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...