Effects of Porphyrin Substituents on Film Structure and Photoelectrochemical Properties of Porphyrin/Fullerene Composite Clusters Electrophoretically Deposited on Nanostructured SnO2 Electrodes

A supramolecular polymeric heterojunction composed of an all-carbon conjugated polymer and fullerenes

Herein, we design and synthesize a novel all-carbon supramolecular polymer host (SPh) containing conjugated macrocycles interconnected by a linear poly(para-phenylene) backbone. Applying the supramolecular host and fullerene C60 as the guest, we successfully construct a supramolecular polymeric heterojunction (SPh⊃C60). This carbon structure offers a means to explore the convex-concave π-π interactions between SPh and C60. The produced SPh was characterized by gel permeation chromatography, mass spectrometry, FTIR, Raman spectroscopy, and other spectroscopies. The polymeric segment can be directly viewed using a scanning tunneling microscope. Femtosecond transient absorption and fluorescence up-conversion measurements revealed femtosecond (≪300 fs) electron transfer from photoexcited SPh to C60, followed by nanosecond charge recombination to produce the C60 triplet excited state. The potential applications of SPh⊃C60 in electron- and hole-transport devices were also investigated, revealing that C60 incorporation enhances the charge transport properties of SPh. These results expand the scope of the synthesis and application of supramolecular polymeric heterojunctions.

Supramolecular Assemblies for Electronic Materials

This work presents a synergy between organic electronics and supramolecular chemistry, in which a host-guest complex is designed to function as an efficacious electronic material. Specifically, the noncovalent recognition of a fullerene, phenyl-C₆₁ -butyric acid methyl ester (PC₆₁ BM), by an alternating perylene diimide (P)-bithiophene (B) conjugated macrocycle (PBPB) results in a greater than five-fold enhancement in electron mobility, relative to the macrocycle alone. Characterization and quantification of the binding of fullerenes by host PBPB is provided alongside evidence for intermolecular electronic communication within the host-guest complexes.

One-Dimensional Fullerene/Porphyrin Cocrystals: Near-Infrared Light Sensing through Component Interactions

Recently, organic donor-acceptor (D-A) cocrystals have attracted special interest as functional materials because of their unique chemical and physical properties that are not exhibited by simple mixtures of their components. Herein, we report the preparation of one-dimensional novel D-A cocrystals from C₆₀ and 5,10,15,20-tetrakis(3,5-dimethoxyphenyl)porphyrin (3,5-TPP); these cocrystals have near-infrared (NIR) light-sensing abilities, despite each of their component molecule individually having no NIR light-sensing properties. Micrometer-sized rectangular columnar C₆₀-3,5-TPP cocrystals were produced by a simple liquid-liquid interfacial precipitation method. The cocrystals exhibit a new strong transition in the NIR region indicative of the existence of charge-transfer interactions between C₆₀ and 3,5-TPP in the cocrystals. The C₆₀-3,5-TPP cocrystals showed n-type transport characteristics with NIR light-sensing properties when the cocrystals were incorporated in bottom-gate/bottom-contact organic phototransistors, revealing that organic cocrystals with suitable charge-transfer interaction are useful as functional materials for the creation of novel NIR-light-sensing devices.

Photoconductive Curved-Nanographene/Fullerene Supramolecular Heterojunctions

This study presents synthesis and characterizations of two novel curved nanographenes that strongly bind with fullerene C₆₀ to form photoconductive heterojunctions. Films of the self-assembled curved nanographene/fullerene complexes, which served as the photoconductive layer, generated a significant photocurrent under light irradiation. Gram-scale quantities of these curved nanographenes (TCR and HCR) as the "crown" sidewalls can be incorporated into a carbon nanoring to form molecular crowns, and the molecular structure of C₆₀ @TCR is determined by single-crystal X-ray diffraction. The UV/Vis absorption and emission spectra, and theoretical studies revealed their unique structural features and photophysical properties. Time-resolved spectroscopic results clearly suggest fast photoinduced electron transfer process in the supramolecular heterojunctions.

