High pressure studies of the acridine/fluorene photoreaction: Vibration assisted tunneling

Abstract

We report a multifaceted investigation of the hydrogen transfer photoreaction in acridine-doped fluorene crystals at higher temperature. The purpose is to elucidate the role of vibrationally assisted tunneling in this reaction system. Raman experiments were conducted at various pressures and 77 K to document the change of vibrational frequency for the promoting mode(s). Upon compression, a line with a large pressure coefficient emerges from under the strong phonon mode at 96.5 cm−1. Through polarization studies under pressure, we have identified it as a molecular butterfly mode of B1 symmetry. We have measured the reaction rate at 150 K in order to examine the effect of a suggested promoting mode at ∼440 cm−1. The reaction rate again increases exponentially with pressure, but with a significantly higher pressure coefficient than that at 1.4 and 77 K. Mode patterns based on a recently published [J. Phys. Chem. 98, 12 223 (1994)] normal coordinate analysis of fluorene are used to help establish the promoting modes for this reaction. This consideration suggests that the 95 and 238 cm−1 modes are likely promoting modes in addition to the 125 cm−1 libration. A computation of the Franck–Condon factor for the H-transfer process indicates that a small population of a high overtone of a promoting mode may make a disproportionally large contribution to the reaction rate. This calculation fails to account for the greater pressure coefficient of the reaction rate at higher temperature. Instead, such an increase may come partly from a greater compressibility at higher temperature.