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The photodeamination reaction of the anthrol molecule generating the high-activity quinone methide intermediate has been investigated ( J. Org. Chem. 2017, 82, 6006-6021), though lacking careful explanation for its reaction mechanism. In our research, the mechanism of the anthrol molecule photodeamination induced by excited-state intramolecular proton transfer was reported for the first time. Absorption and emission spectra calculated for the work presented here agreed well with experimental results. To propose a molecular-level explanation of the photodeamination reaction, we calculated bond parameters, corresponding infrared vibrational frequencies, Mayer bond orders, and visualized frontier molecular orbitals of different molecules, and the hydrogen bond strengthening behavior in excited states was confirmed, enhancing the excited state intramolecular proton transfer of the anthrol molecule. Moreover, we concluded that photoisomerization weakened the bond strength between nitrogen and carbon atoms, which promoted the photodeamination reaction. Finally, for visually and quantificationally revealing the photodeamination mechanism, we have established the three-dimensional potential energy surfaces for the deamination reaction in different electronic states and calculated the corresponding reaction Gibbs free energies, it can be confirmed that the photodeamination reaction of the anthrol molecule must be induced by a proton transfer reaction in the excited state. [ABSTRACT FROM AUTHOR]

The nonlinear optical response of some O-doped polycyclic aromatic hydrocarbons (PAHs) is systematically investigated in the present work aiming to understand the influence of structural effects on their nonlinear optical response. In that view, the third-order nonlinear optical properties of these PAHs were measured under 4 ns visible (532 nm) and infrared (1064 nm) laser excitation. The O-doped PAHs were found to exhibit large saturable absorption and negative sign nonlinear refraction under visible excitation, increasing both with the addition of naphthalene units and with the number of O atoms. Their nonlinear optical response was found to be negligible under infrared excitation. Similar measurements performed on thin films of these PAHs have shown that they maintain their large nonlinear optical response even in the solid state, confirming their high potential for optoelectronic and photonic applications. [ABSTRACT FROM AUTHOR]

We explore by means of modeling how absorptive-dispersive mixing between the second- and third-order terms modifies the imaginary χtotal(2) responses from air/water interfaces under conditions of varying charge densities and ionic strength. To do so, we use published Im(χ(2)) and χ(3) spectra of the neat air/water interface that were obtained either from computations or experiments. We find that the χtotal(2) spectral lineshapes corresponding to experimentally measured spectra contain significant contributions from both interfacial χ(2) and bulk χ(3) terms at interfacial charge densities equivalent to less than 0.005% of a monolayer of water molecules, especially in the 3100 to 3300 cm-1 frequency region. Additionally, the role of short-range static dipole potentials is examined under conditions mimicking brine. Our results indicate that surface potentials, if indeed present at the air/water interface, manifest themselves spectroscopically in the tightly bonded H-bond network observable in the 3200 cm-1 frequency range. [ABSTRACT FROM AUTHOR]

Nanosecond transient absorption and theoretical calculations have been used to investigate the intermolecular hydrogen abstractions from alcohols and 1-naphthol by the lowest excited triplet (T1) of 1-chloro-4-nitronaphthalene upon excitation of the compound in organic solvents. The hydrogen abstraction of T1 from hydroxy group of 1-naphthol takes place through an electron transfer followed by a proton transfer through hydrogen bonding interaction with rate constants of ~109 M-1 s-1. Hydrogen-bonding is crucial in this process, indicated by the observation of a half reduction for T1 yield when increasing the concentration of 1-naphthol. The hydrogen abstraction in this way can be decelerated by increasing solvent polarity and hydrogen-bonding donor ability. The T1 of 1-chloro-4-nitronaphthalene can undergo one-step H atom abstraction from alkyl hydrogen in alcoholic solvents, with rate constants of ~104 M-1 s-1, and produce radical intermediates with the absorption maximum at 368 nm. DFT calculation results indicate both oxygens of the nitro group are active sites for hydrogen abstraction, and the difference of activation barriers for formation of two radical isomers is only 1.0 kcal/mol. [ABSTRACT FROM AUTHOR]

Variations in the electromagnetic wavelengths of absorption and reflection of molecules do not only make life colorful but also are often of central importance in solar energy conversion and optoelectronic information processing and transfer. Dithizone and its derivatives and complexes are, to our knowledge, probably the most "colorful" compound, being responsive to no less than seven different external stimuli that effect color change. Apart from a more detailed discussion of the cyclic voltammetry and observed electrochromism in the SCH3-substituted free ligand and mercury complex, concentratochromism, solvatochromism, halochromism, thermochromism, and chronochromism in the ligand, and photochromism in the carboxy-functionalized phenylmercury(II) complex are also presented here. Color changes are either associated with proton transfer or loss, or isomerization. [ABSTRACT FROM AUTHOR]

Excited state ultrafast conformational reorganization is recognized as an important phenomenon that facilitates light-induced functions of many molecular systems. This report describes the femtosecond and picosecond conformational relaxation dynamics of middle-ring and terminal ring twisted conformers of the acetylene π-conjugated system bis(phenylethynyl)benzene, a model system for molecular wires. Through excitation wavelength dependent, femtosecond-transient absorption measurements, we found that the middle-ring and terminal ring twisted conformers relax at femtosecond (400-600 fs) and picosecond (20-24 ps) time scales, respectively. Actinic pumping into the red flank of the absorption spectrum leads to excitation of primarily planar conformers, and results in very different excited state dynamics. In addition, ultrafast Raman loss spectroscopic studies revealed the vibrational mode dependent relaxation dynamics for different excitation wavelengths. To corroborate our experimental findings, DFT and time-dependent DFT calculations were carried out. The Franck-Condon simulation indicated that the vibronic structure observed in the electronic absorption and the fluorescence spectra are due to progressions and combinations of several vibrational modes corresponding to the phenyl ring and the acetylenic groups. Furthermore, the middle ring torsional rotation matches the room-temperature electronic absorption, in stark contrast to the terminal ring torsional rotation. Finally, we show that the middle-ring twisted conformer undergoes femtosecond torsional planarization dynamic, whereas the terminal rings relax on a few tens of picosecond time scale. [ABSTRACT FROM AUTHOR]

Brookite TiO2 exhibits promising photocatalytic activity in photoreduction; however, it is least known due to poor stability. We have synthesized Ti3+-modified brookite TiO2/graphene nanocomposites with specific facets exposed successfully. They show highly improved photoreduction activity for water splitting into H2 with excellent stability. The well-crystallized brookite TiO2 nanorods are surrounded by four reductive (211) facets with high conduction band potential and growth along the [001] direction, indicating that they have high photoreduction ability because more reductive electrons will be excited. By combining spectroscopic techniques and electrochemical analysis methods, the outstanding activity could be linked with the synergistic effects of highly exposed (211) facets, Ti3+ defects, and graphene. Their formation mechanism and the effects in the enhanced photoreduction activity have been discussed in detail. In addition to promoting the separation of photogenerated e--h+ pairs effectively, the midgap state was introduced and the absorption ability was improved. [ABSTRACT FROM AUTHOR]

