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This document is the Accepted Manuscript version of a Published Work that appeared in final form in The Journal of Physical Chemistry Letters, copyright © 2021 American Chemical Society after peer review and technical editing by the publisher. We demonstrate that spin-vibronic coupling is the most significant mechanism in vibrational coherence transfer (VCT) from the singlet (S1) to the triplet (T1) state of the [Pt2(P2O5H2)4]4– complex. Our time-dependent correlation function-based study shows that the rate of intersystem crossing (kISC) through direct spin–orbit coupling is negligibly small, making VCT vanishingly small due to the ultrashort decoherence time (2.5 ps). However, the inclusion of the spin-vibronic contribution to the net kISC in selective normal modes along the Pt–Pt axis increases the kISC to such an extent that VCT becomes feasible. Our results suggest that kISC for the S1 →T2 (τISC = 1.084 ps) is much faster than the S1 → T1 (τISC = 763.4 ps) and S1 → T3 (τISC = 13.38 ps) in CH3CN solvent, indicating that VCT is possible from the low-lying excited singlet (S1) to the triplet (T1) state through the intermediate T2 state. This is the first example where VCT occurs solely due to spin-vibronic interactions. This finding can pave the way for new types of photocatalysis.

DFT calculations on the photoisomerization of hydrazones of 1,2,4-oxadiazole derivatives to 1,2,5-triazoles have been performed showing that the reaction occurred through the first excited singlet state. The Z isomer gave the reaction through a hydrogen atom transfer of the hydrazonic nitrogen atom to the nitrogen atom in four position on the oxadiazole ring. In this case, the isomerization was a concerted reaction. The E isomer could undergo the same reaction. However, it could not be a concerted reaction but required the presence of a ring opening intermediate.

Reactions of electronically excited singlet oxygen (1O2) with the radical cations of guanine (9HG•+), 9-methylguanine (9MG•+), 2′-deoxyguanosine (dGuo•+), and guanosine (Guo•+) were studied in the ...

The permanent dipole moments of 2-cyanoindole (cyanoindole = CNI) in its ground and lowest excited singlet states have been determined from rotationally resolved electronic Stark spectroscopy under jet-cooled conditions. From the orientation of the transition dipole moment and the geometry changes upon electronic excitation the lowest excited singlet state could be shown to be of Lb-symmetry. The general statement, that the La-state has the larger permanent dipole moment of the two lowest excited singlet states, will be challenged in this contribution. On the basis of the different electronic nature of the first excited singlet state the behavior of 2-, 3-, 4- and 5-CNI is discussed. The excited state lifetime of isolated 2-CNI in the gas phase has been determined to be 9.4 ns. This value is compared to the excited state lifetime in ethyl acetate solution of 2.6 ns, which was quantified with a Strickler–Berg analysis. Using water as solvent shortens the 2-CNI lifetime to

[Image: see text] Singlet fission (SF) involves the conversion of one excited singlet state into two lower excited triplet states and has received considerable renewed attention over the past decade. This Perspective highlights recent developments and emerging concepts of SF in solid-state crystalline materials. Recent experiments showed the crucial role of vibrational modes in speeding up SF, and theoretical modeling has started to define an optimal energetic landscape and intermolecular orientation of chromophores for highly efficient singlet fission. A critical analysis of these developments leads to directions for future research to eventually find singlet fission chromophores with excellent optoelectronic properties.

The complex Ag(I)(flavonolate)(PPh3)2 has been prepared and characterized including the crystal structure. This compound shows in degassed solution two emission bands, a fluorescence at λ max=505 nm and a phosphorescence at λ max=780 nm. Both emissions originate from lowest-energy flavonolate intraligand excited singlet and triplet states, respectively. In the presence of oxygen, the phosphorescence disappears and the complex becomes light-sensitive. The flavonolate ligand undergoes a photooxidation by oxygen.

