Studies of the temperature dependence of the phosphorescence spectra and decay rates of aromatic carbonyl molecules in mixed organic crystals

Abstract

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

Microwave-induced delayed phosphorescence studies of the total and radiationless decay processes of3ππ* aromatic carbonyls

“…The values of/3 estimated here are considerably smaller than those given previously, but still much larger than the mixing coefficients ),, due to spin-orbit coupling. The /3 estimated here are even more favourable to the previously reached conclusion that the intrinsic mixing (direct configurational and vibronic) of the 3nrr* state into the 37rrr* state is rather small and therefore the photochemical activities of the arm* aromatic carbonyls at room temperature are likely due to the thermal population of the higher an~r* state rather than the mixed 3n~* character [24]. This conclusion supports the suggestion made by Wagner et al based on the studies of photochemical reacitivty of the aromatic carbonyls [30,31].…”

Substituted acetophenones. Importance of activation energies in mixed state models of photoreactivity

“…This is easily confirmed. Using the parameters employed above, viz., V = 100 cm~_^, b2 = 0.01, AE = 960 cm-1, and = 9 cm-1, eq 1 yields log kTBl¿ -1.9 at 77 Kand -1.7 at 300 K, a twofold variation. On the other hand eq 6 yields log /crad -7.8 at 77 K and -2.0 at 300 K, a variation of almost a million.…”

Origin of the unusual triplet-state properties of xanthones

“…The dominance of T2 emission for xanthone at 77 K is consistent with the extremely small AET value of 29 cm-1, which was determined from the separation between 3(nx*) and 3(xx*) 0-0 bands, and the fact that 3(nx*) radiative rates are much larger than those for 3(xx*) states. 10 Figure 3 shows the phosphorescence spectrum of 3,6dimethylxanthone in n-hexane at 4.2, 77, and 164 K. As in the xanthone case, the emission spectrum changes considerably with temperature. At 4.2 K a complex vi- bronic pattern is observed (all of the dimethyl derivatives exhibit multiple site structure at 4.2 K which further complicates the spectra).…”