Electronic structure and photophysics of a supermolecular iron complex having a long MLCT-state lifetime and panchromatic absorption

Abstract

Exploiting earth-abundant iron-based metal complexes as high-performance photosensitizers demands long-lived electronically excited metal-to-ligand charge-transfer (MLCT) states, but these species suffer typically from femtosecond timescale charge-transfer (CT)-state quenching by low-lying nonreactive metal-centered (MC) states. Here, we engineer supermolecular Fe(II) chromophores based on the bis(tridentate-ligand)metal(II)-ethyne-(porphinato)zinc(II) conjugated framework, previously shown to give rise to highly delocalized low-lying 3MLCT states for other Group VIII metal (Ru, Os) complexes. Electronic spectral, potentiometric, and ultrafast pump–probe transient dynamical data demonstrate that a combination of a strong σ-donating tridentate ligand and a (porphinato)zinc(II) moiety with low-lying π*-energy levels, sufficiently destabilize MC states and stabilize supermolecular MLCT states to realize Fe(II) complexes that express 3MLCT state photophysics reminiscent of their heavy-metal analogs. The resulting Fe(II) chromophore archetype, FeNHCPZn, features a highly polarized CT state having a profoundly extended 3MLCT lifetime (160 ps), 3MLCT phosphorescence, and ambient environment stability. Density functional and domain-based local pair natural orbital coupled cluster [DLPNO-CCSD(T)] theory reveal triplet-state wavefunction spatial distributions consistent with electronic spectroscopic and excited-state dynamical data, further underscoring the dramatic Fe metal-to-extended ligand CT character of electronically excited FeNHCPZn. This design further prompts intense panchromatic absorptivity via redistributing high-energy absorptive oscillator strength throughout the visible spectral domain, while maintaining a substantial excited-state oxidation potential for wide-ranging photochemistry––highlighted by the ability of FeNHCPZn to photoinject charges into a SnO2/FTO electrode in a dye-sensitized solar cell (DSSC) architecture. Concepts enumerated herein afford opportunities for replacing traditional rare-metal–based emitters for solar-energy conversion and photoluminescence applications.

Accessing Photoredox Transformations with an Iron(III) Photosensitizer and Green Light

“…The scarcity of second- and third-row transition metals such as ruthenium, iridium, rhodium, or osmium, coupled with a global desire for sustainable chemistry, has led to the resurgence of research centered on Earth-abundant metal-based photosensitizers. − Several photosensitizers based on copper, , molybdenum, nickel, , tungsten, , zirconium, , chromium, − cobalt, , or iron − have recently been reported as promising candidates in cutting-edge research fields. In particular, luminescent complexes of iron are often considered a “holy grail” because of iron’s high abundance in the Earth’s crust, low toxicity, and low environmental impact.…”

Light‐Driven Multi‐Charge Separation in a Push‐Pull Ruthenium‐Based Photosensitizer – Assessed by RASSCF and TDDFT Simulations

“…Efficient light‐harvesting can be achieved by designing systems with an absorption range covering as much of the visible spectral range as possible. One approach to achieve such panchromatic absorption is to combine multiple chromophoric units in large (supra)molecular architectures [7–11] . Although a broad absorption range can easily be achieved via this strategy, the main challenge is that the different chromophoric units need to be capable to funnel excitation energy towards a defined acceptor unit and/or to lead to generation of separated charges which are transferred to a defined acceptor, independent of the chromophoric unit excited by the absorption event.…”

Ultrafast and long-time excited state kinetics of an NIR-emissive vanadium(iii) complex I: synthesis, spectroscopy and static quantum chemistry

“…[13][14][15][16][17][18] The 3d transition metals possess a weaker ligand eld splitting 10 and smaller SOCs 11 posing severe challenges to the design of the excited state landscape, 14 yet several recent breakthroughs have been reported, e.g. on copper(I), 23,24 nickel(0,II), 25,26 cobalt(III), 27 iron(II,III), [28][29][30][31] chromium(0/III) [32][33][34][35] and vanadium(III). 36 Beyond the conventionally exploited MLCT excited states, 12 LMCT states of the low-spin d 5 electron conguration of iron(III) 18 and spin-ip states of the d 3 electron conguration of chromium(III) 15 are emerging as novel paradigmatic excited states useful for photoapplications.…”