Synthesis and molecular properties of acceptor-substituted squaraine dyes

A broad series of more than 20 acceptor-substituted squaraines was synthesized that feature different acceptor functionalities at the central squaraine four-membered ring. The influence of these acceptor units on the reactivity of semisquaraine precursors and stability of the respective squaraines were explored. Thereby the dicyanovinyl group was found to be the most versatile acceptor group that enabled various modifications at the donor moiety of the squaraine scaffold, leading to an extended series of dicyanovinyl-functionalized squaraines. The variation of donor units afforded a set of NIR fluorophores that cover a wavelength region from the visible at about 650 nm far into the NIR up to 920 nm with fluorescence quantum yields between 0.93 and 0.11 and outstanding optical brightness. This excellent optical property is related to a rigid molecular scaffold that is fixed in an all-cis configuration by the additional dicyanovinyl acceptor unit. The change of the molecular symmetry from C(2h) to C(2v) upon functionalization of the squaraine core with dicyanovinyl acceptor group has been confirmed in solution by electro-optical absorption (EOA) spectroscopy, revealing permanent ground-state dipole moments μ(g) in the range between 4.3 and 6.4 D. These dipole moments direct an antiparallel packing of the molecules in the solid state according to single-crystal X-ray analyses achieved for four dicyanovinyl-functionalized squaraines. The structural properties, the EOA results, as well as the band shapes of the optical spectra indicate that these polymethine dyes are cyanine-type chromophores. It is worth noting that the orientation of the dipole moment vectors is orthogonal to the orientation of the transition dipole moment vectors, which is an uncommon but characteristic feature of this rather novel class of polymethine dyes. With regard to applications of these dyes in organic solar cells, their redox properties were also studied by cyclic voltammetry.