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The excited state dipole moments of two positional isomers of dicyanobenzene have been determined from thermochromic shifts of the absorption and fluorescence emission spectra in ethyl acetate solution and compared to the results of ab initio calculations. We found that the dipole moments of excited states from thermochromic shifts closely resemble the ab initio values of the isolated molecule, while the results of conductor-like screening model (COSMO) using the same wave function model as for the isolated molecule, shows considerable deviations from the experimental values. It is shown that the dipole moments of the two cyano groups add up vectorially for both the ground and excited states. • Dipole moment of two isomeric dicyanobenzenes determined in ground and excited state from thermochromic shifts. • Comparison of the experimental results to SCS-CC2 calculations shows a very good agreement for excited state dipole moments of the isolated molecules. • Bad agreement with the results of COSMO calculations. • Additivity rule for dipole increments works in the ground state and the excited state. [Display omitted] [ABSTRACT FROM AUTHOR]

The excited state dipole moment of anisole has been determined in the gas phase from electronic Stark spectroscopy and in solution using thermochromic shifts in ethyl acetate. Electronic excitation increases the anisole dipole moment in the gas phase from 1.26 D in the ground state to 2.19 D in the electronically excited singlet state, leaving the orientation of the dipole moment practically unchanged. These values are compared to solution phase dipole moments. From variation of the fluorescence emission and absorption maxima with temperature, an excited state dipole moment of 2.7 D was determined. Several solvent polarity functions have been used in combination with experimentally determined cavity volumes at the respective temperatures. Both gas phase and condensed phase experimental dipole moments are compared to the results of ab initio calculations at the CC2 level of theory, using the cc-pVTZ basis set for the isolated molecule and using the COnductor-like Screening MOdel (COSMO), implemented in Turbomole, for the solvated anisole molecule. [ABSTRACT FROM AUTHOR]

Based on Baird́s 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. [ABSTRACT FROM AUTHOR]

Variable-temperature absorption and emission data over the range 80–300 K have been collected for acetylene and buta-1,3-diyne derivatives with polycyclic aromatic hydrocarbons as substituents, in 2-methyltetrahydrofuran (2-MeTHF). The influence of temperature on fluorescence lifetime and quantum yield has also been investigated. A strong coupling between solvation dynamics and solute structure is observed. Moreover, the dependency of fluorescence quantum yield on temperature indicates that the energy of activation of conformational changes for studied compounds is comparable or slightly larger than the activation energy of viscous flow of 2-MeTHF. Moreover, contrary to diphenyl derivatives of buta-1,3-diyne for which conformation changes are barrierless, polycyclic aromatic hydrocarbons derivatives are characterized by a relatively high activation energy. [ABSTRACT FROM AUTHOR]

Variable-temperature absorption and emission data over the range 80–300 K have been collected for 1,2-diphenylacetylene (DPA) and its derivatives, as well as 1,4-diphenylbuta-1,3-diyne (DPB) derivatives in 2-methyltetrahydrofuran (2-MeTHF). Moreover, the influence of temperature on fluorescence lifetime and quantum yield has also been investigated. A strong coupling between solvation dynamics and solute structure is observed in the low-temperature regime as well as at intermediate temperature where the structure of the compound is evolving in the course of excited-state relaxation. Moreover, the dependency of fluorescence quantum yield on temperature indicates that the energy of activation of radiationless transition for 1,2-diphenylacetylene and its derivatives is larger than the activation energy of viscous flow of 2-MeTHF, while for 1,4-diphenylbuta-1,3-diyne derivatives the opposite is observed. This finding implies that the conformational change of DPA and its derivatives in the excited state in 2-MeTHF shows an activation energy, contrary to DPB and its derivatives for which conformational changes are barrierless. [ABSTRACT FROM AUTHOR]

In this study, a range of organic dyes were prepared to investigate difference between five- and six- number-membered ring transition states for internal proton transfer in excited state. Different strength intramolecular hydrogen bond in the target dyes was demonstrated by X-ray crystallography, 1 H NMR spectroscopy and ultraviolet/visible spectroscopy. The fluorescence measurement suggested excited state intramolecular proton transfer occurred via six number-membered ring transition state, while it could not occur through five number-membered ring transition state. The molecular geometry optimization of the target dyes further accounted for the difference of internal proton transfer in excited state occurred by five- or six- number-membered ring transition state. [ABSTRACT FROM AUTHOR]

