Osamu Ito

In this account, studies on the photosensitizing electron transfer of nanocarbons, such as fullerenes, single‐walled carbon nanotubes (SWCNTs), and carbon nanohorns (CNH), performed in our laboratory for about 15 years in the early 21st century have been briefly reviewed. These novel nanocarbons act as excellent electron acceptors, when they are linked to light‐absorbing electron donors, such as porphyrins or phthalocyanines. For such molecule–nanocarbon hybrids, the direct confirmation of fast, transient, electron‐transfer phenomena must be performed with time‐resolved spectroscopic methods, such as transient absorption spectral measurements, in addition to fluorescence time‐profile measurements in the wide‐wavelength regions. Careful use of these methods affords useful information to understand photoinduced electron‐transfer mechanisms. In addition, kinetic data obtained by these methods can assist in the construction of light‐active devices, such as photovoltaic cells and solar H₂‐generation systems.

To solve the energy demands for a sustainable society, it is crucial to use clean solar energy. Thus, finding methods for efficient light energy conversion is an important scientific goal. For this aim, molecular devices employing composites of nanocarbons such as fullerenes, single wall carbon nanotubes (SWCNTs), and graphene are promising. In this review, we survey light-induced electron-transfer processes of these nanocarbons hybridized with photosensitizers, which have proved to be more favorable than ordinary molecules. Fullerenes connected with suitable electron donors yield characteristic long-lived radical ion pairs, suitable for constructing artificial photosynthetic systems. For SWCNTs, a combination of photosensitizers is also crucial to achieve efficient photo-induced electron transfer processes. Furthermore, in the case of diameter-sorted semi-conductive SWCNTs, each tube can be treated as a “molecule”, thus, allowing molecular orbital calculations to probe the geometry as well as the electronic structures of the photosensitizernanotube hybrids. The two-dimensional graphene has afforded a new reaction field with a wide π-system for the attached molecules. For the confirmation of electron transfer processes, transient absorption methods in the wide wavelength regions have been used effectively. The kinetic data obtained in solution are quite useful to understand the electron transfer mechanisms, which afford useful guiding principles to construct efficient light-energy harvesting devices such as photoelectrochemical and photovoltaic solar cells.

Photosensitized electron-transfer processes of fullerenes hybridized with electron donating or other electron accepting molecules have been surveyed in this review on the basis of the recent results reported mainly from our laboratories. Fullerenes act as photo-sensitizing electron acceptors with respect to a wide variety of electron donors; in addition, fullerenes in the ground state also act as good electron acceptors in the presence of light-absorbing electron donors such as porphyrins. With single-wall carbon nanotubes (SWCNTs), the photoexcited fullerenes act as electron acceptor. In the case of triple fullerene/porphyrin/SWCNT architectures, the photoexcited porphyrins act as electron donors toward the fullerene and SWCNT. These mechanisms are rationalized with the molecular orbital considerations performed for these huge supramolecules. For the confirmation of the electron transfer processes, transient absorption methods have been used, in addition to time-resolved fluorescence spectral measurements. The kinetic data obtained in solution are found to be quite useful to predict the efficiencies of photovoltaic cells.

Photoinduced charge separation processes of three-layer supramolecular hybrids, fullerene–porphyrin–SWCNT, which are constructed from semiconducting (7,6)- and (6,5)-enriched SWCNTs and self-assembled via π–π interacting long alkyl chain substituted porphyrins (tetrakis(4-dodecyloxyphenyl)porphyrins; abbreviated as MP(alkyl)₄) (M = Zn and H₂), to which imidazole functionalized fullerene[60] (C₆₀Im) is coordinated, have been investigated in organic solvents. The intermolecular alkyl–π and π–π interactions between the MP(alkyl)₄ and SWCNTs, in addition, coordination between C₆₀Im and Zn ion in the porphyrin cavity are visualized using DFT calculations at the B3LYP/3-21G(*) level, predicting donor–acceptor interactions between them in the ground and excited states. The donor–acceptor nanohybrids thus formed are characterized by TEM imaging, steady-state absorption and fluorescence spectra. The time-resolved fluorescence studies of MP(alkyl)₄ in two-layered nanohybrids (MP(alkyl)₄/SWCNT) revealed efficient quenching of the singlet excited states of MP(alkyl)₄ (¹MP*(alkyl)₄) with the rate constants of charge separation (k_CS) in the range of (1–9) × 10⁹ s⁻¹. A nanosecond transient absorption technique confirmed the electron transfer products, MP˙ ⁺(alkyl)₄/SWCNT˙⁻ and/or MP˙⁻(alkyl)₄/SWCNT˙⁺ for the two-layer nanohybrids. Upon further coordination of C₆₀Im to ZnP, acceleration of charge separation via¹ZnP* in C₆₀Im→ZnP(alkyl)₄/SWCNT is observed to form C₆₀˙⁻Im→ZnP˙⁺(alkyl)₄/SWCNT and C₆₀˙⁻Im→ZnP(alkyl)₄/SWCNT˙⁺ charge separated states as supported by the transient absorption spectra. These characteristic absorptions decay with rate constants due to charge recombination (k_CR ) in the range of (6–10) × 10⁶ s⁻¹, corresponding to the lifetimes of the radical ion-pairs of 100–170 ns. The electron transfer in the nanohybrids has further been utilized for light-to-electricity conversion by the construction of proof-of-concept photoelectrochemical solar cells.

