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

Abstract

In order to better understand the dependence of charge recombination rate vs. temperature kcR(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-l,8-dicarboximide, and the acceptor Ais 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 ate strikingly well simulated with a model that includes methoxy group rotation, which occurs at a tate of 2.6-10 4 S -I at 210 K and speeds up to 1.25-107 s -~ at 295 K, corresponding to an energy barrier of 38 k J/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 aplanar anda 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 kcR(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.

Covalent Radical Pairs as Spin Qubits: Influence of Rapid Electron Motion between Two Equivalent Sites on Spin Coherence

“…When the g -factors of the two radicals comprising the RP are comparable, as is the case for A 2 3+• and A 3+• ( g = 2.0025) and for D +• ( g = 2.0028), RP-ISC is driven by the difference in electron–nuclear hyperfine interactions of each radical, Δ a , eq : , where a 1 i and a 2 j are the individual hyperfine couplings in radicals 1 and 2 and m 1 i and m 2 j are the corresponding nuclear spin quantum numbers. Using the measured hyperfine couplings of A 2 3+• and A 3+• (this work) and that of D +• (Table S3), Δ a = 0.85 mT and 0.94 mT for D +• –C–A 2 3+• and D +• –C–A 3+• , respectively. If 2 J < Δ a , RP-ISC mixes all three triplet states with the corresponding singlet state. ,,, TREPR spectroscopy was used to certify that 2 J < a for D +• –C–A 2 3+• and D +• –C–A 3+• .…”

“…It is well-known that the value of B 1/2 obtained from the MFE plot is a function of the total effective hyperfine interaction of all the nuclei within radicals 1 and 2, a eff 1 and a eff 2 , respectively, that drive RP-ISC as determined by eqs and : where i = 1 or 2, I is the nuclear spin quantum number, and j is summed over all nuclear spins within radical i . Using eqs and , and the hyperfine couplings of A 2 3+• , A 3+• , and D +• (Table S3), the estimated hyperfine contribution to B 1/2 is 3.7 mT for both D +• –C–A 2 3+• and D +• –C–A 3+• . This results from the fact that even though the hyperfine splittings of A 2 3+• are about half those of A 3+• , the dimeric acceptor has twice as many nuclear spins.…”

“…Importantly, the fits are constrained such that the reconstructed species-associated spectra of the singlet and triplet RPs are identical given that their energy difference is far below TREPR Measurements. When the g-factors of the two radicals comprising the RP are comparable, as is the case for A 2 3+• and A 3+• (g = 2.0025) and for D +• (g = 2.0028 61 ), RP-ISC is driven by the difference in electron−nuclear hyperfine interactions of each radical, Δa, eq 3: 32,62…”

Time-Resolved EPR Spectra of Spin-Correlated Radical Pairs: Spectral and Kinetic Modulation Resulting from Electron−Nuclear Hyperfine Interactions

“…In the case of the triad D − C − A , two reference molecules, D − C and A , were chemically converted into their corresponding radicals, D +• − C and A −• , and their individual cw EPR spectra are presented in Figure a. As studied earlier, the donor cation exhibits a resolved array of hyperfine peaks at 295 K thanks to motional averaging of the two possible methoxy group orientations. At 210 K, the conformational dynamics of D +• − C is frozen, and only a broad Gaussian-like profile remains.…”

Probing the orientation of porphyrin oligomers in a liquid crystal solvent – a triplet state electron paramagnetic resonance study

“…Especially for electron and energy transfer reactions, determining the orientation of the ZFS tensor axes within the molecular frame is important since the mutual ZFS tensor orientation of the donor and acceptor moieties has a direct impact on the reaction efficiencies and deactivation rates [21,22], i.e. the electron transfer rates depend on the conformational distribution of the donor and acceptor molecules (conformational gating) which can be controlled by LCs [23][24][25].…”