There are several factors that affect the dynamics of adsorbed hydrogen atoms on a carbon surface. Using density functional calculations we show that coadsorption can be a highly influential factor. The diffusion of hydrogen adsorbed on graphene is explored in the presence of H-containing molecules. Without coadsorbates the diffusion barrier of H on graphene is 0.94 eV, while with water/ammonia it is 0.85/0.12 eV. The low barrier in the case of ammonia is attributed to the formation of a stable intermediate state NH4, while such a stable state is not found in the case of water. In addition, hydrogen fluoride, hydrogen sulfide, arsine, and phosphine were also considered. We found that stronger hydrogen-hydride bonds lead to lower diffusion barriers of H on graphene. [ABSTRACT FROM AUTHOR]

This work discusses the protein conformational complexity of the B800-850 LH2 complexes from the purple sulfur bacterium Allochromatium vinosum, focusing on the spectral characteristics of the B850 chromophores. Low-temperature B850 absorption and the split B800 band shift blue and red, respectively, at elevated temperatures, revealing isosbestic points. The latter indicates the presence of two (unresolved) conformations of B850 bacteriochlorophylls (BChls), referred to as conformations 1 and 2, and two conformations of B800 BChls, denoted as B800R and B800B. The energy differences between average site energies of conformations 1 and 2, and B800R and B800B are similar (~200 cm-1), suggesting weak and strong hydrogen bonds linking two major subpopulations of BChls and the protein scaffolding. Although conformations 1 and 2 of the B850 chromophores, and B800R and B800B, exist in the ground state, selective excitation leads to 1 → 2 and B800R → B800B phototransformations. Different static inhomogeneous broadening is revealed for the lowest energy exciton states of B850 (fwhm ~195 cm-1) and B800R (fwhm ~140 cm-1). To describe the 5 K absorption spectrum and the above-mentioned conformations, we employ an exciton model with dichotomous protein conformation disorder. We show that both experimental data and the modeling study support a two-site model with strongly and weakly hydrogen-bonded B850 and B800 BChls, which under illumination undergo conformational changes, most likely caused by proton dynamics. [ABSTRACT FROM AUTHOR]

The medium-resolution gas-phase infrared (IR) spectra of 1-bromo-1-fluoroethene (BrFC=CH2, 1,1-C2H2BrF) were investigated in the range 300-6500 cm-1, and the vibrational analysis led to the assignment of all fundamentals as well as many overtone and combination bands up to three quanta, thus giving an accurate description of its vibrational structure. Integrated band intensity data were determined with high precision from the measurements of their corresponding absorption cross sections. The vibrational analysis was supported by high-level ab initio investigations. CCSD(T) computations accounting for extrapolation to the complete basis set and core correlation effects were employed to accurately determine the molecular structure and harmonic force field. The latter was then coupled to B2PLYP and MP2 computations in order to account for mechanical and electrical anharmonicities. Second-order perturbative vibrational theory was then applied to the thus obtained hybrid force fields to support the experimental assignment of the IR spectra. [ABSTRACT FROM AUTHOR]

Cavity ring-down absorption spectroscopy (CRDS) is employed to investigate one-photon dissociation of (COCl)2 at 248 nm obtaining a primary Cl2 elimination channel. A ratio of vibrational population is estimated to be 1:(0.12 ± 0.03):(0.011 ± 0.003) for the v = 0, 1, and 2 levels. The quantum yield of Cl2 molecular channel is obtained to be 0.8 ± 0.4 initiated from the X˜1Ag ground state surface (COCl)2 via internal conversion. The obtained total quantum yield is attributed to both primary ((COCl)2 + hν → 2CO + Cl2) and secondary reactions (dominated by Cl + COCl → Cl2 + CO). The former is estimated to share a yield of >0.14, while the latter contributes up to 0.66. The photodissociation pathway to the molecular products is calculated to proceed via a four-center transition state (TS) from which Cl2 is eliminated synchronously. Installation of the mirrors with reflectivity of 99.995% in the CRDS apparatus prolongs the ring-down time to 70 μs, thus allowing for the contribution from 17% up to 66% of the total Cl2 yield from secondary reaction depending on the reaction temperature. Despite uncertainty in determining the product yield, the primary Cl2 dissociation channel eliminated from (COCl)2 is observed for the first time. [ABSTRACT FROM AUTHOR]

Novel dye sensitizers are highly expected in the development of dye-sensitized solar cells (DSSCs) because dye sensitizers can significantly affect the power conversion efficiency (PCE). Here, the molecular docking strategy is applied to design panchromatic dye sensitizers for DSSCs to improve light-harvesting efficiency covering the full solar spectrum. Considering the broad absorption bands of tetraanthracenylporphyrins (TAnPs) and tetraazuleneporphyrins (TAzPs), based upon porphyrin dye sensitizer YD2-o-C8, the panchromatic dye sensitizers coded as H2(TAnP)-α, H2(TAzP)-γ, H2(TAzP)-ε, and H2(TAzP)-δ are designed by the substitution of the porphyrin-ring in YD2-o-C8 with TAnPs and TAzPs moieties at different positions. The geometries, electronic structures, and excitation properties of the designed dye sensitizers are investigated using density functional theory (DFT) and time-dependent DFT methods. The analysis of geometries, conjugation lengths, electronic structures, absorption spectra, transition configurations, exciton binding energies, and free energy variations for electron injection and dye regeneration supports that the designed molecules are effective to be applied as potential candidates of dye sensitizers for DSSCs. Among the designed dye sensitizers, H2(TAzP)-γ and H2(TAnP)-α must have the better performance in DSSCs. [ABSTRACT FROM AUTHOR]

The 2-cyclohexen-1-one (2CHO) molecule serves as a prototype for understanding the photochemical properties of conjugated enones. We have recorded the cavity ringdown (CRD) absorption spectrum of 2CHO vapor at room temperature over the 360-380 nm range. This portion of the spectrum encompasses the S1(n,π*) ← S0 vibronic band system in the region of the C=C and C=O stretch fundamentals. We have assigned about 40 vibronically resolved features in the spectrum, affording fundamental frequencies for 7 different vibrational modes in the S1(n,π*) state, including the C=C (1554 cm-1) and OC-CH (1449 cm-1) stretch modes. The C=O stretch character is spread over at least four different vibrational modes in the S1(n,π*) state, with fundamentals spanning the 1340-1430 cm-1 interval. This finding stems from a significant reduction in C=O bond order upon excitation, which leads to near-coincidence of the C=O stretch and several CH2 wag frequencies. Such complexities make 2CHO an ideal candidate for testing excited-state computational methods. We have used the present spectroscopic results to test EOM-EE-CCSD harmonic-frequency predictions for the S1(n,π*) state. We have also benchmarked the performance of less costly computational methods, including CIS(D) and TDDFT. For certain density functionals (e.g., B3LYP and PBE0), we find that the accuracy of TDDFT frequency predictions can approach but not meet that of EOM-EE-CCSD. [ABSTRACT FROM AUTHOR]

