Effect of protonation on the photophysical properties of meso-tetra(sulfonatophenyl) porphyrin P.J. Gonc¸alves a , L. De Boni b , N.M. Barbosa Neto b , J.J. Rodrigues Jr. a,b , S.C. Zı ´lio b , I.E. Borissevitch a, * a Faculdade de Filosofia Cieˆncias e Letras de Ribeira˜ o Preto, Universidade de Sa˜ o Paulo, Departamento de Fı´sica e Matema´ tica, Av. Bandeirantes 3900, Vila Monte Alegre, 14040-901, Ribeira˜ o Preto, SP, Brazil b Instituto de Fı´sica de Sa˜ o Carlos, Universidade de Sa˜ o Paulo, Caixa Postal 369, 13560-970 Sa˜ o Carlos, SP, Brazil Received 8 October 2004; in final form 8 March 2005 Available online 14 April 2005 Abstract We present an experimental investigation on the photophysical properties of meso-tetra(sulfonatophenyl) porphyrin (TPPS 4 ) in its bi-protonated and deprotonated states in water solutions at pH 4.0 and 7.0, respectively. The Z-scan technique at 532 nm with single 70 ps pulses as well as with pulse trains, was used in association with other complementary techniques such as UV–Vis absorp- tion spectroscopy and time resolved fluorescence. The saturation behavior and the time evolution of nonlinear optical absorption were analyzed with a five-energy-level diagram. Photophysical parameters such as ground and excited states absorption cross- sections, together with the fluorescence lifetime and intersystem crossing time were determined. Ó 2005 Elsevier B.V. All rights reserved. 1. Introduction Application of porphyrins and porphyrin-like com- pounds in medicine has increased significantly over the last decade due to interesting properties of natural and synthetic porphyrins and their derivatives, especially due to their efficacy in photodynamic therapy (PDT) of cancer [1,2], psoriasis, atheromatous plaque, viral and bacterial infections including HIV [3,4], and blood substitutes (see review [5] and references therein). Among them the synthetic water-soluble meso-tetrakis (p-sulfonatofenyl) porphyrin (TPPS 4 ) is studied and used in clinical experiments as a promising sensitizer for PDT [6,7]. Besides this TPPS 4 possesses nonlinear optical absorption [8], which can lead to its application in photonics devices such as optical limiters [9] and switches [10]. The porphyrin efficacy in PDT, as well as their non- linear optical characteristics, depends on the porphyrin excited (singlet and triplet) state lifetimes and quantum yields [1,2], which in turn depend on the environmental characteristics, such as pH, ionic strength etc. So, due to the presence of nitrogen atoms, the TPPS 4 molecule can be protonated in acid media, which changes drastically its spectral and energetic properties [11–13]. Thus, in order to apply these materials in photonics and medicine with higher efficacy, a complete determination of their photophysical parameters and their dependence on external conditions are required. The present work investigates the influence of proton- ation on characteristics of excited states of TPPS 4 . To accomplish this task, we have employed the Z-scan tech- nique with both single 70 ps pulses and the entire Q- switch envelope containing about 20 mode-locked 70 ps pulses separated by 13 ns intervals. We have also 0009-2614/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.03.100 * Corresponding author. Fax: +55 016 633 9949. E-mail addresses: iourib@ffclrp.usp.br, iouribor@usp.br (I.E. Borissevitch). www.elsevier.com/locate/cplett Chemical Physics Letters 407 (2005) 236–241