Solvent-controlled photoinduced electron transfer between porphyrin and carbon nanotubes.

beta-Cyclodextrin (beta-CD)-modified multiwalled carbon nanotubes (MWCNTs) were successfully prepared by reaction of surface-bound carboxylic chloride groups of MWCNTs with aminoethyleneamino-deoxy-beta-CD (ENCD) and comprehensively characterized by FTIR, Raman, and X-ray photoelectron spectroscopy and thermogravimetric and differential thermal analysis. The beta-CD-modified MWCNTs (ENCD-MWCNTs) are highly water-soluble, with a solubility of ca. 9.0 mg x mL(-1). Furthermore, the photoinduced electron transfer (PET) process between tetrakis(4-carboxyphenyl)porphyrin (TCPP) and ENCD-MWCNTs was investigated by means of fluorescence, fluorescence decay, transient absorption spectroscopy, and cyclic voltammetry. Obvious quenching processes were observed upon addition of both MWCNT-COOH and ENCD-MWCNTs to the aqueous solutions of TCPP, indicating that the PET process between TCPP and carbon nanotubes takes place upon irradiation. When 1-adamantane acetic acid was added to the aqueous solutions of TCPP/MWCNT-COOH and TCPP/ENCD-MWCNTs, respectively, the former fluorescence remains, while the latter fluorescence recovers. On the contrary, the fluorescence intensity of TCPP in the DMF solution was hardly decreased upon addition of ENCD-MWCNTs, whereas its fluorescence was quenched in the presence of MWCNT-COOH. The observations indicate that the CD cavities play a vital role on the control of the PET process.