Electrochemistry of Platinum(II) Porphyrins: Effect of Substituents and π-Extension on Redox Potentials and Site of Electron Transfer Ping Chen, † Olga S. Finikova, ‡ Zhongping Ou,* ,†,§ Sergei A. Vinogradov,* ,‡ and Karl M. Kadish* ,† † Department of Chemistry, University of Houston, Houston, Texas 77204-5003, United States ‡ Department of Biochemistry and Biophysics, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States § Department of Applied Chemistry, Jiangsu University, Zhenjiang 212013, P. R. China * S Supporting Information ABSTRACT: Fourteen platinum(II) porphyrins with differ- ent π-conjugated macrocycles and different electron-donating or electron-withdrawing substituents were investigated as to their electrochemical and spectroscopic properties in non- aqueous media. Eight compounds have the formula (Ar 4 P)Pt II , where Ar 4 P = the dianion of a tetraarylporphyrin, while six have π-extented macrocycles with four β,β′-fused benzo or naphtho groups and are represented as (TBP)Pt II and (TNP)Pt II where TBP and TNP are the dianions of tetrabenzoporphyrin and tetranaphthoporphyrin, respectively. Each Pt(II) porphyrin undergoes two reversible one-electron reductions and one to three reversible one-electron oxidations in nonaqueous media. These reactions were characterized by cyclic voltammetry, UV-visible thin-layer spectroelectrochemistry and in some cases by ESR spectroscopy. The two reductions invariably occur at the conjugated π-ring system to yield relatively stable Pt(II) π-anion radicals and dianions. The first oxidation leads to a stable π-cation radical for each investigated porphyrin; but in the case of tetraarylporphyrins containing electron-withdrawing substituents, the product of the second oxidation may undergo an internal electron transfer to give a Pt(IV) porphyrin with an unoxidized macrocycle. The effects of macrocycle structure on UV-visible spectra, oxidation/reduction potentials, and site of electron transfer are discussed. ■ INTRODUCTION Platinum(II) porphyrins possess unique optical character- istics 1,2 and have been widely studied for their possible applications in medicine and technology. For example, the highly emissive triplet states of Pt(II) porphyrins have been used extensively in sensing 3-11 and biological imaging 12-21 of oxygen as well as in the construction of organic light-emitting devices. 22-28 Our own interest in platinum porphyrins has focused in part on their applications 8,10,19-21 and in part on elucidating oxidation-reduction potentials while at the same time spectroscopically characterizing the products of each redox reaction in nonaqueous media. 29,30 Recently, we reported the first evidence for the reversible electrochemical conversion between a Pt(II) and a Pt(IV)) porphyrin. 30 The investigated compound was (TPP)Pt II , where TPP is the dianion of tetraphenylporphyrin. This study is extended in the present article to include Pt(II) tetraarylpor- phyrins with different electron-donationg or electron-with- drawing substituents as well as substituted tetrabenzoporphyr- ins and tetranaphthoporphyrins, which have symmetrically extended π-ring systems. Although it has been well- documented that substituents at the β-pyrrolic positions of a porphyrin macrocycle affect both redox potentials and the site of electron transfer, 31,32 a detailed electrochemical study under the same experimental conditions of multiple Pt(II) porphyrins, having significantly different macrocyclic structures, has not been reported. In the present work, eight of the investigated compounds have the formula (Ar 4 P)Pt II , where Ar 4 P is the dianion of a tetraarylporphyrin (Chart 1), while six have a π-extended macrocycle with four β,β′-fused benzo or naphtho groups and are represented as (TBP)Pt II and (TNP)Pt II , where TBP and TNP are the dianions of tetrabenzoporphyrin and tetranaph- thoporphyrin, respectively (Chart 2). The electron transfer reactions of each Pt(II) porphyrin were characterized by cyclic voltammetry, UV-visible thin-layer spectroelectrochemistry, and, in some cases, by ESR spectroscopy. The effects of substituents and macrocycle extension on UV-visible spectra, redox potentials, and site of electron transfer are discussed. ■ EXPERIMENTAL SECTION Materials. (TPP)Pt II 5 was purchased from Frontier Scientific, Inc. and used as received. All other porphyrins (1-4 and 6-14) were synthesized as described previously (see refs 29, 33, and 34, and references therein). Dichloromethane (CH 2 Cl 2 , 99.8%) was purchased from EMD Chemicals Inc. and used as received. Benzonitrile (PhCN) was purchased from Sigma-Aldrich Co. and freshly distilled over P 2 O 5 before use. Tetra-n-butylammonium perchlorate (TBAP) was Received: February 14, 2012 Published: May 24, 2012 Article pubs.acs.org/IC © 2012 American Chemical Society 6200 dx.doi.org/10.1021/ic3003367 | Inorg. Chem. 2012, 51, 6200-6210