Hiromichi Kataura

Encapsulation of Carotenoids in Single-wall Carbon Nanotubes and their Optical Properties

In order to develop next generation technologies such as all-optical switching devices, data processing using an optical-neural network, etc., it is necessary to develop materials which have the following three features; (1) large third-order optical nonlinearity, (2) ultrafast optical response, and (3) sufficient robustness for device applications. π-conjugated molecules have the potential to fulfill all the three requirement as they satisfy the first and second requirements. However, π-conjugated polyene molecules are not stable enough for use as nonlinear optical device materials. They easily degrade under ambient conditions when reacting with radical species (e.g., singlet oxygen), and also trans-to-cis isomerization occurs under illumination of light or as a result of heat treatment. Encapsulation of organic molecules inside single-wall carbon nanotubes (SWCNTs) has recently been reported. Air stable n-type SWCNT was produced by encapsulating tetrathiafulvalene (TTF) or tetramethyltetraselenafulvalene (TMTSF) inside the tubes. Takenobu et al. proposed that organic molecules inside the tubes should be protected from any external reactive species by the surrounding tube wall. Therefore we expected the degradation problem of π-conjugated molecules might be overcome by encapsulating them inside carbon nanotubes. Carotenoids are very important natural pigments in plants and animals, and they have been extensively used as model systems for the study of π-conjugated polyene molecules. Figure 1a shows the chemical structure of βcarotene. We recently succeeded to encapsulate βcarotene in SWCNT (Figure 1b shows a schematic illustration of β-carotene inside SWCNT). The physical properties and filling rate of β-carotene inside SWCNT were evaluated from the results of Raman and optical absorption measurements. (Figure 2 shows the Raman spectra of SWCNTs before and after the encapsulation procedure of β-carotene.) We found that the stability of βcarotene to the UV light is highly improved by encapsulation in SWCNT. The presence of a surrounding tube wall would protect β-carotene from the attack of radical species existing outside the wall, and isomerization would be prohibited since there would not be enough space for β-carotene to change its conformation inside the nanotube. It is reported that the encapsulation of polyacetylene will change the density of state of SWCNT. Therefore we expect that the encapsulation of polyene molecules inside SWCNTs can change physical properties of SWCNT as well as those of encapsulated molecules. Therefore the encapsulation technique presented here will contribute to various applications of SWCNTs since it can be used not only to stabilize polyene molecules but also to optimize the electronic and optical properties of SWCNTs. We found that several other carotenoids, lycopene and astaxantin, can be also encapsulated in SWCNTs with a method similar to that used in β-carotene. Thus we expect that functionalized carotenoids would also be able to enter easily into SWCNT. As with functionalized fullerene peapods, simple chemical transformation of the organic groups would enable us to introduce different chemical functionalities into the cavity of SWCNTs.