# Towards panchromatic Fe(ii) NHC sensitizers via HOMO inversion

Anil Reddy Marri, Bogdan Marekha, Thomas Penfold, Stefan Haacke,
Philippe Gros

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Anil Reddy Marri, Bogdan Marekha, Thomas Penfold, Stefan Haacke, Philippe Gros. Towards panchromatic Fe(ii) NHC sensitizers via HOMO inversion. Inorganic Chemistry Frontiers, 2023, 10, pp.118-126. 10.1039/D2QI01903E. hal-03856124

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Cite this: DOI: 10.1039/d2qi01903e

# Towards panchromatic Fe(II) NHC sensitizers via HOMO inversion†

Anil Reddy Marri, a,b Bogdan Marekha, c,d Thomas Penfold, e
Stefan Haacke *c and Philippe C. Gros *a

A combined experimental and theoretical study of iron(II) complexes with pyridyl N-heterocyclic carbene ligands is presented, with a new focus on the effect of extending the ligands with thiophenes of variable number. In agreement with recent theoretical predictions by the Jakubikova group, these ligands induce a substantial mixture of the occupied t<sub>2g</sub> and π HOMO orbitals, as manifested by a near 80 nm red-shift of the <sup>1</sup>MLCT transition and a 2–3 fold increase of the molar extinction coefficient. The thiophene moieties permit delocalisation of the MLCT state on the ligands, which results in excited state lifetimes in the 13–18 ps range, almost twice as much as for the reference compound lacking the thienyl substituents. Relaxation into <sup>3</sup>MC states remains the main <sup>3</sup>MLCT quenching mechanism, and the effect of the number of thiophenes in lengthening the <sup>3</sup>MLCT lifetime is qualitatively consistent with a reduction in the <sup>3</sup>MLCT–<sup>3</sup>MC energy gap.

Received 2nd September 2022,

Accepted 10th November 2022

DOI: 10.1039/d2qi01903e

rsc.li/frontiers-inorganic

## Introduction

The replacement of noble metals by earth-abundant substitutes is at the forefront of current research<sup>1-4</sup> aimed at developing sustainable photoactive materials required for solar energy conversion or photocatalysis. Iron is by far the most abundant metal in Earth's crust and appears a promising candidate to replace the ruthenium gold standard that is in the same column of the periodic table.

Ruthenium polypyridine complexes have been applied extensively for sensitization in Dye-sensitized solar cells (DSSCs).<sup>5-10</sup> Their photophysical and redox properties facilitate efficient light harvesting through long-lived Metal-to-Ligand Charge Transfer (MLCT) states which are responsible for electron transfer into semiconductors and efficient ground state regeneration after excitation.<sup>11-13</sup>

In contrast Fe(II)-polypyridines have short-lived MLCT states due to an ultrafast intersystem crossing populating their low-lying metal centred (MC) states.<sup>14-17</sup> To date increasing the ligand field strength has been the strategy of choice to increase

the energy of the unreactive low-lying ligand field states. Ligands based on σ-donating NHC (N-heterocyclic carbenes). The MLCT lifetime has reached tens of picoseconds for complexes with tridentate C^N C^{18-20} and bidentate C^N ligands,<sup>21-24</sup> and nanoseconds for compounds bearing tridentate C^C C^C ligands,<sup>25</sup> making it possible for iron to be used in photochemical applications such as iron-sensitized DSSCs.<sup>26-31</sup> However, apart from the recently published Fe(II) complex with phenN,N'^C ligands,<sup>32</sup> this improvement in lifetime was obtained at the sacrifice of achieving the panchromatic absorption required for solar energy conversion applications. Recently, "HOMO inversion" has emerged as a promising concept in the field.<sup>33-35</sup> This approach aims to adjust the filled orbital energetic levels in order to mix metal t<sub>2g</sub> and ligand molecular orbitals thus switching the HOMO localization from the metal to the ligand (Fig. 1). This approach was demonstrated experimentally by the Herbert group<sup>35</sup> to design new panchromatic iron(II) complexes using ligands combining amido donors with benzannulated phenanthridine and quinoline. The covalent amido-Fe bond deeply strengthened the

