Synthesis of Novel Quinaldine-Based Squaraine Dyes: Effect of Substituents and Role of Electronic Factors Kuthanapillil Jyothish, Kalliat T. Arun, and Danaboyina Ramaiah* Photosciences and Photonics DiVision, Regional Research Laboratory (CSIR), TriVandrum 695 019, India d_ramaiah@rediffmail.com; rama@csrrltrd.ren.nic.in Received August 11, 2004 ABSTRACT Condensation of squaric acid with quinaldinium salts containing electron-donating substituents gave only the semisquaraines. However, with salts possessing electronegative and electron-withdrawing groups, the squaraine dyes were isolated in quantitative yields. The semisquaraines formed undergo condensation with highly nucleophilic salts yielding the unsymmetrical squaraine dyes. These results demonstrate the role of electronic factors and provide valuable information for the design of efficient squaraine-based sensitizers that can have potential applications in photodynamic therapy. Squaraines form a class of dyes possessing sharp and intense absorption bands in the red to near-infrared region. 1 The photophysical and photochemical properties of these have been studied extensively, 1,2 because their absorption and photochemical characteristics make them highly suitable for a number of industrial applications. These include, photo- receptors in copiers, 3 photoconductors in organic solar cells, 4 and IR absorbers in organic optical disks. 5 The semisquaraine derivatives and the squaraine dyes also find application as sensors for metal ions and biologically important molecules. 6 Due to the very low intersystem crossing efficiency of these dyes, 1,2 their potential as sensitizers in photodynamic therapy (PDT) 7 has not yet been explored. Our objective has been to explore the use of suitable squaraine dyes as sensitizers for PDT. In this context, we (1) (a) Law, K. Y. J. Phys. Chem. 1987, 91, 5184. (b) Law, K. Y. Chem. ReV. 1993, 93, 449. (c) Liang, K.; Law, K. Y.; Whitten, D. G. J. Phys. Chem. B 1997, 101, 540. (d) Das, S.; Thomas, K. G.; George, M. V. Mol. Supramol. Photochem. 1997, 1, 467. (2) (a) Liang, K.; Farahat, M. S.; Perlstein, J.; Law, K. Y.; Whitten, D. G. J. Am. Chem. Soc. 1997, 119, 830. (b) Kamat, P. V.; Das, S.; Thomas, K. G.; George, M. V. J. Phys. Chem. 1992, 96, 195. (c) Das, S.; Thomas, K. G.; Kamat, P. V.; George, M. V. J. Phys. Chem 1994, 98, 9291. (d) Chen, H.; Farahat, M. S.; Law, K. Y.; Whitten, D. G. J. Am. Chem. Soc. 1996, 118, 2584. (3) (a) Law, K. Y.; Bailey, F. C. J. Imaging Sci. 1987, 31, 172. (b) Tam, A. C.; Balanson, R. D. IBM J. Res. DeVelop. 1982, 26, 186. (4) (a) Loufty, R. O.; Hsiao, C. K.; Kazmaier, P. M. Photogr. Sci. Eng. 1983, 27, 5. (b) Merritt, V. Y. IBM J. Res. DeVelop. 1978, 22, 353. (c) Piechowski, A. P.; Bird, G. R.; Morel, D. L.; Stogryn, E. L. J. Phys. Chem. 1984, 88, 934. (d) Liang, K. N.; Law, K. Y.; Whitten, D. G. J. Phys. Chem. 1995, 99, 16704. (5) (a) Fabian, J.; Nakazumi, H.; Matsuoka, M. Chem. ReV. 1992, 92, 1197. (b) Emmelius, M.; Pawlowski, G.; Vollmann, H. W. Angew. Chem., Int. Ed. Engl. 1989, 28, 1445. (6) (a) Xie, J.; Comeau, A. B.; Seto, C. T. Org. Lett. 2004, 6, 83. (b) Ros-Lis, J. V.; Garcia, B.; Jimenez, D.; Martinez-Manez, R.; Sancenon, F.; Soto, J.; Gonzalvo, F.; Valldecabres, M. C. J. Am. Chem. Soc. 2004, 126, 4064. (c) Kukrer, B.; Akkaya, E. U. Tetrahedron Lett. 1999, 40, 9125. (d) Ros-Lis, J. V.; Martinez-Manez, R.; Soto, J. Chem. Commun. 2002, 2248. (7) (a) Dougherty, T. J. Photochem. Photobiol. 1987, 45, 879. (b) Kessel, D. Photodynamic Therapy of Neoplastic Disease; CRC Press: Boca Raton, FL, 1990; Vol. 2. (c) Moser, J. G. Photodynamic Tumor Therapy: 2nd and 3rd Generation Photosensitizers; Harwood Academic Publishers: Amsterdam, 1998. (d) Bonnett, R. Chemical Aspects of Photodynamic Therapy; Gordon and Breach Science Publishers: The Netherlands, 2000. ORGANIC LETTERS 2004 Vol. 6, No. 22 3965-3968 10.1021/ol048411y CCC: $27.50 © 2004 American Chemical Society Published on Web 09/30/2004