Nonradiative transitions in benzene

Calculations of nonradiative rates from excess energy levels of theS ₁ singlet state in benzene and deuterobenzene are presented. It is shown that calculations with a complete basis set lead to non-Condon effects which lie in the range of a factor of about 2–3 for theS ₁?S ₀ transition. Non-Condon matrix elements as well as non-Condon generating functions are developed. The results suggest that the development of more elaborate and more far-reaching concepts of the non-Condon models is necessary.     

Nonradiative transitions in benzene

A symmetric Rosen-Morse potential was assumed for the potential of the out-of-plane vibrations of benzene. Matrix elements and the respectiveS ₁ ?S ₀ nonradiative rates were calculated for several values of the anharmonicity constants and compared with the results obtained with a harmonic potential. Anharmonicities of the out-of-plane modes in benzene are found to decrease the values of the matrix elements and rates, thus leaving unresolved, a discrepancy between experimental and theoreticalS ₁ ?S ₀ rates for benzene. However, in molecules with distorted acceptor modes whose frequencies are higher in the optically excited, as compared to the acceptor state, increases of several orders of magnitude of the nonradiative rate could occur on introduction of Rosen-Morse type anharmonicities.     

Radiationless decay processes in benzene; an external deuterium isotope effect

C₆H₆ or C₆D₆ and CHCl₃ form weak complexes in glassy 3-methyl pentane solution at 77°K. Substitution of CDCl₃ for CHCl₃ greatly increases the phosphorescence efficiency and decay time of the benzene and suppresses the photochemical production of hexatrienes. The observations can be understood if there is vibronic coupling of the triplet benzene with the CH motion in the CHCl₃, which accelerates the rate of its radiationless decay.     

Nonradiative decay mechanism of fluoren-9-ylidene malononitrile ambipolar derivatives

We report recent results on the nonradiative decay (NRD) of fluoren-9-ylidene malononitrile (FM) ambipolar derivatives (FMDs). 2,7- and 3,6-disubstituted FMDs present distinctive photophysics. Charge separation was found dominant for excited state relaxation. The radiative decay (RD) is sensitive to changes in temperature and solvent medium only for the case of 3,6-FMDs. Excited state deactivation of carbazole-containing 3,6-FMD (CPAFM36) was exclusively nonradiative in polar solvents with excited state lifetimes shorter than 10 ps. The charge separation/recombination mechanism of the corresponding FMDs is suggested to fall in the inverted Marcus region of electron transfer. Given the electron-withdrawing properties of the FM unit, its ambipolar derivatives are suggested as potential candidates for air-stable organic thin-film transistors and molecular organic photovoltaics.     

Nonradiative decay mechanisms of the biologically relevant tautomer of guanine

We present the excited-state potential energy profiles of the biologically relevant 9H-keto-amino tautomer of guanine with respect to the radiationless decay via the out-of-plane deformation of the six-membered ring as well as the dissociation of NH bonds. The CASPT2//CASSCF method is employed for the reaction-path calculations. The reaction path for the out-of-plane deformation in the (1)pi pi* state leads in a barrierless way to a conical intersection with the electronic ground state. For the NH dissociation via the (1)pi sigma* state, the 9H-keto-amino tautomer is shown to have lower energy barriers than the 7H tautomers which we have studied recently. These two radiationless decay mechanisms explain the unexpected missing of the biologically relevant form in the resonant two-photon ionization spectrum of guanine in a supersonic jet. It is suggested that these ultrafast deactivation processes provide the biologically relevant tautomer of guanine with a high degree of photostability.     

Non-radiative decay and mode mixing in benzene

The non-radiative decay of mixed vibronic states is studied using the Fermi resonance 6²l⁰−11² states in S₁ benzene as a prototype. A strong dependence on the mixing coefficient is exhibited, and the results are compared with the experimental data of Stephenson et al. to elucidate the nature of the mixed state.     

Channel-three decay in benzene a picosecond fluorescence investigation

Fluorescence quantum yields, decay times and spectra have been measured following excitation of benzene vapour above the channel-three threshold. Experiments have been carried out on the static gas and on molecules jet-cooled to a vibrational temperature of 160 K. The excitation source was a recently developed high-power, picosecond, narrow-bandwidth laser continuously tunable from 224 to 252 nm. Channel-three decay was observed for all vibrational levels, even those not excited via line-broadened transitions. In addition it was observed that fluorescence decay curves resulting from excitation just above the channel-three threshold were non-exponential. It is proposed that the cause of absorption line broadening — intramolecular vibrational redistribution — is not directly related to channel-three, which is postulated to be coupling to a state X, which could be an isomeric form of benzene or a hidden singlet electronic state.     

