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

Giavazzi, Davide; Schumacher, Marvin; Grisanti, Luca; Anzola, Mattia; Maiolo, Francesco Di; Zablocki, Jennifer; Lützen, Arne; Schiek, Manuela; Painelli, Anna

doi:10.1039/d2tc05555d

J. Mater. Chem. C

2023

DOI: 10.1039/d2tc05555d

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

Davide Giavazzi

Marvin Schumacher

Luca Grisanti

et al.

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...

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2024

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Cited by 3 publications

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Ultrafast Excitonic Transitions in Enantiopure and Racemic Squaraine Thin Films

Bernhardt,

Rieland,

Wang

et al. 2024

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While chirality is a prevalent character of 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 signal, 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. Highly resembling transient responses of enantiopure thin films under excitations at different photon energies strongly indicate that a single type of aggregate dominates the linear optical response, that is, a strong red‐shifted (J‐like) and weak blue‐shifted (H‐like) absorption band. On the other hand, the transient properties of the racemic thin film deviate from this pattern and remain largely ambiguous. 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 might arise from intermolecular charge transfer with potential contributions from the coupling of excitons to the vibrations. This non‐radiative relaxation pathway explains 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 and underline the significance of non‐Frenkel exciton states for the optical properties of anilino squaraine dyes.

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Ultrafast Excitonic Transitions in Enantiopure and Racemic Squaraine Thin Films

Bernhardt,

Rieland,

Wang

et al. 2024

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Genesis of the Chirality of Polythiophene Aggregates from Classical Molecular Dynamics

Romagnoli,

Padula

2024

J. Phys. Chem. C

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Conjugated polymers are widely used in electronic devices whose performance is highly dependent on their electronic properties. Properties and performance are influenced by the supramolecular organization of the chromophores constituting the polymer backbone, and identifying structure−property relationships is crucial to design better devices. Polythiophenes (PTs) are among the most-studied polymers, and their ordered aggregation in solution is often induced by the introduction of chiral side chains. This allows using chiroptical spectroscopy, such as electronic circular dichroism, to monitor aggregation, but since experimental atomistic demonstrations of aggregate arrangements are lacking, computational models have been developed to identify the most likely structure. In this work, we simulate the aggregation of chiral polythiophenes by using classical molecular dynamics. Standard simulations show the formation of aggregates with herringbone arrangements, typical of polycyclic aromatic hydrocarbon crystals and also known for P3HT, the most used polythiophene, rather than the chiral H aggregates proposed in the literature. We exploited Hamiltonian Replica Exchange simulations to explore a larger portion of the free energy surface in the search for chiral conformations that would explain the experimentally observed chiroptical response. We identified cisoid and transoid helical structures with intramolecular chirality, without the formation of π−stacked helical aggregates commonly suggested, which resulted unstable. Given past studies of the spectroscopic response of these structures, we assign the genesis of the chirality of polythiophene aggregates to intramolecular arrangements of polymer chains, rather than to interactions among different chains organized in a chiral fashion.

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Phase-cycling and double-quantum two-dimensional electronic spectroscopy using a common-path birefringent interferometer

Timmer,

Lünemann,

Gittinger

et al. 2024

Optica

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Ultrafast spectroscopy provides unique access to the coherent dynamics of atomic, molecular, and solid state quantum systems. A most powerful, yet experimentally challenging tool for this is two-dimensional electronic spectroscopy (2DES), allowing to isolate excitation pathways and to selectively probe coherent and incoherent couplings by controlling the phase of the ultrashort pulses that interact with the system. Its experimental implementation can in principle greatly be simplified by employing inherently phase stable birefringent in-line interferometers (TWINS), which, however, are thought to lack sufficient phase control. Here, we demonstrate an adaptation of TWINS providing full phase-cycling capabilities for 2DES. This is demonstrated by recording rephasing, non-rephasing, zero-quantum, and double-quantum 2DES on a molecular J-aggregate. This easy-to-implement extension opens up new experimental possibilities for TWINS-based 2DES in multidimensional all-optical and photoemission spectroscopy and microscopy.

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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...

Cited by 3 publications

References 64 publications

Ultrafast Excitonic Transitions in Enantiopure and Racemic Squaraine Thin Films

Ultrafast Excitonic Transitions in Enantiopure and Racemic Squaraine Thin Films

Genesis of the Chirality of Polythiophene Aggregates from Classical Molecular Dynamics

Phase-cycling and double-quantum two-dimensional electronic spectroscopy using a common-path birefringent interferometer

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