Ultrafast Excited-State Dynamics of [Re(L)(CO)3(bpy)]n Complexes: Involvement of the Solvent
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Version 2 2018-08-28, 11:50
Version 1 2016-02-24, 21:21
Posted on 2018-09-20 - 18:54
Ultrafast excited-state dynamics of [Re(L)(CO)3(bpy)]n (L = Cl, Br, n = 0; L = 4-ethyl-pyridine (Etpy), n = 1+; bpy = 2,2′-bipyridine) have been investigated in dimethylformamide (DMF) solution by fluorescence up-conversion (FlUC) and UV−vis transient absorption (TA) with ∼100 fs time resolution. TA was also measured in the [1-ethyl-3-methyl-imidazolium]BF4 ionic liquid. The complexes show a very broad fluorescence band at 540−550 nm at zero time delay, which decays with 100−140 fs (depending on L) by intersystem crossing (ISC) to a ππ* intraligand (3IL) and a Re(L)(CO)3 → bpy charge-transfer (3CT) excited states. A second emission decay component (1.1−1.7 ps), apparent in the red part of the spectrum, is attributed to 3IL → 3CT conversion, leaving phosphorescence from the lowest 3CT state as the only emission signal at longer time delays. The triplet conversion is slower in DMF than acetonitrile, commensurate with solvation times. Full assignment of the excited-state absorption at long delay times is obtained by TD-DFT calculations on the lowest triplet state, showing that the 373 nm band is the sole diagnostics of bpy reduction in the CT excited state. Bands in the visible are due to Ligand-to-Metal-Charge-Transfer (LMCT) transitions. Time-resolved UV−vis absorption spectra exhibit a units-of-ps rise of all absorption features attributed to 3IL → 3CT conversion as well as electronic and vibrational relaxation, and a ∼15 ps rise of only the 373 nm ππ*(bpy•−) band, which slows down to ∼1 ns in the ionic liquid solvent. It is proposed that this slow relaxation originates mainly from restructuring of solvent molecules that are found very close to the metal center, inserted between the ligands. The solvent thus plays a key role in controlling the intramolecular charge separation, and this effect may well be operative in other classes of metal-based molecular complexes.
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Nahhas, Amal El; Cannizzo, Andrea; Mourik, Frank van; Blanco-Rodríguez, Ana María; Záliš, Stanislav; Vlček, Antonín; et al. (2016). Ultrafast Excited-State Dynamics of [Re(L)(CO)3(bpy)]n Complexes: Involvement of the Solvent. ACS Publications. Collection. https://doi.org/10.1021/jp101999m
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AUTHORS (7)
AN
Amal El Nahhas
AC
Andrea Cannizzo
FM
Frank van Mourik
AB
Ana María Blanco-Rodríguez
SZ
Stanislav Záliš
AV
Antonín Vlček
MC
Majed Chergui
KEYWORDS
3 CT statetime delayRetriplet conversionLMCTintramolecular charge separation3 CTfluorescence up-conversionDMFSolvent Ultrafast excited-state dynamicsUltrafast Excited-State Dynamicssolvation timesfluorescence bandUV4- ethyl-pyridine3 IL373 nm bandFull assignmentISCvibrational relaxationTD-DFT calculationsabsorption featuresunits-of-ps riseemission signalexcited-state absorptiontriplet stateTAdelay timestime delaysbpy reductioncomplexes showmetal center