Triplet Energy Transfer (EnT)-Promoted 1,3-Dipolar Cycloaddition Reactions of -(Trimethylsilyl)methylphthalimide with Electron-Deficient Alkynes and Alkenes.

Triplet energy transfer (EnT)-promoted photochemical pathways, arisen by visible light and its photosensitizers, have gained significant attention as a complementary strategy for initiating organic transformations in photochemical reactions that are unlikely to occur through a single electron transfer (SET) process. In the present study, we investigated the triplet EnT-promoted 1,3-dipolar cycloaddition reactions of -(trimethylsilyl)methylphthalimide with electron-deficient alkynyl and alkenyl dipolarophiles. The triplet excited state of -(trimethylsilyl)methylphthalimide, promoted by the triplet EnT from thioxanthone (TXA) photosensitizer, underwent sequential intramolecular SET and carbon-to-oxygen migration of the silyl group to form azomethine ylide.

Photocatalytic Conversion of CO to Formate/CO by an (η--Cymene)Ru(II) Half-Metallocene Catalyst: Influence of Additives and TiO Immobilization on the Catalytic Mechanism and Product Selectivity.

The catalytic efficacy of the monobipyridyl (η--Cymene)Ru(II) half-metallocene, [(-Cym)Ru(bpy)Cl] was evaluated in both mixed homogeneous (dye + catalyst) and heterogeneous hybrid systems (dye/TiO/Catalyst) for photochemical CO reduction. A series of homogeneous photolysis experiments revealed that the (-Cym)Ru(II) catalyst engages in two competitive routes for CO reduction (CO to formate conversion via Ru-hydride vs CO to CO conversion through a Ru-COOH intermediate). The conversion activity and product selectivity were notably impacted by the p value and the concentration of the proton source added.

Reaction-Environment-Dependent Photoaddition Reactions of -Phenyl Amino Acid Esters Possessing a Silyl Group with Fullerene C: Selective Formation of Aminomethyl-1,2-dihydrofullerenes vs Fulleropyrrolidines.

The current study investigates SET-promoted photoaddition reactions of the silyl-group-containing -phenylglycinates and -phenylalaninates, -((trimethylsilyl)methyl)--phenyl-substituted glycinates and alaninates, respectively, with fullerene C to explore how the types of amino acid esters (AAEs) and molecular oxygen affect the photoaddition reaction efficiencies and chemoselectivity of in situ formed radical cations of AAEs. The results showed that under deoxygenated (N-purged) conditions, photoreactions of -phenylglycinates with C produced aminomethyl-1,2-dihydrofullerenes through the addition of α-amino radicals arising by sequential SET and desilylation processes from initially formed secondary anilines to C. In oxygenated conditions, photoreactions of -phenylglycinates with C, albeit less efficient, took place to form fulleropyrrolidines through a pathway involving 1,3-dipolar cycloaddition of azomethine ylides to C assisted by in situ formed O.

Systematic radical species control by electron push-pull substitution in the perylene-based D-π-A compounds.

Organic radical materials have been mainly reported on the stabilization of radical species because of their high energy and reactivity, while design strategies for controlling radical species beyond stabilization have remained challenging. Here, we report the electronic push-pull control spanning the neutral to the radical state of a series of perylene-based donor-π-acceptors (D-π-A). By introducing electron-withdrawing and -donating R groups to the donor of D-π-A, the observed intramolecular interactions controllable at the HOMO level led to the exploration of radical species.

Singlet fission dynamics modulated by molecular configuration in covalently linked pyrene dimers, Anti- and Syn-1,2-di(pyrenyl)benzene.

Covalently linked dimers (CLDs) and their structural isomers have attracted much attention as potential materials for improving power conversion efficiencies through singlet fission (SF). Here, we designed and synthesized two covalently ortho-linked pyrene (Py) dimers, anti- and syn-1,2-di(pyrenyl)benzene (Anti-DPyB and Syn-DPyB, respectively), and investigated the effect of molecular configuration on SF dynamics using steady-state and time-resolved spectroscopies. Both Anti-DPyB and Syn-DPyB, which have different Py-stacking configurations, form excimers, which then relax to the correlated triplet pair ((TT)) state, indicating the occurrence of SF.

Silyl Tether-Assisted Photooxygenation of Electron-Deficient Enaminoesters: Direct Access to Oxamate Formation.

