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Carotenoids (Cars) in bacterial photosynthesis are known as accessory light harvesters and photoprotectors. Recently, the singlet fission (SF) reaction initiated by Car photoabsorption has been recognized to be an effective excitation deactivation channel disfavoring the light harvesting function. Since the SF reaction and the triplet sensitization reaction underlying photoprotection both yield triplet excited state Cars (Car*), their contribution to the overall Car* photoproduction are difficult to disentangle. To tackle this problem, we resorted to the triplet excitation profiles (TEPs), i.e., the actinic spectra of the overallCar* photoproduction. The TEPs combined with the conventional fluorescence excitation spectra allowed us to extract the neat SF contribution, which can serve as a spectroscopic measure for the SF reactivity. This novel spectroscopic strategy was applied to analyze the light harvesting complexes (LHs) from Tch. tepidum and Rba. sphaeroides 2.4.1. The results unambiguously showed that the SF reaction of Cars proceeds with an intramolecular scheme, even in the case of LH1-RC from Rba. sphaeroides 2.4.1 likely binding a secondary pool of Cars. Regarding the SF-reactivity, the geometric distortion in the conjugated backbone of Cars was shown to be the structural determinant, while the length of the Car conjugation was suggested to be relevant to the effective localization of the geminate triplets to avoid being annihilated. The SF reaction scheme and structure-activity relationship revealed herein will be useful not only in deepening our understanding of the roles of Cars in photosynthesis, but also in enlightening the applications of Cars in artificial light conversion systems.

The hyperpolarizabilities of conjugated polymers are studied within a tight-binding model. Exact evaluation of y' '(3') is carried out numerically for polymer chains of varying lengths, cispolyacetylene and polythiophene. EA'ects of solitons on y' ' are considered. Among many interesting properties of conjugated polymers, the unusually large optical nonlinearity has received much attention recently. Aside from possible optoelectronic applications, the third-order susceptibility g' ' also provides unique information about the excited states. It is therefore useful to calculate g' ' from theoretical models and to compare it with experiment.Due to mathematical complications not many realistic calculations are available. Model calculations' involving electron-electron interactions have been carried out only for very short chains, whereas the remarkable chain-length dependence of y' ' persists for moderately long chains.Recently, Wu performed a Green's-