<figure><img src="image_3.png" /><figcaption>Fig. 1 Energetic diagrams illustrating the HOMO inversion concept.</figcaption></figure>

<figure><img src="image_4.png" /></figure>

<footnote><sup>a</sup>Université de Lorraine, CNRS, L2CM, F-54000 Nancy, France.</footnote>

<footnote>E-mail: philippe.gros@univ-lorraine.fr</footnote>

<footnote><sup>b</sup>Department of Chemistry, North Carolina State University, 851 Main Campus Drive, Raleigh, North Carolina, 27695-8204, USA</footnote>

<footnote><sup>c</sup>Université de Strasbourg, CNRS, IPCMS, F-67000 Strasbourg, France</footnote>

<footnote><sup>d</sup>Max-Planck-Institute for Medical Research, D-69120 Heidelberg, Germany</footnote>

<footnote><sup>e</sup>Chemistry-School of Nat. & Env. Sci., Newcastle University, Newcastle Upon-Tyne, UK</footnote>

<footnote>†Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2qi01903e</footnote>
Research Article

<figure><img src="image_5.png" /><figcaption>**Fig. 2** Complexes studied in this work. The counterion is PF₆⁻ throughout.</figcaption></figure>

ligand field subsequently producing long lived excited states with up to 2.5 ns lifetime.

The group of E. Jakubikova³³ has reported a computational work on a series of iron(II) polypyridine complexes and predicted that the ligand π-delocalization should promote notable changes in absorption features and suggested that the same effect could be expected with pyridylNHC Fe(II) complexes. To our knowledge besides this exclusively computational work, the effect of π-delocalization and the impact of HOMO inversion on the photophysical and electronic properties of pyridylNHC Fe(II) complexes has not been investigated yet.

Herein we report the synthesis of complexes **CTh**, **CTh2** and **CTh3** designed with thiophene and fused polythiophenes conjugated with the central pyridine ring of the pyridylNHC ligand (Fig. 2) and the investigation of their photophysical properties by means of ultra-fast transient absorption. Complex **C0** is the proto-typical [Fe(bmip)₂]²⁺, where bmip = 2,6-bis(3-methyl-imidazole-1-ylidine)-pyridine, published by the group of Wärnmark.¹⁸,³⁶ We demonstrate that, as compared to **C0**, extending the delocalization promotes an important red-shift of the pyridine-MLCT absorption band and an impressive increase of the molar extinction coefficients. The excited state lifetimes were also increased substantially by comparison with the unsubstituted complexes while they were slightly impacted by the conjugation extent.

Results and discussion

Synthesis of ligands and complexes

The imidazolium salts **L1**, **L2** and **L3**, that are the required precursors of the pyridyl-NHC ligands to be coordinated to iron, were first synthesized (Scheme 1, see ESI† for experimental details). **L1** was obtained following our reported procedure.²⁷ **L2** and **L3** were obtained in good yields respectively by quaternization of pyridylimidazoles **3** and **4** resulting from a Suzuki coupling of the bromo derivative **1**²⁷ with the appropriate preformed thienyl boron pinacol esters that were obtained by a lithiation-borylation sequence on the parent thienyl compounds (see ESI†).³⁷,³⁸ The target complexes were finally

<figure><img src="image_6.png" /><figcaption>**Scheme 1** Synthesis of ligand precursors and thienyl substituted complexes.</figcaption></figure>

obtained in good to moderate yield applying an *in situ* coordination protocol. FeCl₂ was first reacted with the appropriate ligand precursor, then *t*-BuOK was added to generate the carbene moiety. The coordination yield was found dependent on the number of thiophenes attached to the central pyridine. This agrees with the weakening of the pyridine nitrogen coordination ability by conjugation with the extended π-system.

