Photochemical Generation of 2, 3-Naphthoquinodimethane Derivatives. An extremely Facile Forbidden Thermal Reaction and Evidence for a Low-Energy Doubly Excited State

The 2, 3-naphthoquinodimethanes 5 and 6 are generated by irradiation of the condensed cyclobutenes 7 and 8 in rigid organic glasses at 77 K. Upon warming to about 120 K, 6 rapidly undergoes a “forbidden” electrocyclic ring closure to 8. If viscosity is low, bimolecular processes contribute to the destruction of 6. In contrast, 5 is stable in fluid solution up to about 200 K and in a plastic briefly up to room temperature, and its destruction is purely intermolecular, with no detectable return to 7. UV and PES measurements on 7 and 8 reveal the presence of substantial interactions between the n+ lone-pair orbital and the a orbital of the cyclobutene bond formed in the reaction 6 → 8, and indicate that the striking ease of the “forbidden” ring closure in 6 compared with 5 may be due to similar σ hyperconjugation in the transition state (n_-σ interaction). Another possible contribution is due to the inductive effect of the nitrogens, which makes the “antiaromatic” transition state of the forbidden process look somewhat like a cyclobutadiene dication rather than like unperturbed cyclobutadiene. The UV-visible absorption, linear dichroic, and polarized fluorescence and excitation spectra of 5 and 6 are in excellent agreement with π-electron calculations using singly and doubly excited configurations. Comparison of spectral shapes of the visible bands of 5 and 6 and of their measured radiative lifetimes suggests that an additional very weak low-energy transition is present in 5. The calculations strongly suggest that this transition largely involves a HOMO, HOMO → LUMO.LUMO double excitation. © 1979, American Chemical Society. All rights reserved.