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. 2026 Aug 24;17(39):18915–18923. doi: 10.1039/d6sc05359a

Excited state distortions reverse the decay lifetime trend in Fe(ii) phenanthroline complexes

Jessica Toigo a, Ujas Acharya a, Ka-Ming Tong a, Giliandro Farias b, Saeid Kamal a, Brian O Patrick a, Michael O Wolf a,✉
PMCID: PMC13501468  PMID: 42639400

Abstract

The correlation between increased ligand field strength and longer decay lifetimes in iron(ii) complexes is well established. In this work, we observe an unusual inverse relationship between lifetime and ligand field strength, where a stronger ligand field leads to a shorter lifetime. Two phenanthroline-iron(ii) complexes containing imidazol-2-ylidene (FephenImi) and benzimidazol-2-ylidene (FephenBzi) ligands are reported. Transient absorption experiments in acetonitrile reveal longer 3MLCT and 3MC lifetimes for FephenImi of 9 and 195 ps, respectively, compared to 2 and 53 ps, respectively, for FephenBzi. The 3MLCT lifetime is extended in dichloromethane for FephenImi to 27 ps. The differences in the excited-state dynamics are rationalized using DFT surface analysis. A closer-to-perfect octahedral geometry of FephenImi in the excited state increases the energy of the 3MC state and leads to a kinetic barrier between the triplet states. In contrast, the smaller axial angles in FephenBzi facilitate the 3MLCT → 3MC deactivation pathway despite the higher ligand field strength. These results provide a new perspective on geometric distortions in the excited states that may have an impact on the conversion between triplet states and increase the decay lifetime in iron(ii) complexes.

Excited state geometric distortions in iron(ii) phenanthroline NHC complexes show an unexpected excited state decay lifetime trend.graphic file with name d6sc05359a-ga.webp

Introduction

Photocatalysis relies on the availability of sensitizers that efficiently convert light into chemical reactivity.1–5 Many ruthenium(ii) and iridium(iii) complexes display the desired characteristics and are commonly used as photosensitizers.6–8 Although iron is 106–107 times more abundant in the Earth's crust than these noble metals, the use of iron(ii) in photocatalysis is limited by low-lying triplet metal-centred (3MC) states that result in ultrafast decay lifetimes.9–13 The lifetimes of iron(ii) complexes may be extended when strong field ligands are used to limit the population of deactivating MC states.11–15 However, in some cases, the decay lifetime shows a greater dependence on electronic coupling between deactivating states than ligand field strength.16–21 Here we show that phenanthroline N-heterocyclic carbene (NHC) ligands result in an unusual inversion in the decay lifetime trend, where a higher ligand field strength results in a shorter decay lifetime. The kinetic barrier between the 3MC and triplet metal-to-ligand charge-transfer (3MLCT) states dictates the excited-state dynamics in this case.

In general, strong σ-donating and π-accepting ligands increase ligand-field splitting, destabilizing the MC states leading to longer excited-state lifetimes (Fig. 1A).14,22,23 Extensions of the decay lifetime using this strategy are exemplified by the groundbreaking work of Wärnmark, Bauer, Gros, Heinze, and others, who have reported first-row NHC-based iron(ii) complexes using this approach.17,24–31 Computational and experimental studies show a correlation between the number of ligating NHCs present and the excited-state lifetime; longer lifetimes are observed with an increasing number of NHC units.29,32,33

Fig. 1. (A) Different NHC ligands showed initial extension of decay lifetimes in iron(ii) complexes.11,17,28 (B) A strong σ-donating ligand combined with phenanthroline (phen) leads to inversion of states in an iron(ii) complex and an increased MLCT lifetime.25 (C) A cis tetracarbene iron(ii) complex introduces a 3MLCT/3MC barrier, increasing the decay lifetime.10 (D) New iron(ii) complexes containing phenanthroline NHC ligands investigated here.

