A laser flash photolysis study of pivalothiophenone triplets: steric and electronic effects in thione photoreaction kinetics

Upon laser pulse excitation (Aex = 532 nm) into the lowest-lying '(n,a*) band system, pivalothiophenones in benzene solutions give rise to short-lived triplets (Ama: = 325-335 nm, em: = (1 1-15) X lo3 M-' cm-I) with quantitative intersystem crossing efficiencies. The triplet yields decrease slightly (by 10-30%) upon changing A, to 308 nm (Le., upon excitation into S2). Kinetic data are presented for intrinsic triplet lifetimes, self-quenching, and quenching by oxygen, di-tert-butylnitroxy radical, and various reagents capable of interacting with the triplets via energy, electron, or hydrogen-atom transfer and by biradical formation (possibly leading to cycloaddition). The mechanisms of the quenching processes are discussed. Relative to rigid aromatic thiones, namely, xanthione and thiocoumarin, the interaction of pivalothiophenone triplets with most of the quenchers are kinetically inefficient. This is interpreted primarily as a manifestation of the steric crowding at positions a to the thiocarbonyl group.

Laser flash photolysis study of triplets of cyclobutanethiones

Five cyclobutanethiones with different chromophores at the 3-position were examined for triplet state behaviour in benzene using laser excitation into their low lying nπ*1 band systems. A weak transient absorption attributable to the triplet state is observed in all these cases. Results concerning triplet lifetimes, intersystem crossing yields (S1 → T1), self-quenching kinetics and kinetics of energy transfer to all-trans-1,6-diphenyl-1,3,5-hexatriene and oxygen and quenching by di-t-butyl nitroxide (DTBN) are presented. Intersystem crossing yields estimated with reference to p,p′-dimethoxythiobenzophenone are roughly unity in all five cases. Self-quenching rates are found to be less than diffusion limited and this is attributed to steric crowding at the α positions (dimethyl group). The rates of oxygen and DTBN quenching compare well with those reported for several other thiones in the literature. No transients other than the triplet were detected in the above cyclobutane-thiones.

Triplet-state Photophysics and Transient Photochemistry of Cyclic Enethiones A Laser Flash Photolysis Study

The triplets of four cyclic enethiones, including thiocoumarin, have been investigated by nanosecond laser flash photolysis. Data are presented for transient spectra and kinetics associated with triplets, quantum yields of intersystem crossing and singlet oxygen photosensitization. The quenching of the thiocoumarin triplet (A:, = 485 nm, E:,, = 8.8 x lo3 dm3 mol-' cm-'in benzene) by several olefins, amines and hydrogen donors occurs with rate constants of 107-5 x lo9 dm3 mol-' s-'; the lower limits of quantum yields ( c#+~) for the related photoreactions, estimated from ground-state depletion, are generally small (0.0-0.1 1 in benzene, except for good hydrogen donors, namely, p-methoxythiophenol and tri-n-butylstannane) . The radical anion of thiocoumarin (A,,, = 405-435 nm) is formed in two stages upon triplet quenching by triethylamine in acetonitrile; the fast component is the result of direct electron transfer to the triplet and the slower component is assigned to secondary photoreduction of the thione ground state by the a-aminoalkyl radical derived from the triethylamine radical-cation.

Dynamics of charge transfer in the excited amine complexes of the fullerenes C60 and C70.: A picosecond laser flash photolysis study

In benzene solution, C60 and C70 interact weakly in the ground state with amines having favourable oxidation potentials. Picosecond time-resolved absorption measurements show that on photoexcilation, the weak complexes undergo charge separation to produce ion pairs which in turn undergo fast geminate recombination either to produce the triplet state of the fullerenes or give back the ground slate of the complex, depending on the oxidation potential of the amine. Free-ion yield is generally negligible.

