Ultrafast Spectroscopic Signatures of Coherent Electron-Transfer Mechanisms in a Transition Metal Complex

Abstract

The prevalence of ultrafast electron-transfer processes in light-harvesting materials has motivated a deeper understanding of coherent reaction mechanisms. Kinetic models based on the traditional (equilibrium) form of Fermi's Golden Rule are commonly employed to understand photoinduced electron-transfer dynamics. These models fail in two ways when the electron-transfer process is fast compared to solvation dynamics and vibrational dephasing. First, electron-transfer dynamics may be accelerated if the photoexcited wavepacket traverses the point of degeneracy between donor and acceptor states in the solvent coordinate. Second, traditional kinetic models fail to describe electron-transfer transitions that yield products which undergo coherent nuclear motions. We address the second point in this work. Transient absorption spectroscopy and a numerical model are used to investigate coherent back-electron-transfer mechanisms in a transition metal complex composed of titanium and catechol, [Ti(cat)3](2-). The transient absorption experiments reveal coherent wavepacket motions initiated by the back-electron-transfer process. Model calculations suggest that the vibrationally coherent product states may originate in either vibrational populations or coherences of the reactant. That is, vibrational coherence may be produced even if the reactant does not undergo coherent nuclear motions. The analysis raises a question of broader significance: can a vibrational population-to-coherence transition (i.e., a nonsecular transition) accelerate electron-transfer reactions even when the rate is slower than vibrational dephasing?

Behind the scenes of spin-forbidden decay pathways in transition metal complexes

“…transitions must be accompanied by a change in angular momentum. 163 It thus predicts that the rate of ISC is larger for transitions of 1 (π, π * ) 3 (n, π * ) type than for 1 (π, π * ) 3 (π, π * ) (and vice versa). This rule is based on the electronic part of Eq.…”

Nonlinear Photocurrent Spectroscopy of Layered 2D Perovskite Quantum Wells

“…Contributions of the excitons to the 2D NLPC spectrum are obscured by the broad component of the NLPC signal in Figure a–d. For this reason, the spectra in Figure e–h have been processed with an algorithm designed to separate resonance Raman and fluorescence components of spontaneous emission spectra (i.e., a numerical method for separating narrow and broad components of a spectrum). , This approach, which is outlined in the Supporting Information, is appropriate for the present application because (i) the excitons and continuum states possess different line widths and (ii) the broad line shape associated with the continuum states is not readily subtracted by fitting the spectra with phenomenological functions (e.g., sum of Gaussians, polynomials, etc.). At all delay times, the 2D spectra in Figure e–h possess diagonal and off-diagonal peaks at the wavelengths associated with the n = 2 and 3 quantum wells (i.e., noise induces weaker features at other wavelengths).…”

Simulating Coherent Multidimensional Spectroscopy of Nonadiabatic Molecular Processes: From the Infrared to the X-ray Regime

“…TA studies of ultrafast relaxation dynamics are very common; recent examples are refs and − , and femtosecond time-resolved pump–probe optical microscopy techniques are emerging , (for recent reviews see refs − ). Here, we denote special attention to studies that employed sophisticated spectral probe conditions.…”