Electronic properties of complexes

The UV-Vis absorption spectra of the three complexes in deaerated acetonitrile solution are shown in Fig. 3 and the data concerning the lowest energy bands are collected in Table 1. The complexes display three distinct absorption bands. The band at higher energy (250–350 nm) is assigned to ligand-centered π-π* transitions, while the two other bands at lower energy, correspond to Fe-NHC transitions (340–420 nm) and Fe-pyri-

<figure><img src="image_7.png" /><figcaption>**Fig. 3** UV-Vis spectra of complexes in acetonitrile. Inset: normalized absorbance for the lowest energy band.</figcaption></figure>
<table><caption>**Table 1** Spectroscopic and electrochemical properties of complexes</caption><thead><tr><th>Complex</th><th>λ<sub>max</sub> (nm)<br>(ε(M<sup>-1</sup> cm<sup>-1</sup>))</th><th>E<sub>ox</sub> (Fe<sup>II</sup>/Fe<sup>III</sup>)<br>(V SCE<sup>-1</sup>)</th><th>E<sub>Red</sub> (V SCE<sup>-1</sup>)</th><th>ΔE</th></tr></thead><tbody><tr><td>C0<sup>18,22</sup></td><td>284 (31 433)<br>390 (9077)<br>457 (15 955)</td><td>0.80 (rev)</td><td>-1.95 (irr)</td><td>2.75</td></tr><tr><td>CTh</td><td>319 (51 619)<br>399 (12 519)<br>516 (39 330)</td><td>0.68 (rev)</td><td>-1.69 (qr)</td><td>2.37</td></tr><tr><td>CTh2</td><td>328 (61 027)<br>401 (18 354)<br>526 (44 300)</td><td>0.71 (rev)</td><td>-1.56 (qr)</td><td>2.27</td></tr><tr><td>CTh3</td><td>359 (51 200)<br>535 (30 240)</td><td>0.73 (rev)</td><td>-1.52 (irr)</td><td>2.25</td></tr></tbody></table>

dine MLCT transitions (410–650 nm). In **CTh3**, the very broad π–π* absorption band of the dithienothiophene moiety masks the Fe-NHC MLCT band.

As a general trend, the substitution of complex C0 with thiophenes promoted a notable bathochromic effect on both the π–π* and Fe-pyridine MLCT bands and the amplitude of the red-shift was directly related to the number of thiophene units (Δλ = 78 nm going from C0 to CTh3). In contrast, the thiophenes had no impact on the Fe-carbene MLCT band the transition energy of which remained constant.

Focusing on the lowest-energy portion of the spectrum, the MLCT Fe-pyridine band, **CTh**, **CTh2** and **CTh3** present a broad and intense absorption, ranging from 450 to 620 nm. Among all, **CTh3** showed the broadest and most red shifted absorption (535 nm, ε = 30 240 M⁻¹ cm⁻¹) and **CTh2** the highest extinction coefficient (526 nm, ε = 44 300 M⁻¹ cm⁻¹).

The cyclic voltammetry data (Table 1) globally confirm the UV-Vis absorption spectroscopy results. The electrochemical gap markedly decreased when comparing **CTh** to **C0** in agreement with the 59 nm redshift of the MLCT band upon addition of one thiophene unit on the central pyridine. Also corroborating the absorption spectra, a decrease of the gap was observed albeit in a lesser extent going from **CTh** to **CTh3**.

Table 2 gathers the computed energy and oscillator strengths for the lowest singlet excited states of C0, CTh, CTh2 and CTh3 that are associated with the lowest band in the absorption spectrum. Consistent with the experimental observations, a red shift of the lowest transitions of CTh, CTh2 and

<table><caption>**Table 2** Computed energies of the HOMO and LUMO energies and lowest singlet excited states of C0, CTh, CTh2 and CTh3 calculated at the ground state optimized geometry using TDDFT(B3LYP*)</caption><thead><tr><th>Complex</th><th>λ (nm) (f)</th><th>ε<sub>HOMO</sub>/eV</th><th>ε<sub>LUMO</sub>/eV</th><th>ΔE/eV</th></tr></thead><tbody><tr><td rowspan="2">C0</td><td>518 (0.00163)</td><td rowspan="2">-5.33</td><td rowspan="2">-2.02</td><td rowspan="2">3.31</td></tr><tr><td>518 (0.00162)</td></tr><tr><td rowspan="2">CTh</td><td>615 (0.00340)</td><td rowspan="2">-5.29</td><td rowspan="2">-2.41</td><td rowspan="2">2.88</td></tr><tr><td>614 (0.00370)</td></tr><tr><td rowspan="2">CTh2</td><td>628 (0.00290)</td><td rowspan="2">-5.32</td><td rowspan="2">-2.56</td><td rowspan="2">2.76</td></tr><tr><td>627 (0.00260)</td></tr><tr><td rowspan="2">CTh3</td><td>637 (0.00430)</td><td rowspan="2">-5.33</td><td rowspan="2">-2.62</td><td rowspan="2">2.71</td></tr><tr><td>637 (0.00340)</td></tr></tbody></table>

