Shipp James D, Fernández-Terán Ricardo J, Auty Alexander J, Carson Heather, Sadler Andrew J, Towrie Michael, Sazanovich Igor V, Donaldson Paul M, Meijer Anthony J H M, Weinstein Julia A
Department of Chemistry, University of Sheffield. Sheffield S3 7HF, U.K.
Department of Physical Chemistry, University of Geneva, CH-1205 Geneva, Switzerland.
ACS Phys Chem Au. 2024 Oct 29;4(6):761-772. doi: 10.1021/acsphyschemau.4c00073. eCollection 2024 Nov 27.
Donor-bridge-acceptor complexes (D-B-A) are important model systems for understanding of light-induced processes. Here, we apply two-color two-dimensional infrared (2D-IR) spectroscopy to D-B-A complexes with a -Pt(II) acetylide bridge (D-C≡C-Pt-C≡C-A) to uncover the mechanism of vibrational energy redistribution (IVR). Site-selective C isotopic labeling of the bridge is used to decouple the acetylide modes positioned on either side of the Pt-center. Decoupling of the D-acetylide- from the A-acetylide- enables site-specific investigation of vibrational energy transfer (VET) rates, dynamic anharmonicities, and spectral diffusion. Surprisingly, the asymmetrically labeled D-B-A still undergoes intramolecular IVR between acetylide groups even though they are decoupled and positioned across a heavy atom usually perceived as a "vibrational bottleneck". Further, the rate of population transfer from the bridge to the acceptor was both site-specific and distance dependent. We show that vibrational excitation of the acetylide modes is transferred to ligand-centered modes on a subpicosecond time scale, followed by VET to solvent modes on the time scale of a few picoseconds. We also show that isotopic substitution does not affect the rate of spectral diffusion, indicating that changes in the vibrational dynamics are not a result of differences in local environment around the acetylides. Oscillations imprinted on the decay of the vibrationally excited acceptor-localized carbonyl modes show they enter a coherent superposition of states after excitation that dephases over 1-2 ps, and thus cannot be treated as independent in the 2D-IR spectra. These findings elucidate the vibrational landscape governing IR-mediated electron transfer and illustrate the power of isotopic labeling combined with multidimensional IR spectroscopy to disentangle vibrational energy propagation pathways in complex systems.
供体-桥-受体复合物（D-B-A）是理解光诱导过程的重要模型体系。在此，我们将双色二维红外（2D-IR）光谱应用于含有-Pt(II)乙炔桥（D-C≡C-Pt-C≡C-A）的D-B-A复合物，以揭示振动能量重新分布（IVR）的机制。桥的位点选择性碳同位素标记用于解耦位于Pt中心两侧的乙炔模式。D-乙炔基与A-乙炔基的解耦使得能够对振动能量转移（VET）速率、动态非谐性和光谱扩散进行位点特异性研究。令人惊讶的是，即使不对称标记的D-B-A中的乙炔基团被解耦并位于通常被视为“振动瓶颈”的重原子两侧，它们之间仍会发生分子内IVR。此外，从桥到受体的布居转移速率既具有位点特异性又与距离有关。我们表明，乙炔模式的振动激发在亚皮秒时间尺度上转移到以配体为中心的模式，随后在几皮秒的时间尺度上发生VET到溶剂模式。我们还表明，同位素取代不会影响光谱扩散速率，这表明振动动力学的变化不是乙炔周围局部环境差异的结果。印刻在振动激发的受体局部羰基模式衰减上的振荡表明，它们在激发后进入状态的相干叠加，该叠加在1-2皮秒内退相，因此在2D-IR光谱中不能被视为独立的。这些发现阐明了控制红外介导电子转移的振动态势，并说明了同位素标记与多维红外光谱相结合在解开复杂体系中振动能量传播途径方面的强大作用。
