Influence of structural dynamics on charge recombination rates in photogenerated radical ion pairs: Evidence from EPR spectroscopy and computation

In order to better understand the dependence of charge recombination rate vs. temperature k_CR(T) within a linear donor-chromophore-acceptor (D-C-A) molecular triad, the structural dynamics of the cation radical D ^+̇-C is studied individually using variable-temperature electron paramagnetic resonance (EPR) spectroscopy and electronic structure calculations. Here, the donor D is p-methoxyaniline, the chromophore C is 4-(N-piperidinyl)-naphthalene-1,8-dicarboximide, and the acceptor A is naphthalene-l,8:4,5-bis(dicarboximide). The EPR spectra of D^+̇-C exhibit marked changes in their overall shape throughout the 190-295 K temperature range. These spectra have hyperfine splittings that are strikingly well simulated with a model that includes methoxy group rotation, which occurs at a rate of 2.6 · 10⁴ s^-1 at 210 K and speeds up to 1.25 · 10⁷ s^-1 at 295 K, corresponding to an energy barrier of 38 kJ/mol. This considerable barrier reflects the partial conjugation between MeO and the aromatic ring and is confirmed by the calculated energy of a series of D^+̇-C rotamers. The simulations also reveal that inversion of the anilino N center emerges at T > 250 K and can be represented by a planar and a pyramidal conformation with the equilibrium constant K = [pyramidal]/[planar] increasing from 0.029 at 250 K to 0.56 at 295 K. In the same temperature range, the charge recombination rate of D ^+̇-C-A^-̇ accelerates abruptly and can be separated into two components, according to the above planar/pyramidal equilibrium. The k_CR(T) of the pyramidal conformation has an activation energy of 41 kJ/mol, virtually the same as the barrier of MeO rotation. These results show that the intramolecular structural dynamics of the radical cation within D ^+̇-C-A control the overall charge recombination reaction with this radical ion pair.