Zero-field splitting of the ground and excited triplet states of 2-naphthylphenylcarbene studied by hole-burning on triplet-triplet fluorescence excitation spectra

Spectral holes were burned in the two main lines of the triplet-triplet 0-0 fluorescence excitation spectrum of 2-napththylphenylcarbene in n-hexane at 1.8 K. For the line assigned to pseudo-E/trans conformer, the central hole at the frequency of burning light has several narrow components separated by 0.3 ± 0.05 and 1.0 ± 0.1 GHz and a satellite doublet line on the low energy side, 14.5 ± 0.5 and 15.5 ± 0.5 GHz away. For the line assigned to the psuedo-Z/cis conformer, the central hole is accompanied by holes burned on the low energy side, 0.8 ± 0.1, 5.2 ± 0.1, 11.9 ± 0.1 and 16.2 ± 0.1 GHz away. The hole-burning pattern is explained by a model taking into account the zero-field splitting (ZFS) of the ground T0 and excited T1 triplet states as well as the selectivity of excitation relaxation by the intersystem crossing channel. Analysis provides information about the ZFS parameters of the T1 state. For the pseudo-E/trans 2-NPC they are: D0 = 0.47 ± 0.02 cm−1, E0 = 0.017 ± 0.003 cm−1, D1 = 0.038 ± 0.003 cm−1 and E1 = 0.005 ± 0.001 cm−1.