Electron Transfer. Shunichi Fukuzumi

indicating that there is no significant interaction between attached porphyrins and CSCNTs in the ground states [99].

The fluorescence lifetime of CNC–(H₂P)_n was determined to be 3.0 ± 0.1 ns, which is much shorter than that of ref‐H₂P (14.1 ± 0.1 ns) [99]. The fluorescence emission at 650 nm was also quenched in CNC–(H₂P)_n [99]. The short fluorescence lifetime of CNC–(H₂P)_n and an efficient fluorescence quenching of porphyrins in CNC–(H₂P)_n as compared to the ref‐H₂P may result from the photoinduced electron transfer from the singlet excited state of H₂P (¹H₂P^*) to CNC in CNC–(H₂P)_n. The occurrence of photoinduced electron transfer to afford the charge‐separated (CS) state of CNC–(H₂P)_n was confirmed by nanosecond laser flash photolysis measurements in Figure 4.8, where the absorption bands in the visible and near infrared (NIR) regions are attributed to H₂P^·+, which are clearly different from the triplet–triplet absorption of ref‐H₂P [99]. The formation of the CS state was also confirmed by EPR measurements under photoirradiation of CNC–(H₂P)_n in frozen N,N‐diemthylformamide (DMF) at 153 K. The observed isotropic EPR signal at g = 2.0044 agrees with that of ref‐H₂P^·+ produced by one‐electron oxidation with [Ru(bpy)₃]³⁺ (bpy = 2,2′‐bipyridine) in deaerated CHCl₃ [99]. The EPR signal corresponding to the reduced carbon‐based nanomaterials was too broad to be detected, probably due to delocalization of electrons in CNC [99].

Figure 4.7 (a) Synthetic procedure of CNC–(H₂P)_n. (b) TEM image of CNC–(H₂P)_n.

      Source: Ohtani et al. 2009 [99]. Reproduced with permission of John Wiley & Sons.

The CS state of CNC–(H₂P)_n detected in Figure 4.8a decays obeying clean first‐order kinetics: the first‐order plots at different initial CS concentrations afford linear correlations with the same slope (Figure 4.8b) [99]. Thus, the decay of the CS state results from back electron transfer in the nanohybrid rather than intermolecular back electron transfer from CNC^·− to H₂P^·+. The CS lifetime was determined from the first‐order plots in Figure 4.8b to be 0.64 ± 0.01 ms, which is the longest lifetime ever reported for electron donor‐attached nanocarbon materials [99]. Such a long CS lifetime may be ascribed to the efficient electron migration in the CNCs following CS.

Figure 4.8 (a) Transient absorption spectra of (a) CNC–(H₂P)_n taken at 20 and 1.8 ms after laser excitation at 426 nm and (b) ref‐H₂P in deaerated DMF at 298 K taken at 100 ms and 1.6 ms after laser excitation at 426 nm. (c) Decay time profiles and (d) first‐order plots at 470 nm with different laser powers (5, 3, 2, and 1 mJ/pulse).

      Source: Ohtani et al. 2009 [99]. Reproduced with permission of John Wiley & Sons.