Taft, Catalan, and Laurence et al. were employed to assess the effect of non-specific and specific interactions on the photophysical properties of 1NO4S. All three solvation energy relationships showed that the non-specific interactions mainly control the photophysical properties of 1NO4S while the contribution of the specific interactions are weak. Molecular encapsulation of 1NO4S by α-cyclodextrin (α-CD), β-cyclodextrin (β-CD), γ-cyclodextrin (γ-CD), methyl-β-cyclodextrin (Mβ-CD), 2-hydroxypropyl-β-cyclodextrin (HPβ-CD) in aqueous solution has been studied by steady state and time resolved fluorescence techniques. Steady state fluorescence measurements show 1:1 inclusion of 1NO4S with different cyclodextrin derivatives with an association constant of 7.9 ± 4.0, 59.6 ± 10.0, 0.4 ± 0.5, 169.8 ± 25.0, and 359.0 ± 20.0 M−1 for 1NO4S complexes with α-CD, β-CD, γ-CD, Mβ-CD or HPβ-CD, respectively following the fluorescence intensity changes of the anionic form (RO−*) at 435.0 nm. Association constants obtained following the fluorescence enhancement of the neutral form (ROH*) were the same as determined following fluorescence intensity decrease of RO−* at 372 nm. Data derived from time resolved measurements gave very close value for the association constants as determined from steady state fluorescence measurements.

Boron dipyrromethene (BODIPY) being a chromophoric dye molecule has been extensively studied in dye sensitized solar cell (DSSC), imaging, and dye laser application. BODIPY is a stable boron complex with a broad absorption and photoluminescence (PL) in the visible region of solar radiation. This class of molecules show anomalous photophysical properties, which varies with concentration of the molecule. Here, we report the thorough analysis of photophysical study after synthesized the BODIPY molecule, which made up of two thiophene BODIPY cores comprising a phenyl spacer in between, coded as TG2 dye molecule. Steady state and time resolved concentration dependent PL reveal that, at very low concentration (nM) TG2 shows both S1 (first excited state) and S2 (second excited state) PL. Importantly, at higher concentration (mM) TG2 found to exist in aggregated state. Optical absorption and PL studies confirmed that TG2 exists in mixture of J-/H- aggregate state. Moreover, femtosecond transient absorption (fs-TA) analysis has been studied following 400 nm optical pump, which reveals that the excited singlet state converted to the mixed aggregated state with in an average time constant of 6 ps.

Novel stable zinc (II) thiol-phthalocyaninate gold nanoparticle nanoconjugates (PcS-AuNP) with a reproducible and homogeneous distribution size of 16 ± 4 nm were efficiently prepared by means of a ligand exchange reaction. The nanoconjugates were characterized by TEM, DLS, FT-IR and UV–vis. PcS-AuNP were a stable system at least for two months in darkness. Singlet molecular oxygen generation was observed when gold plasmon and phthalocyanine were independently excited at 510 nm and 610 nm, respectively. The PcS-AuNP system obtained is a photosensitive material in a wide range of the UV–vis spectrum (300–400 nm, 500–530 nm and 610–799 nm). © 2018 Elsevier B.V.

The excited-state intramolecular proton transfer (ESIPT) dynamics of 2-(4′-Amino-2′-hydroxyphenyl)-1H-imidazo-[4,5-c]pyridine (AHPIP-c) has been studied by using density-functional theory and time-dependent density-functional theory method. Three kinds of different polar aprotic solvents, including acetonitrile (strong polar), tetrahydrofuran (weak polar) and methylcyclohexane (non-polar) have been chosen to explore solvent effects on these molecules. The calculated absorption and fluorescence spectra agree well with the experimental results for the three solvents and the dual fluorescence emission mechanism is well explained. The electron density ρ(r) and Laplacian ∇2ρ(r) at the bond critical point (BCP) have been calculated using the Atoms-In-Molecule (AIM) theory, which prove that the intramolecular hydrogen bond (O1 H2⋯N3) exists in the S0 state. The geometric parameters and the infrared vibrational spectra in the O H stretching vibrational region have been calculated, which manifests the hydrogen-bond is strengthened in the S1 state. The molecular electrostatic potential surface and frontier molecular orbitals analysis demonstrate that the proton transfer prefer occurring on excited state because of the charge redistribution upon photo-excitation. The results of potential energy curves, further confirm that the proton transfer process is more likely to conduct in the S1 state due to the lower potential energy barrier than that in the S0 state. In addition, we also find that ESIPT reaction is more easily to occur as the solvent polarity decreases. Therefore, we believe that solvent effect could play an important role in controlling excited state behaviors of AHPIP-c molecules.

