Electrochemistry of a Double-Decker Lutetium(III) Phthalocyanine in Aqueous Media. The First Evidence for Five Reductions Karl M. Kadish,* ,² Takashi Nakanishi, ²,‡ Aysegu 1 l Gu 1 rek, § Vefa Ahsen, | and Ismail Yilmaz* ,², ⊥ Department of Chemistry, UniVersity of Houston, Houston, Texas 77204-5003, Department of Applied Chemistry, Faculty of Engineering, Nagasaki UniVersity, Bunkyo-cho, Nagasaki 852-8521, Japan, Marmara Research Center, Department of Chemistry, TUBITAK, P.O. Box. 21, Gebze, Kocaeli 41470, Turkey, Department of Chemistry, Gebze Institute of Technology, P.O. Box. 141, Gebze, Kocaeli 41400, Turkey, and Department of Chemistry, Technical UniVersity of Istanbul, Maslak, Istanbul 80626, Turkey ReceiVed: July 10, 2001 The double-decker lutetium(III) phthalocyanine, Lu[(RS) 4 Pc] 2 , where R ) C 12 H 25 , was synthesized and electrochemically characterized under several solution conditions. The neutral complex undergoes a single reversible one-electron oxidation and up to four reversible one-electron reductions in nonaqueous solutions of CH 2 Cl 2 or benzonitrile containing 0.1 M tetra-n-butylammonium perchlorate (TBAP), while one oxidation and five reductions are seen in aqueous 0.5 M KCl for a gel-like cast film membrane of Lu[(RS) 4 Pc] 2 in a tetraoctylphosphonium bromide (4C 8 P + Br - ) matrix at a basal plane pyrolytic graphite electrode. The half- wave potentials for reduction of Lu[(RS) 4 Pc] 2 in CH 2 Cl 2 , 0.1 M TBAP are located at E 1/2 ) 0.04, -1.03, -1.36 and -1.73 V, while the single oxidation is located at E 1/2 ) 0.43 V vs SCE. These potentials, when measured vs Fc/Fc + , are negatively shifted by 0.20-0.46 V from the E 1/2 values obtained in the 0.5 M KCl solution where the electrogenerated anions of the double-decker complex are stabilized by the cations of the 4C 8 P + Br - matrix and five reductions are seen at E 1/2 ) 0.29, -0.66, -0.94, -1.22, and -1.41 V vs SCE. The fifth reduction has been theoretically predicted but had never before been observed for any double- decker lutetium(III) phthalocyanine or related complex. A cast film of Lu[(RS) 4 Pc] 2 displays only one oxidation in the 0.5 M KCl solution (at E 1/2 ) 0.78 V vs SCE), and no reductions at all are seen in the absence of a matrix. An almost identical oxidation potential of E 1/2 ) 0.80 V is seen for oxidation of Lu[(RS) 4 Pc] 2 in KCl when dissolved in the cationic matrix. These half-wave potentials for oxidation in the cationic film, when measured vs Fc/Fc + , are thus more positive by more than 0.27-0.32 V than E 1/2 values measured in CH 2 Cl 2 or PhCN and this is consistent with a decreased degree of solvent or anion binding to {Lu[(RS) 4 Pc] 2 } + in the 4C 8 P + Br - matrix, as compared to what is observed in solution. Introduction Double-decker phthalocyanines and especially the lutetium derivatives have been the subject of extensive investigations as to their electrochromic properties, 1 their intrinsic semiconduc- tivity, 2 and their third-order optical nonlinearity. 3 Lu(Pc) 2 , where Pc represents the dianion of phthalocyanine, has been shown to undergo up to seven redox processes, five of which have been characterized with respect to the color changes shown in Chart 1. The solution 4 and film 5 electrochemical behavior of double- decker lutetium(III) phthalocyanines have also been extensively investigated under different experimental conditions. Some electrochemical studies were carried out in solutions of meth- ylene chloride (CH 2 Cl 2 ) or benzonitrile (PhCN) containing tetra- n-butylammonium perchlorate (TBAP) or tetra-n-butylammo- nium hexafluorophosphate (TBAPF 6 ), 4 while others were carried out in aqueous LiClO 4 , KClO 4 , or KCl solutions on cast, Langmuir-Blodgett, or spin-coated films of Lu(Pc) 2 , 1g,a,c,f Lu- (R 4 Pc) 2 , or Lu(R 8 Pc) 2 , where R ) 15-crown-5, 5d,f C n H 2n+1 - OCH 2 , 5b,f or C 3 H 7 O. 5e The present paper reports the synthesis and electrochemical characterization of Lu[(RS) 4 Pc] 2 (1), where R ) C 12 H 25 (see Chart 2), under several different solution and experimental conditions. These were (1) in solutions of CH 2 Cl 2 or PhCN containing 0.1 M TBAP and (2) in solutions of 0.5 M KCl as a cast film of Lu[(RS) 4 Pc] 2 on a bare pyrolytic graphite electrode (BPG) and as a cast film of the same compound incorporated into a cationic matrix of 4C 8 P + Br - . The latter measurements provide the first data for the multiple reduction of any double- decker phthalocyanine in aqueous media. Organic gels, 6 a gel-like membrane of a tetraoctylphospho- nium bromide (4C 8 P + Br - ) lipid analogue, 7 and a multilayer of synthetic ammonium lipids 8-10 are all known to provide suitable microenvironments for examining the electrochemistry of redox active molecules, and this has recently been utilized for measuring the reduction potentials and ion-pairing reactions of easily reducible C 60 at an electrode surface in an aqueous medium. 7,9 The same methodology is used in the present paper for examining the multiple reductions of Lu[(RS) 4 Pc] 2 and enabled, for the first time, an experimental determination of ² University of Houston. ‡ Nagasaki University. § TUBITAK. | Gebze Institute of Technology. ⊥ Technical University of Istanbul. CHART 1 9817 J. Phys. Chem. B 2001, 105, 9817-9821 10.1021/jp012636y CCC: $20.00 © 2001 American Chemical Society Published on Web 09/15/2001