Ground State Nuclear Magnetic Resonance Chemical Shifts Predict Charge-Separated Excited State Lifetimes

Abstract

Dichalcogenolene platinum(II) diimine complexes, (L E,E ′)Pt(bpy), are characterized by charge-separated dichalcogenolene donor (L E,E ′) → diimine acceptor (bpy) ligand-to-ligand charge transfer (LL′CT) excited states that lead to their interesting photophysics and potential use in solar energy conversion applications. Despite the intense interest in these complexes, the chalcogen dependence on the lifetime of the triplet LL′CT excited state remains unexplained. Three new (L E,E ′)Pt(bpy) complexes with mixed chalcogen donors exhibit decay rates that are dominated by a spin−orbit mediated nonradiative pathway, the magnitude of which is proportional to the anisotropic covalency provided by the mixed-chalcogen donor ligand environment. This anisotropic covalency is dramatically revealed in the 13 C NMR chemical shifts of the donor carbons that bear the chalcogens and is further probed by S K-edge XAS. Remarkably, the NMR chemical shift differences also correlate with the spin−orbit matrix element that connects the triplet excited state with the ground state. Consequently, triplet LL′CT excited state lifetimes are proportional to both functions, demonstrating that specific ground state NMR chemical shifts can be used to evaluate spin−orbit coupling contributions to excited state lifetimes.

Chromophore-radical excited state antiferromagnetic exchange controls the sign of photoinduced ground state spin polarization

“…The differences in the nature of the ESP for mPh-Pt and 6-Me-mPh-Pt correlate with the bridge-dependent energy gaps between 2 T 1 and the other states ( 4 T 1 , 2 NN, 4 NN) in this energy region, which are affected by CAT/SQ-bridge bond torsions. Since the degree of charge separation in the excited state manifolds of mPh-Pt and 6-Me-mPh-Pt is near unity, 13 the J SQ-NN value for LZn(SQ-6-Me-mPh-NN) can be used as a proxy for J SQ-NN in the excited state of 6-Me-mPh-Pt . The J SQ-NN exchange interaction that we obtain for LZn(SQ-6-Me-mPh-NN) is related to the 2 T 1 – 4 T 1 energy gap (Δ E D1Q ) in the three-spin excited states of mPh-Pt and 6-Me-mPh-Pt according to Δ E D1Q = 1.5 J SQ-NN when J bpy ˙ -SQ ≫ J SQ-NN .…”

“…The triplet state of the (bpy)M(CAT) parent chromophore is otherwise inaccessible due to non-competitive intersystem crossing (ISC) rates compared to efficient non-radiative relaxation back to the singlet ground state. 13,15 …”

“…The triplet state of the (bpy)M(CAT) parent chromophore is otherwise inaccessible due to non-competitive intersystem crossing (ISC) rates compared to efficient non-radiative relaxation back to the singlet ground state. 13,15 Chromophores with appended radicals have been used to provide high-resolution information regarding the nature of their low-energy excited states, [11][12][16][17] illustrate how multiple, pairwise, excited state magnetic exchange interactions influence photophysical processes, 11 and highlight how the excited state chromophore triplet -radical magnetic exchange interaction and associated excited state dynamics can affect electron spin polarization (ESP). 11, 16-17, 22-23, 25-34 In most systems of this type, ESP is a result of the spin selectivity of the transitions between nearly degenerate 2 T 1 and 4 T 1 states, and ESP is observed when these states are relatively long lived.…”

Metal–Dithiolene Bonding Contributions to Pyranopterin Molybdenum Enzyme Reactivity

“…This cofactor is also found in anaerobic tungsten enzymes, and it may be one of the most ancient cofactors in biology [5]. The study of metal-dithiolene compounds (metallodithiolenes) has undergone a recent renaissance, with their synthesis, geometric structure, spectroscopy, bonding, and electronic structure having been recently highlighted [4,[6][7][8][9][10][11][12][13][14][15][16][17][18][19][20]. Here, we briefly review the discovery of metallodithiolene compounds [13,21].…”

Metal Ion Control of Photoinduced Electron Spin Polarization in Electronic Ground States

“…The antiferromagnetic exchange interaction mediated by a m -Ph bridge has previously been determined from solid state magnetic susceptibility measurements on LZn II SQ- m-Ph -NN ( J SQ‑NN = −32 cm –1 ) . Given the nearly complete charge separation in the LL′CT excited state manifolds of 1-Pd and 1-Pt , the 2 T 1 – 4 T 1 energy gap is ≈1.5 J SQ‑NN = 48 cm –1 . By comparison, MCD measurements yield a markedly larger J bpy •– ‑SQ = 1400 cm –1 .…”