Two-photon fluorescence excitation spectrum of a naphthalene norbornylog: Implications for electron transfer

Abstract

Polarized two-photon fluorescence excitation spectra of 1,4-dimethoxy-2,3-dimethyl naphthalene and of a 1,4-dimethoxy-2,3-norbornolog naphthalene, in 0.002 M cyclohexane solution, are reported in the range 665-545 nm at 293 K. Intensity predictions for the 1L b and 1L a states based upon alternate hydrocarbon pseudoparity rules are confirmed: the weak 1L b origin is seen as is a strong 1L b vibronic origin attributed to the naphthalene 4b 2u vibration, but the 1L a origin is not identified. However, consideration of the one-photon absorption spectrum then reveals that the 1L a origin is higher in energy than is the 1L b origin, confirming that the fast through-bridge photoinduced electron transfer seen in naphthalene—dicyanoethylene norbornologs is an electronically forbidden but vibronically allowed process. As far as we are aware, these are the first examples of such processes.