We were delighted to catch-up with Professor Pi Tai Chou, Chair Professor of the Chemistry Department, and Director of the Center for Emerging Material and Advanced Devices, at National Taiwan University. Professor Chou gained his Ph.D. in Chemistry and Biochemistry from Florida State University, and was a postdoctoral fellow at the University of California at Berkeley. He is also fellow of the Royal Society of Chemistry (FRSC) and the recipient of a number of awards, including Taiwan Outstanding Research Awards, Academic Achievement Awards, National Chair Awards, International Asian and Oceanian Photochemistry Association (APA) Award (Japan) and The World Academy Sciences (TWAS) Chemistry Prize.
In this article, Professor Chou talks about his research on anomalous photophysical and photochemical phenomena.
What kind of research are you undertaking?
My current research interest is to gain a fundamental understanding of anomalous photophysical and photochemical phenomena. This could lead to new or modified theories being developed, or an obscure theory currently in limbo being revitalized. In addition research along this leading frontier can lead to new emerging materials being strategically designed for optoelectronic or bio-medical applications.
There are three major research directions that I am currently undertaking. The first is the excited-state intramolecular proton transfer reaction (ESIPT), in which the correlation among hydrogen-bonding (H-bonding) strength, proton transfer dynamics and the thermodynamics is investigated. We apply steady-state and time-resolved absorption spectroscopy from t=∞ to 100 fs to resolve the relaxation dynamics and hence the corresponding spectral temporal evolution. These results enable the establishment of empirical relationships between the H-bonding strength, and the kinetics and thermodynamics of ESIPT. These relationships provide rational design principles of proton-transfer molecules in lighting application, covering panchromatic emission from visible to near infrared region.
Another research area is to probe the structure-emission relationship. The emerging materials possessing thermally activated delayed fluorescence (TADF), involve the spatially separated electron donor (D) and acceptor (A), for which their relative spatial localization becomes important. Using Fourier transformed transient absorption spectroscopy in UV-Vis and IR, we would be able to structurally resolve the D/A type charge transfer state, so that the mutual location and the interaction can be probed. This technique can also be applied to probe the excited-state structural deformation, such as photo-induced butterfly type motion and ring-opening reaction, etc.
We also pay close attention to the excited-state relaxation dynamics of late transition metal complexes that play a pivotal role in phosphorescence organic light emitting diodes (POLEDs). Our in-depth investigation at the molecular level has established important relationships amongst relaxation dynamics, phosphorescence yield, emission hue, and molecular framework. The external versus internal heavy atom effect, the role of metal to ligand charge transfer in enhancing intersystem crossing, and harnessing d-d versus metal-to-ligand transition to suppress radiationless deactivation and fine-tuning spin-orbit coupling have been formulated.
What is innovative about your research?
I am originally trained as a spectroscopist, focusing on the spectroscopy and dynamics of UV-Vis-NIR absorption and emission. Strictly, I can be classified as a physical chemist. However, I have also extensive experience in organic chemistry. I can therefore take advantage of chemical design, which the physical chemist cannot normally access. For example, multiple proton transfer is key to mimic the proton relay inside the water tunnel of biological systems. However, without ingenious design of molecules capable of undergoing multiple proton transfer, the associated dynamics and mechanism cannot be resolved. The combination of chemical design, synthesis and state-of-the-art photophysical techniques has led to several breakthroughs during my research career. These include the breakdown of Kasha’s rule to harvest highly excited energy, by-passing the energy gap law based on full J-aggregate alignment to achieve champion emission yield in the near IR and white light generation in a single type molecule.
How have our instruments helped your research?
As a matter of fact, to a well experienced spectroscopist, steady state fluorescence and the corresponding excitation spectra provides profound and valuable information for both ground and excited states properties. Accordingly, Edinburgh Instruments fluorometers, which I have been using for more than 15 years, help me considerably. Their excellent calibration in both excitation and emission regions in terms of wavelength provide very reliable data. This, together with various accessories such as time correlated single photon counting technique, phosphorescence lifetimes, temperature control, etc., gives full power for the spectroscopies and dynamics. There are a number of fluorimeters that I have used through my 35 year research career and amongst these, I would rate Edinburgh Instruments fluorometers as the most robust. Our recent upgrades for the absolute emission quantum yield measurement using an integrating sphere helps us a lot in the solid-state research.
With over 500 scientific papers published, 15 review articles, 6 book chapters and being named in 18 patents or disclosures, we are proud to have such a good relationship with Professor Chou, and look forward to more brilliant work in the future.
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