Enhancement of solubility via esterification: Synthesis and characterization of octakis (ester)-substituted phthalocyanines Barbaros Akkurt, Esin Hamuryudan * Chemistry Department, Istanbul Technical University, Faculty of Science and Letters, Maslak, TR-34469 Istanbul, Turkey Received 4 November 2007; received in revised form 30 January 2008; accepted 1 February 2008 Available online 8 February 2008 Abstract Metal-free and metallophthalocyanines (M ¼ Zn, Ni and ClFe) carrying eight hydroxyethylsulfanyl groups at peripheral positions were prepared from 4,5-bis(hydroxyethylsulfanyl)phthalonitrile. The reactivity of the hydroxyethyl groups was demonstrated by esterification of phthalocyanine derivatives with pyridine-4-carboxylic acid and also ferrocenecarboxylic acid in the presence of dicyclohexylcarbodiimide com- bined with N,N-dimethylaminopyridine or p-toluenesulfonic acid and acetic anhydride. Unlike the parent phthalocyanines, the symmetrically functionalized phthalocyanines with eight ester units were soluble in common organic solvents such as CHCl 3 , THF, pyridine and DMF, spar- ingly soluble in ethanol and acetone and insoluble in water and hexane. The newly synthesized compounds were characterized by elemental analysis, IR, UVevis, FAB-MS and 1 H NMR spectroscopy. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: Phthalocyanine; Esterification; Ferrocenecarboxylic acid; Pyridinecarboxylic acid; Zinc 1. Introduction Among tetrapyrrole compounds, phthalocyanines which are full-aromatic planar molecules due to their 18-p electron structure are not only capable of undergoing classical dis- placement reactions, but they can also be substituted with a great deal of functional groups. In addition to their extensive use as dyes and pigments, this versatility provides many appli- cations, such as liquid crystals, catalysts, electrochromic and photochromic substances, data storage systems, photodynamic cancer therapy agents, photoactive units, chemical sensors, and nonlinear optical devices [1e3]. Solubility is a key feature for phthalocyanines and most of their behaviors are best investigated in soluble form. Because parent unsubstituted metal-free and metallophthalocyanines and most of the metal polymeric phthalocyanines are insoluble in common organic solvents, the synthesis of new phthalocy- anine systems should be essentially designed in such a way so that the final phthalocyanine derivatives are sufficiently soluble to perform the desired activities. A common means for preparing soluble phthalocyanines is to attach functional groups like tertiary butyl or hexyl groups, amide groups, carboxylic acid and sulfonic acid groups [1e16], bulky and crown ether groups [17e23], azo groups [24], etc. at the pe- ripheral and axial positions of phthalocyanine ring. Compared to unsubstituted parent metal phthalocyanines, ester-contain- ing porphyrazines and phthalocyanines are highly soluble in chlorinated hydrocarbons [25e28]. A further step for phthalocyanine esters is the possibility to form supramolecular structures with donor groups on the ester moiety. For instance, pyridyl-containing esters can coordinate metal ions with their nitrogen [29]. Another potential feature is to quaternize the terminal group, which is useful in providing an additional aqueous solubility [30]. The applications of ester-containing phthalocyanines are very variable. For example, some esters [31] showed gas- sensor response against NO x gases, whereas some showed characteristics of liquid crystals with glassy transitions [23,32]. Furthermore, some patents and publications reported * Corresponding author. Tel.: þ90 212 285 6836; fax: þ90 212 285 6386. E-mail address: esin@itu.edu.tr (E. Hamuryudan). 0143-7208/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.dyepig.2008.02.001 Available online at www.sciencedirect.com Dyes and Pigments 79 (2008) 153e158 www.elsevier.com/locate/dyepig