Titanocene as a New Acceptor (A) for Arylamine Donors (D) in D−π–A Chromophores

Abstract

Charge transfer (CT) transitions are relevant in the fields of solar energy conversion and nonlinear optical materials. Herein, a series of complexes with an alkynyl linkage between an aryl amine donor and a TiIV (titanocene) acceptor is reported. Each complex displays a strong (15000 < ε < 24,000 M–1cm–1), low-energy (520 < λ < 560 nm) absorption ascribed to an amine to TiIV ligand-to-metal CT. This characterization is supported by UV–vis spectroscopy, cyclic voltammetry, and TD-DFT calculations. These complexes are not photostable; therefore, an alternate architecture, wherein the amine donor is appended to the titanocene cyclopentadienyl ligand, has been designed. The molar absorptivity of the amine to TiIV CT in this latter architecture is lower (2100 M–1 cm–1), indicating weaker donor–acceptor coupling. This architecture is indeed much more photostable.

Oxidatively stable ferrocenyl-π-bridge-titanocene D–π-A complexes: an electrochemical and spectroscopic investigation of the mixed-valent states

“…In this report we discuss two strategies to enhance oxidative stability: 1) appending the donor group to the Cp ligand (i.e, (Me2CpC2Fc)2TiCl2, Fig. 2) 17,18 thus avoiding weaker titanium alkynyl bonds that cleave upon oxidation (vida infra) or 2) complexing a copper halide in between the alkynes, which was previously utilized by Lang et al to generate the oxidatively stable TMS Cp2Ti(C2Fc)2CuBr species (Fig. 2).…”

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

“…Thus, we have turned our attention to the complexes with organic amine donor ligands appended to titanocenes. Our previous communication on these complexes reported on their absorption spectra and lack of photostability . Herein, we report on their room-temperature solution-phase photochemistry, their low-temperature emission properties, and an in-depth computational investigation.…”

“…Our group recently investigated a series of titanocene complexes with absorption bands involving CT to Ti IV (Figure ). − For example, titanocenes with ethynylferrocene ligands, e.g., Cp 2 Ti­(C 2 Fc) 2 , have a low-energy visible absorbance band ascribed to Fe II -to-Ti IV metal-to-metal charge transfer (MMCT) . Titanocenes with alkynylarylamine ligands have a low-energy absorbance ascribed to a LMCT .…”

“… − For example, titanocenes with ethynylferrocene ligands, e.g., Cp 2 Ti­(C 2 Fc) 2 , have a low-energy visible absorbance band ascribed to Fe II -to-Ti IV metal-to-metal charge transfer (MMCT) . Titanocenes with alkynylarylamine ligands have a low-energy absorbance ascribed to a LMCT . In the case of MMCT from ferrocene ligands, the presence of MC states clearly impacts the excited-state lifetimes of the MMCT states, as demonstrated by time-resolved transient-absorption spectroscopy and time-dependent density functional theory (TDDFT) investigations .…”

Synthesis and characterization of a tert-butyl ester-substituted titanocene dichloride: t-BuOOCCp2TiCl2

“…One architecture used in compounds of this type is the Donor-bridge-Acceptor (D--A) architecture, where absorption of a photon results in the transfer of charge from an electron-rich donor portion of the molecule to an electron-poor acceptor portion through a conjugatedlinkage (Ji et al, 2018). Alkynyl titanocenes utilizing titanocene acceptors and ferrocenyl or arylamine donors are promising candidates for sensitizers in DSSCs (Turlington et al, 2016;Pienkos et al, 2016Pienkos et al, , 2018Livshits et al, 2019). In photovoltaic technologies, the sensitizer must be attached to a semiconductor substrate, commonly TiO 2 , using an anchoring group exhibiting a high binding affinity for the substrate (Zhang & Cole, 2015;Kalyanasundaram & Grä tzel, 1998).…”