A Comprehensive Approach to Exciton Delocalization and Energy Transfer

Abstract

Electrostatic intermolecular interactions lie at the heart of both the Forster model for resonance energy transfer (RET) and the exciton model for energy delocalization. In the Forster theory of RET, the excitation energy incoherently flows from the energy donor to a weakly coupled energy acceptor. The exciton model describes instead the energy delocalization in aggregates of identical (or nearly so) molecules. Here, we introduce a model that brings together molecular aggregates and RET. We will consider a couple of molecules, each described in terms of two diabatic electronic states, coupled to an effective molecular vibration. Electrostatic intermolecular interactions drive energy fluxes between the molecules, that, depending on model parameters, can be described as RET or energy delocalization. At variance with the standard Forster model for RET and of the exciton model for aggregates, our approach applies both in the weak and in the strong coupling regimes and fully accounts for the quantum nature of molecular vibrations in a nonadiabatic approach. Coupling the system to a thermal bath, we follow RET and energy delocalization in real time and simulate time-resolved emission spectra.

Solid state solvation: a fresh view

“…53 Specifically, to account for the relaxation of the photoexcited dye we couple the vibrational coordinate of the dye to a thermal bath of harmonic oscillators. 54,55 As discussed in the ESI,† we adopt the Redfield approach and assume a constant spectral density for the bath degrees of freedom, so that a single parameter γ enters the definition of the molecular relaxation process, that we set to γ = 5 ps −1 . Finally, the relaxation of the solvent, that enters the model as a classical overdamped coordinate, is described by the Smoluchowsky equation.…”

Aggregates of Cyanine Dyes: When Molecular Vibrations and Electrostatic Screening Make the Difference

“…The physics of aggregates is driven by intermolecular electrostatic interactions. As discussed in recent literature, − ,,,− essential-state models lend themselves quite naturally to introduce intermolecular interactions. The diabatic basis set for the dimer is the direct product of electronic basis states false| N ⟩ , false| Z 1 ⟩ and | Z 2 false⟩ of the monomer (see Table S4).…”

“…The delicate issue is how to discriminate the two kinds of interactions. Indeed this is not possible by adopting the diabatic basis, since the third term of eq accounts for static and dynamical interactions at the same time. ,,, …”

“…Indeed this is not possible by adopting the diabatic basis, since the third term of eq 3 accounts for static and dynamical interactions at the same time. 42,43,53,56 Following an approach developed some years ago for aggregates of quadrupolar dyes, 43 a step-by-step procedure is adopted. First, a mean-field Hamiltonian is defined to accurately describe the ground-state properties of the dyes inside the aggregate.…”

A marvel of chiral squaraine aggregates: chiroptical spectra beyond the exciton model

“…ESMs for polar and multipolar dyes lend themselves quite naturally to address ES intermolecular interactions in molecular aggregates, releasing the main approximations of the exciton model. 32,48,50,53,54 Specifically, modeling SQ aggregates we release the dipolar approximation and describe each molecule in terms of a ground and two excited states. More interestingly, however, since intermolecular interactions are defined on the diabatic basis, the resulting model fully accounts for the molecular polarizability, allowing the charge distribution on each dye to readjust in response to the ES potential generated by the surrounding molecule.…”