By Belaine Mamo
At 4pm on Tuesday March 4th, the Physics and Astronomy Department welcomed Professor of Chemistry Claudia Avalos where she spoke about her research at New York University in a presentation entitled “Spin Exchange Interactions and Spin Polarization in Pentacene-Radical Dyads” for their weekly colloquium.
After completing her undergraduate degree in Chemistry at California State University Chino, Dr. Avalos went on to earn her PhD at UC Berkeley where she looked to enhance nuclear magnetic resonance (NMR) signals. NMR essentially uses a magnetic field to disrupt nuclei with a different magnetic field creating emission of electromagnetic signals.
To do this, she studied diamond nitrogen-vacancy centers — NVCs are nanoscale magnetic field sensors which can be used to detect NMR signals — and their spins to see how they are used to sense magnetic fields and enhance NMR signals. Investigating spin interactions in diamonds deepens the understanding of how different spins can affect properties of nitrogen-vacancy centers, and, in doing this, better magnetic field sensors can be created. Through optical pumping, the process of using light irradiation to move an intended material into a different electron spin state, nitrogen-vacancy centers can become spin polarized, meaning the spin can be aligned towards a different direction. By transferring this polarization to nuclei outside of the diamond lattice, researchers can boost NMR signals.
What is the reason for polarization of nuclei in the first place? Dr. Avalos noted how NMR signals are in fact quite weak, making it difficult to investigate low concentrations of nuclei (i.e. detect and study their nuclear spins). Thus, it is essential to polarize nuclei to boost signals in order to detect certain materials. With optical polarization in particular, these signals are boosted further and could eliminate the need for microwave irradiation.
Following her work at Berkeley, Dr Avalos completed her postdoctoral studies at the Lausanne École Polytechnique in Switzerland, where she used another method, called transient electron paramagnetic resonance (EPR), to transfer spin polarizations. Transient EPR is one type of continuous wave EPR where a constant frequency is applied to a material, and, as the magnetic field varies, the electron spin state changes. As the magnetic field strengthens, the electrons resonate with the microwave frequency, and some of this microwave energy is absorbed and creates a signal. This (along with other types of EPR) give insight into the interaction between the electron’s spin and surrounding spins.
Dr. Avalos’ specific study at Lausanne utilized transient EPR to create non-Boltzmann polarization (simply, not using temperature to polarize) on electron spins and to transfer this polarization to nuclei using microwaves. Compared to non-Boltzmann processes, she explains that Boltzmann polarization methods provide minimal boosts to NMR signals.
Now a professor at NYU, Dr. Avalos currently researches optical and magnetic resonance spectroscopies, which are methods of studying the structure and function of materials using light, to understand spin dynamics of materials. She looks to understand the relationships, structure and function of photoactive (responds to light) materials, like organic chromophores. More specifically, Dr. Avalos studies chromophore radical systems, which are molecules that absorb light and reflect back color that are connected to stable radicals (molecules with unpaired electrons that have comparatively long lives). She notes that organic systems have longer coherence times and can display ground state polarization at room temperature, which is helpful — we want to transfer radiation without microwaves.
Dr. Avalos is interested in how spin interactions can disrupt molecular systems’ optical properties. In the case of organic chromophore-radicals, in influencing spin (both their orbit and exchange) interactions, it can be possible to generate a large electron spin polarization in ground state! This can be transferred to nuclei with microwave irradiation, thereby boosting NMR signals.
Dr. Avalos explained how, in her lab, she takes a particle she wants to polarize (essentially, to isolate the spins she wants to change/study), and then introduces stable radicals. By designing these experiments, Dr. Avalos hopes to figure out how to design molecules that can be optically pumped into ground state.
Overall, the Avalos lab looks to understand deeper how spin interactions have influence on molecular systems by looking at materials including chromophore-radicals, and organic semiconductor materials, which also contain radicals. Dr. Avalos notes that “[it would be] even more exciting if we could come up with [a method] that could be applied towards boosting NMR signals in solution”. Though this particular process is a bit more complicated, it has promising applications to pharmaceutical research. With boosts to NMR signals, NMR analysis could be conducted with lower concentrations, saving pharmaceutical companies money and resources.
On the other hand, by studying spin interactions in semiconductors, researchers can discover which defects enhance properties we desire, and lessen those we don’t. This can be instrumental in designing new types of doped materials, which are just materials with intentional impurities to induce specific functions in the semiconductor. This can be particularly helpful in the cases of organic light emitting diodes (OLEDs, a kind of LED!) and solar material functions.
After the talk, she welcomed speaking with students and attendees of the event.
Keep an eye out for a Physics & Astronomy colloquium every Tuesday afternoon!