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, 4,7dimethyl-phen (dmp); L = Cl, n = 0; L = imidazole, n = 1þ) 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 •f 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 f 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 (e5 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 (g15 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.

Origin of electronic absorption spectra of MLCT-excited and one-electron reduced 2,2′-bipyridine and 1,10-phenanthroline complexes

“…Excited-state spectra were obtained as described previously [26,27,52]. In short, spectra in the visible spectral range were measured using 400 nm pumping ($100 fs, 1 kHz) and white-light continuum probe beam (340-680 nm) that was generated by focusing a small part of the 800 nm fundamental output of the Ti:S laser into a 1 mm thick CaF 2 window.…”

“…Nevertheless, the comparisons with the reduced-state spectra can be made because: (a) the ground state absorption set in at 450 nm, so that any new features at longer wavelengths are not affected by the ground state absorption; (b) the features that show up at wavelengths shorter than 450 nm are all new excited-state absorption bands, since they overwhelm the ground state absorption in this region (shown in black). Excited-state spectra (blue, olive) were measured at a sufficiently long time delay (>40 ps) after excitation, which (given the ultrafast rate of relaxation processes in these systems [26,27,40]) ensures that they represent the relaxed lowest excited state.…”

“…The excited-state spectrum of [ReCl(CO) 3 (bpy)] displays the typical [4,26,27,40] strong narrow band at 373 nm, and a weaker broad band at $466 nm with a shoulder at $440 nm. Whereas the intense near-UV bands of the excited and reduced states are comparable in appearance, the shapes of the corresponding absorption features in the visible indicate different origins.…”

“…Calculated m(CO) vibrational energies match well the experimental values determined spectroelectrochemically, Table 1. Excited-state calculations in DMF were described before [26,27,48]. Fig.…”

“…The nature of electronic transitions of reduced metal a-diimine complexes has not been studied by quantum chemical calculations and their generally accepted assignment to intraligand N,N ÅÀ pp ⁄ transitions was based on similarities with the spectra of free reduced ligands. Excited-state triplet-triplet electronic transitions were theoretically investigated only very recently for [Ru(bpy) 3 ] 2+ [25] and a series of rhenium tricarbonyl bpy and phen complexes [26,27], using open-shell time-dependent DFT techniques (TD-DFT). Important contributions from LMCT excitations and character mixing appear to be common features of their excited-state transitions.…”