Multimolecular assemblies on high surface area metal oxides and their role in interfacial energy and electron transfer

High surface area metal oxides offer a unique substrate for the assembly of multiple molecular components at an interface.

Self-assembling porphyrins and phthalocyanines for photoinduced charge separation and charge transport

Large π-conjugated compounds are promising building blocks for organic thin-film electronics such as organic light-emitting diodes, organic field-effect transistors, and organic photovoltaics. Utilization of porphyrins and phthalocyanines for this purpose is highly fascinating because of their excellent electric, photophysical, and electrochemical properties as well as intense self-assembling abilities arising from π-π stacking interactions. This paper focuses on fundamental aspects of self-assembled structures that have been obtained from porphyrin and phthalocyanine building blocks and more complex composites for photoinduced charge separation and charge transport toward the potential applications to organic thin-film electronics.

Porphyrin-modified electrodes for solar energy conversion

This mini review presents our recent developments in porphyrin-modified electrodes for solar energy conversion. Various porphyrins have been assembled on nanostructured semiconducting electrodes to achieve efficient photocurrent generation. First, porphyrins have been organized with fullerenes onto semiconducting electrodes to elucidate the relationship between the molecular structures, film structures, and photoelectrochemical properties of the modified electrodes. Formation of hole and electron-transporting highways in the porphyrin/fullerene composite film led to the remarkable enhancement of photocurrent generation. Second, porphyrin-modified single-walled carbon nanotubes have also been assembled onto semiconducting electrodes. The degree of chemical functionalization by the bulky porphyrins was found to have a large impact on the photoelectrochemical properties. Third, asymmetrically π-elongated porphyrins have been successfully employed in dye-sensitized solar cells to improve the cell performance as well as the light-harvesting properties. The power conversion efficiency of the fused porphyrin cell was improved by 50% compared to the reference cell using the corresponding unfused porphyrin. Finally, carboxyquinoxalino derivatives of zinc porphyrin have been further developed to extend the concept of asymmetrically π-elongated porphyrins. The maximum power conversion efficiency of 5.2% was obtained by using 5,10,15,20-tetrakis(2,4,6-trimethylphenyl)-6′-carboxyquinoxalino[2,3-b]porphyrinatozinc(II).

Near unity photon-to-electron conversion efficiency of photoelectrochemical cells built on cationic water-soluble porphyrins electrostatically decorated onto thin-film nanocrystalline SnO₂ surface

Thin transparent SnO(2) films have been surface modified with cationic water-soluble porphyrins for photoelectrochemical investigations. Free-base and zinc(II) derivatives of three types of cationic water-soluble porphyrins, (P)M, viz., tetrakis(N-methylpyridyl)porphyrin chloride, (TMPyP)M, tetrakis(trimethylanilinium)porphyrin chloride, (TAP)M, and tetrakis(4'-N-methylimidazolyl-phenyl)porphyrin iodide, (TMIP)M, (M = 2H or Zn) are employed. The negative surface charge and the porous structure of SnO(2) facilitated binding of positively charged porphyrins via electrostatic interactions, in addition to strong electronic interactions in the case of (TMPyP)M binding to nanocrystalline SnO(2). The SnO(2)-porphyrin binding in solution was probed by absorption spectroscopy which yielded apparent binding constants in the range of 1.5-2.6 × 10(4) M(-1). Both steady-state and time-resolved fluorescence studies revealed quenching of porphyrin emission upon binding to SnO(2) in water suggesting electron injection from singlet excited porphyrin to SnO(2) conduction band. Addition of LiClO(4) weakened the ion-paired porphyrin-SnO(2) binding as revealed by reversible emission changes. Over 80% of the quenched fluorescence was recovered in the case of (TMPyP)M and (TAP)M compounds but not for (TMIP)M suggesting stronger binding of the latter to SnO(2) surface. Photoelectrochemical studies performed on FTO/SnO(2)/(P)M electrodes revealed incident photon-to-current conversion efficiencies (IPCE) up to 91% at the peak maxima for the SnO(2)-dye modified electrodes, with very good on-off switchability. The high IPCE values have been attributed to the strong electrostatic and electronic interactions between the dye, (TMPyP)M and SnO(2) nanoparticles that would facilitate better charge injection from the excited porphyrin to the conduction band of the semiconductor. Electrochemical impedance spectral measurements of electron recombination resistance calculations were supportive of this assignment.

