Living Long and Prosperous: Productive ILCT States from a Re(I) Terpyridine Photosensitiser with Enhanced Light Absorption and How to Tune Them

Ricardo J. Fernández-Terán 1,2 , Laurent Sévery 1

  • 1 Department of Chemistry, University Of Zurich, Zurich
  • 2 Department of Chemistry, University of Sheffield, Sheffield

Abstract

The ground- and excited-state properties of six Re(I) κ²N-tricarbonyl [1] and six κ³N-dicarbonyl [2] complexes with 4'-(4-substituted-phenyl)-terpyridine ligands bearing substituents of different electron donating abilities were evaluated. Significant modulation of the electrochemical potentials and a nearly fourfold variation of the triplet metal-to-ligand charge transfer (³MLCT) lifetimes were observed when going from CN to OMe.

With the more electron-donating NMe₂ group, we observed in the κ²N complex the appearance of a very strong absorption band, red shifted by ca. 100 nm with respect to the other complexes in the series. This was accompanied by a dramatic enhancement of the excited-state lifetime (380 ns vs 1.5 ns), and a character change from ³MLCT to intraligand charge transfer (³ILCT), despite the remote location of the substituent. This change, however, was absent in the corresponding κ³N-dicarbonyl counterparts.

The dynamics and characters of the excited states of all complexes were assigned by combining transient IR spectroscopy, IR spectroelectrochemistry and (TD-)DFT calculations. Selected complexes were evaluated as photosensitisers for hydrogen production, with the κ²N-NMe₂ complex resulting in a stable and efficient photocatalytic system reaching TON(Re) values of over 2100, representing the first application of the ³ILCT state of a Re(I) carbonyl complex in a stable photocatalytic system.

[1] R. Fernández-Terán and L. Sévery, Inorg. Chem. 2021, 60 (3), 1334–1343.

[2] R. Fernández-Terán and L. Sévery, Inorg. Chem. 2021, 60 (3), 1325–1333.

κ²N-Tricarbonyl Complexes: From MLCT to ILCT by Remote Substitution

  • A systematic modification in the triplet lifetimes was observed: more electron-donating substituents increase the lifetime.
  • The ground-state electrochemical potentials of the k2N-tricarbonyl complexes are also systematically changed: complexes with electron-donating substituents are stronger (photo)reductants.
  • The complexes 2a-e are emissive and show a strong rigidochromic blue shift upon cooling to 77 K (in 2-MeTHF), characteristic of 3MLCT excited states.

Synthetic route and structures of the studied complexes

HOMO and LUMO orbitals of the complexes with more electron-donating substituents (2e-f) and the unsubstituted/reference complex (2d)

Charge density difference isosurfaces of the S1 excitation of the complexes with more electron-donating substituents (2e-f) and the unsubstituted/reference complex (2d)

We evidenced a systematic shift in the UV-Vis absorption spectra, where an additional very intense transition was observed in the case of the NMe2-substituted complex (2f).

Low-temperature emission spectra allowed us to determine the singlet-triplet energy gap ΔGST, crucial to obtaining the excited-state redox potentials.

TRIR spectra of complexes 2a-e (R1= CN to OMe) exhibited the typical features of an MLCT excited state (i.e. blue-shifted excited-state absorption bands)

TRIR spectra of complex 2f (R1 = NMe2) shows features attributed to an intraligand charge transfer (ILCT) state, which compare favourably with the difference spectrum of the 1st reduction (IR-SEC, blue dashed line).


A significantly longer lifetime (380 ns for 2f, vs 2 ns for the unsubstituted complex, 2d) was observed in this case.

Hydrogen evolution from 2f as a photosensitizer. Conditions: [2f] = 50 μM, [Co] = 0.5 mM, [dmgH2] = 3.5 mM, 1
M TEOA, 0.1 M TfOH in DMF, and λexc = 453 nm.

 

Sustained hydrogen evolution was observed when combining complex 2f with a standard Cobalt-DMG catalyst in DMF with triethanolamine (TEOA) as sacrificial electron donor.

 

The initial electron transfer step in this photocatalytic system is believed to consist of oxidative quenching of the [Re]* photosensitiser, followed by very quick regeneration by TEOA.

 

This shows that ILCT states can be used to harvest more sunlight through a ca. 100 nm red shift of the absorption, while mantaining similar redox potentials and longer lifetimes than MLCT states in the same series of complexes.

κ³N-Dicarbonyl Complexes: Coordination Environment Prevents Access to ILCT States

Synthetic route and structures of the studied complexes

  • The {Re(CO)2}+ moiety with a κ3N-terpy coordination framework shows panchromatic absorption throughout the entire UV-Vis spectrum.

