Redox Chemistry in Molecular Electronics and Photosynthesis


In an earlier Advanced Application, we discussed the synthesis of nanometer-sized electronic components for use in nanocircuits. Of course, the ultimate lower size limit for electronic devices are those composed of single molecules. This field of work is called molecular electronics. The long-term goal for researchers in this field is to develop single molecules that can work like computers. Computers like the one you are using to read this tutorial are built from a series of individual components (wires, switches, logic and memory elements, and input/output components). Molecular electronic devices also require components with functions analogous to those in conventional computers.

Professor Dewey Holten and his collaborators, use ultrafast laser spectroscopy to study the way energy flows through molecular electronic devices and strategies for controlling that energy flow and improving its efficiency. The molecular device shown below consists of an "output" connected to an "input" by a "wire", and moreover it has an "on/off switch". When the switch is "on" energy that is received as "input" can be sent to the "output", but when the switch is "off" energy received at the "input" is not sent to the "output". The oxidation state of the zinc porphyrin (the portion of the molecule contained in the green box) determines whether the switch is "on" or "off". When the zinc porphyrin "switch" is in its neutral, ground state (the "on" state) light energy that is absorbed at the input (in purple) is transferred via the molecular "wire" (in orange) to the part of the output (in red) as evidenced by the emission of light from the output (this light emission process is called fluorescence).

Figure 1. A molecular electronic device called an optoelectronic switch. Energy is allowed to flow from the "input" to the "output" through the "molecular wire" when the "switch" is turned "on". The "on/off" state of the switch depends on the oxidation state of the molecular components in the switch.
[From Chem. Mater. 13: 1023-1034 (2001).]
So how do you turn the switch "off"?

The answer is by using light to adjust the oxidation state of the zinc porphyrin group. Turning the switch "off" requires two steps shown below.



In the first step, the Ru2+bpy3 must absorb a photon of light with wavelength 500 nm, which causes the oxidation of the Ruthenium 2+ ion. The Ru2+ ion transfers an electron to the bpy3 groups. This process of oxidizing the ruthenium, to form Ru3+ with light, is called photo-oxidation. Once the ruthenium has been oxidized, the absorption of a photon of light with wavelength 550 nm by the Zn-porphyrin part of the molecule causes the oxidation of the Zn-porphyrin. The excited Zn-porphyrin transfers an electron to the Ru3+. In this state ((Zn-porphyrin)+-Ru2+bpy3-) the switch is turned "off" and energy is not allowed to flow from the input to the output. Eventually, the switch will "relax" back to its "on" or ground state ((Zn-porphyrin)-Ru2+bpy3). So if you want to maintain the "switch" in the "off" state it is necessary to continually supply radiation with 500 nm and 550 nm wavelengths.

Photosynthesis Professor Holten's group also studies dynamics of the early events in photosynthesis. A protein called chlorophyll (which occurs in plants and in some photosynthetic bacteria) contains a molecular group similar to the Zinc porphyrin used as a switch in the molecule above. Absorption of light energy by the magnesium porphyrin in chlorophyll also causes photoxidation (as in the case of the molecular switch discussed above) and a series of complex electron transfer steps that we will not discuss here. Many of these electron transfer steps occur on the picosecond (10-12 s) timescale. Professor Holten's laser spectroscopy laboratory is ideally equipped to study these very fast events. The energy harnessed in this process is used to promote the redox process that is ultimately the source of all the food that we eat. In this redox reaction, the carbon in CO2 is reduced to make carbohydrates, which have the empirical formula CH2O, while oxygen results from the oxidation half-reaction. The overall process is:



Can you write the two half-reactions for this process?
Answer


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