Non-Localized Ligand-to-Metal Charge Transfer Excited States in (Cp)₂Ti(IV)(NCS)₂

Abstract

The bent d(0) titanium metallocene (Cp)(2)Ti(NCS)(2) exhibits an intense phosphorescence from a ligand-to-metal charge transfer triplet excited state at 77 K in an organic glass substrate and a poly(methyl methacrylate) plastic substrate. Quantum chemical calculations and spectroscopic studies show that the orbital parentage of this triplet state arises from the promotion of an electron from an essentially nonbonding symmetry adapted pi molecular orbital located on the NCS(-) ligands to a d(z)2-(y)2 orbital located on the Ti metal. Standard infrared spectroscopy of (Cp)(2)Ti(NCS)(2) in its ground electronic state at 77 K reveals a pair of closely spaced absorptions at (2072 cm(-1), 2038 cm(-1))(glass) and (2055 cm(-1), 2015 cm(-1))(plastic) that are assigned, respectively, to the symmetric and antisymmetric CN stretching modes of the two coordinated NCS(-) ligands. Low-temperature (77 K) time-resolved infrared spectroscopy that accesses the phosphorescing triplet excited state on the ns time scale shows an IR bleach that is coincident with the two ground state CN stretching bands and an associated grow-in of a pair of new IR bands at slightly lower energies (2059 cm(-1), 2013 cm(-1))(glass) and (2049 cm(-1), 1996 cm(-1))(plastic) that are assigned, respectively, to the symmetric and antisymmetric CN stretches in the emitting triplet state. These transient IR bands decay with virtually identical lifetimes to those observed for the phosphorescence decays when measured under identical experimental conditions. Singular value decomposition analysis of the time-resolved infrared data shows that the observed transient IR features arise from the same electronic manifold as measured through luminescence studies. The close similarity between the ground state and excited-state CN stretching bands in (Cp)(2)Ti(NCS)(2) indicates that symmetry breaking does not occur in forming the charge-transfer triplet excited-state manifold; i.e., electron density is withdrawn from a delocalized pi MO spread across both NCS(-) ligands. Calculations at several levels of theory reveal a delocalized ligand-to-metal charge transfer excited triplet manifold. These calculations closely reproduce the relative intensity ratios and frequencies of the symmetric and antisymmetric transient infrared vibrations in the CN region. This study is the first time-resolved infrared investigation of a ligand-to-metal charge-transfer excited state and the first to be performed at cryogenic temperatures in thin-film organic glass and plastic substrates.

Ligand-to-Metal Charge-Transfer Photophysics and Photochemistry of Emissive d0 Titanocenes: A Spectroscopic and Computational Investigation

“…These orbitals do not change meaningfully when calculated using ωB97XD/6-311+G­(d) instead of MN15/LANL2DZ. Qualitatively similar orbital orderings have been observed for Cp 2 Ti­(SCN) 2 . The lowest-energy singlets and triplets are dominated by a HOMO-to-LUMO transition (Charts S1–S3).…”

“…In the case of X = halide, emission was ascribed to a Cp-to-Ti LMCT triplet excited state, although the authors also considered emission from an X-to-Ti LMCT triplet excited state . Emission from Cp 2 Ti­(SCN) 2 was ascribed to a SCN-to-Ti LMCT triplet excited state based on the TDDFT data . For the arylalkynylamine complexes investigated herein, prior evidence that the lowest-energy absorption generates an LMCT excited state and further quantum-chemical calculations (vide infra) suggest that emission is from an arylalkynylamine-to-Ti IV 3 LMCT state.…”

Using picosecond and nanosecond time-resolved infrared spectroscopy for the investigation of excited states and reaction intermediates of inorganic systemsBased on the presentation given at Dalton Discussion No. 6, 9?11th September 2003, University of York, UK.

“…23 A rare example of a ligand field (LF) excited state is presented in the TRIR study of [W(CO) 5 (py)] in a glass matrix at 77 K. 28 Brozik and co-workers have recently exploited the use of 77 K glasses to obtain the IR spectrum of the LMCT excited state of [Ti(η 5 -C 5 H 5 ) 2 (NCS) 2 ]. 29 Peripheral IR reporter groups such as carboxylic acids and esters on substituted diimine ligands have also been utilised to indirectly probe charge transfer processes using both ns-and ps-TRIR. 30 TRIR studies in the fingerprint region have also probed 31 32 Intercalating ligands such as dppz provide a convenient method for binding the metal complex into the DNA double helix.…”

Luminescent Cationic Group 4 Metallocene Complexes Stabilized by Pendant N-Donor Groups

“…24 It was found that at a low temperature (77 K) in the solid state, titanocene dihalides (X = F, Cl, Br) are luminescent with relatively long lifetimes (10 2 to 10 3 μs), which confirmed earlier studies on electronic structures of bent titanocenes showing a significant LMCT character of the lowest excited states. 25 Assignment of excited states in the related titanocene Cp 2 Ti(NCS) 2 was later addressed by a combination of DFT calculations and timeresolved IR spectroscopy in a study by Kenney et al 26 Only recently, photoredox catalysis using visible-light irradiation was demonstrated by Gansaüer and Flowers with Cp 2 TiCl 2 on reactions such as reduction of epoxides and 5-exo cyclizations of unsaturated epoxides. 27 This approach using energy of the 3 LMCT excited state of the metallocene with a concomitant electron transfer for chemical transformations is indeed a perspective topic for future studies.…”