Fig. 1

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To obtain inversion between 3MC and 3MLCT states in photoactive iron(ii) complexes that absorb in the visible region requires lowering the energy of the charge-transfer band (Fig. 1B).34–40 This so-called “seesaw” effect acts to decrease the energy of the 3MLCT state relative to the 3MC state, extending the decay lifetime. The use of ligands containing conjugated groups, such as anthracene and pyrene, has proven to be beneficial to increase the decay lifetimes in iron(iii) and iron(ii) complexes.34–37,41 More recently, Berkefeld et al. reported a system in which both a conjugated moiety (phenanthroline) and a strong σ-donating cyclometalated ligand are coordinated to the metal center.42 This approach substantially increased the MLCT lifetime to 1 ns, one of the longest decay lifetimes reported to date.

We were intrigued by the possibility of applying a different perspective on the photophysics of iron(ii) complexes focusing on excited state distortions. The extent of distortion in complexes containing the same number of σ-donating ligands, but with different connectivity, has been shown to influence the 3MLCT → 3MC conversion (Fig. 1C).18,24 This distortion depends on the antibonding interactions in the d orbital containing the unpaired electron. We hypothesized that if the lowest triplet excited state was less distorted, better overlap between ligand and metal orbitals would raise the energy of the 3MC state.43,44 Inspired by Berkefeld's work, we modelled two phen-containing iron(ii) complexes with different NHC ligands, FephenImi and FephenBzi, using density functional theory (DFT). The calculations indicated a higher-energy 3MC state in FephenImi than in FephenBzi, despite the higher ligand field strength reported for benzimidazol-2-ylidene containing complexes.13,14,28 This difference is attributed to a greater distortion in the 3MC geometry of FephenBzi compared to FephenImi, resulting from distinct coordination symmetries.

We investigated whether excited state geometric distortions caused by different coordination symmetries would outweigh ligand field strength in the decay lifetime of iron(ii) complexes. This study is the first case in which increasing the ligand field strength leads to a decreased excited-state lifetime. These results highlight that DFT can predict decay-lifetime trends by investigating the symmetry of the coordination sphere in the excited state.

Results and discussion

Computational modelling and molecular structures

We used DFT to obtain an initial assessment of the complexes' electronic properties. The structures of complex cations containing phenanthroline and imidazol-2-ylidene (FephenImi) and benzimidazol-2-ylidene (FephenBzi) were initially optimized using the O3LYP ZORA-def2-TZVP(-f) level of theory (Tables S6–S8). Our initial goal was to evaluate how the NHC ligand strength could influence the “seesaw” effect described above. The extension of the π-system in the diimine was anticipated to lower the energy of the MLCT state, while the NHC portion would alter the energy of the MC state depending on the ligand field strength. The highest occupied molecular orbitals (HOMOs) in these complexes have contributions from the dxy orbitals with minimal contributions from the ligand orbitals (Fig. S18 and S19). The lowest unoccupied molecular orbital (LUMO) shows mixed contributions from ligand and metal orbitals. FephenBzi shows less contribution from the metal orbitals to the LUMO, suggesting they are higher in energy than in FephenImi. The greater contribution of the d-metal orbitals to the LUMO indicates that the NHC ligands used herein have a lower ligand field strength than those in Berkefeld's work (2022).22,42 This impacts the excited-state character and decay dynamics discussed below.

We then proceeded with the synthesis of the ligands and complexes using procedures adapted from the literature (see the SI for more details).45,46FephenImi and FephenBzi complexes were obtained with yields of 19 and 27%, respectively (Scheme S1). The identity of all complexes was confirmed with 1H and 13C{1H} NMR spectroscopies, single crystal X-ray diffraction, and HR-ESI mass spectrometry. HR-ESI mass spectra show peaks corresponding to the molecular ions [Fe(phenImi)2]2+ and [Fe(phenBzi)2]2+ with characteristic iron isotopic patterns (Fig. S1–S4). The 1H NMR spectra of the complexes display a single set of peaks corresponding to the ligand consistent with a meridional geometry. The peaks in the 1H NMR spectra at 10.92 ppm for FephenImi and 10.79 ppm for FephenBzi (Fig. S5–S12) corresponding to the protons in the 2 position of the imidazolium salt are absent in the spectra of the complexes. Carbene formation is supported by the characteristic peaks in the 13C{1H} NMR spectra of the complexes at 196.3 ppm (FephenImi) and 211.0 ppm (FephenBzi).

