Distinctive features of exciton self-trapping in quasi-one-dimensional molecular chains (J-aggregates)

Spectroscopic study of excitons in molecular chains (S120 J-aggregates), fo rmed in a solid matrix, disclosed well-pronounced features which cannot be described by the conventional theory of exciton dynamics in a strictly one- dimensional system: the existence of an energy barrier for self-trapping an d the dependence of the spectral polaronic shift on the chain fragment leng th L available for exciton motion. It is shown that these unusual features of exciton self-trapping are inherent in a quasi-one-dimensional system whe re one-dimensional electronic motion is combined with three-dimensional lat tice deformation. Polaronic phenomena in such systems are of a complicated threshold character: some critical length Lo limits from below the region o f the existence of free excitons in stable or metastable states. The charac ter of self-trapping with decreasing L is dictated by another critical para meter: depending on its value, an exciton goes to a polaronic state either with a usual jump of spectral and spatial characteristics or in a continuou s way. These properties of the quasi-one-dimensional self-trapping naturall y explain the spectroscopic picture observed. (C) 2000 Elsevier Science B.V . All rights reserved.