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The triplet-triplet fluorescence of perprotonated and perdeuterated diphenylcarbene (DPC-hlo and -dlo) 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 broad bands 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 threedifferent geometriesof DPC having CZ,, C2, and Cl symmetries. The computed triplet-triplet transition energies which indicate an hypsochromic shift going from planar CZ, to nonplanar C1 structures compare favorably with the experimental data. The fluorescence decays of DPC-hlo and -dl0 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 TFT1 transition going from CZ, to C1 structures, as due to the inversion between the two excited triplets corresponding to the n r * 1 and nr*2 configurations. The zero-field splitting (ZFS) parameter D(T1) of DPC in the first excited triplet T I has been estimated from the magnetic field effect on the fluorescence decays. D(T1) = 0.007 and 0.20 cm-l for measurements carried out respectively on the sharp and on the broad fluorescence. Tentative ZFS calculations give D(T1) = 0.0155 cm-l for the Cb structure and D(T1) = 0.01 15 cm-l for the CI 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 r 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.

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