Radiationless Transitions in Polyatomic Molecules. II. Triplet-Ground-State Transitions in Aromatic Hydrocarbons

Abstract

The theory of Part I is applied to nonradiative transitions from the lowest triplet state to the ground state of aromatic hydrocarbons. A previously communicated empirical relation between triplet energy, triplet lifetime, and the relative number of hydrogen atoms per molecule is substantiated and its physical implications are discussed. It is transformed into a relation between the Franck-Condon factor of the transitions and the triplet energy. In this form it can be compared with the theoretical expressions derived in Part I. No satisfactory theoretical representation of the empirical formula could be obtained on the basis of a harmonic-oscillator description of the normal modes of the molecules. However, introduction of anharmonicity leads to excellent agreement between theory and experiment. A one-parameter formula is derived which accounts with good accuracy for the dependence of the triplet lifetime on the triplet energy and the number of carbon and hydrogen atoms in the molecule. This formula shows that the Franck-Condon factors relevant to the radiationless triplet-ground-state transition are governed by CH or CD stretching modes, which behave in this respect as completely degenerate for a given molecule. The single adjustable parameter is related to the anharmonicities of these modes in the ground and triplet state. The analysis confirms that the purely radiative triplet lifetime of most if not all aromatic hydrocarbons is close to 30 sec and that the Franck-Condon factor is the only parameter in the expression for the nonradiative triplet decay constant which varies considerably between different aromatic hydrocarbons. Finally the temperature dependence of the Franck-Condon factors is considered. A theoretical treatment indicates a very small temperature dependence below 400°K. This result seems to be borne out by an analysis of recent experiments, leading to the conclusion that the observed temperature effects are associated with bimolecular processes. A notable exception is benzene, where the triplet lifetime is temperature-dependent down to very low temperatures, possibly due to a Jahn-Teller distortion.

Triplet state dynamics on isolated conjugated polymer chains

“…In contrast, with the ladder polymer MeLPPP, a reasonable triplet quantum yield was observed, consistent with the observation of phosphorescence on photoexcitation of this polymer in low temperature glasses [15,16] or films [44]. Although other factors may be involved, we wish to suggest that, as with typical aromatic systems [45][46][47], the decrease in the S 1 -T 1 energy separation on going from MEH-PPV, through PFO to MeLPPP increases the rate of ISC. Recently, a similar energy gap dependence has been reported for nonradiative decay from the triplet state of Pt-containing conjugated polymers and monomers [48].…”

Direct observation of subpicosecond vibrational dynamics in photoexcited myoglobin

“…Taken together, our observations indicate the following scenario: an initial nonradiative transition from the excited state to the ground electronic state occurs through a manifold of highly excited, high frequency overtones and combination bands with the most favorable FC overlap 33,34 . These modes, which are extremely broad and non Raman-resonant (hence not visible in the present experiment) relax on a sub-picosecond time scale through rapid intramolecular vibrational energy redistribution.…”

A benchmark theoretical study of the electronic ground state and of the singlet-triplet split of benzene and linear acenes

“…For example, singlet-triplet gaps can be used to evaluate the strength of electron-electron correlation in luminescent conjugated polymers. 32 The singlet-triplet energy gaps of benzene and linear acenes have been measured decades ago, from phosphorescence spectra, and are well summarized in a 1967 paper, 33 or in books 34 by Birks. Note that most S 0 -T 1 transition energies of aromatic compounds were measured at various temperatures in glasslike solid crystals, or in solutions.…”

“…The discrepancies can be larger in the case of ͑symmetry forbidden͒ weak vibrational 0-0 components, as was for instance the case with benzene. 33 The greatest care is needed with experimental values obtained for hexacene, which can only be synthesized in a polymethylmethacrylate matrix, where it remains stable up to 12 h at most. 36 Heptacene can also be synthesized in the same matrix ͑under an inert atmosphere͒, where it can be maintained only up to 4 h; 37 no one has ever managed to measure the ST gap of this compound so far.…”