Conformational Analysis and Dynamics of 9-Propylfluorene and 9-Ethylfluorene

Abstract

The S1 excited state and cation ground state of jet-cooled 9-propylfluorene have been studied using resonance enhanced multiphoton ionization (REMPI) and zero electron kinetic energy (ZEKE) photoelectron spectroscopy. Three conformations of 9-propylfluorene have been experimentally identified. Quantum chemical calculations have been used to calculate the ground state and cation energies of each conformer as well as the barrier to conformer interconversion via a bond rotation. ZEKE spectroscopy has also been used to search for conformer interconversion upon vibronic excitation to S1, but no evidence for a dynamic barrier crossing has been found, even at the highest energy probed (3210 cm-1). Similar studies previously carried out on 9-ethylfluorene were also extended to higher energy, and no evidence of dynamic barrier crossing was found at energies as high as 3525 cm-1. Statistical density of state calculations have been carried out to predict the relative populations of each conformer as a function of excess energy, and the Rice−Ramsperger−Kassel−Marcus (RRKM) isomerization rates have also been calculated.