Excitons on a 1d Periodic Conjugated Polymer Chain: Two Electronic Structures of Polydiacetylene Chains

This is the authors' abstract. We don't add key points for this paper.

Crystalline polydiacetylenes (PDA) provide a model system for conjugated polymers. Monomer crystals where the polymer content is very low give access to the electronic structure of a 1D straight periodic isolated chain. PDA are known to exist as crystals in two electronic structures, the so-called 'blue' and 'red' phases. In 3BCMU crystals, blue and red chains coexist, so that the electronic properties of both types of chains in the same environment can be compared. In both cases fluorescence originates from the 1 B u exciton state. The blue exciton lifetime is 140 fs, its fluorescence quantum yield is small (10-4) and the relaxation is dominated by non radiative processes via low lying A g states as in polyenes. On the contrary, for red chains the exciton state is highly fluorescent (quantum yield 0.30), its lifetime is 52 ps at 15 K, implying a short radiative lifetime <100 ps. Recent results on the temperature dependence of lifetime and fluorescence yield imply the presence of states, thermally accessible from the exciton state, but non radiant to the ground state. This is consistent with a non planar conformation of the chain.