Metal-free near-infrared (NIR) active unsymmetrical squaraine dyes, RSQ1 and RSQ2, with benzodithiophene (BDT) π-spacer and cyanoacrylic acid acceptor were synthesized by utilizing palladium catalyzed direct (hetero)arylation reaction. Methyl and 2-ethylhexyl groups were strategically placed at the BDT unit for RSQ1 and RSQ2, respectively, to investigate the effect of alkylated π-spacer on dye aggregation on the TiO surface and recombination reactions at TiO/dye/electrolyte interface. These dyes have strong absorption (ε > 10 M cm) in near-infrared (NIR) region and exhibit similar optical and electrochemical properties as they have same conjugated framework. RSQ2 performed better than RSQ1 owing to its higher open-circuit voltage (V) and fill factor (ff) in spite of having comparable short-circuit current density (J). The panchromatic incident photon-to-current conversion efficiency (IPCE) response was also observed for both the dyes. RSQ2 showed power conversion efficiency (PCE) of 6.72% with short-circuit current density (J) of 18.53 mA/cm, open circuit voltage (V) of 0.538 V, and fill factor (ff) of 67.4%, without any coadsorbent. Attenuation of the charge recombination for RSQ2 was revealed by electrochemical impedance analysis (EIS) and open-circuit potential decay transients (OCVD), which attributes to its higher V and ff in comparison to RSQ1.

A new procedure for the mild, practical, and scalable Diels-Alder reaction of tropones with arynes is reported. Differently substituted tropones undergo selective [4 + 2] cycloaddition with arynes generated in situ by the fluoride-induced 1,2-elimination of 2-(trimethylsilyl)aryl triflates, allowing the formation of functionalized benzobicyclo[3.2.2]nonatrienone derivatives in moderate to good yields. In addition, the photophysical properties of the cycloadducts are presented.

The position of the anchoring group is systematically changed with a series of alkyl group wrapped donor–acceptor–donor (D–A–D) based squaraine dyes, 4-SQ to 7-SQ, for the use in dye-sensitized solar cells (DSSCs). By this approach, the orientation as well as the self-assembly of the sensitizers can be controlled on the semiconducting TiO2 surface. All of the dyes functionalized with hydrophobic alkyl groups at sp3-C and N atoms of the indoline units that is far away from the TiO2 surface to control the self-assembly of dyes and passivate the surface. Controlling both the orientation as well as the self-assembly of the sensitizers synergistically enhances the V oc of the DSSC device by imparting the dipole moment on the TiO2 surface and minimizing the interfacial charge recombination process of electrons from TiO2 to the oxidized electrolyte, respectively. Further, the presence of a meta-carboxyl group with respect to the N atom of the indoline donor unit for the dyes 4-SQ and 6-SQ makes them nonconductive for the charge injection process, which sheds light on the importance of through-space electron transfer for the device performance. Emission from the relaxed twisted state was found to be a deactivation pathway for 4-SQ on TiO2 and ZrO2, which revealed the importance of structural factors that promote spatial interaction between the sensitizer and metal oxide surface. Computational studies showed the systematic changes in the dipole moment for the sensitizers 4-SQ, 5-SQ, and 6-SQ upon anchoring to the TiO2 surface. The DSSC device performance varied with the position of anchoring groups in the sensitizers. The DSSC device performance of 5-SQ indicates a J sc value of 11.35 mA cm–2, V oc of 0.698 V, and ff of 77% corresponding to a power conversion efficiency of 6.08% in the presence of 3 equiv of coadsorbent CDCA, which is nearly 1.5 times higher than 6-SQ (V oc 0.7 V, J sc 7.76 mA cm–2, ff 76%, and η 4.14%) and 2.6 times higher than 4-SQ (V oc 0.658 V, J sc 4.42 mA cm–2, ff 78%, and η 2.28%). IPCE studies revealed the importance of orientation for the charge injection and self-assembly of dyes, as devices with 5-SQ and 6-SQ as a sensitizer showed 94 and 77% response at 578 nm, respectively, which correspond to the aggregated structure of the dye. Mott–Schottky and IPCE experiments showed that the orientation of sensitizers could modulate the V oc due to the shift in the flat band potential of TiO2.

