Correlated electronic and nuclear motion plays a crucial role in the photophysics and photochemistry of fundamental natural processes, including photosynthesis and vision, and governs several technologically-relevant phenomena in molecular photonics and electronics. The newly proposed two-dimensional electronic-vibrational (2D-EV) spectroscopy [1] offers a new perspective on the relaxation of photoexcited systems, governed by the subtle interplay of electronic and vibrational degrees of freedom. We enter this fresh and timely field providing a theoretical tool for the simulation of 2D-EV spectra of conjugated dyes in solution. Based on a minimal description of the electronic system, a powerful interpretative tool for a sound understanding of 2D-EV spectra is described, accounting for electron-vibration coupling and for solvation dynamics. In particular, we calculate 2D-EV spectra of solvated organic dyes modeled in terms of a reduced set of electronic diabatic states (the essential states) non-adiabatically coupled to molecular vibrations. An effective overdamped coordinate, whose dynamics is described by the Smoluchowski diffusion equation, accounts for polar solvation. Results are discussed for two dyes with distinctively different spectroscopic behavior: 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM) [2] and 8-(N,N-dibutylamino)-2-azachrysene (AAC) [3]. Linear absorption and fluorescence spectra of DCM are well reproduced based on a minimal two-state model. The same model leads to 2D-EV spectra in good agreement with the recent experimental data by Oliver and coworkers for DCM in DMSO [1]. On the opposite, linear spectra of AAC show a subtle interplay between a locally-excited (LE) and charge-transfer (CT) excitation, calling for a three-state model. Calculated 2D-EV spectra for AAC show a qualitatively different behavior, demonstrating that the experimental data for DCM do not support a LE/CT interplay. This resolves the long-lasting discussion about the nature of low-lying excitations of DCM in favor of the simplest picture.
[1 ] Oliver TAA, Lewis NHC, Fleming GR (2014) Correlating the motion of electrons and nuclei with two-dimensional electronic-vibrational spectroscopy. Proc Natl Acad Sci USA 111(28):10061–10066. [2] Boldrini B, Cavalli E, Painelli A, Terenziani F (2002) Polar dyes in solution: A joint experimental and theoretical study of absorption and emission band shapes. J Phys Chem A 106(26):6286–6294. [3] Sissa C, et al. (2013) Tuning the nature of the fluorescent state: a substituted polycondensed dye as a case study. Chem Eur J 19(3):924–935.