Formation of benzene excimer following UV excitation of the neat liquid is monitored with femtosecond spectroscopy. A prompt rise component in excimer transient absorption, which contradicts the classical scenario of gradual reorientation and pairing of the excited monomers, is observed. Three-pulse experiments in which the population of evolving excimers is depleted by a secondary dump pulse demonstrate that the excimer absorption band is polarized along the interfragment axis. The experiments furthermore prove that the subsequent 4-fold increase in excimer absorption over ~50 ps is primarily due to an increase in the transition dipole of pairs which are formed early on, and not to excited monomers forming excimers in a delayed fashion due to unfavorable initial geometry. Results are analyzed in light of recent studies of local structure in the liquid benzene combined with advanced electronic structure calculations. The prompt absorption rise is ascribed to excited states delocalized over nearby benzene molecules, which are sufficiently close and nearly parallel in the pure liquid. Such low-symmetry structures, which differ considerably from the optimized structures of isolated benzene dimer and solid benzene, are sufficiently abundant in liquid benzene. Electronic structure calculations confirm the orientation of transition dipoles of the excimers along the interparticle axis and demonstrate how slow refinement of the intermolecular geometry leads to a significant increase in the excimer absorption strength. [ABSTRACT FROM AUTHOR]

A model for simulating the transient electronic absorption spectra of donor-acceptor dyads undergoing ultrafast intramolecular charge transfer in solution has been developed. It is based on the stochastic multichannel point-transition approach and includes the reorganization of high-frequency intramolecular modes (treated quantum mechanically) and of low frequency intramolecular and solvent modes (described classically). The relaxation of the slow modes is assumed to be exponential with time constants taken from experiments. The excited-state dynamics is obtained by simulating the population distribution of each quantum state after optical excitation and upon electronic and vibrational transitions. This model was used to simulate the transient electronic absorption spectra measured previously with a pyrylium phenolate in acetonitrile. A very good agreement between the simulated and measured spectra was obtained assuming a three-level model including the ground state, the optically excited state, and a dark state with large charge-transfer character and a substantially different geometry relative to that of the optically excited state. The merit of this approach to disentangle the contributions of both population changes and relaxation processes to the ultrafast spectral dynamics will be discussed. [ABSTRACT FROM AUTHOR]

The excited-state heterolysis of acridinol-based derivatives leads to the release of the OH- ion and the formation of the corresponding acridinium cations. To evaluate the parameters that control the reaction barriers, the kinetics of excited-state OH- release from a series of acridinol photobases were studied using transient absorption spectroscopy. The rate constants were obtained in three solvents (methanol, butanol, and isobutanol), and the data were modeled using Marcus theory. The intrinsic reorganization energies obtained from these fits were found to correlate well with the solvent reorganization energies calculated using dielectric continuum model, suggesting that the excited-state OH- release occurs along the solvent reaction coordinate. Furthermore, the ability of acridinol photobases to photoinitiate chemical reactions was demonstrated using the Michael reaction between dimethylmalonate and nitrostyrene. [ABSTRACT FROM AUTHOR]

Accurate and rapid quantum mechanical prediction of solvatochromic shifts, particularly in systems where charge transfer plays a significant role, is important for many aspects of molecular and material design. Although the semiempirical INDO/SCI approach is computationally efficient and performs well for charge-transfer states, the availability of implicit solvent approaches has been limited. Here, we implement the COSMO solvent model with a perturbative state-specific correction to the excited-state energies with the INDO/SCI method. We show that for a series of prototypical π-conjugated molecules, our newly implemented INDO/SCI/COSMO model yields more accurate absorption energies and comparably accurate solvatochromic shifts to those computed using TD-ωB97XD and CIS with COSMO solvation at a substantially lower computational cost. [ABSTRACT FROM AUTHOR]

We report on a method for performing ultrafast infrared (IR) vibrational spectroscopy using fluorescence detection. Vibrational dynamics on the ground electronic state driven by femtosecond mid-infrared pulses are detected by changes in fluorescence amplitude resulting from modulation of a two-photon visible transition by nuclear motion. We examine a series of coumarin dyes and study the signals as a function of solvent and excitation pulse parameters. The measured signal characterizes the relaxation of vibrational populations and coherences but yields different information than conventional IR transient absorption measurements. These differences result from the manner in which the ground-state dynamics are projected by the two-photon detection step. Extensions of this method can be adapted for a variety of increased-sensitivity IR measurements. [ABSTRACT FROM AUTHOR]

Light-harvesting architectures that afford strong absorption across the near-ultraviolet to near-infrared region, namely, panchromatic absorptivity, are potentially valuable for capturing the broad spectral distribution of sunlight. One previously reported triad consisting of two perylene monoimides strongly coupled to a free base porphyrin via ethyne linkers (FbT) shows panchromatic absorption together with a porphyrin-like S1 excited state albeit at lower energy than that of a typical monomeric porphyrin. Here, two new porphyrin-bis(perylene) triads have been prepared wherein the porphyrin bears two pentafluorophenyl substituents. The porphyrin is in the free base (FbT-F) or zinc chelate (ZnT-F) forms. The zinc chelate (ZnT) of the original triad bearing nonfluorinated aryl rings also was prepared. The triads were characterized using static and time-resolved optical spectroscopy. The results were analyzed with the aid of molecular-orbital characteristics obtained using density functional theory calculations. Of the four triads, FbT is the most panchromatic in affording the most even distribution of absorption spectral intensity as well as exhibiting the largest wavelength span (380-750 nm). The triads exhibit fluorescence yields (0.35 for FbT-F in toluene) that are substantially greater than for the porphyrin benchmarks (0.049 for FbP-F). The singlet excited-state lifetimes (τS) for the triads in toluene decrease in the order FbT-F (2.7 ns) > FbT (2.0 ns) > ZnT (1.2 ns) ∼ ZnT-F (1.1 ns). The τS values in benzonitrile are FbT (1.3 ns) > FbT-F (1.2 ns) > ZnT-F (0.6 ns) > ZnT (0.2 ns). Thus, the free base triads exhibit relatively long (1.2-2.7 ns) excited-state lifetimes in both polar and nonpolar media. The combined photophysical characteristics indicate that FbT and FbT-F are the best choices for panchromatic light-harvesting systems. Collectively, the findings afford insights into the effects of electronic structure on the panchromatic behavior of ethynyl-linked porphyrin-perylene architectures that can help guide next-generation designs and utilization of these systems. [ABSTRACT FROM AUTHOR]

Pyrazolate-bridged dinuclear Pt(II) complexes represent a series of molecules with tunable absorption and emission properties that can be directly modulated by structural factors, such as the Pt-Pt distance. However, direct experimental information regarding the structure of the emissive triplet excited state has remained scarce. Using time-resolved wide-angle X-ray scattering (WAXS), the excited triplet state molecular structure of [Pt(ppy)(μ-t-Bu2pz)]2 (ppy = 2-phenylpyridine; t-Bu2pz = 3,5-di-tert-butylpyrazolate), complex 1, was obtained in a dilute (0.5 mM) toluene solution utilizing the monochromatic X-ray pulses at Beamline 11IDD of the Advanced Photon Source. The excited-state structural analysis of 1 was performed based on the results from both transient WAXS measurements and density functional theory calculations to shed light on the primary structural changes in its triplet metal-metal-to-ligand charge-transfer (MMLCT) state, in particular, the Pt-Pt distance and ligand rotation. We found a pronounced Pt-Pt distance contraction accompanied by rotational motions of ppy ligands toward one another in the MMLCT state of 1. Our results suggest that the contraction is larger than what has previously been reported, but they are in good agreement with recent theoretical efforts and suggest the ppy moieties as targets for rational synthesis aimed at tuning the excited-state structure and properties. [ABSTRACT FROM AUTHOR]