The intermolecular hydrogen bonding dynamics between the functionalized activated carbons (AC) and ethyl mercaptan (ETM) were investigated using DFT and TDDFT methods, respectively. The ground-state and excited-state structures, electronic excitation energies and corresponding oscillation strengths of the low-lying electronically excited states for the functionalized ACs and their hydrogen-bonded complexes I, II, III and IV were calculated. It is demonstrated that the stability trend of forming hydrogen-bonded complexes is I > II and III > IV, which coincides with the results from the interaction energies calculation EHB. Moreover, according to Zhao's rule on the excited hydrogen bonding dynamics, it is found that the hydrogen bonds in the complexes I, II and III are significantly strengthened, with the excitation energy of a related excited state being red shifted, the hydrogen bond interactions can facilitate ETM adsorption onto AC-COOCO, AC-CCC and AC-CO, respectively. The intermolecular hydrogen bond of complex IV in the excited state is noteworthily weakened, in which the excitation energy of a related excited state is blue shifted. Therefore, the hydrogen bond interaction in complex IV is against ETM adsorption onto AC-OH in the excited state. The photoexcitation for complex IV should be controlled during the absorption process. [ABSTRACT FROM AUTHOR]

[Display omitted] • Thermochromic excited state dipole moment determination. • Discussion on state mixing in bicyclic chromophores under electric field influences. • Discussion of problems using thermochromic fluorescence spectroscopy. The excited state dipole moment of the lowest excited singlet state of 2,3-benzofuran in ethylacetate solution is determined using thermochromic spectroscopy and compared to the values which are obtained for the isolated molecule from electronic Stark spectroscopy (Hebestreit et al., 2020) and to the results of ab initio calculations at coupled cluster level of theory. It is shown that the dipole moment from thermochromic shifts in solution deviates considerably from that of the isolated molecule. This finding can be traced back to a field induced mixing by the strong reaction field of the solvent, which exceeds the external Stark field applied in gas phase measurements by a factor of 10.000. Depending on the dipole moments of the perturbing state and its energy gap to the excited state, dipole moments from thermochromic shifts (and consequently also of solvatochromic shifts) might be considerably different to gas phase values. However, the alteration of the electronic configuration of the molecule via the reaction field of the solvent can be deperturbed in the high-field limit. [ABSTRACT FROM AUTHOR]

Abstract: The excited state properties of two amino substituted uracils, 5-aminouracil (5AU) and 5-dimethylaminouracil (5DMAU) have been studied by steady-state and time-resolved fluorescence spectroscopy. In addition, the electronic transitions corresponding to photon absorption and emission were calculated using the TD-DFT/PCM method (with PBE0 and CAM-B3LYP functionals), explicitly including water molecules of the first solvent shell. The fluorescence decays, recorded by fluorescence upconversion, reveal that the emitting states of both molecules are substantially longer-lived than that of uracil. However, while the 5AU fluorescence decays on a picosecond timescale, the 5DMAU fluorescence contains a weak, long-lived component, continuing beyond 100ps. Calculations show that the excited state deactivation of 5AU is governed by the existence of a minimum on the ππ* excited state potential energy surface. For 5DMAU, on the other hand, a very long-lived but weakly fluorescent intramolecular charge transfer (ICT) state, involving a significant charge transfer between the dimethylamino group and the pyrimidine ring, is found. This stable ICT state is characterized by a rotation of the dimethylamino group towards a quasi-perpendicular geometry. [Copyright &y& Elsevier]

