Electronic Tuning of Photoexcited Dynamics in Heteroleptic Cu(I) Complex Photosensitizers

Abstract

Photoexcited dynamics of heteroleptic Cu­(I) complexes as noble-metal-free photosensitizers are closely intertwined with the nature of their ligands. By utilizing ultrafast optical and X-ray transient absorption spectroscopies, we characterized a new set of heteroleptic Cu­(I) complexes [Cu­(PPh3)2(BPyR)]+ (R = CH3, H, Br to COOCH3), with an increase in the electron-withdrawing ability of the functional group (R). We found that after the transient photooxidation of Cu­(I) to Cu­(II), the increasing electron-withdrawing ability of R barely affects the internal conversion (IC) (e.g., Jahn-taller (JT) distortion) between singlet MLCT states. However, it does accelerate the dynamics of intersystem crossing (ISC) between singlet and triplet MLCT states and the subsequent decay from the triplet MLCT state to the ground state. The associated lifetime constants are reduced by up to 300%. Our understanding of the photoexcited dynamics in heteroleptic Cu­(I) complexes through ligand electronic tuning provides valuable insight into the rational design of efficient Cu­(I) complex photosensitizers.

Long Excited-State Lifetimes in Three-Coordinate Copper(I) Com-plexes via Triplet–Triplet Energy Transfer to Pyrene-Decorated Isocy-anides

“…9,10 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. [11][12][13][14][15][16] 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 βdiketiminate (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.…”

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 .…”