Long-lived Electron-transfer State Research Articles

TiO2–rGO nanocomposites with high rGO content and luminescence quenching through green redox synthesis

We developed TiO2–rGO nanocomposites with high exfoliated rGO content upto 47.1% (TG4) by weight hitherto not reported, through green redox synthesis. Progressive reduction of TiO2 nanocrystal size and their in situ intercalation occurred on increasing rGO via one pot synthesis, not reported previously and forming highly stable Ti–O–C bonds. With increase in rGO content, the exfoliation in the nanocomposite was also limited to single-few layered graphene oxide sheets through use of ascorbic acid avoiding restacking, and thereby the recombination of photogenerated carriers. TG4 showed 50% photoluminescence (PL) quenching indicating long-lived electron transfer state with rGO and apparent rate of nonradiative excited state decay of 1.15 × 108 s−1. Details of lattice orientation, Raman spectra, electronic states and PL lifetime decay are discussed to elucidate effective charge carrier separation in the nanocomposite. Near complete photodegradation of Alizarin Yellow GG azo dye, an industrial pollutant, could be obtained under direct sunlight using TG4 as catalyst.

Visible Light-Activatable Oxidase Mimic of 9-Mesityl-10-Methylacridinium Ion for Colorimetric Detection of Biothiols and Logic Operations.

In this work, 9-mesityl-10-methylacridinium ion (Acr+-Mes) is found to act as an effective photocatalyst mimicking the function of oxidase. Upon visible light illumination, the excited Acr+-Mes is able to exhibit superior enzymatic catalytic activity for small molecular substrates as well as protein biomacromolecule (cytochrome c). The experiment results demonstrate that the Acr+-Mes oxidase mimic shows higher affinity to 3,3',5,5'-tetramethylbenzidine (TMB) than natural horseradish peroxidase or the reported molecule oxidase mimic. The reaction mechanism is ascribed to the strong oxidation property of the long-lived electron-transfer state (Acr•-Mes•+) and the electron transfer from Acr•-Mes radical to dissolved oxygen to generate superoxide radicals, which can easily oxidize various substrates. On the basis of these observations, the light-activatable Acr+-Mes with an oxidase-like activity as the probe is utilized for cost-effective, sensitive, and highly selective colorimetric detection of two biothiols (L-cysteine and L-glutathione). The lowest detectable concentrations of L-Cys and L-GSH is 100 nM, which is lower than that of most of the reported methods for biothiols. Beyond this, we construct a series of visual molecular logic gates (AND, INH, and NOR) using the oxidase mimic-involved reaction systems.

Study of the role of oxygen vacancies as active sites in reduced graphene oxide-modified TiO2.

In recent years, substantial efforts have been devoted to exploring reduced graphene oxide/TiO2 (RGO/TiO2) composite materials; however, there is still a paucity of reports on the construction of reduced graphene oxide/TiO2 with oxygen vacancies (RGO/TiO2-OV) via a facile two-step wet chemistry approach. In this work, we show a proof-of-concept study follow RGO introduced into TiO2 with oxygen vacancies, the role of oxygen vacancies as active sites in reduced graphene oxide-modified TiO2. The photocatalytic performance and related properties of blank-TiO2, blank-TiO2 with oxygen vacancies (blank-TiO2-OV), RGO/TiO2, and RGO/TiO2-OV were comparatively studied. It was found that due to the incorporation of RGO, RGO/TiO2 and RGO/TiO2-OV exhibit a higher photocatalytic performance under simulated solar light irradiation than their counterparts without rGO. More importantly, it was found that blank-TiO2 has a higher photocatalytic activity than blank-TiO2-OV under simulated solar light irradiation. However, RGO/TiO2 shows a lower photocatalytic activity than rGO/TiO2-OV. By a series of combined techniques, we found that the introduction of a component, such as RGO, with the matched energy band to TiO2 could lead to the formation of a long-lived electron-transfer state, thus prolonging the lifetime of the photogenerated charge carriers. Furthermore, during the photocatalytic process, RGO could tune the role of oxygen vacancies in TiO2 from recombination centers to active sites. These findings are of great significance for the design of effective photocatalytic materials in the field of solar energy conversion.

Organic Ion-Pair Charge-Transfer (IPCT) Nanoparticles: Synthesis and Photoinduced Electrochromism.

We report fabrication of photoinduced-electrochromic (or photochromic) organic nanoparticles that can be regarded as a new class of photoswitchable nanomaterials. Ion association between dicationic benzyl viologen (BzV2+) and tetraphenylborate anion (TPB-) in water produces organic ion-pair charge-transfer-type (IPCT) nanoparticles with a mean diameter of 15-20 nm. Electrostatic binding of BzV2+ with TPB- results in the prominent appearance of a new broad absorption band at 400-550 nm, which can be assign to the CT transition between these species. The IPCT nanoparticles show an interesting photoinduced electrochromic behavior caused by the electron transfer (ET) from TPB- to BzV2+, giving an intense blue-color dispersion. The feed charge stoichiometry (= [TPB-]/(2 × [BzV2+])) does not influence the kinetics of the photoinduced electrochromism, but it remarkably controls the reverse ET reaction (or retention time) in the nanoparticles; in the case of neutral stoichiometry, a very long-lived ET state can be observed, which is mainly due to decomposition of the oxidized TPB (sacrificial donor) and the resultant absence of the counterpart of benzyl viologen radical cations (BzV+•) for back ET reaction. On the other hand, the excess presence of TPB- does not cause the decomposition of oxidized TPB in the nanoparticle, making the reverse ET reaction faster, but a moderately long-lived ET state is still observable, which is probably originated from efficient electron diffusion or transport between molecules inside the nanoparticle.