**CTh3** compared to C0 is observed and this increases with the delocalization of the MLCT state. The HOMO–LUMO gap is slightly larger than experimentally measured, but exhibits the same trend as the cyclic voltammetry data in Table 1.

The HOMO and LUMO have been plotted for all the complexes (Fig. 4 and ESI†). Fig. 4 clearly evidenced a significant enhancement of the mixing between the ligand and metal orbitals in the HOMO as the thiophene units are included. The Fe 3d orbitals contribution to the HOMO decreased with the increase of the number of thienyl units in the substituent.

### Excited state dynamics and relaxation

Temporal evolution of the fs-TAS signal of the **CTh** series of molecules bears a number of common features and therefore will be illustrated on the example of **CTh3** (Fig. 5, see the ESI† for data on the other molecules). In particular, upon optical excitation of the Fe-pyridine MLCT transition by the pump pulse the differential transient spectra in the 360–680 nm spectral range reveal two negative and two positive features. The former ones (<390 nm, 500–580 nm) come from the depletion of the population of the initial state (ground state bleaching, GSB) and peak around the absorption maxima of the Fe-pyridine and Fe-NHC MLCT transitions (Fig. 5B). In turn, the positive features originate from the excited state absorption (ESA) of the states populated by the pump pulse. Notably, the high-energy ESA (400–470 nm) shows twice the intensity of the low energy ESA feature (>580 nm).

The initially observed spectral features exhibit an ultrafast evolution on the sub-200 fs time scale. Namely, the high-energy ESA undergoes a pronounced blue-shift, which is most evident by the zero-crossing point moving from 500 nm at 0 ps to ~475 nm at 200 fs (Fig. 5B and 480 nm trace in Fig. 5C). The low-energy ESA shows similar development. Also, a somewhat delayed rise of the low-energy GSB band suggests that it overlaps with the ultrafast evolving ESA. Further temporal evol-

<figure><img src="image_8.png" /><figcaption>**Fig. 4** HOMO–LUMO plots for all the complexes including the contribution of Fe 3d orbitals in HOMO.</figcaption></figure>

<figure><img src="image_9.png" /></figure>
Research Article

<figure><img src="image_10.png" /><figcaption>**Fig. 5** Fs-TAS data for CTh3. (A) Two-dimensional color-plot as a function of pump-probe delay and probe wavelength. Positive signals (induced absorption) are in shades of red, whereas negative ones (bleach) are in shades of blue. Contour lines are drawn at 10% intervals of the absolute maximum value. The data around 535 ± 10 nm were omitted due to large noise of scattered pump light overlaying the TAS signal. The time axis is logarithmic above 1 ps delay. (B) Transient spectra at selected delays. Inverse-signed steady-state UV-vis absorption spectrum is shown for comparison (dashed line). (C) Kinetic traces at selected probe wavelengths.</figcaption></figure>

ution shows little spectral variation and rather reflects intensity decay on the ~15 ps time scale. The ESA and GSB features decay concomitantly thereby evidencing complete relaxation of the excited state population back to the ground state.

To provide a quantitative framework to the observed fs-TAS results and given the concerted nature of temporal evolution of different transient spectral features, we performed a global analysis³⁹ using GloTarAn software⁴⁰ assuming a sequential decay model convoluted with a Gaussian instrument response function (IRF). The data could be satisfactorily described by a sum of 4 exponential functions (see the ESI† for the discussion on the relevance of the 4th exponential function). The corresponding decay time constants are reported in Table 3 and the evolution associated difference spectra (EADS) for CTh3 are shown in Fig. 6.

