Fluorescence and ultraviolet absorption spectra and structure of coumaran and its ring-puckering potential energy function in the S-1(π,π*) excited state

Abstract

The fluorescence excitation (jet cooled), single vibrational level fluorescence, and the ultraviolet absorption spectra of coumaran associated with its S-1(pi,pi(*)) electronic excited state have been recorded and analyzed. The assignment of more than 70 transitions has allowed a detailed energy map of both the S-0 and S-1 states of the ring-puckering (nu(45)) vibration to be determined in the excited states of nine other vibrations, including the ring-flapping (nu(43)) and ring-twisting (nu(44)) vibrations. Despite some interaction with nu(43) and nu(44), a one-dimensional potential energy function for the ring puckering very nicely predicts the experimentally determined energy level spacings. In the S-1(pi,pi(*)) state coumaran is quasiplanar with a barrier to planarity of 34 cm(-1) and with energy minima at puckering angles of +/- 14 degrees. The corresponding ground state (S-0) values are 154 cm(-1) and +/- 25 degrees. As is the case with the related molecules indan, phthalan, and 1,3-benzodioxole, the angle strain in the five-membered ring increases upon the pi ->pi(*) transition within the benzene ring and this increases the rigidity of the attached ring. Theoretical calculations predict the expected increases of the carbon-carbon bond lengths of the benzene ring in S-1, and they predict a barrier of 21 cm(-1) for this state. The bond length increases at the bridgehead carbon-carbon bond upon electron excitation to the S-1(pi,pi(*)) state give rise to angle changes which result in greater angle strain and a nearly planar molecule. (c) 2006 American Institute of Physics.