1-Hydroxy-2-acetonaphthone, compound 1b is known to undergo excited-state intramolecular proton transfer (ESIPT) in the lowest excited singlet state in solution, giving weak fluorescence with a large Stokes-shift. We converted compound 1b to the corresponding difluoroboronated (BF2) complex 2b, and investigated the photophysical features in solution and the solid state along with those of the related compound 1d. The other compounds 1a, 1c and 1d were unsuccessful in forming the corresponding BF2 complexes. Blue fluorescence from compounds 1a, 1b and 1d and yellow one from compound 2b in the solid state were recorded. Compounds 1a, 1b and 1d moderately fluoresced with quantum yields of ca. 0.2 in both solution and solid state. Compound 2b showed fluorescence in the solid state with a quantum yield of 0.18 and in solution with a quantum yield of 0.65. The present study introduces potentials of ESIPT molecules, the related compounds and the corresponding difluoroboronated complexes to solid-state fluorophores.

sEDA, pEDA, and cSAR descriptors of the substituent effect were determined for >30 monosubstituted benzenes in the first excited singlet S1 state at the LC-ωB97XD/aug-cc-pVTZ level. It was found that in the S1 state, the σ- and π-valence electrons are a bit less and a bit more affected, respectively, than in the S0 state, but basically, the effect in both states remains the same. In the S0 and S1 states, the d(C–X) distances to the substituent’s first atom and the ring perimeter correlate with the sEDA and pEDA in the appropriate states, respectively. The energies and the gap of the frontier orbitals in the two states are linearly correlated and for the HOMO(S1), LUMO(S1), and HOMO(S1)–LUMO(S1) gap correlate also with the pEDA(S1) and cSAR(S1) descriptors. In all studied correlations, three similar groups of substituents can be distinguished, for which correlations (i) are very good, (ii) deviate slightly, and (iii) deviate significantly. Comparison of the shape of the HOMO(S0) and HOMO(S1) orbitals shows...

Summary The concept of excited-state aromaticity is receiving much attention in that completely reversed aromaticity in the excited state (so-called aromaticity reversal) provides crucial insight into photostability, photoreactivity, and its application to the photosynthetic mechanism and photoactive materials. Despite this significance, experimental elucidation of excited-state aromaticity is still unsolved, particularly for the excited singlet state. Here, as an unconventional approach, time-resolved IR (TRIR) spectroscopy on aromatic and anti-aromatic hexaphyrin congeners shed light on excited-singlet-state aromaticity. The contrasting spectral features between the Fourier transform IR and TRIR spectra reveal the aromaticity-driven structural changes, corroborating aromaticity reversal in the excited singlet states. Our paradigm for excited-state aromaticity, the correlation of IR spectral features with aromaticity reversal, provides another fundamental key to understanding the role of (anti)aromaticity in the stability, dynamics, and reactivity in the excited singlet state of π-conjugated molecular systems.

Kinetics of singlet fission in organic semiconductors, in which the excited singlet state (S_1) spontaneously splits into a pair of triplet (T) excitons, is known to be strongly influenced by back geminate annihilation of TT-pairs. We show that this influence can be properly described only by taking into account the diffusive exciton migration. The migration effect is treated in the model of two kinetically coupled states: the intermediate state of interacting TT-pairs and the state of migrating excitons. Within this model the singlet fission (including magnetic field effects) is studied as applied to the fluorescence decay kinetics (FDK) I_{S_1}(t) for S_1-state. The analysis shows that migration strongly affects the FDK resulting, in particular, in the universal long-time dependence I_{S_1}(t) \sim t^{-3/2}. The model accurately describes the FDK, recently observed for a number of systems. Possible applications of the considered model to the analysis of mechanisms of migration, using experimentally measured FDK, are briefly discussed., 7 pages, 2 figures, misprint in Eq.(1) and some misprints in the text of the paper are corrected

We report a new approach to search for structures of minimum energy conical intersection (MECIs) automatically. Gradient projection (GP) method and single component artificial force induced reaction (SC-AFIR) method were combined in the present approach. As case studies, MECIs of benzene and naphthalene between their ground and first excited singlet electronic states (S 0 /S 1 -MECIs) were explored. All S 0 /S 1 -MECIs reported previously were obtained automatically. Furthermore, the number of force calculations was reduced compared to the one required in the previous search. Improved convergence in a step in which various geometrical displacements are induced by SC-AFIR would contribute to the cost reduction.

A series of cyclo-meta-phenylene congeners with a variation of interphenylene bridges was synthesized by adopting concise synthetic routes to investigate the structure–fluorescence relationships of...

