This news item was created by students Alena Headd, Heidi Messimer, Brent Myers, Scott Shilke, Steven Shilke, and Juli Smith as part of their Chemistry 210 Semester Project in WS99 under the guidance of Prof. Rainer Glaser.

Glaser's "Chemistry is in the News"
To Accompany Wade Organic Chemistry 4/e.
Chapter 15. Ultraviolet Spectroscopy.


For each of the following questions, please refer to the following article:

POWER PLANTS
By David Tenenbaum, The Why Files, University of Wisconsin

 Editorial

As concern about the Greenhouse Effect increases, scientists have been searching for renewable power sources that do not use carbonaceous resources such as natural gas, coal and oil. The sun's light energy, if harnessed efficiently, would be one of our most abundant renewable energy sources. Photovoltaic cells fulfill this function of providing a clean source of power; however, they are expensive to produce and their maximum efficiency is only 20%. Researchers at Los Alamos National Laboratory (LANL) in New Mexico are hoping to far surpass that level with bio-solar cells that are inexpensive to produce and that mimic natural photosynthesis.

Inside the plant cell, light energy is captured to provide the fixation of CO2 into sugars for respiration. The function of bio-solar cells is to convert light or photon energy into an electric current instead of sugars. They capture a photon of light from the sun in the same way that chlorophyll in plants captures light. Chlorophyll and the pigments used in bio-solar cells are organic molecules arranged in rings and chains of conjugated bonds. They are composed of chains of conjugated alkenes and conjugated systems of cyclic pyrines arranged into porphyrin rings. Depending on the complexity of their conjugated system, these molecules absorb certain wavelengths of light and reflect and fluoresce others. This is why leaves are green.

Instead of the silicon semiconductor used in PV cells, bio-solar cells use multiple dye layers of different colors to harness light from the sun and convert it into usable energy. The dyes are layered to make the film as dark as possible in order to achieve maximum light absorption and to maximize the wavelengths of light that can be absorbed from the sun. The dyes transfer their captured energy through their conjugated systems to the zeolite or TiO2 matrix that holds the dyes. Electrons flow from the matrix, through the circuit (where they do work) and back to the dye molecules to replace electrons that are leaving.

 Questions

  1. What is the summary equation for photosynthesis?
  2. Answer: 6CO2 + 12H2O ------------ > C6H12O6 + 6O2 + 6H2O

  3. In comparison to standard photovoltaic cells, what do the pigments in the bio-solar cells replace?
  4. Answer: Silicon Semiconductor

  5. What does the wavelength of incident light have to do with the operation of bio-solar cells?
  6. Answer: The wavelength of ultraviolet light absorbed depends on the electronic energy differences between orbitals in the conjugated pi bonds of the molecule. The higher the energy difference, the longer the wavelength of light needed to bring an elec tron from the Highest Occupied Molecular Orbital (HOMO) to the Lowest Unoccupied Molecular Orbital (LUMO). Sigma bonds are very stable and respond only to wavelengths of 200 nm or less, while electrons in pi bonds are easily excited into the LUMO or antibonding state. In pi bonds, the wavelength needed depends upon the length of the conjugated system. Ethylene, with only one pi bond, has an energy difference of 164 kcal and requires light of 171 nm for the electron to jump orbitals. Hexatriene, a more complex molecule with three conjugated pi bonds, has an energy difference of just 108 kcal and requires light of 258 nm. In bio-solar cells, the pigments are chosen to absorb a wide range of incident light by selecting compounds with different sizes and complexities of conjugated systems.

  7. Why do you think scientists developed silicon-based photovoltaic cells before the more obvious plant-mimicking bio-solar alternative now being studied?

  8. Answer: The photovoltaic effect was first described in 1839, long before the structure of chlorophyll or porphyrins - and how they interact with light - was understood.

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