STUDY OF THE ELECTRONIC-STRUCTURE AND SPECTRA OF DIPHENYLCARBENE CONFORMERS IN THEIR GROUND-STATE AND LOWER EXCITED-STATES

The triplet-triplet fluorescence of perprotonated and perdeuterated diphenylcarbene (DPC-h10 and -d10) in n-hexane at 4.2-25 K are both composed of a sharp spectrum and a broad blue-shifted one. They are attributed to two different conformers, the sharp spectrum corresponding to DPC in a quasi-planar geometry. The excitation spectra of both emissions consist of broadbands with a large Stokes shift. A detailed CS-INDO CI investigation (configuration interaction based on an intermediate neglect differential overlap method adapted to conformation and spectroscopy problems) is performed for three different geometries of DPC having C2v, C2, and C1 symmetries. The computed triplet-triplet transition energies which indicate an hypsochromic shift going from planar C2v to nonplanar C1 structures compare favorably with the experimental data. The fluorescence decays of DPC-h10 and -d10 are nonexponential and attributed to the emission from different sublevels of the first excited triplet state. The measured decays of the broad fluorescence are longer than those of the sharp one. This is in agreement with CS-INDO CI calculations which predict a decrease of the oscillator strength of the T0-T1 transition going from C2v to C1 structures, as due to the inversion between the two excited triplets corresponding to the n-pi*1 and npi*2 configurations. The zero-field splitting (ZFS) parameter D(T1) of DPC in the first excited triplet T1 has been estimated from the magnetic field effect on the fluorescence decays. D(T1) = 0.007 and 0.20 cm-1 for measurements carried out respectively on the sharp and on the broad fluorescence. Tentative ZFS calculations give D(T1) = 0.01 55 cm-1 for the C2v structure and D(T1) = 0.01 15 cm-1 for the C1 structure. The measured and calculated D(T1) values are always smaller than the D(T0) = 0.405 cm-1 value of DPC in the ground triplet state, which indicates the delocalization of the pi electron density onto the aromatic rings upon excitation. It is therefore reasonable to detect a specific chemistry of this carbene in the presence of an intense laser which creates a high concentration of DPC in the first excited triplet T1.