ChemInform Abstract: Selective Photocatalytic Reactions with Organic Photocatalysts

AbstractReview: 189 refs.

Selective photocatalytic reactions with organic photocatalysts

Selective photocatalytic oxygenation of various substrates has been achieved using organic photocatalysts via photoinduced electron-transfer reactions of photocatalysts with substrates and dioxygen under visible light irradiation. Photoinduced electron transfer from benzene to the singlet-excited state of the 3-cyano-1-methylquinolinium ion has enabled the oxidation of benzene by dioxygen with water to yield phenol selectively. Alkoxybenzenes were obtained when water was replaced by alcohols under otherwise the same experimental conditions. Photocatalytic selective oxygenation reactions of aromatic compounds have also been achieved using an electron donor–acceptor linked dyad, 9-mesityl-10-methylacridinium ion (Acr+–Mes) acting as a photocatalyst and dioxygen as an oxidant under visible light irradiation. The oxygenation reaction is initiated by intramolecular photoinduced electron transfer from the mesitylene moiety to the singlet-excited state of the acridinium moiety of Acr+–Mes to afford an extremely long-lived electron-transfer state. The electron-transfer state can oxidize and reduce substrates and dioxygen, respectively, leading to selective oxygenation and halogenation of substrates. C–C bond formation of substrates has also been made possible by using Acr+–Mes as a photocatalyst.

Formation of a long-lived electron-transfer state in mesoporous silica-alumina composites enhances photocatalytic oxygenation reactivity

A simple donor-acceptor linked dyad, 9-mesityl-10-methylacridinium ion (Acr(+)-Mes) was incorporated into nanosized mesoporous silica-alumina to form a composite, which in acetonitrile is highly dispersed. In this medium, upon visible light irradiation, the formation of an extremely long-lived electron-transfer state (Acr(•)-Mes(•+)) was confirmed by EPR and laser flash photolysis spectroscopic methods. The composite of Acr(+)-Mes-incorporated mesoporous silica-alumina with an added copper complex [(tmpa)Cu(II)] (ClO(4)(-))2 (tmpa = tris(2-pyridylmethyl)amine) acts as an efficient and robust photocatalyst for the selective oxygenation of p-xylene by molecular oxygen to produce p-tolualdehyde and hydrogen peroxide. Thus, incorporation of Acr(+)-Mes into nanosized mesoporous silica-alumina combined with an O(2)-reduction catalyst ([(tmpa)Cu(II)](2+)) provides a promising method in the development of efficient and robust organic photocatalysts for substrate oxygenation by dioxygen, the ultimate environmentally benign oxidant.

Determination of the Structural Features of a Long-Lived Electron-Transfer State of 9-Mesityl-10-methylacridinium Ion

Extensive efforts have been devoted to developing electron donor-acceptor systems that mimic the utilization of solar energy that occurs in photosynthesis. X-ray crystallographic analysis shows how absorbed photon energy is stabilized in those compounds by structural changes upon photoinduced electron transfer (ET). In this study, structural changes of a simple electron donor-acceptor dyad, 9-mesityl-10-methylacridinium cation (Acr(+)-Mes), upon photoinduced ET were directly observed by laser pump and X-ray probe crystallographic analysis. The N-methyl group in Acr(+) was bent, and a weak electrostatic interaction between Mes and a counteranion in the crystal (ClO(4)) was generated by photoinduced ET. These structural changes correspond to reduction and oxidation due to photoinduced ET and directly elucidate the mechanism in Acr(+)-Mes for mimicking photosynthesis efficiently.

Formation of a long-lived electron-transfer state of a naphthalene–quinolinium ion dyad and the π-dimer radical cation

An electron donor-quinolinium ion dyad, 2-phenyl-4-(1-naphthyl)quinolinium ion (QuPh+-NA), has been synthesized based on a rational design. The X-ray crystal structure of QuPh+-NA indicates that the dihedral angle between the NA and QuPh+ moieties of QuPh+-NA is nearly perpendicular. The one-electron reduction potential (E(red)) was observed as a well-defined reversible wave at -0.90 V versus SCE. The one-electron reduced species (QuPh*-NA) was detected by ESR. The electron self-exchange rate constant (k(ex)) between QuPh+-NA and QuPh*-NA has been determined from the ESR linewidth alternation. The reorganization energy (lambda) of the electron self-exchange was determined to be 0.42 eV from the k(ex) value. Femtosecond laser irradiation of QuPh+-NA at 355 nm results in formation of the ET state (QuPh*-NA*+) within 0.5 ps via photoinduced ET from NA to the singlet-excited state of QuPh+. The transient absorption bands at 420 nm and 700 nm are assigned to the QuPh and NA*+ moieties, respectively. The nanosecond laser excitation of QuPh+-NA affords the broad absorption band at 1000 nm and is due to the pi-dimer radical cation formed between QuPh*-NA*+ and QuPh+-NA. The intramolecular back electron-transfer process was too slow to compete with the intermolecular back electron-transfer reaction judging from the decay time profile of QuPh*-NA*+, which obeyed second-order kinetics.