Abstract: Direct photolysis of many PCBs has a non quenchable component which survives oxygen quenching. In this study, the nature of the excited states of PCB 138 and PCB 77 was analyzed to understand the origins of reaction from singlet as well as triplet excited states. Data on luminescence quenching, luminescence profiles, reaction quantum yields, and product distributions as functions of excitation wavelength were collected. The most striking and unambiguous observation was the excitation wavelength dependence of the vibronic structure of a fluorescence. Dependence of the apparent quantum yields on the reaction conditions (concentration, intensity) was also recognized during this study. Upon correlating all collected data, sufficient evidence for slow vibrational relaxation in a 2,2′-dichloro congener emerged. This could be seen to open opportunities for reaction from the singlet in competition with intersystem crossing. The slow vibrational relaxation can be understood through analogy to quantitatively modeled biphenyl spectra, to arise from steric hindrance and solvent resistance to rotation about the central C–C bond. [Copyright &y& Elsevier]

Abstract: Two new Ru(II) complexes, [Ru(bpy)2(1-COO-iqu)]+ (2; bpy=2,2′-bipyridine, 1-COO-iqu− =isoquinoline-1-carboxylate) and [Ru(bpy)2(3-COO-iqu)]+ (3; 3-COO-iqu− =isoquinoline-3-carboxylate), were prepared and their crystal structures solved. The ground and excited state properties of 2 and 3 were characterized and compared to those of [Ru(bpy)3]2+ (1). The presence of the oxygen atom in the Ru(II) coordination sphere makes 2 and 3 easier to oxidize than 1. The Ru→bpy MLCT absorption and emission of 2 and 3 are red-shifted relative to that of 1 in CH2Cl2, and the E 00 energies were estimated to be 1.89eV and 1.95eV from the low temperature emission of 2 and 3, resulting in excited state oxidation potentials of −1.03V and −1.10V vs SCE, respectively. In addition to the short-lived emissive 3MLCT state, a long-lived species is observed in the transient absorption of 3 in DMSO (τ =49μs) and pyridine (τ =44μs), assigned to a solvent-coordinated complex. This intermediate is not observed for 3 in non-polar solvents or for 2. The absence of the solvent coordinated intermediate in 2 is explained by the stronger Ru–O bond afforded by the lower conjugation in that extends onto the carboxylic acid in the 1-COO-iquo−ligand, compared to that in the 3-COO-iqu−ligand in 3. Transient absorption experiments also show that the 3MLCT excited state of 3 is able to reduce methyl viologen. [ABSTRACT FROM AUTHOR]

Abstract: Silaazulene (SIAZ) skeletons where one of the C atoms in the azulene skeleton is replaced by Si atom have been characterized from a viewpoint of the excited state by means of ab initio complete active space self-consistent field (CASSCF) calculations. SIAZs studied in the present study are 2-silaazulene (2SIAZ) and 6-silaazulene (6SIAZ). The model reactions for the characterizations of 2SIAZ and 6SIAZ are an S1–S0 internal conversion (IC) process, which has been extensively studied on that of parent azulene. The initial processes of 2SIAZ and 6SIAZ upon electronic excitation into S1 are similar to that of azulene, i.e. change from an aromatic geometry into a non-aromatic planar one. Contrary to the case of azulene, however, 2SIAZ and 6SIAZ in S1 are further stabilized so as to take a non-planar geometry where the local geometry around the Si atom is non-planar. At the conical intersection between S1 and S0 (S1/S0-CIX) of 6SIAZ, only the seven-membered ring takes a non-planar geometry. On the other hand, both the five-membered and the seven-membered rings are non-planar at the S1/S0-CIX of 2SIAZ. Based on these computational findings, we characterize the SIAZ skeletons from a viewpoint of the excited state. [Copyright &y& Elsevier]

