2-Benzoylxanthone (BzX) was synthesized, characterized and used as type II photoinitiator (PI) in combination with triethylamine for the polymerization of methylmethacrylate (MMA). The photophysical/photochemical behaviour of the photoinitiator, the involved excited state and the reaction with the amine co-initiator was studied by means of absorption and nanosecond time-resolved absorption spectroscopy. Upon irradiation with 266 or 355 nm laser light, the triplet state (3)BzX* (lambda(max) = 355 nm and 530 nm) was generated as the only transient (lifetime of 22.7 mu s) in nitrogen saturated MeCN solution. (3)BzX* was confirmed through quenching experiments with oxygen, 2-methylbutadiene, perylene and MMA and spectral similarity to benzophenone triplet ((BP)-B-3*). The quantum yield of its formation (phi(T) = 0.8) and the molar absorption coefficient (epsilon = 7500 L mol(-1) cm(-1)) was measured in MeCN. The triplet was photoreduced by triethylamine (TEA) via photoinduced electron/proton transfer giving the corresponding ketyl and alpha-amino ethyl radical (center dot CHMe-NEt2). From the reduction potential of BzX measured via cyclic voltametry (two cathodic peaks at -1.63 V and -1.91 V vs. Ag/AgCl), an exergonic electron transfer reaction results. The ketyl radical resembles the well-known benzophenone ketyl. In conclusion, the triplet (3)BzX* corresponds to an n -> pi* transition localized on the benzoyl substituent and resembles the benzophenone triplet (BP)-B-3* but deviates from the triplet state of xanthone (X-3*). This is supported through DFT/B3LYP calculations, viz., (i) fully ground and triplet state geometry optimizations show that charge and spin densities are localized on the benzoyl group, and (ii) calculation of the electronic transitions via TD-DFT at B3LYP/631G+(d) and PB1BPE/631G+(d) levels of theory shows the local character. Agreement with experiment is better by applying the conductor like polarized continuum model (CPCM) to consider the solvent effect (MeCN).The effectiveness of BzX as photoinitiator for the polymerization of MMA is found to be double that of the unsubstituted xanthone (X). The photopolymerization rates (R-p) were found to be 7.06 x 10(-4) mol L-1 s(-1) in the case of BzX and 3.04 x 10(-4) mol L-1 s(-1) in the case of BX. This is attributed to the fact that triplet (3)BzX* behaves like the benzophenone triplet (BP)-B-3* and deviates from that of X-3*, i.e., it is the localization of the triplet excitation on the benzoyl subunit which renders BzX a good type-II photoinitiator. (C) 2011 Elsevier B.V. All rights reserved.

The aim of this study was to assess the difference of fluorescence signals of cement and calculus using a 405 nm excitation wavelength. A total number of 20 freshly extracted teeth was used. The light source used for this study was a blue LED with a wavelength of 405nm. For each tooth the spectra of calculus and cementum were measured separately. Fluorescence light was collimated into an optical fibre and spectrally analyzed using an echelle spectrometer (aryelle 200, Lasertechnik Berlin, Germany) with an additionally bandpass (fgb 67, Edmund Industrial Optics, Karlsruhe, Germany). From these 40 measurements the median values were calculated over the whole spectrum, leading to two different median spectra, one for calculus and one for cementum. For further statistical analysis we defined 8 areas of interest (AOI) in wavelength regions, showing remarkable differences in signal strength. In 7 AOIs the intensity of the calculus spectrum differed statistically significant from the intensity of the cementum spectrum (p < 0.05). A spectral difference could be shown between calculus and cement between 600nm and 700nm. Thus, we can conclude that fluorescence of calculus shows a significant difference to the fluorescence of cement. A differentiation over the intensity is possible as well as over the spectrum. Using a wavelength of 405nm, it is possible to distinguish between calculus and cement. These results could be used for further devices to develop a method for feedback controlled calculus removal.

