Vibrational coherence in the excited state dynamics of Cr(acac)3: probing the reaction coordinate for ultrafast intersystem crossing

Abstract

Vibrational coherence was observed following excitation into the lowest-energy spin-allowed 4 A 2 / 4 T 2 ligand-field absorption of Cr(acac) 3 . The transient kinetics were fit to a rapidly damped 164 cm À1 oscillatory component, the frequency of which is not associated with the ground state of the molecule. The signal is assigned as an excited-state vibrational coherence; the timescale of the event suggests that this vibrational coherence is retained during the 4 T 2 / 2 E intersystem crossing that immediately follows 4 A 2 / 4 T 2 excitation. DFT calculations indicate that the 164 cm À1 oscillation likely corresponds to a combination of Cr-O bond stretching in the ligand-field excited state as well as large amplitude motion of the ligand backbone. This hypothesis is supported by ultrafast timeresolved absorption measurements on Cr(t-Bu-acac) 3 (where t-Bu-acac is the monoanionic form of 2,2,6,6-tetramethyl-3,5-heptanedione) -an electronically similar but more sterically encumbered molecule -which exhibits a 4 T 2 / 2 E conversion that is more than an order of magnitude slower than that observed for Cr(acac) 3 . These results provide important insights into the nature of the reaction coordinate that underlies ultrafast excited-state evolution in this prototypical coordination complex.

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

“…6, 8, and Supplementary Information section 6) we find that injecting additional vibrational energy accelerates the decay of 3 MLCT* into CSSs by 14 ± 2%; this acceleration is to be expected since we already know from pump-probe data that 3 MLCT decays to the CSSs faster in the presence of excess vibrational energy (c.f. lifetimes of 3 MLCT* compared to 3 MLCT), similarly to a range of other Pt-acetylide charge transfer systems in solution. 35,36 Taking into account this rate acceleration, we then proceed to fit the off-diagonal kinetic response from Figure 3c.…”

“…The immediate effect of the IR pump is to excite (C C) v=0 into v = 1, resulting in the bleach-transient peak pair which forms the dominant signal in both panels. 26 The vibrational lifetimes of ( 13 C) and ( 12 C) in 3 MLCT extracted from the decay of this transient response ( Supplementary Fig. 11) are ( 13 C) = 1.0 ± 0.4 ps, ( 12 C) = 3.6 ± 0.4 ps, somewhat shorter than in the ground state.…”

“…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.…”

Using coherence to enhance function in chemical and biophysical systems

“…5b. Persistent coherence through an electronic state change is observed in studies of chromium acetylacetonate 86 , where excitation into the lowest-energy spin-allowed ligand-field absorption of this compound results in rapid intersystem crossing from a quartet to doublet spin excited state in < 100 fs. Coherent oscillations produced by excitation of the initial quartet state do not decohere during the radiationless transition; see Fig.…”