ZERO-FIELD SPLITTING OF THE 1ST EXCITED TRIPLET-STATE OF DIBENZOCYCLOHEPTADIENYLIDENE - A CARBENE TO BIRADICAL TRANSFORMATION UPON ELECTRONIC EXCITATION

The quasi-line fluorescence and excitation spectra of dibenzocycloheptadienylidene (DBC) in n-hexane at 20 K, obtained by site-selective laser experiments, do not present mirror-image symmetry. The fluorescence decay of matrix-isolated DBC is nonexponential and attributed to the emission from different sublevels of the first excited triplet state. In the presence of a magnetic field, the lifetime of the slow component decreases. Its dependence as a function of a weak magnetic field can be calculated for different values of the zero-field splitting parameter D. The best fitting value is \D\ = 0.02 cm-1. The D value in the first excited triplet state is significantly smaller than in the ground state where \D\ is known to be 0.3932 cm-1. The decrease of the D value is interpreted with very simple molecular orbital theory. The excitation of DBC takes electron density off the carbene center and delocalizes it into the aromatic rings.