Two-Dimensional Infrared Spectroscopy Resolves the Vibrational Landscape in Donor–Bridge–Acceptor Complexes with Site-Specific Isotopic Labeling

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 trans-Pt(II) acetylide bridge (D–C≡C–Pt–C≡C–A) to uncover the mechanism of vibrational energy redistribution (IVR). Site-selective ¹³C 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.