Multistimuli‐Responsive Materials from Benzothiadiazole‐Based Charge‐Transfer Chromophores: Interdependence of Optical Properties and Aggregation

Abstract

A family of multipolar V-shaped chromophores bearing an A'-D-A-D-A' structure were investigated, where 2,1,3-benzothiadiazole, acting as the central electron acceptor (A), is linked to two thiophene-based electron-donating moieties (D) end-capped with alkyl-cyanoacetate substituents (A'). The spectroscopic properties of thin films and powders were investigated and were compared to solutions and nanoaggregates. The optical properties strongly depended on the aggregation state and could be tuned by means of thermal treatment (annealing) of the films and through mechanical treatment (grinding) of the powders, demonstrating that these materials are multistimuli responsive. The spectroscopic analysis revealed the partial formation of J-aggregates both in the films and in powders, despite the weak luminescence. This behavior was interpreted as due to the partial formation of poorly fluorescent red-shifted aggregates acting as a sink in an efficient excitation energytransfer process from J-aggregates.

Aggregates of polar dyes: beyond the exciton model

“…15,16 Deviations from the exciton model are mainly recognized in the intensity of optical transitions as well as of CD spectra. 25 However, in aggregates of non-polar dyes with a large quadrupolar character, large deviations from the exciton model are observed: [26][27][28][29][30] in these systems, the dense excitation spectrum at the molecular level, and the large molecular polarizability, make the approximations of the exciton model critical, with impressive effects on linear and nonlinear optical spectra.…”

Solid‐State Emission from Mono‐ and Bichromophoric Boron Dipyrromethene (BODIPY) Derivatives and Comparison with Fluid Solution

“…The situationd escribed here is not unlike that reportedf or J-aggregates assembled from benzothiadiazole-basedc harge-transfer complexes. [54] Here, weakly emissive red aggregates are believed to act as excitation traps for the more strongly fluorescent J-aggregates.…”

Optical spectra of organic dyes in condensed phases: the role of the medium polarizability

“…10 Intermolecular interactions in aggregates, supramolecular complexes and crystals have been discussed in different contexts, underlying how mutually interacting polarizable and/or polar molecules lead to specific spectroscopic features that cannot be reconciled with the standard exciton models. [11][12][13][14][15][16][17][18] But even in comparatively simple systems, where the dye is dissolved in dilute solutions, in polymeric matrices or glasses, environmental effects may be quite impressive, ranging from the solvatochromism of polar dyes, 19,20 to symmetry breaking phenomena [21][22][23][24] Essential-state models (ESMs) were proposed and successfully applied to describe low-energy spectral properties of CT dyes in different environments. 10,14,[21][22][23] ESMs are a family of parametric Hamiltonians that only account for few electronic molecular states, usually corresponding to the main resonating structures that characterize each dye, coupled to a few effective vibrational modes, to account for the geometry relaxation accompanying the CT process.…”