Sub-50-fs photoinduced spin crossover in [Fe(bpy)3]2+

Abstract

It is known that excitation by visible light of the singlet metal-to-ligand charge-transfer ((1)MLCT) states of Fe(II) complexes leads to population of the lowest-lying high-spin quintet state ((5)T) with unity quantum yield. Here we investigate this so-called spin crossover (SCO) transition in aqueous iron(II)tris(bipyridine). We use pump-probe transient absorption spectroscopy with a high time resolution of <60 fs in the ultraviolet probe range, in which the (5)T state absorbs, and of <40 fs in the visible probe range, in which both the hot MLCT state and the (5)T state absorb. Our results show that the (5)T state is impulsively populated in less than 50 fs, which is the time we measured for the depopulation of the MCLT manifold. We propose that non-totally-symmetric modes mediate the process, possibly high-frequency modes of the bipyridine (bpy) ligand. These results show that even though the SCO process in Fe(II) complexes represents a strongly spin-forbidden (ΔS = 2) two-electron transition, spin flipping occurs at near subvibrational times and is intertwined with the electron and structural dynamics of the system.

Directing the path of light-induced electron transfer at a molecular fork using vibrational excitation

“…This observation is not surprising since IVR in these organometallic complexes is typically extremely rapid in the Franck -Condon region. 4,35 and therefore the excess vibrational energy populates lower-frequency intramolecular modes that are nonetheless not readily accessible at room temperature (see Supplementary Fig. 7 for the results of a temperature-dependence study of charge separation dynamics).…”

“…Indeed, nuclear and electronic wave functions often strongly interact with each other, leading to exotic phenomena whereby vibrational motion can mediate and dictate the rate and efficiency of electronic transitions. [2][3][4][5] With overwhelming evidence that nuclear-electronic (vibronic) interactions play a crucial role in a broad range of systems, including light harvesting and conversion in photosynthetic organisms, 6-11 this phenomenon represents a tremendous opportunity to acquire deeper knowledge and control over the structural traits that govern molecular function and reactivity.…”

“…Indeed, nuclear and electronic wave functions often strongly interact with each other, leading to exotic phenomena whereby vibrational motion can mediate and dictate the rate and efficiency of electronic transitions. [2][3][4][5] With overwhelming evidence that nuclear-electronic (vibronic) interactions play a crucial role in a broad range of systems, including light harvesting and conversion in photosynthetic organisms, 6-11 this phenomenon represents a tremendous opportunity to acquire deeper knowledge and control over the structural traits that govern molecular function and reactivity.Recent efforts led by Rubtsov et al,12,13 Bakulin et al, 14 and our group 15,16 have shown that one way to experimentally exploit vibronic coupling to modulate molecular function is to introduce targeted vibrational excitation along specific reaction co-ordinates using ultrafast mid-infrared (IR) pulses. This approach enables promoting or suppressing electronic transitions, notably photo-induced electron transfer (ET), an elementary process that pervades the natural world.…”

Photoaquation Mechanism of Hexacyanoferrate(II) Ions: Ultrafast 2D UV and Transient Visible and IR Spectroscopies

“…75,80 Intramolecular energy redistribution occurs at extremely short time scales, reaching even values shorter than the high frequency vibrational time scales, 73,81 while intersystem crossing events also occur at very short time scales. 74 It is therefore expected that the Kasha Rule will also be verified for photochemical reactions, i.e. the significant chemistry involves only the lowest states.…”

Photon‐Gated Spin Transistor

“…[13,14] To achieve optical manipulation of magnetic ordering, much work has been done, providing the possibility to manipulate spins in an optical way, such as the optical excitation of magnetic precession, [15] the all-optical magnetization reversal, [16] and the photon-induced phase transition and spin transition, [17][18][19] confirming the optical control of the spins. So far, gate operation of spin-FET features has been realized by electric field [20][21][22] , magnetic field [11] and interface resonance [23,24] , which can be considered as an electrical-field effect in nature.…”