Hayato Sakai

Supramolecular Structures and Photoelectronic Properties of the Inclusion Complex of a Cyclic Free‐Base Porphyrin Dimer and C60

A cyclic free-base porphyrin dimer H4-CPD(Py) (CPD = cyclic porphyrin dimer) linked by butadiyne moieties bearing 4-pyridyl groups self-assembles to form a novel porphyrin nanotube in the crystalline state. The cyclic molecules link together through nonclassical C-H⋅⋅⋅N hydrogen bonds and π–π interactions of the pyridyl groups along the crystallographic a axis. H4-CPD(Py) includes a C60 molecule in its cavity in solution. In the crystal structure of the inclusion complex (C60⊂H4-CPD(Py)), the dimer “bites” a C60 molecule by tilting the porphyrin rings with respect to each other, and there are strong π–π interactions between the porphyrin rings and C60. The included C60 molecules form a zigzag chain along the crystallographic b axis through van der Waals contacts with each other. Femtosecond laser flash photolysis of C60⊂H4-CPD(Py) in the solid state with photoexcitation at 420 nm shows the formation of a completely charge-separated state {H4-CPD(Py)·+ + C60·−}, which decays with a lifetime of 470 ps to the ground state. The charge-carrier mobility of the single crystal of C60⊂H4-CPD(Py) was determined by flash photolysis time-resolved microwave conductivity (FP-TRMC) measurements. C60⊂H4-CPD(Py) has an anisotropic charge mobility (Σμ = 0.16 and 0.13 cm2 V(−1)  s(−1)) along the zigzag chain of C60 (which runs at 45° and parallel to the crystallographic b axis). To construct a photoelectrochemical cell, C60⊂H4-CPD(Py) was deposited onto nanostructured SnO2 films on a transparent electrode. The solar cell exhibited photovoltaic activity with an incident photon to current conversion efficiency of 17%.

Systematic Control of the Excited‐State Dynamics and Carrier‐Transport Properties of Functionalized Benzo[ghi]perylene and Coronene Derivatives

A series of benzo[ghi]perylene (Bp) and coronene (Cor) derivatives substituted with electron-withdrawing methoxycarbonyl (COOMe) or electron-donating methoxyl (MeO) groups was synthesized. The electrochemical, spectroscopic, and photophysical properties of these compounds were investigated by cyclic voltammetry, steady-state and time-resolved spectroscopy, and quantum-yield measurements. Introduction of suitable substituents onto the aromatic rings enabled control of electrochemical and spectroscopic behavior. Examination of excited-state dynamics revealed that fluorescence quantum yields increased with increasing number of COOMe groups in both Bp and Cor derivatives, consistent with the findings of DFT calculations. Single-crystal analysis allowed the performance of field-effect transistors containing single crystals of the derivatives to be rationalized.

Preparation and structural control of metal coordination-assisted supramolecular architectures of porphyrins. Nanocubes to microrods

We have successfully prepared metal coordination-assisted porphyrin assemblies such as nanocubes, nanorods and microrods by controlling the synthetic conditions. The internal structures and lifetimes of the excited states are also quantitatively discussed.

High‐Yield Excited Triplet States in Pentacene Self‐Assembled Monolayers on Gold Nanoparticles through Singlet Exciton Fission

One of the major drawbacks of organic-dye-modified self-assembled monolayers on metal nanoparticles when employed for efficient use of light energy is the fact that singlet excited states on dye molecules can be easily deactivated by means of energy transfer to the metal surface. In this study, a series of 6,13-bis(triisopropylsilylethynyl)pentacene-alkanethiolate monolayer protected gold nanoparticles with different particle sizes and alkane chain lengths were successfully synthesized and were employed for the efficient generation of excited triplet states of the pentacene derivatives by singlet fission. Time-resolved transient absorption measurements revealed the formation of excited triplet states in high yield (172±26 %) by suppressing energy transfer to the gold surface.

