True Quantum Yield Research Articles

Photolysis of free chlorine and production of reactive radicals in the UV/chlorine system using polychromatic spectrum LEDs as UV sources

Recently, ultraviolet light-emitting diodes (UV-LEDs) and chlorine combined system has been employed as an emerging advanced oxidation process. However, UV-LEDs were commonly considered as monochromatic UV sources. In this study, the obvious quantum yields of chlorine photolysis under 265 nm and 280 nm LEDs irradiations were investigated with treating LEDs as polychromatic UV sources. Particularly, Φobs-poly of HOCl and OCl⁻ for 265 nm LED were found to be 1.50 and 0.70 mol E−1, respectively, whereas Φobs-poly of HOCl and OCl⁻ for 280 nm LED were 1.28 and 0.64 mol E−1, respectively. It was identified that Φobs-poly were 5.66–14.63 % lower than Φobs-mono. This suggests that obvious quantum yield using peak emission wavelength would overestimate the true quantum yield. The production of radical species in LED UV/chlorine systems were determined by the degradation of BA, and illustrated by a mathematical model. Different trends were observed for 265 nm and 280 nm LED UV/chlorine systems as pH increased from 5.0 to 10.0. As pH increased, the formation of OH continuously decreased in both 265 nm and 280 nm LED systems. The formation of Cl increased at neutral pH and more Cl and OH were formed due to the higher molar absorbance coefficient at 280 nm. The chlorine dose-dependent effects on radical productions at pH of 5.0, 7.5 and 10.0 were also assessed. At pH of 5.0, OH was the main radical product and had linear correlation with chlorine dose. At pH of 7.5, the productions of OH and Cl showed similar profiles that increased rapidly at low chlorine dosage and then slowed down.

Simple photoreduction of carbon dioxide to formic acid and true quantum yield.

There is a need to develop techniques for conversion of carbon dioxide to useful products such as formaldehyde, formic acid, methanol, and hydrocarbons. Carbon dioxide can be converted into these products using either photochemically, electrochemically, thermochemical or hydrogenation by bacteria. Formate is of interest due to the possibility of being used in liquid fuel cells, as an additive in pyrolysis vapors and as a precursor for biological fuels. In this work, conversion of carbon dioxide to formic acid under acidic conditions and formate under basic or neutral conditions was accomplished through photoreduction using an inexpensive setup consisting of titanium dioxide, metal phthalocyanines and inexpensive incandescent sources. The yield of formic acid based on anion chromatography was 1.54%. This work also discusses and presents a true quantum yield determined using chemical actinometry which was near 2.0%. Detailed studies of the photoreduction process showed that the amount of sensitizer, light intensity and pH affect the amount of formate generated.

Absolute Method to Certify Quantum Yields of Photon Upconversion via Triplet-Triplet Annihilation.

For the consistent development of the field of photon upconversion via triplet-triplet annihilation (TTA-UC), it is pivotal to know the true quantum yield of TTA-UC emission. Although TTA-UC quantum yields have been determined by common relative measurements using quantum yield standards, there is still a discrepancy between the reported values even for the benchmark sensitizer-emitter pair of platinum(II) octaethylporphyrin (PtOEP) and 9,10-diphenylanthracene (DPA). Here, to resolve this situation, we show a method to obtain the absolute quantum yield of TTA-UC photoluminescence. The difficulty in obtaining absolute TTA-UC quantum yield by the integrating sphere measurement is to accurately calibrate the contribution of reabsorbed upconverted emission by triplet sensitizers. The reabsorption correction is successfully carried out by comparing sensitizer phosphorescence with and without the integrating sphere. An absolute TTA-UC quantum yield of the PtOEP-DPA pair is obtained as 36%, which shows a good agreement with the relative TTA-UC quantum yield. An absolute TTA-UC quantum yield of another red-to-blue TTA-UC pair, platinum(II) meso-tetraphenyltetrabenzoporphyrin (PtTPBP) and 2,5,8,11-tetra-tert-butylperylene (TTBP), is obtained as 27%. These absolute TTA-UC quantum yields can be used as certified values to check the measurement setup and sample condition for determining relative TTA-UC quantum yields in each laboratory.

