High-Efficiency Photoinduced Charge Separation in Fe(III)carbene Thin Films

Abstract

Symmetry-breaking charge separation in molecular materials has attracted increasing attention for optoelectronics based on single-material active layers. To this end, Fe(III) complexes with particularly electron-donating N-heterocyclic carbene ligands offer interesting properties with a ²LMCT excited state capable of oxidizing or reducing the complex in its ground state. In this Communication, we show that the corresponding symmetry-breaking charge separation occurs in amorphous films of pristine [Fe(III)L₂]PF₆ (L = [phenyl(tris(3-methylimidazol-2-ylidene))borate]^-). Excitation of the solid material with visible light leads to ultrafast electron transfer quenching of the ²LMCT excited state, generating Fe(II) and Fe(IV) products with high efficiency. Sub-picosecond charge separation followed by recombination in about 1 ns could be monitored by transient absorption spectroscopy. Photoconductivity measurements of films deposited on microelectrode arrays demonstrated that photogenerated charge carriers can be collected at external contacts.

Scheme 1. State Energy Diagram (left) Representing Symmetry Breaking Charge Separation (SBCS) and Charge Recombination (CR) upon Photoexcitation of [Fe(III)L₂]⁺ and Molecular Structure of the Chromophore (right)

Energies of the excited (²LMCT) and charge separated states are solution-based values (in acetonitrile). Time constants refer to the solid chromophore as a PF₆^– salt.

Figure 1

(a) Optical microscopic and (b) scanning electron microscopy (SEM) images of the [Fe(III)L₂]PF₆/IDE film. The inset is the picture of the IDE sample with the white circle indicating the illuminated area (2.5 mm diam). (c) Emission (red lines) and absorption spectra (blue lines) of [Fe(III)L₂]PF₆ in DMF solution (dashed lines) and as a solid film (solid lines).

Figure 2

Selected fs-TA spectra of the [Fe(III)L₂]PF₆/IDE film measured at early (a) and late (b) times (λ_ex = 500 nm). The dashed line is the expected differential spectrum for the SBCS reaction (see Figure S3). (c) Kinetics at 360 nm (²LMCT excited state and Fe(II)), 550 nm (Fe(III) ground state), and 730 nm (Fe(IV)). Solid lines are fit results from global analysis.

Figure 3

Photoconductivity of the [Fe(III)L₂]PF₆/IDE film (at room temperature). (a) I/V curves (white light, 0.58 mW/cm²) in Ar and O₂ (scans 1–3). (b) Spectral response (Ar, 3 V bias). (c) Photoresponse in Ar and O₂ (white light, 30s/30s on/off cycles).