INSTABILITY OF SELF-TRAPPED FRENKEL EXCITON-STATES IN ONE-DIMENSIONALMICROCRYSTALLITES

We present a theoretical investigation of the instability of self-trap ped Frenkel excitons in one-dimensional (1D) microcrystallites compose d of molecules for which the intermolecular interaction can be suitabl y described by the Lenard-Jones potential. Using the tight-binding app roach, we have found that with decreasing microcrystallite size the se lf-trapped exciton (STE) becomes less dominant with respect to the fre e exciton due to the decrease in the self-trap depth. For the microcry stallite size below a certain value, the STE state practically disappe ars due to the widening of the trapping range over the whole microcrys tallite. The characteristic feature of the 1D system is that, for the range of values used for the material parameters, the STE state has a minimum energy lower than the free exciton band bottom, regardless of the trapping range. Moreover the self-trapping barrier does not exist between the free exciton band and the STE level. From a similar calcul ation we have also found that these situations differ from those of tw o-dimensional (2D) and three-dimensional (3D) systems, and the trappin g range in the 2D and 3D systems is always narrower than that in the 1 D system. By comparison with experimental results, it is suggested tha t the STEs in both bulk aromatic crystals and microcrystallites can be described better by a 1D model. (C) 1998 Elsevier Science B.V. All ri ghts reserved.