The complexes adopt a distorted octahedral geometry according to X-ray diffraction analysis (Fig. 2A, B and Tables S1–S4). Single crystals were obtained using complexes containing bromide as the counterion for FephenImi and hexafluorophosphate for FephenBzi. The bite angles in the single crystals are 159.6(2)° and 159.67(10)° and the N–Fe–N axial angles are 175.4(2)° and 178.43(13)°, respectively for FephenImi and FephenBzi. We evaluated the degree of distortion using OctaDist, where bond length deviation (ζ), angular distortion (Σ) and trigonal twisting (Θ) quantify deviations from an ideal octahedral geometry.47 Both complexes exhibit similar values for these metrics, with FephenBzi being slightly more distorted than FephenImi (Table S9). The average bond length in FephenImi between the metal and coordinating nitrogen atoms is 1.97 Å, with the Fe–N bond trans to the NHC ligand being 0.14 Å longer on average than the Fe–N bond of the phenanthroline in the axial position. The bond elongation is related to the trans influence of the carbene donor.17 This influence is greater in FephenBzi due to the stronger σ-donating character of the benzimidazole-2-ylidene group. In this case, the Fe–N bond length trans to the carbene is 0.163 Å longer than the Fe–N bond length in the axial position. The Fe–C bond length is 1.93 Å in FephenImi and 1.924(3) Å in FephenBzi, similar to those in other iron(ii) diimine NHC complexes.17,25–29,42 The molecular packing of FephenImi and FephenBzi was also examined (Fig. S13 and S14). The crystal lattice of FephenBzi exhibits π⋯π interactions between the NHC ligands with phenyl–phenyl distances of 3.458 Å and a phenyl-imidazole-2-ylidene distance of 3.544 Å; whereas no significant intermolecular interactions were observed in FephenImi.

Fig. 2. Perspective drawing of the X-ray structures of FephenImi (A) and FephenBzi (B). Ellipsoids are plotted at the 50% probability level. Solvent molecules, H atoms and counterions are removed for clarity. Degree of octahedral distortion evaluated using Octadist through the bond length deviation (ζ), angular distortion (Σ), and trigonal twisting (Θ) parameters.

Fig. 2

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Electronic properties dictated by ligand field strength

Cyclic voltammograms of the complexes showed an anodic shift in the FeIII/II oxidation wave in FephenBzi relative to FephenImi. The voltammograms in acetonitrile are displayed in Fig. 3A, S15 and S16, with data summarized in Table 1, S5 and S6.48 Both complexes showed an electrochemically reversible FeIII/II anodic event at E1/2 = 0.43 V vs. Fc+/0 for FephenImi and 0.55 V vs. Fc+/0 for FephenBzi. The higher oxidation potential in FephenBzi is related to the higher π-accepting character of the benzimidazole-2-ylidene group compared to FephenImi resulting in a lowering of the energy of the occupied d orbitals. These complexes are chemically stable in the iron(ii) form under ambient conditions, in contrast to the one reported by Berkefeld, which requires inert conditions because of its greater susceptibility to oxidation to iron(iii) (−0.88 V vs. Fc+/0, Table 1, entry 3).42FephenImi displays a chemically irreversible event at −1.77 V vs. Fc+/0. FephenBzi shows two chemically irreversible events at E1/2 = −1.65 and Epa = −1.91 V vs. Fc+/0. The second process is accompanied by a sharp feature at −1.70 V related to the adsorption of the reduced species on the electrode. Limiting the scan to −1.78 V vs. Fc+/0 did not improve the reversibility of the first reduction event in FephenBzi (Fig. S17). These processes are assigned to ligand reductions, similar to those in other phenanthroline-based iron(ii) complexes.49,50 The potential differences between the redox events (ΔEechem) are 2.19 V in FephenImi and 2.21 and 2.47 V in FephenBzi.

Fig. 3. (A) Cyclic voltammograms of FephenImi and FephenBzi (1 mmol L−1) in argon-sparged 0.1 M (nBu4N)(PF6) in dry acetonitrile at 298 K and 100 mV s−1 scan rate. (B) UV-vis absorption spectra of phenImiH, phenBziH, FephenImi and FephenBzi in acetonitrile at 298 K.

Fig. 3

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Table 1. Electrochemical potentials (in V vs. Fc+/0) of FephenImi, FephenBzi and a previously reported NHC complex of iron(ii).

