Figure 7: Photodynamics study.

(a) Fluorescence spectra of Py-1-SWNT (blue line), PhI-SWNT (orange line), Py-2-SWNT (red line) and Py-ref (green line) in DMF recorded after excitation at 340 nm. The concentrations of the pyrene moieties of Py-1-SWNT, Py-2-SWNT, and Py-ref were adjusted to afford an absorbance of 0.03 at 340 nm. The inset shows the low emission intensity portion of the spectra. (b–d) Femtosecond TA spectra of (b) PhI-SWNT, (c) Py-1-SWNT, and (d) Py-2-SWNT in DMF. Spectra recorded using various time delays between 0.1 and 30 ps are shown. The excitation wavelength used was 350 nm. (e) A schematic charge separation mechanism. HOMO and LUMO energy levels of 1-phenylpyrene were determined based on the first oxidation and reduction potentials as measured using cyclic voltammetry. The HOMO level of the pyrene dimer in Py-1-SWNT is considered higher than that of the pyrene monomer by 0.30 V because the stabilization energy of pyrene dimer radical cation is reported to be ca. 0.3 eV²⁹. The LUMO level of the pyrene dimer was estimated by adding the optical bandgap energy to the HOMO. Note here that the optical bandgap of the pyrene dimer is 2.5 eV, which was determined according to the absorption edge of the pyrene dimer moiety shown in Fig. 2. The energy levels of the lowest unoccupied conduction band (c₁) of the SWNTs are taken from the literature^30,31. The energy levels of the highest occupied valence band (v₁) are estimated by adding the optical bandgap energies of the SWNTs (determined from the emission peaks) to the c₁ level.