The synthesis of 1,8-naphthalimide groups containing imidazolium salts/ionic liquids using I  , PF 6  , TFSI  anions and their photophysical, electrochemical and thermal properties Saliha Ozdemir a , Canan Varlikli a, * , Ilker Oner a , Kasim Ocakoglu a, b , Siddik Icli a a Ege University, Solar Energy Institute, 35100 Bornova-Izmir, Turkey b Mersin University, Tarsus Technical Education Faculty, 33480 Tarsus-Mersin, Turkey article info Article history: Received 23 September 2009 Received in revised form 8 January 2010 Accepted 12 January 2010 Available online 4 February 2010 Keywords: Ionic salt Imidazolium Naphthalimide Photostability Quenching Ionic liquid abstract 1,8-Naphthalimide groups containing imidazolium iodide salts of different alkyl chain length on the imidazole group were synthesized. Hexafluorophosphate and bis(trifluoromethanesulfonyl)imide salts were obtained by ion exchange reactions. The synthesized salts were characterized using a variety of spectroscopic techniques. Fluorescence emission quenching of iodide and bis(trifluoromethanesulfonyl) imide salts were studied using a ruthenium dye, whilst that of hexafluorophosphate salts were performed using tris(8-hydroxyquinoline)aluminum. Calculated fluorescence quenching rate constants were as high as 10 14 M 1 s 1 and values of the Gibbs free energy of electron transfer were [32.04/33.68], [17.52/20.74] and [26.50/30.89] kcal mol 1 for the iodide, hexafluorophosphate and bis(trifluoromethanesulfonyl) imide salts, respectively. The thermal stability of the synthesized salts was high especially in the case of bis (trifluoromethanesulfonyl)imide salts for which values of 470e498  C were achieved. Photostability studies of hexafluorophosphate and bis(trifluoromethanesulfonyl)imide salts were also performed; calculated photodegradation half-life values ranged from 19.25 to 48.12 h. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction In recent years, ionic liquids (ILs) have received much attention because of their unique characteristics such as low volatility, non- flammability, high thermal stability, high ionic conductivity and broad operating temperature range [1e6]. Typically, ionic liquids are liquid at low temperature (<100  C) and consist of a bulky, asymmetric organic cation (e.g. imidazolium or pyridinium ions, also alkylphos- phonium and alkylammonium ions) and a smaller inorganic anion. The compounds possess high electrochemical stability and high ionic conductivity [7e12]. Appropriate choices of the anion/cation combination permit the tuning of properties of ionic liquids such as miscibility with water and other solvents, dissolution ability, polarity, viscosity and density. Ionic liquids based on imidazolium salts have been studied extensively as non-volatile electrolytes in nc-TiO 2 based dye sensi- tized solar cells (DSSCs), owing to their good stability and high ionic conductivity [13e17]. Recently, imidazolium ionic liquids and organic salts have been used in organic light-emitting devices (OLEDs) as a means of enhancing the efficiency of such devices as a result of improved charge injection [18,19]. It has been reported that using imidazolium ionic liquid in thin film transistors as an electrolyte component improves device characteristics [20]; investigations into the use of imidazolium based ILs for capacitors, fuel cells and batteries have been carried out [4,21e25]. Naphthalimide derivatives, which are of interest in various scien- tific and technological fields, were chosen because of their excellent photophysical and photochemical properties. The compounds enjoy use as dyes for organic solar cells [26], fluorescent dyes for solar energy collectors [27], organic light-emitting diodes and organic field effect transistors as n-type semiconductors [28,29], fluorescent markers for medical and biological purposes [30] (such as potential HIV drugs, cell makers, DNA-cleaving agents), laser dyes [31] and liquid-crystal additives for electro optical displays of the guest-host type [32,33]. This paper concern the synthesis of a series of novel imidazolium salts with the naphthalimide functional group attached to the imi- dazolium cation, employing iodide (I  ), hexafluorophosphate (PF 6  ) and bis(trifluoromethanesulfonyl)imide (TFSI  ) as anions. The salts were characterized using spectroscopic, electrochemical and thermal methods; some of the salts can be considered as ionic liquids since they have melting points <100  C. The major focus of this work was to investigate novel imidazolium salts/ionic liquids * Corresponding author. Tel.: þ90 232 3886025; fax: þ90 232 3886027. E-mail address: canan.varlikli@ege.edu.tr (C. Varlikli). Contents lists available at ScienceDirect Dyes and Pigments journal homepage: www.elsevier.com/locate/dyepig 0143-7208/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.dyepig.2010.01.005 Dyes and Pigments 86 (2010) 206e216