Compound E 1/2 (L0/˙−)a E 1/2 (FeIII/II)a ΔEechem [V]b ΔEopt [eV]c
FephenImi −1.77 +0.43 2.19 2.29
FephenBzi −1.65, −1.91d +0.55 2.20, 2.46 2.36
FephenPhe −2.46 −0.88 1.58 —
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a

Potential in V referenced to Fc+/0.

b

Calculated from the difference between Epa and E1/2.

c

Calculated from UV-vis spectra.

d

Irreversible wave.

e

Berkefeld et al. (2022).42

Both complexes display broad absorption bands in the visible region that are not present in the ligands (Fig. 3B). FephenImi shows a band peaking at 542 nm with shoulders at 639 and 432 nm. A blue-shifted absorption is observed in FephenBzi, with a λmax of 525 nm. Both bands are attributed to MLCT [d(Fe) → π*(phen)] states as predicted by time-dependent density functional theory (TD-DFT) and supported by spectroelectrochemistry, shown below (Fig. 4A, B, S20–S26, Tables S10 and S11). The optical bandgaps (ΔEopt) of 2.29 eV (FephenImi) and 2.36 eV (FephenBzi) are close to the ΔEechem values obtained from cyclic voltammetry, supporting the MLCT assignment (Table 1). A strong absorption band (ε ∼105 L mol−1 cm−1) is observed at 300 nm in FephenBzi, which is red-shifted relative to that in FephenImi (284 nm). These bands are assigned to π(phen) → π*(phen) transitions.51

Fig. 4. (A) Spectroelectrochemistry following metal-based oxidation (red, scanning from 0 to +0.7 V vs. Fc+/0) and ligand-based reduction (blue, scanning from 0 to −1.8 V vs. Fc+/0) of FephenImi in acetonitrile at 298 K. (B) Spectroelectrochemistry following metal-based oxidation (red, scanning from 0 to +0.8 V vs. Fc+/0) and ligand-based reduction (blue, scanning from 0 to −1.8 V vs. Fc+/0) of FephenBzi in acetonitrile at 298 K. Femtosecond transient absorption spectra of FephenImi (C) and FephenBzi (D) in acetonitrile at 298 K.

Fig. 4

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Simulated absorption spectra obtained from TD-DFT support the MLCT assignment (Tables S10 and S11). Difference electron density maps of transitions between 509 and 451 nm (Fig. S20, states S6–S11) in FephenImi exhibit electron density depletion in the iron d-orbitals, and density gain in the phenanthroline π* orbitals. This is also observed for transitions between 483 and 429 nm in FephenBzi (Fig. S21, states S5–S11). State S1 of FephenImi at 604 nm has an oscillator strength of 0.0237 and is composed of 93% of a HOMO → LUMO transition. State S1 is blue-shifted to 573 nm in FephenBzi, exhibiting an oscillator strength of 0.0381 and has a HOMO → LUMO transition contribution of 91%. The blue shift is caused by the higher ligand field experienced by FephenBzi and is consistent with both UV-vis and electrochemical data. Both low-lying singlet states show major MLCT and minor MC character. High-energy transitions are primarily composed of π(phen) → π*(phen) and π(NHC) → π*(phen) transitions. Overall, the simulated spectra are in good agreement with the experimental data (Fig. S22).

Spectroelectrochemistry and excited state evolution

The MLCT character of the lowest absorption band is further corroborated by spectroelectrochemistry experiments.52 The changes in the UV-vis spectra were monitored when the potential was scanned past the redox events identified in the cyclic voltammetry experiments discussed above. The depletion of the 540 and 523 nm bands of both complexes upon oxidation and reduction supports the MLCT assignment (Fig. 4A and B). Bands at 311 and 768 nm increase in optical density (OD) when FephenImi is oxidized from iron(ii) to iron(iii) (Fig. S23). Weaker bands at 338, 357 and 389 nm and two broad bands at 622 and 687 nm appear when the ligand is reduced in FephenImi (Fig. S24). Similarly, bands at 365 and 700 nm increase in OD as iron(ii) is oxidized in FephenBzi, and bands at 386, 428, 584 and 681 nm appear when the complex is reduced (Fig. S25 and S26).