In accordance with the qualitative analysis of the experimental data, the first sub-100 fs component revealed by the global analysis accounts for the initial blue-shift of both high

<figure><img src="image_11.png" /><figcaption>**Fig. 6** Evolution associated difference spectra (EADS) obtained from global analysis of CTh3 fs-TAS data. The corresponding decay time constants are indicated in the legend.</figcaption></figure>

and low-energy ESA bands. The corresponding spectral profile of EADS also suggests that the high-energy part of the ESA feature is very intense as it largely compensates the GSB signal around the pump wavelength. The other sub-ps component bears some residual features of this transient blue-shift of the high energy ESA and corresponds to a significant part of the intensity decay of the low energy ESA on a sub-picosecond time scale (Fig. 5B). The two slower components mostly reveal the complete intensity decay back to the ground state with very little changes in band shape.

The physical picture of the photodynamics of these molecules upon MLCT optical excitation that emerges from these results resembles previous results from us¹⁹,²⁰,⁴¹⁻⁴³ and others¹⁸,⁴⁴ on similar tridentate complexes with 4 NHC ligands. Namely, the initially created ¹MLCT state is converted through an ultrafast (<100 fs) spin-crossover to a hot non-emissive (no noticeable stimulated emission observed) ³MLCT state which then undergoes a sub-ps vibrational relaxation. This thermalized triplet state then returns to the ground state on the ps time-scale. In particular the last, τ₄ ~ 18 ps, step corresponds to the return to the ground state, involving ultrafast relaxation through lower energy ³MC states, in accordance with the relaxation cascade observed for the parent pyridylNHC Fe (II) complexes.¹⁸,¹⁹,⁴¹⁻⁴³ In particular the reference complex C0 was studied in detail,¹⁸,³⁶ demonstrating a 9.0 ps lifetime of the ³MLCT state (corresponding to τ₄).

The τ₃ = 5.2 ps is a relevant time constant since it is needed for an improved quality of the fits, in particular in the low energy bleach part (see Fig. S18†). However, unlike the 0.43 ps component, its EADS does not show a significant difference in spectral shape with the 17.9 ps EADS, unlike similar time components we found in previous work with diazinyl-NHC ligands.²⁰ This suggests two alternatives for its assignment. It could be due to a residual slower thermalization process which involves vibrational modes that are less strongly coupled to the optical transitions and which therefore leads to little spectral variation. Alternatively, τ₃ and τ₄ may correspond

<table><caption>**Table 3** Time constants derived from the analysis of the TAS dataª</caption><thead><tr><th></th><th>CTh</th><th>CTh2</th><th>CTh3</th></tr></thead><tbody><tr><td>τ<sub>1</sub> (ps)</td><td>&lt;0.01</td><td>0.030</td><td>0.045</td></tr><tr><td>τ<sub>2</sub> (ps)</td><td>0.17</td><td>0.21</td><td>0.43</td></tr><tr><td>τ<sub>3</sub> (ps)</td><td>3.0</td><td>4.7</td><td>5.2</td></tr><tr><td>τ<sub>4</sub> (ps)</td><td>13.2</td><td>15.5</td><td>17.9</td></tr></tbody></table>

<figure><img src="image_12.png" /></figure>

<footnote>ª For the reference complex C0, τ₄ is 9 ps.¹⁸,³⁶</footnote>
to two different ³MLCT–³MC relaxation pathways in the multi-
dimensional excited state landscape. As a side remark, a paral-
lel population of both ³MLCT and ³MC on the sub-picosecond
time scale³⁶,⁴³,⁴⁵ cannot be excluded, since population of the
latter states go commonly un-noticed in UV-VIS transient
absorption experiments.⁴²

Concerning the influence of the number of fused thiophene
rings in the **CTh** series of molecules on the excited state
dynamics one can see from the results of the global analysis
(Table 3) as well from the bleach recovery kinetics (Fig. 7) that
increasing the number of fused thiophene rings and thereby
promoting the electron delocalization in the MLCT state leads
to a moderate slow-down of all the steps of the excited state
relaxation. Nonetheless, this delocalization effect is apparently
not enough to alter the overall relaxation cascade mechanism
observed for related Fe-NHC complexes.