The aim of this study was to assess the detection of calculus by Laser Induced Breakdown Spectroscopy (LIBS). The study was performed with an Nd:YVO4 laser, emitting pulses with a duration of 8 ps at a wavelength of 1064 nm. A repetition rate of 500 kHz at an average power of 5 W was used. Employing a focusing lense, intensities of the order of 10(11) W/cm(2) were reached on the tooth surface. These high intensities led to the generation of a plasma. The light emitted by the plasma was collimated into a fibre and then analyzed by an echelle spectroscope in the wavelength region from 220 nm - 900 nm. A total number of 15 freshly extracted teeth was used for this study. For each tooth the spectra of calculus and cementum were assessed separately. Comprising all single measurements median values were calculated for the whole spectrum, leading to two specific spectra, one for calculus and one for cementum. For further statistical analysis 28 areas of interest were defined as wavelength regions, in which the signal strength differed regarding the material. In 7 areas the intensity of the calculus spectrum differed statistically significant from the intensity of the cementum spectrum (p < 0.05). Thus it can be concluded that Laser Induced Breakdown Spectroscopy is well suited as method for a reliable diagnostic of calculus. Further studies are necessary to verify that LIBS is a minimally invasive method allowing a safe application in laser-guided dentistry.

Electron transfer in non-polar media (alkanes, alkyl chlorides) exhibits some essential peculiarities. For instance the reaction of hetero-substituted aromatics with parent solvent radical cations results in the parallel formation of metastable donor radical cations and fragmentation products, in comparable amounts. The fragmentation products originate from a dissociative donor radical cation which decays extremely rapidly, i.e., in a few femtoseconds. This phenomenon is explained in terms of intramolecular dynamic motions which cause changes of the electron density (pi- and n-orbitals) in dependence on the deformation angle between the substituents and the aromatic ring. Hence femtosecond dynamics is reflected in the nanosecond time range and can be observed with real-time spectroscopy. Therefore, the process is named free electron transfer (FET) which corresponds to an unhindered electron jump occurring in the first approach of the reactants. This dynamic controlled process is compared with the classical electron transfer theories which are based on equilibrium kinetics. From the FET mechanism some new aspects for chemical reaction kinetics can be derived (critical review, 69 references).

A number of N,N′-linked benzoannelated isothiazol-3(2H)-one 1,1-dioxides, not available via oxidation of isothiazolium salts, were obtained with good yields by reaction of N-amino heterocycles with 2-chlorosulfonylbenzoyl chloride and evaluated for their inhibitory activity toward human leukocyte elastase (HLE) and acetylcholinesterase (AChE). 2-(Phthalimid-1-yl)-1,2-benzisothiazol-3(2H)-one 1,1-dioxide and 2-(2-methyl-4-oxo-3(4H)-quinazolinyl)-1,2-benzisothiazol-3(2H)-one 1,1-dioxide were found to be inhibitors of HLE and tested as potential precursors of nitrogen-centered radicals using 266 nm laser flash photolysis.

The Letter deals with the photogeneration of thionaphthol triplets and interprets these results by quantum chemical calculations using DFT method.

This work provides an in-depth look at the bimolecular free-electron transfer (FET) from bisubstituted (amine and -CR(2)SiMe(3) groups) aromatic molecules to the solvent radical cations of n-BuCl. Because of the low rotational barriers, the substrates obtain all possible arrangements in solution. The electron jump is an unhindered process that does not require a defined encounter complex. The resulting radical cations show great conformer diversity because they directly inherit the geometry of their mobile precursors. One part of the radical cations is unstable and dissociates instantly, but the other one is metastable (microsecond lifetime). The two substituents reduce the barrier of internal rotation, resulting in stabilization of the otherwise good leaving group -SiMe(3). The amine group governs the reactivity of the system because it receives most of the electron density of the fluctuating highest MOs: primary and secondary amine groups lead to both instant and delayed formation of aminyl radicals; tertiary amines cause the rapid loss of an alpha-H(+) to yield alpha-aminoalkyl radicals.

The electron transfer from various monosubstituted naphthyl derivatives (naphtols, NpOH; naphtylamines, NpNH2; and thionaphtols, NpSH) to parent n-BuCl radical cations was studied by means of pulse radiolysis. The experiments reveal the synchronous and direct formation of two types of transients: the metastable solute radical cation (NpXH(*+), X = heteroatom) and the corresponding heteroatom-containing radical (NpX(*)) in comparable amounts. This is explained in terms of the free (unhindered) electron transfer in nonpolar solvents, which is a bimolecular process reflecting femtosecond time scale events of intramolecular dynamic motions accompanied by significant changes of the electron distribution within the donor molecule.

