Laser Spectroscopy of Jet-Cooled Molecules
Laser Spectroscopy of Jet-Cooled Molecules
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Why cold, isolated molecules?
By incorporating a vapor sample into a supersonic jet expansion, collisional cooling brings the molecules to ~2 K internal temperature and isolated in a molecular beam. Once the molecular beam travels through the vacuum chamber to the probe region, tunable high-power laser(s) pulses intercept the molecular beam to probe the cold molecules. The wavelength (or photon energy) of the laser is tuned through individual electronic or vibrational resonances, and either ion counts or fluorescence are detected in response. The result is a UV-vis or IR spectrum with individual quantum state precision, revealing otherwise hidden information about the molecular structure, electronic structure and orbitals, and excited state dynamics. In addition, due to the extremely sharp peaks in these spectra, individual conformational or structural isomers can be resolved and their spectra and structures resolved.
Importantly, the individual features in these highly-resolved spectra are molecular vibrational states that can be individually assigned to particular vibrational modes and quanta. The "vibronic" peaks in UV-vis jet spectra are associated only with vibrations that are activated by that specific electronic transition, revealing only those types of nuclear motion that reflect the geometry change induced by the new excited electron configuration. This inherently distills down the usual 3N-6 number of unique vibrational modes (N = number of atoms), to only those coupled to the electronic transition. Thus, nuclear dynamics triggered by a UV-visible absorption event such as photochemistry, isomerization, or vibrational energy transfer can be isolated and characterized allowing for a better understanding of the connection between electronic structure and nuclear dynamics.
Vibrational-electronic energy migration through natural pigments
Nature utilizes multi-pyrrolic compounds to accomplish a variety of light-mediated functions including light-harvesting in photosynthesis and light-sensing to initiate growth and development in plants and fungi. Chlorophyll is an example of a cyclic tetrapyrrole for example, and it is the ubiquitous pigment that makes plant life green due to their red and blue absorptions. Cyanobacteria and some algae utilize linear tetrapyrroles, or bilins, as their primary pigment allowing for more flexibility in structure, conjugation, and therefore overall light-harvesting. In fact, most of those bilin-containing proteins absorb in the center of the solar spectrum where chlorophylls show an absence of absorption, giving them a chance to compete for sunlight in, often times, a chlorophyll dominant environment.
The inherent flexibility of bilins lend them stronger vibronic activity (nuclear motion triggered by a electronic transition) compared to chlorophyll, and recent works have shown that those vibrations can be utilized to mediate energy transfer across separate bilins in proximity to one another. However, only certain high frequency modes (800-1600 cm-1) have been identified as vibronic coupling modes to facilitate this unique coherent energy transfer mechanism.
In our lab, we investigate the simplest bilin subunits, free dipyrroles, to minimize the complexity of the many torsional coordinates that arise in the full bilin tetrapyrrole (from 6 to 2) while maintaining the electronic character of them. By subjecting the dipyrrole molecule to the cold environment of the supersonic jet, those specific vibrational modes and their extent of excitation can be precisely identified and quantified to help us understand the connection of the electronic character of the molecule to the activated nuclear motion responsible for their unique "vibronic communication" in vivo.
Light screening properties of bio-inspired sunscreens
Like humans, plants, animals, fungi, and bacteria are susceptible to UV-induced damage of epidermal tissue, DNA, etc. due to the bond-breaking energies of UV radiation. In a similar way that humans produce melanin as a natural sunscreen to absorb and down-convert that UV energy to heat, other organisms have been shown to do the same with different organic compounds which are made through unique biosynthetic pathways.
Given their biosynthesis, these bio-inspired UV sunscreens provide a natural alternative to current synthetic sunscreens, which often present environmental issues such as bleaching of coral reefs. Purifying these natural sunscreens and probing them via laser spectroscopy in a supersonic jet allows us to assess the connection between chemical structure and their ability to screen and dissipate UV energies. Specifically, we are addressing three criteria necessary for an effective sunscreen: (1) strong absorption in the UV-A and UV-B regions; (2) extended, broad absorption covering as much of the UV range as possible; and (3) fast dissipation of the absorbed energy into heat to recycle the sunscreen for another photon absorption event.
One class of natural suncreens found in plants is the sinapate family of compounds. Below shows resonant two-photon ionization (R2PI) spectra demonstrating the development of the UV absorption as the compound is systematically derivatized to the natural compound, sinapoyl malate . The spectra is found to broaden out with more and more extensive vibronic activity due to the complexity of the conjugated system and flexibility.
J.C. Dean, R. Kusaka, P.S. Walsh, F. Allais, and T.S. Zwier, “Plant Sunscreens in the UV-B: Ultraviolet Spectroscopy of Jet-Cooled Sinapoyl Malate, Sinapic Acid, and Sinapate Ester Derivatives,” J. Am. Chem. Soc., 136, 14780–14795 (2014).
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