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Ones and twos: The two-photon absorption cross sections (σ(2)) of singlet diradical hydrocarbons with phenalenyl peripheral groups (see scheme) are larger than those of closed-shell aromatic hydrocarbons by an order of magnitude or more and are comparable to those of π-conjugated compounds with strong electron donor or acceptor substituents. A significant increase in cross section was also observed in the resonance-enhanced region.
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Kamada et al. (2007) studied this question.
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