Laser flash photolysis was used to investigate the oxidation of methionine (Met) and tyrosine (Tyr) residues in methionine-enkephalin (MetEnk, TyrGlyGlyPheMet) by the triplet state of 4-carboxybenzophenone (4CB). Quenching of the 4CB triplet by model amino acids and peptides (Tyr, TyrGly, TyrGlyGly, PheMet, TyrMet, MetTyr, MetEnk and TyrGlyGlyPheLeu) occurred with rate constants close to the diffusion-controlled limit, kq=(1−3)×109M−1s−1. Experimental transient spectra, that were resolved into components, revealed the presence of various electron-transfer intermediates, i.e. ketyl radicals (CBH) and ketyl radical anions (CB−) of 4CB, tyrosyl radicals (TyrO), and (S∴N)+ radical cations. Based on the concentration profiles obtained from spectral deconvolutions, quantum yields of the transients were determined. For the quenchers containing only the tyrosine residue, the quantum yields of the tyrosyl radicals were found to be close to unity and equal to the sum of the quantum yields for the formation of 4CB ketyl radicals and 4CB ketyl radical anions. For quenchers containing both tyrosine and methionine residues, the formation quantum yields of tyrosyl radicals were decreased from the value of 1 to approximately 0.7–0.8 which was equal to the sum of the quantum yields for the formation of CBH and CB−. The above observations are discussed in terms of competitive oxidation reactions involving Tyr and Met followed by the intramolecular electron transfer from the tyrosine residue to the sulfur-centered radical cation on the methionine residue.

In this paper, the bimolecular free (unhindered) electron transfer (FET) from various trityl-containing compounds to the solvent radical cations of n-BuCl is described. In good agreement with the previously studied cases, the FET involving trityl-derived compounds results in the formation of two different types of the radical cation, which undergo the subsequent fragmentation via two alternative reaction channels. This unusual effect is caused by the intramolecular rotational motion in the ground-state molecules around the arrow-marked bond Ar-//-X-CPh 3 (Ar = aromatic moiety; X = S, O, NH, CH 2), since such oscillations are directly connected with the electron distribution within the molecule. An unhindered electron jump from the donor trityl compound to the solvent radical cation, taking place in the subfemtosecond time range, generates the solute radical cation with the inherited geometry and the electron distribution of its precursor. Among the whole variety of produced radical cations, two extreme conformer states can be distinguished, namely, a planar and a twisted state. The planar type represents the structures with minimum energy, whereas the twisted type is destabilized by the increased value of the rotational barrier in the ionized state. The difference in the energetic profiles between planar and twisted radical cations plays a crucial role in their subsequent fragmentation. The planar radical cation follows the thermodynamically favored pathway generating ArX (*) and Ph 3C (+). A distinct part of the twisted radical cation dissociates faster than it relaxes into the more preferable planar conformation and, therefore, produces a thermodynamically unfavorable couple of products: ArX (+) and Ph 3C (*). This fragmentation channel is exclusively caused by FET. The undertaken quantum chemical calculations enable the judgment of the energetics of the different dissociation channels of the radical cations of the trityl derivatives.

The ion-molecule reaction between solvent parent radical cations and hetero group-substituted aromatics in non-polar media shows a characteristic product distribution, which depends strongly on the mobility of the substituent. This electron transfer phenomenon seems to reflect femtodynamics of the donor molecule, which is appearing in the nanosecond time range.