After mapping out the electrochemical and basic photophysical properties, we focused on the excited-state dynamics of the complexes. A femtosecond pulsed laser was used to excite FephenImi at 550 nm and FephenBzi at 525 nm to monitor the transient absorption. The complexes exhibited similar optical evolutions, but with distinct time constants (Fig. S27–S33). FephenImi shows a ground-state bleach (GSB) band at 530 nm and excited-state absorption (ESA) bands at 364 and 740 nm (Fig. 4C). All spectral features decay within 1 ns, with isosbestic points at 390 and 681 nm. FephenBzi shows a GSB at 525 nm and ESAs at 372 and 680 nm. In this case, the dynamics of both complexes overall resemble the resulting spectra of the oxidized species and do not display the characteristic ESAs associated with the reduced species.

Global analysis of the fs-TA data suggests distinct decay paths for the two complexes. The analysis yielded a triexponential decay for FephenImi and a biexponential decay for FephenBzi. The 0.2 ps component in FephenImi is assigned to vibrational relaxation of the singlet state followed by intersystem crossing to a low-lying excited state (Fig. 5A). This occurs within the instrument response time and is consistent with other observations for d6 low-spin species.28,53–57 The decay-associated spectra (DAS) with a lifetime of 9 ps show ESAs at 359 and 376 nm and a broad band from 580 to 700 nm. These bands resemble the features observed in the spectroelectrochemical difference spectra of the oxidized iron species and the reduced ligand respectively (Fig. 4A). We thus assign this component to the 3MLCT state.29,34 The DAS of FephenBzi with a lifetime of 2 ps displays a GSB band at 510 nm along with ESAs at 389, 587, and 675 nm (Fig. 5B). The 3MLCT lifetime is shorter than that of FephenImi and other similar iron(ii) NHC complexes. Analysis in dichloromethane solutions showed distinct effects on the decay lifetime. The 3MLCT lifetime is increased to 27 ps for FephenImi and decreased to 1 ps for FephenBzi (Fig. S29–S33). Overall, the results in both solvents differ significantly from what has previously been reported for other complexes in the literature, where a stronger ligand field leads to a longer lifetime.14,24,28,29

Fig. 5. DAS of FephenImi obtained from fs-TA in de-aerated acetonitrile at 298 K prompted by a 550 nm excitation source (A). Schematic representation of the potential energy curves estimated by DFT/TD-DFT methods of FephenImi (B). DAS of FephenBzi obtained from fs-TA in de-aerated acetonitrile at 298 K prompted by a 525 nm excitation source (C). Schematic representation of the potential energy curves estimated by DFT/TD-DFT methods of FephenBzi (D). The crossing point between 3MLCT and 3MC is marked with an X.

Fig. 5

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The 3MC decay lifetime of complex FephenImi is more than two times longer compared to that of the iron(ii) bipyridine NHC analog.29 The DAS with a lifetime of 195 ps shows a positive OD at 367 nm and a negative OD from 380 to 700 nm. The observed features are consistent with spectroelectrochemical results for metal oxidation, and this DAS is assigned to 3MC decay (Fig. 4A, red trace).24,29,52 The decay lifetime of the 3MC state is nearly four times shorter in FephenBzi. The DAS corresponding to a lifetime of 53 ps also matched with the oxidized FephenBzi complex. In this case, the 3MC lifetime is shorter with increasing ligand field strength as it may be influenced by the reorganization energy.25,29,58 The 3MC lifetimes are similar in dichloromethane, with values of 202 ps for FephenImi and 54 ps for FephenBzi (Fig. S33).

