Time-resolved investigation of excitation energy transfer in carbon nanotube-porphyrin compounds.
Résumé
Single-Wall Carbon Nanotubes (SWNTs) non-covalently functionalized with organic molecules have proven to be a very promising light-harvesting system. The goal is to combine the unique transport and optical properties of nanotubes with the wide tunability of organic molecules properties. Among those molecules, porphyrins, a precursor of photosynthetic centers, show a strong absorption in the visible that is already used in dye sensitized hybrid solar cells [1]. Porphyrins link to SWNTs through π-stacking interaction which preserves most of the intrinsic properties of the SWNTs, in contrast with covalent functionalization. Moreover, the coupling is still strong enough to induce new functionalities. It was recently shown that efficient excitation energy transfer (EET) can occur in such system [2,3]. The very high quantum yield reported in this system suggests that the EET mechanism must occur on an ultrafast time-scale [4]. In order to characterize the nature of the EET mechanism, we investigate its dynamics at a subpicosecond time scale, by means of pump-probe spectroscopy. We show that the ground state recovery time of porphyrin in the compound is reduced by several orders of magnitude compared to the case or free porphyrin (Fig. 1). Concomitantly, a strong bleaching signal is observed on the optical resonances of the nanotubes showing an ultrafast population build-up upon excitation of the porphyrin (Fig. 2). We conclude that the energy transfer occurs on a time scale shorter than 100fs and that the surrounding of SWNTs by porphyrins does not affect the relaxation dynamics. References [1] W. M. Campbell, et al., The Journal of Physical Chemistry C, 111, 11760 (2007). [2] G. Magadur, et al., ChemPhysChem, 9, 1250 (2008).pp. 200-205. [3] J. P. Casey, et al., Journal of Materials Chemistry, 18, 1510 (2008). [4] C. Roquelet, et al., Applied Physics Letters, 97, 141918 (2010).