We report on the design and synthesis of new D-π-A organic sensitizers incorporating D35 as the electron donor unit and (benzyloxy)benzene as the π- linker. The structure and yield of the synthesized dyes were confirmed via The 1H NMR, 13C NMR, and mass spectrometry analyses. The density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations were used to get insights on the electronic properties of the synthesized dyes, indicating their capability as photosensitizers. The photophysical characteristics, electrochemical properties, and photovoltaic performance of the sensitizers were investigated. The cyclic voltammetry measurements showed that the oxidation potentials of the excited states of the dyes are more negative than the conduction band edge of anatase TiO2 and their ground state oxidation potentials are more positive than the CoII/III redox shuttle, indicating their potential as photosensitizers. The dye containing benzyloxy group at the para position to the anchoring group showed higher power conversion efficiency (PCE), higher light harvesting efficiency (LHE), wider range of absorption, lower bandgap, higher incident photon-to-current conversion efficiency (IPCE), and higher excitation binding energy than the counterpart dye containing benzyloxy group in the ortho position.

Serotonin (5-HT) is an important monoamine neurotransmitter that plays a vital role in the regulation of various cognitive and behavioural functions including sleep, mood, pain, depression, anxiety, aggression, learning etc. From the nanotoxicity and neurotoxicity point of view, interaction studies between serotonin and nanoparticles are highly essential to develop an indispensable understanding on the effect of nanoparticles on monoamine neurotransmitters. In the present work, steady-state and time-domain fluorescence measurements were carried out along with binding energy measurements through X-ray photoelectron spectroscopic (XPS) technique to understand the interaction of 5-HT with silver nanoparticles (AgNPs). The emergence of a week red absorption band and quenching of fluorescence intensity along with decrease in full width half maximum, but not the excited state decay time of 5-HT in the addition of AgNPs suggest the formation of a non-fluorescent complex in the ground state that indicates static quenching along with radiative energy transfer among them. This is also confirmed by the low energy shift of Ag transition in 3d3/2 and 3d5/2 in XPS measurements and a coupling of Lb transition with the surface plasmons of AgNPs in the excitation spectra. The obtained thermodynamic parameters from the fluorescence quenching reveal the nature of interaction between 5-HT and AgNPs is hydrophobic. The increase in binding constant with increasing temperature suggested an enhancement in the strength of this interaction due to rise in the mobility of the molecules and hence increases the sensitivity and association of 5-HT with AgNPs.

Through solving the time-dependent Schrodinger equation for the nuclear motion, quantum dynamical simulations of the water/TiO2-rutile (110) system were performed. Five-dimensional potential energy surfaces (PES) for the electronic ground and an excited state previously calculated on CASSCF (Complete Active Space Self-Consistent Field) level of theory served as the starting point for our investigations [J. Mitschker, T. Kluner, Phys. Chem. Chem. Phys. 17 (2015) 268–275]. Through investigating the motion of the water molecule by a selected series of three- and four-dimensional simulations, a precise picture of the photodissociation mechanism is obtained. Furthermore, calculations of photodissociation probabilities reveal the most favoured pathway for a complete dissociation of the adsorbate including the excitation and relaxation process as well as temperature effects.

We present a detailed photophysical properties of a synthesized charge transfer (CT) molecule (2E, 4E)-ethyl 2-cyano-5-(4-(dimethylamino) phenyl) penta-2, 4-dienoate (ECDPPD) in bulk medium and its spectral modulation inside β-Cyclodextrin cavity. Enhancement of acceptor strength of 4-(dimethylamino) cinnamaldehyde by replacing the moderate acceptor in the form of a Formyl group by two acceptors known to display greater electron withdrawing ability along with extra conjugation which may be responsible for higher dipole moment both in ground as well as in excited state. The observed excited CT process and its solvent polarity dependent emissive properties is due to simply increasing the ‘end-to-end’ length between donor and acceptor along with introduction of two different acceptors in the same site. Low fluorescence quantum yield and shorter fluorescence lifetime is due to the operation of active non-radiative channels through flexible bonds supported by viscosity and temperature dependent spectral measurement. The molecule ECDPPD easily interacts with β-CD with the formation of inclusion complex with a definite stoichiometry where the photophysics are modulated and hence can be used as fluorescence microenvironment reporter for example to evaluate the polarity as well as the micro viscosity of various regions of protein and other bimolecular systems.

