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

Complexes with ligand-to-metal charge-transfer (LMCT) excited states involving d metals represent a new design for photocatalysts. Herein, the photochemistry and photophysics of d titanocenes of the type CpTi(CR), where CR = ethynylphenyl (CPh), 4-ethynyldimethylaniline (CDMA), or 4-ethynyltriphenylamine (CTPA), have been investigated. CpTi(CPh) and CpTi(CDMA) have also been characterized by single-crystal X-ray diffraction. The two aryl rings in CpTi(CDMA) are nearly face-to-face in the solid state, whereas they are mutually perpendicular for CpTi(CPh). All three complexes are brightly emissive at 77 K but photodecompose at room temperature when irradiated into their lowest-energy absorption band. The emission wavelengths and photodecomposition quantum yields are as follows: CpTi(CPh), 575 nm and 0.65; CpTi(CTPA), 642 nm and 0.42; CpTi(CDMA), 672 nm and 0.25. Extensive benchmarking of the density functional theory (DFT) model against the structural data and of the time-dependent DFT (TDDFT) model against the absorption and emission data was performed using combinations of 13 different functionals and 4 basis sets. The model that predicted the absorption and emission data with the greatest fidelity utilized MN15/LANL2DZ for both the DFT optimization and the TDDFT. Computational analysis shows that absorption involves a transition to a LMCT state. Whereas the spectroscopic data for CpTi(CTPA) and CpTi(CDMA) are well modeled using the optimized structure of these complexes, CpTi(CPh) required averaging of the spectra from multiple rotamers involving rotation of the Ph rings. Consistent with this finding, an energy scan of all rotamers showed a very flat energetic surface, with less than 1.3 kcal/mol separating the minimum and maximum. The computational data suggest that emission occurs from a LMCT state. Optimization of the LMCT state demonstrates compression of the C-Ti-C bond angle, consistent with the known products of photodecomposition.