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Ground State Nuclear Magnetic Resonance Chemical Shifts Predict Charge-Separated Excited State Lifetimes † † † † † Jing Yang, Dominic K

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Ground State Nuclear Magnetic Resonance Chemical Shifts Predict Charge-Separated Excited State Lifetimes † † † † † Jing Yang, Dominic K Article Cite This: Inorg. Chem. 2018, 57, 13470−13476 pubs.acs.org/IC Ground State Nuclear Magnetic Resonance Chemical Shifts Predict Charge-Separated Excited State Lifetimes † † † † † Jing Yang, Dominic K. Kersi, Casseday P. Richers, Logan J. Giles, Ranjana Dangi, † ‡ § § Benjamin W. Stein, Changjian Feng, Christopher R. Tichnell, David A. Shultz,*, † and Martin L. Kirk*, † ‡ Department of Chemistry and Chemical Biology and College of Pharmacy, The University of New Mexico, Albuquerque, New Mexico 87131-0001, United States § Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204, United States *S Supporting Information ABSTRACT: Dichalcogenolene platinum(II) diimine com- ′ plexes, (LE,E )Pt(bpy), are characterized by charge-separated ′ → dichalcogenolene donor (LE,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 (LE,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 13C 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. ■ INTRODUCTION even eliminate first order out-of-state SOC contributions to 1 Intersystem crossing (ISC) represents a fundamental non- electronic relaxation via ISC. To gain additional insight into the nature of nonradiative T1 radiative decay mechanism for relaxation between electronic → Downloaded via UNIV OF NEW MEXICO on January 15, 2019 at 16:46:04 (UTC). − S ISC, we have synthesized a series of transition metal states of different spin multiplicity.1 4 As a result of the change 0 complexes where the T → S relaxation to the electronic in spin state, ISC is an inherently spin-forbidden process.4 The 1 0 ground state is governed by low-symmetry distortions that See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. − spin forbiddenness can be overcome via the spin orbit derive from the chemical nature of the E/E′ donor atoms in a operator, which mixes states of different spin multiplicity. ′ ′ ′ series of (LE,E )Pt(bpy) (LE,E = dichalcogenolene, bpy = 4,4 - The ability to obtain molecular-level control over excited states di-tert-butyl-2,2′-bipyridine) complexes (Figure 1). In previous that relax via nonradiative decay channels is central to work,1 we suggested a mechanism to explain why complexes increasing our ability to harness technologically relevant with at least one heavy chalcogen underwent intersystem photoprocesses for the advancement of nanoscale molecule- crossing, while the dioxolene-containing complex (1-O,O′) did 5−7 8−13 based photonics, optoelectronics, solar energy con- not. This mechanism involves symmetry-allowed and energeti- 14−18 4 → version devices, photochemical reactivity, and photo- cally favorable SO-induced ISC from S1 T2 followed by 19−21 driven molecular motors. In marked contrast to radiative subsequent relaxation to T1 for heavy-chalcogen containing triplet → singlet decay (i.e., phosphorescence), which is readily complexes, a process that is energetically uphill for 1-O,O′. correlated with molecular structure,4,13 there exists a dearth of The consequence is that 1-O,O′ undergoes rapid radiationless − structure property relationships directed toward understand- decay from S1, while the remaining complexes emit weakly ing the mechanism of SOC-promoted ISC. The typical from T1. In the present paper, we explain the chalcogen- approach for increasing the rate of spin-forbidden nonradiative 4 decay is to employ a heavy atom with a large SOC constant. Received: July 24, 2018 However, symmetry restrictions can dramatically alter and Published: October 9, 2018 © 2018 American Chemical Society 13470 DOI: 10.1021/acs.inorgchem.8b02087 Inorg. Chem. 2018, 57, 13470−13476 Inorganic Chemistry Article Figure 2. Jablonski diagrams for the observed photoprocesses in 1- ′ − O,O (left) and the 2-O,S 5-S,Se complexes (right). S1 and T1 → (arising from the HOMO LUMO transition) and S2 and T2 (arising from the HOMO−1 → LUMO transition) are all LL′CT − excited states. Spin orbit promoted intersystem crossing from S1 to − T1 is symmetry forbidden, while spin orbit promoted intersystem crossing from S1 to T2 is symmetry allowed. Note that intersystem ′ crossing from S1 to T2 in 1-O,O does not occur due to the T2 state