Behavior of Biradicals Generated from a Norrish Type-I Reaction of 2,2-Diphenylcycloalkanones. Chain-Length Dependence and Magnetic Field Effects.

The reaction course of acyl-diphenylmethyl biradicals (alpha-oxo-omega, omega-diphenyl-alpha, omega-alkanediyl biradicals), O=C-up arrow-(CH2)(n-2)-(CPh2)-Ph-up arrow ((BR)-B-3-n), generated from the Norrish type-I reaction of 2,2-diphenylcycloalkanones (CK-n) with various ring sizes in methanol, is switched from intramolecular disproportionation (n = 6, 7), giving a diphenylalkenal, to acylphenyl recombination (n greater than or equal to 9), affording a cyclophane derivative. The behavior of (BR)-B-3-9 derived from CK-9 was studied in detail; the photolysis of CK-9 afforded an open-chain and a cyclic decarbonylation product together with an unsaturated aldehyde and a 4-methylene-2,5-cyclohexadienyl ketone (a pre-cyclophane). The photolysis of this ketone gave the same products that arose from the photolysis of CK-9, presenting the possibility that the decarbonylation products and a part of the aldehyde are formed as secondary products during irradiation. The magnetic field dependence of the lifetimes of (BR)-B-3-n (n = 12 and 13) generated from CK-12 and 13, respectively, was measured in methanol by means of a pulsed-laser excitation technique. The rate constants for intersystem crossing showed a maximum at a relatively low field strength, which decreased with increasing the field strength to level off to an asymptotic value at > 1.5 kG. The results show that the role of hyperfine coupling in intersystem crossing is less important in these systems, presumably because of the presence of a carbonyl oxygen and the absence of a hydrogen atom at the radical center.