Research

Our research encompasses (1) the primary reactions of photosynthesis and (2) tetrapyrroles chromophores and arrays for applications in solar-energy conversion, molecular materials, and photomedicine. The techniques employed include static and laser-based femtosecond to seconds time-resolved absorption and emission spectroscopy.

Primary Processes in the Photosynthetic Reaction Center. The goal of our photosynthesis research is to achieve a molecular-level understanding of the primary charge separation process in the bacterial reaction center. In this pigment-protein complex, light energy is converted into chemical potential energy by a series of fast electron transfers from the photoexcited bacteriochlorophyll special dimer P to BA, HA, QA, and finally QB with a quantum yield of ~1. We have been preparing and studying mutants with altered free energies of the charge separated states that modulate the yields of charge separation versus recombination and give electron transfer fully down the normally inactive B branch.
Figure 1: Wild-Type Reaction Center and Photochemical Events

Multiporphyrin Arrays. We are studying energy and electron transfer reactions in arrays of porphyrins and other chromophores that serve as light-harvesting systems, molecular wires, optoelecronic switches, and other functional elements for applications in solar-energy conversion, molecular photonics, and materials chemistry. Figure 2 illustrates the energy flow in a host-guest complex consisting of a cyclic hexameric array of porphyrins and a central guest tetrapyrrole chromophore. These studies encompassed ultrafast transient absorption spectrocscopy, static and time-resolved fluorescence spectroscopy, and kinetic modelling.

Tetrapyrrole Chromophores. We are investigating the electronic and photophysical properties of tetrapyrrole (porphyrin, chlorin, bacteriochlorin) chromophores for a variety of applications in solar-energy conversion and photomedicine (photodynamic therapy and optical molecular imaging). The various applications require specifically tuned photophysical properties such as absorption and fluorescence wavelengths and intensities, and excited-state lifetimes. Figure 3 illustrates several chlorin molecules and the analysis of the molecular-orbital characteristics that underpins tuning of the photophysical and redox properties. Figure 4 shows absorption spectra of representative porphyrin, chlorin and bacteriochlorin chromophores that utilize different regions of the solar spectrum and have different surface-attachment tethers. Figure 5 compares absorptance spectra of the tetrapyrrole chromophores with the solar spectrum, as part of a study to optimize solar-cell performance characteristics.

Selected Publications

• H. L. Kee, P. D. Laible, J. A. Bautista, D. K. Hanson, D. Holten, and C.Kirmaier, "Determination of the Rate and Yield of B-side Quinone Reduction in Rhodobacter capsulatus Reaction Centers," Biochemistry, 45, 7314 (2006).
• H-E. Song, C. Kirmaier, L. Yu, D. F. Bocian, J. S. Lindsey, and D. Holten, "Effects of Multiple Pathways on Excited-State Energy Flow in Self-Assembled Wheel-and-Spoke Light-Harvesting Architectures," J. Phys. Chem. B, 110, 11931 (2006).
• G. M. Hasselman, D. F. Watson, J. R. Stromberg, D. F. Bocian, D. Holten, J. S. Lindsey, and G. J. Meyer, "Theoretical Solar-to-Electrical Energy-Conversion Efficiencies of Perylene-Porphyrin Light-Harvesting Arrays," J. Phys. Chem. B, 110, 25430 (2006).
• H-E. Song, J. Cissell, T. P. Vaid, and D. Holten, "Photophysics of Reduced Silicon Tetraphenylporphyrin," J. Phys. Chem. B, 111, 2138 (2007).
• W. Akers, F. Lesage, D. Holten, S. Achilefu, "In Vivo Resolution of Multiexponential Decays by Whole-Body Time-Resolved Diffuse Optical Tomography," Molecular Imaging, 6.4, 237 (2007).
• H. L. Kee, C. Kirmaier, Q. Tang, J. R. Diers, C. Muthiah, M. Taniguchi, J. K Laha, M. Ptaszek, J. L. Lindsey, D. F. Bocian, and D. Holten, "Effects of Substituents on Synthetic Analogues of Chlorophylls. Part 2: Redox Properties, Optical Spectra and Electronic Structure," Photochem. Photobiol., 83, 1125-1143 (2007).
• J. R. Stromberg, A. Marton, H. L. Kee, C. Kirmaier, J. R. Diers, C. Muthiah, M. Taniguchi, J. S. Lindsey, D. F. Bocian, G. J. Meyer, and D. Holten, "Examination of Tethered Porphyrin, Chlorin, and Bacteriochlorin Molecules in Mesoporous Metal-Oxide Solar Cells," J. Phys. Chem. C., 111, 15464-15478 (2007).
• C. Muthiah, H. L. Kee, J. R. Diers, D. Fan, M. Ptaszek, D. F. Bocian, D. Holten, and J. S. Lindsey, "Synthesis and Excited-State Photodynamics of a Chlorin–Bacteriochlorin Dyad: Through-Space Versus Through-Bond Energy Transfer In Tetrapyrrole Arrays," Photochem. Photobiol., 84, 786-801 (2008).
• M. Taniguchi, D. L. Cramer, A. D. Bhise, H. L. Kee, D. F. Bocian, D. Holten, and J. S. Lindsey, "Accessing the Near-Infrared Spectral Region with Stable, Synthetic, Wavelength-Tunable Bacteriochlorins," New J. Chem., xx, xxxx (2008).
• J. I. Chuang, S. G. Boxer, D. Holten, and C. Kirmaier, "Temperature Dependence of Electron Transfer to the M-Side Bacteriopheophytin in Rhodobacter capsulatus Reaction Centers," J. Phys. Chem. B., xx, xxxx (2008).