Js. Demelo et al., THE PHOTOPHYSICAL BEHAVIOR OF 3-CHLORO-7-METHOXY-4-METHYLCOUMARIN RELATED TO THE ENERGY SEPARATION OF THE 2 LOWEST-LYING SINGLET EXCITED-STATES, The Journal of chemical physics, 107(16), 1997, pp. 6062-6069
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.