The recently synthesized spiro[cyclohexadiene-dihydroacridines] consisting of perpendicularly arranged aroylcyclohexadiene and N-methyl-dihydroacridine moieties were found to have photochromic properties. The reversible photoisomerization from the spiro compound toward a colored merocyanine caused by C-C bond cleavage In the cyclohexadiene was studied by stationary and time-resolved measurements of their optical spectra, The course of the absorption under UV and visible irradiation, respectively, and HPLC analysis of the photoproducts result in the determination of elicitation energy-dependent quantum yields for the merocyanine formation and, in reverse, the ring closure, as well as degradation. Whereas the thermal back reaction completely recovers the spiro compound (k approximate to 6.8 x 10(-4) s(-1), T = 22 degrees C), degradation of the merocyanine under if radiation at 480 nm has a probability of about 6%. Picosecond-resolved measurements of the fluorescence and the transient absorption show that photoisomerization occurs via the first excited singlet state within 100 ps depending on the activation barrier.

In this paper, the absorption and fluorescence spectra of xanthene, 9-trimethylsilyl xanthene (1) and 9-methyl-9-trimethylsilyl xanthene (2) are presented and discussed with the help of CNDO/S and ab initio calculations. introduction of the trimethylsilyl group at the 9-position of the xanthene molecule considerably shifts the short-wavelength band (246 nm) to the red and reduces the fluorescence quantum yields (Phi(fl) = 0.03 for xanthene, 0.008 for 1 and < 10(-4) for 2 in MeCN) and the fluorescence lifetimes (tau(fl) = 7.4 ns for xanthene, 220 ps for 1 and < 100 ps for 2 in MeCN), while it does not affect seriously the long-wavelength band and the singlet excited state energies (Es = 97.7 kcal/mol for xanthene, 94.5 kcal/mol for 1 and 97.4 kcaVmoI for 2 in MeCN). Ab initio calculations predict a 'roof-like' structure for 1 with folding angles 30 degrees for the So and 20 degrees for the S1 state. Laser (248, 266 and 308 nm) and lamp photolysis (254 nm) of 1 and 2 (MeCN or cyclohexane) results in [1,3]-trimethylsilyl rearrangement into the ortho-position of the xanthene moiety in the sense of a photo-Fries type reaction. The corresponding photo-Fries intermediates (exocyclic cyclohexatrienes: trimethyl-(1H-xanthen-1-yl)-silane, 1CHT and trimethyl-(9-methyl-lH-xanthen-1-yl)-silane, 2CHT) are formed within the 20 ns laser pulse and show absorption spectra peaking up at 4 10 and 403 nm, respectively. Additionally, small amounts of the corresponding 9-xanthyl radicals were detected as a result of the C-Si bond rupture. Using ps-laser flash photolysis (266 nm laser, MeCN) we observed a broad absorption spectrum peaking up at 960 mn and decaying monoexponentially with a lifetime of 130 ps, close to the measured fluorescence lifetime. We assigned therefore this transient to the singlet excited state of 1 (S-1 -> S-n, absorption). We assume the S, state as the origin of the photo-Fries rearrangement, giving via C-Si bond dissociation a singlet gerninal radical pair (9-xanthyl radical(+) (SiMe3)-Si-center dot)- In the next step, the radical pair undergoes predominantly in-cage recombination to the persistent photo-Fries intermediates with high quantum yields (Phi(1CHT) = 0.70 and Phi(2CHT) = 0.50), while to a lesser extent it escapes the solvent cage (Phi = 0.30 for 1) and undergoes typical free radical reactions (e.g., scavenging with O-2). The estimation of the above quantum yields was possible only after determination of the absorption coefficients (8) of the photo-Fries intermediates [epsilon(ICHT)(410 nm) = 16,800 m(-1) cm(-1) and epsilon(2CHT)(403 nm) = 15,900 W cm(-1) using three independent methods; this represents the first example in the literature. (c) 2006 Elsevier B.V All rights reserved.

Two aspects of the chemical repair reaction occurring in the presence of thiols (RSH) were investigated by pulse radiolysis method in aqueous solutions. Kinetics of the repair process of the pyrimidine-derived radicals (Pyr(center dot)) of defined structures was studied for various thiols. The repair rate constants k(repair) were in the range of 1.5 x 10(6) dm(3) mol(-1) s(-1) to 7.6 x 10(7) dml mol(-1) s(-1) depending on the structure of the repaired pyrimidinyl radical. Reactivity of thiyl radicals (RS center dot) towards pyrimidine C5-C6 double bond was investigated. It is now reliably documented that the RS center dot radicals add to the C5-C6 unsaturated bond in pyrimidines (Pyr) with the rate constants k(A) = (1.0-3.0) x 10(7) dm(3) mol(-1) s(-1), whereas the elimination takes place with k(E) = (0.7-2.0) x 10(5) s(-1).

