Franck-Condon factors in intermolecular intersystem crossing

It is concluded from the energy dependence of the rate constant for intersystem crossing between guests and host molecules in mixed crystals that Franck-Condon factors for inter- and intramolecular radiationless transitions are virtually identical.     

Matrix elements for intersystem crossing

A new formulation of intersystem-crossing matrix elements is presented which includes correction for prompt scattering and avoids the Herzberg-Teller expansion for nonadiabatic coupling.     

An analysis of heavy-atom perturbation of intersystem crossing as a mechanistic tool in photochemistry : Probing for singlet and triplet dominated reactions

A kinetic analysis of heavy-atom enhanced intersystem crossing is presented, with the object of using this perturbation method for determining whether the majority of a photoproduct originates from an excited singlet or an excited triplet precursor. Expressions are derived which relate the quantum efficiencies of product formation (φ_r) and S₁→T₁ intersystem crossing (φ_isc) with potential quenching or enhancement by the heavy-atom. These allow limits to be placed on the possible quenching or enhancement that may be observed in particular cases (see Fig. 4 and 6) and thereby make an interpretation of the data less ambiguous than has been suggested in recent discussions. A particularly useful observation is that for those molecules (for example, aromatics) having triplets which are relatively unaffected by heavy-atoms, quenching of a product by more than 50% demonstrates that a majority of this product must be derived from the excited singlet state.     

Vibrational relaxation by reversible intersystem crossing

The time- and energy-resolved fluorescence spectra of CO obtained under selective excitation of the A ¹Π (υ′ = 1) level have been studied. The apparent vibrational relaxation in the singlet manifold is shown to be mainly due to the three-step process: the singlet → triplet crossing from υ′ = 1 level, vibrational relaxation in the triplet manifold and reverse crossing to the A ¹Π (υ′ = 0) level. The decay form may be fitted by assuming the relaxation rate constants in triplet (and singlet) manifolds of the order of 10⁵ s^?1 Torr^?1 i.e. smaller by one order of magnitude than previously proposed.     

Studies of intersystem crossing dynamics in acetylene

We report a new ab initio study of the acetylene T3 potential energy surface, which clarifies the nature of its energy minimum, and present computed equilibrium geometries and diabatic frequencies. This information enables the computation of harmonic vibrational overlap integrals of T3 vibrational levels with the S1 3nu3 state. The results of this calculation support the interpretation of two local perturbations of S1 3nu3, revealed in ultraviolet laser-induced fluorescence/surface electron ejection by laser excited metastables spectroscopy and Zeeman anticrossing measurements, respectively, as arising from two rotational submanifolds of a single T3 vibrational state. We present plausible assignments for this state as a guide for future experimental work.     

Mechanism of collision-induced intersystem crossing in CO

We have studied inert-gas pressure effects on the fluorescence decay in CO selectively excited to the υ = 0 to 7 vibronic levels of the A ¹Π electronic state. It is shown that the dependence of the quenching cross section σ_isc on the average value of the ST mixing coefficient (β²) has a quasi-logarithmic form. A simple two-level model describing semiquantitatively this behavior is proposed.     

On the intersystem crossing of pentacene in p-terphenyl

Applying methods developed for single molecule spectroscopy to small ensembles, we have recorded high-resolution fluorescence-excitation spectra for pentacene in all substitutional sites of a p-terphenyl single crystal. The difference in intersystem crossing efficiency for pentacene molecules in the various substitutional sites is discussed on the basis of these spectra and data from optically detected magnetic resonance experiments.     

Temperature dependence of intersystem crossing of pyrene

The temperature dependence of the rate constant for intersystem crossing from S₁ of pyrene was determined by means of a method proposed previously. In contrast to the conclusions of Stevens, Thomaz and Jones, and of Kropp, Dawson and Windsor, we conclude that both temperature-independent and temperature-dependent intersystem crossing processes exist. These two processes contribute equally at ≈ 150 K.     

The nature of internal conversion and intersystem crossing in exciplexes

Experimental data obtained by the authors themselves and other published data on the dependence of the rate constants of internal conversion and intersystem crossing in exciplexes (back electron transfer) on the value of energy gap were critically revisited. The conclusion that these processes occur according to the mechanism of radiationless diabatic quantum transitions, rather than the preceding thermally activated solvent and reactant reorganizations was substantiated.Translated from Khimiya Vysokikh Energii, Vol. 39, No. 2, 2005, pp. 114–125.Original Russian Text Copyright © 2005 by Kuzmin, Soboleva, Dolotova, Dogadkin.This revised version was published online in April 2005 with a corrected cover date.     

