Site-Selective Orbital Interactions in an Ultrathin Iron-Carbene Photosensitizer Film.

We present the first experimental study of the frontier orbitals in an ultrathin film of the novel hexa-carbene photosensitizer [Fe(btz)3]3+, where btz is 3,3'-dimethyl-1,1'-bis(p-tolyl)-4,4'-bis(1,2,3-triazol-5-ylidene). Resonant photoelectron spectroscopy (RPES) was used to probe the electronic structure of films where the molecular and oxidative integrities had been confirmed with optical and X-ray spectroscopies. In combination with density functional theory calculations, RPES measurements provided direct and site-selective information about localization and interactions of occupied and unoccupied molecular orbitals. Fe 2p, N 1s, and C 1s measurements selectively probed the metal, carbene, and side-group contributions revealing strong metal-ligand orbital mixing of the frontier orbitals. This helps explain the remarkable photophysical properties of iron-carbenes in terms of unconventional electronic structure properties and favorable metal-ligand bonding interactions-important for the continued development of these type of complexes toward light-harvesting and light-emitting applications.

Introduction

Photofunctional transition metal complexes are used for a wide range of molecule-based light-harvesting and light-emitting applications that are currently under development.[1] This includes solar energy conversion systems that cover both molecular photovoltaics and solar fuel applications, which take advantage of favorable and often unique excited-state dynamics and photo-induced charge-transfer properties of such complexes.[2] Current efforts aim to replace rare and expensive metals in benchmark complexes (e.g., Ru2+ polypyridyl dyes[3]) with earth-abundant alternatives based on, for example, iron or copper.[4,5]

Iron N-heterocyclic carbene (NHC) complexes have recently emerged as a promising new class of photosensitizers[6] capable of driving ultrafast electron injection on surfaces[7] as well as bimolecular photoredox processes in solution.[8] Specifically, the recently reported [Fe(btz)3]3+ complex (Scheme ), where btz is 3,3′-dimethyl-1,1′-bis(p-tolyl)-4,4′-bis(1,2,3-triazol-5-ylidene), has a ∼100 ps excited-state lifetime and is effectively the first reported photoluminescent iron-based light-harvesting and light-emitting complex.[9] This constitutes a significant improvement over traditional iron complexes which suffer from ultrafast decay of initially excited charge-transfer states.[10] This improvement is generally attributed to the electronic interaction between the iron center and the carbene ligands, where the ligands help to prolong the excited-state lifetime of the charge-transfer states.

Scheme 1

(a) Chemical Structure of Fe(btz)3.[14] (b) Geometry of a Single Ligand Showing the Bistriazole (btz) Core Carbene Moiety and Toluene Side-Groups

The [Fe(btz)3]3+ complex has been characterized as a low spin-3d5 complex,[9] with the main metal and ligand frontier molecular orbital (MO) interactions schematically represented in Figure . An increased understanding of the influence of metal–ligand bonding and the charge-transfer properties is thus critical for further molecular design development. Particularly interesting for further investigations are the noninnocent contributions of the NHC–ligand σ and π orbitals with strong σ-donation and π-backbonding interactions with the metal center. This additional detail about the fundamental electronic properties enables comparisons with existing investigations of prototype iron-based photoactive complexes.[11−13] There is therefore great interest in gaining better insight into the character and localizations of key MOs in this genre of FeIII complexes.

Figure 1

Schematic diagram of the molecular valence electronic structure of FeIII(btz)3 (in the quasi-octahedral ligand field-splitting convention) showing key orbital interactions between Fe 3d, carbene-type (btz) ligand (L), and toluene (tol) side-groups. Included are (i) the FeIII t2g hole (empty circle), (ii) the carbene σ lone pair bonding interaction with the unoccupied Fe eg levels, and associated antibonding destabilization of the Fe eg level (blue lines), (iii) the Fe t2g back-bonding interactions with carbene (btz) π levels together with associated antibonding interactions (orange lines), and (iv) toluene (tol) side-group occupied π and unoccupied π* orbitals.

X-ray absorption spectroscopy (XAS) has proven to be a powerful tool to probe frontier orbital covalency, including spectral features resulting from complex multistate effects and ligand–metal interactions.[12,15] Resonant X-ray photoelectron spectroscopy (RPES) provides complementary information, connecting unoccupied and occupied states, and has been used to study the electronic structure of components of solar energy devices,[16−18] to deconstruct the valence band structure by the resonant enhancement of MOs,[19−21] and even to measure ultrafast charge transfer.[22,23] By using RPES alongside density functional theory (DFT) calculations, this paper explores the nature and localization of occupied and unoccupied MOs for an ultrathin film of [Fe(btz)3]3+ through selective enhancement of orbitals localized to different regions in the molecule.

