Topical questions in the photophysics of J aggregates

The photo-physics of J aggregates exhibits a great deal of intriguing optical properties due to the collective optical emission that takes place from a large number of coherently coupled molecules such in linear aggregates. In this work we consider two main phenomena that contribute to define the coherence properties at low temperature: the exciton self trapping and the orientational disorder. The interaction of a one dimensional Frenkel exciton with phonons gives rise to the formation of self-trapped states without the need of overcoming an energy barrier. We develop a numerical scheme to take into account the presence of self trapped excitons in a linear aggregate of finite length and their interaction with an acoustic phonon thermal bath in order to study exciton relaxation on J aggregates after an impulsive optical excitation. We are able to calculate the population dynamics and the radiative life time. We show that the existence of self trapped states is to be taken into account for a correct description of the photophysics of J aggregates. In particular, for the case of BIC aggregates, a quantitative agreement with the experimental data on the temperature dependence of the radiative life time is obtained even considering a simple model of thermalized self trapped excitons. The estimated self trapping energy is in good agreement with the observed Stokes shift. In the second part of the work we study the Frenkel exciton dynamics in a one-dimensional molecular aggregate with static disorder in both the on-site energies and transition dipole moments. Using a tensorial generalisation of the coherent-potential approximation, we calculate the linear absorption spectra, the exciton density of states, and the coherence length responsible for the linear optics. In particular, we consider the purely orientational disorder in the transition dipoles, and show that our theory agrees well with the numerical simulations. In addition to features shared by other disordered aggregate models, we show that the strength of orientational disorder affects the anisotropy degree of the optical response with the main components of the optical susceptibility tensor being characterised by non equal coherence lengths. (C) 2004 WILEY VCH Verlag GmbH & Co. KGaA, Weinheim.