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The bis{4-{2-[2-(dialkylamino)thiazol-5-yl]ethenyl}-2,6-dihydroxyphenyl}squaraines 12a,b were synthesized from ethyl carbonochloridate (1) in six steps (Scheme). The donor ± acceptor ± donor systems 12a,b are dark blue dyes with absorption maxima in the NIR region, unless the measurements are performed in the presence of EtOH. In the latter case, the long-wavelength band disappears, and the absorption in the UV region is strongly enhanced. The l max values in CHCl 3 and CHCl 3 /EtOH differ by more than 450 nm. The completely reversible effect can be rationalized by the reversible degradation of intramolecular H-bonds and a consequent torsion between the acceptor and the donor moieties. In contrast to 1,3-diarylsquaraines, which represent blue or blue-green pigments or dyes, we prepared some time ago 1-aryl-3-stilbenyl-and 1,3-bis(stilbenyl)squaraines, which absorb in the near infrared (NIR) [10 ± 13]. It turned out that the extension of the conjugation provokes first a strong bathochromic effect, but surprisingly further extension of the conjugation leads to a hypsochromic effect [10]. Thus, a donor ± acceptor distance as in stilbenoid moieties seems to be highly appropriate for an absorption in the NIR region, which is characterized by a strong intramolecular charge transfer (ICT). We studied in this context 2-(dialkylamino)thiazol-5-yl substituents, which are bound via ethenediylphenylene segments to the squaraine center. The heterocyclic building blocks were chosen to improve the photoconductivity induced by NIR radiation. Squaraines exhibit normally a good hole transport [1b]. The heterocyclic moieties should ameliorate the efficiency for the electron transport.

Extended Conjugation in Stilbenylsquaraines Unsymmetrical 1‐aryl‐3‐stilbenylsquaraines 2a–e and 1,3‐bis(stilbenyl)squaraines 3a–c were prepared by CC coupling reactions of the corresponding substituted arenes with derivatives of squaric acid (Schemes 2 and 3). Dialkylamino and alkoxy groups enhance the solubility of these dyes and enlarge the intramolecular charge transfer of these donor–acceptor–donor systems. The extended conjugation of the stilbene units – in comparison with arene building blocks – leads to significant bathochromic shifts in the Vis/NIR absorption spectra.