"Spectroelectrochemical Detection of Proton-coupled Redox Processes in Novel tatpp-based Photocatalysts"

R. O. LEZNA; N.R. DE TACCONI; F.M. MACDONNELL

Faraday Discussions Volume 140

Workshop; Electrocatalysis - Theory and Experiments at the Interface; 2008

Royal Society of Chemistry

Ruthenium (II) complexes of the pyridyl polyazine ligands such as dppz, tpphz and tatpp have garnered considerable attention due to the unusual acceptor properties of these ligands.  In particular the tatpp unit (9,11,20,22-tetraaza tetrapyrido[3,2-a:2’3’-c:3”,2”-l:2”’,3”’]-pentacene), capable of reversible multi-electron storage upon visible irradiation1, will be shown to assemble on the electrode surface and therefore is of potential use in electrocatalysis or in thin film photocatalysis.             The spectroelectrochemical behavior of Ru(II) and Zn(II) complexes of the tatpp ligand in solution and as thin films will be compared and contrasted.  Particularly, zinc complexes are electrochemically organized in highly ordered, ultrathin blue films by cycling their solutions in the potential encompassing the electroreduction of the tatpp ligand, while the related ruthenium complexes (with bipyridine or phenanthroline as additional ligands) will remain, under cycling, as solution species.  Among the latter, the mononuclear ruthenium complex [(bpy)2Ru(tatpp)]2+ (from now on 12+), will be presented as a representative complex along with the characterization of its reduction products in the pH range 10.5 to 2. The study is a combination of voltammetry and UV-visible spectro-electrochemistry carried out on a slow potential scan without and with potential modulation (low frequency AC waveform of 11 Hz and 50 mVp-p amplitude).  Under basic conditions, the radical [(bpy)2Ru(tatpp·-)]+(1·+) is the first electrogenerated species that converts to [(bpy)2Ru(Htatpp-)]+ (H1+) and then to [(bpy)2Ru(H2tatpp)]2+(H212+) by a follow-up proton-coupled reduction. Partial dimerization of radical 1·+ is also observed in basic media in agreement with previous results obtained in acetonitrile2.  Neutral or acidic conditions favor the formation of the protonated, radical species, [(bpy)2Ru(Htatpp·)]2+ (H1·2+), which rapidly undergoes disproportionation.  At more negative potentials, potential modulated spectroelectrochemistry was able to demonstrate that the doubly-reduced complexes (e.g., H212+, H1+) undergo a two proton-two electron reductions to give the quadruply reduced and protonated species H412+ and/or H31+ throughout the pH range investigated.  The spectral detection of a rapid comproportionation of H412+ with the initial 12+ complex, leading to the regeneration of intermediate double-reduced species H212+, will also be presented.             The study of these proton-coupled redox processes and related chemical reactions in tatpp-based surface films and the use of these electroreactions in electrocatalysis are currently being investigated.  (1) (a) Konduri, R.; Ye, H.; MacDonnell, F.M.; Serroni, S.; Campagna, S.; Rajeshwar, K. Angew. Chem., Int. Ed. 2002, 41, 3815. (b) Konduri, R.; de Tacconi, N.R.; Rajeshwar, K.; MacDonnell, F.M.  J. Am. Chem. Soc. 2004, 126, 11621. (c) de Tacconi, N. R; Lezna, R. O.; Konduri, R.;  Ongeri, F.; Rajeshwar, K.; MacDonnell, F.M. Chem. Eur. J. 2005, 11, 4327. (2) de Tacconi, N.R.; Chitakunye, R.; MacDonnell, F. M.; Lezna, R. O. J. Phys. Chem. A 2008, 112, 497.