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Polymer fluorescence: even-parity states, alternation, and heteroatoms

Polymer fluorescence indicates a lowest singlet excitation S1 of odd parity, while an even-parity S1 favors radiationless decay. The optical gap Eg defines the lowest odd-parity singlet, 11Bu, and the two-photon gap Ea fixes the 21Ag state. The 1B/2A ordering of conjugated polymers reflects different single-particle gaps, 4t(delta) for alternating chains, at nearly constant correlations. Topological and heteroatom contributions to the effective alternation of poly-p- phenylenevinylenes and polythiophenes are discussed, polymer spectra is analyzed in terms of molecular (pi) -electron theory. Even-parity states at Ea and biexcitons derived from two- electron excitation across Eg indicate intermediate correlations and alternations, thereby limiting the accuracy of band, strong-correlation, and large-(delta) descriptions. Correlated states in the dimer limit illustrate the 2A/1B crossover of Hubbard or excitonic systems.