A comparative study of ultraviolet photoconductivity relaxation in zinc oxide (ZnO) thin films deposited by different techniques

Photoresponse characteristics of ZnO thin films deposited by three different techniques namely rf diode sputtering, rf magnetron sputtering, and electrophoretic deposition has been investigated in the metal-semiconductor-metal (MSM) configuration. A significant variation in the crystallinity, surface morphology, and photoresponse characteristics of ZnO thin film with change in growth kinetics suggest that the presence of defect centers and their density govern the photodetector relaxation properties. A relatively low density of traps compared to the true quantum yield is found very crucial for the realization of practical ZnO thin film based ultraviolet (UV) photodetector.

Modulation of trans-to-cis photoisomerization and photoluminescence of 1,2-bis(4-pyridyl)ethylene or 4-styrylpyridine coordinated to fac-tricarbonyl(5-chloro-1,10-phenathroline)rhenium(I)

Photochemical and photophysical properties of fac-[Re(CO)3(Clphen)(trans-L)]+ complexes, Clphen=5-chloro-1,10-phenathroline and L=1,2-bis(4-pyridyl)ethylene, bpe, or 4-styrylpyridine, stpy, were investigated to complement the understanding of intramolecular energy transfer process in tricarbonyl rhenium(I) complexes having an electron withdrawing group attached to polypyridyl ligands. These new compounds were synthesized, characterized and the photoisomerization quantum yields were accurately determined by 1H NMR spectroscopy. The true quantum yields for fac-[Re(CO)3(Clphen)(trans-bpe)]+ were constant (Φ=0.55) at all investigated irradiation wavelengths. However, for fac-[Re(CO)3(Clphen)(trans-stpy)]+, similar true quantum yields were observed only at higher energy irradiation (Φ313 nm=0.53 and Φ365 nm=0.57), but it decreased significantly at 404nm (Φ=0.41). These results indicated different deactivation pathways for the trans-stpy complex photoisomerization. Quantum yields decreased as the 3ILtrans-L and 3MLCTRe→NN excited states become closer and the behavior was discussed in terms of the excited state energy gaps. Additionally, luminescence properties of photoproducts, fac-[Re(CO)3(Clphen)(cis-L)]+, were also investigated in different environments to analyze the relative energy of the 3MLCTRe→Clphen excited state for each compound.

Photoconductivity characteristics of ZnO nanoparticles assembled in nanogap electrodes for portable photodetector applications

In the present report, ZnO nanoparticles of size ∼9 nm have been assembled by optimized dielectrophoresis process on the pre-fabricated nanogap (60 nm) electrodes. The fabricated ZnO nanoparticles based nano-device was studied for its I– V characteristic and typical non-linear semiconducting behavior was observed. When illuminated with ultraviolet (UV) radiation of wavelength 365 nm, a tremendous change in conductivity of almost one order of magnitude was observed indicating a high sensitivity of the fabricated nano-device. Temporal photoresponse characteristics were studied at a fixed level of UV illumination intensity (1.0 mW/cm 2) and the response time under 10 ms was recorded. The photoresponse curve exhibited a perfect rise and recovery without any slow component. Absence of slow component in the photoresponse characteristic suggested that shallow trapping centers were inactive and the photoconductivity could be due to the true quantum yield of photogenerated charge carriers through interband and deep trapping centers transitions.

1H NMR spectroscopy as a tool to determine accurate photoisomerization quantum yields of stilbene-like ligands coordinated to rhenium(I) polypyridyl complexes