Abstract: The isomerization pathways of 4,4′-azobenzene disulfonate in the S0 and T1 states have been studied by using density functional theory (DFT) with B3LYP method at the levels of 6-31G(d,p) and 6-311++G(d,p), respectively. There are two isomerization pathways in the S0 state. One is the inversion of one CNN angle combined with certain degree of rotation around the CN bond, and it is worthy to notice that the potential energy profile includes three sequential transition states. The other pathway is the rotation of CNNC dihedral angle involved inversion of one CNN angle, while the two isomers are connected through the only one transition state. Calculation indicates that the molecular structures at the highest points on the potential energy profiles of two pathways are identical, and the energy barriers are the same, 20.52kcal/mol. In the T1 state, there exists the rotation pathway (rotation of CNNC dihedral angle) and its energy barrier is 4.11kcal/mol. In the excited states (T1, S1, T2, and S2), the potential energy profiles of the vertical excitation are obtained by time dependent density functional theory (TD-DFT) at the B3LYP/6-311++G(d,p) level. The photoexcitation at 342nm results in the reactant molecule populated in the S2 state, but isomerization does not occur directly on the S2 state due to the high energy barrier. It could undergo a rapid relaxation to the minimum of S1 state, and then the isomerization occurs using the inversion or rotation pathway. The results show that the rapid energy redistribution among the various vibrations renders the concentration of such amounts of energy in the inversion coordinate very improbable, while there are two possible photoisomerization pathways by the rotation pathway. The isomerization can easily occur through the S0/S1 conical intersection and the S0–T1–S0 crossing to reach the product. The primary isomerization pathways for 4,4′-azobenzene disulfonate can go through the inversion and rotation forms in the S0 state and the rotation mechanism in the excited state. [Copyright &y& Elsevier]

Abstract: Photoinduced charge transfer (CT) and excited state properties in the mixed coaggregates of 1,3,5-triphenyl-2-pyrazoline (TPP) and 1,4-dicyanonaphthalene (DCN) are investigated theoretically, using time-dependent density functional theory (TD-DFT) as well as the two-dimensional (2D) site (transition density matrix) and three-dimensional (3D) cube (transition density and charge difference density) representations. The calculated results indicate that a strong absorption band stems from the S0 →S4 transition. There are electron–hole coherences between TPP and DCN monomers, which are shown by 2D site representation. Direct visual evidence revealed by 3D cube representations indicates that photoinduced CT mechanism for the mixed coaggregates of TPP and DCN is the mixture of intermolecular and intramolecular CT in the vertical absorption. Some phenyl group of TPP monomer not only serves as the electronic donor in the intramolecular CT, but also in the intermolecular CT process. [Copyright &y& Elsevier]

Abstract: Both transient photolysis and steady-state photo-degradation experiments were performed to gain insight into the kinetics and mechanisms of degradation of p-nitroaniline (p-NA) in acetonitrile (MeCN) solutions. Complete degradation of p-NA was observed at diverse irradiation conditions under 254nm UV light. Once H2O2 was added into the experimental system, degradation of p-NA was enhanced remarkably. The removal rate increased rapidly with increment of the irradiation time and reached 90% at 30min. p-NA could be totally removed after 90min in UV/H2O2 process. In the presence of O2 and H2O2, removal rate increased linearly with increment of the irradiation time and reached 90% at 10min. p-NA could be totally removed after 20min in UV/(O2 +H2O2) process. For transient photolysis, excited states of p-NA were observed after 355 and 266nm laser flash photolysis (LFP). The transient absorption spectra were recorded and bimolecular rate constant of 6.89×109 M−1 s−1 was calculated for the self-quenching of 3p-NA*. Production of 3p-NA* in MeCN and H2O mixed solution was also studied. LFP of p-NA with addition of H2O2 was investigated for the first time. [Copyright &y& Elsevier]

Abstract: The excited states of a flavin-related compound, lumiflavin, were studied by the symmetry-adapted cluster (SAC)-configuration interaction (CI) method. The absorption peaks observed in the experimental spectrum were theoretically assigned. Transition energy of some low-lying n–π* states were obtained. The energy minimum structures of the first singlet and triplet excited states were calculated by the SAC-CI method. The structural changes upon excitation were at most 0.05Å. The solvation effect on the absorption energy in aqueous solution was investigated using polarizable continuum model (PCM) and by including water molecules into the computational model. The solvatochromic shift of the second peak (31A′ state) originates from both microscopic (hydrogen bonding) and macroscopic (electronic polarization of solvent) solvation effects. [Copyright &y& Elsevier]

