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 tetraethoxyediylene 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-degrees-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.