The triplet excited-state formation efficiency in a flavin derivative was increased by the introduction of iodine into the molecular framework. The transient absorption measurements showed that the intersystem crossing rate was 1.1 × 1010 s-1, significantly faster than in the parent flavin compound. Furthermore, the photocatalytic efficiency of iodoflavin was evaluated using the oxidation of benzyl alcohol as a model reaction. The benzaldehyde product yields were higher when iodoflavin was used as a photocatalyst, showing that the increased triplet yield directly translates into improved photocatalysis. The iodoflavin catalyst also allowed the use of higher substrate concentrations (since the undesired electron transfer from singlet excited state was minimized), which is expected to improve the practical aspects of photocatalysis by flavins. [ABSTRACT FROM AUTHOR]

We study the linear and nonlinear optical properties of a well-known acid-base indicator, bromophenol blue (BPB), in aqueous solution by employing static and integrated approaches. In the static approach, optical properties have been calculated using time-dependent density functional theory (TD-DFT) on the fully relaxed geometries of the neutral and different unprotonated forms of BPB. Moreover, both closed and open forms of BPB were considered. In the integrated approach, the optical properties have been computed over many snapshots extracted from molecular dynamics simulation using a hybrid time-dependent density functional theory/molecular mechanics approach. The static approach suggests closed neutral ⇒ anionic interconversion as the dominant mechanism for the red shift in the absorption spectra of BPB due to a change from acidic to basic pH. It is found by employing an integrated approach that the two interconversions, namely open neutral ⇒ anionic and open neutral ⇒ dianionic, can contribute to the pH-dependent shift in the absorption spectra of BPB. Even though both static and integrated approaches reproduce the pH-dependent red shift in the absorption spectra of BPB, the latter one is suitable to determine both the spectra and spectral broadening. Finally, the computed static first hyperpolarizability for various protonated and deprotonated forms of BPB reveals that this molecule can be used as a nonlinear optical probe for pH sensing in addition to its highly exploited use as an optical probe. [ABSTRACT FROM AUTHOR]

Recent experiments replacing oxygen atoms by sulfur in thymine have revealed that absorption and intersystem crossing properties of these derivatives are strongly dependent on the position and number of the substitutions, affecting their potential performance for photodynamical therapy. Using multireference quantum chemical methods (CASPT2 and DFT/MRCI), we calculated absorption spectra and spin-orbit coupling matrix elements for thymine (Thy), 2-thiothymine (2tThy), 4-thiothymine (4tThy), and 2,4-dithiothymine (2,4dtThy), to investigate this relation between structure and photophysics. The simulations showed that a simple 4-electrons/4-orbital minimum model can explain the main experimentally observed spectral features. Moreover, the computational estimate of intersystem crossing lifetimes in this sequence of molecules revealed that the experimental value attributed to thymine in water might be underestimated by a factor 20, most probably due to an overlap of singlet/triplet absorption signals in the transient absorption spectrum. The difference between the absorptivity of 2tThy and 2tThd was also investigated, but no conclusive explanation could be found. [ABSTRACT FROM AUTHOR]

The photophysics of a norbornyl-bridged covalent tetracene (Tc) dimer BT1 and a monomer analogue Tc-e were studied in room-temperature nonpolar solvents. Notably in BT1, a Davydov-split band is observed in UV absorption, heralding interchromophore electronic interactions. Emission spectra indicate an acene-like vibronic progression mirroring the lowest-energy visible absorption. For BT1, this argues against excited-state excimer formation. Evidence of intramolecular singlet fission (SF) comes from a comparison of time-resolved emission decay signals collected for BT1 versus Tc-e in toluene. In BT1, the multiexcitonic 1TT state is produced in 70 ns in 6% yield. A ratio of fission versus fusion rate constants provides an experimental measure of the SF reaction free energy at 52 meV in good agreement with previous calculations. The low SF yield corroborates our expectations that orbital symmetry effects on diabatic coupling for SF are important for dimers that cannot rely on more favorable thermodynamics. [ABSTRACT FROM AUTHOR]

We have determined absorption of the near UV light (290-345 nm) by nitric acid (HNO3) deposition on sapphire window surfaces as a function of the HNO3 pressure, by using Brewster angle cavity ring-down spectroscopy. Apparent monolayer HNO3 surface absorption cross sections have been obtained; they range between (1.7 ± 1.1) × 10-19 and (0.29 ± 0.03) × 10-19 cm2/molecule. When nitric acid cross section values on sapphire surfaces were divided by those on fused silica surfaces for which only molecular HNO3 adsorption was reported, a new absorption band appeared in the 320-345 nm region. The shape of this absorption band is similar to that reported for surface nitrate (NO3-) at quartz/water interfaces, but is red-shifted by about 10 nm. Our study suggests that a small percentage (<7%) of adsorbed HNO3 formed by HNO3 deposition on sapphire surfaces is dissociated into surface nitrate on the time scale of about 5-7 min. Background transmission changes in the 320-350 nm region after exposing clean sapphire surfaces with many repeated HNO3 deposition/evacuation cycles are consistent with surface nitrate formation. We obtained nitrate surface absorption cross section data over 320-350 nm range. We also modeled photolysis rates of HNO3/NO3- on urban grimes. Atmospheric implications of the results are discussed. This paper was published ASAP on May 2, 2016, with an incorrect Figure 6 and errors in Figure 7. The corrected version was reposted on May 3, 2016. [ABSTRACT FROM AUTHOR]

Controlling stimulated emission is of importance because it competes with absorption and fluorescence under intense laser excitation. We performed resonant nonlinear optical spectroscopy measurements using femtosecond pulses shaped by π- or π/2-step phase functions and carried out calculations based on density matrix representation to elucidate the experimental results. In addition, we compared enhancements obtained when using other pulse shaping functions (chirp, third-order dispersion, and a time-delayed probe). The light transmitted through the high optical density solution was dominated by an intense stimulated emission feature that was 14 times greater for shaped pulses than for transform limited pulses. Coherent enhancement depending on the frequency, temporal, and phase characteristics of the shaped pulse is responsible for the experimental observations. [ABSTRACT FROM AUTHOR]