 

  • The ground- and excited-state properties of these complexes were examined by TRIR spectroscopy and a vast array of methods.

 

  • Contrary to their tricarbonyl counterparts, the κ3N-terpy dicarbonyls are non-emissive.

 

  • Their electronic properties were systematically varied, and it was shown that the excited states preserve their MLCT character irrespective of the substituent.

 

  • The ground-state electrochemical potentials of the κ3N-dicarbonyl complexes are similar to those of their κ2N-tricarbonyl counterparts, but they are stronger excited-state reductants.

 

  • The red shifts in the νCO frequencies highlight the increased electronic density around the metal centre.

Systematic variation in the triplet excited-state lifetimes of the tricarbonyl (2a-e) and dicarbonyl (3a-f) complexes. Complex 2f (R1 = NMe2) is an outlier given its very long lifetime (380 ns) and different excited-state character.

  • All dicarbonyl complexes had short lifetimes, as expected given the fact that their absorption is significantly red shifted (~720 nm) compared to that of the tricarbonyls (~380 nm).

 

  • TRIR spectroscopy confirmed the 3MLCT character of the excited states of all these complexes, also in agreement with DFT calculations.

TRIR spectroscopy of the dicarbonyl complexes (3a-f) confirms their MLCT character in both the singlet and triplet excited state manifolds.

Conclusions and Outlook

Conclusions:

  • The coordination geometry around the Re(I) metal centre plays a central role in determining the photophysical properties.

  • The κ3N-terpy dicarbonyl complexes have red-shifted and panchromatic absorption, are non-emissive and are stronger excited-state reductants, but their lifetimes are much shorter.

  • The κ2N-NMe2 substituted complex (2f) has a significantly longer lifetime, due to a switch in the excited-state character from MLCT to ILCT.

  • Complex 2f acts as a stable, robust and efficient photosensitiser. Mechanistic studies of ILCT-mediated photocatalysis are underway.

  • The observed substituent effects give valuable feedback for the design of new photosensitisers.

Outlook:

  • Mechanistic studies of ILCT-mediated photocatalysis are underway.
  • Bond-specific IR excitation to control the balance between ILCT vs MLCT states.
  • Protonation state tuning of ILCT vs MLCT excitation characters.
  • Solvent-controlled state ordering.

Acknowledgements:

We thank Prof. Dr. Roger Alberto and his group for the use of the photochemical H2 evolution detection system and laboratory facilities. Dr. Benjamin Probst-Rüd, Prof. Dr. Bernhard Spingler, Dr. Jan Helbing and Dr. Gökçen Tek are acknowledged for insightful discussions. 

 

This research was funded through the Swiss National Science Foundation (Sinergia Project CRSII2 160801/1, and AP Energy Grant PYAPP2 160586), and the University Research Priority Program (URPP) for Solar Light into Chemical Energy Conversion (LightChEC) of the University of Zurich.

Introduction and Motivation - Why, how and what?

(Open each tab to read more and see all the figures!)

The conversion of solar energy to fuels requires efficient light harvesting as a first step.

We focused on enhancing the light harvesting properties of the widely used Re(I) carbonyl-based photosensitisers.

The ground- and excited-state properties of these complexes were studied and compared to better understand the effect of substitution at a remote position and the coordination environment.

A quantitative comparison of Re(I) κ2N-terpy tricarbonyl and κ3N-terpy dicarbonyl complexes was performed.

 

The substituents in a position far away from the metal centre were systematically varied to ascertain the applicability of this approach for tuning the properties of these complexes.

 

Functional groups with strongly donating and withdrawing electronic properties were evaluated.

 

A vast array of experimental (UV-Vis, Emission, time-resolved IR spectroscopy, electrochemistry, FT-IR spectroelectrochemistry, photocatalytic hydrogen monitoring) and theoretical methods (DFT and TD-DFT) were used to study these systems in detail.

Summary of the studied complexes with different electron-donating and withdrawing substituents


Pulse sequence for pump-probe spectroscopy. In the present study, time-resolved IR (TRIR) was used to monitor the dynamics and properties of these complexes.

Transient infrared (TRIR) spectroscopy provides a unique and detailed picture of the changes in electronic density around the metal centre in this complexes in the shortest timescales (fs to µs).

 

Electrochemistry and Infrared Spectroelectrochemistry (IR-SEC) provide unique information about the energetics and spectroscopic signatures of the intermediates involved in electron transfer.