Vibronic quantum effects in fluorescence and photochemistry. Competition between vibrational relaxation and photochemistry and consequences for photochemical control

In this paper we measured the fluorescence quantum yield (Phi(F)) and the reaction quantum yield (Phi(PC)) of a photochromic molecule (flindersine) as a function of the vibronic level (n) excited within a given sequence. We found that Phi(F) decreased and Phi(PC) increased with an increase in the quantum number of the vibronic level excited within a sequence. On the basis of a previously proposed model, this behavior was interpreted as resulting from competition between vibrational relaxation and photochemistry at each vibronic level. This model was broadened, and a new equation developed which, alone, or in combination with fluorescence data, permits determination of (1) the molar extinction coefficient of the partially produced colored form, (2) the quantum yield of vibrational relaxation, Phi(V), and the complementary Phi(PC) at each vibronic level, (3) the photochemical reaction rate constant, kpc, (4) the nonradiative internal-conversion rate constant from S-1 to S-0, k(NR). and (5) the vibrational relaxation rate constant among the n levels of S-1, k(V). The k(PC) value (1.7 x 10(10) s(-1)) is comparable to k(V) (4.0 x 10(10) s(-1)) and k(NR) (2.3 x 10(10)s(-1)). The data and model account fbr the significant decrease in Phi(F) with an increase in the value of n excited. Therefore, from the results here as well as those from our previous works, we propose the theory that for molecules undergoing excited-state photochemistry, there will he a vibronic-level dependence for Phi(PC) and Phi(F) and potentially for the triplet state yield Phi(T) as well. It also appears that there can be a vibronic-mode and electronic-state dependence for these parameters. The nature of the photochemistry could also well be mode-dependent.