Excited state distortions facilitate decay

DFT potential-surface analyses point to different kinetic barriers between the excited states in the complexes. Energies of the lowest singlet, quintet, and triplet MLCT and MC states were calculated along the corresponding Fe–Nphen elongation pathways (Fig. 5B and C). The potential surface curves for the first singlet (1MLCT) and 3MLCT states were obtained from TD-DFT calculations. For the lowest triplet state and quintet states, identified as the 3MC and 5MC states, potential surface curves were obtained from unrestricted Kohn-Sham (UKS) calculations. The quintet state is higher in energy than the 3MC state and is not the preferred decay path. This is expected based on observations in other NHC iron(ii) complexes.14,24,28,29 The lowest-lying triplet state has 3MLCT character at short Fe–Nphen distances in both complexes. The 3MC state decreases in energy as the distance between Fe and Nphen increases, taking over as the lowest-lying triplet state. This results in a crossing between triplet states that occurs at longer bond distances in FephenImi than in FephenBzi. Additionally, the 3MC state lies lower in energy for FephenBzi at 1.435 eV than for FephenImi at 1.479 eV at the Franck–Condon geometry. We expect the stabilization of the 3MC state in FephenBzi to facilitate the conversion between the 3MLCT and 3MC states, thus supporting the 3MLCT → 3MC → 1GS path. This was also observed in Fe(ii) complexes in which the cis/trans configurations gave different reorganization energies, and where the 3MLCT state is nested within the 3MC state.17,18,24

A closer look at the orbitals at the 3MLCT and 3MC geometries rationalized the observed potential energy kinetic barrier. We previously described the HOMO in the ground state with two paired electrons, with the dxy2 configuration (Fig. S18 and S19). The 3MC state results in two unpaired electrons in distinct d orbitals, dxy1 and dx2−y21 (Fig. 6A). The angle along the N–Fe–N bond axis in this geometry is closer to 180° in FephenImi (177.44°) compared to that in FephenBzi (158.60°). Similarly, the distortion parameters ζ, Σ, and Θ are lower for FephenImi (0.700, 107.95, and 315.60, respectively) than for FephenBzi (0.734, 124.54, and 472.46, respectively). The less distorted geometry found in FephenImi increases the electron–electron repulsion along the coordination axes, increasing the 3MC energy as observed in Fig. 5A. More importantly, the symmetry difference between the complexes results in antibonding interactions along the N–Fe–N axis in FephenBzi while in FephenImi the dx2−y21 orbital is located along the N–Fe–C axis. This increases the Fe–N bond length and decreases the 3MC energy in FephenBzi. The 3MLCT state originates from a dxy → π*(phen) transition for both complexes (Fig. 6B). Consequently, the conversion from the 3MLCT to the 3MC state requires a π*(phen) → dx2−y21 transition. This electronic coupling relies on the superposition of the π*(phen) and dx2−y21 orbitals.24 Complex FephenImi in the 3MLCT geometry shows larger N–Fe–C and N–Fe–N angles of 161.00° and 172.70°, respectively, compared to FephenBzi (152.20° and 165.34°). The reduced octahedral distortion in the FephenImi excited state is further supported by the ζ, Σ and Θ distortion parameters of the 3MLCT geometry, which are similar to those of the 1GS structure (Table S9). We hypothesized that the more orthogonal geometry in the excited state results in lower electronic coupling between π*(phen) and dx2−y21 orbitals, creating a higher kinetic barrier during the conversion. As a result, a longer decay time is found for FephenImi compared to FephenBzi.

Fig. 6. (A) 3MC state geometries of FephenImi (left) and FephenBzi (right) with the dx2−y21 orbitals with α spin density. (B) 3MLCT state geometries of FephenImi (left) and FephenBzi (right) with the π*1 (phen) orbitals with α spin density. Degree of octahedral distortion evaluated using Octadist through the bond length deviation (ζ), angular distortion (Σ), and trigonal twisting (Θ) parameters.

Fig. 6

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Understanding the origin of the distortion in the excited state may guide new ligand designs that limit the population of the deactivating states. Previous studies of iron(ii) complexes that possess the same average ligand field strength have shown that different connectivity of ligands can influence excited-state distortion.24,59 Antibonding interactions involving the orbital containing the unpaired electron control the energy of the 3MC state. If the N–Fe–N axis is aligned with the orbital containing the unpaired electron, a more distorted geometry will lower the energy of the 3MC state. However, manipulating cis/trans geometries and positioning the N–Fe–N axis in a specific way might not always be synthetically accessible.

