Exciton Coupling Effects in Polymeric cis-Indolenine Squaraine Dyes

Abstract

Dicyanovinylene-substituted cis-indolenine squaraines were polymerized by a Ni-mediated Yamamoto homocoupling reaction. Preparative recycling GPC was used to obtain polymer fractions with different molecular weight distributions in order to investigate the influence of molecular weight on the optical properties. In this attempt, conjugated cyclic trimers could also be isolated that are, to the best of our knowledge, the first of their kind. These trimers and the squaraine polymers were investigated by cyclic voltammetry, absorption spectroscopy, and static and time-resolved fluorescence spectroscopy. While the trimers show multiple fluorescence bands with, for squaraines, an exceptional huge Stokes' shift, the polymers show broad absorption in the red to near infrared (NIR) region with narrow fluorescence and only the common minor Stokes' shift. The spectral features of the squaraines were interpreted to be caused by excitonic coupling. By comparison of experimental and computed (CNDO/S2 CIS) spectra detailed, structural models of the polymers were derived that consist of helix (H-aggregate behavior) and zigzag motifs (Jaggregate behavior). ■ INTRODUCTIONThe goal of this work is to synthesize polysquaraine dyes that are based on dicyanovinylene indolenine squaraine monomers and to study their optical and electrochemical properties. Squaraines (Scheme 1) are intensely colored dyes that are generated by 1,3-dicondensation reactions of nucleophilic compounds such as heterocycles or other electron-rich aromatics with squaric acid. This results in a donor− acceptor−donor structure with a strong and narrow cyaninelike absorption (ε > 10 5 M −1 cm −1 ) and fluorescence in the red to near-infrared (NIR) region. These unique absorption and fluorescence properties make squaraines suitable for a variety of applications in nonlinear optics, 1−7 data imaging, 8 organic photovoltaics, 9−17 or as fluorescence labels 18−21 and sensors for anions, 22 cations, 23−27 and neutral molecules. 28,29 There are several highly recommended reviews that give more-detailed insight into this remarkable class of dye. 30−33 The absorption maximum of squaraine dyes can be influenced by the nucleophilic component (e.g., anilines, pyrroles, indolenines and its heteroatom analogues), by replacing one oxygen of squaric carbonyl group by, e.g., sulfur 34,35 or methylene active moieties, such as dicyanomethylene 9,34,36 or barbituric acid, 34 and finally by adding electron-donating substituents to the nucleophilic (hetero)cycle component. We recently investigated the latter aspect, using triarylamine substituents attached to a series of indolenine squaraine dyes. 37 These triarylamine−squaraine conjugates display a bathochromic shift of the absorption maximum and versatile redox behavior. A totally different way to shift the absorption of squaraine dyes is to synthesize oligomers or polymers that then show band shifts and broadenings due to excitonic coupling of localized squaraine states. In this context, we recently prepared polymers based on indo...

Exciton Transport in Molecular Aggregates – From Natural Antennas to Synthetic Chromophore Systems

“…However, the [SQB] n polymer shows a quite unusual spectroscopic behaviour. [224] As can be seen in Figure 14, the shape of the absorption band manifold is strongly dependent on the solvent. No correlation with any solvent property (polarity, viscosity, Hildebrand parameter, etc.)…”

“…Similar to the SQA oligomers, the dimer [SQB] 2 , the trimer [SQB] 3 and the polymer [SQB] n display a progressive red-shift of the lowest energy absorption band compared to the monomer. [223,224] The monomer and the trimer also show high fluorescence quantum yields and were used as efficient emitters in NIR OLED devices. [225,226] Again, as in the SQA cases the exciton coupling (estimated from the bandwidth) decreases slightly with the number of SQB chromophores (dimer: 800 cm −1 , trimer: 636 cm −1 , polymer: 620 cm −1 ).…”

Squaraine‐Based Polymers: Toward Optimized Structures for Optoelectronic Devices

“…Synthesis of pi‐conjugated polymers comprising squaraine building blocks is a promising approach to materials with small optical band‐gap featuring attractive properties for device applications . In particular, Ajayaghosh and Eldo reported the polycondensation of 1,4‐dialkoxydivinylbenzene‐bridged pyrrole derivatives (1‐methyl‐ and 1‐dodecylpyrrole) and squaric acid to access a series of squaraine‐based polymers with optical band gaps as low as 0.79 eV and excellent absorption range spanning the UV, visible, and near infrared region (300–1300 nm) .…”

Energy Transfer Between Squaraine Polymer Sections: From Helix to Zigzag and All the Way Back

“…[72][73][74][75][76][77][78][79][80] Unlike conjugated polymers that are based on very small monomers such as styrene (e.g., MEH-PPV) or thiophene (e.g., P3HT) and whose polymer properties are totally different from those of the monomers, 81 squaraine homo-and copolymers are based on squaraine dyes which already show a strong absorption in the red region of the visible spectrum. [32][33][34][35][36]82 The optical properties of the squaraine polymer, although distinct from those of the monomeric dye, can be explained by exciton coupling of localized squaraine chromophore transition moments which in general leads to broadened and red-shifted spectra reflecting the excitonic manifold of states.…”