Physical, Theoretical & Computational

Structure and Spectroscopy of Linear Polyenes: Finite and Infinite (#456)

Linearly conjugated polyenes form a unique class of chromophoric species. Interest in these molecules spans theoretical chemistry, spectroscopy, and photophysics and finds applications in photobiology, biotechnology and materials science. The infinite chain polyene polyacetylene continues to attract experimental and theoretical attention as the prototypical conducting polymer. In contrast to aromatic species, linear polyenes exhibit bond alternation, multiple conformations, and Z,E isomers. The effects of electron correlation on pi electronic state structures and energies do not conform to simple molecular orbital descriptions. Most notably, the lowest excited singlet state of linear polyenes is not the strongly allowed HOMO to LUMO excitation but a state that has dominant contributions from excitations involving two electrons. This low-energy, dark state has the same symmetry as the ground state, which has significant consequences for the photophysics and photochemistries of these systems. The original experimental studies of Kohler and co-workers on model polyenes have been extended to carotenoids and infinite polyenes. The early semi-empirical theoretical treatments of Karplus and co-workers have been expanded to ever improving ab initio methods, though significant challenges remain in obtaining accurate theoretical descriptions of the excited states of both short and long polyenes. The current state of polyene research will be discussed in three sessions involving: (1) polyene spectroscopy, (2) theory of finite and infinite polyenes. (3) polyene-based photovoltaics and the structures and dynamics of the excited states of carotenoids in photosynthesis and vision.
Last update: Dec 28, 2015