Photoinduced ω-bond cleavage was investigated in seven newly synthesized aromatic ketones having five-membered rings (furan and thiophene) and/or phenyl group connected to the phenylthiyl moiety with C S bond. Following direct laser excitation of these ketones in acetonitrile, the transient absorption spectra that appeared were shown to be due to the corresponding triplet states. No chemical reactions occurred upon direct excitation. However, triplet sensitization of these aromatic ketones using xanthone revealed no formation of their triplet states. Instead absorption spectra of the phenylthiyl radical (PTR) appeared as a result of C S bond cleavage. These observations indicate that higher triplet states were reactive leading to C S bond dissociation. The C S bond dissociation mechanism upon triplet sensitization for the studied carbonyl compounds was discussed considering energy diagrams of the higher excited states.

Based on a new designed hydroxylflavone-quinoline fluorophore, a novel fluorescent probe FQDNP for detecting thiophenol with large Stokes shift has been developed recently. Despite the excited state intramolecular proton transfer mechanism has been mentioned in the report, the detailed excited state processes for the compound FQOH produced by FQDNP reacting with PhSH are still ambiguous. Typically, in which direction will the proton of hydroxyl group transfer in the excited state? Oxygen or nitrogen atom, which one is more likely to interact with the proton in hydroxyl group by an intramolecular hydrogen bonding? In this work, we have explored the excited state processes of the probe detecting for thiophenol with the density functional theory (DFT) and the time-dependent density functional theory (TDDFT) methods. The calculated photo-induced changes in the optimized geometry parameters, intramolecular hydrogen bonding energies, stretching vibration frequencies, as well as the energy barriers for FQOH imply the proton of hydroxyl group is more likely to transfer to nitrogen atom rather than to oxygen atom in the S1 state. The calculations on the probe FQDNP reveals the fluorescence is quenched by the twisting and photoinduced electron transfer process. Our calculations provide a detailed description for the excited state processes of the probe sensing to thiophenol, which is expected to be applied in designing more effective fluorescent probes.

The excited-state intramolecular proton transfer (ESIPT) mechanism and photophysical properties of a series of 2-(1H-1,3-benzodiazol-2-yl)-3-hydroxy-4H-benzo[h]chromen-4-one (HBC4H) derivatives are explored theoretically. From these, a new explanation of ESIPT mechanism of 3-hydroxy-4H-benzochromone derivatives has been proposed. Firstly, three isomers are optimized in toluene solvent. through the analysis of the hydrogen bond parameters and infrared vibration spectrum, it is found that the hydrogen bonds of HBC4H should be strengthened in the S1 state. In order to explain the mechanism of ESIPT, the Boltzmann distribution and torsion forces of the three isomers are deliberated, and the searched transition state (TS) structures are depicted. It is discovered that the potential barriers are all lower than 5.00 kcal/mol in the S1 state, and HBC4H-Ia and HBC4H-Ic may coexist in the S0 state, and more significantly, among the three isomers, HBC4H-Ia may be the most favorable form of ESIPT.

We measured the heterodyne transient grating (HD-TG) responses of the BiVO4 photoanode for the first time. We demonstrated that the responses observed in acetonitrile, an inactive solvent, consisted of the recombination arising from the surface trapped holes and the thermal diffusion component. In addition, we found two components in the responses observed in aqueous solution under the applied bias condition, which were assigned to the recombination arising from the surface trapped holes and mixed component corresponding to the competition between back electron/hole recombination and water oxidation, respectively. The observed responses showed the strong dependence of the signal intensities on the applied voltage and the retardation of the recombination, resulting from the band bending.

Photosensitizers, which produce singlet oxygen O2(1Δg) under light irradiation, have attracted much attention due to their application potential in medicine and material science. In this respect, we recently reported on cationic metallocene complexes with pendant N-donor moieties, which exhibited enhanced luminescence in the solid-state originating from their triplet states with luminescence quantum yields, ΦL, up to approximately 0.52, unprecedented for this class of compounds. This finding suggested that these complexes can be efficient singlet oxygen photosensitizers. Herein, we evaluate the photogeneration of O2(1Δg) by several Ti, Zr and Hf metallocenes in dichloromethane solution, including recently described cationic derivatives. After excitation by UV A light, cationic Zr and Hf metallocenes stabilized by N-donor imine and pyridine moieties exhibited green, yellow or orange luminescence from long-lived 3LMCT excited states (ΦL ∼ 0.02 – 0.40 in argon-saturated solutions) and efficient O2(1Δg) production (ΦΔ ∼ 0.29 – 0.77 in air-saturated solutions). In contrast, parent neutral Zr complexes and cationic Ti derivatives with enhanced absorption of visible light, exhibited lesser ability to produce O2(1Δg) (ΦΔ

