Solvent-Induced Aggregation and Cation-Controlled Self-Assembly of Tripodal Squaraine Dyes: Optical, Chiroptical and Morphological Properties

Chiroptical Properties of Planar Benzobisthiazole-Bridged Squaraine Dimers

Five chiral squaraine dimers were synthesized by fusing chiral indolenine semisquaraines with three different benzobisthiazole bridges. The thereby created squaraine dimers show a strong splitting of the lowest energy absorption bands caused by exciton coupling. The intensities of the two exciton transitions and the energetic splitting depend on the angle of the two squaraine moieties within the chromophore dimer. The electric circular dichroism spectra of the dimers show intense Cotton effects whose sign depends on the used squaraine chromophores. Sizable anisotropies g_abs of up to 2.6 × 10^-3 could be obtained. TD-DFT calculations were used to partition the rotational strength into the three Rosenfeld terms where the electric-magnetic coupling turned out to be the dominant contribution while the exciton chirality term is much smaller. This is because the chromophore dimers are essentially planar but the angle between the electric transition dipole moment of one squaraine and the magnetic transition dipole moment of the other squaraine strongly deviates from 90°, which makes the dot product between the two moment vectors and, thus, the rotational strength substantial.

Solvent-Dependent Functional Aggregates of Unsymmetrical Squaraine Dyes on TiO2 Surface for Dye-Sensitized Solar Cells

Alkyl group wrapped donor-acceptor-donor (D-A-D) based unsymmetrical squaraine dyes SQ1, SQ5, and SQS4 were used to evaluate the effect of sensitizing solvents on dye-sensitized solar cell (DSSC) efficiency. A drastic change in DSSC efficiency was observed when the photo-anodes were sensitized in acetonitrile (bad solvent when considering dye solubility) and chloroform (good solvent) with an Iodolyte (I^-/I₃^-) electrolyte. The DSSC device sensitized with squaraine dyes in acetonitrile showed better photovoltaic performance with enhanced photocurrent generation and photovoltage compared to the device sensitized in chloroform. In a good sensitizing solvent, dyes with long hydrophobic alkyl chains are deleterious forming aggregates on the TiO₂ surface, which results in an incident photon-to-current conversion efficiency (IPCE) response mostly from monomeric and dimeric structures. Meanwhile, a bad sensitizing solvent facilitates the formation of well-packed self-assembled structures on the TiO₂ surface, which are responsible for a broad IPCE response and high device efficiencies. The photoanode sensitized in the bad sensitizing solvent showed enhanced V_OC values of 642, 675, and 699 mV; J_SC values of 6.38, 11.1, and 11.69 mA/cm²; and DSSC device efficiencies of 3.0, 5.63, and 6.13% for the SQ1, SQ5, and SQS4 dyes in the absence of a coadsorbent (chenodeoxycholic acid (CDCA)), respectively, which were further enhanced by CDCA addition. Meanwhile, the photoanode sensitized in the good sensitizing solvent showed relatively low photovoltaic V_OC values of 640, 652, and 650 mV; J_SC values of 5.78, 6.79, and 6.24 mA/cm²; and device efficiencies of 2.73, 3.35, and 3.20% for SQ1, SQ5, and SQS4 in the absence of CDCA, respectively, which were further varied with equivalents of CDCA. The best DSSC device efficiencies of 6.13 and 3.20% were obtained for SQS4 without CDCA, where the dye was sensitized in acetonitrile (bad) and chloroform (good) sensitizing solvents, respectively.

Axially chiral indolenine derived chromophore dimers and their chiroptical absorption and emission properties

Yamamoto homocoupling of two chiral oxindoles led to the atropo-diastereoselective formation of an axially chiral oxindole dimer. This building block served as the starting material for the syntheses of axially chiral squaraine and merocyanine chromophore dimers. These dimers show pronounced chiroptical properties, this is, outstandingly high ECD signals (Δε up to ca. 1500 M⁻¹ cm⁻¹) as a couplet with positive Cotton effect for the P-configuration around the biaryl axis and a negative Cotton effect for the M-configuration. All investigated dimers also exhibit pronounced circularly polarised emission with anisotropy values of ca. 10⁻³ cgs. Time-dependent density functional calculations were used to analyse the three contributions (local one electron, electric–magnetic coupling, and exciton coupling) to the rotational strength applying the Rosenfeld equation to excitonically coupled chromophores. While the exciton coupling term proves to be the dominant one, the electric–magnetic coupling possesses the same sign and adds significantly to the total rotational strength owing to a favourable geometric arrangement of the two chromophores within the dimer.

From an axially chiral oxindole, squaraine and merocyanine chromophore dimers with pronounced chiroptical properties were prepared.