The synthesis, optical characterization and computational modeling of seven benzo[c][1,2,5]thiadiazole (BTD) donor-acceptor dyes are reported. These dyes have been studied using electrochemical analysis, electronic absorption, emission, and Raman and resonance Raman spectroscopies coupled with various density functional theoretical approaches. Crystal structure geometries on a number of these compounds are also reported. The optical spectra are dominated by low energy charge-transfer states; this may be modulated by the coupling between donor and acceptor through variation in donor energy, variation of the donor-acceptor torsion angle, and incorporation of an insulating bridge. These modifications result in a perturbation of the excitation energy for this charge-transfer transition of up to ~2000 cm-1. Emission spectra exhibit significant solvatochromisim, with Lippert-Mataga analysis yielding Δµ between 8 and 33 D. Predicted λmax, ε, and Raman cross sections calculated by M06L, B3LYP, PBE0, M06, CAM-B3LYP, and ωB97XD DFT functionals were compared to experimental results and analyzed using multivariate analysis, which shows that hybrid functionals with 20-27% HF best predict ground state absorption, while long-range corrected functionals best predict molecular polarizabilities. [ABSTRACT FROM AUTHOR]

We elucidated the photochromic spiro-4a,5-dihydropyrrolo[1,2-b]pyridazine/betaine (DPP/betaine) system by comparing state-of-the-art density functional theory calculations with nanosecond/millisecond UV-vis absorption spectroscopy, as well as steady-state absorption and cyclization kinetics. Time-dependent density functional theory calculations are employed to examine the transformations occurring after photoexcitation. This study shows that the photochromic spiro-4a,5-dihydropyrrolo[1,2-b]pyridazine and spiro-1,8a-dihydroindolizine (DHI) systems react according to similar pathways. However, notable differences exist. Although photoexcitation of the spiro-DPP system also leads to cis-betaines, which then isomerize to trans-betaines, we found two distinct classes of cis isomers (cis-betaine rotamer-1 and cis-betaine rotamer-2), which do not exist in spiro-1,8a-dihydroindolizine. Similar to our previous study on the spiro-DHI/betaine system, a complicated potential-energy landscape between cis and trans isomers exists in the spiro-DPP system, consisting of a network of transition states and intermediates. Because the spiro-DPP/betaine is even more complicated than the spiro-DHI/betaine system, (substituted) photochromic systems featuring a 4a,5-dihydropyrrolo[1,2-b]pyridazine functional unit will require thorough in silico design to function properly as logical gates or in devices for information storage. [ABSTRACT FROM AUTHOR]

Resonance Raman spectroscopy provides much stronger Raman signal levels than its off-resonant counterpart and adds selectivity by excitation tuning. Raman preresonance of benzene has been well studied. On-resonance studies, especially at phonon-allowed absorptions, have received less attention. In this case, we observe resonance of many of the vibration modes associated motion of the carbons in the ring while tuning over the ¹B2u absorption, including the related ν9 (CC stretch Herzberg notation, ν14 Wilson notation) and ν10 (CH-parallel bend Herzberg notation, ν15 Wilson notation) vibrational modes along with the ν2 (CC-stretch or ring-breathing Herzberg notation, ν1 Wilson notation) mode and multiples of the ν18 (CCC-parallel bend Herzberg notation, ν6 Wilson notation) vibrational mode. The ring-breathing mode is found to mix with the b2u modes creating higher frequency composites. Through the use of an optical parametric oscillator (OPO) to tune through the ¹B2u absorption band of liquid benzene, a stiffening (increase in energy) of the vibrational modes is observed as the excitation wavelength nears the ¹B2u absorption peak of the isolated molecule (vapor) phase. The strongest resonance amplitude observed is in the 2 × ν18 (e2g) mode, with nearly twice the intensity of the ring-breathing mode, ν2. Several overtones and combination modes, especially with ν2 (a1g), are also observed to resonate. Raman resonances on phonon-allowed excitations are narrow and permit the measurement of vibrations not Raman-active in the ground state. [ABSTRACT FROM AUTHOR]

The bimolecular reactions that follow 267 nm ultraviolet photolysis of ICN in acetonitrile solution have been studied using transient absorption spectroscopy on the picosecond time scale. Time-resolved electronic absorption spectroscopy (TEAS) in the ultraviolet and visible spectral regions observes rapid production and loss (with a decay time constant of 0.6 ± 0.1 ps) of the photolytically generated free CN radicals. Some of these radicals convert to a solvated form which decays with a lifetime of 8.5 ± 2.1 ps. Time-resolved vibrational absorption spectroscopy (TVAS) reveals that the free and solvated CN-radicals undergo geminate recombination with I atoms to make ICN and INC, H atom abstraction reactions, and addition reactions to solvent molecules to make C3H3N2 radical species. These radical products have a characteristic absorption band at 2036 cm-1 that shifts to 2010 cm-1 when ICN is photolyzed in CD3CN. The HCN yield is low, suggesting the addition pathway competes effectively with H atom abstraction from CH3CN, but the delayed growth of the C3H3N2 radical band is best described by reaction of solvated CN radicals through an unobserved intermediate species. Addition of methanol or tetrahydrofuran as a cosolute promotes H atom abstraction reactions that produce vibrationally hot HCN. The combination of TEAS and TVAS measurements shows that the rate-limiting process for production of ground-state HCN is vibrational cooling, the rate of which is accelerated by the presence of methanol or tetrahydrofuran. [ABSTRACT FROM AUTHOR]

We present the first study detailing the capture and aggregation of hyperthermal CO2 molecules by amorphous solid water (ASW) under ultra-high vacuum conditions at 125 K, near the amorphous/crystalline transition. Using time-resolved in situ reflection-absorption infrared spectroscopy (RAIRS), CO2 molecules with translational energies above 3.0 eV are observed to directly embed underneath the vacuum-solid interface to become absorbed within the ice films despite an inability to adsorb at 125 K; this behavior is not observed for crystalline films. Upon embedding, the mobility of CO2 within 125 K amorphous ice and the strength of its intermolecular interactions result in its segregation into clusters within the ice films. Tracing the kinetics of CO2 embedding events under different energetic conditions allows for elucidation of the underlying dynamics, and we draw comparison with other projectiles we have studied to promote generalized conclusions in regard to empirical prediction of a projectile's embedding probability. Through application of a classical model of the entrance barrier for projectiles colliding with amorphous ice, we provide direct evidence for a unified connection between embedding probability and projectile momentum; an account of all embedding data measured by our group traces a unified barrier model. This work highlights the interplay between translational energy and momentum accommodation during collisions with ice in high speed gas flows. [ABSTRACT FROM AUTHOR]

Many photoactive biomolecules are anions and exhibit ππ* optical transitions but with a degree of charge transfer (CT) character determined by the local environment. The phenolate moiety is a common structural motif among biochromophores and luminophores, and nitrophenolates are good model systems because the nitro substituent allows for CT-like transitions. Here we report gas-phase absorption spectra of o- and p-nitrophenolate·H2O complexes to decipher the effect of just one H2O and compare them with ab initio calculations of vertical excitation energies. The experimental band maximum is at 3.01 and 3.00 eV for ortho and para isomers, respectively, and is red-shifted by 0.10 and 0.13 eV relative to the bare ions, respectively. These shifts indicate that the transition has become more CT-like because of localization of negative charge on the phenolate oxygen, i.e., diminished delocalization of the negative excess charge. However, the transition bears less CT than that of m-nitrophenolate·H2O because this complex absorbs further to the red (2.56 eV). Our work emphasizes the importance of local perturbations: one water causes a larger shift than experienced in bulk for para isomer and almost the full shift for ortho isomer. Predicting microenvironmental effects in the boundary between CT and non-CT with high accuracy is nontrivial. However, in agreement with experiment, our calculations show a competition between the effects of electronic delocalization and electrostatic interaction with the solvent molecule. As a result, the excitation energy of ortho and para isomers is less sensitive to hydration than that of the meta isomer because donor and acceptor orbitals are only weakly coupled in the meta isomer. [ABSTRACT FROM AUTHOR]