Short-lived triplets of aliphatic thioketenes

The photophysical behavior of the triplets of three aliphatic thioketenes, namely di-tert-butylthioketene (1), 2,6-di-tert-butylcyclohexylthioketene (2) and 2,2,6,6-tetramethylcyclohexylthioketene, has been studied in fluid solutions at room temperature by nanosecond laser flash photolysis. Upon 532 nm laser excitation into the S1 state, the thioketenes in concentrated benzene solutions produce very short-lived transient absorptions (τ < 5 ns; λmax ≈ 450 nm) attributable to their triplets. The photogeneration of the latter under S1 excitation has also been established by energy transfer to all-trans-1,6-diphenyl-1,3,5-hexatriene. The factors which render the triplet lifetimes short are shown to be intrinsic in origin (rather than self-quenching). Unlike thiocarbonyl compounds in general, the thioketenes posses low intersystem crossing yields (less than 0.1 in benzene). From the kinetics of the quenching of a series of sensitizer triplets by 1 and 2, the thioketene triplet energies are estimated to be 43 – 44 kcal mol−1.

Photochemistry of Far Red Responsive Tetrahydroquinoxaline-Based Squaraine Dyes

The photochemical and redox properties of two newly synthesized tetrahydroquinoxaline-based squaraine dyes (SQ) are investigated Using femto- and nanosecond laser flash photolysis, pulse radiolysis, and cyclic voltammetry. In acetonitrile and dichloromethane, these squaraines exist its monomers in the zwitterionic form (lambda(max) approximate to 715 nm, epsilon(max) approximate to 1.66 x 10(5) M-1 cm(-1) in acetonitrile). Their excited sin-let states ((1)SQ*) exhibit a broad absorption hand at 480 nm, with singlet lifetimes of 44 and 123 ps for the two dyes. Both squaraines exhibit poor intersystem crossing efficiency (Phi(ISC) < 0.001). Their excited triplet states ((3)SQ*), however, Ire efficiently generated by triplet-triplet energy transfer Using triplet excited 9,10-dibromoanthracene. The excited triplet states of the squaraines dyes exhibit it broad absorption hand at ca. 560 nm (epsilon(triplet) approximate to 4.2 x 10(4) M-1 cm(-1)) and undergo deactivation via triplet-triplet annihilation and ground-state quenching processes. The oxidized forms of the investigated squaraines (SQ(center dot+)) exhibit absorption maxima at 510 and 610 nm.

Photoinduced electron transfer in zinc porphyrin-crown ether supramolecule with Eu3+ electron acceptor in crown void

Photoinduced electron transfer (ET) reactions in the zinc porphyrin-crown ether (ZnPCE) supramolecule, in which one crown ether moiety containing Eu3+ as electron acceptor is covalently linked to zinc porphyrin (ZnP), were studied by flash photolysis. In methanol solutions, highly efficient charge separation occurs via intramolecular ET from (ZnP)-Zn-3 to Eu3+ encapsulated in the crown ether void (k(1) = (3 +/- 1) X 10(3) s(-1)) and intramolecular ET from 3ZnP to uncomplexed Eu2+ (k(2) = (2.5 +/- 0.5) X 10(3) s(-1)). Intermolecular ET from Eu2+ escaped from the crown ether void to ZnP.+ (k(tau) = (4.3 +/- 0.7) X 10(8) M(-1) s(-1)) seems to be the main pathway of charge recombination.

Photophysical properties of the fullerenes, C60 and C70

Photophysical properties of the singlet and triplet states of the fullerenes, C60 and C70, have been investigated in toluene, benzene, hexane and ethanol solutions by pico- and nano-second laser flash photolysis techniques. Lifetimes, extinction coefficients, quantum efficiencies of the formation of the excited states and such properties have been determined. True triplet�triplet absorption spectra of C60 and C70 are reported. C60, unlike C70, is found to undergo photoionization in ethanol solution giving solvated electrons.

Direct and sensitized (energy and electron transfer) geometric isomerization of stilbene within zeolites: a comparison between solution and zeolite as reaction media

The trans- and cis-stilbenes upon inclusion in NaY zeolite are thermally stable. Direct excitation and triplet sensitization results in geometric isomerization and the excited state behavior under these conditions are similar to that in solution. Upon direct excitation, a photostationary state consisting of 65% cis and 35% trans isomers is established. Triplet sensitization with 2-acetonaphthone gave a photostationary state consisting of 63% cis and 37% trans isomers. These numbers are similar to the ones obtained in solution. Thus, the presence of cations and the confined space within the zeolite have very little influence on the overall chemistry during direct and triplet sensitization. However, upon electron transfer sensitization with N-methylacridinium (NMA) as the sensitizer within NaY, isomerization from cis-stilbene radical cation to trans-stilbene occurs and the recombination of radical ions results in triplet stilbene. Prolonged irradiation gave a photostationary state (65% cis and 35% trans) similar to triplet sensitization. This behavior is unique to the zeolite and does not take place in solution. Steady state fluorescence measurements showed that the majority of stilbene molecules are close to the N-methylacridinium sensitizer. Diffuse reflectance flash photolysis studies established that independent of the isomer being sensitized only trans radical cation is formed. Triplet stilbene is believed to be generated via recombination of stilbene radical cation and sensitizer radical anion. One should be careful in using acidic HY zeolite as a medium for photoisomerization of stilbenes. In our hands, in these acidic zeolites isomerization dominated the photoisomerization. (C) 2002 Elsevier Science B.V. All rights reserved.