Influence of solvent on carbene intersystem crossing rates

The influence of coordinating solvents on singlet-to-triplet carbene intersystem crossing (ISC) rates has been studied with diphenylcarbene (DPC) and para-biphenyltrifluoromethylcarbene (BpCCF 3) by using ultrafast time-resolved spectroscopy. DPC has a triplet ground state in all of the solvents considered, and the concentration of singlet carbene at equilibrium is too small to be measured. It is found that the lifetime of (1)DPC is extended in acetonitrile, benzene, tetrahydrofuran, dichloromethane, and halobenzene solvents relative to cyclohexane. The solvent effect does not well correlate with bulk measures of solvent polarity. The singlet-triplet energy separation of BpCCF 3 is close to zero. The data demonstrates that BpCCF 3 has a triplet ground state in benzene, fluorobenzene, and hexafluorobenzene. Halogenated solvents are found to dramatically retard the rate of ISC in (1)BpCCF 3. We postulate that the empty p orbital of a singlet carbene coordinates with a nonbonding pair of electrons of a halogen atom of the solvent to form a pseudoylide solvent complex, stabilize the singlet carbene, and decrease the singlet-triplet (S-T) energy gap. The "golden rule" of radiationless transitions posits that the smaller the energy gap between the two states, the faster their rate of interconversion. To explain the apparent violation of the golden rule of radiationless transitions for the carbene ISC processes monitored in this study, we propose that the significantly different specific solvation of the singlet and triplet carbenes imposes a Franck-Condon-like factor on the ISC process. Those solvents that most solvate the singlet carbene will also have the greatest structural difference between singlet carbene-solvent complex and their triplet spin isomer-solvent complex, the smallest S-T gap, and the slowest ISC rate. Alternatively, one can propose that a highly solvated singlet carbene must desolvate prior to ISC, and that this requirement decelerates the radiationless transition.     

Hexafluoroacetone photochemistry: Internal conversion versus intersystem crossing

The hexafluoroacetone photodecomposition quantum yield is calculated as a function of pressure at 24° and 313 nm using the established mechanism of excited state processes and rate parameters determined by previous luminescence and photodecomposition studies. Agreement with experiment can be obtained if internal conversion increases relative to intersystem crossing as the excitation energy is increased.     

Reduced equations of motion for collision-induced intersystem crossing

A model is proposed for collision-induced intersystem crossing in “intermediate case” and small molecules. The collisions are assumed to cause dephasing (T₂) among the zero order singlet and triplet molecular states. The combined effect of the intramolecular spin-orbit coupling (μ) and the collisional dephasing, results in the experimentally observable relaxation of populations (T₁). The basic assumption of the present model is that the duration of a collision τ_c is short compared to the intramolecular coupling (μτ_c ? 1). Reduced equations of motion for the molecular density matrix are derived and conditions for observing nonexponential relaxations are discussed. The model demonstrates the equivalence of T₁ and T₂ processes, depending on our choice of a basis set.     

Inverse deuterium isotope effect in the intersystem crossing of diphenylcarbene

The singlet-to-triplet intersystem crossing rate (k_st) of diphenylcarbene (DPC) is found to exhibit an inverse isotope effect in various solvents. An off-resonance coupling model between the initial singlet state and a sparse triplet vibronic manifold accounts for k_ST showing both an inverse isotope effect in a given solvent as well as an inverse energy gap effect in a solvent series.     

Time-dependent approaches for the calculation of intersystem crossing rates

We present three formulas for calculating intersystem crossing rates in the Condon approximation to the golden rule by means of a time-dependent approach: an expression using the full time correlation function which is exact for harmonic oscillators, a second-order cumulant expansion, and a short-time approximation of this expression. While the exact expression and the cumulant expansion require numerical integration of the time correlation function, the integration of the short-time expansion can be performed analytically. To ensure convergence in the presence of large oscillations of the correlation function, we use a Gaussian damping function. The strengths and weaknesses of these approaches as well as the dependence of the results on the choice of the technical parameters of the time integration are assessed on four test examples, i.e., the nonradiative S(1) ? T(1) transitions in thymine, phenalenone, flavone, and porphyrin. The obtained rate constants are compared with previous results of a time-independent approach. Very good agreement between the literature values and the integrals over the full time correlation functions are observed. Furthermore, the comparison suggests that the cumulant expansion approximates the exact expression very well while allowing the interval of the time integration to be significantly shorter. In cases with sufficiently high vibrational density of states also the short-time approximation yields rates in good agreement with the results of the exact formula. A great advantage of the time-dependent approach over the time-independent approach is its excellent computational efficiency making it the method of choice in cases of large energy gaps, large numbers of normal modes, and high densities of final vibrational states.     

Triplet to ground state intersystem crossing and CIS-trans isomerization in stilbene

The T₁ ? S₀ intersystem crossing process in stilbene is considered adopting a formulation in which the isomerization coordinate can be treated separately. Energy surfaces in agreement with experimental and theoretical results are employed and pertinent spin-orbit coupling is evaluated. The calculated decay rate is in good agreement with the experimental results based on the azulene quenching effect. The qualitative feature that quantum yield for cis formation tends to be higher than the counterpart trans formation is explained. Deuterium effects are discussed and explanations for them are suggested.     

Competition between intersystem crossing and internal conversion in isolated large molecules

The excess energy and deuteration dependence of the radiationless decay rate in “isolated” aromatic hydrocarbons (anthracene, 9,10-dimethylanthracene, phenanthrene and fluorene) suggest that S₁→S₀ internal conversion dominates over S₁→T intersystem crossing for molecules with very large excess vibrational energies.