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)Cl₂]Cl and 4-ethynyl-N-isopropyl-1,8-naphthalimide (HC₂NAP^iPr) yields [Co(cyclam)(C₂NAP^iPr)Cl]Cl (1), which reacts with LiC₂Y at -78 °C to afford [Co(cyclam)(C₂NAP^iPr)(C₂D)]Cl with D as C₆H₄-4-NMe₂ (2a), NAP^iPr (2b), Ph (2c), and C₆H₄-4-N(4-MeOPh)₂ (2d). Molecular structures of 1 and 2a were established using single-crystal X-ray diffraction, while the redox properties and fluorescence profiles of compounds 1 and 2 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 1, 2a, and 2d 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 2a and 2d 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 2a and 4 ps in 2d). 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 2a and 2d.