lying higher in energy than S1 for this complex. the excited state manifold to yield a triplet excited state. When excited state ISC does occur, the nature of the E/E′ donor atoms are observed to control the rate of nonradiative decay to the S0 electronic ground state. Recently, we compared the ′ ′ ′ relaxation behavior of 1-O,O , which is observed to undergo Figure 1. (Top) Structures of (LE,E )Pt(bpy) complexes, 1-O,O , 2- τ ≈ 29 ′ ′ fast, nonradiative relaxation ( NR 650 ps) from its LL CT O,S, 3-S,S, 4-O,Se, 5-S,Se, where E,E = O,S; S,S; O,Se; S,Se, Pt = +2 1 8 singlet excited state ( A1,S1), to complexes that possess oxidation state (d ), diamagnetic, square-planar transition metal ion, L ′ = dichalcogenolene donor (depicted in red), and bpy = diimine heavier chalcogen E/E donors and undergo ISC to emissive ′ τ ≥ Φ ≈ 1,28 acceptor (depicted in blue). The carbons bearing the chalcogens are LL CT triplet states ( T1 100 ns; em 0.09). As labeled Ci. See the Supporting Information for NMR chemical shift summarized in Figure 2, the chalcogen dependence on ′ assignments. (Bottom) Electronic absorption spectra of (LE,E )Pt- relaxation pathway is a consequence of exergonic, symmetry- (bpy) complexes as solutions in methylene chloride. 1 → 3 allowed SOC-promoted ISC from S1 to T2 ( A1 B1), 3 → 3 1 followed by B1 A1 IC to the T1 excited state for 2-S,S. 1 → 3 1 ′ dependent lifetimes for the weakly emissive, heavy chalcogen- Since the A1 B1 ISC is energetically uphill for 1-O,O , − fi → containing complexes (2-O,S 5-OSe). Speci cally, we present this pathway is unavailable resulting in direct S1 S0 IC being → an extraordinary empirical correlation of SOC-induced T1 markedly faster. 13 − S0 nonradiative decay rates with the C NMR chemical shift Spin Orbit Coupling Contributions to ISC Rates. A ff ′ ff di erences between (LE,E )Pt(bpy) dichalcogenolene quater- fundamental di erence between spin-forbidden ISC and spin- nary carbons, which are labeled as Ci in Figure 1. This provides allowed internal conversion (IC) is that the square of the SOC a unique ground state spectroscopic predictor (NMR) of ⟨φ | |φ ⟩2 matrix element (i.e., S0 HSOC T1 ), which connects the two molecular excited state lifetimes that also directly probes the states of different spin multiplicity, factors into the rate magnitude of the SOC matrix element that connects the T1 equation for ISC.30 Under the assumption that all the excited state with the S0 ground state. contributions (e.g., vibronic coupling and Franck−Condon factors) to the rate equation except ⟨φ |H |φ ⟩2 are ■ RESULTS AND DISCUSSION S0 SOC T1 identical for a series of complexes, the rate of intersystem Ligand-Dependent Nonradiative Lifetimes. A distin- crossing (kNR) back to the ground state can be expressed as eq guishing feature of these chromophores is the presence of a 1,4,30 1: dichalcogenolene → diimine ligand-to-ligand charge transfer ′ ̂ 2 (LL CT) band in the visible region of the spectrum (Figure ⟨|φφLi|⟩ 22−25 ′ ST01 1). With the exception of 1-O,O catecholate com- kNR ∝ ∑ 2 26,27 ()EE− plex, these complexes display phosphorescent emission i TS10 (1) ′ with LL CT excited state lifetimes that are dominated by which reduces to eq 21,4 when the sum over all indices x, y, z is nonradiative rate constants, which are markedly greater than relegated to only one direction (e.g. x). the radiative rate constants.1,28 The chromophoric π-systems, ′ ff ̂ 2 1-O,O and 2-S,S, both possess e ective C2v symmetry when ⟨|φφSTLx|⟩ neglecting the presence of the t-Bu substituents. In the C k ∝ 01 2v NR 2 (2) point group, the HOMO and LUMO for 1-O,O′ and 2-S,S ΔE − Δ both possess b1 symmetry. Thus, the ground state term for The T1 S0 energy gaps, E, are essentially the same for the these complexes is A1, and this is also the term symbol for the entire series of compounds studied here, indicating that the ′ 1 ff lowest-energy S1 and T1 LL CT excited states. Inspection of nonradiative relaxation rate di erences across the series do not Figure 2 indicates that the nature of the chalcogen donor set result from an energy-gap law dependence. For C2v 3-S,S, there − 3 coordinated to the Pt ion determines whether ISC can occur in is no spin orbit matrix element that connects the T1 ( A1) and 13471 DOI: 10.1021/acs.inorgchem.8b02087 Inorg. Chem. 2018, 57, 13470−13476 Inorganic Chemistry Article 1 ̂ sulfur(3p) atomic orbital coefficient (c ) in the LCAO (linear S0 ( A1) states.
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