  As narrow optical bandgap materials, semiconducting single-walled carbon
nanotubes (SWCNTs) are rarely regarded as charge donors in photoinduced
charge-transfer (PCT) reactions. However, the unique band structure and unusual
exciton dynamics of SWCNTs add more possibilities to the classical PCT
mechanism.In this work, we demonstrate PCT from photoexcited semiconducting
(6,5) SWCNTs to a wide-bandgap wrapping
poly-[(9,9-dioctylfluorenyl-2,7-diyl)-alt-(6,6')-(2,2'-bipyridine)] (PFO-BPy)
via femtosecond transient absorption spectroscopy. By monitoring the spectral
dynamics of the SWCNT polaron, we show that charge transfer from photoexcited
SWCNTs to PFO-BPy can be driven not only by the energetically favorable E33
transition but also by the energetically unfavorable E22 excitation under high
pump fluence. This unusual PCT from narrow-bandgap SWCNTs toward a wide-bandgap
polymer originates from the up-converted high-energy excitonic state (E33 or
higher) that is promoted by the Auger recombination of excitons and charge
carriers in SWCNTs. These insights provide new pathways for charge separation
in SWCNT-based photodetectors and photovoltaic cells.
