The variation of the optical absorption of carbon nanotubes with their geometry has been a long standing question at the heart of both metrological and applicative issues, in particular because optical spectroscopy is one of the primary tools for the assessment of the chiral species abundance of samples. Here, we tackle the chirality dependence of the optical absorption with an original method involving ultra-efficient energy transfer in porphyrin/nanotube compounds that allows uniform photo-excitation of all chiral species. We measure the absolute absorption cross-section of a wide range of semiconducting nanotubes at their S22 transition and show that it varies by up to a factor of 2.2 with the chiral angle, with type I nanotubes showing a larger absorption. In contrast, the luminescence quantum yield remains almost constant.The versatility of the physical properties of Single-Wall carbon Nanotubes (SWNTs) with respect to their geometry (the so-called (n, m) chiral species) is very attractive for applications [1][2][3][4], but on the other hand, the uncontrolled mixtures of species produced by regular synthesis methods blur out their specific properties. Post-growth sorting methods now allow to enrich samples in some specific species [5], but they also miss a tool for the quantitative assessment of their outcome. Optical techniques such as absorption, photoluminescence (PL) or resonant Raman spectroscopies are the primary tools to this end. However, these techniques can neither give a quantitative estimate of the species concentration nor their relative abundance to-date because they miss the knowledge of the (n, m)-dependence of the optical cross-section at the nanotubes' resonances (S 11 and S 22 ). Although several studies pointed that the optical properties of carbon nanotubes depend on their chiral angle, they all actually dealt with a combination of physical parameters (such as absorption cross-section, Raman scattering cross-section or PL quantum efficiency). As a result, the literature gives quite contradictory or inconclusive results, some of them pointing to a larger abundance of near armchair nanotubes (interpreted as energetically favored in the growth process) whereas other studies concluded for a larger optical cross-section for large chiral angles [6][7][8][9][10].Here, we propose an original method for assessing the chirality dependence of the absorption cross-section of semiconducting carbon nanotubes, by means of noncovalent functionalization with tetraphenyl porphyrin (TPP) molecules (Inset of Figure 1). This functionalization gives rise to an extremely efficient energy transfer [11] that allows to excite uniformly the whole set of carbon nanotubes regardless of their chirality. By comparison with the PL signal obtained in the regular excitation scheme (on the intrinsic S 22 transition of the SWNTs) of the same sample, we can single out the contribution of the absorption cross-section in the chiral dependence of the PL intensity. We show that the main variation of this absorption cross-sect...

Single-wall carbon nanotubes noncovalently functionalized with porphyrin molecules have proven to be a very promising light harvesting system either for energy or charge transfer. In this work, we investigate the dynamics of this coupling at a subpicosecond time-scale, by means of transient absorption spectroscopy. We show that the ground state recovery time of the porphyrin is reduced by several orders of magnitude in the compound compared to the case of pristine porphyrin. Concomitantly, a strong bleaching signal is observed on the optical resonances of the nanotubes showing an ultrafast population buildup upon excitation of the porphyrin. We conclude that the energy transfer occurs on a time-scale shorter than 100 fs. Two-color measurements show that higher excited states of the nanotubes are populated on the same time-scale raising the point of the transfer mechanism. We briefly discuss two possible mechanisms.