Abstract
Two-dimensional electronic-vibrational spectroscopy (2DEVS) is an emerging spectroscopic technique which exploits two different frequency ranges for the excitation (visible) and detection (infrared) axes of a 2D spectrum. In contrast to degenerate 2D techniques, such as 2D electronic or 2D infrared spectroscopy, the spectral features of a 2DEV spectrum report cross correlations between fluctuating electronic and vibrational energy gaps rather than autocorrelations as in the degenerate spectroscopies. The center line slope of the spectral features reports on this cross correlation function directly and can reveal specific electronic-vibrational couplings and rapid changes in the electronic structure, for example. The involvement of the two types of transition moments, visible and infrared, makes 2DEVS very sensitive to electronic and vibronic mixing. 2DEV spectra also feature improved spectral resolution, making the method valuable for unraveling the highly congested spectra of molecular complexes. The unique features of 2DEVS are illustrated in this paper with specific examples and their origin described at an intuitive level with references to formal derivations provided. Although early in its development and far from fully explored, 2DEVS has already proven to be a valuable addition to the tool box of ultrafast nonlinear optical spectroscopy and is of promising potential in future efforts to explore the intricate connection between electronic and vibrational nuclear degrees of freedom in energy and charge transport applications.
Highlights
The method of two-dimensional electronic–vibrational spectroscopy (2DEVS), introduced in 2014 by Oliver et al.,1 was proposed as a technique for correlating the evolution of the nuclear degrees of freedom (DoF) with that of the electronic DoF in optically excited molecules and complexes
It soon became clear that oscillatory structures in 2DEV spectra of molecular complexes could reveal the role of vibronic interactions in facilitating energy flow
The good spectral resolution in 2DEV spectra along the detection (IR) axis is not surprising, but in many 2DEV spectra, enhanced resolution along the excitation axis as compared to linear absorption or 2D electronic spectra is noticeable. This has enabled the clear identification of the involvement of specific vibrational modes in relaxation over particular energy gaps in an excitonic manifold,3 for example, or of vibronic coupling across specific metal-to-ligand charge transfer (MLCT) states
Summary
The method of two-dimensional electronic–vibrational spectroscopy (2DEVS), introduced in 2014 by Oliver et al., was proposed as a technique for correlating the evolution of the nuclear degrees of freedom (DoF) with that of the electronic DoF in optically excited molecules and complexes. The good spectral resolution in 2DEV spectra along the detection (IR) axis is not surprising, but in many 2DEV spectra, enhanced resolution along the excitation (visible) axis as compared to linear absorption or 2D electronic spectra is noticeable This has enabled the clear identification of the involvement of specific vibrational modes in relaxation over particular energy gaps in an excitonic manifold, for example, or of vibronic coupling across specific metal-to-ligand charge transfer (MLCT) states.. This has enabled the clear identification of the involvement of specific vibrational modes in relaxation over particular energy gaps in an excitonic manifold, for example, or of vibronic coupling across specific metal-to-ligand charge transfer (MLCT) states.4 This improved resolution along the excitation axis enables tracking the temporal evolution of the excitation frequency distribution at specific vibrational frequencies that can be related back to individual components or electronic states in heavily overlapped spectral regions of multi-component systems.. There are multiple aspects of similarity in these three 2D spectroscopies, such as the exploitation of photon polarization to enhance information content, and clearly, the three methods should be regarded as complementary with the choice of the method dictated by the questions asked
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