Photochemistry and Photophysics of Charge-Transfer Excited States in Emissive d¹⁰/d⁰ Heterobimetallic Titanocene Tweezer Complexes

Abstract

Transition-metal complexes that undergo ligand-tometal charge transfer (LMCT) to d 0 metals are of interest as possible photocatalysts due to the lack of deactivating d−d states. Herein, the synthesis and characterization of nine titanocene complexes of the formula Cp 2 Ti(C 2 Ar) 2 •MX (where Ar = phenyl, dimethylaniline, or triphenylamine; and MX = CuCl, CuBr, or AgCl) are presented. Solid-state structural characterization demonstrates that MX coordinates to the alkyne tweezers and CuX coordination has a greater structural impact than AgCl. All complexes, including the parent complexes without coordinated MX, are brightly emissive at 77 K (emission max between 575 and 767 nm), with the coordination of MX redshifting the emission in all cases except for the coordination of AgCl into Cp 2 Ti(C 2 Ph) 2 . TDDFT investigations suggest that emission is dominated by arylalkynyl-to-titanium 3 LMCT in all cases except Cp 2 Ti(C 2 Ph) 2 •CuBr, which is dominated by CuBr-to-Ti charge transfer. In roomtemperature fluid solution, only Cp 2 Ti(C 2 Ph) 2 and Cp 2 Ti(C 2 Ph) 2 •AgCl are emissive, albeit with photoluminescent quantum yields ≤2 × 10 −4 . The parent complexes photodecompose in room-temperature solution with quantum yields, Φ rxn , between 0.25 and 0.99. The coordination of MX decreases Φ rxn by two to three orders of magnitude. There is a clear trend that Φ rxn increases as the emission energy increases. This trend is consistent with a competition between energy-gap-law controlled nonradiative decay and thermally activated intersystem crossing between the 3 LMCT state and the singlet transition state for decomposition.

Zinc(II) Complexes with Triplet Charge-Transfer Excited States Enabling Energy-Transfer Catalysis, Photoinduced Electron Transfer, and Upconversion

“…The ILCT states of our Zn II complexes complement the different types of photoactive excited states reported recently for first-row and other Earth-abundant transition-metal complexes, 28 which includes the classical MLCT states for d 6 complexes (Cr 0 , Mn I , Fe II , Co III ), 19 , 24 , 25 , 115 − 117 square-planar d 8 compounds (Ni II ) 26 , 118 and four-coordinate d 10 complexes (Cu I ), 3 , 78 ligand-to-metal charge-transfer (LMCT) states for Ti IV , Zr IV , 119 , 120 Mn IV , Fe III and Co III , 121 − 126 121 − 126 metal-centered (MC) states for V III , Cr III and Co III , 127 − 132 as well as ligand-to-ligand charge-transfer (LLCT) excited states for two-coordinate Cu I complexes. 29 − 33 Given these findings, it seems reasonable to conclude that Zn II complexes with charge-transfer and triplet excited states would perhaps deserve greater attention in future studies aiming to discover new photophysics and photochemistry in first-row transition-metal complexes.…”

Charge‐Transfer and Spin‐Flip States: Thriving as Complements

“…A different strategy to avoid this problem entirely is using metal centers with a d 0 or d 10 electronic configurations such as Ti IV , Zr IV , or Cu I [10, 15, 107–115] . As a consequence of their full or empty d‐shells, no excited MC states exist.…”

“…[101][102][103][104][105][106] A different strategy to avoid this problem entirely is using metal centers with a d 0 or d 10 electronic configurations such as Ti IV , Zr IV , or Cu I . [10,15,[107][108][109][110][111][112][113][114][115] As a consequence of their full or empty d-shells, no excited MC states exist. For linear Cu I complexes, nonradiative deactivation of the LL'CT states could be shut down completely, thereby resulting in emission quantum yields of > 99.9 % in solution.…”

Organometallic Intermediates in the Synthesis of Photoluminescent Zirconium and Hafnium Complexes with Pyridine Dipyrrolide Ligands

“…58,59 Owing to advances in the development of group 4 metallocene catalysts for polymerization reactions, 60,61 the optical properties of emissive zirconocene and hafnocene species have been steadily improved by the introduction of more rigid cyclopentadienyl systems, for example in ansa-metallocenes, 54 and continue to be studied for photochemical applications. 62,63 More recently, d 0 chromophores with electron-rich pincer ligands have garnered increased attention. 64 The availability of ligands with a wide variety of different donor atoms combined with the high structural rigidity imparted by the pincer framework makes this class of compounds a prime target for the development of new photoluminescent molecules.…”