EVIDENCE BY OPTICAL SPECTROSCOPY AND EPR FOR THE LIGHT-INDUCED CONFORMATIONAL ISOMERIZATION OF 2-NAPHTHYLPHENYLCARBENE IN N-HEPTANE SHPOLSKII MATRIX

The quasi-line fluorescence and excitation spectra of 2-naphthylphenyl carbene (2-NPC) in n-hexane and 12-heptane Shpolskii matrices at 4.2 K are composed of several spectra with origins shifted by 30-300 cm(-1) from each other. They have been attributed to the triplet-triplet tra nsition of the syn and anti conformers of 2-NPC, each one being trappe d in several different sites of the matrix. This attribution has been confirmed by the EPR experiments performed at 77 K. Furthermore, the f luorescence and EPR spectra reveal the light-induced transformation of the syn 2-NPC into the anti 2-NPC conformer in the n-heptane matrix. The fluorescence decays of 2-NPC in both matrices at 4.2 K are nonexpo nential and attributed to the emission from different sublevels of the first excited triplet state T-1. The zero-field splitting parameter D (T-1) = 0.011 cm(-1) in the T-1 state of 2-NPC has been estimated from the magnetic field effect on the fluorescence decays. cm This value i s much smaller than the D(T-0) = 0.41 cm(-1) value in the ground state T-0 of 2-NPC determined by EPR, which indicates the delocalization of the pi electron density onto the aromatic rings upon excitation.