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.
Highlights
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
Exciting progress in experimental techniques is making investigations related to the following questions tractable: how do specific molecular coordinates control photoexcited energy transfer dynamics? What role do vibronic couplings and coherences play during photoexcited energy transfer and relaxation? Third-order nonlinear Fourier transform (FT) techniques such as two-dimensional (2D) electronic (ES) and 2D infrared (IR) spectroscopy have greatly advanced our understanding of coherent molecular phenomena in solution[24,25,26,27,28,29,30,31,32,33]
Using polarization-selective 2D electronic–vibrational (2D EV) spectroscopy of N34− at a single τ2 time delay, we discovered that two excited electronic states with 1MLCT character (1MLCTA and 1MLCTB) are vibronically coupled to charge donor and acceptor vibrations within the excited triplet manifold, likely facilitating ultrafast photoexcited charge transfer[34]
Summary
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. A wealth of experimental and computational studies on Ru-centered complexes have measured their photoexcited dynamics following singlet metalto-ligand charge transfer (MLCT) excitation, such as ultrafast intersystem crossing (ISC) These past studies have established that the excited triplet MLCT states can provide sufficiently longlived charge separation to yield a useful chemical potential for energy harvesting applications[12,13,14,15,16,17,18]. Time-dependent 3D EV features, such as frequency shifts of low-frequency modes as generally indicated by L1 moving to L1’ in the ω2 frequency space, report on the temporal evolution and nonequilibrium relaxation of the vibronic eigenstates during photoexcited processes (depicted by the sphere coloration gradient)
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