A computational probe granting insight into intra and inter-stacking interactions in squaraine dye derivatives

In order to decipher structure function relationships regarding the optoelectronic properties of organic functional materials, two sets of anilinosquaraine dye derivatives were analysed computationally. 2,4-Bis[4-(N,N-diisobutylamino)-2,6-dihydroxyphenyl]squaraine exhibits orthorhombic and monoclinic polymorphs (motif-1 and motif-2) in bulk and spin coated films, respectively, and concomitant J- and H-aggregate spectral features. In the second set, in spite of variation in the n-alkyl chain of the anilinosquaraine derivatives (n-propyl vs. n-butyl; motif-3 and motif-4), both acquired triclinic morphology and intermolecular charge transfer had been identified as the origin of their panchromaticity. The differences between these motifs were closely inspected at the theoretical level including geometrical parameters, internal reorganization energies, charge transfer integrals, drift mobilities, interaction energies and lattice energies along with a comprehensive Hirshfeld surface analysis. A correlation was established between C⋯C type intra-stack interactions with the observed red-shifted absorption patterns in the order motif-1 > motif-3 > motif-4. Hydrophobic interactions, π-π interactions and hydrogen bonds were found to influence the crystal packing patterns and concomitantly the overall molecule planarity and thereby the extent of π-bond delocalization/resonance. Insights into intra-stack and inter-stack interactions based on crystal packing effects are provided, which support and potentially guide the experimentalists' key ideas for shaping and encompassing the applications of promising squaraine derivative materials.

Synthesis of near-infrared absorbing and fluorescent bis(pyrrol-2-yl)squaraines and their halochromic properties

1,2-Squaraines (partially conjugated neutral structure) showed blue-shifted λmax compared to the corresponding 1,3-squaraines (fully conjugated zwitterionic structure) and the mono-protonated thiophene-fused 1,3-squaraine showed λmax at 1007 nm.

Indenoquinaldine-Based Unsymmetrical Squaraine Dyes for Near-Infrared Absorption: Investigating the Steric and Electronic Effects in Dye-Sensitized Solar Cells

A series of near-infrared (NIR)-responsive unsymmetrical squaraine dyes (ISQ1-3) incorporating a fused indenoquinaldine-based donor have been designed and synthesized. C₁₂ alkyl chains were incorporated at the sp³ -hybridized carbon center of the indene unit of the indenoquinaldine in an out-of-plane orientation to control dye aggregation on the surface of titanium dioxide, and indole (ISQ1), benzo[e]indole (ISQ2), and quinoline (ISQ3) moieties were included as the donor component bearing the anchoring carboxy group to extend the absorption in the NIR region and to systematically study the effect of the electronic modification on the performance of dye-sensitized solar cells (DSSC). All the dyes exhibit intense absorption (ϵ≥10⁵  m^-1  cm^-1 ) in the NIR region, and the dye-adsorbed TiO₂ films exhibit broad panchromatic absorption. The incident photon-to-current efficiency (IPCE) spectrum of the ISQ3-based DSSC device displays a panchromatic IPCE response up to 880 nm. Additionally, the ISQ3-sensitized device provides the best efficiency of 4.15 % with a short circuit current density (J_SC ) of 10.02 mA cm^-2 , open-circuit voltage (V_OC ) of 0.58 V, and fill factor (ff) of 72 % in the presence of 10 equivalents of 3α,7α-dihydroxy-5β-cholanic acid (CDCA). Electrochemical impedance spectroscopy analysis showed attenuated charge recombination in the ISQ3-sensitized DSSC, which contributes to its higher value of V_OC compared with the other dyes.

Supramolecular helical nanostructures from self-assembly of coil-rod-coil amphiphilic molecules incorporating the dianthranide unit

Coil-rod-coil amphipathic oligomers composed of a rigid dianthranide unit and a hydrophilic branched oligoether as the coil segment were synthesized. These amphiphilic molecules self-assemble into clew-like aggregates composed of fibres or helical nanofibers in aqueous solution. Subsequently, supramolecular polymers were produced from the above objects through charge-transfer interactions by adding 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (4F-TCNQ). Interestingly, temperature-sensitive supramolecular chirality was induced by lateral methyl units located at the interface of the rigid and flexible segments. However, upon addition of the electron-acceptor molecule, 4F-TCNQ, strong donor-acceptor interactions restrict any change in supramolecular chirality with temperature.