Luminescence quenching of [Ru(bpy)(CN)4]2−, [Ru(phen)(CN)4]2−, [Ru(bpy)3]2+ and [Ru(phen)3]2+ by hexavalent chromium ion in aqueous media were investigated by steady-state and time-resolved measurements. Effect of pH on the luminescence quenching process is also investigated. It has been found that luminescence changes due to the interaction of all Ru(II) complexes with Cr(VI) ions were high at pH values less than or equal to 5.0 and negligible at pH ≥ 8.0. The experimental data of the quenching process were analyzed using Stern–Volmer (S–V) equations. The steady state luminescence emission data shows an upward curvature, indicating that the quenching is preceded by both dynamic and static quenching. For all studied complexes, the lifetime of the excited 3MLCT state decreases with increasing Cr(VI) concentration implying that the diffusion is mainly responsible for the quenching process. The Stern–Volmer dynamic quenching constants (KD) and the bimolecular quenching rate constant for the quenching process (kq) were calculated by applying the Stern–Volmer equation to the luminescence decay data. The bimolecular quenching rate constants (Kq) were found to increase in the order [Ru(bpy)(CN)4]2−

Azapodophyllotoxin, the nitrogen analogue of antitumor podophyllotoxin, are new series of fascinating aspirants of anticancer drugs. The potential bioactive azapodophyllotoxin 5-(2-Hydroxyethyl)-9-(3,4,5-trimethoxyphenyl)-6,9-dihydro-[1,3]dioxolo[4,5–g]furo[3,4–b]quinolin-8(5H)-one (HTDQ) has been found to be highly effective against various cancer cell lines as compared to its minimal toxicity against normal cell lines. With an ulterior motive of exploring fluorescence properties of this promising antineoplastic candidate in biological environment, the intricate details of its ground and excited state solvation and solvation dynamics are investigated using steady state and Time resolved spectroscopic techniques. The experimental observations are substantiated by DFT and TD-DFT based quantum chemical calculations along with in silico molecular dynamics simulation. The solvatochromic behaviour of HTDQ reveals its sensitivity towards solvent polarity and proticity. The charge delocalization within HTDQ framework in ground state (G.S) entirely differs from that of in excited state (E.S). The change in polarity of the medium induces a peculiar short range intramolecular charge delocalization in G.S itself, forming a zwitterionic structure and endorses the unusual absorption characteristics. Whereas, in the E.S, it undergoes a long range intramolecular charge transfer (ICT) . In non-polar solvents, the long range ICT induces stacking interaction between two dipolar HTDQ molecules leading to the formation of a new short-lived less emissive excimer species. However, in aprotic and protic polar solvents, the scenario is individually different due to the presence of H-bond donor (hydroxyl hydrogen atom) as well as acceptor sites (oxygen atoms) in the molecular framework of HTDQ. In aprotic solvents, the formation of H-bonds occurs due to presence of hydrogen acceptor site in the solvent that boost up the emission intensity to its maxima. In polar protic solvents, HDTQ subsumes both of its H-bond donor and acceptor sites and further strengthens the H-bonding interactions, which essentially stabilizes the probe molecule within the solvent shell and open up the non-radiative decay channels thereby reducing the emission intensity. HTDQ exhibits bathochromic shift in the emission spectra with increasing solvent polarity, with some exceptional shifts in 1,4 dioxane and THF, which may stem from another interesting conformational polarization of non-polar chair form to polar boat form of the solvent itself. Thus, the different spectroscopic aspects dealt herein demystify the impact of solvent milieu on the solute–solvent interactions and the interesting emissive nature of this potential therapeutic HTDQ that may provide impetus to develop new class of theragnostic agent with promising imaging capability and improved anti-neoplastic activities.

In this work, we explore the substituent effect, which is a tunable parameter for the excited-state intra-molecular proton transfer (ESIPT) process, on the photo-physical properties and ESIPT process of benzo[a]imidazo[5,1,2-cd]indolizines via theoretical methods. The calculations in geometric parameters, O–H stretching vibration frequencies, hydrogen bonding energies and energy curves along with the proton transfer path demonstrate the substitution exhibits different effects on the hydrogen bonding behaviors in the ground and excited state, thereby on the ESIPT process. In addition, the simulated absorption and emission spectra are in agreement well with the experimental results. Compared to those for the analogs bearing fluorine and methyl substitutions, the relatively higher quantum yield and the blue-shift emission wavelength of methoxy group substituted compound have been attributed to the less extent of charge transfer in the excited state.

