Reversible Interconversion between Singlet and Triplet 2-Naphthyl ...

L J. Am. Chem. Soc. PAGE EST: 11.5 Zhu et al.chemical loss of CO 2 to give 2-vinylnaphthalene (VN), whichunfortunately has no IR absorptions with an intensity commensuratewith that of CdO stretches, and hence it is notsurprising that it does not manifest itself clearly as a photofragment.However, in the flash photolysis studies described inthe accompanying paper of Wang et al., 16 VN was observed asa final product of the photolysis of MDA.ConclusionsOn irradiation into its weak visible absorption band, 3 NCCis converted to a photoproduct with a broad absorption at 420nm. Concurrently, the ESR signals of 3 NCC disappear, whichindicates that the photoproduct is a singlet species. On photolysisat 450 nm or upon standing overnight at 12 K in the dark, 3 -NCC is re-formed almost quantitatively. We showed, by acombination of different spectroscopic and theoretical methods,that the above photoproduct is identical to the singlet state ofthe same carbene, 1 NCC, which is marginally protected fromspontaneous decay to the triplet ground state by a pronouncedconformational change of the COOMe group. The ester groupis coplanar with the naphthylcarbene moiety in 3 NCC butassumes a perpendicular conformation in 1 NCC to avoiddestabilization of the formally vacant carbenic 2p atomic orbitalby the electron-withdrawing carbonyl group. A very similar caseof “conformational control of the spin state” has recently beenfound in dimethylenenepyrrole diradicals. 108 However, to ourbest knowledge, the example of 2-naphthyl(carbomethoxy)carbeneis the first case in which a metastable singlet state and itsslow thermal conversion to the triplet ground state have beenobserved spectroscopically.Acknowledgment. This work is part of Project 2028-047212.96 of the Swiss National Science Foundation and GrantCHE-9301025 of the U.S. National Science Foundation. Weare very indebted to Prof. Matthew Platz (Ohio State University)for raising the problem of the enigmatic photoproduct of 3 NCCand for his continued interest in the resolution of the problem.We thank him as well as Prof. John Toscano (Johns HopkinsUniversity) for the open exchange of data and manuscripts whichmade it possible to present our results jointly. Finally, wegratefully acknowledge the help of Dr. Stephan Matzinger(University of Fribourg) in locating the oxirene transition statefor the NCC-NMC interconversion and for the IRC calculationson the phenyl derivative.Supporting Information Available: Tables containing theB3LYP/6-31G* optimized Cartesian coordinates, total energies,and thermal corrections of all stationary points (minimums andtransition states) discussed in this study, in ASCII format. Thismaterial is available free of charge via the Internet athttp://pubs.acs.org.JA983277W