Reported in this contribution are the synthesis and characterization of both monoacetylide and unsymmetric bis-acetylide compounds based on Co III (cyclam) (cyclam = 1,4,8,11,tetraazacycloctetradecane). Refluxing [Co(cyclam)Cl 2 ]Cl with HC 2 R in the presence of organic amine resulted in the formation of trans-[Co(cyclam)(C 2 R)Cl]Cl (R =-4-C 6 H 4 NO 2 (1a),-ferrocenyl (1b) and-4-C 6 H 4 SC 2 H 4 SiMe 3 (1c)) in satisfactory yields. Subsequent reactions between LiC 2 Ph and trans-[Co(cyclam)(C 2-4-C 6 H 4 NO 2)Cl]Cl resulted in a mixture of trans-[Co(cyclam)(C 2-4-C 6 H 4 NO 2)(C 2 Ph)]Cl (2a) and trans-[Co(cyclam)(C 2 Ph) 2 ]Cl (trace). Molecular structures of the new compounds were established using single crystal X-ray diffraction. Cyclic voltammetric study revealed that these compounds typically display three Co-based couples: an irreversible 1eoxidation, and two 1ereductions.

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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 (HC2NAPiPr) yields [Co­(cyclam)­(C2NAPiPr)­Cl]Cl (1), which reacts with LiC2Y at −78 °C to afford [Co­(cyclam)­(C2NAPiPr)­(C2D)]Cl with D as C6H4-4-NMe2 (2a), NAPiPr (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.

Electron transfer (ET) in donor–bridge–acceptor (DBA) compounds featuring alkyne bridges depends strongly on the torsion angle between the donor and acceptor.