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 aromaticityand any associated optical absorptionssimply 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 compounds electron count. This creates a dye with tunable optical absorption of near-infrared lighta type of radiation critical to applications involving organic solar cells and photodynamic cancer therapies.
Conjugated molecules 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 hemiporphyrazinea new aromatic species that quantum calculations predict would have similar or stronger near-infrared absorption bands.
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More information: Muranaka, A., et al. /π hemiporphyrazine: a redox switchable near-infrared dye. Journal of the American Chemical Society 134, 190193 (2012).