Experimental Methods

Films of Fe(btz)3(PF6)3 were dropcast onto clean gold substrates and were characterized by lab source X-ray photoelectron spectroscopy (XPS) (Physical Electronics PHI X-Tool) and optical absorption spectroscopy (PerkinElmer Lambda 1050 spectrophotometer). RPES and near-edge X-ray absorption fine structure (NEXAFS) measurements were carried out at the HE-SGM beamline/endstation[24] at the Helmholtz-Zentrum Berlin BESSY II synchrotron facility. Binding energy (BE) scales were calibrated to the gold fermi edge. Photon energy scales were calibrated using photoemission peaks generated from the first and second order light for the measurements over the C and N absorption edges. Further information regarding the instrumentation, sample preparation, and analysis methods can be found in the Supporting Information.

Results

Figure presents the optical absorbance spectra of a [Fe(btz)3]3+ thin film on a gold substrate for comparison with the previously characterized solutions of the complexes.[9,14] The oxidation state of [Fe(btz)3]3+ remains unchanged upon drop-casting. This is supported by XPS of the same films. Both measurements are further discussed in the Supporting Information. We are therefore proceeding with confidence that the [Fe(btz)]3+ films behave as previously characterized.[9,14] This significant finding highlights the potential of [Fe(btz)3]3+ for application in devices where solid thin films of the complex would be required.

Figure 2

Absorbance of solution (colored area) and thin film (circles and lines) of [Fe(btz)3]3+. Each curve is normalized to the peak at the longest wavelength. Inset: Photo of the as-prepared thin film with 1 mm scale bar.

As a basis for understanding the general electronic structure, an MO diagram of [Fe(btz)3]3+ is presented in Figure showing results from spin-polarized (unrestricted) DFT calculations performed on the B3LYP*/SDD/MeCN//B3LYP*/6-311G(d)/MeCN level of theory (detailed in the Supporting Information). These calculations provide a more complete MO perspective of the characteristic 3d5 electronic configuration of the [Fe(btz)3]3+ complex and the frontier MO structure that we have previously reported.[9,25] Further details of the MO structure is presented in Supporting Information Figures S9–S12. The overall characteristics corroborate the previous findings in terms of the main metal and ligand MO features, including the low/spin (t2g)5 population of the Fe(III) as well as significant metal–ligand mixing of several of the frontier orbitals, highlighting the noninnocent character introduced by the NHC–ligands for these complexes. The right-hand half of Figure provides simple assignments of the experimental NEXAFS and RPES measurements with dominant MO contributions from the ground-state calculations (further supported by comparisons with information from explicit XAS calculations detailed in the Supporting Information), highlighting key resonant transitions from core levels to unoccupied states as discussed below.

Figure 3

Selected valence MOs from spin-polarized electronic structure calculations of [Fe(btz)3]3+ (left) together with associated assignments of experimental NEXAFS and RPES features to characteristic MO types (right). Alignment of experimental and calculated energy scales was done using the position of the πtoluene orbital (β-268).

N 1s and C 1s RPES maps presented in Figure provide detailed site-specific insight into ligand MO features. Constant BE line profiles extracted from each map are included (showing NEXAFS constructed from photoelectrons originating in specific occupied orbitals). These are compared to conventional NEXAFS measured using the partial yield detector and simulated spectra (constructed from quantum chemical calculations—details can be found in the Supporting Information). The line shapes predicted by the calculations closely match the experimental spectra, providing support to the following assignments of spectral features.

Figure 4

N 1s (a) and C 1s (b) RPES maps showing the valence band photoemission as a function of the incident photon energy. Alongside each are the constant BE line profiles extracted from the maps and NEXAFS, measured using a partial yield detector and simulated NEXAFS. The inset is the schematics of one of the six identical carbene rings and attached toluene groups with the different species labeled.

