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

Abstract

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.

New Synthetic Route for Cobalt(III) Dissymmetric Bisalkynyl Complexes Based on Cobalt(III)(cyclam)(C2NAPMes)

“…Comparison of mono‐alkynyl ( 1a ) to the bis‐alkynyl species ( 3a – c ), reveals that the first Co reduction shifts to significantly more negative potentials upon addition of a second alkynyl. This trend is consistent with previous observations and is attributed to the increased donor strength of an alkynyl vs. a chloro in the axial position . Comparison of the HC 2 NAP Mes reduction potential (–1.52 V) to those of 1a and 3a – c validates the assignment of the reversible reduction at –1.66 V being localized on the naphthalimide moiety.…”

“…The quasi‐reversible oxidation couple at 0.22 V for the complex 3a is assigned to the donor (–C 6 H 4 ‐4‐NMe 2 ) based on related literature examples of [Co(cyclam)(C 2 C 6 H 4 ‐4‐NMe 2 ) 2 ] + and [Co(cyclam)(C 2 C 6 H 4 ‐4‐NMe 2 )(C 2 C 6 F 5 )] + . Consistent with observations for [Co(cyclam)(C 2 NAP i Pr)(C 2 C 6 H 4 ‐4‐NMe 2 )]Cl, this assignment suggests that the HOMO and LUMO for compound 3a reside on the donor and acceptor, respectively, and the electrochemical HOMO‐LUMO gap [ E g = E 1/2 ( D ) – E 1/2 ( A )] can be estimated as 1.88 V.…”

“…Furthermore, the similarity between the emission of 1a and 3a suggests that the C 6 H 4 ‐4‐NMe 2 donor does not significantly affect the electronic states of Co bound ‐C 2 NAP Mes . Similar findings were reported for the analogous compounds bearing NAP i Pr, with fluorescent quantum yields (Φ fl ) in CH 2 Cl 2 on the order of 0.43–0.66 % for the Co III (cyclam)(C 2 NAP i Pr) compounds and 42 % for HC 2 NAP i Pr . The dimerized forms of NAP R ligand, namely NAP R ‐C 4 ‐NAP R (R = n Bu or Ph), were reported to have Φ fl on the order of 83 % and 101 %, respectively .…”

“…The + . [22] Consistent with observations for [Co(cyclam)(C 2 NAPiPr)(C 2 C 6 H 4 -4-NMe 2 )]Cl, [20] this assignment suggests that the HOMO and LUMO for compound 3a reside on the donor and acceptor, respectively, and the electrochemical HOMO-LUMO gap [E g = E 1/2 (D) -E 1/2 (A)] can be estimated as 1.88 V.…”

“…Described in this contribution are several new [Co III (cyclam)-(C 2 NAP R )Cl] + type complexes (R = Mes, Me, Pen, 2-ethhex, Oct) that exhibit improved crystallinity and reactivity over [Co III (cyclam)(C 2 NAPiPr)Cl] + . [20] Careful tuning of the alkyl group on the naphthalimide moiety revealed that -C 2 NAP Mes based complexes performed the best in the aforementioned categories. The increased stability afforded by the mesityl group allows for selective synthesis of the [Co III (cyclam)(C 2 NAP Mes )(C 2 Ar)] + type complexes under weak base conditions through a triflate intermediate.…”