Photo-induced electron transfer dynamics and its mid-IR modulation of an ethyne bridged donor-acceptor complex

Electron transfer (ET) in donor-bridge-acceptor (DBA) complexes involving alkyne bridges features structural simplicity derived from the rigid and nearly linear donor-acceptor (D-A) geometry, and complexity derived from the torsion angle distribution with its associated influence on the D-A coupling and conjugation. Here, we report on the study of ET processes in DMA-C2-NAP, featuring a dimethylaniline (DMA) donor, ethyne bridge (C2), and N-isopropyl-1,8-naphthalimide (NAP) acceptor. Transient UV-vis and UV-mid-IR spectroscopy experiments were performed and analyzed using TD-DFT computations that targeted the torsion angle dependent properties of the compound in various electronic states. We also modulated the rate of charge separation (CS) in DMA-C2-NAP by exciting a CC stretching mode of the bridge (ν_CC) in a 3-pulse experiment: a UV pump was followed by a mid-IR pump pulse (at ν_CC), and spectral changes in the visible region that were caused by the mid-IR pulse were detected. We compared the results for DMA-C2-NAP with those obtained recently for DMA-C4-NAP, which features a butadiyne bridge (K. C. Mendis et. al., Phys. Chem. Chem. Phys., 2024, 26(3), 1819-1828). As in the DMA-C4-NAP species, we found a symmetry-based relationship between the S₁ and S₂ excitation probabilities on their torsion angle. The D-A coupling is found to be larger in DMA-C2-NAP than in DMA-C4-NAP, producing a fast single-component CS process with a rate of (0.67 ps)^-1, as opposed to the two-component CS processes in DMA-C4-NAP (0.63 and 4.3 ps) attributed to CS in compounds with different torsion angles. Faster CS in DMA-C2-NAP is also realized because of its narrower torsion angle distribution in the ground state. The 3-pulse experiments found a strong dependence of the transient spectra on the time delay between the mid-IR pulse and the UV pump. The presence of vibronic coupling that involves ν_CC in the S₂ state allows us to track the CS process. A global matrix-based analysis found a 1.3-fold reduction in the CS rate in DMA-C2-NAP upon mid-IR excitation of ν_CC. It is predicted that narrowing the torsion angle distribution for the alkyne-bridged compounds will produce changes in the CS rates, which holds potential for greater rate modulation.