J-aggregates are of significant interest for organic materials conceived by supramolecular approaches. Their discovery in the 1930s represents one of the most important milestones in dye chemistry as well as the germination of supramolecular chemistry. The intriguing optical properties of J-aggregates (in particular, very narrow red-shifted absorption bands with respect to those of the monomer and their ability to delocalize and migrate excitons) as well as their prospect for applications have motivated scientists to become involved in this field, and numerous contributions have been published. This Review provides an overview on the J-aggregates of a broad variety of dyes (including cyanines, porphyrins, phthalocyanines, and perylene bisimides) created by using supramolecular construction principles, and discusses their optical and photophysical properties as well as their potential applications. Thus, this Review is intended to be of interest to the supramolecular, photochemistry, and materials science communities.

J-aggregates: from serendipitous discovery to supramolecular engineering of functional dye materials.

Photoacoustic imaging (PAI) is an emerging tool that bridges the traditional depth limits of ballistic optical imaging and the resolution limits of diffuse optical imaging. Using the acoustic waves generated in response to the absorption of pulsed laser light, it provides noninvasive images of absorbed optical energy density at depths of several centimeters with a resolution of ∼100 μm. This versatile and scalable imaging modality has now shown potential for molecular imaging, which enables visualization of biological processes with systemically introduced contrast agents. Understanding the relative merits of the vast range of contrast agents available, from small-molecule dyes to gold and carbon nanostructures to liposome encapsulations, is a considerable challenge. Here we critically review the physical, chemical and biochemical characteristics of the existing photoacoustic contrast agents, highlighting key applications and present challenges for molecular PAI.

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Contrast agents for molecular photoacoustic imaging

This critical review provides an overview on the formation of π-stacks of functional dyes in solution, aiming to acquaint young researchers with this topical research field and to stimulate further advance in supramolecular dye chemistry. Different mathematical models that have been proposed and applied for the description of aggregation equilibria of π-systems in solution are discussed. The factors that have significant impact on the structural features of aggregates and the thermodynamics of π–π stacking such as electrostatic interactions, size and geometry of the dye molecules are covered in this review. A comparison of the binding strength is made for different classes of functional π-conjugated systems, from simple benzene to more extended polycyclic hydrocarbon molecules, including triphenylenes and hexabenzocoronenes, heteroaromatic porphyrins and phthalocyanines, quadrupolar naphthalene and perylene bisimides, dipolar or even zwitterionic merocyanines and squaraines, and some macrocyclic dyes. Solvent effects on binding constants are analysed by linear free energy relationships with various solvent polarity scales (98 references with multiple entries).

Self-assembled π-stacks of functional dyes in solution: structural and thermodynamic features

The cyanine platforms including cyanine, hemicyanine, and squaraine are good candidates for developing chemosensors because of their excellent photophysical properties, outstanding biocompatibility, and low toxicity to living systems. A huge amount of research work involving chemosensors based on the cyanine platforms has emerged in recent years. This review focuses on the development from 2000 to 2015, in which cyanine, hemicyanine, and squaraine sensors will be separately summarized. In each section, a systematization according to the type of detection mechanism is established. The basic principles about the design of the chemosensors and their applications as bioimaging agents are clearly discussed. In addition, we emphasize the advances that have been made in improving the detection performance through incorporation of the chemosensors into nanoparticles.

Recent Development of Chemosensors Based on Cyanine Platforms

Challenges and breakthroughs in recent research on self-assembly

This feature article highlights the recent developments in the field of squaraine chemistry Attempts have been made to address the relevance of squaraine dyes as a class of functional organic materials useful for electronic and photonic applications Due to the synthetic access of a variety of squaraine dyes with structural variations and due to the strong absorption and emission properties which respond to the surrounding medium, these dyes have been receiving significant attention Therefore, squaraine dyes have been extensively investigated in recent years, from both fundamental and technological viewpoints

Squaraine dyes: a mine of molecular materials

Let's do the twist: Tripodal squaraines self-assemble from acetonitrile to form hollow spheres, the complexation of which with Ca 2+ or Mg 2+ results in extended networks. An analogous chiral dye exhibits a bisignate CD couplet and a helical morphology upon Ca 2+ binding (see figure). Thus, the molecular chirality of a functional dye is expressed through specific cation binding and manifested in the form of supramolecular helicity.

Self‐Assembly of Tripodal Squaraines: Cation‐Assisted Expression of Molecular Chirality and Change from Spherical to Helical Morphology

Controlled self-assembly of squaraines to 1D supramolecular architectures with high molar absorptivity.

The synthesis, characterisation, optical, chiroptical, aggregation, and alkaline earth metal cation assisted self-assembly properties of tripodal squaraine dyes have been described. In the tripodal geometry, these dyes exhibit three absorption bands around 650, 620 and 580 nm in contrast to the single, sharp absorption of a simple dye (SQ) at 640 nm. The fluorescence quantum yield of the squaraine dyes are 25-30 times lower when compared to that of SQ, which indicates intramolecular exciton interaction as a result of the confinement of the dyes. The evaporation of an acetonitrile solution of the dye resulted in the formation of vesicular objects as confirmed by AFM and TEM analyses. However, evaporation of an acetonitrile solution containing 10-12 % water gave short fibrous aggregates. In the presence of Ca 2+ or Mg 2+ ions, the dyes exhibit an intense and sharp absorption band at 547 nm with a concomitant decrease of the native absorption. Interestingly, the dye with chiral groups failed to give a circular dichroic signal during aggregation in solvent mixtures, whereas strong signals were observed in the presence of Ca 2+ and Mg 2+ ions. AFM and TEM analyses of the corresponding cation complexes revealed the formation of worm like nanohelices. However, addition of EDTA to the Ca 2+ or Mg 2+ complex exhibited a reversal of the absorption, emission, and circular dichroic spectra to that of the native dye, indicating decomplexation. AFM analyses revealed the transformation of the helices to particles. These observations reveal the difference in the nature and properties of the simple aggregates formed in solvent mixtures and those formed in the presence of cations. In the present study, we were able to establish the importance of specific cation binding in controlling the size, shape, and properties of the hierarchical assemblies of tripodal squaraines.

Parayalil Chithra

Papers

Squaraine dyes: a mine of molecular materials

Self‐Assembly of Tripodal Squaraines: Cation‐Assisted Expression of Molecular Chirality and Change from Spherical to Helical Morphology

Controlled self-assembly of squaraines to 1D supramolecular architectures with high molar absorptivity.

Solvent-induced aggregation and cation-controlled self-assembly of tripodal squaraine dyes: optical, chiroptical and morphological properties.

Self-Assembly of Tripodal Squaraines: Cation-Assisted Expression of Molecular Chirality and Change from Spherical to Helical Morphology.