Squaraine dyes have been paid much attention because of the prominent light absorption property and electron-donating characters. The range of absorption depends on the substitution on 1- and 3-position of cyclobutene core. The installing π-expanded heterocyclic components provides squaraine dyes with near-infrared (NIR) absorption capability, leading to the development of NIR materials for organic photovoltaics (OPVs).1,2 NIR absorbing donor materials are expected to lead not only the improvement of the light-harvesting feature of OPVs, but also the development of transparent OPVs. In this paper, synthesis and properties of NIR absorbing squaraine dyes bearing benzo[c,d]indolenine moiety were studied for their application to donor materials for OPVs Conventional squaraine dyes with benzo[c,d] indolenine moiety (SQBI) and their analogues with dicyanovinylene group on 2-position of cyclobutene (SQBICN) were synthesized by the condensation of a benzo[c,d]indolenium salt with corresponding squaric acid. Absorption maxima of SQBI were found at 901 nm in chlorobenzene. Whereas, absorption bands of SQBICN was bathochromically shifted due to the effect of dicyanovinylene group and their maxima were observed around 954 nm. Thus, these dyes show sharp and intense absorption in the near-infrared region as expected. Thin films of these dyes prepared on glass substrates exhibit bathochromically shifted absorptions in comparison to those in solutions (Δλmax = 40-70 nm), indicating that the microcrystal which expresses the exciton coupling between adjacent squaraines occurs in film states. X-ray crystallographic analysis revealed that SQBI and dicyanovinylene-substituted SQBICN adopt transoid- and cisoid-like structure, respectively. Energy levels of their frontier orbitals meet requirements for thermodynamically favorable electron transfer into conventional fullerene-based acceptors. The OPVs having these dyes as electron donors exhibited the NIR response in reflect of their absorption property, indicating that these dyes work as donor materials in OPVs. In conclusion, benzo[c,d]indolenine component confers the NIR absorbing property on squaraine dyes. These dyes show favorable energy levels of frontier orbitals and crystallinity on the thin-film state. The test OPV devices revealed their potential for the use of donor materials working on the NIR region. (329 words/max 500 words)
1. Maeda, T.; Nitta, S.; Nakao, H.; Yagi, S.; Nakazumi, H. J. Phys. Chem. C 2014, 113, 16618.
2. Maeda, T.; Shima, N.; Tsukamoto, T.; Yagi, S.; Nakazumi, H. Synth. Met. 2011, 161, 2481.