Photophysical study of Zn phthalocyanine in binary solvent mixtures A. Staicu a,⇑ , A. Pascu a , M. Boni a , M.L. Pascu a , M. Enescu b a Laser Department, National Institute for Lasers, Plasma, and Radiation Physics, Atomistilor 409, 077125 Magurele, Bucharest, Romania b UFR-ST Laboratoire Chrono-Environnement UMR CNRS 6249, Université de Franche-Comté, 16 Route de Gray, 25030 Besancon Cedex, France article info Article history: Available online 19 December 2012 Keywords: Zn phthalocyanine Photophysics DMSO/water solvent abstract Photophysical properties of phthalocyanines are important in photodynamic therapy, where these com- pounds are proposed as photosensitizing agents. We report here some significant solvent effects on the photophysical properties of Zn phthalocyanine (ZnPc) observed in binary solvent mixture dimethyl sulf- oxide/water at several ratios of cosolvents. The absorbance of ZnPc at the maximum of Q band has a sharp drop in intensity for a water mass percent in the solvent mixture larger than 40%. The same characteristic shows also the quantum yield of fluorescence. A particular result is the increase of singlet oxygen lifetime for water percentage raise up to 20% in the solvent mixture. The effects are discussed in connection with the particular solvent microenvironment, involving DMSO/water clusters formation and the strong inter- action between the solute and the solvent. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction Phthalocyanines constitute a class of organic compounds with wide application in different fields as dyes and pigments, solar cells [1], optical materials [2], sensors, electronic devices [3]. They are also proposed as photosensitizers in photodynamic therapy (PDT). The photophysical properties of phthalocyanines are impor- tant to be determined in order to establish their efficiency in PDT. It is known that unsubstituted phthalocyanines show in aqueous solution a high aggregation tendency [3]. Sulphonated phthalocyanine photosensitizers for PDT were widely synthesized because of the advantage of their water solubility [4–6]. New methods of drug delivery developed in the last years as liposomal formulation, encapsulation or conjugation with nanoparticles [7–9] were employed also for phthalocyanines in order to over- come the water solubility problem. These allow a reconsideration of the potential role of basic metallophthalocyanines as photosen- sitizers, especially taking into account their higher efficiency in generating active species. Among metallated phtahlocyanines, Zn phthalocyanine (ZnPc) has a high quantum yield of singlet oxygen generation and it was used in different drug delivery vehicles as liposomes and nanopar- ticles [7–9]. On the other hand, DMSO is a solvent that is often used as drug delivery support due to its efficient penetration in the tissues [10]. DMSO/water is one of the most interesting solvent mixtures, show- ing a strong non-ideal behavior. Shin et al. observed [11,12] by mass spectrometry that the cluster formation between water and DMSO molecules is nonlinearly dependent on the solvent compo- sition; at certain ratios of mixing, drastic changes in the micro- scopic solvent structure take place. It was also demonstrated that in such a solvent mixture the solvation depends on the microscopic solvent structures, implying that solute species interact with the solvent clusters, rather than with individual solvent molecules. Re- ports were also made regarding theoretical studies on the binary mixture DMSO/water in order to explain the nonlinear and non- aditive behavior of cluster formation [13,14]. On the other side, the studies of Marcus [15] showed only milder deviations in cosol- vent aggregation. Several reports were made regarding studies about photosensi- tive dyes in binary solvent mixtures and their photophysical behavior. Absorption spectra of a benzoporphyrin derivative in DMSO/water mixture showed, starting from about 40% upwards water in solvent, the dimer formation [16]. In the case of diphenyl- hexatriene in solvent mixtures containing water and DMSO, the absorption of the compound versus water content of the solvent showed a steep drop at about 46% water in solvent assigned to aggregates formation [17]. Studies on rotational diffusion of cou- marins in aqueous DMSO solutions were performed by Inamdar et al. [18]. Also, studies on hypericin aggregation in DMSO/water mixture were recently reported [19]. We report here a photophysical study of ZnPc in binary DMSO/ water solvent mixture. Different spectroscopic techniques were used: absorption spectroscopy for monitoring the UV–vis spectral changes, laser induced fluorescence spectroscopy to assess inten- sity and quantum yield of the ZnPc fluorescence, time-resolved phosphorescence of singlet oxygen spectroscopy to check the 0022-2860/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molstruc.2012.12.010 ⇑ Corresponding author. Tel./fax: +40 21 4575739. E-mail address: angela.staicu@inflpr.ro (A. Staicu). Journal of Molecular Structure 1044 (2013) 188–193 Contents lists available at SciVerse ScienceDirect Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc