Pyrrolyl Squaraines–Fifty Golden Years

Multivariate Synthetic Strategy for Improving Crystallinity of Zwitterionic Squaraine-Linked Covalent Organic Frameworks with Enhanced Photothermal Performance

Two-dimensional covalent organic frameworks (2D COFs) offer a designable platform to explore porous polyelectrolyte frameworks with periodic ionic skeletons and uniform pore channels. However, the crystallinity of ionized 2D COF is often far from satisfactory as the electrostatic assembly of structures impedes the ordered layered arrangement. Here, a multivariate synthetic strategy to synthesize a highly crystalline squaraine (SQ)-linked zwitterionic 2D COF is proved. A neutral aldehyde monomer copolymerizes with squaric acid (SA) and amines in a controlled manner, resulting in the ionized COF with linkage heterogeneity in one tetragonal framework. Thus, the zwitterions of SQ are spatially isolated to minimize the electrostatic interaction and maintain the highly ordered layered stacking. With the addition of 85%-90% SA (relative to a total of aldehydes and SA), a fully SQ-linked zwitterionic 2D COF is achieved by the in-situ conversion of imine to SQ linkages. Such a highly crystalline SQ-linked COF promotes absorptivity in a full spectrum and photothermal conversion performances, and in turn, it exhibits enhanced solar-to-vapor generation with an efficiency of as high as 92.19%. These results suggest that synthetically regulating charge distribution is desirable to constitute a family of new crystalline polyelectrolyte frameworks.

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.

Computational Study of the Influence of Nitrogen-Containing Unsaturated Heterocyclic Substituents on Electronic and Spectroscopic Properties of Squaraine Derivatives

A comprehensive understanding of the influence of the different structural elements for the molecular properties is crucial for improving the material design procedures in the field of photosensitive dyes. The present study provides a detailed analysis of the influence of the number of heteroatoms (nitrogens) and the lengths of the π-electron skeleton on the one- and two-photon absorption of the symmetric squaraine dyes. Extended computational study covers the conventional vertical excitation calculations within the TD-DFT formalisms as well as several advanced single-reference methods including double excitations such as CIS(D), SAC-CI, and ADC(2). Additionally, the weaknesses of the vertical approach are investigated by including the geometry relaxation upon excitation via adiabatic and 0-0 treatment.

Potential Use of Squarates and Croconates as Singlet Fission Sensitizers

The geometrical and electronic structures of 44 squarate and croconate derivatives are computationally studied by quantum chemistry methods, in the pursuit of new singlet fission sensitizers. A non-negligible singlet open-shell diradical character is observed for most of the studied molecules, which can be controlled through chemical substitution as well as by the size of the central ring. Such a diradical character is related to small singlet-triplet energy gaps, facilitating the accomplishment of the singlet fission energetic requirements. In general, the present results indicate that squarates hold superior singlet fission capabilities than croconates, although we have identified several derivatives within both families as promising singlet fission sensitizers.

Adsorptive Properties of Poly(1-methylpyrrol-2-ylsquaraine) Particles for the Removal of Endocrine-Disrupting Chemicals from Aqueous Solutions: Batch and Fixed-Bed Column Studies

The adsorptive properties of poly(1-methylpyrrol-2-ylsquaraine) (PMPS) particles were investigated in batch and column adsorption experiments as alternative adsorbent for the treatment of endocrine-disrupting chemicals in water. The PMPS particles were synthesised by condensing 3,4-dihydroxycyclobut-3-ene-1,2-dione (squaric acid) with 1-methylpyrrole in butanol. The results demonstrated that PMPS particles are effective in the removal of endocrine disrupting chemicals (EDCs) in water with adsorption being more favourable at an acidic pH, and a superior sorption capacity being achieved at pH 4. The results also showed that the removal of EDCs by the PMPS particles was a complex process involving multiple rate-limiting steps and physicochemical interactions between the EDCs and the particles. Gibbs free energy of −8.32 kJ/mole and −6.6 kJ/mol, and enthalpies of 68 kJ/mol and 43 kJ/mol, were achieved for the adsorption E2 and EE2 respectively The removal efficiencies of the EDCs by PMPS particles were comparable to those of activated carbon, and hence can be applied as an alternative adsorbent in water treatment applications.

The History of Azulenyl Squaraines

The synthesis of the first azulenyl squaraine dye was reported in 1966 and the paucity of activity in the scientific literature since that time has given the false impression that they have remained relatively unexplored as organic dyes. In contrast, a wealth of research activity on azulenyl squaraines has been recorded in the patent literature and it is the purpose of this review to bring these reports to the attention of scientists that have an interest in the chemistry and applications of squaraine dyes. This review shows that it is possible to prepare, via targeted substitution of the azulene ring, a dye with an absorption maximum value ranging from 650 to 840 nm.