El Nahhas, Amal and Consani, Cristina and Blanco-Rodríguez, Ana María and Lancaster, Kyle M. and Braem, Olivier and Cannizzo, Andrea and Towrie, Michael and Clark, Ian P. and Záliš, Stanlislav and Chergui, Majed and Vlček, Antonín, Jr. (2011) Ultrafast Excited-State Dynamics of Rhenium(I) Photosensitizers [Re(Cl)(CO)_(3)(N,N)] and [Re(imidazole)(CO)_(3)(N,N)]^+: Diimine Effects. Inorganic Chemistry, 50 (7). pp. 2932-2943. ISSN 0020-1669 http://resolver.caltech.edu/CaltechAUTHORS:20110411-105636044
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Femto- to picosecond excited-state dynamics of the complexes [Re(L)(CO)_(3)(N,N)]^n (N,N = bpy, phen, 4,7-dimethyl-phen (dmp); L = Cl, n = 0; L = imidazole, n = 1+) were investigated using fluorescence up-conversion, transient absorption in the 650−285 nm range (using broad-band UV probe pulses around 300 nm) and picosecond time-resolved IR (TRIR) spectroscopy in the region of CO stretching vibrations. Optically populated singlet charge-transfer (CT) state(s) undergo femtosecond intersystem crossing to at least two hot triplet states with a rate that is faster in Cl (~100 fs)^(−1) than in imidazole (~150 fs)^(−1) complexes but essentially independent of the N,N ligand. TRIR spectra indicate the presence of two long-lived triplet states that are populated simultaneously and equilibrate in a few picoseconds. The minor state accounts for less than 20% of the relaxed excited population. UV−vis transient spectra were assigned using open-shell time-dependent density functional theory calculations on the lowest triplet CT state. Visible excited-state absorption originates mostly from mixed L;N,N^(•−) → Re^(II) ligand-to-metal CT transitions. Excited bpy complexes show the characteristic sharp near-UV band (Cl, 373 nm; imH, 365 nm) due to two predominantly ππ*(bpy^(•−)) transitions. For phen and dmp, the UV excited-state absorption occurs at 305 nm, originating from a series of mixed ππ* and Re → CO;N,N•− MLCT transitions. UV−vis transient absorption features exhibit small intensity- and band-shape changes occurring with several lifetimes in the 1−5 ps range, while TRIR bands show small intensity changes (≤5 ps) and shifts (~1 and 6−10 ps) to higher wavenumbers. These spectral changes are attributable to convoluted electronic and vibrational relaxation steps and equilibration between the two lowest triplets. Still slower changes (≥15 ps), manifested mostly by the excited-state UV band, probably involve local-solvent restructuring. Implications of the observed excited-state behavior for the development and use of Re-based sensitizers and probes are discussed.
|Additional Information:||© 2011 American Chemical Society. Received: November 20, 2010; Published: March 09, 2011. This work was supported by the Swiss NSF via the NCCR MUST, QMUL, European collaboration programs COST Action D35 and ESF-DYNA, STFC, and Czech Ministry of Education Grants ME10124 and LD11082. Work at Caltech was supported by GCEP (Stanford).|
|Official Citation:||Ultrafast Excited-State Dynamics of Rhenium(I) Photosensitizers [Re(Cl)(CO)3(N,N)] and [Re(imidazole)(CO)3(N,N)]+: Diimine Effects Amal El Nahhas, Cristina Consani, Ana Mara Blanco-Rodrguez, Kyle M. Lancaster, Olivier Braem, Andrea Cannizzo, Michael Towrie, Ian P. Clark, Stanislav Zli, Majed Chergui, Antonn Vlek Jr. Inorganic Chemistry 2011 50 (7), 2932-2943|
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|Deposited By:||Jason Perez|
|Deposited On:||19 Apr 2011 17:35|
|Last Modified:||26 Dec 2012 13:09|
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