Vibronic coherence evolution in multidimensional ultrafast photochemical processes

Abstract

The complex choreography of electronic, vibrational, and vibronic couplings used by photoexcited molecules to transfer energy efficiently is remarkable, but an unambiguous description of the temporally evolving vibronic states governing these processes has proven experimentally elusive. We use multidimensional electronic-vibrational spectroscopy to identify specific time-dependent excited state vibronic couplings involving multiple electronic states, high-frequency vibrations, and low-frequency vibrations which participate in ultrafast intersystem crossing and subsequent relaxation of a photoexcited transition metal complex. We discover an excited state vibronic mechanism driving long-lived charge separation consisting of an initial electronically-localized vibrational wavepacket which triggers delocalization onto two charge transfer states after propagating for ~600 femtoseconds. Electronic delocalization consequently occurs through nonadiabatic internal conversion driven by a 50 cm−1 coupling resulting in vibronic coherence transfer lasting for ~1 picosecond. This study showcases the power of multidimensional electronic-vibrational spectroscopy to elucidate complex, non-equilibrium energy and charge transfer mechanisms involving multiple molecular coordinates.

The role of mixed vibronic Qy-Qx states in green light absorption of light-harvesting complex II

“…In this work, we utilize recent advances in multidimensional spectroscopy, namely the advent of two-dimensional electronic-vibrational (2DEV) spectroscopy 30 , to study the origin and involvement of these highly mixed states-inaccessible to more conventional spectroscopies. A major advantage of this technique is the improved spectral resolution, afforded by infrared (IR) detection, which has successfully allowed for insight into ultrafast energy transfer, charge transfer, and proton transfer dynamics 6,19,[31][32][33][34] . IR detection also inherently makes this technique especially sensitive to the mixing of vibronic states because such mixing significantly alters vibrational transition moments.…”

Two-dimensional electronic–vibrational spectroscopy: Exploring the interplay of electrons and nuclei in excited state molecular dynamics

“…Of course, 2DEV spectroscopy can function across a range of excitation and detection conditions dependent on the light sources employed and the compatibility of pulse shaping and detection capabilities. For example, Gaynor et al utilized a spectrally broadened UV pump source for the 2DEVS studies of the solar cell dye, [Ru-(dcbpy) 2 (NCS) 2 ] (dcbpy = 4,4 ′ -dicarboxy-2,2 ′ -bipyridine) (N3), 4,24 while Song et al demonstrated a 2DEV spectrometer with excitation frequencies ranging from 490 to 950 nm. 25 In a later paper by Gaynor et al, the applicability of a broadband mid-IR source was demonstrated; 26 however, the experimental implementation of broadband probes remains limited.…”

“…2 However, it soon became clear that oscillatory structures in 2DEV spectra of molecular complexes could reveal the role of vibronic interactions in facilitating energy flow. 3 A parallel investigation of vibronic effects in ultrafast charge transfer (CT) via 2DEVS was pioneered by Gaynor et al 4 A comparison of 2DEVS with the degenerate third-order 2D infrared [5][6][7][8] and 2D electronic [9][10][11] spectroscopies (2DIRS and 2DES,…”

“…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. 4 This improved resolution along the excitation axis also 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. 19 Of course, 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.…”

The role of mixed vibronic Qy-Qx bands in the light-harvesting dynamics of the major antenna complex, LHCII

“…A major advantage of this technique is the improved spectral resolution, afforded by IR detection, which has successfully allowed for insight into ultrafast energy transfer, charge transfer, and proton transfer dynamics. 6,19,[31][32][33][34] IR detection also inherently makes this technique especially sensitive to the mixing of vibronic states because such mixing significantly alters vibrational transitions moments. Additionally, the further sensitivity provided by polarization-dependent 2DEV spectroscopy has been demonstrated in the spectral assignments of monomeric Chl a and b 35 , as well as in unveiling the role of vibronic coupling in a solar cell dye.…”