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.

Linear π-gelators self-assemble into entangled fibers in which the molecules are arranged perpendicular to the fiber long axis. However, orientation of gelator molecules in a direction parallel to the long axes of the one-dimensional (1-D) structures remains challenging. Herein we demonstrate that, at the air-water interface, an oligo(p-phenylenevinylene)-derived π-gelator forms aligned nanorods of 340 ± 120 nm length and 34 ± 5 nm width, in which the gelator molecules are reoriented parallel to the long axis of the rods. The orientation change of the molecules results in distinct excited-state properties upon local photoexcitation, as evidenced by near-field scanning optical microscopy. A detailed understanding of the mechanism by which excitation energy migrates through these 1-D molecular assemblies might help in the design of supramolecular structures with improved charge-transport properties.

Three different squaraine tethered bichromophoric podands 3a-c with one, two, and three oxygen atoms in the podand chain and an analogous monochromophore 4a were synthesized and characterized. Among these, the bichromophores 3a-c showed high selectivity toward alkaline earth metal cations, particularly to Mg(2+) and Ca(2+) ions, whereas they were optically silent toward alkali metal ions. From the absorption and emission changes as well as from the Job plots, it is established that Mg(2+) ions form 1:1 folded complexes with 3a and 3b whereas Ca(2+) ions prefer to form 1:2 sandwich dimers. However, 3c invariably forms weak 1:1 complexes with Mg(2+), Ca(2+), and Sr(2+) ions. The signal output in all of these cases was achieved by the formation of a sharp blue-shifted absorption and strong quenching of the emission of 3a-c. The signal transduction is achieved by the exciton interaction of the face-to-face stacked squaraine chromophores of the cation complex, which is a novel approach of specific cation sensing. The observed cation-induced changes in the optical properties are analogous to those of the "H" aggregates of squaraine dyes. Interestingly, a monochromophore 4a despite its binding, as evident from (1)H NMR studies, remained optically silent toward Mg(2+) and Ca(2+) ions. While the behavior of 4a toward Mg(2+) ion is understood, its optical silence toward Ca(2+) ion is rationalized to the preferential formation of a "Head-Tail-Tail-Head" arrangement in which exciton coupling is not possible. The present study is different from other known reports on chemosensors in the sense that cation-specific supramolecular host-guest complexation has been exploited for controlling chromophore interaction via cation-steered exciton coupling as the mode of signaling.

Inspired by the elegance and complexity of natural helical assemblies, from the nanoscopic DNA double helix and collagen triple helix to microscopic viruses and macroscopic sea shells, chemists have been trying to mimic the structure and functions of biological macromolecules in self-assembled synthetic molecules. [1][2][3][4][5][6][7] The most challenging task in these studies is the control of the overall morphology and the supramolecular chirality of the selfassembled architectures within the nanometer-to-micrometer length scale.[8] Although a few reports on reversible morphology transitions caused by external stimuli have appeared, [9] a spherical-to-helical change induced by a cation was previously unknown. However, in an interesting report Nolte and co-workers described the loss of helicity in a phthalocyanine-derived self-assembly in the presence of potassium ions.[10] Herein we report an unprecedented self-assembly of tripodal squaraine dyes, which form vesicular structures upon evaporation of the solvent from a solution of the dye and helical architectures through the expression of molecular chirality into supramolecular helicity upon a specific cation binding. [11] The supramolecular chemistry of functional dyes is at center stage in the "bottom-up" creation of nanoarchitectures.[12] Squaraines, a class of zwitterionic dyes, are well known for their tendency to form aggregates under the influence of certain solvent mixtures and cations. [13,14] Although these dyes have been studied extensively owing to their fundamental and technological significance, they are rarely exploited for the crafting of supramolecular architectures. The propensity of these dyes to aggregate to form hierarchical assemblies could be enhanced by confining the chromophores to an aromatic platform, thereby making interchromophore interaction possible. As a proof of principle of this hypothesis, we designed the three tripodal squaraines 1 a-c, which we then synthesized by the reaction of 1,3,5-tris(2-(N-methyl-N-phenylamino)ethoxy)benzene with the corresponding N,N-(dialkylaminophenyl)-4-hydroxycyclobut-3-ene-1,2-dione and characterized by spectral analysis. The UV/Vis spectra of these dyes show strong absorption but weak emission. For example, 1 c has an absorption maximum in chloroform at 647 nm (6.2 10 À7 m, e 651 nm = 8.9 10 5 m À1 cm À1 ) with a shoulder at 617 nm, whereas in acetonitrile, in addition to the l max absorption at 651 nm (8.3 10 À7 m, e 651 nm = 2.5 10 5 m À1 cm À1 ) and the shoulder at 614 nm, another band was observed at 578 nm. The emission maximum of 1 c when excited at 570 nm in chloroform or acetonitrile occurred at 662 and 663 nm with quantum yields of 0.01 and 0.004, respectively (relative to bis(4-(dimethylamino)phenyl)squaraine in chloroform as the standard). These observations support intramolecular chromophore interaction in acetonitrile and the consequent exciton coupling of the confined chromophores. [15] The absorption spectra of 1 a-c in acetonitrile-water mixtures of different compositions showed ...