N K-edge NEXAFS and RPES show two absorption resonances at 400.4 and 401.7 eV photon energy. We assign these to absorption into the same unoccupied MO from two distinguishable core level environments labeled Na and Nb,c in the inset in Figure a. This is consistent with both the simulated and measured NEXAFS which are in good agreement where the ratios of the two peaks in the spectra match the 1:2 ratio of the Na/Nb,c atoms. It is also consistent with our interpretation of the XPS results (see Supporting Information). This unoccupied state is well described by the calculated MO β-281, which is evenly distributed around the nitrogen atoms (see Figure ). The RPES line profiles show coupling of these NEXAFS features with the first three distinct occupied MO features at binding energies 3.6 eV, 6.0 eV, and 7.5 eV, which are enhanced in the maps via participant decay.[26,27] The first 3.6 eV feature shows strong enhancement/coupling with the Na atom and little coupling with the Nb/Nc atoms. The feature at 6.0 eV BE is equally enhanced on excitation into either nitrogen environment. Finally, the 7.5 eV enhancement shows stronger coupling with the Na environment relative to the Nb,c environment. Together, the results for the N 1s measurements highlight the atomic specificity of the probed valence MOs pertaining to their general molecular distribution and nodal structure, where there are clearly discernible differences for the different nitrogen environments on the btz rings.

The carbon NEXAFS (Figure b) represents a complex convolution of many different core levels and unoccupied orbitals, but the calculated spectra and RPES can aid in deconstructing it. The dominant feature is a strong absorption at 285.0 eV corresponding to transitions from C on the toluene ligands into a set of π* MOs such as β-294 (almost exclusively localized on the toluene). The simulated spectra highlight that although this absorption feature is dominated by absorption into the toluene ring, the two carbons in the carbene ring also contribute. The absorption feature at 286.3 eV, labeled “x,” appears to mainly originate from the toluene carbons (both the ring and methyl).

Constant BE line profiles have been extracted from the C 1s map—this time intercepting the strong enhancement at BE = 4.2 eV, attributed to the π orbital of the toluene, and the same 6 eV feature highlighted in the N map. In the πtol (BE ≈ 4 eV) line profile, we see enhancement of the πtol* and “x” features. On the 6 eV line, we see a new feature centered at 286.8 eV, labelled as “y”. Unfortunately, we are unable to confidently identify the MO corresponding to this feature because the relative energy scales between experiment and theory drift apart as we stray further from the lowest unoccupied MO (LUMO).

Figure presents resonantly enhanced valence band spectra, measured for Fe, N and C resonances, on a common BE scale. This provides a basis for an in-depth analysis of occupied valence features and how these correlate between different elements/functional groups. Assignments of resonantly enhanced features require caution, for example, to distinguish between participant processes observed as constant BE features and resonant enhancement of Auger decay processes seen as constant kinetic energy features. Although it is common to interpret RPES in the simple MO perspective used here, it should also be noted that valence band features may also be affected by more complex multielectron state effects.[28]

Figure 5

Valence band resonant enhancements on a shared BE scale. (a) 709 and 712 eV represent Fe absorption into t2g and eg, respectively. (b,c) show the nitrogen and carbon RPES maps. Constant photon energy line profiles are extracted from (marked with dotted lines) and plotted under each map.

The C and N valence band spectra in Figure are extracted from the RPES maps at the photon energies previously highlighted to be of interest (Na/Nb,c and πtol*/x/y). The relevant sections of the maps are insets in the figure where these constant photon energy slices are marked. Fe maps are not presented because of severe radiation damage seen after extended resonant excitation of the Fe atom. Key excitation energies were therefore selected, and NEXAFS measurements were used to confirm that the radiation damage was minor. We also include valence band spectra measured off-resonance for comparison—this aids in differentiating features enhanced by participant decay from the effect of variance in photoionization cross-sections with photon energy.

In the nitrogen and carbon data, three enhancements of occupied orbitals are highlighted (labeled a–c). Enhancements “a” and “c” are evident in the nitrogen RPES plots measured at photon energies of 400.4 and 401.7 eV. These enhancements are only weakly visible in the carbon RPES, and even then, only for the 286.9 eV excitation data (corresponding to feature “y” in the resonant NEXAFS—Figure b). Enhancement “a” (BE ≈ 3.8 eV) is consistent with the calculated MO β-277, which has notable density on the iron atom, the innermost carbon, and the outermost nitrogen (Na) atoms. Enhancement “c” (BE ≈ 6.0 eV) is well represented by the β-258 MO, which is localized across both Na and Nb,c, the Fe center, and the carbene carbons. It follows that enhancement “y” is a transition from carbon atoms in the carbene rings.

RPES enhancement “b” (BE = 4.2 eV, πtoluene), which is well represented by MO β-286, features strongly in the two line profiles at hν = 285 eV (π*) and hν = 286.3 eV (“x”). This supports that these two resonances correspond to LUMO levels localized on the toluene ligands.