Near-Infrared Photoelectrochemical Conversion via Photoinduced Charge Separation in Supramolecular Complexes of Anionic Phthalocyanines with Li+@C60

Two phthalocyanines possessing carboxylate groups ((TBA)4H2Pc·1 and (TBA)4H2Pc·2) form 1:2 supramolecular complexes with lithium cation-encapsulated C60 (Li(+)@C60) [H2Pc·1(4-)/(Li(+)@C60)2 and H2Pc·2(4-)/(Li(+)@C60)2] in a polar mixed solvent. From the UV-vis spectral changes, the binding constants (K) were estimated as ca. 10(12) M(-2). Upon the photoexcitation of constructed supramolecular complexes, photoinduced electron transfer occurred to form the charge-separated (CS) state. The lifetime of the CS state was determined to be 1.2 ms for H2Pc·2(4-)/(Li(+)@C60)2, which is the longest CS lifetime among the porphyrinoid/fullerene supramolecular complexes. H2Pc·1(4-)/(Li(+)@C60)2 also afforded the long-lived CS state of 1.0 ms. The spin state of the long-lived CS states was determined to be a triplet, as indicated by the EPR signal at g = 4. The reorganization energy (λ) and the electronic coupling term were determined to be λ = 1.70 eV, V = 0.15 cm(-1) from the temperature dependence of the rate constant for the charge recombination of the CS state of H2Pc·1(4-)/(Li(+)@C60)2. The energy of the CS state (0.49 eV) is much smaller than the reorganization energy, indicating that the back-electron-transfer process is located in the Marcus normal region. The small electronic coupling term results from the spin-forbidden back electron transfer due to the triplet CS state. Supramolecular complexes of anionic zinc phthalocyanines with Li(+)@C60 were also prepared and investigated. The ZnPc·4(4-)/Li(+)@C60 supramolecular nanoclusters were assembled on the optically transparent electrode (OTE) of nanostructured SnO2 (OTE/SnO2) to construct the dye-sensitized solar cell. The IPCE (incident photon-to-photocurrent efficiency) values of OTE/SnO2/(ZnPc·4(4-)/Li(+)@C60)n were much higher than the sum of the two IPCE values of the individual systems OTE/SnO2/(Li(+)@C60)n and OTE/SnO2/(ZnPc·4(4-))n, covering the near-infrared region.

Molecular nanoarchitectures composed of porphyrins and carbon nanomaterials for light energy conversion

In this review, we report the recent advances in the construction of composite molecular nanoarchitectures of porphyrins and nanoscale carbon materials such carbon nanotubes (CNT), graphenes and polycyclic aromatic hydrocarbons (PAH) for photoinduced electron transfer and light energy conversion. First, we state novel single-wall carbon nanotubes (SWCNT)-driven aggregation of protonated porphyrins to produce supramolecular assemblies in the form of macroscopic bundles. Then, photoinduced electron transfer in self-assembled single-walled carbon nanotube (SWCNT)/zinc porphyrin (ZnP) hybrids utilizing (7,6)- and (6,5)-enriched SWCNTs having different band gaps is reported. Further, we discuss the structural and photoelectrochemical properties of porphyrin-based molecular assemblies of other carbon materials such as stacked-cup carbon nanotubes (SCCNT), carbon nanohorns (CNH) and graphenes. Finally, novel supramolecular patterning formation composed of triphenylene core-centered porphyrin hexamers for electronics is discussed.

Coronenetetraimide-centered cruciform pentamers containing multiporphyrin units: synthesis and sequential photoinduced energy- and electron-transfer dynamics.