In this work, the use of proton nuclear magnetic resonance, 1H NMR, was fully described as a powerful tool to follow a photoreaction and to determine accurate quantum yields, so called true quantum yields ( Φ true), when a reactant and photoproduct absorption overlap. For this, Φ true for the trans– cis photoisomerization process were determined for rhenium(I) polypyridyl complexes, fac-[Re(CO) 3(NN)( trans-L)] + (NN = 1,10-phenanthroline, phen, or 4,7-diphenyl-1,10-phenanthroline, ph 2phen, and L = 1,2-bis(4-pyridyl)ethylene, bpe, or 4-styrylpyridine, stpy). The true values determined at 365 nm irradiation (e.g. Φ NMR = 0.80 for fac-[Re(CO) 3(phen)( trans-bpe)] +) were much higher than those determined by absorption spectral changes ( Φ UV–Vis = 0.39 for fac-[Re(CO) 3(phen)( trans-bpe)] +). Φ NMR are more accurate in these cases due to the distinct proton signals of trans and cis-isomers, which allow the actual determination of each component concentration under given irradiation time. Nevertheless when the photoproduct or reactant contribution at the probe wavelength is negligible, one can determine Φ true by regular absorption spectral changes. For instance, Φ 313 nm for free ligand photoisomerization determined both by absorption and 1H NMR variation are equal within the experimental error (bpe: Φ UV–Vis = 0.27, Φ NMR = 0.26; stpy: Φ UV–Vis = 0.49, Φ NMR = 0.49). Moreover, 1H NMR data combined with electronic spectra allowed molar absorptivity determination of difficult to isolate cis-complexes.

Photoswitches and Luminescent Rigidity Sensors Based on fac-[Re(CO)3(Me4phen)(L)]+

The fac-[Re(CO)3(Me4phen)(trans-L)]+ complexes, Me4phen = 3,4,7,8-tetramethyl-1,10-phenanthroline and L = 4-styrylpyridine, stpy, or 1,2-bis(4-pyridyl)ethylene, bpe, were synthesized and characterized by their spectroscopic,photochemical, and photophysical properties. The complexes exhibit trans-to-cis isomerization upon 313, 334, 365,and 404 nm irradiation, and the true quantum yields can be efficiently determined by absorption changes combined with 1H NMR data. For fac-[Re(CO)3(Me4phen)(trans-bpe)]+ similar quantum yields were determined at all wavelengths investigated. However, a lower value (phitrue = 0.35) was determined for fac-[Re(CO)3(Me4phen)(trans-stpy)]+ at404 nm irradiation, which indicates different pathways for the photoisomerization process. The photoproducts, fac-[Re(CO)3(Me4phen)(cis-L)]+, exhibit luminescence at room temperature with two maxima ascribed to the 3ILMe4phen and 3MLCTRe-->Me4phen excited states. The luminescence properties were investigated in different media, and the behavior in glassy EPA at 77 K showed that the contribution of each emissive state is dependent on the excitation wavelength. The photochemical and photophysical behavior of the complexes were rationalized in terms of the energy gap of excited states and can be exploited in photoswitchable luminescent rigidity sensors.

Quantum yield of P700+ photodestruction in isolated photosystem I complexes of the cyanobacteriumArthrospira platensis

The photostability of P700 cation radical (P700+) was studied by evaluating the quantum yields of P700(+) photodestruction in photosystem I (PSI) complexes of the cyanobacterium Arthrospira platensis. The time courses of P700+ photodestruction in PSI trimers and monomers have been measured in aerobic conditions under selective excitation of far-red absorption band of P700+ by intense light of laser diodes. Long-term exposure of PSI complexes to 808 or 870 nm laser light caused destruction of P700+ and antenna chlorophylls. The true integral quantum yield of P700+ photodestruction calculated from these data was less than 0.7-1.4 x 10(-8). Illumination of PSI complexes by 650 nm light caused destruction of antenna chlorophylls with true quantum yield of about 6-7 x 10(-6) and damage of P700 with apparent quantum yield 2-3 x 10(-8). Preferential photodestruction of the long-wavelength antenna chlorophyll absorbing at 710 nm as compared with bulk chlorophylls was observed. About three orders of difference in magnitude between quantum yields of P700+ and bulk chlorophyll photodestruction indicates that P700+ is extremely photostable for functioning as an efficient quencher of singlet excitation energy in PSI.