Non-condon effects on radiationless triplet decay in benzene and naphthalene

Model calculations that avoid the Condon approximation are reported for T₁$\text{}\text{?}$ar S₀ intersystem crossing in normal and perdeuterated benzene and naphthalene. In both benzenes and in naphthalene-d₈, skeletal bending vibrations coupling T₁ to T₂ are found to be more efficient as accepting modes than CH-stretching vibrations.     

Nonradiative decay dynamics of methyl-4-hydroxycinnamate and its hydrated complex revealed by picosecond pump-probe spectroscopy

The lifetimes of methyl 4-hydroxycinnamate (OMpCA) and its mono-hydrated complex (OMpCA-H(2)O) in the S(1) state have been measured by picosecond pump-probe spectroscopy in a supersonic beam. For OMpCA, the lifetime of the S(1)-S(0) origin is 8-9 ps. On the other hand, the lifetime of the OMpCA-H(2)O complex at the origin is 930 ps, which is ～100 times longer than that of OMpCA. Furthermore, in the complex the S(1) lifetime shows rapid decrease at an energy of ～200 cm(-1) above the origin and finally becomes as short as 9 ps at ～500 cm(-1). Theoretical calculations with a symmetry-adapted cluster-configuration interaction (SAC-CI) method suggest that the observed lifetime behavior of the two species is described by nonradiative decay dynamics involving trans → cis isomerization. That is both OMpCA and OMpCA-H(2)O in the S(1) state decay due to the trans → cis isomerization, and the large difference of the lifetimes between them is due to the difference of the isomerization potential energy curve. In OMpCA, the trans → cis isomerization occurs smoothly without a barrier on the S(1) surface, while in the OMpCA-H(2)O complex, there exists a barrier along the isomerization coordinate. The calculated barrier height of OMpCA-H(2)O is in good agreement with that observed experimentally.     

Fluorescence decay times and quantum yields for benzene in the solid and vapour phases

Fluorescence decay-tune measurements of solid benzene at very low temperature give a value of 83.8 ns which is similar, within experimental error, to the high-pressure value of benzene vapour reported in the literature. The agreement in the fluorescence quantum yield is discussed     

On the “third decay channel” and vibrational redistribution problems in benzene derivatives

Fluorescence lifetimes and yields have been measured for toluene, aniline-h₇ and aniline-d₅ vapors in the spectral range corresponding to a rapid increase of the non-radiative rates. Non-exponential decays and discrepancies between lifetime and yield dependence on ν_exc are observed. Results are discussed on the ground of a model assuming: (a) strong anharmonic coupling of “active” and “non-active” modes, (b) specific “third channel” threshold for each vibrational progression and (c) absence of rapid vibrational redistribution.     

Model study of the abrupt excess-energy dependence of radiationless decay in benzene and azabenzenes

Model studies are reported aimed at accounting for the abrupt dependence of radiationless decay-rate constants on excess energy (known as channel-three decay in the case of S₁ benzene) in singlet and triplet manifolds of benzene and azabenzenes. The favored model involves L_a(ππ*) state, strongly distorted along an out-of-plane coordinate, crossing the ground-state potential at an energy close to the minimum of the lowest excited state. The results are compared with experimental observations on benzene and three azabenzenes. Implications for photochemical reactions are also discussed.     

Excitation and decay processes in helium clusters studied by fluorescence spectroscopy

Excitation and decay processes of helium clusters are investigated with fluorescence methods. The results differ remarkably from that obtained for the heavier rare gas clusters. They are discussed in view of the unusual structural and electronic properties of helium.     

Theoretical explanation of nonexponential OH decay in reactions with benzene and toluene under pseudo-first-order conditions

