RH(III)-PHOTOSENSITIZED INTERCONVERSION OF NORBORNADIENE AND QUADRICYCLANE

The utility of two Rh(III) diimine complexes, Rh(phen)(3)(3+) and Rh(p hi)(2)(phen)(3+) (phen = 1,10-phenanthroline, phi = 9,10-phenanthreneq uinone diimine), as sensitizers for the interconversion of norbornadie ne (N) and quadricyclane (Q) has been investigated using steady-state photochemical and laser flash photolysis (LFP) techniques. Irradiation of acetonitrile solutions of Rh(phen)(3)(3+) and N causes slow conver sion to Q. The reaction is reversible; irradiation of Rh(phen)(3)(3+) in the presence of Q leads to N. Irradiation of acetonitrile solutions of Rh(phi)2(phen)3+ and Q yields N. However, this reaction is irrever sible; irradiation of the Rh(In) complex in the presence of N fails to afford Q. Irradiation of methanol solutions of either Rh(III) complex in the presence of N or Q affords minor amounts of two methanol-C7 ad ducts but fails to quench the N-Q interconversion reaction. The result s are consistent with N-Q interconversion via an exciplex intermediate . The Rh(III)-sensitized deazatization of two cyclic azoalkane derivat ives (Azo-N, Azo-Q) of N and Q was also investigated. Deazatization wa s achieved by Rh(phen)(3)(3+) but not Rh(phi)(2)(phen)(3+) sensitizati on. The results are consistent with a mechanism involving triplet ener gy transfer, but the involvement of exciplex intermediates cannot be r uled out. Bimolecular rate constants for quenching of the Rh(III) exci ted states by N, Q, Azo-N, and Azo-Q were determined by LFP.