According to Hund’s rule, the lowest triplet state (T1) is lower in energy than the lowest excited singlet state (S1) in closed-shell molecules. The exchange integral lowers the energy of the tripl...

When a molecular electron donor interacts with multiple electron acceptors, quantum coherence can enhance the electron transfer (ET) rate. Here we report photodriven ET rates in a pair of donor–acceptor (D-A) compounds that link one anthracene (An) donor to one or two equivalent 1,4-benzoquinone (BQ) acceptors. Subpicosecond ET from the lowest excited singlet state of An to two BQs is about 2.4 times faster than ET to one BQ at room temperature, but about 5 times faster at cryogenic temperatures. This factor of 2 increase results from a transition from ET to one of two acceptors at room temperature to ET to a superposition state of the two acceptors with correlated system–bath fluctuations at low temperature.

The rotationally resolved electronic spectrum of 4-cyanoindole and some N-D and C-D deuterated isotopologues has been measured and analyzed. Dipole moments in the ground and electronically excited state have been determined, using electronic Stark spectroscopy. From the geometry changes upon excitation, orientation of the transition dipole moment, and the values for the permanent dipole moments, the lowest excited singlet state could be shown to be of La symmetry. The excited state lifetime of isolated 4-cyanoindole has been determined to be 11 ns, while for the ringdeuterated isotopologues lifetimes between 5 and 6 ns have been found. The different behavior of 3-, 4-, and 5-cyanoindole is discussed on the basis of the different electronic nature of the electronically excited singlet states.

Time-resolved fluorescence of [Au(CN)2−]3 in water reveals a coherent vibration of 74 cm-1. A massive non-Condon effect, proved unambiguously by time-resolved fluorescence spectra, demonstrates that the wave packet motion arises from the coherent bond formation.

The dynamics of ferulic acid (3-(4-hydroxy-3-methoxyphenyl)-2-propenoic acid) and caffeic acid (3-(3,4-dihydroxyphenyl)-2-propenoic acid) in acetonitrile, dioxane and water at pH 2.2 following photoexcitation to the first excited singlet state are reported. These hydroxycinnamic acids display both strong ultraviolet absorption and potent antioxidant activity, making them promising sunscreen components. Ferulic and caffeic acids have previously been shown to undergo trans-cis photoisomerization via irradiation studies, yet time-resolved measurements were unable to observe formation of the cis-isomer. In the present study, we are able to observe the formation of the cis-isomer as well as provide timescales of relaxation following initial photoexcitation.

A hybrid Bethe–Salpeter/time-dependent density-functional-theory method is described that aims at improving the performance of the GW/Bethe–Salpeter-equation (GW/BSE) method in general and for excited triplet states in particular. The static screened exchange W used in the BSE is combined with the correlation kernel of the underlying density functional in a manner that retains a proven feature of the BSE, that is, the correct description of charge–transfer excitations. The performance of the new method, labeled cBSE, is assessed using G0W0 or evGW quasiparticle energies, and an improved performance is observed. The cBSE approach shows nearly equal performance for excited singlet and triplet states, rivaling coupled-cluster theory (in the CC2 approximation) in accuracy at a computational cost that is at least one order of magnitude smaller.

Single crystals of 4-(diisopropylamino)benzonitrile (DIABN) undergo an intramolecular charge transfer (ICT) reaction in the excited singlet state. At 300 K, the fluorescence consists of emissions from the locally excited (LE) and from the ICT state. Upon cooling to 5 K, the ICT fluorescence intensity gradually decreases relative to that of the LE emission and is absent below 60 K. With crystalline 4-(dimethylamino)benzonitrile (DMABN), in contrast, only LE emission is found over the entire range from 300 to 5 K. The phosphorescence spectra of the DIABN and the DMABN crystals do not present any evidence for an additional ICT emission, showing that ICT does not occur in the triplet state. An activation energy E-a of similar to 4 kJ/mol is determined for the LE -> ICT reaction of DIABN crystals, from the temperature dependence of the fluorescence decay times tau(2) and tau(1) E-a is attributed to changes in the molecular conformation of DIABN other than a full rotation of the large diisopropylamino group with respect to the benzonitrile moiety. In a comparison with crystal and solution data, literature results from transient vibrational and absorption spectra are discussed and it is concluded that they cannot be employed to favor the TICT (perpendicular twist) over the PICT (planar) model for DIABN and DMABN.

