Volume 2 12, number 6 CHEMICAL PHYSICS LETTERS 24 September 1993 A Monte Carlo simulation of spectral diffusion in the presence of energetic and substitutional disorder AK Dutta, T.N. Misra and G.B. Talapatra Department of Spectroscopy, Indian Assocration for the Culfivationof Science, Jadavpur, Calcutta 700 032. India Received 27 April 1993; in final form 22 July 1993 The isotropic exchange mediated spectral diffusion process in one- and two-dimensional systems with energetic as well as substitutional disorder has been studied by employing a Monte Carlo simulation technique. The dynamics and the mechanism of the spectral diffusion process have been investigated by monitoring the flow of energy of an excitation started at specified sites with a Gaussian distribution of the energy profile. The importance of acceptor concenlration and the excitation transfer topology for the switching of spectral diffusion has been observed. The concentration-dependent excitation transport efliciency in two dimensions is found to be consistent with the percolation model. 1. Introduction Over the past few years, the dynamics of incoher- ent electronic excitation transport in disordered sys- tems has been a subject of considerable attention [ I- 91. Recently, the process of excitation transfer, also called spectral diffusion, has been investigated for inhomogeneously broadened singlet-triplet transi- tions in orientationally disordered organic solids [ lo-21 1. Spectral diffusion is the process of energy transfer between like species which have different transition energies, i.e. it describes a situation in which an optical excitation initially situated at a spe- cific energy alters its position in energy space in the course of time. Disorder resulting from the fluctua- tion of site energies is called inhomogeneous broad- ening, also commonly referred to as diagonal dis- order. The fluctuation of site energies arises due to variation of local environments. Recently, some work has been reported on spectral diffusion in low-con- centration systems, particularly in glasses and liq- uids [22]. In such systems the dynamic spectral broadening is supposed to arise due to the fluctua- tion of intermolecular interaction. In this Letter we focus on a model for energy migration and trapping (spectral diffusion) in a class of systems where both energetic and substitutional disorder are present. Recently, some work on such systems has been re- ported [ 18-2 11. Some simulation work in connec- tion with spectral diffusion and photo-conductivity on systems possessing only energetic disorder has also been reported 123-251. This Letters deals with a Monte Carlo simulation of a model of spectral diffusion in a binary solid so- lution of a host (H) and a guest (G), where guest molecules have an inhomogeneously broadened dis- tribution of energies due to some kind of disorder. The objective of this work is to develop a simu- lation algorithm for studying the dynamics and the efficiencies of spectral diffusion on acceptor concen- tration, dimensionality of transport and site-selec- tive excitation. In our system, as noted earlier, the guest molecules have a spread in site energy due to random energetic and substitutional disorder. Also, the spatial posi- tions of guest molecules are randomly distributed in the host matrix. Thus, we are dealing with disorder induced excitation transport in a random medium due to fluctuations of site energies. The simulation algorithm, based on the incoherent excitation trans- fer, corresponds to a random walk on the random lattice of distributed excitation energies. 0009-2614/93/$ 06.00 0 1993 Elsevier Science Publishers B.V. All rights reserved. 637