Ultrafast Intramolecular Charge Separation in a Donor–Acceptor Assembly Comprising Bis(η5-cyclopentadienyl)molybdenum Coordinated to an Ene-1,2-dithiolate-naphthalenetetracarboxylicdiimide Ligand

Abstract

The first example of a Donor-spacer-Acceptor tryad, based upon a molybdenum-ene-1,2-dithiolate unit as the Donor and a naphthalene-diimide as the Acceptor, has been synthesized and its photophysical properties investigated. Synthesis required the preparation of a new pro-ligand containing a protected ene-1,2-dithiolate bound through a phenyl linkage to a naphthalenetetracarboxylicdiimide (NDI) group. Deprotection of this pro-ligand by base hydrolysis, followed by reaction with [Cp(2)MoCl(2)], produced the new dyad [Cp(2)Mo(SC(H)C(C(6)H(4)-NDI)S)] (2). Electrochemical studies showed that 2 can be reversibly oxidized to [2](+) and reduced to [2](-), [2](2-), and [2](3-). These studies, augmented by UV/vis, IR, and electron paramagnetic resonance (EPR) spectra of electrochemically generated [2](+) and [2](-), show that the highest occupied molecular orbital (HOMO) of 2 is ene-1,2-dithiolate-based and the lowest unoccupied molecular orbital (LUMO) is NDI-based; these conclusions are supported by density functional theory (DFT) calculations for the electronic ground state on a model of 2 which also showed that these two parts of the molecule are electronically distinct. The dynamics of the excited states of 2 in CH(2)Cl(2) solution were investigated by picosecond time-resolved IR spectroscopy following irradiation by a 400 nm ∼120 fs laser pulse. These investigations were complemented by an ultrafast transient absorption spectroscopic study from 420 to 760 nm of the nature of the excited states of 2 in CH(2)Cl(2) solution following irradiation by a 383 nm ∼120 fs laser pulse. These studies showed that irradiation of 2 at both 400 and 383 nm leads to the formation of the [(Cp)(2){Mo(dt)}(+)-Ph-{NDI}(-)] charge-separated state as a result of a cascade electron transfer initiated by the formation of an (1)NDI* excited state. (1)NDI* rapidly (ca. 0.2 ps) forms the local charge transfer state [Cp(2)Mo(dt)-{Ph}(+)-{NDI}(-)] which has a lifetime of about 1.7 ps and decays to produce the ground state and the charge-separated state [(Cp)(2){Mo(dt)}(+·)-Ph-{NDI}(-)]; the latter has an appreciable lifetime, about 15 ns in CH(2)Cl(2) at room temperature.