The three Cu(I) complexes [Cu(POP)(N^N)]PF6, N^N = 2-(1H-imidazol-2-yl)-5-(naphthalen-2-ylethynyl)pyridine (P1), 2-(1H-imidazol-2-yl)-5-(phenanthren-9-ylethynyl)pyridine (P2), 2-(5-(naphthalen-2-ylethynyl)pyridin-2-yl)-1H-phenanthro[9,10-d]imidazole (P3), were prepared and characterized. The photophysical properties of these Cu(I) complexes is tuned by extending π- system of the N^N ligand through the addition of polyaromatic groups such as naphthalene and phenanthrene into the pyridine ring part of the diimine ligands via an acetylide linker or phenanthrene-fused imidazole ring. The resulting complexes exhibit higher absorption ability in the visible region. The emissions are in the range of 582–601 nm with quantum yields of 2.30–7.70% and the excited lifetimes of 77.4–115.8 μs for all complexes at room temperature. The photophysical properties were further explained by DFT and TDDFT methods.

Excited state photophysics of flavin compounds has attracted great attention due to their involvement in several photobiological processes as a cofactor in photoreceptor enzymes. In this work, we have determined the initial excited state structural dynamics of flavin related compound, lumiflavin (LF), upon ultraviolet (UV) excitation through resonance Raman intensity analysis. A quantitative measurement of resonance Raman cross-sections across the 265 nm absorption band (257–275 nm) of LF have been performed, and Raman excitation profiles (REPs) are constructed for all the Raman active modes. The REPs and absorption cross sections have been simulated using time-dependent wave packet propagation (TDWP) formalism to extract the ultrafast dynamics of LF within 50 fs after photoexcitation. The total internal reorganization energy is estimated to be 470 cm−1, which is clear evidence for the significant structural distortion in the excited state from that of the ground state. By comparing the excited state structural changes between S1 state and 265 nm excited state, we found that different molecular distortions occur following photoexcitation to those two different states. The total reorganization energy is obtained to be 1542 cm−1, in which the maximum contribution comes from the inertial response of water (1070 cm−1). Additionally, our simulation also yields an instantaneous response of the first solvation shell within an ultrafast timescale, τ ∼ 30 fs, following photoexcitation. The current study provides the basis for future investigations to determine the excited state properties of flavin compounds upon UV excitations.

A detailed study of the excited-state tautomerization of 7AI-(NH3)n (n = 1, 2), 7AI-H2O-NH3 and 7AI-NH3-H2O complex in the gas phase were investigated at the CASSCF(10, 9)/6-31G(d, p) level theoretically. The effect of dynamic electron correlation on the barrier height and reaction energy was considered by MRPT2. The excited-state double proton transfer in 7AI-NH3 takes place in a concerted but asynchronous protolysis pathway. However, the triple proton transfer in 7AI cluster along the H-bonded ammonia dimer prefers a two-step process with a distinct intermediate. Each proton transfer step was a concerted but asynchronous path. The formation of H-bonded chain consisting of mixed water-ammonia affected the excited-state proton transfer mechanism. With the stabilization effect of the second solvent molecule, the first solvent molecule with large or small proton-accepting ability made the ESTPT occur in a stepwise or concerted pattern.

Photo-Fries rearrangement of phenyl benzoate is studied using continuous flow for modulating the reactivity of singlet radical pair by changing the viscosity of the solvent. The effect of flow and proximity of the reactants with the light source on the reactivity of radical pair, formed from singlet excited state was investigated in details. In non-viscous solvent, the results from flow synthesis were comparable to batch reactor. In viscous solvents, selectivity of ortho- and para-isomers (o-/p- isomer) of the product could be controlled by changing viscosity as well as the flow rate. Using flow synthesis, ortho- and para-isomer ratio was obtained as high as 8.45 which are twice as compared to batch experiment with in fraction of residence time.

Three linear push-pull compounds comprised of difluoroboron β-diketonate and triphenylamine unit 4,6-Bis(4-(diphenylamino)phenyl)-2,2-difluoro-2H-1,3,2-dioxaborinin-1-ium-2-uide, (E)-6-(4-(Diphenylamino)phenyl)-4(4(diphenylamino)styryl)-2,2-difluoro-2H-1,3,2-dioxaborinin-1ium-2-uide and 4,6-Bis((E)-4-(diphenylamino)styryl)-2,2-difluoro-2H-1,3,2-dioxaborinin-1-ium-2-uide (ABA, ABVA, and AVBVA) have been probed by many spectroscopies. The results clarify the photo-physical property of the compounds where ABVA and AVBVA are linked with vinyl units (V). The solvent-dependent steady absorption and photoluminescence spectra data shows that the vinyl unit and the polarity of the solvent influence the photo-physical properties of the compound. Moreover, the vinyl unit would further enhance the role of the polar environment. The transient absorption measurement then offers excitation relaxation behavior. The results show that excited compounds in cyclohexene go back to the ground state through the intramolecular charge transfer (ICT) state, and the transfer lifetime from the initial excited state to the ICT state would prolong after the introduction of vinyl unit. The relaxation behavior becomes complex when cyclohexene is replaced by tetrahydrofuran. Some excited compounds go directly back to the ground state and others go back to the ground state after passing through the ICT state. In addition, two-photon PL measurements and a Z-scan technique are employed, and this confirms that these compounds have two-photon behavior, but the two-photon absorption section gradually decreases. This decrease is attributed to the spectral shifting affected by the vinyl unit.

