The Added Value of Circular Dichroism to Study Charge Transfer Excitons in Micro-Crystalline Squaraine Aggregates
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KOPO 2022 11th Workshop Conjugated oligomers and polymers
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Organic semiconducting molecules are revolutionizing opto- and bio-electronic applications. For successful device integration it is essential to understand the nature and excitation pathways of molecular excitons correlated to their aggregation pattern in extended or structured thin films.  Prototypical materials are n-alkyl anilino squaraines adopting a slipped ?-stacking associated with characteristic double-hump absorption features.  Such absorption pattern can easily be confused with a Davydov splitting, however, this is contradicted by the crystallography of thin films and especially micro-crystals. Theoretical modeling evidences by an essential state model the hybrid nature of the molecular excitons consisting of an intramolecular Frenkel exciton and an intermolecular charge transfer exciton. The modeling describes well the linear dichroism of micro-crystals spotted by polarized spectro-microscopy.  However, alternative models are suitable to describe the unpolarized double-hump absorbance spectra of extended thin films by accounting for disorder within an exciton model.  As additional unique identifier we introduce chirality to the squaraine compounds by partially replacing the structure-directing n-alkyl side chains with enantiomerically pure citronellyl-derived substituents.  The structurally related chiral squaraines show as intended very similar double-hump absorbance spectra. Additionally, an excitonic circular dichroism can unambiguously be probed by (imaging) Mueller matrix polarimetry,  which will provide deeper insight into the nature of the excitonic transitions.