A non-covalent double-decker binding strategy is employed to construct functional supramolecular single-wall carbon nanotubes (SWCNT)–tetrapyrrole hybrids capable of undergoing photoinduced electron transfer and performing direct conversion of light into electricity. To accomplish this, two semiconducting SWCNTs of different diameters (6,5 and 7,6) were modified via π–π stacking of pyrene functionalized with an alkyl ammonium cation (PyrNH₃⁺). Such modified nanotubes were subsequently assembled via dipole–cation binding of zinc porphyrin with one (1) or four benzo-18-crown-6 cavities (2) or phthalocyanine with four benzo-18-crown-6 cavities at the ring periphery (3), employed as visible-light photosensitizers. Upon charactering the conjugates using TEM and optical techniques, electron transfer via photoexcited zinc porphyrin and phthalocyanine was investigated using time-resolved emission and transient absorption techniques. Higher charge-separation efficiency is established for SWCNT(7,6) with a narrow band gap than the thin SWCNT(6,5) with a wide band gap. Photoelectrochemical studies using FTO/SnO₂ electrodes modified with these donor–acceptor conjugates unanimously demonstrated the ability of these conjugates to convert light energy into electricity. The photocurrent generation followed the trend observed for charge separation, that is, incident-photon-to-current efficiency (IPCE) of a maximum of 12 % is achieved for photocells with FTO/SnO₂/SWCNT(7,6)/PyrNH₃⁺:1.

Unsymmetric dumbbell molecules based on N‐methylpyrrolidine[60]fullerene, oligothienylenevinylenes (nTV; n=2, 4), and N‐methylpyrrolidine[70]fullerene, namely, C₆₀‐nTV‐C₇₀ were synthesized and their photophysical properties were studied. In nonpolar solvents, photoinduced energy‐transfer process predominantly takes place from the singlet excited state of nTV to C₆₀ and C₇₀, as was confirmed by time‐resolved emission and transient absorption spectroscopy. In polar solvent, charge‐separation processes take place instead of energy transfer. The generated charge‐separated radical‐ion pairs decay to the neutral molecules by a fast charge‐recombination process; for n=4, a rate constant of 2×10⁷ s⁻¹ and lifetime of 50 ns were evaluated.

Photoinduced electron transfer in self-assembled via ion-pairing porphyrin–SWCNT nanohybrids has been reported. To accomplish this, two kinds of semiconducting SWCNTs of different diameters, and free-base or zinc porphyrin bearing peripheral positive or negative charges serving as light absorbing photo-active materials are utilized. The donor–acceptor hybrids are held by ion-pairing with the help of oppositely charged pyrene derivatives adhered to the side walls of SWCNTs. Higher charge-separation efficiency is established for thick SWCNT(7,6) with narrower band gap compared with the thin SWCNT(6,5) with wider band gap. Photoelectrochemical studies using FTO/SnO₂ electrodes modified with these donor–acceptor nanohybrids unanimously demonstrated the ability of these nanohybrids to harvest light energy into electricity. Importantly, the photocurrent generation followed the trend observed for charge-separation, that is, incident-photon-to-current conversion efficiency of a maximum of 8% is achieved for photocells with SWCNT(7,6), while such conversion efficiencies are smaller for the cells derived from SWCNT(6,5). These results indicate that higher light energy conversion efficiencies are possible to achieve by the selection of the appropriate SWCNTs with right band gap on combination of appropriate porphyrins.

A supramolecular assembly of zinc porphyrincarbon nanohorns (CNHs) was constructed in a polar solvent. An ammonium cation was covalently connected to the CNH through a spacer (sp) (CNH‐sp‐NH₃⁺) and bound to a crown ether linked to a zinc porphyrin (CrownZnP). Nanohybrids CNH‐sp‐NH₃⁺;CrownZnP and CNH‐sp‐NH₃⁺ were characterized by several techniques, such as high‐resolution transmission electron microscopy, thermogravimetric analysis, X‐ray photoelectron spectroscopy, and Raman spectroscopy. The photoinduced electron‐transfer processes of the nanohybrids have been confirmed by using time‐resolved absorption and fluorescence measurements by combining the steady‐state spectral data. Fluorescence quenching of the ZnP unit by CNH‐sp‐NH₃⁺ has been observed, therefore, photoinduced charge separation through the excited singlet state of the ZnP unit is suggested for the hybrid material, CNH‐sp‐NH₃⁺;CrownZnP. As transient absorption spectral experiments reveal the formation of the radical cation of the ZnP unit, electron generation is suggested as a counterpart of the charge‐separation on the CNHs; such an electron on the CNHs is further confirmed by migrating to the hexylviologen dication (HV²⁺). Accumulation of the electron captured from HV^.+ is observed as electron pooling in solution in the presence of a hole‐shifting reagent. Photovoltaic performance with moderate efficiency is confirmed for CNH‐sp‐NH₃⁺;CrownZnP deposited onto nanostructured SnO₂ films.