Absorption spectra of mixed two-dimensional cyanine aggregates on silver halide substrates

Abstract

Using the coherent potential approximation ͑CPA͒, we study the absorption spectra of two-dimensional molecular aggregates formed from binary random molecular mixtures. In addition to the substitutional randomness, we include Gaussian randomness in the transition frequencies within each of the two classes of molecules. The latter is motivated by the considerable disorder that is typical for two-dimensional aggregates. By comparing to exact diagonalization results for small clusters, we show that the CPA gives an excellent description of the spectra for this kind of disorder, both in the cases of amalgamation and persistence type mixing. Taking into account long-range excitation transfer interactions mediated by the extended molecular transition dipoles, we analyze experimental spectra of amalgamation-type mixed cyanine aggregates adsorbed on AgBr ͕100͖surfaces that are also reported here. We find good agreement between theory and experiment for the position, the width, and the general shape of the absorption line as a function of the mixing ratio. The analysis also allows us to estimate the slip angle characterizing the structure of these aggregates.

Molecular Arrangements in Two-Dimensional J-Aggregate Monolayers of Cyanine Dyes

“…This displacement generates the slip angle α = atan(D/S) ~ 15-17 o which is observed in the vortexes of leaves (Figure 3a), where D is the plane-to-plane intermolecular distance which is typically considered to be ~ 0.35 -0.4 nm. [22,23] Noteworthy to say, that for this particular dye the unique anti-parallel up-down arrangement of adjacent molecules slipped by S ~ 1.3 nm comes from the essentially not-in-line arrangement of three joint cycles of π-electron chromophores (Figure 2a). The observed rectangular ends of stripes could be explained by two alternative conceivable models with the ladder (Figure 1b) and brickwork arrangements (Figure 1c) having different molecular orientation with respect to the aggregates long axis, i.e.…”

Localization properties of a one-dimensional tight-binding model with nonrandom long-range intersite interactions

“…11,12,15 To conclude, we note that our findings, apart from being interesting from the theoretical point of view, are relevant for real physical systems. Thus, some organic materials with planar geometry, in which optically allowed excitations are dipolar Frenkel excitons, [45][46][47][48][49][50][51][52] represent an example; the value of the interaction exponent for the dipole-dipole intersite interaction is = 3, which resembles the weak localization regime in the 1D long-range model when =3/2. 41 Dipoleexchange spin waves in ferromagnetic films provide yet another example where these results are relevant.…”

Optical spectra and localization of excitons in inhomogeneous helical cylindrical aggregates

“…A well-known example is the CPA, 33,34 which previously has been applied with success to calculate spectra of, for instance, isotopically mixed aromatic crystals ͑dichotomic disorder͒, 35,36 one-component systems with Gaussian diagonal disorder, 37,38 and two-component systems with bi-Gaussian diagonal disorder. 39 In this paper, we will use both brute-force numerical simulations and the CPA to calculate the spectra and the density of states. We will extend the usual CPA to account for finite-size effects ͑Sec.…”