| Abstract
| - A complete spectroscopic and photophysical study of three alternating naphthalene-α-thiophene copolymerswas undertaken in solution (room and low temperature) and in the solid state (thin films in a Zeonex matrix).The study comprises absorption, emission, and triplet−triplet spectra together with quantitative measurementsof quantum yield (fluorescence, intersystem-crossing, internal conversion, and singlet oxygen formation)lifetimes and singlet and triplet energies. The overall data allow the determination of the rate constants for allthe decay processes. Comparison between the behavior of analogous 1-naphthyl(oligo)thiophenes and the2,6-naphthalene(oligo)thiophene copolymers allows several important observations. First, the polymers displayhigher fluorescence quantum yields and lower S1∼∼→T1 intersystem-crossing yields than the oligomers.This can be attributed to the presence of the 1,5-dioctyloxynaphthalene groups in the copolymers leading toa more rigid polymer backbone, which decreases radiationless deactivation and increases the radiative efficiency.Second, the singlet and triplet energies are significantly lower in the polymers than with the correspondingoligomers. This implies a lower HOMO−LUMO energy difference in the polymers due to an extendedπ-delocalization. Third, the singlet-to-triplet (S1−T1) energy splitting is higher in the oligomers than with thepolymers, even though the former display higher intersystem-crossing yields. It is suggested that this mayresult from intersystem-crossing in the oligomers involving significant charge-transfer (CT) character (spin-orbit coupling is mediated by CT mixing involving the singlet and triplet states in matrix elements of thetype 〈ΨCT |H‘|3Ψ1〉) of the relevant excited states but that is less important with the polymers. We believethat this may be relevant to understanding the nature of CT states in conjugated copolymers.
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