Fluorescence and electronic absorption spectra of phthalan: Two-dimensional vibrational potential energy surface for the ring-puckering and flapping in the S-1(pi,pi(*)) state

The ring-puckering and ring-flapping vibrations of phthalan in its S-1(pi,p i*) electronic excited state have been studied using fluorescence excitatio n spectroscopy of jet-cooled molecules, dispersed fluorescence spectroscopy , and ultraviolet absorption spectroscopy. This electronic state has A(1) s ymmetry resulting from a B-2--> B-2 orbital transition. Thus type A absorpt ion bands result from A(1)--> A(1) and B-2--> B-2 transitions to the S-1 vi bronic levels. The ring-puckering levels for the S-1(pi,pi*) electronic sta te were determined for both the flapping ground (v(F)=0) and excited states (v(F)=1) and these were used to calculate both one- and two-dimensional po tential energy surfaces which fit the observed spectra. In the S-1(pi,pi*) state phthalan was found to be planar and more rigid than in the ground sta te in terms of the puckering coordinate. However, the molecule is less rigi d along the flapping coordinate. This study shows how several types of spec troscopy and computations must be used in conjunction with each other to at tain a comprehensive analysis of the electronic excited state. (C) 2000 Ame rican Institute of Physics. [S0021-9606(00)00515-8].