THE PHOTOPHYSICAL BEHAVIOR OF 3-CHLORO-7-METHOXY-4-METHYLCOUMARIN RELATED TO THE ENERGY SEPARATION OF THE 2 LOWEST-LYING SINGLET EXCITED-STATES

The photophysical behavior of 3-chloro-7-methoxy-4-methylcoumarin (C1M MC) was studied as a function of the solvent and temperature. The radi ative lifetime r(F)(0) = 1/k(F) is essentially solvent independent and its value (approximate to 4.6 ns) is totally commensurate with the fl uorescence originating from a (1)(pi,pi) state as the lowest excited state. From the fluorescence data obtained in 24 solvents plus nine so lvent mixtures, and the triplet formation quantum yields for three rep resentative solvents, it was found that the internal conversion rate c onstant (k(IC)) dictates the photophysical behavior of C1MMC and chang es of two orders of magnitude occur from nonpolar to polar solvents. F rom the temperature dependence (20 to - 100 degrees C) of the fluoresc ence lifetimes in five solvents it was found that a change of the inte rnal conversion rate constant of the same order of magnitude occurred as above. The rate constants and the activation energies for the radia tionless processes were determined. The results show that the reason f or the dramatic variation of k(IC) is the fact that when the S-2(n,pi ) state is close lying to S-1(pi,pi), there is a decrease of the acti vation energy of the internal conversion process (S-1 similar to simil ar to --> S-0). Increasing solvent polarity increases the energy gap b etween these states, and decreases the magnitude of the effect. Decrea sing the temperature to sufficiently low values, disables the deactiva tion channel. The interpretation of the foregoing results can be satis fied by either a model involving a fast equilibrium between two close lying S-2 and S-1 states or in terms of the so-called ''proximity effe ct.'' (C) 1997 American Institute of Physics.