Multiconfigurational Excitonic Couplings in Homo- and Heterodimer Stacks of Azobenzene-Derived Dyes

Abstract

Molecular excitons play a major role within dye aggregates and hold significant potential for (opto)electronic and photovoltaic applications. Numerous studies have documented alterations in the spectral properties of dye homoaggregates, but only limited work has been reported for heteroaggregates. In this article, dimeric dye stacks were constructed from azobenzene-like dyes with identical or distinct structures, and their excitonic features were computationally investigated. Our results show that strong exciton coupling is not limited to identical chromophores, as often assumed, based on a recently made available Frenkel Exciton Hamiltonian and multiconfigurational plus second-order perturbation theory energetics methodology. Heteroaggregate stacks were found to exhibit different absorption features from the corresponding interacting monomers, indicating considerable coupling interactions between units. We analyzed how such coupling may vary according to various aspects, such as the relative positions of the interacting monomers or the differences in their energetics. Such qualitative and semiquantitative analyses allow the evaluation of the excitonic behavior of these dye aggregates to encourage further efforts toward a deeper understanding of the excitonic properties of tailored dye heteroaggregate systems.

Scheme 1

Scheme 2

Figure 1

Computed monomeric absorption spectra. Superimposed computed absorption spectra of monomeric azobenzene (A) and its derivatives (R, N, H, and M) considered in this work. The spectra are normalized to the absorption maximum of A.

Figure 2

Computed absorption spectra for π-stacked homodimers at d = 3.5 Å. The spectra of each corresponding monomer are reported for ease of comparison. All spectra are normalized to the absorption maximum of monomer A. Monomer spectra follow the same coloring scheme as in Figure 1, while homodimer spectra are reported in black.

Figure 3

Homodimer AA excitonic excitation energies dependence on distance d. Distance d dependence of excitonic excitation energies for the in-phase (|5⟩ and |9⟩) and out-of-phase (|4⟩ and |8⟩) excitonic states for π-stacked AA dimers (black lines). At each distance, we report four excitonic excitation energies, for each of which the label reports the oscillator strength followed by the excitonic state number, e.g., 0.66; 5 means that at d = 3.5 Å, the in-phase state |5⟩ has a computed oscillator strength = 0.66. The continuous lines represent the excitation energy of the monomeric brightest transitions S₀ → S₂ and S₀ → S₄, report the respective oscillator strengths ( and ), and follow the same coloring scheme as in Figure 1.

Figure 4

Homodimers RR, NN, HH, and MM excitonic excitation energies dependence on distance d. Distance d dependence of excitonic excitation energies of the in-phase and out-of-phase excitonic states for π-stacked (a) RR, (b) NN, (c) HH, and (d) MM homodimers (black lines): at each distance, we report two excitonic excitation energies (in-phase and out-of-phase), for each of which the label reports the oscillator strength value followed by the excitonic state number (|n⟩), with the same notation as in Figure 3. The continuous lines represent the excitation energy of the monomeric brightest transitions S₀ → S₂, report the respective oscillator strengths , and follow the same coloring scheme as in Figure 1.

Figure 5

Computed heterodimeric absorption spectra. Computed absorption spectra of all possible heterodimeric models. For each heterodimer, the spectra of the two coupled monomers are reported for ease of comparison. All spectra are normalized to the absorption maximum of monomer A. Monomer spectra follow the same coloring scheme as in Figure 1, while each heterodimer spectrum is in black.

Figure 6

Heterodimers AM and HM excitonic excitation energies dependence on distance d. Distance d dependence of excitonic excitation energies (black lines) of the first two brightest states for π-stacked AM (top) and HM (bottom) dimers. At each distance, the top label reports the oscillator strength value of the corresponding excitonic transition, while the bottom label reports the excitonic state number (|n⟩). The continuous lines represent the excitation energy of the monomeric brightest transitions: S₀ → S₂ and S₀ → S₄ for A and S₀ → S₂ for M and H, along with the respective oscillator strength values, and follow the same coloring scheme as in Figure 1.