Segregated donor-acceptor columns in liquid crystals that exhibit highly efficient ambipolar charge transport

Liquid crystalline donor (i.e., phthalocyanine) was covalently linked to acceptor (i.e, fullerene) to achieve efficient charge-transport properties in a liquid crystalline phase. The columnar structure exhibited highly efficient ambipolar charge-transport character, demonstrating the potential utility of the strategy in organic electronics.

Photoelectrochemical and photocatalytic properties of biphasic organic p- and n-type semiconductor nanoparticles fabricated by a reprecipitation process

The visible-light-responsive photoelectrochemical and photocatalytic properties of nanoparticles of C(60), partially hydrolyzed aluminum phthalocyanine chloride (denoted as AlPc), and a composite of the two are reported. The three types of nanoparticles were obtained through a reprecipitation method from N-methyl-2-pyrrolidone solutions of C(60), aluminum phthalocyanine chloride (AlPcCl), and their mixture, respectively. The nanoparticle composite's ultraviolet-visible absorption, diffuse-reflectance and Fourier transform IR spectra, X-ray diffraction pattern, and scanning electron microscopy image are all similar to the sum of those of the C(60) and AlPc particles, respectively. The nano-ordered composite exhibits p/n junctionlike photoelectrochemical characteristics, which were investigated in comparison with those of vapor-deposited C(60) (n-type), AlPcCl (p-type), C(60)/AlPcCl (n/p), and AlPcCl/C(60) (p/n) electrodes. The nanoparticle composite further shows photocatalytic activity for the decomposition of trimethylamine to carbon dioxide in a suspension system.

Reduced graphene oxide and porphyrin. An interactive affair in 2-D

Photoexcited cationic 5,10,15,20-tetrakis(1-methyl-4-pyridinio)porphyrin tetra(p-toluenesulfonate) (TMPyP) undergoes charge-transfer interaction with chemically reduced graphene oxide (RGO). Formation of the ground-state TMPyP-RGO complex in solution is marked by the red-shift of the porphyrin absorption band. This complexation was analyzed by Benesi-Hildebrand plot. Porphyrin fluorescence lifetime reduced from 5 to 1 ns upon complexation with RGO, indicating excited-state interaction between singlet excited porphyrin and RGO. Femtosecond transient absorption measurements carried out with TMPyP adsorbed on RGO film revealed fast decay of the singlet excited state, followed by the formation of a longer-living product with an absorption maximum around 515 nm indicating the formation of a porphyrin radical cation. The ability of TMPyP-RGO to undergo photoinduced charge separation was further confirmed from the photoelectrochemical measurements. TMPyP-RGO coated conducting glass electrodes are capable of generating photocurrent under visible excitation. These results are indicative of the electron transfer between photoexcited porphyrin and RGO. The role of graphene in accepting and shuttling electrons in light-harvesting assemblies is discussed.

Nanometer-scale dynamics of charges generated by radiations in condensed matter

The dynamics of short-lived charges generated by pulsed radiations such as electron beam (EB) and photon was investigated to elucidate their reactivity, electronic properties, and spatial behavior on a nanometer scale. Chemical reactions of radical cations (hole) and anions (electron) in condensed matter (organic liquids, polymers, and conjugated materials) occupy an important place in postoptical nanolithography and organic electric devices. The spatiotemporal evolution of charges during geminate ion recombination was measured by a highly improved picosecond (ps) pulse radiolysis and incorporated into a Monte Carlo simulation to clarify the key role of the charges in the formation of latent image roughness of chemically amplified resists (CARs). The dynamics and alternating-current (AC) mobility of transient charge carriers in conjugated materials such as polymer and organic crystals were studied by the combination of microwave conductivity and optical spectroscopies, revealing the potential plausibility for high-performance electric devices. Anisotropy measurement and methodology to resolve the sum of mobility into hole and electron components without electrodes have also been demonstrated.