2-(4-(9,9-Dimethylacridin-10(9H)-yl)phenyl)-5-phenyl-1,3,4-oxadiazole has an energy difference between the lowest excited singlet and triplet states (ΔEST) of ca. 0.24 eV. Introduction of two electronegative fluorine atoms onto the acceptor portion of the molecule to give 2-(4-(9,9-dimethylacridin-10(9H)-yl)-3,5-difluorophenyl)-5-phenyl-1,3,4-oxadiazole lowers the energy of the singlet emission with a negligible effect on the corresponding triplet energy, leading to a donor–acceptor compound with decreased ΔEST of ca. 0.13 eV that displays thermally activated delayed fluorescence. Organic light-emitting diodes fabricated using the latter compound display high EQEmax of 21.9% at a luminance of 10 cd/m2 and sky-blue emission, however, they suffer from a large efficiency roll-off at increased luminance.

The bent bond/antiperiplanar hypothesis (BBAH) is used to propose a mechanism-based orbital model for the facial selectivity of sigmatropic hydrogen shifts under both thermal and photochemical conditions. The BBAH analysis of these concerted rearrangements invokes transient vibrationally excited singlet diradicals in both 4 n and 4 n+2 polyenes.

The influence of phenyl linkage and donor strength on the photophysical properties of new derivatives of quinoxaline-containing iminodibenzyl and iminostilbene moieties is studied. The donor-acceptor derivatives showed dual thermally activated delayed fluorescence (TADF) and room temperature phosphorescence (RTP) despite a large energy gap between the excited singlet and triplet states (ca. 0.5 eV). This extremely rare observation is explained by the twisted and rigidified structure of the iminodibenzyl moiety.

Hydrated electrons are among the strongest reductants known. Adding the ascorbate dianion as a sacrificial donor turns the photoionization of 9-anthrolate in water into a catalytic cycle for their in situ production with near-UV light (355 nm). The photoionization step is exclusively biphotonic and occurs via the first excited singlet state of the catalyst. Neither triplet formation nor any photochemical side reactions interfere. The ionization by-product, the anthroxy radical, is inert towards the ascorbate monoanion but is rapidly reduced by the dianion, thereby recovering the starting catalyst. A sufficient amount of the sacrificial donor makes that reduction quantitative and leads to a sustainable generation of hydrated electrons, as is evidenced by electron yields greatly surpassing the catalyst concentration. Control experiments established that the superincrease is indeed due to the catalyst regeneration and not to an ionization of other species involved in the reaction.

International audience; Photoelectron Spectroscopy (PS) and Near-Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy have been used to investigate the occupied and empty density of states of biphenylene films of different thicknesses, deposited onto a Cu(111) crystal. The obtained results have been compared to previous gas phase spectra and single molecule Density Functional Theory (DFT) calculations to get insights into the possible modification of the molecular electronic structure in the film induced by the adsorption on a surface. Furthermore, NEXAFS measurements allowed characterizing the variation of the molecular arrangement with the film thickness and helped to clarify the substrate-molecule interaction.

The quantum yield for the release of leaving groups from o-nitrobenzyl “caged” compounds varies greatly with the nature of these leaving groups, for reasons that have never been well understood. We found that the barriers for the primary hydrogen-atom transfer step and the efficient nonradiative processes on the excited singlet and triplet surfaces determine the quantum yields. The excited-state barriers decrease when the exothermicity of the photoreaction increases, in accord with Bell–Evans–Polanyi principle, a tool that has never been applied to a nonadiabatic photoreaction. We further introduce a simple ground-state predictor, the radical-stabilization energy, which correlates with the computed excited-state barriers and reaction energies, and that might be used to design new and more efficient photochemical processes.

