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

Abstract

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 CC stretching mode of the bridge (ν_CC) in a 3-pulse experiment: a UV pump was followed by a mid-IR pump pulse (at ν_CC), 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_CC 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_CC. 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.