Pyrimidinyl radicals of various structures (Pyr*) were generated in aqueous and alcohol-containing solutions by means of pulse radiolysis to determine the rate constants of their repair reactions by different thiols (RSH = cysteamine, 2-mercaptoethanol, cysteine, and penicillamine): Pyr* + RSH --> PyrH + RS*. C5-OH and C6-OH adduct radicals of the pyrimidines react with thiols with k9 = (1.2-10.0) x 10(6) dm3 mol(-1) s(-1). Similar repair rate constants were found for uracil- and thymine-derived N1-centered radicals, k31 = (1.5-6.1) x 10(6) dm3 mol(-1) s(-1). However, pyrimidine radical anions protonated at their C6 position and C6-uracilyl radicals, with carbonyl groups at their C5 position, react with thiols faster, with k24 = (0.5-7.6) x 10(7) dm3 mol(-1) s(-1) and k14 = (1.4-4.8) x 10(7) dm3 mol(-1) s(-1), respectively. Quantum chemical calculations, at the B3LYP/6-31G(d,p) and self-consistent reaction field polarizable continuum model level point to the combined effects of the energy gap between interacting molecular orbitals, charge distribution within different pyrimidine-derived radicals, and the coefficients of the atomic orbitals as the possible reasons for the differences in the rate constants of repair.

On the bases of picosecond and nanosecond laser flash photolysis with detection by emission and absorption spectroscopy, a quantitative description is given of all deactivation channels of the first excited singlet state of thiophenols ArSH(S(1)) such as fluorescence, intersystem crossing (ISC), chemical dissociation into radicals, and radiation-less internal conversion (IC). For this purpose, the photolysis of thiophenol and its methyl-, methoxy-, and chloro-substituted derivatives was studied in solvents of increasing polarity: 1-chlorobutane, ethanol, and acetonitrile. The fluorescence lifetime of the thiophenols was found to range from some hundreds of picoseconds up to a few nanoseconds, correlating with fluorescence quantum yields between 0.001-0.040, at room temperature. Depending on the substitution pattern of the aromatic ring, the quantum yield of the S-H bond dissociation was found to be between 0.3-0.5, irrespective of the solvent polarity. In laser photolysis, no triplet formation of the investigated compounds could be observed neither by the direct way nor by subsequent sensitization with beta-carotene. As a difference to the total, the radiation-less internal conversion (Phi(IC)>or= 0.5) was found to be the dominating process.

The (free) electron transfer (FET) from electron donor molecules to parent solvent radical cations of alkanes and alkyl chlorides exhibits mechanistic peculiarities that are conditioned by the low polarity of these solvents. Because of the negligible solvation of ions in such systems and the almost complete lack of an activation barrier, the electron jump takes place at the very first encounter of the reactants and, as such, in extremely short times of <or=10(-15) s. Molecular oscillations (deformation, bending) occurring within the femtosecond time domain result directly in significant changes of the pi- or n-electron distribution in the HOMO ground state of the donor molecule, thereby generating a distribution of conformers. This is considered to be a rationale for a possible generation of different product radical cations in the free electron transfer, which exhibit different spin and charge distribution and, consequently different stability. Experimentally, the latter has been verified for aromatic donors, substituted with mobile, i.e., not rigidly fixed heteroatom-centered groups (various phenol type compounds, thiophenols, aromatic amines, benzyltrimethylsilanes etc.). The individually characteristic product distribution could be visualized and quantified by time-resolved spectroscopy in the nanosecond time domain. On the basis of a manifold of experimental data and supported by quantum-chemical calculations, the free electron transfer phenomenon is analyzed and discussed in detail in this summarizing report. The results presented here stand also for a, seemingly paradox, situation in which the products of a diffusion-controlled bimolecular reaction are governed by femtosecond events.