Three novel two-photon absorption chromophores (1BI, 2TAZ, and 3PyB), which have trans-1,4-divinylbenzene π-bridge, methoxy donors, and different acceptors, were synthesized. Their structures were elucidated by means of hydrogen-1 nuclear magnetic resonance, carbon-13 nuclear magnetic resonance, Fourier transform infrared spectroscopy, and high-resolution mass spectrometry. Their photophysical properties, including linear absorption, single-photon excited fluorescence, two-photon absorption, and two-photon excited fluorescence, were systematically investigated in various solvents. The structure-property relationships were studied aided with the density functional theory calculations. In addition, the absorption, single- and two-photon fluorescence spectra of 3PyB-ctDNA were examined to evaluate whether 3PyB could be used as a DNA stain. The possible interaction mechanism of 3PyB-ctDNA was also studied by the NaCl titration and the viscosity experiment. The results indicate that 3PyB may serve as a potential DNA fluorescent probe.

Photophysical properties of some 1-naphthol and 2-naphthol sulfonate derivatives and their efficiency as singlet oxygen photosensitizers in aqueous media are reported. The fluorescence emission is mainly from the deprotonated form (conjugate base) and their maxima cover the range of 433–529 nm. The fluorescence decay of these derivatives was found to be fitted with bi-exponential function with χ2 of about 1.0 ± 0.1 in all cases. Rate constants for oxygen quenching of the excited singlet states of these compounds in H2O are found to be (1.2 ± 0.2) × 1010 M−1 s−1 for the neutral form and much lower for the quenching of their conjugate bases. p K a values were found to be in the range 7.4–9.5 while p K a * are close to zero and about 3.3 for 2-naphthol and 2-naphthol-6-sulfonate using Forster equation and in the range of 1.4–3.0 using the fluorescence titration method obtained from the relative integrated fluorescence intensity. Singlet oxygen quantum yields photosensitized by these photoacids were reported in D2O and found to be in the range 0.13–0.35 for the sulfonated derivatives.

A series of novel highly Stokes shifted near infrared (NIR) emitting donor-π-acceptor (D-π-A) based push-pull chromophore BJH1-BJH6 having 4- (diethylamino) aryl as a donor, separated by π-spacer were designed and synthesized. BJH2 and BJH5 have additional cyano group, which acts as electron trap and alters the photophysical properties significantly. The photophysical properties were recorded in nine different solvents, and as the polarity increases the absorption and emission are red shifted. The solvent polarity plots support positive solvatochromism and charge transfer. Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) computations were utilized to correlate the effect of the π-linker modification on absorption. Stokes shifts were in the range of 47 nm to 240 nm. The highest Stokes shift of 240 nm was observed for BJH3 in acetonitrile. The higher Stokes shift and larger dipole moment ratio confirm the higher polar the excited state. The HOMO-LUMO energy levels obtained by DFT are utilized to establish a correlation between structural modification and photophysical properties. BJH2 and BJH5 having acetonitrile thiophene as a π-linker show selective stabilization of LUMO level without altering HOMO energy levels.

Based on Bairds rule, which states that the excited state of an aromatic compound is antiaromatic, a new method for hydrogenating aromatic molecules by metal-free photochemical reaction employing 1,4-cyclohexadiene (1,4-CHD) as hydrogen source is proposed. Here only light and 1,4-CHD are required to hydrogenate aromatic compounds without metal catalysts or temperature. The proposed mechanism is related to the reduction potential of the aromatic probes on their first triplet excited state. In such methodology, besides the non-aromatic character of the excited state, the aromatization of the hydrogen donor turns this reaction feasible as a hydrogen storage procedure at low cost, expanding the chemical interest for materials like graphene.

Photophysical properties of a newly designed synthetic compound DYMDAP have been investigated in combination with computational calculations. DYMDAP shows a solvent dependent emissive behavior which in non-polar solvents shows two distinct peaks along with a shoulder but with increasing polarity as also proticity of the solvents the intensity of each band varies significantly. Along with the two bands, a new band is observed in polar protic solvent due to the proton transfer (PT) in the excited state of the compound. Only in polar protic solvents, intensity of the PT band increases significantly whereas in non-polar or polar aprotic medium intensity is sparse. By comparing the spectral observations of DYMDAP with two control compounds DYMP and DYMDAB the proton transfer process is assigned to be due to solvent mediated and the proton transfer process is enhanced by the presence of charge transfer moiety.

