Simulation of energy transfer in disordered two‐dimensional mixed J‐aggregates

Abstract

A theoretical model, based on numerical simulations, is provided to analyze the energy transfer in a twodimensional mixed J-aggregate. The J-aggregate is modeled as a square arrangement of parallel donor molecules in which a given concentration of acceptors is randomly substituted. The Frenkel exciton states are calculated by numerical diagonalization of the aggregate Hamiltonian including the static disorder. The scattering rates are calculated using a model of exciton-phonon coupling. The concentration dependences of the absorption and fluorescence line shapes, the localization behavior of the exciton states and the time-resolved fluorescence decays (configurationally averaged) are obtained. The simulations are checked against recent experimental results on mixed J-aggregates of cyanine dyes. . Relaxation dynamics studies in two-dimensional structures were reported on THIATS aggregates [4][5][6]. A computational study of energy transfer and trapping in two-dimensional J-aggregates was performed in order to simulate the luminescent properties of such aggregates [7]. Recently, the luminescence properties, at room temperature, of mixed J-aggregates of two kinds of cyanine dyes were examined [8][9][10]. In general, these aggregates of finite size are characterized by relatively short stacking distances between the chromophores, leading to strong intermolecular interactions because of the long range dipole-dipole couplings. Thus, the cooperative effects are dominant (even at room temperature) and are generally discussed in terms of collective excitation states.In this paper, a theoretical model, based on numerical simulations, is provided to analyze the energy transfer in a two-dimensional mixed J-aggregate featuring the oxacyanine (S9)-thiacyanine (S11) mixed system [10]. The two-dimensional J-aggregate system is modeled as a square arrangement of parallel dipoles (donors) in which a given concentration of molecules, with lower energies (acceptors), is randomly substituted. The intermolecular interactions are calculated using the extended dipole method. First, the general formalism of the calculations is presented to describe and analyze the photophysical properties of the pure J-aggregate. The Frenkel exciton states are calculated by numerical diagonalization of the aggregate Hamiltonian. The static disorder is taken into account by averaging these properties over many configurations. For the dynamic disorder, a model of exciton-phonon coupling is used, assuming a Bose Einstein distribution for the phonon occupation density, to describe the energy transfer (intraband scattering) among the exciton states. For each disordered configuration, the scattering rates are calculated and used in a master equation to obtain the time evolution of the excitonic population after initial excita-

Homogeneously mixed porphyrin J-aggregates with rod-shaped nanostructuresvia zwitterionic self-assembly

“…[7][8][9][10] Co-assembly of analogous J-aggregating dyes could form homogeneously mixed J-aggregates with excitonic properties distinguished from those of individual ones. [11][12][13][14][15][16][17][18][19][20][21][22][23] Such a binary excitonic system of molecular assemblies has been of fundamental interest [11][12][13][14][15][16][17][18][19][20] and is promising for spectral sensitizers 21 and application to photovoltaics. 22,23 However, most studies of mixed J-aggregates remain to be on cyanine dyes.…”