Structurally Induced Chirality of an Achiral Chromophore on Self-Assembled Nanofibers: A Twist Makes It Chiral

Chiroptical properties of cyanine aggregates: hierarchical modelling from monomers to bundles

Some achiral cyanine dyes form well-ordered chiral assemblies exhibiting pronounced Circular Dichroism (CD) and Circularly Polarized Luminescence (CPL). Notably, achiral C8O3 cyanines self-assemble into tubular J-aggregates, which further organize into bundles displaying bisignate CD spectrum - hallmark of an exciton coupled system - and an unusual bisignated CPL. In contrast, the tubular aggregates display a monosignate CD spectrum. The mechanism underlying these intriguing features remains elusive. In the present work, a quantum-mechanical exciton model is proposed to elucidate the (chir)optical behaviour of C8O3 aggregates. A herringbone arrangement of C8O3 dyes within the tubular aggregates well reproduces the observed spectral signatures. The anomalous observation of a singular CD peak in tubular aggregates is ascribed to the intrinsic chirality of the monomeric units inside the aggregate, whereas the CD doublet characterizing the bundles is attributed to the exciton coupling between the constituent tubes. The bisignated CPL signal observed in bundles reveals significant anti-Kasha emission at room temperature and is quantitatively addressed accounting for a very tiny exciton splitting leading to a sizable thermal population of both exciton states. This study provides crucial insights on the complexity of C8O3 aggregation and on the origin of chiroptical response at various aggregation stages.

Development of Polymethine Dyes for NIR-II Fluorescence Imaging and Therapy

Fluorescence imaging in the second near-infrared window (NIR-II) is burgeoning because of its higher imaging fidelity in monitoring physiological and pathological processes than clinical visible/the second near-infrared window fluorescence imaging. Notably, the imaging fidelity is heavily dependent on fluorescence agents. So far, indocyanine green, one of the polymethine dyes, with good biocompatibility and renal clearance is the only dye approved by the Food and Drug Administration, but it shows relatively low NIR-II brightness. Importantly, tremendous efforts are devoted to synthesizing polymethine dyes for imaging preclinically and clinically. They have shown feasibility in the customization of structure and properties to fulfill various needs in imaging and therapy. Herein, a timely update on NIR-II polymethine dyes, with a special focus on molecular design strategies for fluorescent, photoacoustic, and multimodal imaging, is offered. Furthermore, the progress of polymethine dyes in sensing pathological biomarkers and even reporting drug release is illustrated. Moreover, the NIR-II fluorescence imaging-guided therapies with polymethine dyes are summarized regarding chemo-, photothermal, photodynamic, and multimodal approaches. In addition, artificial intelligence is pointed out for its potential to expedite dye development. This comprehensive review will inspire interest among a wide audience and offer a handbook for people with an interest in NIR-II polymethine dyes.

Red Circularly Polarized Luminescence from Dimeric H-Aggregates of Acridine Orange by Chiral Induction

Understanding the mechanism of chirality transfer from a chiral surface to an achiral molecule is essential for designing molecular systems with tunable chiroptical properties. These aspects are explored herein using l- and d-isomers of alkyl valine amphiphiles, which self-assemble in water as nanofibers possessing a negative surface charge. An achiral chromophore, acridine orange, upon electrostatic binding on these surfaces displays mirror-imaged bisignated circular dichroism and red-emitting circularly polarized luminescence signals with a high dissymmetry factor. Experimental and computational investigations establish that the chiroptical properties emerge from surface-bound asymmetric H-type dimers of acridine orange, further supported by fluorescence lifetime imaging studies. Specifically, atomistic molecular dynamics simulations show that the experimentally observed chiral signatures have their origin in van der Waals interactions between acridine orange dimers and the amphiphile head groups as well as in the extent of solvent exposure of the chromophore.