In this article, photophysical characterizations of series of 2,6-unsubstituented 1,4-dihydropyridines (US-DHPs) were described by UV–vis absorption and fluorescence emission spectroscopies in diluted solutions. Their maxima show an intimate relationship with the substituent position on the DHP ring and the solvent nature had an influence on the irradiative lifetime of the lowest energy transition. In order to get insight into these behaviours, theoretical calculations were performed at the density functional theory (DFT) level. The geometrical and electronic structural variations after excitation were analyzed and the natural populatation analysis (NPA) charge distributions in the excited states were calculated for predicting the reactive sites of US-DHPs, which is meaningful for explaining the regioselectivity of their photochemical reactions.

In this work, we present a whole and deep study on the thermal redox and the photophysical and photochemical reactions of a tricarbonyl Re(I) complex coordinating Pterin, fac-ReI(CO)3(pterin)(H2O) (pterin = 2-amino-4-oxo-3H-pteridine). In aqueous solutions, the fluorescence of the complex is attributed to the emitting 1IL state (τemi ∼ 7.6 ns). In MeCN, however, the luminescence was ascribed to an overlapping dual emission from 1IL and 3MLCT states (τemi1 = 8.0 ns and τemi2 = 1.0 μs). Oxygen quenching of the 3MLCT based luminescence occurred with kq = 1.6 × 109 M−1 s−1. In glasses at 77 K, nevertheless, the 3MLCT prevailed over 1IL states. Flash photolysis experiments in aqueous solutions showed the spectrum of 3IL while in MeCN the presence of 3MLCT was evident. Pulse radiolysis experiments under oxidizing and reducing environments were performed in aqueous solutions of the Re-Pterin complex. By performing multivariate curve resolution − alternating least-squares (MCR-ALS), two species were identified under reducing conditions: an intermediate related to the semireduced radical of pterin ligand and the dihydrogenated [Re(CO)3(7,8-PtrH2)(H2O)] product. TD-DFT calculations helped to ascertain the nature of these species. Flash photolysis experiments where the excited states were reduced with triethylamine were in good agreement with pulse radiolysis experiments under reductive conditions. The oxidized transient spectrum was also obtained by pulse radiolysis, which compares very well with those published for Re(II) species, leading us to propose oxidation in the metal core as the reaction product of the transient under oxidizing conditions. The combination of different spectroscopic techniques along with the theoretical calculation allowed elucidating the nature, dynamics and reactivity of the excited states prevail in a Re-Pterin complex. This is of the particular importance considering that equivalent studies have not been reported for any other rhenium complex of substituted pterins nor for complexes of other transition metal ions of pterin derivatives. Fil: Ragone, Fabricio. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas; Argentina Fil: David Gara, Pedro Maximiliano. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Centro de Investigaciones Ópticas. Provincia de Buenos Aires. Gobernación. Comisión de Investigaciones Científicas. Centro de Investigaciones Ópticas. Universidad Nacional de La Plata. Centro de Investigaciones Ópticas; Argentina Fil: Garcia Einschlag, Fernando Sebastian. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas; Argentina Fil: Lappin, Alexander G.. University of Notre Dame; Estados Unidos Fil: Ferraudi, Guillermo J.. University of Notre Dame; Estados Unidos Fil: Wolcan, Ezequiel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas; Argentina Fil: Ruiz, Gustavo Teodosio. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas; Argentina

Fluorescent molecular rotors (FMR) have wide range of applications due to high sensitivity of their emission intensity to microenvironment viscosity. Thioflavin T (ThT), which exhibits FMR properties, is widely used as fluorescent probe for in vitro detection of amyloid fibrils (AF) due to its high affinity and “light-up” feature (fluorescence quantum yield of ThT changes by ∼3 orders of magnitude upon binding to AF). At physiological pH, ThT is positively charged and, therefore, it does not cross blood-brain barrier (BBB). It has been proposed that neutral derivatives of ThT are more likely to cross BBB, which should make them suitable for in vivo applications. However, for in vivo applications as fluorescent imaging agents, the neutral ThT derivatives must retain the FMR properties of ThT. In this paper, we examined whether neutral ThT derivatives exhibit FMR properties by studying the effect of solvent viscosity on their fluorescence intensity and decay lifetime. We observed that while the cationic ThT derivatives possess FMR properties, the neutral forms behave as regular highly-emitting fluorophores. Further, quantum chemical calculations in gas phase showed significant differences in the shape of excited state potential energy surfaces for the neutral and cationic derivatives of ThT. While, for charged ThT derivatives, the E(S1*) energy is minimal for the twisted conformation with dihedral angle φ = 90° between molecular fragments, the coplanar conformation with φ = 0° (or 180°) is more favorable for the neutral derivatives. From our experimental and theoretical studies we conclude that the neutral ThT derivatives lack FMR properties as their photoexcitation does not induce twisting motion coupled with internal charge transfer and, therefore, their specificity as fluorescent imaging agents for AF detection is lower than that of parent ThT.

