Electron Transfer. Shunichi Fukuzumi

and the movement of ClO₄⁻ provide strong evidence for the generation of the ET state of Acr⁺–Mes upon photoexcitation. In contrast to the case of Acr⁺–Mes, no geometrical difference was observed upon photoexcitation of Acr⁺–Ph, which does not afford the ET state [72].

Figure 4.4 (a) UV–vis spectral change in the steady‐state photolysis of a deaerated PhCN solution of Acr⁺–Mes (3.3 × 10^–5 M). Spectra were recorded at 90‐second interval. (b) UV–vis absorption spectra obtained by photoirradiation with high‐pressure mercury lamp of deaerated 2‐MeTHF glasses of Acr⁺–Mes at 77 K. Inset: picture images of frozen PhCN solutions of Acr⁺–Mes before and after photoirradiation at low temperatures and taken at 77 K.

Figure 4.5 (a) Diagram of the reaction cavity: (left) diagram around the N‐methyl group, with numbers indicating the volumes of the divided cavity formed by the dotted line; (right) drawing around ClO₄⁻. (b) Cooperative photoinduced geometrical changes. The dashed line indicates the suggested Mes^·+⋯ClO₄⁻ electrostatic interaction.

      Source: Hoshino et al. 2012 [72]. Reproduced with permission of American Chemical Society.

Immobilization of Acr⁺–Mes has also been achieved by incorporating Acr⁺–Mes cation into nanosized mesoporous silica–alumina (AlMCM‐41), which has cation exchange sites to obtain a nanocomposite (Acr⁺–Mes@AlMCM‐41) [73]. The shape and size of nanosized AlMCM‐41 were controlled by changing the preparation conditions as shown in Figure 4.6, where TEM images reveal a tubular or rod‐like (tAlMCM‐41) morphology in the diameter of 50–100 nm with the length of 0.2–2 μm array (part a) and also a sphere morphology (sAlMCM‐41, part b) [73]. The X‐ray powder pattern of tAlMCM‐41 exhibited a well‐resolved pattern with a prominent peak (100) observed at c. 2θ = 2.56°, indicating a highly ordered material with a hexagonal array [73]. Uniform channels c. 4 nm in diameter exist in a tube. Because the Acr⁺–Mes molecular size is small enough as compared with the pore size of mesoporous silica with its diameter of more than 3 nm, cation exchange with Acr⁺–Mes occurs spontaneously upon mixing Na⁺–exchanged AlMCM‐41 with Acr⁺–Mes in acetonitrile [73]. The cation exchange percentages of tAlMCM‐41 and sAlMCM‐41 by Acr⁺–Mes were determined to be 16% and 18%, respectively [73]. The Acr⁺–Mes incorporated into AlMCM‐41 is stable without leaching out in acetonitrile at room temperature [73].

Upon photoexcitation of Acr⁺–Mes@tAlMCM‐41 suspended in MeCN, photoinduced ET from the Mes moiety to the singlet excited state of the Acr⁺ moiety occurred within 10 ps to produce the ET state as detected by laser flash photolysis and electron paramagnetic resonance (EPR) measurements [60,67]. In contrast to the case in solution (vide supra), no bimolecular decay of the ET state occurs because each Acr⁺–Mes molecule is isolated inside AlMCM‐41 [73]. The lifetime of the ET state of Acr⁺–Mes@tAlMCM‐41 suspended in acetonitrile was determined to be 2.3 seconds at 198 K, which is much longer than that in solution because of the inhibition of bimolecular BET in AlMCM‐41 as illustrated in Figure 4.6 [73]. Thus, incorporation of a simple electron donor–acceptor dyad into AlMCM‐41 has made it possible to elongate the lifetime of the charge‐separated state, which is longer than that of the bacterial photosynthetic reaction center (one second) [74].

Figure 4.6 Transmission electron microscope (TEM) images of (a) tAlMCM‐41 and (b) sAlMCM‐41 (the high‐resolution image of tAlMCM‐41 is inserted in (a)). (c) Reaction scheme of photocatalytic oxygenation of p‐xylene with Acr⁺–Mes and [(tmpa)Cu^II]²⁺ incorporated into sAlMCM‐41.

      Source: Fukuzumi et al. 2012 [73]. Reproduced with permission of PNAS.

The triplet ET state of Acr^·–Mes^·+@tAlMCM‐41 was detected by an EPR spectrum measured at 4 K, which exhibited a fine structure together with a strong sharp signal at g = 4.0 [73]. The distance between two electron spins was determined from the zero‐field splitting parameters to be 7.7 Å, which agrees with the expected distance of 7.2 Å between an sp² carbon atom at the 4 position of the mesityl moiety and sp² carbon atoms at the 3 and 6 positions of the acridinyl moiety [73]. Polycondensation of Acr⁺–Mes‐bridged organosilane in the presence of a nonionic surfactant is also reported to yield a mesostructured organosilica solid with a functional framework that exhibited long‐lived photoinduced CS [75].

      Nano‐sized charge‐separated molecules can also be obtained by using single‐walled carbon nanotubes (SWNTs) [76], which exhibit excellent chemical and physical properties as revealed by various potential applications [77–81]. Extensive efforts have so far been devoted to assemble electron donor and acceptor molecules on SWNTs [82–88]. However, the fine control of size (i.e. length) of SWNTs remains a formidable challenge, because SWNTs have seamless cylindrical structures made up of a hexagonal carbon network, which leads to the difficulty of solubilization/functionalization without treatment with strong acid or vigorous sonication [89–92]. On the other hand, the cup‐stacked carbon nanotubes (CSCNTs) that consist of cup‐shaped nanocarbon (CNC) units, which stack via van der Waals attractions, have merited special attention from the viewpoint of the conventional carbon nanotube alternatives [93–96]. The tube–tube van der Waals energy between CNCs has been counterbalanced by the thermal or photoinduced electron transfer multi‐electron reduction due to electrostatic repulsion, resulting in the highly dispersible CNCs with size homogeneity [97,98].

The CNCs with controlled size have been functionalized with a large number of porphyrin molecules [99]. The general procedure for the synthesis of the porphyrin‐functionalized cup‐shaped nanocarbons [CNC–(H₂P)_n] is shown in Figure 4.7a [99]. The CNCs are first functionalized with aniline as the precursor for further functionalization with porphyrins. The aniline‐functionalized nanocarbons react with the porphyrin derivatives to construct the nanohybrids.

      The structure of the CNCs of the CNC–(H₂P)_n nanohybrids is shown by the TEM in Figure 4.7b, which reveals a CNC with a hollow core along the length of the nanocup with well‐controlled diameter (c. 50 nm) and size (c. 100 nm) [99]. The weight percentage of porphyrins attached to the CNCs was determined by thermogravimetric analysis (TGA) and elemental analysis to be ca. 20% [99]. This corresponds to one functional group per 640 carbon atoms of the nanocup framework for CNC–(H₂P)_n nanohybrid. Thus, the π‐framework of the CNC is not destroyed despite attachment of a large number of porphyrin molecules on the CNC.

      Spectroscopic evidence for the covalent functionalization of CNC–(H₂P)_n nanohybrid was obtained by an intensity increase of the Raman signal at 1353 cm⁻¹ (D band) in the functionalized CNC as compared with the pristine