Controlling the coordination symmetry can be the key to obtaining photoactive iron(ii) complexes. FephenImi shows a similar coordination symmetry to FephenPh reported by Berkefeld, as both Imi and cyclometalated ligands possess strong σ-donating character.42 We propose that the different coordination symmetry in FephenBzi arises from the greater π-accepting character of Bzi, which increases the electron–electron repulsion and shifts the preferred coordination. Thus, the use of σ-donating over π-accepting ligands may be advantageous to minimize excited state distortions and increase the 3MLCT lifetime in asymmetric tridentate iron(ii) photosensitizers. Although these distortions and symmetrical coordination are challenging to optimize using simple coordination-chemistry principles, DFT analysis predicts how distortions will influence the ordering of excited states.

Conclusions

This work demonstrates that coordination symmetry influences the kinetic barrier between the 3MLCT and 3MC states, thereby governing the excited-state decay of iron(ii) complexes. Structural, electrochemical and spectroscopic characterization studies using X-ray diffraction, cyclic voltammetry and UV-vis spectroscopy, respectively, indicate that FephenBzi possesses a stronger ligand field than FephenImi. The non-radiative path was investigated using femtosecond transient absorption spectroscopy and showed longer 3MLCT and 3MC lifetimes for FephenImi than for FephenBzi. DFT surface-potential analysis rationalized the differences observed: FephenImi exhibits a kinetic barrier between the 3MLCT and 3MC states, resulting in a slower relaxation. This barrier is caused by a greater overlap between orbitals in the excited state of FephenImi, which increases the 3MC state energy. FephenBzi shows a more distorted excited-state geometry, which weakens orbital overlap and enables rapid 3MLCT → 3MC → 1GS conversion. Greater excited-state distortion observed in FephenBzi arises from coordination symmetry differences, with antibonding interactions located along the N–Fe–N axis rather than the N–Fe–C axis in FephenImi. These results suggest that favoring σ-donating over π-accepting ligands in asymmetric tridentate iron(ii) photosensitizer environments may minimize excited state distortions and increase the barrier for 3MLCT → 3MC conversion. These results highlight the importance of excited-state geometry, coordination symmetry and energetic barriers in designing long-lived iron(ii) excited states.

Author contributions

JT, UA, KMT, GF and MOW wrote and edited the manuscript. JT and UA carried out the synthesis, characterization and photophysical measurements. KMT and BOP collected and refined the XRD structures. GF conducted DFT calculations. SK carried out time-resolved measurements and global analysis of the spectroscopic data. MOW supervised the research. All authors discussed the progress of the research and reviewed the manuscript.

Conflicts of interest

The authors declare no competing financial interest.

Supplementary Material

SC-017-D6SC05359A-s001
SC-017-D6SC05359A-s002

Acknowledgments

This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC). We would like to acknowledge the Laboratory for Advanced Spectroscopy and Imaging Research (LASIR) for facility access. JT thanks the province of British Columbia for a BC Graduate Scholarship.

Data availability

CCDC 2496009 and 2496010 contain the supplementary crystallographic data for this paper.60a,b

Supplementary information (SI): general procedures and equipment details, synthesis and characterization of ligand precursor and coordination complexes, X-ray data, NMR and HR-ESI mass spectra, cyclic voltammograms and spectroelectrochemistry data, photophysical data, and DFT, TD-DFT and SOC-TD-DFT data. See DOI: https://doi.org/10.1039/d6sc05359a.

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Data Citations

  1. (a) CCDC 2496009: Experimental Crystal Structure Determination, 2026, 10.5517/ccdc.csd.cc2ps9f7 [DOI]
  2. (b) CCDC 2496010: Experimental Crystal Structure Determination, 2026, 10.5517/ccdc.csd.cc2ps9g8 [DOI]

Supplementary Materials

SC-017-D6SC05359A-s001
SC-017-D6SC05359A-s001.pdf (7.1MB, pdf)
SC-017-D6SC05359A-s002
SC-017-D6SC05359A-s002.cif (84.6KB, cif)

Data Availability Statement

CCDC 2496009 and 2496010 contain the supplementary crystallographic data for this paper.60a,b

Supplementary information (SI): general procedures and equipment details, synthesis and characterization of ligand precursor and coordination complexes, X-ray data, NMR and HR-ESI mass spectra, cyclic voltammograms and spectroelectrochemistry data, photophysical data, and DFT, TD-DFT and SOC-TD-DFT data. See DOI: https://doi.org/10.1039/d6sc05359a.

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