Effect of chlorine substitution on triplet state structure of thioxanthone: A time-resolved resonance Raman study

Substitution plays an important role in determining the triplet state reactivity. In this paper, we have studied the effect of chlorine substitution on the triplet state structure and the reactivity of thioxanthone (TX). We have employed time-resolved resonance Raman technique to understand the structure of the lowest triplet excited state of 2-chlorothioxanthone (CTX). The experimental findings have been corroborated with the computational results using density functional theory. Akin to the parent compound (TX), coexistence of two lowest triplet states has been observed in case of CTX, which has been substantiated using resonant probe wavelength dependence study. The relative contribution of 3n-pi* to 3 pi-pi* to the equilibrated triplet state has been found to be more for CTX compared to TX suggesting increase in the triplet state reactivity after the substitution. The above observation has been further supported by the flash photolysis experiments. Copyright (C) 2013 John Wiley & Sons, Ltd.

Thermal Degradation Processes in Poly(xylylene sulfides) Investigated by Comparative Direct Pyrolysis MS and Flash Pyrolysis GC/MS Experiments

The thermal degradation processes of two sulfur polymers, poly(xylylene sulfide) (PXM) and poly(xylylene disulfide) (PXD), were investigated in parallel by direct pyrolysis mass spectrometry (DPMS) and flash pyrolysis GC/MS (Py-GC/MS). Thermogravimetric data showed that these polymers decompose with two separate steps in the temperature ranges of 250-280 and 600-650 degrees C, leaving a high amount of residue (about 50% at 800 degrees C). The pyrolysis products detected by DPMS in the first degradation step of PXM and PXD were terminated by three types of end groups, -CH3, -CH2SH, and -CH=S, originating from thermal cleavage reactions involving a series of homolytic chain scissions followed by hydrogen transfer reactions, generating several oligomers containing some intact xylylene sulfide repeating units. The presence of pyrolysis compounds containing some stilbene-like units in the first degradation step has also been observed. Their formation has been accounted for with a parallel cleavage involving the elimination of H2S from the PXM main chains. These unsaturated units can undergo cross-linking at higher temperatures, producing the high amount of char residue observed. The thermal degradation compounds detected by DPMS in the second decomposition step at about 600-650 degrees C were constituted of condensed aromatic molecules containing dihydrofenanthrene and fenanthrene units. These compounds might be generated from the polymer chains containing stilbene units, by isomerization and dehydrogenation reactions. The pyrolysis products obtained in the Py-GC/MS of PXM and PXD at 610 degrees C are almost identical. The relative abundance in the pyrolysate and the spectral properties of the main pyrolysis products were found to be in generally good agreement with those obtained by DPMS. Polycyclic aromatic hydrocarbons (PAHs) were also detected by Py-GC/MS but in minor amounts with respect to DPMS. This apparent discrepancy was due to the simultaneous detection of PAHs together with all pyrolysis products in the Py-GC/MS, whereas in DPMS they were detected in the second thermal degradation step without the greatest part of pyrolysis compounds generated in the first degradation step. The results obtained by DPMS and PSI-GC/MS experiments showed complementary data for the degradation of PXM and PXD and, therefore, allowed the unequivocal formulation of the thermal degradation mechanism for these sulfur-containing polymers.