Photooxygenation reactions of electron-deficient enaminoesters bearing an oxophilic silyl tether at the α-position of the nitrogen atom using methylene blue (MB) were explored to develop a mild and efficient photochemical strategy for oxidative C-C double bond cleavage reactions via singlet oxygen (O). Photochemically generated O, through energy transfer from the triplet excited state of MB (MB*) to molecular oxygen (O), was added across a C-C double bond moiety of enaminoesters to form perepoxides, which rearranged to form dioxetane intermediates. The cycloreversion of the formed dioxetane via both C-C and O-O bond cleavage processes led to the formation of oxamates.

Influence of picolinate ancillary ligands on unique photophysical properties of Ir(ppz)(LX).

Homoleptic -Ir(ppz) (ppz = phenylpyrazole) and a series of heteroleptic Ir(ppz)(LX) complexes consisting of picolinic acid (pic), 3-hydroxypicolinic acid (picOH), and isoquinolinecarboxylic acid (iq) as ancillary ligands (LX) were synthesised to investigate the influence of the ancillary ligands on the photophysical properties of the complexes. Generally, the role of the ancillary ligand is considered insignificant compared to that of the main ligand. Ir(ppz) showed deep-blue emission with a vibronic structure at 77 K, whereas Ir(ppz)(LX) showed a broad and red-shifted emission.

-Terphenyl linked donor-π-acceptor dyads: intramolecular charge transfer controlled by electron acceptor group tuning.

A series of -terphenyl linked donor-π-acceptor (D-π-A) dyads were prepared to understand the electronic effects of a -terphenyl linker according to the electron-accepting ability change. The energy band gaps of the dyads were controlled by tuning the accepting ability, which resulted in emission colors ranging from blue-green to red. In the Lippert-Mataga plots, intramolecular charge transfer (ICT) behavior was observed, which showed gradually increased ICT characteristics as the accepting ability was increased.

Solvent-modulated proton-coupled electron transfer in an iridium complex with an ESIPT ligand.

Proton-coupled electron transfer (PCET), an essential process in nature with a well-known example of photosynthesis, has recently been employed in metal complexes to improve the energy conversion efficiency; however, a profound understanding of the mechanism of PCET in metal complexes is still lacking. In this study, we synthesized cyclometalated Ir complexes strategically designed to exploit the excited-state intramolecular proton transfer (ESIPT) of the ancillary ligand and studied their photoinduced PCET in both aprotic and protic solvent environments using femtosecond transient absorption spectroscopy and density functional theory (DFT) and time-dependent DFT calculations. The data reveal solvent-modulated PCET, where charge transfer follows proton transfer in an aprotic solvent and the temporal order of charge transfer and proton transfer is reversed in a protic solvent.

Visible-Light-Induced Selective C-C Bond Cleavage Reactions of Dimeric β-O-4 and β-1 Lignin Model Substrates Utilizing Amine-Functionalized Fullerene.

Finding a selective and efficient fragmentation process under ambient conditions is pivotal for the generation of fuels and chemical feedstocks from lignoceullosic biomass. In the present study, visible-light and amine-functionalized fullerene-based photocatalyst-promoted photodegradation reactions of dimeric β-O-4 and β-1 lignin model compounds, containing varying numbers of methoxy substituents on the arene ring, were explored to find and develop mild, eco-friendly photochemical techniques for efficient delignification. The results showed that, in contrast to well-known organic photoredox catalysts, amine-functionalized fullerene photocatalyst promoted photochemical reactions of lignin model compounds could lead to more efficient lignin fragmentation reactions through a pathway involving a selective C-C bond cleavage process, and in addition, C-hydroxyl moiety in lignin model compounds played a significant role in the success of the C-C bond cleavage reaction of lignin model substrates.

Charge transfer induced by electronic state mixing in a symmetric X-Y-X-type multi-chromophore system.

Charge transfer (CT) from electron donor (D) to acceptor (A) plays an important role in photoelectric or electrochemical devices and is a useful concept for a molecule with D and A well distinguishable. Here, we report our finding that even in a molecule with D and A not resolvable, CT can be induced by electronic state mixing (ESM) in a symmetric multi-chromophore system (MCS), namely 1,4-di(1-pyrenyl)benzene (Py-Benz-Py). Unlike Py and Py-Benz, Py-Benz-Py exhibits unique photophysical properties attributable to the reduction of the energy gap between two electronic states induced by ESM.

Electron Push-Pull Effects on Intramolecular Charge Transfer in Perylene-Based Donor-Acceptor Compounds.

A series of asymmetric donor-acceptor (D-A) perylene-based compounds, 3-(,-bis(4'-(R)-phenyl)amino)perylene (-()), were successfully prepared to explore their intramolecular charge transfer (ICT) properties. To induce ICT between the donor and acceptor, diphenylamine (DPA) derivatives (electron donor units) with the same functional groups ( = CN, F, H, Me, or OMe) at both para positions were linked to the C-3 position of perylene to produce five - derivatives. A steady-state spectroscopy study on -()s exhibited a progressively regulated ICT trend consistent with the substituent effect as it progressed from the electron-withdrawing group to the electron-donating group.

