Near-Unity Singlet Fission on a Quantum Dot Initiated by Resonant Energy Transfer

The conversion of a high-energy photon into two excitons using singlet fission (SF) has stimulated a variety of studies in fields from fundamental physics to device applications. However, efficient SF has only been achieved in limited systems, such as solid crystals and covalent dimers. Here, we established a novel system by assembling 4-(6,13-bis(2-(triisopropylsilyl)ethynyl)pentacen-2-yl)benzoic acid (Pc) chromophores on nanosized CdTe quantum dots (QDs). A near-unity SF (198 ± 5.7%) initiated by interfacial resonant energy transfer from CdTe to surface Pc was obtained. The unique arrangement of Pc determined by the surface atomic configuration of QDs is the key factor realizing unity SF. The triplet-triplet annihilation was remarkably suppressed due to the rapid dissociation of triplet pairs, leading to long-lived free triplets. In addition, the low light-harvesting ability of Pc in the visible region was promoted by the efficient energy transfer (99 ± 5.8%) from the QDs to Pc. The synergistically enhanced light-harvesting ability, high triplet yield, and long-lived triplet lifetime of the SF system on nanointerfaces could pave the way for an unmatched advantage of SF.acceptedVersionPeer reviewe

Efficient photocatalytic proton-coupled electron-transfer reduction of O2 using a saddle-distorted porphyrin as a photocatalyst

Photocatalytic O2 reduction reactions proceeded to produce H2O2 using a diprotonated saddle-distorted dodecaphenylporphyrin as a photocatalyst. The quantum yield (12%), the turnover number (3000 for 6 h), and the turnover frequency (500 h−1) are achieved in photocatalytic systems based on free-base porphyrins for the first time. The photocatalytic reaction mechanism has been revealed by ns-laser flash photolysis and kinetic analysis

Zinc Phthalocyanine−Graphene Hybrid Material for Energy Conversion: Synthesis, Characterization, Photophysics and Photoelectrochemical Cell Preparation

Graphene exfoliation upon tip sonication in o-­‐DCB was accomplished. Then, covalent grafting of (2-­‐ aminoethoxy)(tri-­‐tert-­‐butyl) zinc phthalocyanine (ZnPc), to exfoliated graphene sheets was achieved. The newly formed ZnPc-­‐graphene hybrid material was found soluble in common organic solvents without any precipitation for several weeks. Application of diverse spectroscopic techniques verified the successful formation of ZnPc-­‐graphene hybrid materi-­‐ al, while thermogravimetric analysis revealed the amount of ZnPc loading onto graphene. Microscopy analysis based on AFM and TEM was applied to probe the morphological characteristics and to investigate the exfoliation of graphene sheets. Efficient fluorescence quenching of ZnPc in the ZnPc-­‐graphene hybrid material suggested that photoinduced events occur from the photoexcited ZnPc to exfoliated graphene. The dynamics of the photoinduced electron transfer was evaluated by femtosecond transient absorption spectroscopy, thus, revealing the formation of transient species such as ZnPc+ yielding the charge-­‐separated state ZnPc•+–graphene•–. Finally, the ZnPc-­‐graphene hybrid material was integrated into a photoactive electrode of an optical transparent electrode (OTE) cast with nanostructured SnO2 films (OTE/SnO2), which exhibited sta le and reproducible photocurrent responses and the incident photon-­‐to-­‐current conversion efficien-­‐ cy was determine

Controlling Open-Circuit Voltage of Organic Photovoltaic Cells by Inserting Thin Layer of Zn-Phthalocyanine at Pentacene/C_<60> Interface

We demonstrate organic photovoltaic cells composed of multi charge-separation (MCS) interfaces in the active layer for the improvement of power conversion efficiency (η_P). The MCS interfaces are composed of pentacene/C_ and Zn-phthalocyanine (ZnPc)/C_ by inserting thin layer of ZnPc at the CS interface between pentacene/C_. We obtain enhanced η_P and V_ in accordance with the increased energy difference between the lowest unoccupied molecular orbital (LUMO) level of the n-type material (C_) and the HOMO level of the p-type material (ZnPc). By inserting 2 nm ZnPc at the interface, both open-circuit voltage (V_) and short-circuit current density (J_) have been improved simultaneously and the resulting η_P reaches 2.07%

Control of Open-circuit voltage in organic photovoltaic cells by inserting an ultrathin metal-phthalocyanine layer

The authors develop organic photovoltaic cells with multicharge separation (MCS) interfaces by inserting a very thin layer of metal phthalocyanine. The devices with MCS interface allow one to control short-circuit current density (J_) and open-circuit voltage (V_) . The power conversion efficiency (η_p) of the device with MCS interface (Cu-phthalocyanine/C_ and pentacene/C_) is enhanced compared with that of the device with single charge separation interface (pentacene/C_). The enhancement of η_p is attributable to the increase in V_ with maintaining the J_. By using Zn-phthalocyanine, which possesses longer excited lifetime compared with Cu-phthalocyanine, both J_ and V_ have been improved simultaneously and the η_p reaches 2.04%

Geometries and Terahertz Motions Driving Quintet Multiexcitons and Ultimate Triplet-Triplet Dissociations via the Intramolecular Singlet-Fissions

Importance of vibronic effects has been highlighted for the singlet-fission (SF) that convert one high-energy singlet exciton into doubled triplet excitons, as strongly correlated multiexcitons. However, molecular mechanisms of spin conversion processes and ultimate de-couplings in the multiexcitons are poorly understood. We have analyzed geometries and exchange couplings of the photoinduced multiexcitons in the pentacene dimers bridged by a phenylene at ortho and meta positions [denoted as o-(Pc)2 and m-(Pc)2] by simulations of the time-resolved electron paramagnetic resonance spectra. We clarified that terahertz molecular conformation dynamics plays a role on the spin conversion from the singlet strongly coupled multiexcitons 1(TT) to the quintet state 5(TT). The strongly coupled 5(TT) multiexcitons are revealed to possess entirely planar conformations stabilized by mutually delocalized spin distributions, while the intramolecular de-coupled spin-correlated triplet pairs generated at 1 microsecond are also stabilized by distorted conformations resulting in two separately localized biradical characters

Porphyrin-Based Molecular Architectures for Light Energy Conversion

We have constructed a series of supramolecular photovoltaic cells composed of multi-porphyrin arrays such as porphyrin-alkanethiolate monolayer protected-gold nanoparticles [H_2PCnMPC: n is the number of methylene groups in the spacer], porphyrin dendrimers [D_nP_n], and porphyrin-peptide oligomers [P(H_2P)_n] and fullerene (C_) on nanostructured SnO_2 electrodes (OTE/SnO_2). These multi-porphyrin arrays form complexes with fullerene molecules and they form clusters in acetonitrile/toluene mixed solvent. The highly colored composite clusters are assembled as three-dimensional arrays onto nanostructured SnO_2 films [denoted as OTE/SnO_2/(multi-porphyrin array+C_)_m] using an electrophoretic deposition method. These highly organized molecular assembly films attain drastic enhancement of light energy conversion properties as compared to the non-organized reference system. The maximum power conversion efficiency (η) of OTE/SnO_2/(H_2PC15MPC+C_)_m reaches 1.5%, which is 45 times higher than that of the reference system (0.035%)