Ultrafast dynamics and excited state spectra of open-chain carotenoids at room and low temperatures.

Many of the spectroscopic features and photophysical properties of carotenoids are explained using a three-state model in which the strong visible absorption of the molecules is associated with an S0 (1(1)Ag-) --> S2 (1(1)Bu+) transition, and the lowest lying singlet state, S1 (2(1)Ag-), is a state into which absorption from the ground state is forbidden by symmetry. However, semiempirical and ab initio quantum calculations have suggested additional excited singlet states may lie either between or in the vicinity of S1 (2(1)Ag-) and S2 (1(1)Bu+), and some ultrafast spectroscopic studies have reported evidence for these states. One such state, denoted S*, has been implicated as an intermediate in the depopulation of S2 (1(1)Bu+) and as a pathway for the formation of carotenoid triplet states in light-harvesting complexes. In this work, we present the results of an ultrafast, time-resolved spectroscopic investigation of a series of open-chain carotenoids derived from photosynthetic bacteria and systematically increasing in their number of pi-electron carbon-carbon double bonds (n). The molecules are neurosporene (n = 9), spheroidene (n = 10), rhodopin glucoside (n = 11), rhodovibrin (n = 12), and spirilloxanthin (n = 13). The molecules were studied in acetone and CS2 solvents at room temperature. These experiments explore the effect of solvent polarity and polarizability on the spectroscopic and kinetic behavior of the molecules. The molecules were also studied in ether/isopentane/ethanol (EPA) glasses at 77 K, in which the spectral resolution is greatly enhanced. Analysis of the data using global fitting techniques has revealed the ultrafast dynamics of the excited states and spectral changes associated with their decay, including spectroscopic features not previously reported. The data are consistent with S* being identified with a twisted conformational structure, the yield of which is increased in molecules having longer pi-electron conjugations. In particular, for the longest molecule in the series, spirilloxanthin, the experiments and a detailed quantum computational analysis reveal the presence of two S* states associated with relaxed S1 (2(1)Ag-) conformations involving nearly planar 6-s-cis and 6-s-trans geometries. We propose that in polar solvents, the ground state of spirilloxanthin takes on a corkscrew conformation that generates a net solute dipole moment while decreasing the cavity formation energy. Upon excitation and relaxation into the S1 (2(1)Ag-) state, the polyene unravels and flattens into a more planar geometry with comparable populations of 6-s-trans and 6-s-cis conformations.

Femtosecond time-resolved transient absorption spectroscopy of xanthophylls.

Xanthophylls are a major class of photosynthetic pigments that participate in an adaptation mechanism by which higher plants protect themselves from high light stress. In the present work, an ultrafast time-resolved spectroscopic investigation of all the major xanthophyll pigments from spinach has been performed. The molecules are zeaxanthin, lutein, violaxanthin, and neoxanthin. beta-Carotene was also studied. The experimental data reveal the inherent spectral properties and ultrafast dynamics including the S(1) state lifetimes of each of the pigments. In conjunction with quantum mechanical computations the results address the molecular features of xanthophylls that control the formation and decay of the S* state in solution. The findings provide compelling evidence that S* is an excited state with a conformational geometry twisted relative to the ground state. The data indicate that S* is formed via a branched pathway from higher excited singlet states and that its yield depends critically on the presence of beta-ionylidene rings in the polyene system of pi-electron conjugated double bonds. The data are expected to be beneficial to researchers employing ultrafast time-resolved spectroscopic methods to investigate the mechanisms of both energy transfer and nonphotochemical quenching in higher plant preparations.

Origin of the bathochromically shifted optical spectra of meso-tetrathien-2'- and 3'-ylporphyrins as compared to meso-tetraphenylporphyrin.

The UV-Vis and fluorescence spectra of free base and diprotonated meso-tetrathien-2'-ylporphyrins are, when compared to the spectra of meso-tetra-phenyl- or even -thien-3'-ylporphyrins, characterized by surprisingly large red-shifts. A comparison of the optical spectra and the computed rotational barriers for these meso-aryl-substituted porphyrins and a detailed conformational analysis of the single crystal X-ray structure of a diprotonated meso-tetrathien-2'-ylporphyrin suggest that the origin of the altered electronic properties of meso-tetrathien-2'-ylporphyrins are mainly due to the contribution of conformations in which the thienyl groups adopt idealized co-planar arrangements with the porphyrin ring. These conformations allow an efficient extension of the porphyrinic pi-system through conjugation. We synthesized a meso-tetrathien-2'-ylporphyrin with methyl groups in the o-position, thus preventing the formation of conformers with co-planar thienyl groups and a corresponding thien-2'-ylporphyrin with methyl substituents in a distal position that possesses the same steric requirements for thienyl group rotation as the parent compound, to conclusively deduce the influence of the conformers on the electronic structure. A MNDO-PSDCI computation of their optical spectra further supports our key hypothesis. DFT computations of the total energies of the hypothetical diprotonated thien-2'-ylporphyrin conformer with perpendicular thienyl groups and the conformer containing near-co-planar thienyl groups quantify the resonance stabilization energy. Our results support and complement recent photophysical and theoretical studies by Gupta and Ravikanth and Friedlein et al. on thien-2'-yl-substituted core-modified porphyrins and [meso-tetra(5'-bromothien-2'-yl)porphyrinato]Zn(ii), respectively.

Stereoisomers of carotenoids: spectroscopic properties of locked and unlocked cis-isomers of spheroidene.

A systematic optical spectroscopic and computational investigation of a series of locked-cis-isomers of spheroidene has been carried out with the goal being to better understand the relationships between stereochemistry, photochemistry, photophysics and biological function of geometric isomers of carotenoids. The spectroscopic properties of 15,15'-locked-cis-spheroidene, 13,14-locked-cis-spheroidene, 11, 12-locked-cis-spheroidene in solution are compared with those observed for unlocked spheroidene. The locked-cis bonds are incapable of undergoing cis-to-trans isomerization and therefore provide an effective means of exploring the relationship between specific stereoisomers and molecular spectroscopy. Samples of the molecules were purified using a high performance liquid chromatography (HPLC) apparatus equipped with a diode array detector, which records the absorption spectra immediately as the molecules emerge from the column and prior to any isomerization that might occur. For several stable isomers, resonance Raman (rR) spectroscopy was carried out to assign their configurations. Quantum computations of absorption spectra were performed using ZINDO/S and also MNDO-PSDCI methods employing nearly full single and double configuration interaction within the pi-electron manifold. Also, for a few test cases, ground state minimizations were done using density functional methods (B3LYP/6-31G(d)). The MNDO-PSDCI methods coupled with the density functional ground state minimization provide an accurate assignment of the positions of the 2(1)Ag - , 1(1)Bu +, and 1(1)Ag + excited states and also address the nature of the forbidden 1(1)Bu - state, whose location is uncertain for polyenes and carotenoids. We demonstrate that the configurational description of the 1(1)Bu - state is sufficiently unique to preclude assignment of its energy based on the characterization of surrounding excited singlet states. The experimental and computational data also offer important insights into the photochemical and photophysical properties of stereoisomers of carotenoids.