Redox Chemistry in Molecular Electronics
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.
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).
So how do you turn the switch "off"?
|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).]
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?
Return to the Redox Reactions Module
Return to the Advanced Applications Page