Structure Kinetics Correlations in Isostructural Crystals of α-(ortho-Tolyl)-acetophenones: Pinning Down Electronic Effects Using Laser-Flash Photolysis in the Solid State

Aqueous suspensions of nanocrystals in the 200-500 nm size range of isostructural alpha-(ortho-tolyl)-acetophenone (1a) and alpha-(ortho-tolyl)-para-methylacetophenone (1b) displayed good absorption characteristics for flash photolysis experiments in a flow system, with transient spectra and decay kinetics with a quality that is similar to that recorded in solution. In contrast to solution measurements, reactions in the solid state were characterized by a rate limiting hydrogen transfer reaction from the triplet excited state and a very short-lived biradical intermediate, which does not accumulate. Notably, the rate for delta-hydrogen atom transfer of 1a (2.7 X 10(7) s(-1)) in the crystalline phase is 18-fold larger than that of 1b (1.5 X 10(6) s(-1)). With nearly identical molecular and crystal structures, this decrease in the rate of delta-hydrogen abstraction can be assigned unambiguously to an electronic effect by the para-methyl group in 1b, which increases the contribution of the (3)n,pi* configuration relative to the reactive (3)n,pi* configuration in the lowest triplet excited state. These results highlight the potential of relating single crystal X-ray structural data with absolute kinetics from laser flash photolysis.