Squaraine chemistry. A new approach to symmetrical and unsymmetrical photoconductive squaraines. Characterization and solid state properties of these materials

A new approach to 1-(p-dimethylaminophenyl)-2-hydroxycyclobutene-3,4-dione (4), a precursor for the synthesis of symmetrical and unsymmetrical photoconductive squaraines, is described. In situ generation of p-nitrophenyl ketene, by the reaction of p-nitrophenylacetyl chloride and triethylamine in the presence of tetraethoxyethylene in anhydrous ether, results in a (2 + 2) cycloaddition between the ketene and the electron-rich olefin. The cycloadduct generated, 6, was hydrolyzed to give 1-(p-nitrophenyl)-2-hydroxycyclobutene-3,4-dione (5) in 51% yield. Reductive alkylation of 5 by hydrogen in the presence of formaldehyde in DMF at 50–60 °C affords 4 in 81% yield. The overall yield of 4 is 41% from tetraethoxyethylene and is significantly better than other preparative procedures of 4 based on a partial Friedel–Crafts reaction or an arylation reaction of a squaric acid derivative with an aniline, where yields ranging from 8.5% to 24% were reported. Condensations of 4 with 2 equivalents of N,N-dimethylaniline and its derivatives in refluxing 2-propanol in the presence of 6 equivalents of tributyl orthoformate yield symmetrical squaraine 1a and unsymmetrical squaraines 1b–1e. The synthesis is accomplished by a (2 + 2) cycloaddition – reductive alkylation – condensation reaction sequence and represents a new, cost-effective synthesis for photoconductive squaraines where the expensive squaric acid reactant is bypassed. The spectroscopic and the solid state properties of 1a–1e have been studied. Results show that the properties exhibited by 1a–1e are essentially identical to those exhibited by other photoconductive, symmetrical squaraines that are synthesized by other known processes. From the resemblance in solid state properties, 1a–1e are expected to be photoconductive. This has indeed been observed in preliminary experiments.