Control of Chemoselectivity of SET-Promoted Photoaddition Reactions of Fullerene C with α-Trimethylsilyl Group-Containing -Alkylglycinates Yielding Aminomethyl-1,2-dihydrofullerenes or Fulleropyrrolidines.

Knowledge about factors that govern chemoselectivity is pivotal to the design of reactions that are utilized to produce complex organic substances. In the current study, single-electron transfer (SET)-promoted photoaddition reactions of fullerene C with both trimethylsilyl and various alkyl group-containing glycinates and ethyl -alkyl--((trimethylsilyl)methyl)glycinates were explored to evaluate how the nature of -alkyl substituents of glycinate substrates and reaction conditions govern the chemoselectivity of reaction pathways followed. The results showed that photoreactions of C with glycinates, performed in deoxygenated conditions, produced aminomethyl-1,2-dihydrofullerenes efficiently through a pathway involving the addition of α-amino radical intermediates that are generated by sequential SET-solvent-assisted desilylation of glycinate substrates to C.

Inter-Ligand Energy Transfer Process in an Ir-Complex with Expanding π-Conjugated Ligand.

The inter-ligand energy transfer (ILET) process in heteroleptic iridium complex, [Ir(dfppy) (bpy-Im )] , where dfppy=2-(2,4-difluorophenyl)pyridine and bpy-Im =4,4'-bis(1,2-diphenyl-1H-benzo[d]imidazole)-2,2',-bipyridine, was investigated using a femtosecond transient absorption (fs-TA) spectroscopic technique. The photophysical properties of [Ir(dfppy) (bpy-Im )] with significantly expanding π-conjugated ligand are compared to those of [Ir(dfppy) (bpy)] (bpy=2,2'-bipyridine) and a free bpy-Im ligand. The emission spectrum of [Ir(dfppy) (bpy-Im )] shows no shift upon changing the solvent polarity, whereas the free ligand bpy-Im showed bathochromic fluorescence shifts with increasing solvent polarity, which is attributed to intramolecular charge transfer (ICT).

Terphenyl backbone-based donor-π-acceptor dyads: geometric isomer effects on intramolecular charge transfer.

The molecular geometry effects of ortho, meta, and para-terphenyl based donor-π-acceptor (D-π-A) dyads on intramolecular charge transfer (ICT) were studied to investigate structure-ICT relationships. Terphenyl based D-π-A dyads were prepared by two-step palladium catalyzed, Suzuki-Miyaura coupling reactions, in which triphenylamine (TPA) was used as the electron donor and 1,2-diphenyl-benzimidazole (IMI) as the electron acceptor. The photophysical and electrochemical properties of terphenyl backbone-based ortho (O), meta (M), and para (P) dyads were compared.

Electron Push-Pull Effects in 3,9-Bis(-(R)-diphenylamino)perylene and Constraint on Emission Color Tuning.

A series of perylene-based donor-acceptor-donor (D-A-D) compounds, 3,9-bis(-(R)-diphenylamino)perylene (R: CN (), F (), H (), Me (), and OMe ()), was synthesized using 3,9-dibromoperylene with -(R)-diphenylamine, and the intramolecular charge transfer (ICT) on the D-A-D system with regard to the electron push-pull substituent effect was investigated. By introducing various -(R)-diphenylamine derivatives with electron-donating or electron-withdrawing R groups, the energy band gaps of the D-A-D compounds were systematically controlled and the emission colors were efficiently tuned from green to red. As expected, the steady state emission spectra of all D-A-D compounds were observed, as well as the emission color controlled, depending on the Hammett substituent constants (σ).

Photoaddition reactions of -benzylglycinates containing α-trimethylsilyl group with dimethyl acetylenedicarboxylate: competitive formation of pyrroles β-enamino esters.

A study was conducted to gain insight into the preparative potential of photosensitized reactions of acyclic -benzylglycinates containing an α-trimethylsilyl group with dimethyl acetylenedicarboxylate (DMAD). The photosensitizers employed in the reactions include 9,10-dicyanoanthracene (DCA), 1,4-dicyanonaphthalene (DCN), rose bengal (RB) and fullerene C. The results show that photoirradiation of oxygenated solutions containing the photosensitizers, glycinates and dimethyl acetylenedicarboxylate leads to competitive formation of pyrroles and β-enamino-esters.

Chromophore-Catalyst Assembly for Water Oxidation Prepared by Atomic Layer Deposition.