A Jablonski diagram showing the computed energetics of
the key states for the series of complexes is shown in Fig. 8.
While the MLCT states show a distinct red shift, the ³MC
states, which mix very little with the fused thiophene units
(see spin density in Fig. S20†) remain largely unchanged
among the **CTh**, **CTh2** and **CTh3** series compared to **C0**. The
geometry of all the important structures are provided in the
ESI.†

Given the significant structural reorganization between the
³MLCT and ³MC states, the decreasing energy gap (ΔE)
between the two states as the number of thiophene units is
increased may introduce a small barrier to the transformation
between the two states, responsible for the increase in τ₃ and
τ₄ observed experimentally. This counterintuitive effect is high-
lighted in Fig. 9. Indeed, and as in Marcus theory for electron
transfer,⁴⁶ the energy barrier between the states decreases, and

<figure><img src="image_13.png" /><figcaption>Fig. 7 Semi-log plot of normalized kinetics of the bleach recovery of CTh, CTh2 and CTh3. Dots are measured datapoints whereas lines are results of the global fit (see text).</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>**Fig. 8** Computed Jablonski diagrams of the important excited states in (a) C0, (b) CTh, (c) CTh2 and (d) CTh3. Using TDDFT, the energies of the ¹,³MLCT states were computed at the ⁵MLCT optimized geometry and the energies of the ³MC was computed at the ⁵MC optimized geometry (¹MLCT and ³MLCT states have similar geometries). Both geometries were computed using UKS and the relevant geometries are in the ESI.†</figcaption></figure>

<figure><img src="image_15.png" /><figcaption>**Fig. 9** Schematic representation of the ³MLCT and the ³MC states approximated as harmonic potentials along an effective structural coordinate Q. When the energy gap ΔE decreases (left to right), the barrier Vb between ³MLCT and ³MC increases, leading to a longer ³MLCT lifetime.</figcaption></figure>

reaches zero only when ΔE is equal to the structural reorganization energy. A similar behavior was recently also put forward in a study on solvent effects of Fe complexes.⁴⁷

Conclusions

In conclusion, we present here a combined experimental and
theoretical study of iron(II) complexes with pyridyl NHC
ligands conjugated with thiophene and fused polythiophenes.
As predicted theoretically,³³ these ligands induce a substantial
mixture of the occupied t₂g and π HOMO orbitals, as mani-
fested by a near 80 nm red-shift of the ¹MLCT transition and a
2-3-fold increase of the extinction coefficient. The thiophene
moieties allow for a delocalisation of the MLCT state on the
ligands, which results in excited state lifetimes in the 13-18 ps
range, almost twice as much as for the reference compound
lacking the thiophene side groups. Relaxation into ³MC states
remains the main ³MLCT quenching mechanism, and the
effect of the number of thiophenes in lengthening the ³MLCT lifetime is qualitatively consistent with a stabilisation of the ³MLCT energy due to increased charge delocalisation. As a result, the ³MLCT–³MC energy gap is reduced (Fig. 8), which leads to an increased barrier for the excited state structural relaxation involved in the ³MLCT–³MC transition (Fig. 9). Works are in progress to further amplify the mixing between metal and ligand orbitals *via* a wider π-extended delocalization in various parts of the ligands.

# Experimental

## General

Solvents and commercially available reagents were used as received. Thin layer chromatography (TLC) was performed by using silica gel 60 F-254 (Merck) plates and visualized under UV light. Chromatographic purification was performed by using silica gel 60 (0.063–0.2 mm per 70–230 mesh). ¹H (400 MHz) and ¹³C NMR (100 MHz) spectra were taken on a DRX400 Bruker spectrometer at ambient temperature. The chemical shifts (δ), were calibrated by using either tetramethylsilane (TMS) or signals from the residual protons of the deuterated solvents and are reported in parts per million (ppm) from low to high field, NMR signal assignments for L2, L3, CTh, CTh2 and CTh3 are detailed in the ESI.†