Abstract: The structures of trans-[Pd2X4(PH2CH2PH2)2] (X=Cl (1), Br (2) and I (4)) and [Pd2(μ-I)2I2(PH2CH2PH2)2] (3) in the ground and excited states were optimized by the MP2 and unrestricted MP2 methods. It is shown that upon excitation the Pd–Pd distances shorten ca. 0.40Å for 1, 2 and 4 but lengthen ca. 0.10Å for 3. This is rationalized by their electronic structures in the excited states and spectroscopic calculations. On the basis of such structures, we predict the absorption and emission spectra of 1–3 in the CHCl3 solution at the time-dependent density functional theory (TD-DFT) level. Experimental absorption spectra are well reproduced by theoretically simulated spectra. [Copyright &y& Elsevier]

Abstract: Novel positively charged naphthalimide derivatives were synthesized, in which N-methylpyridinium substituent was attached to the dicarboximide moiety. The absorption and fluorescence spectra were studied, fluorescence lifetimes, fluorescence yields and triplet yields were determined in acetonitrile and CH2Cl2. Ion-pairing was found to markedly alter the photophysical properties in the latter solvent. The association with iodide counterion decreased the radiative rate constant. The triplet formation was much more rapid for the 2,3-isomers, whereas the 1,2-derivatives emitted stronger fluorescence. The rate of internal conversion proved to be more than one order of magnitude higher for all N-methylpyridinium derivatives compared with that of the parent compounds, which might indicate that the significantly smaller energy gap between the S1 and S2 excited states led to more efficient excited state relaxation. [Copyright &y& Elsevier]

Abstract: The charge transfer and structural distortions that occurred in the complexes cuprous(I) bis-phenanthroline (Cu(NN)2 +) (NN denotes 1,10-phenanthroline, 2,9-dimethyl-1,10-phenanthroline and 2,9-di(trifluoromethyl)-1,10-phenanthroline) upon excitation with an irradiation of light were studied by density functional theory (DFT). The calculations showed that the electrons transferred from central metal Cu to ligands with the transition of the complexes Cu(NN)2 + from ground state to excited state. As a consequence, the central copper in the excited state of Cu(NN)2 + exhibited similar electronic density to that in the corresponding complex Cu(NN)2 2+. Accompanying with this transfer process, the coordination polyhedra of Cu(NN)2 + became distorted upon excitation. The structural distortion was significantly reduced by increasing the steric bulk of the 2- and 9-positions substituents in the ligands NN, which is helpful for the increase of the life time of the excited state. [Copyright &y& Elsevier]

Abstract: Ab initio and CIS theories were employed to study a new type of perylenequinonoid photosensitizer (PQP)—hypomycin B (HMB), which has only one pair of adjacent hydroxyl group and carbonyl group in its peri-region. Geometries, Mulliken charge population, energy, and photophysical and photochemical process in excited state were discussed in detail. The results indicate that HMB exhibits quite similar intramolecular proton transfer (IPT) to perylenequinone, which offers an indirect understanding that the IPT process in perylenequinonoid derivatives (PQD) is a single proton transfer process. There exists the crossing of the potential energy surface of the singlet and triplet so that the intersystem crossing (ISC) can occur. The ISC will increase triplet quantum efficiency, which is the basis for photosensitization of PQP. [Copyright &y& Elsevier]

To find the effect of conjugate and substituents on the intramolecular proton transfer, fourteen systems are studied using HF and CIS methods. The results indicate that the intramolecular proton transfer barrier decreases firstly, and changes a little for larger conjugate systems when the combined phenyl cycles are added from three to five. Hereby, we conjecture that the larger conjugate systems (more than three combined phenyl cycles) have little effect on the proton transfer barrier. Theoretical studies on substituents effect show that electronic donor groups enhance the H-bond and electronic acceptor groups reduce the H-bond. The position of substituents is another factor in the proton transfer reaction. The effect of all the substituents on the proton transfer barrier is small, so the character of mother molecule is crucial to the proton transfer reaction. Moreover, the linear analysis results show that the charges on the acceptor oxygen and the differences between the labile hydrogen and the acceptor oxygen have good linear relationship with the proton transfer barrier. [Copyright &y& Elsevier]