Donor-acceptor interaction-driven self-assembly of amphiphilic rod-coil molecules into supramolecular nanoassemblies

Rigid-flexible amphiphilic molecules consisting of an aromatic segment based on pyrene and biphenyl units and hydrophilic polyethylene oxide chains self-assemble into lamellar, hexagonal columnar, and two-dimensional columnar nanostructures in the bulk state. In aqueous solution, these molecules self-assemble into nanofibers, spherical micelles, and multilayer nanotubes, depending on the chain or rod length of the molecules. Notably, ordered nanostructures of supramolecular polymers, such as single-layer curving fragments, nanofibers, and nanosheets, were constructed through charge-transfer interactions between the nanoobjects and an electron-acceptor molecule, 2,4,5,7-tetranitrofluorenone. These experimental results reveal that diverse supramolecular morphologies can be controlled by tuning rod-coil molecular interactions or charge-transfer interactions between the donor and acceptor molecules.

A simple approach for the discrimination of surfactants based on the control of squaraine aggregation

The color of squaraine solution shows a "fingerprint" change upon addition of different surfactants. A cross-responsive sensing array based on a small molecular probe was applied to discriminate surfactants with 100% confidence limits. Furthermore, the probe can selectively detect sodium dodecyl sulfate (SDS) with turn-on fluorescence response.

Modulating the photo-exciting process of photosensitizer to improve in vitro phototoxicity by preparing its self-assembly nanostructures

Self-assembled photosensitizer nanostructures preparation by controlling the charge property of drug and ion strength of environment to improve photodynamic activity.

Biosupramolecular nanowires from chlorophyll dyes with exceptional charge-transport properties

Conductive tubes: Self-assembled nanotubes of a bacteriochlorophyll derivative are reminiscent of natural chlorosomal light-harvesting assemblies. After deposition on a substrate that consists of a non-conductive silicon oxide surface (see picture, brown) and contacting the chlorin nanowires to a conductive polymer (yellow), they show exceptional charge-transport properties.

Molecular assembly of a squaraine dye with cationic surfactant and nucleotides: its impact on aggregation and fluorescence response

A novel fluorescent system has been assembled by using ATP, surfactant, and a squaraine dye in an aqueous buffer solution. In the system, a cationic surfactant such as cetyl trimethyl ammonium bromide (CTAB) forms a sphere-like micelle, whose positive charge at the surface of the micelle attracts the negatively charged ATP to form a unique organized nanostructure. Such an organized system is shown to interact with the squaraine dye (SQ) to perturb its aggregate structure, thereby generating the optical response. The nanostructure of the assembly has been characterized by dynamic light-scattering (DLS) and atomic force microscopy (AFM). The unique feature of the developed sensing system is that the analytes ATP form part of the assembly structure. The system utilizes forces such as electrostatic interaction and π-π stacking of the aromatic segment of ATP and SQ to achieve the selective detection of ATP.

Construction of helical J-aggregates self-assembled from a thymidylic acid appended anthracene dye and DNA as a template

The thymidylic acid appended anthracene dye 2,6-bis[5-(3'-thymidylic acid)pentyloxy]anthracene (1) was synthesized, and the self-assembly of 1 and the binary self-assembly of 1 with a complementary single-stranded 20-meric oligodeoxyadenylic acid (dA(20)) were performed in 0.1 x TE buffer solution (i.e., 1.0 x 10(-3) M Tris-HCl, 1.0 x 10(-4) M ethylenediaminetriacetic acid (EDTA)). The characteristic J-band, small Stokes shift (6 nm), Cotton effect, and helical nanofibers 5.1 nm in diameter are observed in UV/Vis, fluorescence, and circular dichroism (CD) spectroscopies and atomic force microscopy (AFM) measurements for the binary self-assembly of 1 and dA(20) in aqueous solution. These observations revealed that the helical J-aggregates, in which the short-axis transition dipoles of the anthracene moieties are aligned in a head-to-tail fashion, are formed from the binary self-assembly of 1 and dA(20). The UV/Vis absorption and CD band of the anthracene L(a) region were found to be strongly dependent on temperature and showed cooperative changes for the binary self-assembly of 1 and dA(20). The self-assembly of the single-component 1 produced right- and left-handed helical nanofibers with diameters ranging from 4.0 to 10 nm. In contrast, for the binary self-assembly, the UV/Vis and fluorescence spectra showed no J-band and the Stokes shift was at approximately 107 nm for the single-component 1 in aqueous solution. In addition, the binary self-assembly of 1 and noncomplementary single-stranded 20-meric oligothymidylic acid (dT(20)) showed a small J-band and the J-band disappeared at 50 degrees C upon heating. On the basis of these observations, we concluded that thymine-adenine base-pair formation induced supramolecular helical J-aggregates in the binary self-assembly of 1 and dA(20) in aqueous solutions.