A series of coronenetetraimide (CorTIm)-centered cruciform pentamers containing multiporphyrin units, in which four porphyrin units are covalently linked to a CorTIm core through benzyl linkages, were designed and synthesized to investigate their structural, spectroscopic, and electrochemical properties as well as photoinduced electron- and energy-transfer dynamics. These systems afforded the first synthetic case of coroneneimide derivatives covalently linked with dye molecules. The steady-state absorption and electrochemical results indicate that a CorTIm and four porphyrin units were successfully characterized by the corresponding reference monomers. In contrast, the steady-state fluorescence measurements demonstrated that strong fluorescence quenching relative to the corresponding monomer units was observed in these pentamers. Nanosecond laser flash photolysis measurements revealed the occurrence of intermolecular electron transfer from triplet excited state of zinc porphyrins to CorTIm. Femtosecond laser-induced transient absorption measurements for excitation of the CorTIm unit clearly demonstrate the sequential photoinduced energy and electron transfer between CorTIm and porphyrins, that is, occurrence of the initial energy transfer from CorTIm (energy donor) to porphyrins (energy acceptor) and subsequent electron transfer from porphyrins (electron donor) to CorTIm (electron acceptor) in these pentamers, whereas only the electron-transfer process from porphyrins to CorTIm was observed when we mainly excite porphyrin units. Finally, construction of high-order supramolecular patterning of these pentamers was performed by utilizing self-assembly and physical dewetting during the evaporation of solvent.

Formation of One‐Dimensional Helical Columns and Excimerlike Excited States by Racemic Quinoxaline‐Fused [7]Carbohelicenes in the Crystal

A series of quinoxaline-fused [7]carbohelicenes (HeQu derivatives) was designed and synthesized to evaluate their structural and photophysical properties in the crystal state. The quinoxaline units were expected to enhance the light-emitting properties and to control the packing structures in the crystal. The electrochemical and spectroscopic properties and excited-state dynamics of these compounds were investigated in detail. The first oxidation potentials of HeQu derivatives are approximately the same as that of unsubstituted reference [7]carbohelicene (Heli), whereas their first reduction potentials are shifted to the positive by about 0.7 V. The steady-state absorption, fluorescence, and circular dichroism spectra also became redshifted compared to those of Heli. The molecular orbitals and energy levels of the HOMO and LUMO states, calculated by DFT methods, support these trends. Moreover, the absolute fluorescence quantum yields of HeQu derivatives are about four times larger than that of Heli. The structural properties of the aggregated states were analyzed by single-crystal analysis. Introduction of appropriate substituents (i.e., 4-methoxyphenyl) in the HeQu unit enabled the construction of one-dimensional helical columns of racemic HeQu derivatives in the crystal state. Helix formation is based on intracolumn π-stacking between two neighboring [7]carbohelicenes and intercolumn CH⋅⋅⋅N interaction between a nitrogen atom of a quinoxaline unit and a hydrogen atom of a helicene unit. The time-resolved fluorescence spectra of single crystals clearly showed an excimerlike delocalized excited state owing to the short distance between neighboring [7]carbohelicene units.

Controllable Electronic Structures and Photoinduced Processes of Bay‐Linked Perylenediimide Dimers and a Ferrocene‐Linked Triad

A series of perylene-3,4,9,10-bis(dicarboximide) (PDI) dimers linked through the bay regions was systematically synthesized to examine the electronic structures and photophysical properties in dependence on the distance and orientation between the two PDI units. The spectroscopic and electrochemical measurements suggested that the coupling value of a directly linked PDI dimer (PDI)2 is much larger than those of para- and meta-phenylene-bridged PDI dimers p-(PDI)2 and m-(PDI)2 . The width of Davydov splitting was quantitatively evaluated to compare the coupling values between the two PDI units in these dimers by absorption spectroscopy in frozen 2-methyl-THF. Excimer formation of PDI dimers induced the strong fluorescence quenching and large red-shifts. Femtosecond transient absorption revealed a broad absorption derived from an excimer in the range from about 600 nm to the near-IR region. The rate constants of formation and decay of the excimer are strongly dependent on the coupling values. Time-resolved measurements on ferrocene-linked p-(PDI)2 revealed a competition between the photoinduced processes of electron transfer and excimer formation in PhCN, which is in sharp contrast with the sole electron-transfer process in toluene.

Photoelectrochemical properties of supramolecular composites of an anionic zinc chlorin and Li+@C60 on SnO2

An electron donor–acceptor supramolecular complex was formed between an anionic zinc chlorin carboxylate (ZnCh-) and lithium-ion-encapsulated [60]fullerene (Li+@C60) by an electrostatic interaction...