Fluorescence and electronic absorption spectra of phthalan: Two-dimensional vibrational potential energy surface for the ring-puckering and flapping in the S1(π,π*) state

Abstract

The ring-puckering and ring-flapping vibrations of phthalan in its S1(π,π*) electronic excited state have been studied using fluorescence excitation spectroscopy of jet-cooled molecules, dispersed fluorescence spectroscopy, and ultraviolet absorption spectroscopy. This electronic state has A1 symmetry resulting from a B2→B2 orbital transition. Thus type A absorption bands result from A1→A1 and B2→B2 transitions to the S1 vibronic levels. The ring-puckering levels for the S1(π,π*) electronic state were determined for both the flapping ground (vF=0) and excited states (vF=1) and these were used to calculate both one- and two-dimensional potential energy surfaces which fit the observed spectra. In the S1(π,π*) state phthalan was found to be planar and more rigid than in the ground state in terms of the puckering coordinate. However, the molecule is less rigid along the flapping coordinate. This study shows how several types of spectroscopy and computations must be used in conjunction with each other to attain a comprehensive analysis of the electronic excited state.