Flow of metal into the flash gap in the last stages of a plane-strain closed-die forging operation

In closed-die forging the flash geometry should be such as to ensure that the cavity is completely filled just as the two dies come into contact at the parting plane. If metal is caused to extrude through the flash gap as the dies approach the point of contact — a practice generally resorted to as a means of ensuring complete filling — dies are unnecessarily stressed in a high-stress regime (as the flash is quite thin and possibly cooled by then), which reduces the die life and unnecessarily increases the energy requirement of the operation. It is therefore necessary to carefully determine the dimensions of the flash land and flash thickness — the two parameters, apart from friction at the land, which control the lateral flow. The dimensions should be such that the flow into the longitudinal cavity is controlled throughout the operation, ensuring complete filling just as the dies touch at the parting plane. The design of the flash must be related to the shape and size of the forging cavity as the control of flow has to be exercised throughout the operation: it is possible to do this if the mechanics of how the lateral extrusion into the flash takes place is understood for specific cavity shapes and sizes. The work reported here is part of an ongoing programme investigating flow in closed-die forging. A simple closed shape (no longitudinal flow) which may correspond to the last stages of a real forging operation is analysed using the stress equilibrium approach. Metal from the cavity (flange) flows into the flash by shearing in the cavity in one of the three modes considered here: for a given cavity the mode with the least energy requirement is assumed to be the most realistic. On this basis a map has been developed which, given the depth and width of the cavity as well as the flash thickness, will tell the designer of the most likely mode (of the three modes considered) in which metal in the cavity will shear and then flow into the flash gap. The results of limited set of experiments, reported herein, validate this method of selecting the optimum model of flow into the flash gap.

Ultrafine powders of SrTiO3 from the hydrothermal preparation and their catalytic activity in the photolysis of water

Ultrafine powders of SrTiO3 are prepared at 100–150°C by the hydrothermal method, starting from TiO2·xH2O gel and Sr(OH)2 and H2O-isopropanol mixed solvent as the medium, The X-ray diffractograms of the powder show line broadening. The minimum crystallite size obtained ranges from 5 to 20nm with 20% H2O-80% C3H7OH as the reaction medium, as estimated from X-ray half-peak widths and TEM studies. The electron diffraction results indicate high concentration of lattice defects in these crystallites. The optical spectra of the particle suspensions in water show that the absorption around the band gap is considerably broadened, together with the appearance of maxima in the far ultraviolet. Aqueous suspensions of SrTiO3 powders, as such, do not produce H2 or O2 on UV irradiation. After coating with rhodium, H2 and O2 are evolved on illumination. However, the turn over number of O2 is lower than the stoichiometrically expected values from the corresponding values of H2. No correlation of the photocatalytic activity with surface area is observed. The activity of Rh-SrTiO3 slowly deteriorates with extended period of irradiation.

EPR study of trap-centres produced on TiO2 particles during water photolysis

The results of an EPR investigation are presented on the paramagnetic trap-centres produced on hydrothermally prepared TiO2 particles during water photolysis at room temperature under band-gap irradiation. The trapped holes correspond to O− species adjacent to cation vacancies that are formed to compensate the hydroxyl ions in the subsurface layers. The trapped electrons are accounted for as Ti3+ in the conduction band or Ti3+ - adjoining oxygen vacancy to form shallow donor states. Although hole-centres are normally stabler than electron-centres, strongly adsorbed donor molecules reverse the stability. Concentration of hole-centres is increased by the presence of platinum on TiO2 surface and electron-centres are not detected on Pt/TiO2 during water photolysis.

Sacrificial photolysis of water on TiO2 fine powders prepared by the hydrothermal method

The often discussed role of surface hydroxylation of TiO2 particles as an essential characterestics for their photocatalytic activity can be verified by preparing TiO2 powders by hydrothermal method since hydroxylated surface layers will be better retained on these particles formed in superheated water. Thus, fine powders of TiO2 (rutile) with high degree of crystallinity are formed from titanium oxychloride in the mixed solvent of water and 2-propanol at 160–230°C and 20–120 atm. The anatase phase is produced from the same medium when sulfate ion impurity is present, with Image . TiO2 powders are washed free of anions and 2-propanol by ultrafiltration and are Pt mounted by a photochemical method. Aqueous suspensions of both forms of TiO2 neither as such nor after Pt-loading, do not produce H2 on band gap illumination whereas, H2 is generated in presence of hole scavengers such as EDTA, TEOA, sulfite or hypophosphite. The effects of hole scavenger concentration, Pt : TiO2 ratio, particulate suspension density and the nature of hole scavengers on H2 production are presented. Platinised rutile powders are equally active as anatase in sacrificial systems.