Three zeaxanthin homologues withconjugation lengths Nof 15, 19, and 23 denoted asZ15, Z19, and Z23 were studied by femtosecondtransient absorption spectroscopy, and the results were compared tothose obtained for zeaxanthin (Z11). The energies of S2decrease from 20 450 cm–1(Z11) to 18 280cm–1(Z15), 17 095 cm–1(Z19), and 16 560 cm–1(Z23). Fitting the Ndependence of the S2energies allowed the estimationof E∞, the S2energy of a hypotheticalinfinite zeaxanthin, to be ∼14 000 cm–1. Exciting the 0–0 band of the S2state producescharacteristic S1–Snspectral profiles in transient absorption spectra with maxima at556 nm (Z11), 630 nm (Z15), 690 nm (Z19), and 740 nm (Z23). The redshift of the S1–Sntransitionwith increasing conjugation length is caused by a decrease in theS1state energy, resulting in S1lifetimes of9 ps (Z11), 0.9 ps (Z15), 0.35 ps (Z19), and 0.19 ps (Z23). Essentiallythe same lifetimes were obtained after excess energy excitation at400 nm, but S1–Snbecomesbroader, indicating a larger conformation disorder in the S1state after 400 nm excitation compared to excitation into the 0–0band of the S2state. An S* signal was observed in allsamples, but only for Z15, Z19, and Z23 does the S* signal decay witha lifetime different from that of the S1state. The S*lifetimes are 2.9 and 1.6 ps for Z15 and Z19, respectively. In Z23the S* signal needs two decay components yielding lifetimes of 0.24and 2.3 ps. The S* signal is more pronounced after 400 nm excitation. [ABSTRACT FROM AUTHOR]

Theangle dependence of strong-field ionization was studied fora set of second period hydrides (BH3, CH4, NH3, H2O, and HF) and third period hydrides (AlH3, SiH4, PH3, H2S, and HCl).Time-dependent configuration interaction with a complex absorbingpotential was used to model ionization by a seven cycle 800 nm cosinesquared pulse. The ionization yields were calculated as a functionof the laser polarization and plotted as three-dimensional surfaces.The general shapes of angular dependence can be understood in termsof ionization from the highest occupied orbitals. Variations in theshapes with laser intensity indicate that ionization occurs not justfrom the highest occupied orbitals, but also from lower-lying orbitals.These deductions are supported by variations in the population analysiswith the intensity of the laser field and the direction of polarization. [ABSTRACT FROM AUTHOR]

Two setups based on CW cavity ring-downspectroscopy were usedat Bruxelles and Rennes to record jet-cooled water dimer absorptionbetween 7188 and 7285, and between 7357 and 7386 cm–1. Some 19 absorption features are reported, significantly more thanin the literature. Limited high-resolution information is availabledue to strong overlap between neighboring vibration-rotation-tunneling(VRT) structures and to spectral broadening induced by short upperstate vibrational predissociation lifetimes, likely to range between100 and 20 ps. Rotational band contours analyses are performed toassign the partly resolved VRT structures to the v1v2v3,vfvb= 000,11; 200,00; 000,20; and 101,00 zero-order vibrational states.Their wavenumbers are found to be 7192.34, 7225.86, 7240.57, and 7256.99cm–1, respectively. Both so-called acceptor-switchingtunneling components are involved in the assignments whose tentativecharacter is discussed. [ABSTRACT FROM AUTHOR]

The photophysical studies of two phenols, derivatives of 3-hydroxybenzo[c]coumarin, were performed in n-nonane matrix at 5 K. Unstructured fluorescence spectrum of the derivative bearing a salicylaldehyde moiety, whose onset is shifted by ca. 3000 cm-1 to lower energy in respect to that of absorption, and short decay time of this emission (0.75 ns) suggested the occurrence of excited-state intramolecular proton transfer (ESIPT). The experimental results were interpreted with the aid of quantum chemistry calculations performed with the DFT and TDDFT/B3LYP/6-31++G(d,p) methods. [ABSTRACT FROM AUTHOR]

Tocreate the whole-rainbow-fluorophores (WRF) having the small Δλem(the difference of λembetween a given fluorophore and nearest neighboring fluorophorehaving the same core skeleton) values (<20 nm) in full visibleregion (λem: 400–650 nm), the high log ε(>4.5), and the high Φf(>0.6), we investigatedmoleculardesign, synthesis, and light-emitting characteristics of the π-conjugatedmolecules (D/A-BPBs) consisting of 1,4-bis(phenylethynyl)benzene (BPB)modified by donor groups (OMe, SMe, NMe2, and NPh2) and an acceptor group (CN). As a result, synthesized 20 D/A-BPBs(1a–5d) were found to be the desiredWRF. To get the intense red fluorophore (Φf>0.7,λem> 610 nm), we synthesized new compounds (5e–5i) and elucidated their photophysicalproperties in CHCl3solution. As a result, 5h, in which a 4-cyanophenyl group is introduced to the para-position of two benzene rings in the terminal NPh2groupof 5d, was found to be the desired intense red fluorophore(log ε = 4.56, Φf= 0.76, λem= 611 nm). The intramolecular charge-transfer nature ofthe S1state of WRF (1a–5d) was elucidated by the positive linear relationship between opticaltransition energy (νem) from the S1stateto the S0state and HOMO(D)–LUMO(A) difference,and the molecular orbitals calculated with the DFT method. It is demonstratedthat our concept (Φf= 1/(exp(−Aπ) + 1)) connected with the relationship betweenΦfand magnitude (Aπ) of π conjugation length in the S1state can beapplied to WRF (1a–5d). It is suggestedthat the prediction of Φffrom a structural modelcan be achieved by the equation Φf= 1/(exp(−((ν̃a– ν̃f)1/2× a3/2) + 1), where ν̃aandν̃fare the wavenumber (cm–1) of absorption and fluorescence peaks, respectively, and ais the calculated molecular radius. From the viewpointof application of WRF to various functional materials, the light-emittingcharacteristics of 1a–5iin dopedpolymer films were examined. It was demonstrated that 1a–5idispersed in two kinds of polymer film (PSTand PMMA) emit light at the whole visible region and have the small Δλemvalues (<20 nm) and the highΦfvalues (>0.6). Therefore, the present D/A-BPBscan be said to be the desired WRF even in the doped polymer film. [ABSTRACT FROM AUTHOR]

