Effect of Steric Hindrance on the Dynamics of Charge Recombination within Geminate Ion Pairs

Abstract

The dynamics of charge recombination within geminate ion pairs formed by electron transfer (ET) quenching of excited aromatic hydrocarbons by aliphatic and aromatic amines was investigated using picosecond transient grating spectroscopy. With aliphatic donors, the rate constant of back ET, k BET , shows a substantial decrease with increasing steric encumbrance around the N atom. No correlation between k BET and the exergonicity of the process was observed. This effect is ascribed to a decrease of the electronic coupling matrix element, V, which is affected by both the distance between the N atom of the donor and the aromatic plane of the acceptor and by the delocalization of the hole upon increasing the bulkiness of the alkyl substituents. With aromatic amines, k BET is substantially slower than with the unhindered amines. This is also explained in terms of a smaller value of V because of charge delocalization.

Investigations of bimolecular photoinduced electron transfer reactions in polar solvents using ultrafast spectroscopy

“…As discussed in more details in Ref. [62], the effect of other parameters, such as the reorganisation energy associated to high frequency modes, on the variation of k CR can be safely ruled out. This result is in contradiction with the G-dependent ET distance model.…”

“…A first result of such direct measurements is that the assumption of k sep = 5 × 10 8 s −1 in ACN is not valid. Indeed, k sep values ranging from ∼10 8 to 1.5 × 10 9 s −1 have been found [11,[60][61][62][63]. Because of this, the indirectly determined k CR values have to be considered with caution.…”

“…Another indication against long distance ET, at least in non-viscous solvents like ACN, has been obtained by measuring the effect of steric hindrance on the CR dynamics of ion pairs [62]. Fig.…”

Simultaneous Generation of Different Types of Ion Pairs upon Charge-Transfer Excitation of a Donor−Acceptor Complex Revealed by Ultrafast Transient Absorption Spectroscopy

“…This results in a large span of CR time constants going from a few hundreds of femtoseconds to values similar to or greater than the time constant for dissociation into free ions, which is on the order of 1 ns in ACN. [55][56][57][58] …”

Photoinduced Electron Transfer Reactions: From the Elucidation of Old Problems in Bulk Solutions Towards the Exploration of Interfaces

“…[3,9,[12][13][14][15][16][17][18] According to the second hypothesis, highly exergonic CS should yield two ions at remote distance, and thus CR should be relatively slow. Our measurements using a donor/acceptor (D/A) pair with ∆G CS = -2.2 eV reveal that more than 90% of the ion pairs undergo CR in less than 1 ps.…”