Self-absorption correction for solid-state photoluminescence quantum yields obtained from integrating sphere measurements

A new method is presented for analyzing the effects of self-absorption on photoluminescence integrating sphere quantum yield measurements. Both the observed quantum yield and luminescence spectrum are used to determine the self-absorption probability, taking into account both the initial emission and subsequent absorption and reemission processes. The analysis is experimentally validated using the model system of the laser dye perylene red dispersed in a polymer film. This approach represents an improvement over previous methods that tend to overestimate the true quantum yield, especially in cases with high sample absorbance or quantum yield values.

Titanium dioxide photosensitised oxidation of α,β-dihydroxybenzyl derivatives in CH 3CN

The TiO 2-photosensitised oxidation in CH 3CN of a series of 1-(X-phenyl)-1,2-ethanediols ( 1) and of 2-(X-phenyl)-1,2-propanediols ( 2) gives the corresponding benzaldehyde and acetophenone, respectively, accompanied by formaldehyde, whereas a series of symmetrically X-ring-substituted 1,2-diphenyl-1,2-ethanediols ( 3) yields the corresponding benzaldehyde (substrate/product molar ratio = 0.5). The adsorption constant values (2400–3900 M −1) are significantly higher than those of benzylic alcohols and can be explained by assuming that the TiO 2 surface adsorption occurs through both OH groups (chelation effect). The relative rates, determined for each series from competitive kinetic experiments, remained nearly constant when the oxidation potential increased, instead of decreasing as observed in homogeneous phase. This behaviour has been attributed to the preferential adsorption of the OH groups, that increases their oxidizability until they favourably compete, as regards electron abstraction, with the aromatic site. The reaction rate thereby becomes much less sensitive to the ring substituent effect. Based on the comparison of true quantum yield values (obtained in the photo-oxidation in the presence of colloidal TiO 2) of ring unsubstituted 1 and 2, the deprotonation pathway of the cation radical was excluded. Analogous to what happens with 2-aminoalcohol radical cations, the most probable path is a heterolytic, concerted and base-catalysed (by basic sites at the TiO 2 surface) C α–C β fragmentation that produces a radical and a final product (carbonyl compound).

The sun-exposed peel of apple fruit has a higher photosynthetic capacity than the shaded peel

To determine whether the sun-exposed peel of apple fruit has a higher photosynthetic capacity than the shaded peel, fruit peel samples were taken in both early July and early September from the exterior part of the canopy of mature 'Liberty'/M.9 trees for measuring oxygen evolution, key enzymes and metabolites involved in photosynthesis, and chlorophyll fluorescence. Compared with the shaded peel, the sun-exposed peel had higher light-saturated oxygen evolution rate and higher light saturation point, but lower apparent and true quantum yields. The activity of ribulose-1,5-bisphosphate carboxylase/oxygenase, glyceraldehyde-3-phosphate dehydrogenase, phosphoribulokinase, stromal fructose-1,6-bisphosphatase, ADP-glucose pyrophosphorylase and sucrose-phosphate synthase (SPS) were higher in the sun-exposed peel than in the shaded peel on both sampling dates except that no significant difference was found in SPS activity between the two peel types in September. No significant difference was detected in the concentration of key metabolites (G6P, F6P, G1P, and PGA) between the sun-exposed peel and the shaded peel, suggesting that the response of the key enzymes to light exposure is well coordinated. Chlorophyll fluorescence quenching analysis showed that the sun-exposed peel had higher PSII quantum efficiency than the shaded peel at each given PFD, which resulted mainly from the higher photochemical quenching coefficient (qP). The sun-exposed peel had higher thermal dissipation capacity, as indicated by larger NPQ and Fo quenching, than the shaded peel at high PFD. In conclusion, the sun-exposed peel of apple fruit has higher activities of the Calvin cycle enzymes and higher rate of electron transport, leading to higher photosynthetic O2 evolution capacity. It appears that the acclimation of the Calvin cycle activities, thermal dissipation, and electron transport in apple peel are well coordinated in response to light exposure.

