Aromatic hydrocarbons radiationless transition

In this section the effect of spin-orbit coupling on radiative and radiationless intercombinational transitions (transitions occurring between states of different multiplicity) will be investigated. We will be particularly concerned with the use of internal and external heavy atoms to induce spin-orbit coupling. The effect of heavy atoms on intercombinational processes occurring in aromatic hydrocarbons, carbonyl compounds, and heterocyclic compounds will be discussed. 

Siebrand. W., Williams, D. F. Isotope rule for radiationless transitions with an application to triplet decay in aromatic hydrocarbons. J. Chem. Phys. 46, 403 (1967). 

Until recently the lifetimes of the triplet states of aromatic hydrocarbons in fluid solution at room temperature had been investigated exclusively by the technique of flash absorption spectroscopy. The lifetimes reported for many hydrocarbons, e.g., anthracene or phenan-threne, had been below 1 msec, and it had been assumed that radiationless conversion processes were so rapid under these conditions that the competing radiative triplet-singlet transition would be too slow to per-... 

In aromatic hydrocarbons, the radiationless transitions from the triplet to the ground state are dominated by CH stretching vibrational modes. In these hydrocarbons perdeuteration reduces kJsc and consequently enhances kp, the rate constant for phosphorescence emission. The lifetime rp, is considerably reduced. The reduction of kjsc (T, 5,) which... 

Totally deuterated aromatic hydrocarbons yield measured phosphorescence lifetimes greater than their protonated analogs.182 This behavior is ascribed to the closer spacing of vibrational levels in deuterated compounds with a consequent decrease in probability for nonradiative T -> S0 transitions. Quantum mechanical tunnelling may also contribute to the rate of the radiationless process with the normal compounds. 

The theory of rotation effects on prolate luminescent molecules in solution and its experimental verification have been developed and compared. Generalized diffusion equations for the rotational motion of an asymmetric rigid motor have been used to given an expression for steady-state fluorescence depolarization. " The radiationless transition from the first excited singlet state of Eosin has been measured by optoacoustic relaxation, and the absolute fluorescence quantum yields of organic dyes in poly(vinyl alcohol) have also been measured by the photoacoustic method. The accuracy of the method has been discussed in the latter paper. Actinometry in flash photolysis experiments has been assisted by new measurements on the extinction coefficient of triplet benzophenone. Matrix-isolation fluorescence spectrometry has been used to detect polycyclic aromatic hydrocarbons from gas chromatography. ... 

The details of the mechanism of decay of states in alkanes retain their interest. The effect of deuterium on fluorescence lifetimes has been discussed in terms of the theory of radiationless transitions. Analysis of fluorescence line shapes and Raman excitation profiles of tetradesmethyl-p-carotene in isopentane has been carried out at 190 and 230K . Solvation occurs over a time scale of about 100 fs whilst vibrational relaxation has a time scale of about 250 fs. The kinetics of the interaction of alcohols with the excited state of triethylamine shows involvement of a charge transfer exciplex . Ionizing radiation is a means of exciting saturated hydrocarbons and the complexity of three component systems containing saturated hydrocarbons, aromatic solvent, and fluorescent solute has been examined. ... 

In radiationless transitions from the triplet state to the ground state of aromatic hydrocarbons, the excess electronic energy goes predominantly into the CH stretching vibrations. Being of high frequency (i) = 3,000 cm ), they are widely spaced and much smaller vibrational quantum numbers are required than for other normal modes of vibration whose wave numbers are at most half... 

A recent tunneling effect model for radiationless transitions (198) has been applied to benzene and other aromatic hydrocarbons. The CH stretching vibrations are considered as dominant for the non-radiatlve process. Rate constants for the radiationless process Sg calculated by theory are of the same order of magni-... 

From experimental results it has been concluded that for aromatic hydrocarbons the radiationless transition is negligible if the energy differ-... 

Siebrand, W., Radiationless Transitions in Polyaromatic Molecules. II. Triplet Ground state Transitions in Aromatic Hydrocarbons, J. Chem. Phys. 1967, 47, 2411 2422. 

V. Excess energy dependence of radiationless transitions in aromatic hydrocarbons experimental results, 186... 

V. EXCESS ENERGY DEPENDENCE OF RADIATIONLESS TRANSITIONS IN AROMATIC HYDROCARBONS EXPERIMENTAL RESULTS... 

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