Ultrafast studies of some diarylcarbenes.

The photochemistry of 5-diazo-10,11-dihydrodibenzo[a,d]diazocycloheptene (DDBC) and 9-diazoanthrone (DAN) were studied by ultrafast time resolved techniques. The excited states of these diazo compounds were observed by UV-Vis spectroscopy and were found to decay in 300 fs. The diazo excited state decays led to the appearance and first direct observation of singlet 5-diazo-10,11-dihydrodibenzo[a,d]cycloheptenylidene (DBC) and 9-anthronylidene (AN). The dynamics of DBC and AN were studied in acetonitrile, cyclohexane and methanol. The lifetimes of (1)DBC are 83 ps and 72 ps in acetonitrile and cyclohexane, respectively. The lifetime of (1)DBC shortens to 9 ps in methanol due to rapid reaction with the solvent. The lifetime of (1)AN is 87 ps and 66 ps in acetonitrile and cyclohexane, respectively. In methanol, the lifetime of (1)AN cannot be determined due to spectral overlap of (1)AN and cation ANH+. The decays of (1)DBC and (1)AN are controlled by intersystem crossing (ISC) in acetonitrile and cyclohexane and the rates of ISC of (1)DBC or (1)AN are similar in these two solvents. This differs from the solvent dependence of other diarylcarbene intersystem crossing rates. This is attributed to the relatively large singlet-triplet (S-T) gaps of these carbenes and this factor dominates the influence of solvent.