Nonlinear Optical Properties of Water-Soluble Polymeric Dyes with Biological Applications O. Varnavski, † R. G. Ispasoiu, † M. Narewal, † J. Fugaro, ‡ Y. Jin, † H. Pass, ‡ and T. Goodson III* ,† Department of Chemistry, Wayne State University, Detroit, Michigan 48202, and Department of Surgery, Wayne State University School of Medicine, Detroit, Michigan 48202 Received November 4, 1999; Revised Manuscript Received March 20, 2000 ABSTRACT: The nonlinear optical properties of two water-soluble polymeric dyes, poly(R-478) (an anionic anthraquinone dye) and poly(S-119) (an anionic azo dye), are investigated utilizing nanosecond and femtosecond optical techniques such as z-scan, nonlinear transmission, and time-resolved luminescence. Poly(R-478) showed large nonlinear refraction when studied with femtosecond laser pulses at 800 nm, with a value for the intensity-dependent refractive index (n I) of 1.26 × 10 -4 cm 2 /GW. The thin film result at 1064 nm indicated a large nonlinear absorption in both polymeric dyes. The origin of the large fast optical nonlinearities in the polymeric system S-119 was investigated by probing the different functional groups of the polymer. The chromophore group (Sunset Yellow) of the S-119 polymer showed a smaller nonlinear response when compared to the polymer result. Significant differences in the electronic dynamics between the parent poly(S-119) and the chromophore Sunset Yellow were observed by time-resolved luminescence spectroscopy. The applications of the NLO effects in these polymers are demonstrated for laser ablation of A549 lung carcinoma cells. Introduction Conjugated polymers are now important materials for potential applications in nonlinear optics (NLO), optical limiting, light emitting diodes, and photorefractive effects. 1-4 The advances in experimentation and con- ceptual understanding obtained from the vast research of NLO polymeric materials provide a strong basis for further investigations of new and useful macromolecular materials. Applications in self-assembled monolayers, NLO effects from surfaces, and the quantitative analyti- cal characterization of materials and substances have already been demonstrated. 5-8 However, there have been no reports concerning the application of NLO effects in organic polymers to the characterization and ablation of biological materials. There are several factors that may inhibit the use of organic NLO polymers in biological systems. There is a synthetic limitation due to the fact that most NLO organic polymers are not soluble in the aqueous phase. It is essential that organic polymers have suitable solubility with the biological system (in water) in order to probe real processes in the biological host. Another limiting factor is that many organic polymers possess structural characteristics that are toxic to the biological system used in the study. Indeed, it is important to analyze the biological material in its full and living condition without the difficult and confusing circum- stance of cell death due to toxicity of the NLO polymer. The third major limitation of applying NLO effects to the characterization of biological systems is that the intensities necessary to detect large NLO effects in many materials are too large and potentially destructive to the biological host. It is crucial to find NLO polymers with very large nonlinear coefficients that require only small average powers (but perhaps larger peak intensi- ties) to be used in order to detect appreciable NLO effects. All of these limitations should be overcome before significant advances in the application of organic polymeric NLO effects (and the understanding of the effects) to biological systems can be accomplished. Recently, there have been advances toward the syn- thetic versatility of novel conjugate polymers with large NLO effects in the aqueous phase. Tripathy and co- workers 9 have reported large NLO effects with an epoxy-based polymer with NLO azo chromophores de- signed to contain anionic groups to induce water solu- bility and self-assembly. Using this polyanion with a polycation, multilayers were prepared on a glass sub- strate by alternating adsorption from dilute aqueous solutions. Heflin et al. 5 have utilized water-soluble NLO polymers for an ionically self-assembled monolayer (ISAM) technique for thin-film deposition. The second- harmonic intensity of the films exhibits the expected quadratic dependence on film thickness up to at least 100 bilayers, corresponding to a film thickness of 120 nm. 5 Tian et al. 10 have investigated the spectroscopic properties of a supramolecular system consisting of two oppositely charged porphyrin monomers, zinc tetrakis- (4-sulfonoatophenyl)porphyrin and zinc tetrakis(N-me- thylpyridyl)porphyrin, by absorption and fluorescence spectroscopy. Photoinduced electron transfer and charge separation in the supramolecular system was studied by time-resolved degenerate four-wave mixing (DFWM), and the results indicated the formation of the charge- separated state within 30 ps and the recombination time is about 340 ps. There have been a number of other reports concerning the NLO effects of self-assembly systems based on water-soluble polymers such as the work by Fox et al. 11 in which molecules with cores of copper and nickel (octaazaphthalocyanines) are fused to four nonracemic helicenes. 11 The Langmuir-Blodgett films constructed from these systems give very large second-order NLO responses. 12 It is worth noting that † Wayne State University. ‡ Wayne State University School of Medicine. * To whom all correspondence should be sent. E-mail: tgoodson@ chem.wayne.edu. 4061 Macromolecules 2000, 33, 4061-4068 10.1021/ma991868g CCC: $19.00 © 2000 American Chemical Society Published on Web 04/27/2000