Photocatalytic hydrogen evolution from carbon-neutral oxalate with 2-phenyl-4-(1-naphthyl)quinolinium ion and metal nanoparticles

Photocatalytic hydrogen evolution has been made possible by using oxalate as a carbon-neutral electron source, metal nanoparticles as hydrogen-evolution catalysts and the 2-phenyl-4-(1-naphthyl)quinolinium ion (QuPh(+)-NA), which forms the long-lived electron-transfer state upon photoexcitation, as a photocatalyst. The hydrogen evolution was conducted in a deaerated mixed solution of an aqueous buffer and acetonitrile (MeCN) [1:1 (v/v)] by photoirradiation (λ > 340 nm). The gas evolved during the photocatalytic reaction contained H(2) and CO(2) in a molar ratio of 1:2, indicating that oxalate acts as a two-electron donor. The hydrogen yield based on the amount of oxalate reached more than 80% under pH conditions higher than 6. Ni and Ru nanoparticles as well as Pt nanoparticles act as efficient hydrogen-evolution catalysts in the photocatalytic hydrogen evolution. The photocatalyst for hydrogen evolution can be used several times without significant deactivation of the catalytic activity. Nanosecond laser flash photolysis measurements have revealed that electron transfer from oxalate to the photogenerated QuPh˙-NA˙(+), which forms a π-dimer radical cation with QuPh(+)-NA [(QuPh˙-NA˙(+))(QuPh(+)-NA)], occurs followed by subsequent electron transfer from QuPh˙-NA to the hydrogen-evolution catalyst in the photocatalytic hydrogen evolution. Oxalate acts as an efficient electron source under a wide range of reaction conditions.

Efficient photocatalytic hydrogen evolution without an electron mediator using a simple electron donor–acceptor dyad

A highly efficient photocatalytic hydrogen evolution system without an electron mediator such as methyl viologen (MV(2+)) has been constructed using 9-mesityl-10-methylacridinium ion (Acr(+)-Mes), poly(N-vinyl-2-pyrrolidone)-protected platinum nanoclusters (Pt-PVP) and NADH (beta-nicotinamide adenine dinucleotide, reduced form) as the photocatalyst, hydrogen evolution catalyst and electron donor, respectively. The photocatalyst (Acr(+)-Mes) undergoes photoinduced electron transfer (ET) from the Mes moiety to the singlet excited state of the Acr(+) moiety to produce an extremely long-lived ET state, which is capable of oxidizing NADH and reducing Pt-PVP, leading to efficient hydrogen evolution. The hydrogen evolution efficiency is 300 times higher than that in the presence of MV(2+) because of the much faster reduction rate of Pt-PVP by Acr(*)-Mes compared with that by MV(*+). When the electron donor (NADH) is replaced by ethanol in the presence of an alcohol dehydrogenase (ADH), NADH is regenerated during the photocatalytic hydrogen evolution.

Persistent Electron-Transfer State of a π-Complex of Acridinium Ion Inserted between Porphyrin Rings of Cofacial Bisporphyrins

A free-base cofacial bisporphyrin, H(4)DPOx, forms a pi-complex with acridinium ion (AcH(+)) by pi-pi interaction in benzonitrile (PhCN). Formation of the H(4)DPOx-AcH(+) pi-complex was probed by UV-vis and NMR spectroscopy. The binding constant between AcH(+) and H(4)DPOx is determined as 9.7 x 10(4) M(-)(1). Photoinduced electron transfer (ET) from the H(4)DPOx to the AcH(+) moiety occurred efficiently in the pi-complex to form the ET state (H(4)DPOx(*)(+)-AcH(*)). The ET state is successfully detected by laser flash photolysis. The lifetime of the ET state is 18 mus in PhCN at 298 K, and the quantum yield of the ET state is 90%. The temperature dependence of the ET state lifetime has been examined in the range from 273 to 353 K. The ET state lifetime exhibited a large temperature dependence, and the linear plot of ln(k(BET)T(1/2)) vs T(-)()(1), in accordance with the Marcus equation, affords the ET reorganization energy (0.54 eV). As a result, a remarkably long-lived ET state has been attained at low temperature, and virtually no decay of the ET state was observed at 77 K. Such an extremely long-lived ET state is indeed detected by steady-state UV-vis absorption spectroscopy.

Misleading effects of impurities derived from the extremely long-lived electron-transfer state of 9-mesityl-10-methylacridinium ion

9-Mesityl-10-methylacridinium ion has an extremely long-lived electron-transfer state, which results in the misleading effects of electron donor impurities contained in benzonitrile to produce the stable acridinyl radical.