High-resolution mass spectrometry (HRMS) data was obtained by using Bruker micrOTOF-Q spectrometer. UV-vis spectra were recorded in a 1 cm path length quartz cell on a LAMBDA 1050 (PerkinElmer), spectrophotometer. Cyclic voltammetry was performed on a Radiometer PST006 potentiostat using a conventional three-electrode cell. The saturated calomel electrode (SCE) was separated from the test compartment using a bridge tube. The solutions of studied complexes (0.2 mM) were purged with argon before each measurement. The test solution was acetonitrile containing 0.1 M Bu₄NPF₆ as supporting electrolyte. The working electrode was a vitreous carbon rod (1 cm²) wire and the counter-electrode was a 1 cm² Pt disc. After the measurement, ferrocene was added as the internal reference for calibration. All potentials were quoted versus SCE. In these conditions the redox potential of the couple Fc⁺/Fc was found at 0.39 V. In all the experiments the scan rate was 100 mV s⁻¹.

## Synthesis of complexes

**CTh.** To an Ar degassed solution of L1²⁷ (88 mg, 0.126 mmol) in 2 mL of anhydrous DMF was added anhydrous FeCl₂ (8 mg, 0.063 mmol). Then *t*-BuOK (0.071 g, 0.630 mmol) was added to the above mixture and stirred at room temperature for 20 min. A saturated aqueous solution of NH₄PF₆ was added (10 ml) and the precipitate was collected by filtration. The crude product was further purified by silica gel column chromatography eluting with an acetone/H₂O/KNO₃(sat) = 10/3/0.5 mixture. The reddish fraction was collected and after evaporation of acetone, the left solution was treated with a saturated solution of NH₄PF₆. The desired complex precipitate

was then filtered, washed with distilled water followed by ether, and dried under vacuum. **CTh** was obtained as a reddish solid (50 mg, 68% yield). ¹H NMR (400 MHz, CD₃CN, δ ppm): 8.12 (d, *J* = 2.19 Hz, 2H), 7.95 (s, 2H), 7.84 (d, *J* = 3.72 Hz, 1H), 7.06 (s, *J* = 3.72 Hz, 1H), 7.04 (d, *J* = 2.19 Hz, 2H), 2.99 (t, *J* = 7.51 Hz, 2H), 2.62 (s, 6H), 1.79 (q, 2H), 148–1.35 (m, 6H), 0.95 (t, *J* = 7.09 Hz, 3H). ¹³C NMR (100 MHz, CD₃CN, δ ppm): 201.1, 154.5, 151.4, 144.5, 138.0, 128.7, 127.2, 127.0, 116.1, 101.6, 35.3, 32.0, 31.8, 30.5, 29.0, 22.9, 14.0. ESI-HRMS calcd for C₄₆H₅₄FeN₁₀S₂ *m/z* = 433.1656. Found: 433.1653 [M – 2PF₆].

**CTh2.** The same procedure was repeated using L2 (80 mg, 0.119 mmol), FeCl₂ (8 mg, 0.06 mmol) and *t*-BuOK (68 mg, 0.597 mmol). **CTh2** was obtained as a purple solid (50 mg, 38% yield). ¹H NMR (400 MHz, CD₃CN, δ ppm): 8.26 (s, 1H), 8.17(d, *J* = 2.10 Hz, 2H), 8.05(s, 2H), 7.76(d, *J* = 5.30 Hz, 1H), 7.52(d, *J* = 5.28 Hz, 1H), 7.05(d, *J* = 2.10 Hz, 2H), 2.64(s, 6H). ¹³C NMR (100 MHz, CD₃CN, δ ppm): 200.8, 154.6, 144.6, 142.2, 141.7, 141.0, 131.7, 127.1, 120.9, 120.7, 117.1, 101.9, 35.3. ESI-HRMS calcd for C₃₈H₃₀FeN₁₀S₄ *m/z* = 405.0438. Found: 405.0493 [M – 2PF₆].

