The photocatalyzed degradation of Acid Blue 40, a textile dye, in aqueous suspension of TiO2 has been studied by monitoring the change in dye concentration and formation of carbon dioxide as a function of irradiation time. Mechanistic studies suggest that in addition to the reaction of the hydroxyl and superoxide radicals with the dye, electron transfer from the photoexcited TiO2 to the dye forms a significant pathway in the photodegradation process.

Photocatalytic degradation of waste water pollutants. Titanium dioxidemediated oxidation of a textile dye, Acid Blue 40

By means of ns laser flash photolysis, the absorption spectra and molar absorption coefficients (Iµ) of the 1,4-naphthosemiquinone radical (NQH˙) and 2,3-dimethyl-1,4-naphthosemiquinone radical (DMNQH˙) have been determined to be 8200 dm3 mol–1 cm–1 at 365 nm and 7100 dm3 mol–1 cm–1 at 368 nm in acetonitrile (MeCN) and MeCN–H2O (4 : 1 v/v) at 295 K. On the basis of the determined absorption spectra and Iµ values, hydrogen atom abstraction (HA) by triplet 1,4-naphthoquinone (NQ) and 2,3-dimethyl-1,4-naphthoquinone (DMNQ) from phenol (PhOH) in MeCN was studied by laser photolysis techniques. It was found that HA by triplet NQ and DMNQ (3NQ* and 3DMNQ*) from PhOH occurred in a collision process with quenching rate constants, kq= 8.6 × 109 and 5.5 × 108 dm3 mol–1 s–1, respectively. On the basis of the obtained values of kq and quantum yields (ΦHA), the efficiencies (ψHA) for HA by both 3NQ* and 3DMNQ* from PhOH were obtained to be unity. The rate constants (kHA) for HA of 3NQ* and 3DMNA* were determined to be 8.6 × 109 and 5.5 × 108 dm3 mol–1 s–1, respectively. The difference in kHA may be derived from (1) the steric hindrance by the methyl groups of DMNQ and (2) the degree of 3(π,π*) character mixed in with the 3(n,π*) of NA and DMNQ. The triplet–triplet absorption spectra of NQ and DMNQ were identified, and their absolute molar absorption coefficients (IµT–Tλ) were determined to be 8200 dm3 mol–1 cm–1 at 365 nm and 5200 dm3 mol–1 cm–1 at 368 nm, respectively, in MeCN on the basis of the quantum yields of intersystem crossing (Φisc; 0.74 for NQ, 0.98 for DMNQ) evaluated by thermal lensing techniques. The deactivation mechanism of 3NQ* and 3DMNQ* in the absence of PhOH was found to be self-quenching by NQ and DMNQ in MeCN with the rate constants, kSQ= 9.7 × 108 and 1.2 × 108 dm3 mol–1 s–1, respectively.

Laser flash photolysis studies on hydrogen atom abstraction from phenol by triplet naphthoquinones in acetonitrile

The photochemistry of a molecular pentad composed of a central anthraquinone (AQ) acceptor flanked by two Ru(bpy)32+ photosensitizers and two peripheral triarylamine (TAA) donors was investigated by transient IR and UV–vis spectroscopies in the presence of 0.2 M p-toluenesulfonic acid (TsOH) in deaerated acetonitrile. In ∼15% of all excited pentad molecules, AQ is converted to its hydroquinone form (AQH2) via reversible intramolecular electron transfer from the two TAA units (τ = 65 ps), followed by intermolecular proton transfer from TsOH (τ ≈ 3 ns for the first step). Although the light-driven accumulation of reduction equivalents occurs through a sequence of electron and proton transfer steps, the resulting photoproduct decays via concerted PCET (τ = 4.7 μs) with an H/D kinetic isotope effect of 1.4 ± 0.2. Moreover, the reoxidation of AQH2 seems to take place via a double electron transfer step involving both TAA+ units rather than sequential single electron transfer events. Thus, the overall charge-re...

http://pubs.acs.org/doi/pdf/10.1021/jacs.7b01605

Intramolecular Light-Driven Accumulation of Reduction Equivalents by Proton-Coupled Electron Transfer

Photoreduction of 9,10-anthraquinone by triethylamine: a Fourier-transform EPR study

From the measurements of phosphorescence spectra and triplet–triplet absorptions of the title compounds, it has been proposed that the lowest triplet states (T1) of the α‐chloro compounds are of mixed nπ*–ππ* or ππ* character, while the nπ* triplet states are the lowest ones for anthraquinone and 2‐chloroanthraquinone (the β‐chloro compound). Compared with anthraquinone and the β‐chloro compound, much shorter lifetimes of the T1 states and small phosphorescence quantum yields were obtained for the α‐chloro compounds. These results have been interpreted in terms of the modification of the geometrical molecular structure by the interaction of the carbonyl group with chlorine atom(s) at the α position(s), causing the T1 states to be of ππ* character with short lifetimes.

The lowest triplet states of anthraquinone and chloroanthraquinones: The 1‐chloro, 2‐chloro, 1,5‐dichloro, and 1,8‐dichloro compounds

Photoreductions of α-chloroanthraquinones followed by photochemical dehydrochlorination in ethanol at room temperature

Photoreduction of anthraquinone (AQ) by triethylamine (TEA) in toluene and ethanol was studied by means of steady-state photolysis and laser photolysis. In toluene, addition of TEA enhanced the quantum yield of photoreduction of AQ, and the reaction was found to originate via the formation of an exciplex between ground-state TEA and the lowest triplet state of AQ: This exciplex changed to a contact ion pair followed by proton transfer, generating anthrasemiquinone radical and triethylamine radical, and 9,10-anthracenediol was obtained as the final photoproduct. Although the mechanism of the photoreduction of AQ in ethanol/TEA was essentially the same as that in toluene/TEA and the final products in ethanol/TEA were 9,10-anthracenediol and its anion, the quantum yield of photoreduction decreased with increase in TEA concentration. It was proposed that the exciplex in ethanol was quenched by ground-state TEA via the formation of a triplex of a radical anion of AQ and a radical cation of TEA dimer bonded by ...

Photoreduction of anthraquinone by triethylamine in ethanol and toluene studied by steady-state photolysis and laser photolysis.

Photolyses of α-bromoanthraquinones (1-bromo-, 1,5-dibromo-, and 1,8-dibromoanthraquinones) with 366-nm light in ethanol at room temperature gave rise to the formation of debrominated 9,10-anthracenediol as a final product. This was interpreted in terms of the sequence of the formation of α-bromo-9,10-anthracenediols followed by the photochemical dehydrobromination yielding the corresponding anthraquinones with one less bromine atom than original ones. Photolysis of 2-bromoanthraquinone gave rise to the formation of 2-bromo-9,10-anthracenediol, and no dehydrobromination was observed. Combined with the values of the quantum yields of photoreduction, the results of the laser photolyses revealed that the rate constant for the hydrogen-atom abstraction from ethanol decreased as an increase of the ππ* character of the lowest triplet states of α-bromoanthraquinones.

Kazuhide Sawada

Papers

Photoreductions of α-chloroanthraquinones followed by photochemical dehydrochlorination in ethanol at room temperature

Photoreduction of anthraquinone by triethylamine in ethanol and toluene studied by steady-state photolysis and laser photolysis.

Photochemical reactions of bromoanthraquinones in ethanol at room temperature studied by steady-state photolysis and laser photolysis.