In this work ten singlet and nine triplet states are studied through multi-reference configuration interactions with singles and doubles (MR-CISD), including Davidson extensivity correction (MR-CISD+Q). For the first time the excited states whose energies are larger than ∼9.5 eV have been calculated using highly correlated methods. The energies, spatial extent (〈r2〉), configurations weights and oscillator strengths (f) have been computed. At the MR-CISD+Q level the excited states energies vary from ∼7.51 to 11.98 eV. The lowest (nσ*) excited singlet state is significantly mixed with the n3pa1 and n3s Rydberg states, while the next (n3s) has a non-negligible mixture with the nσ* state. The next three singlet states obtained result from the (nCl)3(3pe)1 configuration and are almost degenerate. The next (n3pa1) singlet state is significantly mixed with the nσ* state, while the last three have σC–Cl → Rydberg (3s or 3p) as the main configurations. According to the f values the most intense transition is to the 41A1 state, a σ3pa1 Rydberg state mixed with the σσ* and σ3s configurations. Our results indicate that the σσ* configuration is responsible for the high f value of the gs → 41A1 transition. The Rydberg-valence mixing is greatly reduced in the triplet states whose singlet counterparts have significant multiconfigurational character. The 23A1 state (σσ*) does not have its singlet counterpart, while the 41A1 state (σ3pa1 + σσ* + σ3s) does not have its triplet counterpart. The obtained results are in good agreement with experimental results and with previous CASPT2 results.

In this issue of Chem, Basel et al. describe solvent-induced changes in photophysics for a non-conjugated pentacene dimer. The authors clearly demonstrate dynamic relaxation of the excited singlet exciton and find that singlet fission rates can be enhanced in more polar media and are particularly rapid in benzonitrile.

The electronic origin bands A and B of 5-hydroxyindole were measured using rotationally resolved electronic spectroscopy. From comparison of the experimental rotational constants to the results of ab initio calculated structures, we could make the assignment of band A being due to the syn conformer and of band B being due to the anti conformer. These conformers, which differ in the orientation of the hydroxy group with respect to the rest of the molecule, have considerably different S1 state life times. The most probable explanation for this surprising finding is a different conical intersection of the ππ* states of both conformers with the repulsive πσ* state.

Solvatochromic behavior of 1-naphthol (N1) and 2-naphthol (N2) has been studied in different solvents at room temperature (298 K). The ground and first excited singlet state dipole moments are estimated using solvatochromic shift method. Bakhshiev and Kawski and Bilot correlations based on bulk solvent polarity parameters are applied. The results are further verified by using the microscopic solvent polarity parameter E T N . For both molecules investigated, the excited state dipole moments are larger than the corresponding values in the ground state. Moreover, for N1, the values obtained in aprotic solvents are much less than those obtained when protic solvents are included, which underlines the presence of specific interaction in case of protic solvents.

Multireference configuration interaction (MRCI) calculations of the lowest singlet X(1A') and triplet ã((3)A'') states as well as the first excited singlet Ã((1)A'') state have been performed for a series of bromocarbenes: CHBr, CFBr, CClBr, CBr(2), and CIBr. The MRCI calculations were performed with correlation consistent basis sets of valence triple-ζ plus polarization quality, employing a full-valence active space of 18 electrons in 12 orbitals (12 and 9, respectively, for CHBr). Results obtained include equilibrium geometries and harmonic vibrational frequencies for each of the electronic states, along with ã((3)A'') ← X((1)A') singlet-triplet gaps and Ã((1)A'') ← X((1)A') transition energies. Comparisons have been made with previous computational and experimental results where available. The MRCI calculations presented in this work provide a comprehensive series of results at a consistent high level of theory for all of the bromocarbenes.

The electronic absorption and fluorescence spectra of 2-hydroxy-4-methylquinoline (HMQ) have been recorded at room temperature (298 K) in solvents of different polarities. The spectral data have been used to evaluate the ground and first excited singlet state dipole moment using the solvatochromic shift method (Bakhshiev [31] and Kawski–Chamma–Viallet [32,33]). The validity of these methods in the light of proper choice of solvents is discussed.

Carotenoids are essential light-harvesting pigments in natural photosynthesis. They absorb in the blue–green region of the solar spectrum and transfer the absorbed energy to (bacterio-)chlorophylls, and thus expand the wavelength range of light that is able to drive photosynthesis. This process is an example of singlet–singlet excitation energy transfer, and carotenoids serve to enhance the overall efficiency of photosynthetic light reactions. The photochemistry and photophysics of carotenoids have often been interpreted by referring to those of simple polyene molecules that do not possess any functional groups. However, this may not always be wise because carotenoids usually have a number of functional groups that induce the variety of photochemical behaviours in them. These differences can also make the interpretation of the singlet excited states of carotenoids very complicated. In this article, we review the properties of the singlet excited states of carotenoids with the aim of producing as coherent a picture as possible of what is currently known and what needs to be learned.

