title: Fluorescence and ultraviolet absorption spectra and structure of coumaran and its ring-puckering potential energy function in the S1(pi,pi*) excited state.
abstract: The fluorescence excitation (jet cooled), single vibrational level fluorescence, and the ultraviolet absorption spectra of coumaran associated with its S1(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 S0 and S1 states of the ring-puckering (nu45) vibration to be determined in the excited states of nine other vibrations, including the ring-flapping (nu43) and ring-twisting (nu44) vibrations. Despite some interaction with nu43 and nu44, a one-dimensional potential energy function for the ring puckering very nicely predicts the experimentally determined energy level spacings. In the S1(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 (S0) 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 S1, 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 S1(pi,pi*) state give rise to angle changes which result in greater angle strain and a nearly planar molecule.
authors.affiliation: Department of Chemistry, Texas A&M University, College Station, TX 77843-3255, USA.
authors.forename: Juan
authors.initials: J
authors.lastname: Yang
authors.forename: Martin
authors.initials: M
authors.lastname: Wagner
authors.forename: Katsuhiko
authors.initials: K
authors.lastname: Okuyama
authors.forename: Kevin
authors.initials: K
authors.lastname: Morris
authors.forename: Zane
authors.initials: Z
authors.lastname: Arp
authors.forename: Jaebum
authors.initials: J
authors.lastname: Choo
authors.forename: Niklas
authors.initials: N
authors.lastname: Meinander
authors.forename: Ohyun
authors.initials: O
authors.lastname: Kwon
authors.forename: Jaan
authors.initials: J
authors.lastname: Laane