E.C. Lim

Vibronic perturbation of the lowest triplet state and phosphorescence of aromatic ketones

Abstract The polarization of phosphorescence of acetophenone and its derivatives reveal the extent to which the lowest triplet state is perturbed by a nearby triplet state, and it leads to a conclusive assignment of the lowest triplet state. In acetophenone, the lowest triplet state is of n→ π * origin in 3-methylpentane glass and of π → π * origin in EPA glass. All common parasubstituted acetophenones possess the lowest triplet state of π → π * origin both in hydrocarbon and hydroxyl solvents. The Franck-Condon factor dependence of the vibronic coupling is suggested by the observation that low frequency vibrational modes appear to become less efficient in coupling 3 nπ * and 3 ππ * states as the energy gap between these states increases.

Electronic relaxation in naphthalene vapor

Abstract Fluorescence of naphthalene vapor at 0.1 mm and room temperature has been studied as a function of vibrational level and excitation energy. It is shown that the quantum yield of fluorescence from different vibrational levels of the lowest excited singlet states (S 1 ) shows a large variation, with the emission from a b 1g level showing particularly high yield. The quantum yield of the emission from several virbational levels (a b 1g level in particular) of S 1 is greater from naphthalene- h 8 than for naphthalene- d 8 , contrasting the normal deuterium isotope effect on radiationless transitions. It also shown that the quantum yield of fluorescence decrease rather smoothly with increasing excitation energy in the spectral region covering the second and third absorption bands. In the same spectral region, the fluorescence quantum yield of d 8 is greater than that of h 8 , and the ratio of the two yields increases smoothly with increasing energy of excitation.

Phosphorescence of 1-indanone and related compounds

Abstract Dual phosphorescence of 1-indanone and related molecules is shown to arise from two nearby triplet states of different orbital origin.

Pseudo Jahn-Teller effect and luminescence of nitrogen heterocyclic compounds

Abstract The correlation between the solvent dependence of the spectral shape of fluorescence and the solvent dependence of the radiationless transitions strongly suggests that it is the pseudo Jahn-Teller distortions of the lowest excited singlet state which leads to the strong solvent dependence of the fluorescence yield in nitrogen heterocyclic compounds.

Non-Born—Oppenheimer vibronic coupling and deuterium isotope effect on T1 → S0 radiative transitions of aromatic hydrocarbons

Abstract Radiative lifetimes of the lowest triplet states are determined for several aromatic hydrocarbons from the measured fluorescence quantum yield, phosphorescence quantum yield, and lifetime of phosphorescence. It is found that the radiative lifetime is considerably longer for perdeuterated naphthalene and phenanthrene than for the corresponding perprotonated species. A theoretical explanation is presented which involves a new non-Born—Oppenheimer coupling mechanism.

Radiationless transitions in monocyclic diazines

Abstract It is shown that the fluorescence and intersystem crossing in monocyclic diazines originate from the second excited singlet state (S 2 ). The occurrence of these phenomena is justified on the basis of the slow S 2 → S 1 internal conversion resulting from the low density of states.

Polarization of singlet-singlet electronic transitions in substituted naphtahlenes

Abstract The polarization of three low energy-singlet-singlet electronic transitions of several mono-substituted naphthalenes have been measured relative to the transition moment of fluorescence by the method of photoselection. It is shown that substitution of a hydroxy or a methoxy group at the 2 (orβ)-position of naphthalene leads to mixing between the 1 L b and 1 L a states, resulting in the short-axis polarization of the electronic transition to the former state. 1 (or α)- substituted compound exhibit no such mixing, and the transition to the 1 L b state is polarized largely along the long axis of the molecule. Substitution of a cyano group at the 2-position also leads to short-axis polarization of the L b band. These results are consistent with the predictions based on molecular orbital calculations.

Rotational effects on the S0-T1 oscillator strength of pyrazine

Abstract The rotationally resolved S 0 →T 1 transition of jet-cooled pyrazine has been studied using phosphorescence excitation and multiphoton ionization spectroscopy. A computer simulation that divides the relative transition intensity by 2 J +1 successfully reproduces the experimental spectra. The results are interpreted in terms of a possible spin-rotational-orientational selectivity in the singlet-triplet coupling that governs the transition oscillator strength.

Non-planar vibrations and radiationless molecular electronic transitions in nitrogen heterocyclic and aromatic carbonyl compounds

Abstract The importance of non-planar vibrations in radiationless molecular electronic transitions of hetero-aromatic molecules is described and discussed.

Spin-forbidden electronic transitions in non-planar aromatic amines

Abstract Evidence is presented which indicates that the direct spin—orbit coupling between low-lying ππ* states is largely responsible for the efficient S1 → T1 intersystem crossing and T1 → S0 radiative transition in non-planar aromatic amines.