Enhanced Visible Light Absorption in Heteroleptic Cuprous Phenanthrolines

Abstract

This work presents a series of Cu(I) heteroleptic 1,10-phenanthroline chromophores featuring enhanced UVA and visible-light-harvesting properties manifested through vectorial control of the copper-to-phenanthroline chargetransfer transitions. The molecules were prepared using the HETPHEN strategy, wherein a sterically congested 2,9-dimesityl-1,10-phenanthrolne (mesPhen) ligand was paired with a second phenanthroline ligand incorporating extended π-systems in their 4,7-positions. The combination of electrochemistry, static and timeresolved electronic spectroscopy, 77 K photoluminescence spectra, and timedependent density functional theory calculations corroborated all of the experimental findings. The model chromophore, [Cu(mesPhen)(phen)] + (1), lacking 4,7-substitutions preferentially reduces the mesPhen ligand in the lowest energy metal-to-ligand charge-transfer (MLCT) excited state. The remaining cuprous phenanthrolines (2−4) preferentially reduce their π-conjugated ligands in the low-lying MLCT excited state. The absorption cross sections of 2−4 were enhanced (ε MLCTmax = 7430−9980 M −1 cm −1 ) and significantly broadened across the UVA and visible regions of the spectrum compared to 1 (ε MLCTmax = 6494 M −1 cm −1 ). The excited-state decay mechanism mirrored those of long-lived homoleptic Cu(I) phenanthrolines, yielding three distinguishable time constants in ultrafast transient absorption experiments. These represent pseudo-Jahn−Teller distortion (τ 1 ), singlet−triplet intersystem crossing (τ 2 ), and the relaxed MLCT excited-state lifetime (τ 3 ). Effective light-harvesting from Cu(I)-based chromophores can now be rationalized within the HETPHEN strategy while achieving directionality in their respective MLCT transitions, valuable for integration into more complex donor− acceptor architectures and longer-lived photosensitizers.

Sterically Encumbered Aryl Isocyanides Extend Excited-State Lifetimes and Improve the Photocatalytic Performance of Three-Coordinate Copper(I) β-Diketiminate Charge-Transfer Chromophores

“…The development of effective molecular photosensitizers is crucial for various applications in energy-related fields − as well as synthetic organic chemistry. − While precious heavy transition metal-based coordination compounds featuring iridium­(III), , ruthenium­(II), , and platinum­(II) , have traditionally been favored due to their higher efficiency in photocatalysis, there is growing interest in exploring cheaper alternatives such as first-row transition metal-based coordination compounds − and metal-free organic chromophores. − Among the first-row transition metals, copper­(I) complexes have garnered significant attention due to their relatively longer metal-to-ligand charge transfer (MLCT) excited-state lifetimes, attributed to the absence of deleterious metal-centered (MC) states. The majority of copper­(I) complexes that have been studied so far are homoleptic or heteroleptic four-coordinate structures using phenanthroline derivatives or diphosphine ligands. − Another series of four-coordinate structures bearing β-diketiminate (NacNac) ligands has been prepared by our group. − The electron-rich nature of β-diketiminate ligands destabilizes the HOMO energy level, resulting in a smaller HOMO–LUMO gap and broad absorption across the visible region, which is crucial for solar harvesting. However, a series of complexes gave no photocatalytic activity, which we attribute to their short excited-state lifetimes that maximize at ∼2 ns …”

Optical Property Control by the Interligand Charge Transfer Excited State in Brominated Homoleptic and Heteroleptic Aluminum Dinuclear Triple-Stranded Helicates

“…By intentionally reducing the symmetry of these molecules, we anticipate further control over their excited-state properties, including nonradiative deactivation or intersystem crossing. Indeed, the unique photophysical properties of heteroleptic complexes, particularly those exhibiting ligand-centered luminescence such as zinc­(II) and copper­(I) complexes, have been extensively studied. − …”

Long Excited-State Lifetimes in Three-Coordinate Copper(I) Complexes via Triplet–Triplet Energy Transfer to Pyrene-Decorated Isocyanides

“…The most actively investigated classes of copper­(I) photosensitizers are homoleptic four-coordinate complexes with the general formula [Cu­(N^N) 2 ] + , with N^N representing a diimine ligand, and the heteroleptic [Cu­(N^N)­(P^P)] + versions pairing one diimine with one diphosphine ligand. − A common strategy for improving excited-state lifetimes in copper­(I) bis-chelate photosensitizers involves sterically crowded ligand designs, which inhibit excited-state (pseudo) Jahn–Teller distortions that lead to rapid nonradiative decay. We investigated this strategy in a series of four-coordinate heteroleptic copper­(I) complexes bearing diimine ligands and β-diketiminate (NacNac) ligands, which have superior visible absorption compared to typical copper­(I) photosensitizers, dominated by low-energy charge-transfer bands that cover a wide range of the visible and even the near-IR regions. , However, even with sterically crowding alkyl substituents on one or both ligands, these chromophores only have excited-state lifetimes of 2 ns or less .…”