Excited-state proton transfer (ESPT) in 2-naphthol derivatives with electron withdrawing substituent (&z.sbnd;SO2NH&z.sbnd;) dissolved in methanol and methanol–water mixtures is investigated. The analogs studied contain also a carboxyl group, situated at various distances from the proton donating phenolic group. The method of investigation consists in measuring the steady state fluorescence spectra. The parameter R of these spectra (ratio of fluorescence quantum yields at two defined wavelengths) given by Weller’s equation was fitted to the experimental values of R as a function of water concentration in the mixtures. From this procedure, ESPT rate constant and other parameters of this reaction were obtained. It was found that the carboxyl group contained in the molecule of 2-naphthol derivative greatly enhances ESPT reaction in methanol. This effect depends on the flexibility of a chain linking the aromatic system with the carboxyl group. ESPT reaction in five various analogs of 2-naphthol with an electron withdrawing substituent and a carboxyl group was compared. Influence of water addition and the fluorophore concentration, on ESPT was studied in detail. The effect of organic acid addition to the methanolic solution was also investigated. Influence of other organic solvents on ESPT is compared to that of methanol. Our results indicate that proton transfer to the carboxyl group and a preferential binding of other polar molecules play an essential role in the kinetics of proton transfer reaction The role of these catalytic effects is more pronounced in low polar media than in water solutions. By analogy to our model systems, conclusions concerning the proton transfer reaction in biological energy transformation systems can be drawn. [Copyright &y& Elsevier]

In cyclohexane as the solvent, 1-azacarbazole (or α-carboline) (AC) forms fluorescent ground state 1:1 hydrogen-bonded complexes with N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMA) and hexamethylphosphoramide (HMPA). The absorption and fluorescence spectra of the complexes are red shifted with respect to those of the non-bonded AC, and their association constants increase as the hydrogen-bonding acceptor properties of the amides increase. Fluorescence of the AC–DMF and AC–DMA solutions show monoexponential or biexponential decays depending on the monitored emission wavelength. To aid the interpretation of these results, we have also studied the effect that triethylamine and methylethylketone addition produces on the absorption and fluorescence spectra of AC. Triethylamine does not significantly affect the absorption spectrum of AC, but it dynamically quenches its fluorescence. Conversely, methylethylketone behaves similarly as amides do. On the basis of the above results, we assume that, in the ground state, the hydrogen-bonding interaction takes place between the pyrrolic NH group of AC and the carbonyl group of the amide. Hydrogen bonded complexes and non-bonded AC behave in the singlet excited state as independent fluorophores. Singlet excited state of free AC is dynamically quenched by DMF and DMA. The quenching mechanism involves the hydrogen-bonding interaction of the pyrrolic NH group of AC and the lone electron pair of the amide nitrogen atom. For HMPA, probably due to geometrical restrictions, this quenching process is absent. [Copyright &y& Elsevier]

• A theoretical DFT methodology may be used to tailor novel solar cell devices. • Lowest-energy structures at ground and excited states were computed in OPV materials. • Dihedral angles related to shifting of absorption maxima in UV–vis simulated spectra. • Descriptors obtained from electronic structure may aid to improve PV efficiencies. A density functional theory study was performed on a series of organic photovoltaic materials with a promising potential to be implemented as the active layer in a solar cell device. A thorough analysis on the molecular structure at ground and excited state revealed that some of the systems in the series of molecules observed in the ground state is dramatically altered in the first excitation. The dihedral deviations could be addressed as a ruling mechanism behind isomerization and its influence in the opto-electronic properties. The computation of absorption spectra at ground and excited state geometries showed a significant shift in the absorption maxima, presumably due to such geometrical changes. Furthermore, the computation of dipole moments gave us insights into the possible charge transfer from the OPV material to an acceptor in a solar cell device. This is critical when the system is implemented in such a device. Additionally, relevant photovoltaic parameters were collected, and a candidate series of OPV systems was proposed as potential materials for the heterojunctions of a solar cell device. The given theoretical methodology and the photovoltaic data computationally obtained may aid in the in silico design of novel donor materials for the new generation of solar cells. [ABSTRACT FROM AUTHOR]