Photoinduced isomerization and luminescence of fac-[Re(CO)3(ph2phen)(bpe)

The fac-[Re(CO)3(ph2phen)(trans-bpe)]PF 6 complex, ph2phen = 4,7-diphenyl-1,10-phenanthroline and trans-bpe = trans-1,2-bis(4-pyridyl)ethylene, was synthesized, characterized and its photochemical and photophysical behavior was investigated. This complex exhibits trans®cis photoisomerization of the coordinated trans-bpe ligand. The apparent quantum yields in CH3CN, determined by absorption changes, are F313 nm = 0.19 ± 0.02, F365 nm = 0.18 ± 0.04 and F404 nm = 0.18 ± 0.02. Higher true quantum yields (F365 nm = 0.40 ± 0.06) were determined by 1H NMR spectroscopy. The coordination of the trans-bpe ligand to the rhenium(I) polypyridyl complex enables a photosensitized isomerization under a lower energy region irradiation, where the free ligand does not absorb. The increasing luminescence as the photoproduct, fac-[Re(CO)3(ph2phen)(cis-bpe)] +, is formed is ascribed to the change of the lowest lying excited state from 3IL to 3MLCT. The emission exhibits a hypsochromic shift associated with the rigidochromic effect.

Photophysics on Surfaces: Determination of Absolute Fluorescence Quantum Yields from Reflectance Spectra

A method for the calculation of absolute fluorescence quantum yields for dyes attached to solid particles based on reflectance measurements is reported. The same procedure allows calculation of true reflectance spectra (free of fluorescence) for highly fluorescent materials as well. Samples ofcresyl violet were immobilized by adsorption on microgranular cellulose in the concentration range 4.5 x 10(-9) to 3.8 x 10(-6) mol g(-1). Diffuse and total reflectance spectra were recorded with and without insertion of an optical absorption filter between the output of the integrating sphere of a reflectance spectrometer and the photodetector in order to block fluorescence partially. From these data, the relative emission spectrum of the dye, the filter transmission spectrum, and the detector sensitivity, true reflectances and absolute fluorescence quantum yields were recovered. Observed fluorescence quantum yields, affected by dye aggregation and inner filter effects, were concentration and wavelength dependent, ranging approximately between 0.1 and 0.6. The analysis of remission function spectra showed that dye aggregation is negligible up to a concentration of 1.41 x 10(-7) mol g(-1). Fluorescence data were corrected for reemission and reabsorption using a suitable model [Lagorio, M. G.; Dicelio, L. E.; Litter, M. I.; San Roman, E. J. Chem. Soc., Faraday Trans. 1998, 94, 419]. Application of this model to samples showing no aggregation yielded a wavelength-independent true fluorescence quantum yield of 0.60 +/- 0.05, similar to values found in solution. The usage of cresyl violet as a reference for the evaluation of fluorescence quantum yields for weakly fluorescing samples in the solid phase is discussed.

True quantum yields and adsorption constants as tools for a mechanistic study of the TiO 2-sensitized photooxidation of benzylic derivatives

The quantum yields ( Φ) of the colloidal TiO 2-sensitized photooxidation of 4- ( 1) and 3-methoxybenzyl alcohol ( 2) together with 4-methoxybenzyltrimethyl- ( 3) and 4-methoxybenzyltriisopropylsilane ( 4) were determined in CH 3CN, in the presence of HClO 4 for 3 and 4. The true quantum yields ( Φ 0) of 1, 2 and 3, obtained from a Langmuir–Hinshelwood isotherm treatment of Φ at different substrate concentrations, are linearly correlated with I A −1/2, where I A is the light intensity. According to the previously suggested mechanisms for alcohols and silanes, a kinetic scheme that justifies this correlation is suggested. It is shown that the ratio of the slopes (from the Φ 0 versus I A −1/2 plots) for 1 and 2 is equal to the Φ 0 1 /Φ 0 2 ratio at any I A; this ratio depends on the rate constants in the kinetic scheme, in this case principally on the electron transfer constant, k et. On the contrary, the Φ 0 3 /Φ 0 4 ratio depends on k p, the cation radical desilylation rate constant, confirming a steric hindrance to nucleophylic assistance in the CSi fragmentation by the bulky isopropyl group in the SiR 3 moiety. Differently from Φ 0, the adsorption constants on the semiconductor under irradiation ( K D, obtained from the above isotherm treatment) are independent of I A. Moreover, as K D 1 =K D 2 and K D 3 =K D 4 , the structural modifications within the two alcohols and within the two silanes should be far enough away from the adsorption site. For all the substrates, K (the dark adsorption constant) is five times greater than K D, showing that this change does not depend on the substrate structure but is the result of different experimental conditions.

