High-Efficiency Iron Photosensitizer Explained with Quantum Wavepacket Dynamics

Abstract

Fe(II) complexes have long been assumed unsuitable as photosensitizers because of their low-lying nonemissive metal centered (MC) states, which inhibit electron transfer. Herein, we describe the excited-state relaxation of a novel Fe(II) complex that incorporates N-heterocyclic carbene ligands designed to destabilize the MC states. Using first-principles quantum nuclear wavepacket simulations we achieve a detailed understanding of the photoexcited decay mechanism, demonstrating that it is dominated by an ultrafast intersystem crossing from (1)MLCT-(3)MLCT proceeded by slower kinetics associated with the conversion into the (3)MC states. The slowest component of the (3)MLCT decay, important in the context of photosensitizers, is much longer than related Fe(II) complexes because the population transfer to the (3)MC states occurs in a region of the potential where the energy gap between the (3)MLCT and (3)MC states is large, making the population transfer inefficient.

Probing spin–vibronic dynamics using femtosecond X-ray spectroscopy

“…As the dynamics progresses the spectrum begins to exhibit increasing characteristics of the 3 MC spectrum. There are also clear 300 fs oscillations in the transient signal, consistent with those observed in the transient absorption study 11,24 . As assigned above, this corresponds to the wavepacket motion along the ν 6 breathing mode 24 and consequently it is similar to the widely reported wavepacket dynamics of [Fe(bipy) 3 ] 2+ 9 .…”

“…There are also clear 300 fs oscillations in the transient signal, consistent with those observed in the transient absorption study 11,24 . As assigned above, this corresponds to the wavepacket motion along the ν 6 breathing mode 24 and consequently it is similar to the widely reported wavepacket dynamics of [Fe(bipy) 3 ] 2+ 9 . Figure 4 shows kinetic traces integrated over four main regions, namely 7119-7121 eV, 7122-7124 eV, 7131-7135 eV and 7136-signal provides a clear signature of the switching between the two manifolds.…”

“…During the first 1-2 ps, both mechanisms contribute and this gives rise to the initial faster component of the decay dynamics. However at later times the contribution of nonadiabatic coupling is reduced due to a redistribution of population within the 3 MLCT manifold, and this gives rise to the lower population kinetics 24 . This population transfer is slower than the 1 MLCT→ 3 MLCT transition because the population transfer predominantly occurs away from the crossing point between the 3 MLCT-3 MC states, at or near the energy minimum geometry of the 3 MLCT states.…”

Excited‐State Kinetics of an Air‐Stable Cyclometalated Iron(II) Complex

“…In combination with polypyridine ligands in heteroleptic complexes the undesired properties of bis(NHC) ligands to raise the 3 MLCT energy can be partially compensated . For all these compounds of the type [Fe(N^N^N) 2 ] 2+ and [Fe(C NHC^ N ^ C NHC ) 2 ] 2+ experimental and theoretical studies complement and support each other …”

“…[11] For all these compounds of the type [Fe(N^N^N) 2 ] 2 + and [Fe(C NHC^N^CNHC ) 2 ] 2 + experimental andt heoretical studies complement and support each other. [6,13] In contrast, quantumc hemicalp redictions providedb yJ akubikovae tal. and Dixon et al [14,15,16] for the effects of exchanging an N-donor by ac yclometalating ligand could not be confirmede xperimentally so far.A lthough aryl carbanion ligands in the form of 2-phenylpyridine, 1,3-di(2-pyridinyl)benzene, and 6-phenyl-2,2'-bipyridine (Hpbpy already found applications in the context of noble metal complexes, [17,18] air-and waterstablec yclometalatedF e II compounds, allowing for photochemicala pplications, are lacking up to date.…”