Designing a dye you can count on

May 24, 2012
Figure 1: Adding hydroxyl atoms (OH) to a conjugated dye called hemiporphyrazine (bottom structures) enables a redox switching reaction between aromatic (left) and non-aromatic (right) states, setting the stage for ‘on-demand’ absorption of near-infrared light. Credit: 2012 Atsuya Muranaka, RIKEN Advanced Science Institute

Natural substances such as chlorophyll and the heme pigment of red blood cells contain colorful molecules known as porphyrins. They owe their exceptional visual characteristics to a ‘macrocyclic’ chemical structure that links several small rings together into a highly conjugated, aromatic framework. However, chemists who have synthesized porphyrin derivatives have sometimes found that this aromaticity—and any associated optical absorptions—simply disappears.

Now, a research team led by Atsuya Muranaka and Masanobu Uchiyama at the RIKEN Advanced Science Institute, Wako, reports a new way to manipulate the peculiar aromatic properties of macrocycles. The team has found that the aromaticity of a porphyrin-type molecule called hemiporphyrazine can be switched on and off by altering the compound’s electron count. This creates a dye with tunable optical absorption of near-infrared light—a type of radiation critical to applications involving organic solar cells and photodynamic cancer therapies.

Conjugated exhibit aromatic properties only when their number of so-called ‘pi’ electrons is a multiple of the formula 4n+2, where n is an integer. For example, porphyrin rings with 18 pi-electrons are stable and can share electrons aromatically, making them responsive to light. But a porphyrin with 20 pi-electrons readily gives up two electrons and returns to the favored aromatic state.

Hemiporphyrazines, however, are an unusual kind of macrocycle: their particular combination of carbon and nitrogen double bonds produces a non-aromatic structure that is thermally stable with 20 pi-electrons. Despite the promising material characteristics of these porphyrin analogues, their non-aromatic nature currently limits their usefulness. “From a theoretical point of view, it seems easy to take hemiporphyrazines down to 18 pi-electrons,” notes Muranaka. “But so far, no one has succeeded in doing this experimentally.”

The researchers solved this problem by putting four hydroxyl (OH) atoms into hemiporphyrazine to facilitate a redox reaction (Fig. 1). Mixing this compound with a strong oxidizing reagent caused two OH units to lose an electron and turn into double-bonded oxygen atoms, transforming hemiporphyrazine into an aromatic 18 pi-electron system. Consequently, the dye displayed intense near-infrared optical absorption peaks where none existed before. 

The team reverted its hemiporphyrazine to 20 pi-electrons by mixing it with a reducing agent. This reversible system is sure to interest developers of ‘on-demand’ opto-electronic materials. Muranaka says that the next step is to prepare a 22 pi-electron hemiporphyrazine—a new aromatic species that quantum calculations predict would have similar or stronger near-infrared absorption bands.

Explore further: New CMI process recycles magnets from factory floor

More information: Muranaka, A., et al. [18]/[20]π hemiporphyrazine: a redox switchable near-infrared dye. Journal of the American Chemical Society 134, 190–193 (2012).

Related Stories

Wired up and ready to glow

Dec 10, 2010

Thirty years ago, no one believed that elements other than carbon, nitrogen, and oxygen could form double bonds at room temperature. But the discovery of 'kinetic protection' ligands -- large, bulky molecules ...

A diamond ring sparks a paradigm shift

Jun 06, 2011

The sweet smell of benzene gave birth to the term ‘aromatic’ molecules, but it is the chemical bonds within these compounds that have fascinated researchers for almost 200 years. Encasing alternating ...

Economizing chemistry, atom by atom

Feb 03, 2012

In chemistry, downsizing can have positive attributes. Reducing the number of steps and reagents in synthetic reactions, for example, enables chemists to boost their productivity while reducing their environmental ...

Detecting an unexpected delay at ultrafast speed

Aug 05, 2011

Molecules that suddenly transform into new structures when stimulated by photons or electrons play key roles in many chemical and biological processes. Recently, chemists have discovered that adding transition ...

Recommended for you

New CMI process recycles magnets from factory floor

3 hours ago

A new recycling method developed by scientists at the Critical Materials Institute, a U.S. Department of Energy Innovation Hub led by the Ames Laboratory, recovers valuable rare-earth magnetic material from ...

Chemists characterize 3-D macroporous hydrogels

7 hours ago

Carnegie Mellon University chemists have developed two novel methods to characterize 3-dimensional macroporous hydrogels—materials that hold great promise for developing "smart" responsive materials that ...

Substrates change nanoparticle reactivity

13 hours ago

(Phys.org)—Nanoscale materials tend to behave differently than their bulk counterparts. While there are many theories as to why this happens, technological advances in scanning tunneling microscopy (STM) ...

Reviving cottonseed meals adhesives potential

15 hours ago

Cottonseed meal—the leftovers after lint and oil are extracted from cottonseed—is typically fed to ruminant livestock, such as cows, or used as fertilizer. But Agricultural Research Service scientists ...

New concrete composite can heal itself

15 hours ago

In the human body, small wounds are easily treated by the body itself, requiring no further care. For bigger wounds to be healed, the body may need outside assistance. Concrete is like a living body, in that ...

Actuators that mimic ice plants

16 hours ago

Engineers developing moveable robot components may soon take advantage of a trick plants use. Researchers at the Max Planck Institute of Colloids and Interfaces in Potsdam and Harvard University in Cambridge ...

User comments : 0

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.