The spectroscopic properties and dynamic behavior of peridinin in several different solvents were studied by steady-state absorption, fluorescence, and transient optical spectroscopy. The lifetime of the lowest excited singlet state of peridinin is found to be strongly dependent on solvent polarity and ranges from 7 ps in the strongly polar solvent trifluoroethanol to 172 ps in the nonpolar solvents cyclohexane and benzene. The lifetimes show no obvious correlation with solvent polarizability, and hydrogen bonding of the solvent molecules to peridinin is not an important factor in determining the dynamic behavior of the lowest excited singlet state. The wavelengths of emission maxima, the quantum yields of fluorescence, and the transient absorption spectra are also affected by the solvent environment. A model consistent with the data and supported by preliminary semiempirical calculations invokes the presence of a charge transfer state in the excited state manifold of peridinin to account for the observations. The charge transfer state most probably results from the presence of the lactone ring in the π-electron conjugation of peridinin analogous to previous findings on aminocoumarins and related compounds. The behavior of peridinin reported here is highly unusual for carotenoids, which generally show little dependence of the spectral properties and lifetimes of the lowest excited singlet state on the solvent environment.
The vapor phase absorption spectra of cyanogen, cyanoacetylene, dicyanoacetylene, and dicyanodiacetylene have been recorded between 2000 and 1050 Å. Substantial perturbations in the intensity and positions of particular bands upon the addition of helium at 1500 psi pressure served to identify several Rydberg series. These series are assigned on the basis of their quantum defects and symmetry considerations. CNDO/2-CI calculations were used to interpret the valence shell transitions. A very intense intravalence shell band, assigned as a Σu+1 ← 1Σg+ system arising from the lowest π→π* configuration, dominates the spectra of all the molecules. A moderately intense intravalence shell band, assigned as a Πu1 ← 1Σg+ transition arising from the lowest n→π* configuration further characterizes the spectra.
Spectroscopic properties for a series of 2,5-diarylidene-cyclopentanones are reported. Electronic absorption and fluorescence spectra have been measured for the all-E configurations of 2,5-dibenzylidene-cyclopentanone (1), 2,5-bis-(3-phenyl-allylidene)-cyclopentanone (2), and 2,5-bis-(5-phenyl-penta-2,4-dienylidene)-cyclopentanone (3). The absorption spectra have been assigned with the aid of INDO/S calculations. Molecular structures used for the INDO/S calculations were computed with the PM3 Hamiltonian. Agreement between absorption spectra obtained in cyclohexane at room temperature and the theoretical predictions is good. For 1, 2, and 3 the general features of the spectra are similar. The transition to S 1 (weak) is assigned as n f π* (A 2 r A 1 ), to S 2 (strong) as π f π* (B 2 r A 1 ), and to S 3 (moderate) as π f π* (A 1 r A 1 ). The energy gap between S 1 and S 2 is seen to decrease as the length of the polyene chain increases in going from 1 to 3. Fluorescence is not observed for 1 in any of the solvents studied (protic and aprotic). Fluorescence is observed for 2 in protic solvents only. For 3, fluorescence is observed in a number of protic and aprotic solvents. Solvents which are able to induce fluorescence are believed to do so by inverting the order of 1 (nπ*) and 1 (ππ*) states. The influence of hydrogen bonding on the excitation spectra of 2 and 3 is discussed. Solvent-induced shifts in the absorption and fluorescence spectra of 3 in combination with the PM3 calculated ground-state dipole moment (2.8 D) are used to determine the excited-state dipole moment of 3 (6.4 D/protic solvents; 6.6 D/aprotic solvents). Fluorescence quantum yields in different solvents for 3 vary as the fluorescence maxima shift in these solvents, going through a maximum in the mid-frequency range. The variation in quantum yields with different solvents is primarily attributed to changes in the nonradiative rate of decay from S 1 . Excitation, polarized excitation, and fluorescence spectra have been measured for 2 and 3 at 77 K in ethanol/ methanol glass. Vibronic features not observed in the broad spectra obtained in alcohols at room temperature become clearly resolved at 77 K. Evidence is provided that indicates that 2 and 3 undergo excited-state proton-transfer reactions in acetic acid at room temperature.
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