A spectroscopic technique for understanding photochemical reactions

Photochemical reactions - reactions triggered by light - play all sorts of roles, from allowing our eyes to see, to enabling green plants to harvest energy from the sun, and enabling the creation of new nanomaterials.

Using photochemical reactions to our advantage requires a deep understanding of the interplay between the electrons and atomic nuclei within a molecular system after that system has been excited by light. Researchers with the US Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California (UC) Berkeley have reported an advance towards this understanding.

The research group of Graham Fleming, UC Berkeley’s Vice Chancellor for Research, helped develop 2D electronic spectroscopy (2D-ES) in 2007, which enables scientists to follow the flow of light-induced excitation energy through molecular systems with femtosecond (millionth of a billionth of a second) temporal resolution. 2D infrared spectroscopy is a different method which is suitable for studying nuclear vibrational couplings and ground-state structures of chemical and complex biological systems.

Fleming and his research group combined the advantages of 2D-electronic and 2D-infrared spectroscopy to develop a new experimental technique called two-dimensional electronic-vibrational spectroscopy (2D-EV). The technique is the first that can be used to simultaneously monitor electronic and molecular dynamics on a femtosecond time-scale. The results show how the coupling of electronic states and nuclear vibrations affect the outcome of photochemical reactions.

“Combining these two techniques into 2D-EV tells us how photoexcitation affects the coupling of electronic and vibrational degrees of freedom,” said Thomas Oliver, a member of the research group. “This coupling is essential to understanding how all molecules, molecular systems and nanomaterials function.”

2D-EV spectral data tells researchers how photoexcitation of a molecular system affects the coupling of electronic and vibrational degrees of freedom that is essential to understanding how all molecules, molecular systems and nanomaterials function. Image courtesy of Fleming group.

In 2D-EV, a sample is sequentially flashed with three femtosecond pulses of laser light. The first two pulses are visible light that create excited electronic states in the sample. The third pulse is mid-infrared light that probes the vibrational quantum state of the excited system. This combination of visible excitation and mid-infrared probe pulses enables researchers to correlate the initial electronic absorption of light with the subsequent evolution of nuclear vibrations.

“2D-EV’s ability to correlate the initial excitation of the electronic-vibrational manifold with the subsequent evolution of high-frequency vibrational modes, which until now have not been explored, opens many potential avenues of fruitful study, especially in systems where electronic-vibrational coupling is important to the functionality of a system,” Fleming said.

The group used their technique to study the excited-state relaxation dynamics of DCM dye dissolved in a deuterated solvent. DCM is considered a model ‘push-pull’ emitter, meaning it contains both electron donor and acceptor groups, with a long-standing question as to how it fluoresces back to the ground energy state.

“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 before emission,” Oliver said. “The underlying electronic dynamics, which occur on the hundreds of femtoseconds time-scale, drive the far slower ensuing nuclear motions on the excited state potential surface, and serve as an excellent illustration for the unprecedented detail that 2D-EV will afford to photochemical reaction dynamics.”

Nicholas Lewis, Graham Fleming and Tom Oliver developed 2D-EV, a technique that enables electronic and molecular dynamics during a photochemical reaction to be simultaneously monitored on a femtosecond time-scale. Photo by Roy Kaltschmidt.

2D-EV may be used to study several different areas, including rhodopsin, the pigment protein in the retina of the eye that is the primary light detector for vision; and carotenoids, the family of pigment proteins, such as chlorophyll, found in green plants and certain bacteria that absorb light for photosynthesis. For nanomaterials, 2D-EV should be able to shed light on how the coupling of phonons with electrons impacts the properties of carbon nanotubes and other nanosystems.

The technique can also be used to investigate the barriers to electron transfer between donor and acceptor states in photovoltaic systems. Oliver said it is being continually refined to “make it more widely applicable so that it can be used to study lower frequency motions that are of great scientific interest”.

“We think that 2D-EV, by providing unprecedented details about photochemical reaction dynamics, has the potential to answer many currently inaccessible questions about photochemical and photobiological systems,” said Fleming. “We anticipate its adoption by leading laboratories across the globe.”

The research has been published in the Proceedings of the National Academy of Sciences.

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