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

Abstract

Squaraines are quadrupolar molecular dyes forming aggregates with remarkable structure-correlated excitonic properties within the visible to near-infrared spectral range. Upon chiral functionalization a chiroptical response such as circular dichroism is an additional spectroscopic feature. We provide a combined experimental and theoretical survey on chiral aggregates dispersed in a solution of proline-derived anilino squaraines (ProSQs) with varying terminal alkyl chain length (C3 to C12 and C16) directing the aggregation. Different aggregation scenarios with characteristic spectroscopic features appear, intricately depending on the alkyl chain length: concomitant blue- and red-shifted spectroscopic signatures both within the linear and circular absorption for intermediate chain length, and a scenario with dominating blue-shifted signatures for shorter and longer alkyl chain length. Molecular dynamics (MD) simulations on the aggregate structure return the opposite handedness suggesting kinetic control in the experiments. Two modified essential state models (ESM) are applied to calculate the optical spectra with prescribed geometric parameters: a model accounting for just electrostatic intermolecular interactions (ESM-ES) suggests two concomitant aggregates to explain the simultaneous blue- and red-shifted spectral signatures while a model including intermolecular charge transfer (ESM-CT) returns both features for a single aggregate. Due to the complexity of the calculation this is the first time an explicit expression for calculating circular dichroism including intermolecular charge transfer is rendered. However, the ESM-CT model is still limited to tetramers, so finite size effects such as disorder are not fully captured. This could be the reason why the ESM-CT does not reproduce the scenario with dominant blue-shifted spectral signatures. We demonstrate that a dimeric model is sufficient to describe linear absorption but fails for chiroptical properties. Furthermore, a superlinear amplification of circular dichroism intensity with increasing aggregate size is noted for both models. This adds value to the application potential for chiral squaraine aggregates.