Ultrafast Relaxation Dynamics of Osmium–Polypyridine Complexes in Solution

Abstract

We present steady-state absorption and emission spectroscopy and femtosecond broadband photoluminescence up-conversion spectroscopy studies of the electronic relaxation of Os(dmbp) 3 (Os1) and Os(bpy) 2 (dpp) (Os2) in ethanol, where dmbp is 4,4′-dimethyl-2,2′-biypridine, bpy is 2,2′-biypridine, and dpp is 2,3-dipyridyl pyrazine. In both cases, the steady-state phosphorescence is due to the lowest 3 MLCT state, whose quantum yield we estimate to be ≤5.0 × 10. For Os1, the steady-state phosphorescence lifetime is 25 ns. In both complexes, the photoluminescence excitation spectra map the absorption spectrum, pointing to an excitation wavelength-independent quantum yield. The ultrafast studies revealed a short-lived (≤100 fs) fluorescence, which stems from the lowest singlet metal-to-ligand-charge-transfer ( 1 MLCT) state and decays by intersystem crossing to the manifold of 3 MLCT states. In addition, Os1 exhibits a 50 ps lived emission from an intermediate triplet state at an energy ∼2000 cm −1 above that of the long-lived (25 ns) phosphorescence. In Os2, the 1 MLCT− 3 MLCT intersystem crossing is faster than that in Os1, and no emission from triplet states is observed other than the lowest one. These observations are attributed to a higher density of states or a smaller energy spacing between them compared with Os1. They highlight the importance of the energetics on the rate of intersystem crossing.

Ultrafast Photophysics of Transition Metal Complexes

“…This is clear from Table 1, which shows the ISC times we measured for various Fe, Ru, Ni, Pt, Pd, Re, and Os complexes. 37,[48][49][50]53,54 For example, (a) the ISC rate between the lowest singlet and triplet states of a diplatinum complex (Pt 2 POP) was found to lie in the 10−30 ps range and to be solvent-dependent! 48 (b) In complexes such as [Re(L)-(CO) 3 (bpy)] n+ (for L = Cl, Br, or I, n = 0; for L = etpy (ethylpyridine), n = 1), the ISC times (100−150 fs) are significantly longer than in the Fe or Ru complexes, but remarkably, they decreased in the sequence I, Br, Cl, in what appears as an inverse heavy atom effect.…”

“…For example, Os(dmbp) 3 showed slower ISC times ( Figure 4) than Os(bpy) 2 (dpp), which has a higher density of states. 49 The importance of the dynamical aspect of ISC in this temporal regime needs to be stressed. That is, the system explores regions of its configuration space and reaches the points in space where ISC is most favorable due to an energy degeneracy and a favorable coupling of potential surfaces.…”

Characterizing the Solvated Structure of Photoexcited [Os(terpy)2]2+ with X-ray Transient Absorption Spectroscopy and DFT Calculations

“…The normalized X-ray absorption near edge fine structure (XANES) spectrum of GS [Os(terpy) 2 ] 2+ in MeOH (at 1.2 mM concentration) is shown in Figure 2a (black trace). Upon photoexcitation at 527 nm, a fraction of the molecules initially in the 1 A 1 GS is promoted to the singlet metal to ligand charge transfer ( 1 MLCT) state, and relax rapidly to 3 MLCT state on a timescale of sub-ps to tens of ps specific for Os II complexes [18][19][20]. The normalized X-ray absorption near edge fine structure (XANES) spectrum of GS [Os(terpy)2] 2+ in MeOH (at 1.2 mM concentration) is shown in Figure 2a (black trace).…”

Empirical rules of molecular photophysics in the light of ultrafast spectroscopy

“…Over the past few years, we documented a number of cases, especially among coordination chemistry complexes, where emission from higher lying electronic states was observed [54][55][56]. The cases of halogenated rhenium-carbonyl complexes [54], and more so of the osmium complex Os(dmbp) 3 (dmbp = 4,4′-dimethyl-2,2′-biypridine) in ethanol [56] are examples where we detected intermediate 10-50 ps lived emissions. In the latter case, it was due to a higher triplet state lying at an energy ∼2000 cm -1 above the lowest triplet state whose phosphorescence decays in 25 ns, due to a quantum yield of ≤ 5.0 × 10 -3 .…”

“…Further investigations with various transition metal complexes showed completely erratic trends with respect to the value of the SOC of the metal atom, contrary to the expectations based on the above IUPAC definition of the heavy atom effect. This is clear from Table 1, which shows the ISC times we measured for various Fe, Ru, Pt, Re, and Os complexes [33,[55][56][57][58]62], in particularly: a) the ISC rate between the lowest singlet and triplet states of a diplatinum complex (Pt 2 POP) was found to lie in the 10-30 ps range and to be solvent-dependent! [55]; b) In complexes such as [Re(L)(CO) 3 (bpy)]n + (for L = Cl, Br, I, n = 0; for L = Etpy = ethylpyridine, n = 1), the ISC times (100-150 fs) are significantly longer than in the Fe or Ru complexes, but remarkably, they decreased in the sequence I-Br-Cl, in what appears as an inverse-heavy atom effect [54].…”