Comparison of rhenium-porphyrin dyads for CO photoreduction: photocatalytic studies and charge separation dynamics studied by time-resolved IR spectroscopy.

We report a study of the photocatalytic reduction of CO to CO by zinc porphyrins covalently linked to [Re(2,2'-bipyridine)(CO)L] moieties with visible light of wavelength >520 nm. contains an amide CHNHC(O) link from porphyrin to bipyridine (Bpy), contains an additional methoxybenzamide within the bridge CHNHC(O)CH(OMe)NHC(O), while has a saturated bridge CHNHC(O)CH; each dyad is studied with either L = Br or 3-picoline. The syntheses, spectroscopic characterisation and cyclic voltammetry of and are described. The photocatalytic performance of in DMF/triethanolamine (5 : 1) is approximately an order of magnitude better than or in turnover frequency and turnover number, reaching a turnover number of 360. The performance of the dyads with Re-Br units is very similar to that of the dyads with [Re-pic] units in spite of the adverse free energy of electron transfer. The dyads undergo reactions during photocatalysis: hydrogenation of the porphyrin to form chlorin and isobacteriochlorin units is detected by visible absorption spectroscopy, while IR spectroscopy reveals replacement of the axial ligand by a triethanolaminato group and insertion of CO into the latter to form a carbonate. Time-resolved IR spectra of and (560 nm excitation in CHCl) demonstrated electron transfer from porphyrin to Re(Bpy) units resulting in a shift of (CO) bands to low wavenumbers. The rise time of the charge-separated species for is longest at 8 (±1) ps and its lifetime is also the longest at 320 (±15) ps. The TRIR spectra of and are quite different showing a mixture of MLCT, IL and charge-separated excited states. In the case of , the charge-separated state is absent altogether. The TRIR spectra emphasize the very different excited states of the bromide complexes and the picoline complexes. Thus, the similarity of the photocatalytic data for bromide and picoline dyads suggests that they share common intermediates. Most likely, these involve hydrogenation of the porphyrin and substitution of the axial ligand at rhenium.