Light-harvesting function of $\beta$-carotene inside carbon nanotubes explored by femtosecond absorption spectroscopy

Single-walled carbon nanotubes (SWCNTs) encapsulating $\beta$-carotene are attractive components for enhancing optical functionalities of SWCNTs. We have investigated ultrafast dynamics of light-harvesting function of $\beta$-carotene. The relaxation kinetics of $\beta$-carotene is faster in SWCNTs than in nonpolar solvent. The absorbance change due to SWCNTs increases with the encapsulation because of excited energy transfer from $\beta$-carotene. The energy transfer rates from the $1{}^{1}B_u^{+}$ and $2{}^{1}A_g^{-}$ excited states of $\beta$-carotene to SWCNTs are determined to be ${\left(50\mathrm{fs}\right)}^{-1}$ and ${\left(6.6\mathrm{ps}\right)}^{-1}$, respectively.

Figure 1

(Color online) (a) Stationary absorption spectra of SWCNT (black solid line) and Car@SWCNT (blue dashed line) films and the difference absorption spectrum between Car@SWCNT and SWCNT (red dash-dotted line). The inset shows chemical structure of $\beta$-carotene. (b) Time-resolved photoinduced absorbance change of SWCNT and (c) the difference of the absorbance changes between Car@SWCNT and SWCNT.

Figure 2

Temporal dependence of photoinduced absorbance changes of SWCNT (open circles), the difference absorbance changes between Car@SWCNT and SWCNT (closed circles), and $\beta$-carotene in $n$-hexane (open triangles) at probe photon energies of (a) 1.24 and (b) $2.18\mathrm{eV}$. The solid curves are the best fits using exponential functions.

Figure 3

Temporal dependence of photoinduced absorbance changes of SWCNT (open circles) and the difference absorbance changes between Car@SWCNT and SWCNT (closed circles) at probe photon energies of (a) 1.35 and (b) $1.75\mathrm{eV}$. The solid curves are the best fits using exponential functions.

Figure 4

Schematic illustration of energy transfer processes from $\beta$-carotene to semiconducting SWCNT, which occurs after photoexcitation of the $1{}^{1}B_u^{+}$ state of $\beta$-carotene.