Ksenija D. Glusac
Our current demand for energy is large. Even though we are learning how to use available energy more efficiently, the worldwide consumption is expected to rise due to an increase in the human population and the growth of economy. The main energy sources used nowadays, fossil fuels, are not a good solution to our demands for two main reasons: (i) the fuel reserves are expected to dry out in several hundreds of years and (ii) the build-up of atmospheric carbon-dioxide produced from fuel causes global warming and represents a serious environmental problem. Thus, finding alternative energy sources is one of the main challenges facing humanity. One of the most promising energy sources is light from the Sun. The amount of solar radiation that reaches the Earth's surface in only one hour gives enough power to satisfy our energy demands for a whole year. The simplest way to use this energy is to convert it directly into current using photovoltaic devices. However, storage of electricity using batteries is costly. This problem can be avoided if we conducted photocatalytic water splitting and stored the energy in the form of hydrogen and oxygen molecules. The electricity from hydrogen and oxygen can then be obtained using existing fuel cells.
Our group studies flavin-based water oxidation catalysts. For example, the catalyst we are currently investigating consists of ethyl-flavinium perchlorate presented in the scheme above. Our research efforts generally consist of synthetic procedure to make the model compounds. We further investigate the excited state behavior of model compounds using femtosecond optical and mid-IR transient absorption spectroscopy.
Synthesis: Hannah, Renat Vincent and Lorne are our experts for organic synthesis methods. The are currently studying various methods for preparation of ethyl-flavinium salt. One of the procedures involves the following steps:
Spectroscopy: Pavel, James, Dapeng and Katia are our experts for spectroscopic methods. They are currently studying excited state properties of flavin-derivatives using steady-state and time-resolved visible and mid-IR spectroscopies.
Time-Resolved Spectroscopy: Our group is using femtosecond pump-probe spectroscopy in the visible and mid-IR range to study kinetics of catalytic cycle. The scheme below presents the layout of the instrument. It consists of an amplified Ti:Sapphire oscillator that produces 800 nm laser pulses with a width of 100 fs. To select the desired frequencies of pump and probe beams, we use two TOPAS-es. The detection in the visible range is achieved using a CCD camera and in the mid-IR range using 2 x 32 array of MCT detectors.
We recently obtained the transient absorption spectra of our flavinium catalyst in the visible and mid-IR range. The data are presented in the figure below. The visible transient absorption spectrum consists of a positive signal arising from the singlet excited state of flavinium salt and negative signals arising due to ground-state bleach (~520 nm) and stimulated emission (above 700 nm). The lifetime of the flavinium excited state is ~ 500 ps. The TRIR spectra show bleach signals due to C=N and C=O vibrations in the ground state. The positive signals arise due to C=N and C=O vibration of the 1S state of flavinium salt.
R. Khatmullin, D. Zhou, T. Corrigan, E. Mirzakulova, K. D. Glusac, "Thermolysis and Photolysis of 2-ethyl-4-nitro-1(2H)-isoquinolinium hydroperoxide", J. Phys. Org. Chem, 2013, 26, 440-450.
R. Liu, A. Azenkeng, D. Zhou, Y. Li, K. D. Glusac, W. Sun "Tuning Photophysical Properties and Improving Nonlinear Absorption of Pt(II) Diimine Complexes with Extended π-Conjugation in the Acetylide Ligands", J. Phys. Chem. A, 2013, 117, 1907.
R. Liu, D. Zhou, A. Azenkeng, Z. Li, Y. Li, K. D. Glusac, W. Sun "Nonlinear Absorbing Platinum(II) Diimine Complexes: Synthesis, Photophysics and Reverse Saturable Absorption", Chem. Eur. J, 2012, 18, 11440.
D. Zhou, R. Khatmullin, J. Walpitha, N. A. Miller, H. L. Luk, S. Vyas, C. M. Hadad, K. D. Glusac, "The Fast Excited-State Heterolytic C–OH Bond Cleavage of 9-hydroxy-10-methyl-9-phenyl-9,10-dihydroacridine: A Photoemitter of Hydroxide Anions", J. Am. Chem. Soc, 2012, 134, 1130.
E. Mirzakulova, R. Khatmullin, J. Walpita, T. Corrigan, N. M. Vargas-Barbosa, S. Vyas, S. Oottikkal, S. Manzer, C. M. Hadad, K. D. Glusac, "Catalytic Water Oxidation by N(5)-Ethyl-Flavinium Ions", 2012, Nature Chemistry, DOI: 10.1038/NCHEM.1439