Visible-light-driven water splitting was investigated in a dye sensitized photoelectrosynthesis cell (DSPEC) based on a photoanode with a phosphonic acid-derivatized donor-π-acceptor (D-π-A) organic chromophore, 1, and the water oxidation catalyst [Ru(bda)(4-O(CH)P(OH)-pyr)], 2, (pyr = pyridine; bda = 2,2'-bipyridine-6,6'-dicarboxylate). The photoanode was prepared by using a layering strategy beginning with the organic dye anchored to an FTO|core/shell electrode, atomic layer deposition (ALD) of a thin layer (<1 nm) of TiO, and catalyst binding through phosphonate linkage to the TiO layer. Device performance was evaluated by photocurrent measurements for core/shell photoanodes, with either SnO or nanoITO core materials, in acetate-buffered, aqueous solutions at pH 4.

Fluoropolymer-Stabilized Chromophore-Catalyst Assemblies in Aqueous Buffer Solutions for Water-Oxidation Catalysis.

Here, the application of the fluorinated polymer [Dupont AF, a copolymer of 4,5-difluoro-2,2-bis(trifluoromethyl)-1,3-dioxole and tetrafluoroethylene] is described in stabilizing phosphonate-derivatized molecular assemblies on oxide electrodes. In the procedure, the polymer was dip-coated onto the surfaces of oxide electrodes with pre-bound, phosphonate-derivatized chromophores and assemblies, including assemblies for water oxidation. The results of the experiments showed a high degree of stabilization by the added polymer and a demonstration of its use in stabilizing surface-bound assemblies for water-oxidation catalysis.

All-in-One Derivatized Tandem pn-Silicon-SnO/TiO Water Splitting Photoelectrochemical Cell.

Mesoporous metal oxide film electrodes consisting of derivatized 5.5 μm thick SnO films with an outer 4.3 nm shell of TiO added by atomic layer deposition (ALD) have been investigated to explore unbiased water splitting on p, n, and pn type silicon substrates.

A Dye-Sensitized Photoelectrochemical Tandem Cell for Light Driven Hydrogen Production from Water.

Tandem junction photoelectrochemical water-splitting devices, whereby two light absorbing electrodes targeting separate portions of the solar spectrum generate the voltage required to convert water to oxygen and hydrogen, enable much higher possible efficiencies than single absorber systems. We report here on the development of a tandem system consisting of a dye-sensitized photoelectrochemical cell (DSPEC) wired in series with a dye-sensitized solar cell (DSC). The DSPEC photoanode incorporates a tris(bipyridine)ruthenium(II)-type chromophore and molecular ruthenium based water oxidation catalyst.

Self-assembled molecular p/n junctions for applications in dye-sensitized solar energy conversion.

The achievement of long-lived photoinduced redox separation lifetimes has long been a central goal of molecular-based solar energy conversion strategies. The longer the redox-separation lifetime, the more time available for useful work to be extracted from the absorbed photon energy. Here we describe a novel strategy for dye-sensitized solar energy applications in which redox-separated lifetimes on the order of milliseconds to seconds can be achieved based on a simple toolkit of molecular components.

Ligand-to-ligand charge transfer in heteroleptic Ir-complexes: comprehensive investigations of its fast dynamics and mechanism.

To gain new insights into ligand-to-ligand charge-transfer (LLCT) dynamics, we synthesised two heteroleptic Ir(3+) complexes: (Ir(dfppy)2(tpphz)) and (Ir(dfppy)2(dpq)), where dfppy, tpphz, and dpq are 2-(4,6-difluorophenyl)pyridine, tetrapyrido[3,2-a:2',3'-c:3'',2''-h:2''',3'''-j]phenazine, and 2,3-bis-(2-pyridyl)-quinoxaline, respectively. The tpphz and dpq ligands have longer π-conjugation than dfppy. Therefore, the excited ligand-centred (LC) state and the metal-to-ligand charge transfer (MLCT) state of dfppy are higher than those of tpphz and dpq.

Nonaqueous electrocatalytic water oxidation by a surface-bound Ru(bda)(L)₂ complex.

The rate of electrocatalytic water oxidation by the heterogeneous water oxidation catalyst [Ru(bda)(4-O(CH2)3P(O3H2)2-pyr)2], , (pyr = pyridine; bda = 2,2'-bipyridine-6,6'-dicarboxylate) on metal oxide surfaces is greatly enhanced relative to water as the solvent. In these experiments with propylene carbonate (PC) as the nonaqueous solvent, water is the limiting reagent. Mechanistic studies point to atom proton transfer (APT) as the rate limiting step in water oxidation catalysis.