Ab initio nonadiabatic dynamics simulations are reported for thymine with focus on the S(2) --> S(1) deactivation using the state-averaged CASSCF method. Supporting calculations have been performed on vertical excitations, S(1) and S(2) minima, and minima on the crossing seam using the MS-CASPT2, RI-CC2, MR-CIS, and MR-CISD methods. The photodynamical process starts with a fast (

The ground (X1A′) and two lowest lying excited singlet states (11A″ and 21A′) of methyl hypochlorite have been examined using ab initio electronic structure techniques to validate computationally efficient methods, upon which direct dynamics can be based, versus high-level ones, for which direct dynamics would be intractable. Ground-state equilibrium geometries and vibrational frequencies determined using density functional theory (DFT) with the 6-31G(d) basis set are tested against coupled-cluster theory (CCSD(T)) results from the literature. Vertical excitation energies and transition dipole moments calculated at the complete active space self-consistent field CASSCF/6-31+G(d) level of theory are benchmarked against multireference configuration interaction (MRCI) results with the aug-cc-pVXZ (X = D, T, Q) family of basis sets. The excited-state gradients that will govern the classical dynamics are compared for CASSCF/6-31+G(d) versus MRCI/aug-cc-pVXZ (X = D, T). To carry out the ab initio molecular dynamics (AIMD), existing electronic structure codes have been interfaced with the molecular modelling toolkit (MMTK), an open-source program library for molecular simulation applications. We use two examples to demonstrate the use of direct dynamics in MMTK: a canonical ground-state trajectory to sample positions and momenta, and an excited-state microcanonical trajectory based on CASSCF. The work presented here forms the basis for future study of the photodissociation of CH3OCl. As well, the implementation of AIMD within MMTK provides a useful tool for examining a variety of other research problems.

Distyryl pyridine (DSP) photosensitizer for HABI photoinitiator systems was newly designed and eleven methoxy DSP(distyrylpyridines)s were synthesized and evaluated. The results showed that 2, 4-dimethoxy DSP (24DMOP-DSP) was the most efficient among studied methoxy DSPs. Compared with coumarin 1 as reference, the photosensitivity for 24DMOP-DSP as photosensitizer was increased by 150%. Potential of DSP as photosensitizer was supported by the mechanism study, such like photobleaching property of 24DMOP-DSP, derived from configuration change of E to Z, and efficient fluorescence quenching of excited singlet DSP by BCIM.

For an ensemble of different types of luminescence centers with overlapping absorption bands, with no restrictions on the optical densities, we have obtained relations describing the luminescence excitation spectra for each type of center. We consider transformations of the relations in some limiting cases. We suggest a procedure for using the equations obtained to determine the characteristics of the luminescence centers. Some of these procedures have been experimentally implemented in study of intrinsic radiation color centers in lithium fluoride crystals. We have determined the ratios of the luminescence quantum yields for F2 and F3+ color centers, and we have observed that a major role is played by nonradiative transitions in deactivation of the first excited singlet state of F3+ centers.

The photochemistry, photophysical properties, and temperature dependence (–25 to +65 °C) of fluorescence by quantum yields and excited singlet state lifetimes in acetonitrile have been examined for three sets of dialkylbenzene derivatives: Set 1 — ortho-xylene (10), tetralin (11), and indan (12); Set 2 — 2,3-dimethylbenzonitrile (9-23), 5-cyanotetralin (T-23), and 4-cyanoindan (I-23); and Set 3 — 3,4-dimethylbenzonitrile (9-34), 6-cyanotetralin (T-34), and 5-cyanoindan (I-34). Phototransposition reactions occur for 10, 9-23, 9-34, and T-34. Fitting of the temperature-dependent fluorescence data to an Arrhenius expression gave A and Ea values for all substrates studied except I-23 and I-34. The fluorescence intensity of these two compounds was essentially independent of temperature. For the other compounds, the data revealed that the activation barrier separating the excited singlet state (S1) from the reactive intermediate, a prefulvene biradical, was the important one in determining the reaction efficiency. The dominant mode of decay of the reactive intermediate was internal return to the starting material. Moreover, the general observation was made that nitrile substitution ortho to one of the alkyl groups in these dialkylbenzene derivatives reduced the rate at which they were converted to the reactive intermediate and, therefore, also the efficiency of the phototransposition reactions.Key words: phototranspositions, substituted benzenes, temperature-dependent fluorescence, activation parameters.