• Dipole moment of 1-methylindole in its lowest excited singlet state has been determined from thermochromic shifts. • Discrepancies to ab initio values are explained by mixing of the excited states through the strong electric field of the solvent cavity. • The lower singlet state obtains dipole character from the upper state. • The difference to results from Stark spectroscopy are discussed. The permanent dipole moments of 1-methylindole in its ground and lowest excited singlet π π * -state have been determined using the method of thermochromic shifts of the absorption and fluorescence spectra and compared to ab initio calculated dipole moments. While ab initio theory predicts an excited state dipole moment, which nearly equals that of the ground state, the thermochromic shifts give an excited state dipole moment, which is about 2 Debye larger than that of the ground state. Perturbation of the excited state through the large electric field strength of the solvent cavity leads to strong mixing with the next excited singlet state, which has a considerably larger dipole moment. This perturbation is analyzed using the method described by Lombardi [John R. Lombardi J. Phys. Chem. A 1998, 102 , 2817–2823]. The perturbing state could be identified as 1 L a -state, which is the second adiabatic state in the gas phase, but the lowest state in a polar surrounding. [ABSTRACT FROM AUTHOR]

• The impact of solvent polarity on the NLO properties of a nickel-dithiolene complex (1) is investigated. • NLO absorption is studied via Z-scan and femtosecond transient absorption experiment. • The mechanism of selectively tuning optical nonlinearity in metal dithiolene complexes is discussed. The ultrafast optical nonlinear of a dithiolene complex of nickel (bis(1,2-diphenyl-1,2-ethenedithiolato-S,S')-nickel) (1) has been systematically investigated. Results of Z-scan with picosecond and nanosecond pulses show that compound 1 in N,N'-dimethylformamide (DMF) has a selective reverse saturable absorption (RSA), which could be observed with picosecond pulses and would be dramatically suppressed under ns pulses. The pulse-selective response vanishes in dichloromethane (DCM) and toluene solvents, indicating a connection with the decrease of solvent polarity. The ultrafast decay processes of excited state in different solvents are further investigated via femtosecond transient absorption (TA) experiment. The results are studied with global & target analysis and show that the excited state lifetime of 1 is remarkably shortened with the increasing polarity of the solvents, which suppresses the accumulation of excitons on excited-state and result in the substantially decrease of RSA in DMF solvent under nanosecond pulses. Our results are expected to shed light on the mechanism of selectively tuning optical nonlinearity in metal dithiolene complexes on an ultrafast time scale. [ABSTRACT FROM AUTHOR]

• The temperature dependence of Trp fluorescence lifetime in solutions was studied. • A model was suggested of Trp molecules transition from the excited state to the CTS. • Reversal transition from the CTS to the excited state of Trp is considered. • Formation of the quasi-conductive band in the system of hydrogen bonds is considered. The temperature dependence of tryptophan fluorescence lifetime in an aqueous solution, aqueous solutions of glycerol (50 and 75 %, v/v) and dimethyl sulfoxide (DMSO) (50 and 75 %), and 1 M aqueous trehalose solution was studied in the range of –170 to +20 °C. In this temperature range, the fluorescence kinetics in all samples is best approximated by three exponential terms with characteristic times τ 1 ∼ 3 ns, τ 2 ∼ 4 ns, and τ 3 ∼ 15 ns at room temperature. Temperature dependences of fluorescence lifetimes for the fastest and medium components exhibited antisymbatic (antiphase) behavior in the temperature range of –60 to 10 °C. To explain this behavior, a model was suggested that takes into account the tryptophan (Trp) molecule transition from the excited state to the state with separated charges – charge transfer state (CTS), the transitions back to the excited state, and the radiative transitions of the CTS to the ground state. Mathematically, this model is equivalent to the model that we have suggested earlier in [Gorokhov et al. (2018) Biochemistry (Moscow) , 82 , 1615–1623] to describe the transitions between different rotamer forms of tryptophan. The obtained results can be used for interpreting the experimental temperature curves of tryptophan fluorescence lifetime in proteins. [ABSTRACT FROM AUTHOR]