Exciplex Studies‐iv. Radiative and Non‐radiative Relaxation of the Fluorescent State of Indole†

Abstract

Rate constants for both radiative and non-radiative relaxation of the fluorescent state of indole and several methyl-indoles in solution have been calculated from fluorescence lifetime and quantum yield values.Radiative relaxation for indoles is solvent dependent while non-radiative relaxation is attributed to at least two processes, one of which is both solvent and temperature dependent. The latter process measured in aqueous solvents manifests a pattern of compensation of activation enthalpy by activation entropy with an isokinetic temperature for indole of 283°K.A quantitatively consistent description of the excited-state behavior of 1-Me-indole in both free and exciplex state is provided. Small deviations in fluorescence parameters for indole suggest a more complicated set of events.

The Luminescence of the Aromatic Amino Acids

“…As Walker et al (55) have pointed out, the results in the paraffin nonpolar solvent used by the Russian workers may have been complicated by the presence of polar impurities in the solvent. Walker et al (55) have suggested that an inversion of the energies of the two states occurs upon exciplex· formation in polar solvents so that emission occurs from the lLb state in nonpolar solvents ana from the lLa state in polar solvents.…”

“…The thermal profile of quenching above 20 D C may be analyzed in terms of a single dominant quenching process of high activation energy (-12 kcal). (55,92) In nonaqueous solvents, such as dioxane and propylene glycol, where solvated electron production does not occur upon ultraviolet irradiation, the thermal dependence of quantum yield is very much reduced, corresponding to an activation energy of only 4 kcal or less. (92) It is tempting therefore to identify the quenching process of high activation energy observed in water with electron ejection to solvent.…”

Fluorescence of Tryptophan in Keratin

“…The rate constant for radiative decay from the 'L, state of indoles in a non-polar environment is about 0.6-0.8 x l o s s -' (Walker et a/., 1971;Andrews and Forster, 1974) and therefore the contribution from the 'L state to the observed emission would be at least as great as that from the 'La state. The energies of the 'Lb states of indole derivatives are not particularly sensitive to the environment (Strickland et al, 1972).…”

“…The relatively high probability of the 'A-'L, transition is due to the mixing of the 'La state with an allowed 'B state. Lowering the energy of the 'La state by dipolar interactions would be expected to decrease this mixing resulting in a lower radiative probability than that associated with the 'La state of an isolated molecule (Walker et a/., 1971). For example, the rad:ative rate constant for the "'La state" of indole in water is 6.3 x lo7 s -' and that for 3-methylindole in water is 3.9 x lo7 s-' (Privat et a/., 1979) compared with about 2 x loss-', (5x1s lifetime) for the 'La state of 2,3-dimethylindole in 2-methylpentane (Andrews and Forster, 1974).…”

Photosensitized Reduction of Pyrimidine Bases by Tryptophan*

“…It has been observed that the fluorescence yield of indole compounds decreases markedly with temperature (Eisinger and Navon, 1969;Kirby and Steiner, 1979;Walker et al, 1971). Walker et al (1971) proposed that the temperature dependent process involved electron dissociation and this was subsequently confirmed by Hopkins and Lumry (1972) using N,O as a specific electron scavenger.…”