The researchers, led by Shinsuke Takagi from the Tokyo Metropolitan
University and PRESTO of the Japan Science and Technology Agency, have
published their study on their work toward an artificial LHS in a recent
issue of the Journal of the American Chemical Society.
By arranging porphyrin dye molecules on a clay surface using the
“Size-Matching Effect,” researchers have demonstrated an energy transfer
efficiency of approximately 100%, which is an important requirement for
designing efficient artificial light-harvesting systems.
“In order to realize an artificial light-harvesting system, almost 100% efficiency is necessary,” Takagi told PhysOrg.com. “Since light-harvesting systems consist of many steps of energy transfer,
the total energy transfer efficiency becomes low if the energy transfer
efficiency of each step is 90%. For example, if there are five energy
transfer steps, the total energy transfer is 0.9 x 0.9 x 0.9 x 0.9 x 0.9
= 0.59. In this way, an efficient energy transfer reaction plays an
important role in realizing efficient sunlight collection for an
artificial light-harvesting system.”
As the researchers explain in their study, a natural LHS (like those in purple bacteria
or plant leaves) is composed of regularly arranged molecules that
efficiently collect sunlight and carry the excitation energy to the
system’s reaction center. An artificial LHS (or “artificial leaf”)
attempts to do the same thing by using functional dye molecules.
Building on the results of previous research, the scientists chose to
use two types of porphyrin dye molecules for this purpose, which they
arranged on a clay surface. The molecules’ tendency to aggregate or
segregate on the clay surface made it challenging for the researchers to
arrange the molecules in a regular pattern like their natural
counterparts.
“A molecular arrangement with an appropriate intermolecular distance
is important to achieve nearly 100% energy transfer efficiency,” Takagi
said. “If the intermolecular distance is too near, other reactions such
as electron transfer and/or photochemical reactions would occur. If the
intermolecular distance is too far, deactivation of excited dye
surpasses the energy transfer reaction.”
In order to achieve the appropriate intermolecular distance, the
scientists developed a novel preparation technique based on matching the
distances between the charged sites in the porphyrin molecules and the
distances between negatively charged (anionic) sites on the clay
surface. This effect, which the researchers call the “Size-Matching
Rule,” helped to suppress the major factors that contributed to the
porphyrin molecules’ tendency to aggregate or segregate, and fixed the
molecules in an appropriate uniform intermolecular distance. As Takagi
explained, this strategy is significantly different than other attempts
at achieving molecular patterns.
“The methodology is unique,” he said. “In the case of usual
self-assembly systems, the arrangement is realized by guest-guest
interactions. In our system, host-guest interactions play a crucial role
for realizing the special arrangement of dyes. Thus, by changing the
host material, it is possible to control the molecular arrangement of
dyes on the clay surface.”
As the researchers demonstrated, the regular arrangement of the molecules leads to an excited energy transfer efficiency of up to 100%. The results indicate that porphyrin dye molecules and clay host materials look like promising candidates for an artificial LHS.
“At the present, our system includes only two dyes,” Takagi said. “As
the next step, the combination of several dyes to adsorb all sunlight
is necessary. One of the characteristic points of our system is that it
is easy to use several dyes at once. Thus, our system is a promising
candidate for a real light-harvesting system that can use all sunlight.
We believe that even photochemical reaction parts can be combined on
the same clay surface. If this system is realized and is combined with a
photochemical reaction center, this system can be called an ‘inorganic
leaf.’”
By Lisa Zyga
From physorg
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