Corina Scriban

Energetics and dynamics of the low-lying electronic states of constrained polyenes: implications for infinite polyenes.

Steady-state and ultrafast transient absorption spectra were obtained for a series of conformationally constrained, isomerically pure polyenes with 5-23 conjugated double bonds (N). These data and fluorescence spectra of the shorter polyenes reveal the N dependence of the energies of six (1)B(u)(+) and two (1)A(g)(-) excited states. The (1)B(u)(+) states converge to a common infinite polyene limit of 15,900 ± 100 cm(-1). The two excited (1)A(g)(-) states, however, exhibit a large (~9000 cm(-1)) energy difference in the infinite polyene limit, in contrast to the common value previously predicted by theory. EOM-CCSD ab initio and MNDO-PSDCI semiempirical MO theories account for the experimental transition energies and intensities. The complex, multistep dynamics of the 1(1)B(u)(+) → 2(1)A(g)(-) → 1(1)A(g)(-) excited state decay pathways as a function of N are compared with kinetic data from several natural and synthetic carotenoids. Distinctive transient absorption signals in the visible region, previously identified with S* states in carotenoids, also are observed for the longer polyenes. Analysis of the lifetimes of the 2(1)A(g)(-) states, using the energy gap law for nonradiative decay, reveals remarkable similarities in the N dependence of the 2(1)A(g)(-) decay kinetics of the carotenoid and polyene systems. These findings are important for understanding the mechanisms by which carotenoids carry out their roles as light-harvesting molecules and photoprotective agents in biological systems.

Synthesis and Optical Spectroscopy of Oligo(1,6-heptadiynes) with a Single Basic Structure Prepared through Adamantylimido-Based Molybdenum Wittig and Metathesis Chemistry

Linear oligoenes of 1,6-heptadiynes (derived from dialkyl dipropargylmalonates) with a single basic structure and up to 23 conjugated double bonds were synthesized through Wittig-like reactions between bimetallic Mo-alkylidene compounds and aldehyde-capped oligoenes. The relatively rigid and isomerically pure oligoenes have structures with alternating cis,trans conjugated double bonds in which the cis double bond is part of a cyclopentene ring. Molecular weights have been confirmed through MALDI-MS measurements of samples purified by HPLC. Optical spectra of the purified samples show significant vibronic resolution, even in room temperature samples, and are remarkably similar to those of simple polyenes and carotenoids. Therefore, a systematic investigation of the dependence of the allowed electronic transition energies (electronic origins) on conjugation lengths has become possible. Studies of seven allowed transitions for molecules with 5-23 double bonds (= N) indicate asymptotic convergence (with approximately a 1/N dependence) to a common long polyene limit at approximately 16,000 cm(-1). The convergence of these electronic transitions agrees with theoretical treatments of polyene excited-state energies and is consistent with the absorption spectra of analogous diethyl dipropargylmalonate polymers (1/N approximately 0).