howe-siang tan research group

Research

Many chemical reactions and dynamics happen in the femtosecond to picosecond time regime. The only way to directly study systems at such short timescale is through ultrafast laser spectroscopy. Our research group’s interests are in the following area of ultrafast laser spectroscopy:

• Technique Development of Multidimensional Optical Spectroscopy
• Ultrafast Energy Transfer Pathways in Photosynthetic Light Harvesting Complexes

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Ultrafast Energy Transfer Pathways in Photosynthetic Light Harvesting Complexes

The ultimate energy source of plant and animal kingdoms, and the world’s carbon economy is based on the photosynthetic processes in higher plant. In plant photosynthesis, light from the sun is collected by light-harvesting arrays in plant cells and channelled to reaction centres where solar energy is converted to chemical energy to be stored and transported. Light harvesting array complexes are made up of numerous coupled chlorophyll molecules. These chlorophyll molecules get photoexcited and the excitation energy is then transferred onwards to the reaction centre. This ‘funnelling’ process is highly complex, efficient and ultrafast . For example the LHCII complex, ubiquitous in higher plants, consist of 14 chlorophyll a and b molecules and the energy transfer process happens between these molecules in a matter of femto- to picoseconds.

 

2D Spectra of the Excitation Energy Transfer (EET) Process of Plant LHCII at Room Temperature	Structure of LHCII Light Harvesting Complex in higher plants

Using 2D and 3D spectroscopy, we measure the ultrafast excitation energy transfer pathways. In the figure above, room temperature 2D spectra of Plant LHCII are presented. The Chl b and Chl a regions are excited and its subsequent energy transfer is monitored as a function of waiting time Tw.

This work is done in collaboration with Győző Garab and Petar Lambrev from the Biological Research Centre, Hungarian Academy of Sciences.

References

1. P.H. Lambrev, P. Akhtar, and H.-S. Tan, "Insights into the mechanisms and dynamics of energy transfer in plant light-harvesting complexes from two-dimensional electronic spectroscopy". Biochim. Biophys. Acta, Bioenerg. 1861 (4), 148050 (2020). https://doi.org/10.1016/j.bbabio.2019.07.005
2. T.N. Do, A. Huerta-Viga, P. Akhtar, H.L. Nguyen, P.J. Nowakowski, M. Faisal Khyasudeen, P.H. Lambrev, and H.-S. Tan, "Revealing the Excitation Energy Transfer Network of Light Harvesting Complex II by Phenomenological Analysis of Two-Dimensional Electronic Spectra", J. Chem. Phys. 151, 205101 (2019). https://doi.org/10.1063/1.5125744
3. P. Akhtar, C. Zhang, T.N. Do, G. Garab, P.H. Lambrev, and H.-S. Tan. "Two-Dimensional Spectroscopy of Chlorophyll a Excited-State Equilibration in Light-Harvesting Complex II", J. Phys. Chem. Lett. 8, 257-263 (2017). https://doi.org/10.1021/acs.jpclett.6b02615
4. Z. Zhang, P.H. Lambrev, K.L. Wells, G. Garab, H.-S. Tan. "Direct observation of multistep energy transfer in LHCII with fifth-order 3D electronic spectroscopy", Nat. Comm. 6, 7194 (2015). pdf. https://doi.org/10.1038/ncomms8914
5. K.L. Wells, P.H. Lambrev, Z. Zhang, G. Garab and H.-S. Tan. "Pathways of energy transfer in LHCII revealed by room-temperature 2D electronic spectroscopy" Phys. Chem. Chem. Phys. 16, 11640-11646 (2014). pdf. https://doi.org/10.1039/C4CP00876F

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Coherent Multidimensional Optical Spectroscopy: Technique Development

Multidimensional optical spectroscopy is the optical analogue of multidimensional nuclear magnetic resonance (NMR) spectroscopy such as COSY and NOESY. The femtosecond time resolution of ultrafast laser spectroscopy allows experimentalists to look at structural fluctuations of molecular systems and other processes at a much higher time resolution than NMR spectroscopy.
In our lab, we use a pulse shaper assisted pump probe beam geometry setup to perform multidimensional spectroscopy. Phase coherent ultrafast laser pulses produced using optical pulse shaping techniques are used in such experiments. One key component in this technique is the concept of phase cycling. We develop the theory and the selection procedure of phase-cycling schemes for such phase coherent multidimensional optical spectroscopy.

2D Electronic Spectroscopy

Schematic of a 3rd order 2D optical spectroscopy experiment

 

Experimental schematic of a pulse shaper assisted pump-probe 2D spectrometer

 

Using pulse shaper assisted 2D Electronic Spectroscopy in a pump probe beam geometry setup, together with phase cycling, we measure the 2D spectra of Chlorophyll a molecular system.

(a) 2D spectrum with the rephasing and nonrephasing signals retrieved from the 1 X 2 phase-cycling scheme. The peak at the top is the nonrephasing signal while the peak at the bottom is the rephasing signal. (b) 2D spectrum with the rephasing signal retrieved from a 1 X 3 phase-cycling scheme. (c) 2D spectrum with the nonrephasing signal retrieved from a 1 X 3 phase-cycling scheme. (d) Sum of the nonrephasing and the mirror image of the rephasing 2D spectra retrieved from a 1 X 3 phase-cycling scheme giving a pure absorptive peakshape.

3D Electronic Spectroscopy

5th order 3D coherent spectroscopy can be summarized using the following diagram:

Schematic of a 5th order 3D optical spectroscopy experiment

3D spectroscopy in turn provides us with more information on the behavior of the molecules such as recovering three point frequency fluctuation correlation functions (3FFCFs) , measuring high lying excited states, and uncovering hidden couplings.
With some modifications to the data acquisition and pulse shaping programming software, a 2D electronic spectrometer can be used to perform 3D electronic spectroscopy.

Experimental schematic of a pulse shaper assisted pump-probe 2D spectrometer.

Like in the 2D spectroscopy described above, phase cycling is necessary to obtain the desirable 3D spectrum. A 2x2x2x1 phase cycling scheme is used to obtain the purely absorptive 3D spectra of chlorophyll a