A new family of molecules, termed ‘azuleneocyanines’, that can absorb large amounts of near-infrared light—a critical part of the electromagnetic spectrum—has been synthesized by Atsuya Muranaka, Mitsuhiro Yonehara and Masanobu Uchiyama from the RIKEN Advanced Science Institute in Wako1. The work has the potential to advance medical imaging and photodynamic cancer treatments because near-infrared light can penetrate deep into human tissue with little loss of intensity.
The key to the team’s approach is a group of large, cyclic organic molecules known as porphyrins. The numerous carbon- and nitrogen-based double bonds found within these molecules make them extremely sensitive to light radiation and therefore intensely colored. Beginning in the early 20th century, chemists began to alter porphyrin structures to create the class of pigments called phthalocyanines (Fig. 1), which have emerged as important dyes owing to their stability under intense heat and light conditions.
Although many phthalocyanines can absorb near-infrared light with wavelengths between 700 and 800 nanometers, prospective medical applications require dye compounds with enhanced activity in the 700–1100 nanometer region—the so-called ‘optical window’. To meet this challenge, Muranaka and colleagues synthesized a new dye that incorporates azulene—an aromatic molecule containing fused five- and seven-membered hydrocarbon rings—into the phthalocyanine framework. Azulene’s unusual structure gives it unique electron-accepting characteristics that the researchers suspected would lead to an improved dye material.
In their synthesis, the researchers first added cyanide groups to the seven-membered ring of azulene, and then attached two butyl chains to its pentagonal component to improve the product’s solubility. Finally, they linked four modified azulene units together to form the cyclic azuleneocyanine complex—a troublesome process, according to Muranaka, because several hard-to-distinguish structural isomers were produced during the cyclization reaction.
The effort required to produce azuleneocyanine paid off when the researchers observed this compound could absorb intense amounts of light in the optical window region—behavior distinct from other phthalocyanines and the azulene precursor. Theoretical calculations revealed that the seven-membered ring of azulene lowered the energy barrier for electron absorption in the complex, leading to the unprecedented near-infrared activity.
While the high stability and strong absorption capabilities of azuleneocyanine promise to be a boon for near-infrared applications, the researchers also take great pride in the discovery and christening of this molecular family. “Chemists have great enthusiasm for naming molecules,” says Muranaka, “and it’s really exciting for us to name a new class of compounds that we created.”
The corresponding author for this highlight is based at the Advanced Elements Chemistry Research Team, RIKEN Advanced Science Institute
1. Muranaka, A., Yonehara, M. & Uchiyama, M. Azuleneocyanine: A new family of phtalocyanines with intense near-IR absorption. Journal of the American Chemical Society 132, 7844–7845 (2010).
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