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The photophysics of 9,9‘-bianthryl (BA) were investigated by means of fluorescence spectroscopy, nanosecond transient-absorption spectroscopy, and semiempirical calculations. Fluorescence spectra and lifetimes were measured in more than 50 solvents in order to get a detailed picture of the solvent dependence. The results show that the fluorescence lifetime is constant in solvents of low polarity (D < 5) and increases with solvent polarity in more polar solvents. Departures from this trend can be traced to specific solute−solvent interactions. Excited-state singlet−singlet absorption spectra were measured in the ultraviolet range and show a marked solvent dependence. In polar solvents, the spectrum (λmax = 315 nm) is closely related to those of the radical ions of both BA and anthracene. The decay rate constant of this band is identical with that of the fluorescence emission in a range of solvents of varying polarity (D > 5), thus providing direct proof of the charge-separated character of the fluorescent state in polar solvents. The 315 nm band is absent in isooctane, indicating that the fluorescent state is not of charge-separated character in this case. Semiempirical calculations were carried out in order to rationalize the experimental data. Careful consideration of the symmetry character of the electronic states involved and of the solvent effect on these states indicates that two distinct transitions are responsible for the observed fluorescence emission; in nonpolar solvents, a nonpolar state with D 2 symmetry and a torsion angle that is markedly smaller than 90° is the fluorescent state, whereas in polar solvents fluorescence originates from a charge-separated perpendicular state of D 2 d symmetry. This latter state is responsible for the large solvent effects on fluorescence and singlet−singlet absorption. Triplet−triplet absorption and intersystem-crossing efficiency data were also measured in several solvents. They too are solvent-dependent but do not show characteristics of charge separation; they rather are influenced by specific solute−solvent interactions.

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Abstract. The dependence of primary photophysical and photochemical processes, especially of electron photoejection, in phenol and related compounds in aqueous solution on excitation intensity and excitation energy is examined. Theoretical and experimental evidence is presented for the possibility of three pathways for electron ejection: (1) A monophotonic pathway via the fluorescent state, which most probably does not involve the lowest triplet state; (2) a monophotonic pathway requiring higher excitation energies, which takes place in competition with internal conversion to the fluorescent state; and (3) a consecutive biphotonic pathway in which the lowest triplet state absorbs the second photon, and which can become predominant at high intensities, e.g. under flash conditions. It is shown that this model reconciles apparently conflicting results published in the literature.

The effect of addition of the cyclodextrins (CDx) α-CDx and β-CDx on the photophysics and photochemistry of the dimethoxybenzenes (DMB) 1,4-DMB and 1,2-DMB in aqueous solution has been investigated by means of absorption, circular dichroism, fluorescence, and nanosecond transient absorption spectroscopies. The experimental results are discussed in the light of model calculations on the structure and the circular dichroism spectra of the DMB−CDx complexes. The association of 1,2-DMB with both CDxs is much weaker than that of 1,4-DMB owing to steric hindrance. With β-CDx, 1,4-DMB forms 1:1 complexes with a high association constant (K = 630 M-1) in spite of incomplete inclusion. Complexation of 1,4-DMB with α-CDx strongly affects the photophysical properties of the guest:  fluorescence is enhanced and triplet state decay slowed by a factor of 10 or more, depending on the decay reaction mode. Fluorescence enhancement is mainly due to a reduction of the rate constant for internal conversion. The effect of complexation on triplet decay was exploited for a detailed study of the association mechanism in terms of the consecutive formation of 1:1 and 1:2 complexes. Temperature-dependent measurements showed that both association steps are controlled by enthalpy−entropy compensation, the first step being entropy-driven and the second one enthalpy-driven. At lower temperatures (T < 50 °C), 1:2 complex formation predominates. These findings are supported by the circular dichroism spectra and the model calculations. There is no significant effect of complexation with either α- or β-CDx on hydrated electron ejection from 1,4-DMB, but the eaq - decay is accelerated in the presence of β-CDx. This is tentatively attributed to spatial correlation between eaq - and radical cations induced by complexation.

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The photophysical properties of naphthalene were studied in aqueous solution (H2O and D2O), in some organic solvents (ethanol, hexane, and acetonitrile), and in complexes with the cyclodextrins (CDs) α-CD and β-CD, by means of absorption, steady-state and time-resolved fluorescence, circular dichroism, and triplet−triplet absorption spectroscopies. The structures of the CD inclusion complexes were computed using a dynamic Monte Carlo method. The main difference of the photophysics in the pure aqueous with respect to the organic media consists in a reduction of the fluorescence lifetime, τF, by a factor of about 2.5. Consideration of the triplet properties in aqueous and organic media led to the conclusion that this effect is most probably due to a corresponding increase of the intersystem crossing rate, induced by H2O or D2O. Inclusion of naphthalene in the CD hosts has the effect, at high CD concentration (>0.01 M), of increasing τF with respect to the aqueous medium, the value in α-CD being near to that in the organic media and the value in β-CD intermediate. The spectral and kinetic data are consistent with the predominant formation of 1:2 host:guest complexes with α-CD, and of 1:1 complexes with β-CD, although 2:2 complexes with β-CD are also formed and are identified by their excimer like fluorescence. Several experimental results, including the values of τF, and the temperature dependences of fluorescence and triplet−triplet absorption spectra on one hand and of triplet quenching and triplet−triplet annihilation kinetics on the other, point to a considerable structural flexibility of the 1:2 complex with α-CD.