Squaraines based on 2-arylpyrroles

Various derivatives of 2-phenylpyrrole were condensed with squaric acid to give the corresponding squaraines. The products are drawn with the anti geometry rather than the syn geometry generally shown in the past: the arguments for this formulation are given, including the analogy with 2,5-bis(4-ethyl-3,5-dimethylpyrrol-2-yl)-1,4-benzoquinone, for which an X-ray structure is presented. Solutions of the new bis(5-arylpyrrol-2-yl)squaraines have intense, sharp absorption bands shifted to the red. Condensation of squaric acid with arylpyrroles possessing fused ring systems, and condensation with 2-styrylpyrrole, gave chromophores with high values for λmax and ɛmax. Certain of these chromophores appear to be suitable for further structural elaboration to give materials having potential in optoelectronic and photodynamic applications.