Ultrafast studies of some diarylcarbenes

Abstract

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 ¹DBC are 83 ps and 72 ps in acetonitrile and cyclohexane, respectively. The lifetime of ¹DBC shortens to 9 ps in methanol due to rapid reaction with the solvent. The lifetime of ¹AN is 87 ps and 66 ps in acetonitrile and cyclohexane, respectively. In methanol, the lifetime of ¹AN cannot be determined due to spectral overlap of ¹AN and cation ANH⁺. The decays of ¹DBC and ¹AN are controlled by intersystem crossing (ISC) in acetonitrile and cyclohexane and the rates of ISC of ¹DBC or ¹AN 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.