Coordination Environment Prevents Access to Intraligand Charge-Transfer States through Remote Substitution in Rhenium(I) Terpyridinedicarbonyl Complexes

Abstract

Six rhenium(I) 3N-dicarbonyl complexes with 4-(4-substituted phenyl)terpyridine ligands were evaluated in their ground and excited states. These complexes, bearing substituents of different electron-donating strengths-from CN to NMe2-were studied by a combination of transient IR (TRIR), electrochemistry, and IR spectroelectrochemistry, as well as time-dependent density functional theory (TD-DFT). They exhibit panchromatic absorption and can act as stronger photoreductants than their tricarbonyl counterparts. The ground-and excited-state potentials, absorption maxima, and lifetimes (250-750 ps) of these complexes correlate well with the Hammett p substituent constants, showing the systematic effect of remote substitution in the ligand framework. TRIR spectroscopy allowed us to assign the lowest singlet and triplet excited states to a metal-to-ligand charge-transfer (MLCT) character. This result contrasts our previous report on analogous 2N-tricarbonyl complexes, where remote substitution switched the character from MLCT to intraligand charge transfer. With the help of TD-DFT calculations, we dissect the geometric and electronic effects of coordination of the third pyridine, local symmetries, and increasing conjugation length. These results give valuable insights for the design of complexes with longlived triplet excited states and enhanced absorption throughout the visible spectrum, while showcasing the boundaries of the excited-state switching strategy via remote substitution.

Prolonging the Triplet State Lifetimes of Rhenium Complexes with Imidazole‐Pyridine Framework for Efficient CO2 Photoreduction

“…To better understand the driving force of the excited-state of these complexes to accept electrons from sacrificial reductant such as BIH from the viewpoint of thermodynamics, we estimated the excited-state reduction potentials, which were calculated with the equation E red ( 3 [Re I ]*) = E red vs. Fc + /0 + ΔG ST (E red vs. Fc + /0 is the ground state reduction potential, ΔG ST is the singlet-triplet energy gap). [23][24][25] Complexes Re-1À Re-4 were found to have excitedstate reduction potentials ranging from 0.56 to 0.34 V vs. Fc + /0 , as summarized in Table S17. As can be seen from the energy diagram (Figure 6b), these estimated E red ( 3 [Re I ]*) values are close to the redox potential of TEOA, thus limiting electron transfer from TEOA to the excited species.…”

“…[17][18][19][20][21][22] One of advantages with [Re-(CO) 3 (diimine)Cl] complexes is the well-defined structure able to design intentionally, which offers the possibility to enhance the visible light absorption ability and excited (triplet) state lifetime through ligand designing and optimization. [23][24][25][26][27][28] In general, with long excited triplet (T1) lifetime, photocatalysts could have enough time to realize electron transfer between different components so as to dramatically improve the activity. Nevertheless, the T1 is unobservable by conventional photoluminescence spectroscopy because it is readily quenched at the ambient atmosphere, [29] few studies have been conducted so far in the realm of photoinduced CO 2 RR in which T1 of the mononuclear Re catalysts is involved.…”

Dissecting Conjugation and Electronic Effects in the Linear and Non-Linear Optical Properties of Rhenium(I) Carbonyl Complexes

“…1.55 eV in the absorption energies of the S 1 state of the κ 3 N -dicarbonyl complexes (2a-b), in line with our previous results. 24 A further blue shift of ca. 0.5 eV is observed in the sequence 3 → 4 for both substituents.…”

High selectivity of photocatalytic reduction of CO2 to CO based on terpyridine ligand supported CuI metal organic framework

“…Among them, Fernández-Terán and coworkers adopted 4'-(4substituted-phenyl)-terpyridine bearing substituents of different electrondonating abilities allowed the remote control of the electron density on the ligands. As the result, the readily tuning of ground-and excited-state properties of the resulting coordination compounds shows the potential of the terpyridine frameworks for high activity/selectivity of photocatalytic reduction chemistry (Fernández-Terán and Sévery, 2021b).…”