Studies of Ruthenium Complexes Attached to Oxide Surfaces

1. Studies of Ruthenium Complexes Attached to Oxide Surfaces Catherine E. Dedeian and Hannah R. Nandor Cliff J. Timpson, Daniel D. Von Riesen Roger Williams University, One Old Ferry Road, Bristol, RI 02809 Discussion The carboxylic acid functionality of the ISNA ligand, while attractive for attaching the complexes to oxide substrates, hindered our attempts to isolate the complex in pure form. The t-[RuCl(py)4ISNA]+exhibits a high affinity for silica and/or alumina stationary phases making the crude reaction mixtures virtually impossible to chromatograph either via thin layer or column methods. In all cases, little or no movement of the complex was observed with a wide variety of solvents and solvent mixtures. Subsequent attempts to isolate the desired complex utilizing cation exchange methods (Dowex 50W-X2) were also unsuccessful. Ultimately, it was determined that the desired complex could be attached to electrically conductive ITO slides by exposing the slides directly to the crude reaction mixture. The slides were briefly submerged in the reaction mixture (5min) and then scanned for the presence of a metal-based redox couple using cyclic voltammetry. After 15 minutes, a reversible, metal-based redox feature appeared at E1/2 = 856 mV. We assign this feature to the RuII/III redox couple present in the desired complex t-[RuCl(py)4ISNA]+. After 4 hours, another metal-based redox feature appears with an E1/2 of 361mV. This wave is curious as it is not likely ascribed to any mono-chloride, penta-pyridyl containing derivative, all of which exhibit E1/2's >800 mV. No attempts have been made to identify this feature. After 10 hours, the reaction was terminated. Neither the reaction mixture nor any surface attached films produced from submerging the ITO substrates in the crude reaction mixture revealed any fluorescence when excited at 400nm or 450nm. Thin films on ITO glass slides of t-[Ru(tpy)(bpy)ISNA]2+ were prepared by soaking the ITO slides in an acetone solution of the complex for 24 hours. When attached to the ITO slide, a weak but real emission at 674 nm was observed by placing the slide in an argon degassed acetonitrile solution and irradiating the slide at 450nm. The emission maximum is observed to be red-shifted relative to the 620nm emission observed for the complex free in solution (not covalently linked to the surface). This observed red-shift has been previously observed by Meyer et al.2 and has been ascribed to excited-ground state electronic interactions. Figure 3: Cyclic voltammograms of [Ru(tpy)(bpy)ISNA]+2 (v.s. Ag/AgCl) Abstract The synthesis, characterization and study of photo/redox active molecules that can be attached to oxide surfaces has been the focus of a number of studies over the last decade. Recent efforts in our labs have been aimed at producing monolayer films of ruthenium metal complexes bound to non-conductive float glass (SiO2) and electrically conductive oxide substrates (SnO2:Sb, In2O3:Sn). The work presented here will detail our recent efforts to produce, characterize, and modify monolayer films of trans-[RuCl(pyridine)4ISNA]+ (ISNA = isonicotinic acid) and [Ru(tpy)(bpy)(ISNA)]2+ (tpy = 2,2':6',2''-terpyridine, bpy = 2,2'-bipyridine) on both conductive and non-conductive oxide surfaces. Introduction Ruthenium polypyridyl compounds have received considerable attention over the last decade as photosensitizers in liquid-based photovoltaics. Recently, we have begun to focus on complexes which contain the ligand isonicotinic acid (ISNA) as the carboxylic acid functionality has been shown to covalently bind to oxide substrates. Reported here is our attempts to prepare and characterize trans-[RuCl(pyridine)4ISNA](PF6) both in solution and covalently attached to conductive and non-conductive surfaces. The results of these studies are compared to those obtained for the related [Ru(tpy)(bpy)ISNA]2+ complex prepared from previously reported procedures. 