Employing Long-Range Inductive Effects to Modulate Metal-to-Ligand Charge Transfer Photoluminescence in Homoleptic Cu(I) Complexes

Abstract

Four Cu(I) bis(phenanthroline) photosensitizers formulated from a new ligand structural motif (Cu1−Cu4) coded according to their 2,9-substituents were synthesized, structurally characterized, and fully evaluated using steady-state and time-resolved absorption and photoluminescence (PL) measurements as well as electrochemistry. The 2,9-disubstituted-3,4,7,8-tetramethyl-1,10-phenanthroline ligands feature the following six-membered ring systems prepared through photochemical synthesis: 4,4-dimethylcyclohexyl (1), tetrahydro-2H-pyran-4-yl (2), tetrahydro-2H-thiopyran-4-yl (3), and 4,4-difluorocyclohexyl (4). Universally, these Cu(I) metal-to-ligand charge transfer (MLCT) chromophores display excited-state lifetimes on the microsecond time scale at room temperature, including the three longest-lived homoleptic cuprous phenanthroline excited states measured to date in de-aerated CH 2 Cl 2 , τ = 2.5−4.3 μs. This series of molecules also feature high PL quantum efficiencies (Φ PL = 5.3−12% in CH 2 Cl 2 ). Temperature-dependent PL lifetime experiments confirmed that all these molecules exhibit reverse intersystem crossing and display thermally activated delayed PL from a 1 MLCT excited state lying slightly above the 3 MLCT state, 1050−1490 cm −1 . Ultrafast and conventional transient absorption measurements confirmed that the PL originates from the MLCT excited state, which remains sterically arrested, preventing an excessive flattening distortion even when dissolved in Lewis basic CH 3 CN. Combined PL and electrochemical data provided evidence that Cu1−Cu4 are highly potent photoreductants (E ox * = −1.73 to −1.62 V vs Fc +/0 in CH 3 CN), whose potentials are altered solely based on which heteroatoms or substituents are resident on the 2,9-appended ring derivatives. It is proposed that longrange electronic inductive effects are responsible for the systematic modulation observed in the PL spectra, excited-state lifetimes, and the ground state absorption spectra and redox potentials. Cu1−Cu4 quantitatively follow the energy gap law, correlating well with structurally related cuprous phenanthrolines and are also shown to triplet photosensitize the excited states of 9,10diphenylanthracene with bimolecular rate constants ranging from 1.61 to 2.82 × 10 8 M −1 s −1 . The ability to tailor both photophysical and electrochemical properties using long-range inductive effects imposed by the 2,9-ring platforms advocates new directions for future MLCT chromophore discovery.

Changing Directions: Influence of Ligand Electronics on the Directionality and Kinetics of Photoinduced Charge Transfer in Cu(I)Diimine Complexes

“…In the design of effective systems for harnessing and utilizing diffuse solar energy, it is imperative to understand where photoinduced charge-transfer states localize so that they can be extracted to provide electrons for productive redox chemistry. Recent elegant synthesis and spectroscopic studies have shown how to manipulate metal-to-ligand, ligand-to-metal, and ligand-centered charge-transfer states in coordination complexes based on earth-abundant, often first-row transition metals to dramatically extend excited-state lifetimes. − The work presented here complements these studies by providing a foundational understanding of the directionality of MLCT states in earth-abundant molecular photosensitizers, potentially circumventing the need for extremely long-lived excited states because the photogenerated electrons are exactly where they need to be. Here, we have shown that by adding common electron-donating or -withdrawing phenanthroline ligands, we can predictively direct the MLCT state to one ligand or another.…”

Electronic Structures and Photoredox Chemistry of Tungsten(0) Arylisocyanides

“…There is widespread ongoing work aimed at identifying earth-abundant transition metal photosensitizers with excited-state redox properties that are comparable to or better than those of their precious metal counterparts. Noteworthy developments from several laboratories have emerged over the past decade − and have been subjects of recent reviews. − We will restrict discussions to our work on arylisocyanide complexes of Group 6 metals.…”

Photo‐Catalyzed α‐Arylation of Enol Acetate Using Recyclable Silica‐Supported Heteroleptic and Homoleptic Copper(I) Photosensitizers

“…In addition, photocatalytic yields were almost unchanged in homogeneous and heterogenous conditions. That aspect, coupled to the developed methodology that yields robust functionalized surfaces, highlight that the anchoring methodology is optimal but that better copper(I) photosensitizer could be useful to burst the door open to efficient and recyclable photocatalysts [3i] …”