CIRCULAR-POLARIZATION OF THE PHOSPHORESCENCE OF ALPHA,BETA-ENONES - EFFECT OF TRIPLET-TRIPLET COUPLING - EVIDENCE FOR NONDISTORTED 3-PI-PI-STAR STATES

For a series of alpha-beta-enones, mostly of steroidal type, the circular polarizations of the phosphorescence (CPP) data are reported along with the phosphorescence spectra, quantum yields and lifetimes, and the S0 --> 1n-pi-* circular dichroism (CD) and absorption spectra. The data were obtained in organic glasses (77-92 K). From the large values of the degree of CPP (g(lum)), and of the phosphorescence lifetime it is concluded that in all compounds the phosphorescent state is of the 3-pi-pi-* type and acquires its radiative properties predominantly from spin-orbit coupling with the 1n-pi-* state. Whereas for many enones the order of magnitude of g(lum) equals the degree of circular polarization of the S0 --> 1n-pi-* transition (g(abs)), the sign does not. According to the magnitude of the CD, the enones are divided in two groups: the molecules in group I ([g(abs)[ < 15 x 10(-3) are assumed to have approximately planar enone chromophores, those in group II ([g(abs)[ greater-than-or-equal-to 15 x 10(-3)) essentially nonplanar ones. In contrast to group I, in group II g(lum) are systematically opposite in sign. It is argued that in the nonplanar enones 1n-pi-*-1-pi-pi-* coupling is an important mechanism for the generation of optical activity, leading to oppositely signed CD in the S0 --> 1n-pi-* and S0 --> 1-pi-pi-* bands-as experimentally observed. Since the n-pi-*-pi-pi-* separation is much smaller in the triplet than in the singlet manifold, the coupling of the triplet states is enhanced. For the level ordering E(3-pi-pi-*) < E(3n-pi-*) < E(1n-pi-*) < E(1-pi-pi-*) it necessarily leads to opposite signs of g(lum) and g(abs) - without the necessity of invoking geometry changes in the 3-pi-pi-* state. Together with the fact that group II consists of a structurally heterogeneous collection of enones, this implies that, at least in glassy matrices at low temperatures, the spectroscopic 3-pi-pi-* state of alpha,beta-enones does not exhibit appreciable distortion relative to the ground state.