2,4 Figure 4: Emission profiles of [Ru(tpy)(bpy)ISNA]+2 • Conclusions • While it appears t-[RuCl(py)4ISNA]+ can be produced from the general synthetic approach developed by Coe, isolation and purification of the desired product is hindered by the carboxylic acid present on the ISNA ligand. • Monitoring the progress of the formation of t-[RuCl(py)4ISNA]+ by cyclic voltammetry indicates that at least two different products are formed, both of which adhere to the ITO electrodes and exhibit metal based reversible redox waves at E1/2 = 856 mV and E1/2 = 361 mV. • The peak at 856 mV is assigned to t-[RuCl(py)4ISNA]+ by comparison to analogous studies.1 The peak at 361 mV has not been identified. • Surface coverages calculated from electrochemical measurements for both t-[RuCl(py)4ISNA]+ and [Ru(terpy)(bpy)ISNA]2+ were found to be on the order of 1 x 10-11 mol cm-2 which is significantly lower than previously reported Ruthenium polypyridyl complexes. (1 x 10-10 mol cm-2). • The complex t-[RuCl(py)4ISNA]+ shows no emission when excited at 400 nm or 450 nm either in solution or when bound to an ITO / glass electrode. • The complex t-[Ru(tpy)(bpy)ISNA]2+ exhibits an emission in acetonitrile solution at 620 nm when excited at 450 nm. This emission is red-shifted to 674 nm upon surface attachment to the ITO/glass substrate. This shift to lower energy is attributed to excited-ground state interactions. Methods and Materials All reagents were obtained from the Sigma-Aldrich Chemical Company. Starting material trans-[RuCl(py)4NO](PF6)2 was synthesized using procedures reported by Coe.1 The target complex trans-[RuCl(py)4ISNA](PF6) was obtained in crude form via the procedure shown in Figure 2. Isonicotinic acid was added and the reaction was stirred at room temperature for 10 hours. Reaction progress was monitored by removing small aliquots from the reaction mixture and scanning the aliquots by cyclic voltammetry. When the reaction was deemed complete, diethyl ether was added to forcibly precipitate the crude reaction mixture. The crude solid was vacuum filtered and dried by drawing air over the solid and then stored in a vacuum desiccator for a minimum of 24 hours. Prefabricated semiconductive In2O3:Sn (ITO) slides were purchased from Delta Technologies (Stillwater, Minnesota). The slides were pre-treated prior to exposing them to solutions containing the redox active metal complexes by soaking them for 20 seconds in 10% v/v HNO3.2,4 Thin films of the metal complexes were obtained by submerging each slide in a ~millimolar solution of the desired complex dissolved in acetone for various time intervals to allow attachment of the complexes. Fluorescence measurements were obtained using a Jobin-Yvon Horiba Fluorolog-3. Electrochemical measurements in acetonitrile /0.1M supporting electrolyte (TBAH) solutions were obtained using a Bioanalytical Systems BAS CV-50W electrochemical workstation. Absorption (UV-visible) measurements were obtained using an HP8452 Diode Array UV-visible system. Figure 5: Emission profile of t-[RuCl(py)4ISNA]+1 References 1. Coe, B.J. Inorg. Chem. 1995; 34(3); 593-602. 2. Meyer, T.J. Inorg. Chem. 1994, 33, 3952-3964 3. Chou, M.H. Inorg. Chem. 1992,31, 2318-2327 4. Timpson, C.J. Ph.D. thesis. 1995, University of North Carolina, Chapel Hill 5. Gholamkhass, B. Inorg. Chem. 2001,40, 756-765 6. Qu, P. Langmuir 2000, 16, 4662-4671 Figure 2: Synthesis of [RuCl(py)4ISNA]+ Figure 1: Complexes Studied Acknowledgments CED and HRN gratefully acknowledge: Our research advisors C.Timpson and D. Von Riesen Our ACS-SA advisor S. O’Shea D. Futoma for opening the lab at night N. Breen for assistance with fluorescence studies S. Hira for assistance with electrochemistry studies Roger Williams University for support and funding Roger Williams University Student Senate for funding Table 1: Electrochemical and Spectroscopical Data for t-[RuCl(py)4ISNA]+ and [Ru(tpy)(bpy)(ISNA)]+2 www.rwu.edu