Ultrafast Charge Separation in a Photoreactive Rhenium-Appended Porphyrin Assembly Monitored by Picosecond Transient Infrared Spectroscopy

• Published in: Journal of the American Chemical Society Vol. 128; no. 13; pp. 4253 - 4266
• Main Authors: Gabrielsson, Anders, Hartl, František, Zhang, Hong, Lindsay Smith, John R, Towrie, Michael, Vlček, Antonín, Perutz, Robin N
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 05-04-2006
• Subjects: Atomic and molecular physics; Exact sciences and technology; Fluorescence and phosphorescence; radiationless transitions, quenching (intersystem crossing, internal conversion); Molecular properties and interactions with photons; Physics; Radiationless transitions, quenching; Rhenium Carbonyls Complexes; Fluorescence spectroscopy; Infrared spectroscopy; Charge separation; Organic ligand; Radiative lifetimes; Experimental study; Quantum yield; Luminescence decay; Transition elements Complexes; Transition elements Carbonyls Complexes; Molecular assembly; Complexes Mixed; Luminescence quenching; Metalloporphyrin; Zinc Complexes
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja0539802

Rhenium(bipyridine)(tricarbonyl)(picoline) units have been linked covalently to tetraphenylmetalloporphyrins of magnesium and zinc via an amide bond between the bipyridine and one phenyl substituent of the porphyrin. The resulting complexes, abbreviated as [Re(CO) 3 (Pic)Bpy-MgTPP][OTf] and [Re(CO) 3 (Pic)Bpy-ZnTPP][OTf], exhibit no signs of electronic interaction between the Re(CO)3(bpy) units and the metalloporphyrin units in their ground states. However, emission spectroscopy reveals solvent-dependent quenching of porphyrin emission on irradiation into the long-wavelength absorption bands localized on the porphyrin. The characteristics of the excited states have been probed by picosecond time-resolved absorption (TRVIS) spectroscopy and time-resolved infrared (TRIR) spectroscopy in nitrile solvents. The presence of the charge-separated state involving electron transfer from MgTPP or ZnTPP to Re(bpy) is signaled in the TRIR spectra by a low-frequency shift in the ν(CO) bands of the Re(CO)3 moiety similar to that observed by spectroelectrochemical reduction. Long-wavelength excitation of [Re(CO) 3 (Pic)Bpy-MTPP][OTf] results in characteristic TRVIS spectra of the S1 state of the porphyrin that decay with a time constant of 17 ps (M = Mg) or 24 ps (M = Zn). The IR bands of the CS state appear on a time scale of less than 1 ps (Mg) or ca. 5 ps (Zn) and decay giving way to a vibrationally excited (i.e., hot) ground state via back electron transfer. The IR bands of the precursors recover with a time constant of 35 ps (Mg) or 55 ps (Zn). The short lifetimes of the charge-transfer states carry implications for the mechanism of reaction in the presence of triethylamine.