OH radical reactions with benzene and toluene have been studied in the 200-600 K temperature range via the CBS-QB3 quantum chemistry method and conventional transition-state theory. Our study takes into account all possible hydrogen abstraction and OH-addition channels, including ipso addition. Reaction rates have been obtained under pseudo-first-order conditions, with aromatic concentrations in large excess compared to OH concentrations, which is the case in the reported experiments as well as in the atmosphere. The reported results are in excellent agreement with the experimental data and reproduce the discontinuity in the Arrhenius plots in the 300 K < T < 400 K temperature range. They support the suggestion that the observed nonexponential OH decay is caused by the existence of competing addition and abstraction channels and by the decomposition of thermalized OH-aromatic adducts back to reactants. We also find that the low-temperature onset of the nonexponential decay depends on the concentration of the aromatic compounds and that the lower the concentration, the lower the temperature onset. Under atmospheric conditions, nonexponential decay was found to occur in the 275-325 K range, which corresponds to temperatures of importance in tropospheric chemistry. Branching ratios for the different reaction channels are reported. We find that for T > or = 400 K the reaction occurs exclusively by H abstraction. At 298 K, ipso addition contributes 13.0% to the overall OH + toluene reaction, while the major products correspond to ortho addition, which represents 43% of all possible channels.     

A “reduced” model theory for molecular decay processes

It is shown how a many-level radiationless model may be replaced by an effective one involving only the states of interest. When the coupling between the states of interest and the dissipative quasicontinuum is assumed to be not a constant one, the effective interaction can exhibit a non-markovian feature even in the statistical limit. Then, irradiation fields the intensities of which are not negligible compared to the excited state linewidth, are shown to reduce the coupling between the system of interest and its effective “thermal bath”. The previsions of the theory are checked by comparison with results obtained directly from the diagonalization of the Morokuma and Freed hamiltonian.     

The processes of benzene alkylation with n-alkenes on the sulfocation exchangers

The processes of benzene alkylation with n-alkenes C₂–C₃ on sulfocation exchangers are developed. The processes are deprived of the strong corrosion aggressiveness, characteristic of the industrial processes with use of AlCl₃ or BF₃, and do not need the periodic burning out of the formed contaminants, that is characteristic of the processes with application of zeolites. Alkylation proceeds in the liquid phase at moderate temperatures: 70–90°C in the case of alkylation with propene and n-butenes, and 130°C for the alkylation with ethylene. With the use of alkane-alkene fractions C₂–C₃ the conversion of alkenes process exceeds 99%, the alkylbenzene to dialkylbenzenes ratio reaches 20–25%. Peralkylation of dialkylbenzenes it not required.     

Nonradiative Excited-State Decay via Conical Intersection in Graphene Nanostructures

Chemical groups are known to tune the luminescent efficiencies of graphene-related nanomaterials, but some species, including the epoxide group (−COC−), are suspected to act as emission-quenching sites. Herein, by performing nonadiabatic excited-state dynamics simulations, we reveal a fast (within 300 fs) nonradiative excited-state decay of a graphene epoxide nanostructure from the lowest excited singlet (S₁) state to the ground (S₀) state via a conical intersection (CI), at which the energy difference between the S₁ and S₀ states is approximately zero. This CI is induced after breaking one C−O bond at the −COC− moiety during excited-state structural relaxation. This study ascertains the role of epoxide groups in inducing the nonradiative recombination of the excited electron-hole, providing important insights into the CI-promoted nonradiative de-excitations and the luminescence tuning of relevant materials. In addition, it shows the feasibility of utilizing nonadiabatic excited-state dynamics simulations to investigate the photophysical processes of the excited states of graphene nanomaterials.     

Temperature effect on the competition between the processes of 9-cyanophenanthrene exciplex decay

The activation parameters of internal conversion, intersystem crossing, and dissociation into radical ions were determined for the exciplexes of 9-cyanophenanthrene with 1,3,5- and 1,2,3-trimethoxybenzene. The activation enthalpies for the different processes of exciplex decay were found to be close for a given exciplex in a given solvent, thus allowing a linear approximation for the temperature dependence of the logarithm of the exciplex lifetime.Translated from Khimiya Vysokikh Energii, Vol. 39, No. 1, 2005, pp. 26–31.Original Russian Text Copyright © 2005 by Dogadkin, Dolotova, Soboleva, Kuzmin, Plyusnin, Pozdnyakov, Grivin.     

A phase-space approach to the T1 right arrow-wavy S0 radiationless decay in benzene: the effect of deuteration

The influence of full deuteration on the T1 right arrow-wavy S0 intersystem crossing in benzene is studied by a phase space approach. A full treatment of all the vibrational modes in the molecule leads to a ratio of the rate between the two isotopomers which is very close to the experimental value. Several aspects of the results are compared to previous estimates, and the effects of anharmonicity on the rates and accepting modes are examined. This first successful application of the method to a real physical system encourages the possibility of establishing a routine procedure for simple calculations of transition rates even for relatively large molecules.