A series of four unsymmetrical squaraine dyes, XSQ1-4, were synthesized using a fused fluorenylindolenine-based donor unit for dye-sensitized solar cells (DSSCs). The fused structure of fluorenylindolenine helped in moving the absorption toward the near-infrared (NIR) region, and the two sp-C centers available on this donor were utilized to incorporate out-of-plane alkyl chains in opposite directions to control the dye-dye interactions on the TiO surface. High extinction coefficient (ε ≥ 10 M cm) for absorbing NIR photons and suitable highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels with respect to the conduction band of TiO and electrolyte for charge injection and dye regeneration processes, respectively, make these dyes potential sensitizers for DSSCs. Introduction of branched alkyl groups in the π-framework helped in controlling dye aggregation to reduce exciton quenching and assisted in TiO surface passivation to avoid the charge recombination process. Furthermore, having a naphthyl group on the indole part of the anchoring group containing segment helped to red-shift the absorption spectrum of dyes 15 nm toward the NIR region (XSQ3-4). Among all of the dyes under investigation, XSQ2 gave the best photovoltaic performance, having a short-circuit current density ( J) of 13.99 mA cm, open-circuit voltage ( V) of 0.66 V, and a fill factor (ff) of 0.71, with a device performance (η) of 6.57%. Electrochemical impedance spectroscopy revealed higher electron lifetime on TiO for XSQ2, which helps to avoid the charge recombination process.

A series of near-infrared (NIR)-responsive unsymmetrical squaraine dyes (ISQ1-3) incorporating a fused indenoquinaldine-based donor have been designed and synthesized. C alkyl chains were incorporated at the sp -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≥10 m cm ) in the NIR region, and the dye-adsorbed TiO 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 ) of 10.02 mA cm , open-circuit voltage (V ) 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 compared with the other dyes.

Multispectral photodetectors (MSPs) and circularly polarized light (CPL) sensors are important in opto‐electronics, photonics, and imaging. A capacitive photodetector consisting of an interdigitated electrode coated with carbon dot/anthraquinone‐polydiacetylene is constructed. Photoexcitation of the carbon dots induces transient electron transfer to the anthraquinone moieties, and concomitant change in the film dielectric constant and recorded capacitance. This unique photodetection mechanism furnishes wavelength selectivity that is solely determined by the absorbance of the carbon dots incorporated in the anthraquinone‐polydiacetylene matrix. Accordingly, employing an array of polymerized‐anthraquinone photodetector films comprising carbon dots (C‐dots) exhibiting different excitation wavelengths yielded optical “capacitive fingerprints” in a broad spectral range (350–650 nm). Furthermore, circular light polarization selectivity is achieved through chiral polymerization of the polydiacetylene framework. The carbon dot/anthraquinone‐polydiacetylene capacitive photodetector features rapid photo‐response, high fidelity, and recyclability as the redox reactions of anthraquinone are fully reversible. The carbon dot/anthraquinone‐polydiacetylene platform is inexpensive, easy to fabricate, and consists of environmentally friendly materials.

Polydiacetylenes have attracted significant interest for their unique chromatic properties and applications in sensing, imaging, and optics. Here, it is demonstrated that aminoanthraquinone‐substituted diacetylenes exhibit distinct aggregation‐dependent chromatic properties, affected by the alignment of both the aminoanthraquinone headgroups and diacetylene sidechains. Specifically, it is shown that aminoanthraquinone‐diacetylene monomers adopt different film organizations depending upon the polarity of the solvent employed for predissolution. In particular, a yellow aminoanthraquinone‐diacetylene phase, which undergoes photopolymerization upon ultraviolet irradiation, is produced upon dissolution in polar organic solvents prior to deposition and drying on solid substrates. In contrast, a red phase that can not be polymerized is observed when the monomers are predissolved in apolar solvents. Microscopic and spectroscopic analyses indicate that the optical properties of the films are determined by the degree of overlap between the aminoanthraquinone headgroups as well as the alignment of the diacetylene sidechains; both factors are intimately affected by interactions of the monomers with solvent molecules. It is shown that the aggregation‐dependent diacetylene films exhibit remarkable solvochromic, thermochromic, and mechanochromic properties. The aminoanthraquinone‐substituted diacetylenes may facilitate chromatic tuning of polydiacetylene systems in the solid state, determined by solvent‐ and intermolecular interactions and concomitant self‐assembly of the pendant sidechains and aromatic headgroups.

Strategies to diminish both charge recombination and aggregation of dyes on the photoanode by functionalizing the sensitizer with alkyl groups is the best approach to achieve high dye‐sensitized solar cell (DSSC) efficiency. Development of such a photoanode with NIR‐active dyes which is compatible with a cobalt electrolyte is important to enhance the photovoltaic performance. In this report, alkyl‐group‐wrapped donor‐acceptor‐donor (D‐A‐D) based unsymmetrical squaraine dyes have been used for DSSC device characterization with a cobalt electrolyte. Surface passivation of photoanode was varied systematically by the extent of functionalization with alkyl groups to avoid charge recombination. DSSC device performance of 5.92 % was achieved for an alkyl‐group‐wrapped squaraine dye with a cobalt electrolyte. Hence, appending the alkyl groups on the donor unit of squaraine dyes helps passivating the photoanode, whereas introducing hydrophilic groups provides a leaky surface where oxidized electrolyte species reach the titanium‐metal‐oxide surface which promotes the charge recombination process.