**CTh3.** The same procedure was repeated using L3 (57 mg, 0.079 mmol), FeCl₂ (5 mg, 0.039 mmol) and *t*-BuOK (44 mg, 0.392 mmol). **CTh3** was obtained as a purple solid (13 mg, 27% yield). ¹H NMR (400 MHz, CD₃CN, δ ppm): 8.30 (s, 1H), 8.17 (d, *J* = 2.12 Hz, 2H), 8.06 (s, 2H), 7.71 (d, *J* = 5.29 Hz, 1H), 7.53 (d, *J* = 5.30 Hz, 1H), 7.06 (d, *J* = 2.09 Hz, 2H), 2.65 (s, 6H). ¹³C NMR (100 MHz, CD₃CN, δ ppm): 200.8, 154.6, 144.5, 144.2, 143.3, 140.8, 133.1, 131.2, 129.7, 127.2, 122.4, 122.0, 117.1, 101.8, 35.3. ESI-HRMS calcd for C₄₂H₃₀FeN₁₀S₆ *m/z* = 461.0159. Found: 461.0175 [M – 2PF₆].

## Computational details

All complexes were optimized in the ground state and ³MC and ³MLCT states using DFT within the approximation of the B3LYP* exchange and correlation functional⁴⁸ and a def2-SVP basis set⁴⁹ as implemented within the ORCA quantum chemistry package.⁵⁰ While this is only valid for the lowest state of each multiplicity, both the ³MC and ³MLCT are the lowest triplets at their respective optimized geometries. Numerical frequencies calculations were carried out on the geometries optimized to ensure no negative frequencies. TDDFT calculations, used to calculate the excited state energies of each complex used the Tamm–Dancoff approximation⁵¹ and were also performed using B3LYP*. All simulations were performed using a polarisable continuum model with the parameters of acetonitrile.

## Fs-TAS measurements

The home-built transient absorption spectrometer has been described in detail elsewhere⁵²,⁵³ and only substantial parameters will be given here. The pump pulses were tuned to be in resonance with the absorption maxima of the studied compounds (510 nm for **CTh**, 525 nm for **CTh2** and 535 nm for **CTh3**). The probe light covering 360–700 nm range was generated by focusing 40 fs 800 nm pulses of the primary laser

<figure><img src="image_16.png" /></figure>
source in a 2 mm thick CaF₂ plate mounted on a motorized
linear stage to reduce thermal effects. Sample solutions (1 mM
in dry acetonitrile) were circulated in a flow cell with 200 µm
optical pathlength (Starna). The probe beam was focused to *ca.*
60 µm Gaussian spot diameter (1/*e*²). The pump spot size was
*ca.* 150 µm in diameter (1/*e*²) and typical maximal energy den-
sities were 0.25 mJ cm⁻² (in the center of the Gaussian beam
profile) to keep the fraction of excited molecules below 5%.
Solvent subtraction and correction for wavelength-dependent
time-zero were performed using previously established
protocols⁵²,⁵³ using the neat solvent data acquired under iden-
tical conditions.

Author contributions

A. R. M. performed the synthesis and ground state characteriz-
ation of ligands and complexes. B. M. performed transient
absorption spectroscopy experiments and analysed the results
with S. H., T. P. performed DFT computations and analysed
the results. P. C. G. conceived the project and wrote the manu-
script with B. M., T. P. and S. H.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The Strasbourg team is grateful to Dr O. Crégut for expert help
in laser maintenance and development of data acquisition
software, and for the funding provided by the ITI 2021-28
program of the University of Strasbourg, CNRS and Inserm,
supported by IdEx Unistra (ANR 10 IDEX 0002), by SFRI
STRAT'US project (ANR 20 SFRI 0012) and EUR QMAT ANR-17-
EURE-0024 under the framework of the French Investments
for the Future Program. The L2CM thanks the French Agence
Nationale de Recherche (ANR-16-CE07-0013-02), Lorraine
Université d'Excellence (IMPACT N4S) and the European
Regional Development Funds (Programme opérationnel
FEDER-FSE Lorraine et Massif des Vosges 2014-2020/"Fire
Light" project: "Photo-bio-active molecules and nanoparticles"
for financial support and grants to A. R. M. The L2CM also
warmly thanks S. Parant for the electrochemical experiments
and F. Dupire from the mass spectrometry MassLor platform
of Lorraine. TJP acknowledges the EPSRC through grant EP/
V010573/1 for support.

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