Bimolecular photoinduced electron transfer in non-polar solvents beyond the diffusion limit

Abstract

Electron transfer (ET) quenching dynamics in non-polar solvents are investigated using ultrafast spectroscopy with a series of six fluorophore/quencher pairs, covering a driving-force range of more than 1.3 eV. The intrinsic ET rate constants, k 0 , deduced from the quenching dynamics in the static regime, are of the order of 10 12 − 10 13 M -1 s -1 , i.e. at least as large as in acetonitrile, and do not exhibit any marked dependence on the driving force. A combination of transient electronic and vibrational absorption spectroscopy measurements reveal that the primary product of static quenching is a strongly coupled exciplex that decays within a few picoseconds. More weakly coupled exciplexes with a longer lifetime are generated subsequently, during the dynamic, diffusion-controlled, stage of the quenching. The results suggest that static ET quenching in non-polar solvents should be viewed as an internal conversion from a locally-excited state to a charge-transfer state of a supermolecule rather than as a non-adiabatic ET process.

Insights Into the Mechanism of Energy Transfer with Poly(Heptazine Imide)s in a Deoximation Reaction

“…We utilized 1 i as a quencher in time‐resolved fluorescence measurements to determine the fluorescence quenching rate constant. Rate constant k Q is found to be (4.656±0.648) ⋅ 10 9 M −1 s −1 in this case (Figure 4c), which is comparable to the diffusion‐limited reaction rate in diluted solvent media [26,27] . This means quenching is possible only in a rather tight contact by orbital overlapping, unambiguously defining the reaction to proceed via the Dexter mechanism.…”

Lifetime Broadening and Impulsive Generation of Vibrational Coherence Triggered by Ultrafast Electron Transfer

“…In the present case, the CS dynamics is the same in polar and apolar solvents, revealing that the reaction coordinate involves only intramolecular modes. This result goes along with previous observations of ultrafast ET processes occurring on a faster time scale than that of solvent relaxation and/or exhibiting no significant dependence on the solvent polarity. ,− In agreement with the two-dimensional Sumi–Marcus model, ET can proceed entirely via intramolecular coordinates if the barrier along these coordinates is low enough to be overcome on a shorter time scale than that of solvent motion.…”

“…In fact, this effect might not be so uncommon and could, for example, underlie the spectral changes often observed upon addition of high concentrations of quenchers to solutions of chromophores. 45,53 Observation of such broadening could help identifying ultrafast ET processes that are difficult to resolve in time-domain experiments.…”

“…Although this lifetime broadening is known in the gas phase, − or at very low temperatures, − such clear correlation between the excited-state lifetime and the absorption bandwidth was, to the best of our knowledge, never reported so far for organic systems in room-temperature solution. In fact, this effect might not be so uncommon and could, for example, underlie the spectral changes often observed upon addition of high concentrations of quenchers to solutions of chromophores. , Observation of such broadening could help identifying ultrafast ET processes that are difficult to resolve in time-domain experiments.…”

Molecular Dynamics Simulations of Bimolecular Electron Transfer: Testing the Coulomb Term in the Weller Equation

“…22,23 It is also generally accepted that ET in this case is not complete, and the product should be considered as an exciplex rather than a pair of ions. [24][25][26][27][28][29] More direct determination of ∆G ET using time-resolved calorimetry was shown to be hampered by the estimation of the entropy change upon ET. [30][31][32] These controversies on the nature and magnitude of C, along with the difficulty of accessing it experimentally, spurred the use of molecular dynamics (MD) simulations.…”