We report the photophysics of two bent-core liquid crystals (BLCs), C56H67NO9 (BLC1) and C55H65NO9 (BLC2), containing a Schiff-base and two long alkyl chains at its two ends. The ground state dipole moment and the electronic structures of the BLCs were calculated using density functional theory (DFT). The dipole moments of the liquid crystals were estimated experimentally via solvatochromic shift of the absorption and fluorescence spectra. The dipole moments obtained experimentally are in quite good agreement with theoretically calculated values. In the fluorescence spectra of both the BLCs show an interesting feature, i.e., dual emission. This dual emission is explained via the presence of two tautomeric forms (keto and enol) of the BLCs in the excited state. The band at similar to 390-450 nm is assigned to the emission due to the keto-form, whereas the band at similar to 340-370 nm is due to the emission from the enol-form. Irrespective of the solvents used, the keto-band is more intense than the enol-emission-band. This intense emission from the keto-form of the tautomers is demonstrated via the excited state intramolecular proton transfer (ESIPT). From the time resolved fluorescence spectroscopy studies, a longer lifetime is obtained for the keto-band. The associated vibrational states observed in the emission spectrum are responsible for this longer lifetime. Our finding of the dual emissive nature of the liquid crystals in the visible range is potentially useful for high temperature liquid crystal display and sensing applications. (C) 2018 Elsevier B.V. All rights reserved.

Singlet and triplet excited states dynamics of the photosynthetic pigment chlorophyll a in various solvents have been investigated by sub-ns pump-probe spectroscopic measurements with a sub-ns time resolution and a temporal window up to 400 μs. The singlet and triplet lifetimes of chlorophyll a were determined respectively to be 5.2–7.0 ns and 1.2–12 μs, depending on solvents. On a basis of a global analysis of time-resolved spectroscopic data, we estimated an yield of the intersystem crossing from the singlet to triplet excited states chlorophyll a to be about 30% depending not on surrounding environments. The value is much lower than the previously reported values.

Solvent dependence of trans → cis photoisomerization of N,N ′-diacetylindigo (DAI) was investigated by femtosecond transient absorption (TA) spectroscopy. A strong and sharp singlet excited state absorption band centered at 595 nm was observed in the TA spectrum and the differential absorbance (ΔAbs) and the wavelength at the maximum of the band were periodically modulated by coherent vibrational wavepacket motions. However, even in the “reactive solvent”, ethyl acetate (EtAc), where the isomerization takes place, the formation of cis -DAI was unobservable in our experimental time window of 2 ns which means that isomerization is occurring in a longer time scale. Preceding studies suggest that intermolecular hydrogen-bond (H-bond) with the solvent quenches the excited state and blocks the isomerization. Interestingly, we have observed that dimethylformamide (DMF) blocked the isomerization more efficiently than methanol. The excited state lifetime was shorter in DMF compared to EtAc solution suggesting that weak H-bonding with the formyl group of DMF is sufficient to prevent the slow trans → cis photoisomerization of DAI.

Regularities of ultrafast charge recombination kinetics in photoinduced intramolecular electron transfer in polar solvent are investigated. The multichannel stochastic model is used to simulate kinetics of charge separation and ensuing ultrafast charge recombination. The rate constant of ultrafast charge recombination occurring in non-equilibrium mode is shown to depend on the free energy gap of the preceding charge separation stage. This is a clear demonstration of violation of one of the fundamental principles of formal chemical kinetics, namely, the postulate of independence of elementary chemical reactions. The simulations revealed two main trends: (i) the charge recombination rate constant typically increases with decreasing the charge separation free energy gap, (ii) the maximum of the charge recombination rate constant, as a function of the charge recombination free energy gap, |ΔGCR|, shifts toward smaller values of |ΔGCR| with decreasing the free energy gap at the previous, charge separation, stage of the reaction. The prediction of ultrafast charge recombination in donor–acceptor dyads with small charge recombination free energy gap is shown to be supported by the experimental data on the kinetics of charge recombination into the first excited state followed the charge separation from the second excited state.