A tailor made squaraine dye upon binding with Ca2+ self-assembles to form a spherical micellar assembly that reorganises to thermodynamically stable 1D cylindrical rods 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.

Inspired by the elegance and complexity of natural helical assemblies, from the nanoscopic DNA double helix and collagen triple helix to microscopic viruses and macroscopic sea shells, chemists have been trying to mimic the structure and functions of biological macromolecules in self-assembled synthetic molecules. [1][2][3][4][5][6][7] The most challenging task in these studies is the control of the overall morphology and the supramolecular chirality of the selfassembled architectures within the nanometer-to-micrometer length scale.[8] Although a few reports on reversible morphology transitions caused by external stimuli have appeared, [9] a spherical-to-helical change induced by a cation was previously unknown. However, in an interesting report Nolte and co-workers described the loss of helicity in a phthalocyanine-derived self-assembly in the presence of potassium ions.[10] Herein we report an unprecedented self-assembly of tripodal squaraine dyes, which form vesicular structures upon evaporation of the solvent from a solution of the dye and helical architectures through the expression of molecular chirality into supramolecular helicity upon a specific cation binding. [11] The supramolecular chemistry of functional dyes is at center stage in the "bottom-up" creation of nanoarchitectures.[12] Squaraines, a class of zwitterionic dyes, are well known for their tendency to form aggregates under the influence of certain solvent mixtures and cations. [13,14] Although these dyes have been studied extensively owing to their fundamental and technological significance, they are rarely exploited for the crafting of supramolecular architectures. The propensity of these dyes to aggregate to form hierarchical assemblies could be enhanced by confining the chromophores to an aromatic platform, thereby making interchromophore interaction possible. As a proof of principle of this hypothesis, we designed the three tripodal squaraines 1 a-c, which we then synthesized by the reaction of 1,3,5-tris(2-(N-methyl-N-phenylamino)ethoxy)benzene with the corresponding N,N-(dialkylaminophenyl)-4-hydroxycyclobut-3-ene-1,2-dione and characterized by spectral analysis. The UV/Vis spectra of these dyes show strong absorption but weak emission. For example, 1 c has an absorption maximum in chloroform at 647 nm (6.2 10 À7 m, e 651 nm = 8.9 10 5 m À1 cm À1 ) with a shoulder at 617 nm, whereas in acetonitrile, in addition to the l max absorption at 651 nm (8.3 10 À7 m, e 651 nm = 2.5 10 5 m À1 cm À1 ) and the shoulder at 614 nm, another band was observed at 578 nm. The emission maximum of 1 c when excited at 570 nm in chloroform or acetonitrile occurred at 662 and 663 nm with quantum yields of 0.01 and 0.004, respectively (relative to bis(4-(dimethylamino)phenyl)squaraine in chloroform as the standard). These observations support intramolecular chromophore interaction in acetonitrile and the consequent exciton coupling of the confined chromophores. [15] The absorption spectra of 1 a-c in acetonitrile-water mixtures of different compositions showed ...

Fabrication of nano-sized objects is one of the most important issues in nanoscience and nanotechnology. Soft nanomaterials with flexible properties have been given much attention and can be obtained through bottom-up processing from functional molecules, where self-assembly based on supramolecular chemistry and designed assembly have become crucial processes and techniques. Among the various functional molecules, dyes have become important materials in certain areas of nanotechnology and their selfassembling behaviors have been actively researched. In this short review, we briefly introduce recent progress in self-assembly of optical molecules and dyes, based mainly on supramolecular concepts. The introduced examples are classified into four categories: selfassembly of (i) low-molecular-weight dyes and (ii) polymeric dyes and dye self-assembly (iii) in nanoscale architectures and (iv) at surfaces.