Probing Vibronic Coupling in Molecular Oligomers with 1−0 Resonance Tip-Enhanced Raman Spectroscopy

Here we demonstrate the selective excitation and nanoscale imaging of vibronic coupling in artificially constructed molecular oligomers using 1−0 resonance tip-enhanced Raman spectroscopy (TERS). By tuning the laser energy to resonate with a specific vibronic transition (<i>i.e.</i>, |S₀, v = 0⟩ → |S₁, v = 1⟩) of a single zinc phthalocyanine (ZnPc) molecule, we achieve selective enhancement of the corresponding vibrational mode by over an order of magnitude. Applying the 1−0 resonance TERS to an artificially constructed molecular dimer, we investigate the vibronic coupling of the localized intramolecular vibrational modes with the delocalized excitonic modes. We find that the TERS enhancement is governed by the effective transition dipoles of the excitonic modes, with superradiant excitons yielding significantly stronger signals than subradiant ones. Spatially resolved TERS imaging patterns further reveal that the local Raman responses are dictated by the transition dipole configurations of the excitonic modes. Leveraging this knowledge, the 1−0 resonance TERS signal can be further increased with the growing number of molecules in a coherently coupled linear chain. Our findings establish a powerful methodology for interrogating vibronic coupling effects in molecular aggregates at the single-molecule level and offer a new route toward designing high-performance, narrow-band molecular light sources and ultrasensitive vibrational sensors.