Determination of photonic efficiency and quantum yield of formaldehyde formation in the presence of various TiO2 photocatalysts

Illumination of aqueous TiO2 suspentions yields hydroxyl radicals, which can be trapped by methanol producing formaldehyde (HCHO). In this work, the photonic efficiency and quantum yield of HCHO formation in colloidal TiO2 solutions and, P25 and UV 100 suspensions have been determined. Differences in photocatalytic activity of the three photocatalysts have been found and are discussed. The photonic efficiency of HCHO formation in the presence of P25 and UV 100 depends on the concentration of TiO2 and the pH. The critical concentration is 2.5 g/L. Below this, the photonic efficiency with P25 is higher than with UV 100, and vice versa. Optimum pH values for P25 and UV 100 giving the maximum photonic efficiency are 7.7 and 10.4, respectively. Compared to P25 and UV 100, the true quantum yield of HCHO formation in colloidal TiO2 solution varies a little with pH and virtually does not change with the amount of loading of TiO2. The true quantum yield varies as the inverse square root of light intensity. The quantum yield increases from 0.02 to 0.08 when the absorbed photon flux decreases from 8.1 x 10(-7) Ein/L s to 4.9 x 10(-8) Ein/L s. A simple model is presented to explain the experimental observation.

The relationship between photosystem II efficiency and quantum yield for CO(2) assimilation is not affected by nitrogen content in apple leaves.

Bench-grafted Fuji/M.26 apple (Malus domestica Borkh.) trees were fertigated with different concentrations of nitrogen by using a modified Hoagland's solution for 45 d. CO(2) assimilation and photosystem II (PSII) quantum efficiency in response to incident photon flux density (PFD) were measured simultaneously in recent fully expanded leaves under low O(2) (2%) and saturated CO(2) (1300 micromol mol(-1)) conditions. A single curvilinear relationship was found between true quantum yield for CO(2) assimilation and PSII quantum efficiency for leaves with a wide range of leaf N content. The relationship was linear up to a quantum yield of approximately 0.05 mol CO(2) mol(-1) quanta. It then became curvilinear with a further rise in quantum yield in response to decreasing PFD. This relationship was subsequently used as a calibration curve to assess the rate of non-cyclic electron transport associated with Rubisco and the partitioning of electron flow between CO(2) assimilation and photorespiration in different N leaves in response to intercellular CO(2) concentration (C(i)) under normal O(2) conditions. Both the rate of non-cyclic electron flow and the rate of electron flow to CO(2) or O(2) increased with increasing leaf N at any given C(i). The percentage of non-cyclic electron flow to CO(2) assimilation, however, remained the same regardless of leaf N content. As C(i) increased, the percentage of non-cyclic electron flow to CO(2) assimilation increased. In conclusion, the relationship between PSII quantum efficiency and quantum yield for CO(2) assimilation and the partitioning of electron flow between CO(2) assimilation and photorespiration are not affected by N content in apple leaves.

159 The Relationship between Actual Photosystem II Efficiency and Quantum Yield for CO2 Assimilation is Not Affected by Nitrogen Content in Apple Leaves