Surfacemolecular imprinting in layer-by-layer (SMI-LbL) film isknown as a facile and effective strategy to build imprinting sitesthat are more accessible to template molecules compared with molecularimprinting in polymers. Herein, we accomplished the formation of SMI-LbLfilm on electrospun nanofibers for the first time. The SMI-LbL nanofiberswere prepared by a template-induced LbL process on the polyacrylonitrile(PAN) nanofiber substrates, followed by postinfiltrating and photo-cross-linkingof photosensitive agent 4,4′-diazostilbene-2,2′-disulfonicacid disodium salt (DAS). The obtained nanofiber mat maintained thenanofibrous structure and showed rapid absorption and extraction oftemplate molecules of meso-tetra(4-carboxyphenyl)–porphine(Por). The binding capacity of Por reached 2.1 mg/g when 3.5 bilayerswere deposited on the nanofibers. After six cycles of extraction andreabsorption, the binding capacity of Por remained at 83%. Moreover,the absorption results of the targeted templated molecule of Por andthe control molecule of Fast Green, which had a very similar chemicalstructure and charge status to Por, indicated the specific absorptionfor template molecule of Por. Thus, a surface molecular imprintednanofiber mat with high selectivity of the templated molecule hasbeen demonstrated. [ABSTRACT FROM AUTHOR]

Thetrace gas glyoxal (CHOCHO) forms from the atmospheric oxidationof hydrocarbons and is a precursor to secondary organic aerosol. Wehave measured the absorption cross section of disubstituted 13CHO13CHO (13C glyoxal) at moderately high (1cm–1) optical resolution between 21 280 and23 260 cm–1(430–470 nm). The isotopicshifts in the position of absorption features were found to be largestnear 455 nm (Δν = 14 cm–1; Δλ= 0.29 nm), whereas no significant shifts were observed near 440 nm(Δν < 0.5 cm–1; Δλ <0.01 nm). These shifts are used to investigate the selective detectionof 12C glyoxal (natural isotope abundance) and 13C glyoxal by in situcavity enhanced differentialoptical absorption spectroscopy (CE-DOAS) in a series of sensitivitytests using synthetic spectra, and laboratory measurements of mixturescontaining 12C and 13C glyoxal, nitrogen dioxide,and other interfering absorbers. We find the changes in apparent spectralband shapes remain significant at the moderately high optical resolutiontypical of CE-DOAS (0.55 nm fwhm). CE-DOAS allows for the selectiveonline detection of both isotopes with detection limits of ∼200pptv (1 pptv = 10–12volume mixing ratio), and sensitivitytoward total glyoxal of few pptv. The 13C absorption crosssection is available for download from the Supporting Information. [ABSTRACT FROM AUTHOR]

Thelight-absorbing fraction of organic molecules in ambient aerosols,known as “brown carbon,” is an important yet poorlycharacterized component. Despite the fact that brown carbon couldalter the radiative forcing of aerosols significantly, identificationof specific chromophores has remained challenging. We recently demonstratedthat charge transfer (CT) complexes formed in organic molecules couldbe responsible for a large fraction of absorption observed in water-extractedambient particulate matter.1 In the presentstudy, we use excitation–emission matrix fluorescence spectroscopyto further corroborate the importance of CT complexes in definingaerosol optical properties. Monotonically increasing and decreasingquantum yields, decreasing Stokes shifts, and red-shifting emissionmaxima are observed from ambient particulate matter collected in Athens,Georgia, strongly suggesting that a superposition of independent chromophoresis not sufficient to explain brown carbon absorption and fluorescence.Instead, we show that a model in which such chromophores are energeticallycoupled to a dense manifold of CT complexes is consistent with allof the observations. Further, we suggest that a significant fractionof the observed fluorescence originates from CT complexes and thattheir contribution to brown carbon absorption is likely greater thanwe reported previously. [ABSTRACT FROM AUTHOR]

Twoterrylene chromophores, 2,5,10,13-tetra(tert-butyl)terrylene(1) and 2,5-di(tert-butyl)terrylene(2), were synthesized and studied todetermine their singlet exciton fission (SF) efficiencies. Compound 1crystallizes in one-dimensional stacks, whereas 2packs in a slip-stacked, herringbone pattern of dimers motif. Stronglyquenched fluorescence and rapid singlet exciton decay dynamics areobserved in vapor-deposited thin films of 1and 2. Phosphorescence measurements on thin films of 1and 2show that SF is only 70 meV endoergic for thesechromophores. Femtosecond transient absorption experiments using lowlaser fluences on these films reveal rapid triplet exciton formationfor both 1(τ = 120 ± 10 ps) and 2(τ = 320 ± 20 ps) that depends strongly on film crystallinity.The transient absorption data are consistent with formation of anexcimer state prior to SF. Triplet exciton yield measurements indicatenearly quantitative SF in thin films of both chromophores in highlycrystalline solvent-vapor-annealed films: 170 ± 20% for 1and 200 ± 30% for 2. These results showthat significantly different crystal morphologies of the same chromophorecan both result in high-efficiency SF provided that the energeticsare favorable. [ABSTRACT FROM AUTHOR]

Thespectroscopic, electrochemical, and photophysical propertiesof the new complex [P,N-{(C6H5)2(C5H4N)P}Re(CO)3Br]are reported. The UV–vis spectrum in dichloromethane showsan absorption maximum centered at 315 nm and a shoulder at 350 nm.These absorption bands have been characterized to have MLCT character.Excitation at both wavelengths (maximum and shoulder) leads to anemission band centered at 550 nm. Cyclic voltammetry experiments showtwo ill-defined irreversible oxidation waves around .50 and 1.80V that are assigned to ReI/ReIIand ReII/ReIIIcouples whereas an irreversible reduction signalcentered at −1.80 V is likewise assigned to a ligand reductionprocess. These results support the proposal of the MLCT nature ofthe states implied by the emission of the complex. The luminescentdecay fits to a biexponential function, where the lifetimes and emissionquantum yields are dependent on the solvent polarity. DFT calculationssuggest that dπ → π*pyridineand dπ→ π*phenylexcited states may account forthe existence of two decay lifetimes. [ABSTRACT FROM AUTHOR]

We report on the photophysics of N,N′-di(t-butoxycarbonyl)indigos(tBOC indigos),finding that reversible photochemical trans–cisand cis–transisomerization reactions proceedwith high quantum yields (0.10–0.46). Absorption of wavelengthsin the 500–600 nm region induces trans–cisisomerism, while blue light leads to the reverse cis–transprocess. Like their parent indigos, trans-BOC indigoshave low fluorescence yields (∼1 × 10–3), while the cisisomers have no measurable emission.These compounds are the first examples of photoisomerizable indigoiddyes in which photochemical isomerism effectively outcompetes radiativedecay processes. Though indigo dyes typically have poor solubilityin organic solvents, tBOC indigos can be dissolved at concentrationsup to 8 w% in common organic solvents like acetone. Furthermore, unlikeother photoisomerizable indigoids, tBOC indigos are not sensitiveto quenching by proton and electron donors. These features, combinedwith high quantum yields of reversible photoisomerism induced by relativelylow-energy photons (∼2 eV), make tBOC indigo derivatives potentiallyinteresting for photochromic applications, such as photomechanicallyactuated materials. [ABSTRACT FROM AUTHOR]

