Unsymmetrical Bis-Alkynyl Complexes Based on Co(III)(cyclam): Synthesis, Ultrafast Charge Separation, and Analysis.

Donor-bridge-acceptor (D-B-A) systems with a polarizable bridge can afford rapid photoinduced electron transfer dynamics that may be susceptible to rate modulation by infrared excitation. We describe the synthesis, characterization, and electronic structure of a class of readily assembled D-B-A structures linked by a cobalt cyclam bridge. The reaction between [Co(cyclam)Cl]Cl and 4-ethynyl--isopropyl-1,8-naphthalimide (HCNAP) yields [Co(cyclam)(CNAP)Cl]Cl (), which reacts with LiCY at -78 °C to afford [Co(cyclam)(CNAP)(CD)]Cl with D as CH-4-NMe (), NAP (), Ph (), and CH-4-N(4-MeOPh) (). Molecular structures of and were established using single-crystal X-ray diffraction, while the redox properties and fluorescence profiles of compounds and were examined using voltammetric and steady-state emission techniques, respectively. The electronic structures and photophysical properties of these compounds were studied using density functional theory and time-dependent density functional theory methods. The excited-state dynamics of compounds , , and were explored using femtosecond transient absorption spectroscopy with 400 nm excitation and detection in both the visible and mid-IR spectral regions. Formation of a long-lived excited state was complete within 20 ps of excitation in all three compounds. Ultrafast spectral changes observed in and within the first 20 ps indicated the formation of a charge separated state (CS state, D-B-A) with characteristic times of less than 0.1 and 0.25 ps, respectively. The CS state undergoes rapid charge recombination (8 ps in and 4 ps in ). The CS dynamics is facilitated by the Co center, which mixes the bright NAP-centered electronic state with a pure CS state. The mixing strength depends on the donor energetics and conformation, which significantly influences the charge transfer dynamics in and .