The temperature dependence of the phosphorescence spectra, decay rates and the So -->T2(3n ~r*) absorption spectra were studied for aromatic carbonyl molecules in mixed organic crystals. The energy separation, AET, between the %r~r* lowest excited triplet (7'1) states and the higher 3n~r* states (T2) was estimated for several systems from the temperature dependence of the phosphorescence spectra and decay rates. It was found that the decay rates of the aromatic carbonyl molecules in the mixed crystal systems studied are determined by (1) thermal population to the 3nTr* states and (2) increased radiationless transition rates at higher temperatures.
INTRODUCTIONIn the two companion papers we have discussed the results of PMDR [1] and E.P.R.[2] studies of the properties of the lowest excited triplet (T1) states of a series of aromatic carbonyl molecules. In order to supplement these studies we have investigated the temperature dependence of the phosphorescence spectra and lifetimes and the S 0-->T absorption spectra of the aromatic carbonyl molecules in mixed crystal systems. The motives and the objectives of the present study are the following :(1) The magnetic and decay properties of the T 1 states of many aromatic carbonyl molecules were discussed in terms of the vibronic mixing of the 3~r~r* and 3nrr* states [1,3,4]. In order to discuss these properties quantitatively it is necessary to determine the energy difference (AET) between the 3rrrr* (T1) and the 3nrr* (T2) states of these systems. We have attempted to estimate AE r from S 0-->T absorption spectra and temperature dependence of the phosphorescence spectra and lifetimes.(2) In discussing the photochemical properties of the aromatic carbonyl molecules, Wagner et al. [5] suggested the possible role of the thermal equilibrium between the lowest 3rrrr* and the nearby 3nrr* states. Mixed crystal systems can provide information on such equilibrium processes.(3) The radiationless transition from the lowest excited triplet state has been the topic of wide theoretical and experimental investigations in the past decade [6]. Temperature dependence of the radiationless processes has been investigated experimentally [7,8] and theoretically [9], but the previously investigated systems were limited to aromatic hydrocarbons such as benzene [7] and naphthalene [8]. It seems desirable to obtain more data on the temperature Downloaded by [McGill University Library] at 11:15 03 December 2014