Fluorescence spectra of various dilute solutions not inuenced by the apparatus were obtained using appropriate correction curves. Moreover, the validity of the obtained correction curves is checked by comparing measured uorescence spectra of standard organic dyes with their spectra reported in various sources. The corrected uorescence spectra of newly synthesized 3,4,9,10-perylenetetracarboxylic acid derivatives are presented. The observed inuence of the changes in the chemical structure of the side groups on their optical spectra is explained by the appearance of the charge transfer state below the rst excited singlet state in N,N’-bis(4-diphenylaminophenyl)-3,4,9,10-perylenetetracarboxylic acid diimide (BDFAPhPI) solution, resulting in a huge decrease of the uorescence efcienc y compared to the N,N’-diphenyl-3,4,9,10-perylenetetracarboxylic acid diimide (BPhPI) solution.

The syntheses and reactions of photoresponsive dendrimers are described. Dendrimers with photoreversible stilbene cores undergo mutual cis–trans isomerization in organic solvents to give photostationary state mixtures of cis - and trans -isomers. Even stilbene dendrimers with molecular weights as high as 6500 underwent mutual cis–trans isomerization within the lifetime of the excited singlet state. The large dendron group surrounding the photoreactive core may affect the excited state properties of the core to induce the efficiency of photoisomerization and/or reduce the fluorescence efficiency. The photochemistry of stilbene dendrimers, with various types of dendron groups, azobenzene dendrimers and other photoresposive dendrimers is discussed.

Fluorescence from the second excited singlet state (S 2 ) of 3-hydroxyflavone (3HF) has been observed for the first time in supercritical carbon dioxide (sc-CO 2 ) environment. Steady-state experiments reveal that the intramolecular proton transfer is less effective from the S 2 state of 3HF compared to that from the S 1 state.

The excited singlet states of trans-3-aminostilbene and its N-methyl derivatives are strongly fluorescent in cyclohexane solution and have large singlet state dipole moments. Addition of low concen...

Several reaction pathways on the potential energy surface for the reaction CH 3 O + ClO are investigated using ab intio methods. As a result we find two reaction pathways to be most probable: (i) the formation of CH 2 O + HOCI via hydrogen abstraction from the methoxy radical and (ii) the formation of the low-lying adduct CH 3 OOCl. The other adduct CH 3 OClO is calculated to lie about 25 kcal/mol higher in energy than CH 3 OOCl. CH 3 OOCl can be considered to be a sink for Cl and ClO radicals on the ground-state surface. We have also performed MRD-CI calculations for the excited states of CH 3 OOCl and CH 3 OClO to study their photochemistry. In CH 3 OOCl we calculated a strong transition (1la → 14a) at 4.80 eV corresponding to a σ(Cl-O)-σ*(Cl-O) excitation. We find all low-lying excited singlet and triplet states in the energy range between 2.5 and 5.5 eV to be highly repulsive toward Cl-O and O-O cleavage, and thus photodissociation pathways forming the reactive radicals Cl and ClO from CH 3 OOCl are very probable.

Synthesis, thermal and photochemical reactions of vinylidenecyclopropanes are described. Vinylidenecyclopropanes have been prepared by the reaction of 1, 1-disubstituted 2, 2-dibromocyclopropanes with alkenes under phase transfer conditions in a stereospecific manner. Cis-trans photoisomerization of C 2, C 3-disubstituted vinylidenecyclopropanes proceeded via their excited singlet and triplet states and via their radical cations. Direct photoreaction of C 2, C 2, C 3, C 3-tetrasubstituted vinylidenecyclopropanes afforded rearranged products, 1, 2, 3-butatriene derivatives. Triplet-sensitized photoreaction of vinylidenecyclopropanes with electron-deficient alkenes gave vinylidenecyclopentanes. 9, 10-Dicyanoanthracene-sensitized photoreaction of vinylidenecyclopropanes with organic carbonitriles gave vinylidene-l-pyrrolines via photoinduced electron transfer. Oxidation of vinylidenecyclopropanes by oxone or m-CPBA gave cyclobutanone derivatives via methylenecyclopropanes with elimination of carbon monoxide. Thermal rearrangement from 1-diarylvinylidene- 2 -vinylcyclopropanes occurred at relatively low temperature to give vinylidenecyclopentenes. Addition of dihalocarbenes to vinylidenecyclopropanes proceeded regioselectively to give cyclopropylidenecyclopropanes which rearranges to 1- (dihalomethylene) - spiropentanes. These reactions are in sharp contrast with those of methylenecyclopropanes. It has been demonstrated that the thermal and photochemical bond cleavage of vinylidenecyclopropanes occurred mainly at their C 1-C 2 positions.