Luminescence of Hg(3P1) atoms trapped inmixed Ar/Xe matrices containing a small amount of Xe is reported.Broad emission bands, strongly red-shifted from absorption are recordedwhich are assigned to strong complexes formed between the excitedmercury Hg* and xenon atoms. Molecular dynamics calculations are performedon simulated Xe/Ar samples doped with Hg to follow the behavior ofHg* in the mixed rare gas matrices leading to exciplex formation.The role of Xe atoms in the first solvation shell (SS1)around Hg was investigated in detail, revealing the formation of twokinds of triatomic exciplexes; namely, Xe–Hg*–Xe andAr–Hg*–Xe. The first species exists only when two xenonatoms are present in SS1with specific geometries allowingthe formation of a linear or quasi-linear exciplex. In the other geometries,or in the presence of only one Xe in SS1, a linear Ar–Hg*–Xeexciplex is formed. The two kinds of exciplexes have different emissionbands, the most red-shifted being that involving two Xe atoms, whoseemission is very close to that observed in pure Xe matrices. Simulationsgive a direct access to the analysis of the experimental absorption,emission, and excitation spectra, together with the dynamics of exciplexesformation. [ABSTRACT FROM AUTHOR]

Two absorption systems of C5H3+starting at 350 and 345 nm were detected following mass-selectivedeposition of m/e= 63 ions in a6 K neon matrix. These are assigned to the 11A1← X 1A1electronic transitionof 1,2,3,4-pentatetraenylium H2CCCCCH+(isomer B+) and 11B2← X 1A1of penta-1,4-diyne-3-yliumHCCCHCCH+(C+). Theabsorptions of neutral C5H3isomers with onsetsat 434.5, 398.3, 369.0, and 267.3 nm are also detected. The firsttwo systems are assigned to the 12B1← X 2B1and 12A2← X 2B1transitions ofisomer Band C, respectively, and the lattertwo to ethynylcyclopropenyl (A) and 3-vinylidenecycloprop-1-enyl(D) radicals. The structural assignments are based onthe adiabatic excitation energies calculated with the MS-CASPT2 method.A vibrational analysis of the electronic spectra, based on the calculatedharmonic frequencies, supports this. [ABSTRACT FROM AUTHOR]

Photophysicaland photochemical features of [3n]cyclophanes(3nCPs) (n=2–6) in solution were investigated by emission and transientabsorption measurements. The studied 3nCPs show excimerfluorescence without locally excited fluorescence whereas some ofthem emit excimer phosphorescence in rigid glass at 77 K. The probabilityof excimeric phosphorescence from transannular π-electron systemswas shown to strictly depend on the symmetric molecular structures.A feature of intersystem crossing from an excimeric fluorescence stateto the excimeric triplet state was observed. Transient absorptionspectra obtained upon laser flash photolysis of 3nCP revealed formation of the triplet excimer states. Triplet sensitizationof 33CP using xanthone as the sensitizer demonstrated formation oftriplet 33CP via triplet energy transfer whereas from the xanthoneketyl radical formation, it was inferred that triplet xanthone undergoesH atom abstraction from 32CP, providing a benzylic 32CP radical asthe counter species. Based on kinetic and spectroscopic data obtainedupon laser flash photolysis, differences in photochemical reactionsof triplet xanthone between 32CP and 33CP were discussed. [ABSTRACT FROM AUTHOR]

Theabsorption and fluorescence spectra of poly(p-phenylenevinylene)(PPV) oligomers with up to seven repeat unitswere theoretically investigated using the algebraic diagrammatic constructionmethod to second order, ADC(2), combined with the resolution-of-the-identity(RI) approach. The ground and first excited state geometries of theoligomers were fully optimized. Vertical excitation energies and oscillatorstrengths of the first four transitions were computed. The vibrationalbroadening of the absorption and fluorescence spectra was studiedusing a semiclassical nuclear ensemble method. After correcting forbasis set and solvent effects, we achieved a balanced descriptionof the absorption and fluorescence spectra by means of the ADC(2)approach. This fact is documented by the computed Stokes shift alongthe PPV series, which is in good agreement with the experimental values.The experimentally observed band width of the UV absorption and fluorescencespectra is well reproduced by the present simulations showing thatthe nuclear ensemble generated should be well suitable for consecutivesurface hopping dynamics simulations. [ABSTRACT FROM AUTHOR]

Previous THz spectroscopyof the TNT explosive precursor, 2,4-dinitrotoluene(DNT), has been restricted to room temperature (apart from one setof data at 11 K). Here, for the first time, we investigate the spectrumas the temperature is systematically varied, from 7 to 245 K. Manynew features appear in the spectrum on cooling below room temperature.As well as the five absorption lines observed previously, we observefive additional lines. In addition, a new room-temperature line at8.52 THz (281 cm–1) is observed. Six of the linesred-shift with temperature and four of them blue-shift. The blue shiftis explained by interplay between intramolecular and intermolecularhydrogen bonds. The variation in line width and line intensity withtemperature is not systematic, although a conspicuous decrease inline intensity with temperature is observed in all cases. Modelingwith hybrid PBE0 and TPSSh functionals helps identify absorption modes. [ABSTRACT FROM AUTHOR]

Inthis work, we study the photophysical properties of an adenine-basedoligonucleotide using an ensemble of about 200 configurations obtainedfrom molecular dynamics simulations. Specifically, a QM/MM approachis used to obtain the excited-state energies and properties of (dA)20(dT)20with a dimer of π-stacked adeninebases included in the quantum region. The absorption and circulardichroism spectra are computed and analyzed using the algebraic diagrammaticconstruction through second order level of theory method (ADC(2))combined with classical mechanics. We find that the experimentallyobserved red-shifted shoulder in the absorption spectrum is due toexcitonic interactions, while charge-transfer states are present withinthe absorption band at the higher-energy end of the spectrum. Moreimportantly, low-energy states with charge-transfer mixing exist,which could lead to excimers and bonded excimers. These observationssuggest that mixing between charge-transfer and excitonic states playsan important role in the photophysics of oligonucleotides. They alsohighlight the importance of taking into account the conformationalflexibility of the oligonucleotide when investigating photophysicalproperties. [ABSTRACT FROM AUTHOR]

Wehave developed the technique of population-controlled impulsivevibrational spectroscopy (PC-IVS) aimed at providing high-quality,background-free Raman spectra of excited electronic states and theirdynamics. Our approach consists of a modified transient absorptionexperiment using an ultrashort (<10 fs) pump pulse with additionalelectronic excitation and control pulses. The latter allows for theexperimental isolation of excited-state vibrational coherence and,hence, vibrational spectra. We illustrate the capabilities of PC-IVSby reporting the Raman spectra of well-established molecular systemssuch as the carotenoid astaxanthin and trans-stilbeneand present the first excited-state Raman spectra of the retinal protonatedSchiff base chromophore in solution. Our approach, illustrated herewith impulsive vibrational spectroscopy, is equally applicable totransient and even multidimensional infrared and electronic spectroscopiesto experimentally isolate spectroscopic signatures of interest. [ABSTRACT FROM AUTHOR]