Correlating the motion of electrons and nuclei with two-dimensional electronic–vibrational spectroscopy
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Contributed by Graham R. Fleming, May 16, 2014 (sent for review March 24, 2014)

Significance
To have a full understanding of the role of nuclear motions involved in nonradiative relaxation dynamics of complex molecular systems such as carotenoids, nanomaterials, and molecular photoswitches, the ability to correlate the electronic and nuclear degrees of freedom is imperative. We have developed 2D electronic–vibrational spectroscopy to observe this correlation, and this new spectroscopic technique will provide a direct probe of the coupling between electronic and nuclear dynamics at conical intersections, and thus the underlying mechanisms driving the ensuing photochemistry.
Abstract
Multidimensional nonlinear spectroscopy, in the electronic and vibrational regimes, has reached maturity. To date, no experimental technique has combined the advantages of 2D electronic spectroscopy and 2D infrared spectroscopy, monitoring the evolution of the electronic and nuclear degrees of freedom simultaneously. The interplay and coupling between the electronic state and vibrational manifold is fundamental to understanding ensuing nonradiative pathways, especially those that involve conical intersections. We have developed a new experimental technique that is capable of correlating the electronic and vibrational degrees of freedom: 2D electronic–vibrational spectroscopy (2D-EV). We apply this new technique to the study of the 4-(di-cyanomethylene)-2-methyl-6-p-(dimethylamino)styryl-4H-pyran (DCM) laser dye in deuterated dimethyl sulfoxide and its excited state relaxation pathways. From 2D-EV spectra, we elucidate a ballistic mechanism on the excited state potential energy surface whereby molecules are almost instantaneously projected uphill in energy toward a transition state between locally excited and charge-transfer states, as evidenced by a rapid blue shift on the electronic axis of our 2D-EV spectra. The change in minimum energy structure in this excited state nonradiative crossing is evident as the central frequency of a specific vibrational mode changes on a many-picoseconds timescale. The underlying electronic dynamics, which occur on the hundreds of femtoseconds timescale, drive the far slower ensuing nuclear motions on the excited state potential surface, and serve as a excellent illustration for the unprecedented detail that 2D-EV will afford to photochemical reaction dynamics.
Footnotes
- ↵1To whom correspondence should be addressed. E-mail: grfleming{at}lbl.gov.
Author contributions: T.A.A.O., N.H.C.L., and G.R.F. designed research; T.A.A.O. and N.H.C.L. performed research; T.A.A.O. and N.H.C.L. analyzed data; and T.A.A.O. wrote the paper.
The authors declare no conflict of interest.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1409207111/-/DCSupplemental.
Freely available online through the PNAS open access option.
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