Bench-grafted Fuji/M26 apple (Malus domestica Borkh) trees were fertigated with different concentrations of nitrogen by using a modified Hoagland's solution for 45 days. CO2 assimilation and actual photosystem II (PSII) efficiency in response to incident photon flux density (PFD) were measured simultaneously in recent fully expanded leaves under low O2 (2%) and saturated CO2 (1300 ppm) conditions. A single curvilinear relationship was found between true quantum yield for CO2 assimilation and actual PSII efficiency for leaves with a wide range of leaf N content. The relationship was linear up to a quantum yield of approximately 0.05 mol CO2/mol quanta, then became curvilinear with a further rise in quantum yield in response to decreasing PFD. This relationship was subsequently used as a calibration curve to assess the rate of linear electron transport associated with rubisco and partitioning of electron flow between CO2 assimilation and photorespiration in different N leaves in response to intercellular CO2 concentration (Ci) under normal O2 conditions. Both the rate of linear electron flow, and the rate to CO2 or O2 increased with increasing leaf N at any given Ci, but the percentage of linear electron flow to CO2 assimilation remained the same regardless of leaf N content. As Ci increased, the percentage of linear electron flow to CO2 assimilation increased. In conclusion, the relationship between actual PSII efficiency and quantum yield for CO2 assimilation and the partitioning of electron flow between CO2 assimilation and photorespiration are not affected by N content in apple leaves.

Photo-oxidation of some benzylic alcohols sensitized by colloidal TiO2 in CH3CN. A kinetic mechanistic study through quantum yield determinations

The quantum yields (ϕobs) relating to the colloidal TiO2-sensitized photo-oxidation of ring methoxy-substituted benzylic alcohols (D) were determined. The true quantum yields (ϕ0) were obtained from a Langmuir–Hinshelwood isotherm treatment of ϕobs at different [D]. In the light of the suggested photo-oxidation mechanism, an equation was deduced where ϕ0 is expressed as a function of the electron transfer (ket), back electron transfer (k−et) and benzylic deprotonation (kp) rate constants. In particular, the lower ϕ0 value of 3-CH3O-vs 4-CH3O-benzyl alcohol (1, with lower Ep) should principally depend on the difference in ket, while the decrease in ϕ0 on going from alcohol 1 to the α-methyl derivative (4) should be due to the difference in kp (stereoelectronic effect). The adsorption equilibrium constants under irradiation (KD) were also obtained from the above equation and the values are similar for the considered substrates, except for 4. In fact, this substrate shows a lower KD value, probably because of the steric hindrance of the methyl group to the OH adsorption (preferential site) on TiO2. Finally, both the inter- and intramolecular deuterium isotope effect (kH/kD = 1.8 and 1.6, respectively) are consistent with a kinetically significant Cα—H bond breaking following the electron-transfer step. Copyright © 2000 John Wiley & Sons, Ltd.

Terminology, relative photonic efficiencies and quantum yields in heterogeneous photocatalysis. Part I: Suggested protocol

Abstract The term photocatalysis is one amongst several in a quagmire of labels used to describe a photon-driven catalytic process; a simple description of photocatalysis is proposed herein. Other labels such as quantum yield and/or quantum efficiency used in solid/liquid and solid/gas hetero-geneous photocatalytic systems to express process efficiencies have come to refer (incorrectly) to the ratio of the rate of a given event to the rate of incident photons impinging on the reactor walls and typically for broadband radiation. There is no accord on the expression for process efficiency. At times quantum yield is defined; often, it is ill-defined and more frequently how it was assessed is not described. This has led to much confusion in the literature, not only because of its different meaning from homogeneous photochemistry, but also because the description of photon efficiency precludes comparison of results from different laboratories owing to variations in light sources, reactor geometries, and overall experimental conditions. The previously reported quantum yields are in fact apparent quantum yields, i.e. lower limits of the true quantum yields. We address this issue and argue that any reference to quantum yields or quantum efficiencies in a heterogeneous medium is inadvisable until the number of photons absorbed by the light harvester (the photocatalyst) is known. A practical and simple alternative is proposed for general use and in particular for processes employing complex reactor geometries: the concept of relative photonic efficiency (xr) is useful to compare process efficiencies using a given photocatalyst material and a given standard test molecule. A quantum yield can subsequently be calculated since f = xr f phenol, where f phenol denotes the quantum yield for the photocatalyzed oxidative transformation of phenol used as the standard secondary actinometer and Degussa P-25 TiO2 as the standard photocatalyst. For heterogeneous suspensions (only), an additional method to determine quantum yields f is also proposed.