Excitation from Fe 2p to 3d allows us to access both the t2g (which has one electron vacancy) and the unoccupied eg levels. This is consistent with the Fe NEXAFS (Supporting Information S8) where we see a 3 eV splitting between the spectral features attributed to the t2g and eg levels at 709 and 712 eV, respectively. These features were investigated using RPES. When exciting into eg (hν = 712 eV), we see a low BE feature, labeled d, as well as enhancement of various unresolved higher energy valence features, which overlap with the Fe LMM auger decay (at ∼703 eV kinetic energy[29]). When on-resonance with the t2g hole (hν = 709 eV), we see enhancement in the same regions as the “a” (and lower energy shoulder labeled a′) and “c” enhancements. This implies mixing/overlap between the t2g and the highest occupied MO on the btz ligands, consistent with MO β-279. Importantly, the covariation in resonance enhancement features between the Fe and btz in the low BE region (feature “a”) suggests that the t2g levels mix extensively with the πbtz orbitals. This is reminiscent of the well-known Ru–N3 dye where there is significant orbital mixing between the Ru t2g and π-levels on the NCS ligand.[2]

The low BE feature “d” (visible when exciting into the eg) remains intriguing as this enhancement is not apparent, when on the t2g resonance or in the off-resonance spectra. It is also apparent in the N 1s RPES. The C 1s photoemission peaks generated from second-order light mean that we are unable to determine if it is apparent in the carbon RPES. RPES of high-spin solid-state systems, such as hematite,[30] have a feature attributed to the occupied eg at similar ∼1 eV BE we see in the RPES in this manuscript. For the low-spin system studied here, eg should be initially unoccupied and not produce photoemission features. This feature could be an intrinsic low BE feature of the FeIII species or could be due to the presence of a minority species (e.g., some X-ray-induced reduction to FeII). A more detailed investigation of this feature, including complete Fe RPES maps, would therefore be a valuable future study.

The overall experimental indications of strong metal–ligand orbital mixing for several of the frontier MO features involving the highest occupied and some of the low-lying unoccupied orbitals highlight the unique nature of the NHC–ligands in terms of σ/π interactions with the Fe 3d orbitals that has contributed to give these complexes remarkable photophysical properties. For example, the metal–ligand orbital mixing is important in terms of its influence on the energies of the t2g-levels beyond the generic ligand field-splitting model. The combination of the orbital specific and site-selective probing of key MO contributions, both occupied and unoccupied, also provides new insight into the unconventional electronic structure of this low-spin 3d5 complex, where the lowest valence excitations are of ligand-to-metal charge-transfer character, rather than metal-to-ligand character that is common for many Fe(II) and Ru(II) photosensitizers with a d6 ground-state electronic configuration. In particular, the experimental determination of strong metal–ligand mixing is promising for further synthetic developments as it strongly suggests that key electronic properties, such as the Fe(II)/Fe(III) oxidation propensity and low-energy charge-transfer excitation properties, can be tuned via further ligand modifications.

Conclusions

We have made a critical first step toward extending the usefulness of iron-carbene photosensitizers from solution-based applications to light-harvesting and light-emitting applications relying on thin-film technologies. Optical spectroscopy and XPS show that the oxidation state and the electronic configuration of the complex remain intact after drop-casting ultrathin films of the promising hexa-carbene photosensitizer FeIII(btz)3 onto a gold surface. RPES enabled us to probe, in several cases with atomic specificity, the coupling/localization of occupied and unoccupied MOs. These results are interpreted in the context of DFT calculations, providing detailed analysis of the frontier MOs involved in the excitation and emission processes. This reveals significant differences in the valence electronic interactions between the metal ion and the ligand carbene and auxiliary ligand orbitals. These results, in particular, provide direct spectroscopic evidence for strong MO interactions between the Fe 3d-levels and the btz carbene ligands’ frontier σ and π orbitals. This is shown by resonant enhancement of the same MOs at the nitrogen, carbon, and iron absorption edges and by extensive mixing in the calculated MOs. Clear differences in the frontier orbital interactions between the iron and the different nitrogen atoms in the btz ring, revealed by the RPES measurements, highlight significant possibilities to alter the photophysics of the iron-carbene photosensitizers, as well as their interactions via linker and anchor groups, with atomic precision in a systematic and site-specific fashion. From a broader perspective, the improved understanding of the electronic structure properties of these materials thus paves the way for further development toward better photofunctional earth-abundant materials based on iron and related transition metal complexes.