A new organic pigment—touted by researchers as unusual—can switch on by an electrical charge, allowing it to absorb light in the near-infrared (near-IR). This shows promise for traditional photovoltaics as well as sensors. In photosynthesis and organic photovoltaics, pigment molecules convert light into electrical charge, according to the study.1
In their study—led by a team from the University of North Texas (Denton, TX), working with researchers from the Indian Institute of Technology Indore (IIT Indore; India)—the researchers observed the intervalence charge transfer (IVCT), “a phenomenon … in molecular systems comprised of two redox centers differing in oxidation states by one unit.”
Such pigments with intense colors are typically metal-based—for instance, an iron-based Berlin blue or Prussian blue. “In terms of their chemistry, dyes and pigments of this kind are symmetrical molecules, with one side having a higher charge than the other. The sides exchange electrons, and the molecule absorbs light that is at the same wavelength as this energy exchange.” Traditionally, purely organic pigments with such intense colors are rare.
Customizable molecules
The researchers have developed a modular organic molecular system. Organic materials, such as those used in the study, can be easy to modify; traditional metal-based systems cannot. In the study, the researchers chose to target modular construction of dye molecules, which develops customizable molecules that could potentially offer various, different properties.
The new technology’s core was made from a red fluorescent dye molecule. From there, the researchers added a two-part push-pull (or donor acceptor) to the systems on both sides of the core. This stabilized electrical charges under specific conditions. The researchers say that in an uncharged state, “the pigment was just a blue-colored dye molecule. But when an electronic charge was applied, it demonstrated its full capabilities.”
A new, intense absorption band was observed in the study; however, not in the visible-light range. Their new dye absorbed in the near-IR range. Absorption became possible when the two donor-acceptor units resonated with each other.
“The additional, free electron shuffles between the two chemically equivalent entities [reveals] a new charge-transfer peak in the near-IR region,” the researchers say, noting that the molecule had become a mixed valence compound, with similar properties to metal-based dye compounds.
These electronically switchable organic pigments have the potential to help those in the industry to better understand electron transfer, as found in photosynthesis. Additionally, the new technology could be used as an efficient electron-transporting material in photonic devices, as well as for markers used for analyzing electron transitions.
According to the researchers, “the additional, free electron shuffles between the two chemically equivalent entities [reveals] a new charge-transfer peak in the near-IR region.” They added that “the molecule had become a mixed valence compound, with similar properties to metal-based dye compounds.”
REFERENCE
1. F. Khan, Y. Jang, Y. Patil, R. Misra, and F. D’Souza, Angew. Chem., 60, 37, 20518-20527 (Sep. 6, 2021); doi:10.1002/anie.202108293.
