Ultrafast Excited-State Dynamics of Rhenium(I) Photosensitizers [Re(Cl)(CO)_(3)(N,N)] and [Re(imidazole)(CO)_(3)(N,N)]^+: Diimine Effects
Abstract
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).Attached Files
Supplemental Material - ic102324p_si_001.pdf
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Additional details
- Eprint ID
- 23274
- Resolver ID
- CaltechAUTHORS:20110411-105636044
- Swiss National Science Foundation (SNSF)
- Queen Mary University of London
- European collaboration programs COST Action D35 and ESF-DYNA
- Science and Technology Facilities Council (STFC)
- ME10124
- Czech Ministry of Education
- LD11082
- Czech Ministry of Education
- Stanford Global Climate and Energy Project (GCEP)
- Created
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2011-04-19Created from EPrint's datestamp field
- Updated
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2021-11-09Created from EPrint's last_modified field