The X( 1 A') and A( 1 A") electronic states of CFI have been investigated using the CASPT2(18,12) method. Results for the ground-state equilibrium geometry of CFI are in excellent agreement with previous studies. The equilibrium geometry of the excited state shows significant variations in the C-I bond length as a function of basis set in part due to the flatness of the potential energy surface in the region of the minimum. The A( 1 A") ← X( 1 A') adiabatic transition energy for CFI is estimated to be in the range 17000-17500 cm - 1 . A barrier in the exit channel for dissociation to CF + I was located on the excited-state surface, and the barrier height is predicted to be greater than 1525 cm - 1 .

Torsional vibronic bands of the methyl group of m-tolunitrile-H2O and m-tolunitrile-N2O complexes in their S1 (first excited singlet electronic) states were observed under the supersonic-jet condition by laser-induced fluorescence and hole-burning spectroscopies. Torsional potentials determined from the observed frequencies revealed that the complex formation similarly affects for the H2O and N2O complexes. Two isomers were assumed for both the H2O and the N2O complexes: One showed vibrational frequencies of torsion similar to those of the monomer, whereas torsional potentials for the other complexes were lower than that in the monomer and of nearly 6-fold symmetry. The geometries of the complexes were evaluated by molecular-orbital calculations.

The combined temperature and excitation wavelength effects on the photophysical (fluorescence quantum yields and lifetimes) and photochemical behavior of the rotamer mixture of trans-1,2-di(2-naphthyl)ethene (trans-2,2‘-DNE) has been investigated in nonpolar solvent. By using a refined procedure of the principal component analysis (PCA), applied to the fluorescence spectra of the rotamer mixture, the spectral properties and the relative abundances of the three distinct rotamers at room temperature were derived. Furthermore, the rate constants of their radiative and reactive processes were obtained, so suggesting a complete picture of all of the deactivation pathways of the lowest excited singlet state of the trans-2,2‘-DNE rotamers.

Engineering the position of the lowest triplet state (T1) relative to the first excited singlet state (S1) is of great importance in improving the efficiencies of organic light emitting diodes and organic photovoltaic cells. We have carried out model exact calculations of substituted polyene chains to understand the factors that affect the energy gap between S1 and T1. The factors studied are backbone dimerisation, different donor-acceptor substitutions and twisted geometry. The largest system studied is an eighteen carbon polyene which spans a Hilbert space of about 991 million. We show that for reverse intersystem crossing (RISC) process, the best system involves substituting all carbon sites on one half of the polyene with donors and the other half with acceptors., Comment: 7 Pages; 10 Figures

This paper describes the competition of the relaxation processes (fluorescence, isomerization and cyclization) of 5-styryloxazole in the first excited singlet state. The study is carried out under mild conditions and in the first stage of irradiation to compare the photobehaviour with that reported in a previ- ous work on preparative scale. The quantum yields of the radiative and reactive deactivation are compared with those of the analogous compounds containing pentatomic heteroaryl groups.

This review deals with the recent advances in the photochemistry of cyclopropanes, methylenecyclopropanes, and vinylidenecyclopropanes. cis–trans Photoisomerization of 1,2-diarycyclopropanes via excited singlet and triplet states and radical cations, photochemical polar addition to arylcyclopropanes, and photooxygenation of arylcyclopropanes and methylenecyclopropanes giving cyclic peroxides are described. The new photochemistry of vinylidenecyclopropanes including cis–trans photoisomerization, (3+2) photocycloadditions, and photorearrangements is also discussed.