The photochemistry and photophysics of a series of carbenes and nitrenes and their precursors have been investigated using state of the art ultrafast time-resolved spectroscopies and quantum mechanical calculations.
Carbene-related photochemistry and photophysics was discussed in Chapters 2 to 9. 1,2-hydrogen shift in the excited states of diazo compounds was discussed in Chapter 2 and the first direct evidence for the Rearrangements in the Excited State (RIES) mechanism was provided. To pursue other RIES examples, we studied Wolff Rearrangement (WR) in the diazo excited states in Chapter 3, along with a detailed discussion of WR for keto and ester carbenes. In Chapter 4, an ultrafast interconversion for a pair of isomeric ketocarbenes was observed. In Chapter 5, we reinvestigated the photochemistry of 9 diazofluorene and directly observed the formation of an excited singlet carbene in solution for the first time. In Chapter 6, we reported the first carbene solvation dynamics and found that the solvation phenomena are more prominent in polar and hydrogen bonded solvents than in non-polar solvents. The Eisenthal group established a rule for intersystem crossing (ISC) rates of aryl carbenes in 1980s, stating carbene ISC rates correlate with solvent polarities. In Chapter 7, we described some remarkable exceptions to this rule and discussed the factors controlling carbene ISC rates. In Chapter 8, we investigated the photochemistry of arylhalodiazirines and concluded that zwitterions are intermediates in the photochemical formation of arylhalocarbenes. Some other ultrfast studies of diarylcarbenes were discussed in Chapter 9.
In Chapters 10 to 12, photochemistry and photophysics of nitrenes and their precursors were studied. In Chapter 10, we reported the first spectroscopic observation of azide excited states and measured the lifetimes of some extremely reactive nitrenes, o-biphenylylnitrene, 1-naphthyl and 2-naphthylnitrenes at ambient temperature in solution. 2-naphthylnitrene was found to be the shortest-lived singlet nitrene to date (τ = 1.8 ps in acetonitrile). In Chapter 11, we reported the direct observation of phenyl, o biphenylylnitrene and 1 naphthyl nitrenium ions. In Chapter 12, we discovered that the protonation rate of a nitrenium ion in a series of protic solvents is well correlated with the solvent Swain acity parameters.