Correlated vibrational coherence and spectral diffusion analysis of multi-level systems using two-dimensional electronic spectroscopy.

This article focuses on the theoretical and experimental investigation of a minimal three-level "V" system (3LVS) using two-dimensional electronic spectroscopy (2DES), with 6,13-bis(triisopropylsilylethynyl)-pentacene (TIPS-Pn) as the model molecule. The study derives third-order nonlinear optical response functions incorporating frequency fluctuation correlation functions (FFCF) and frequency fluctuation cross correlation functions (FXCF) based on a displaced harmonic oscillator (DHO) model to characterize diagonal and cross-peaks in 2DES spectra. Experimental 2DES measurements and center line slope (CLS) analysis of TIPS-Pn reveal oscillations at 264 cm⁻¹, corresponding to a vibrational mode of the pentacene moiety, with comparable amplitude and phase relationships between diagonal and cross-peaks, consistent with theoretical predictions. The work estimates an effective Huang–Rhys factor (~0.27) quantifying the coupling strength of vibronic transitions to the common vibrational mode and demonstrates that CLS analysis of both diagonal and cross-peaks can quantify correlated vibrational coherences, offering a method to probe coupling in more complex multistate molecular systems.