Ultrafast Excitonic Transitions in Enantiopure Squaraine Thin Films

While chirality is observed in numerous biological and synthetic organic molecules, its selective absorption of circularly polarized light, known as circular dichroism (CD), is typically small due to intrinsically weak coupling between magnetic and electric dipoles. However, thin films of aggregated, enantiopure prolinol-derived squaraine molecules (ProSQ-C16) exhibit an unusually large excitonic CD signal¹, although the underlying mechanism is not yet known.
In this study, we employ steady-state and ultrafast transient absorption spectroscopy to investigate the nature and dynamics of excitons in aggregates of enantiopure and racemic ProSQ-C16 thin films. Nearly identical transient responses of enantiopure thin films under excitations at different photon energies strongly indicate that the linear optical response, including but not limited to the high CD signal, originates from a single type of aggregate, i.e. a strong red-shifted (J-like) and weak blue-shifted (H-like) absorption band that share the same ground state.
Another puzzling aspect is the low fluorescence yield in the enantiopure aggregates of ProSQ-C16 despite the dominating J-like aggregation spectral features. The short lifetime of excited states and coherent oscillations present in the dynamics of the transient absorption signal indicate that the early time dynamics are governed by a transition towards a dark intermediate state, which likely arise from intermolecular charge transfer with potential contributions from the coupling of excitons to low-frequency vibrations. Such non-radiative relaxation pathway provides a natural explanation for the unusually weak fluorescence of the predominately J-like behaving aggregate.
Our findings conclusively show that the chiral aggregate structure has a strong impact on the optical and dynamic response of the excitons in molecular aggregates and underline the significance of non-Frenkel exciton states for the optical properties of anilino squaraine dyes.

[1] Schulz, M.; Zablocki, J.; Abdullaeva, O. S.; Brück, S.; Balzer, F.; Lützen, A.; Arteaga, O.; Schiek, M. Giant Intrinsic Circular Dichroism of Prolinol-Derived Squaraine Thin Films. Nat. Commun. 2018, 9 (1), 2413. https://doi.org/10.1038/s41467-018-04811-7.