Functional polysiloxanes as photoinitiators for UV-curable compositions: Evidence for a polymer effect

Functional silicone copolymers containing one or several types of pendant ester groups including a benzophenone or a thioxanthone chromophore, and/or a tertiary amine were used to prepare novel photo-initiating systems based on the phenone - amine photogeneration of radicals. The influence of the nature and relative amounts of the functional pendant units on the overall efficiency of the various initiating systems was evaluated by measuring the rate of polymerization of an acrylate based composition. By comparing the results obtained from different combinations of polymeric or low molecular weight reactants, a neat polymer effect inducing a greater initiation efficiency is evidenced, especially when a polysiloxane containing only aromatic carbonyl ester groups is associated with free 4-dimethylaminobenzoic esters as hydrogen donors. Time-resolved spectroscopy performed with thioxanthone functional systems indicates that the favourable effect on the apparent polymerization rate is not correlated with the rate of quenching of the triplet excited state by a tertiary amine. The polymer effect can rather be explained by the microheterogeneity of the distribution of the partners in the complex initiation process.     

Reactive cysteine is protonated in the triplet excited state of the LOV2 domain in Adiantum phytochrome3

Phototropin is a plant blue-light sensor protein that possesses a flavin mononucleotide (FMN) as the chromophore in LOV domains. Its photoreaction is an adduct formation between FMN and a nearby cysteine that takes place in the triplet excited state of FMN. In this communication, we revealed that the reactive cysteine is protonated in the triplet excited state of the LOV2 domain of Adiantum phytochrome3 by means of low-temperature FTIR spectroscopy. Its hydrogen-bonding interaction is strengthened in the triplet excited state, presumably with the FMN chromophore. Such strong interaction drives adduct formation on a microsecond time scale.     

A Thioxanthone Sensitizer with a Chiral Phosphoric Acid Binding Site: Properties and Applications in Visible Light-Mediated Cycloadditions

A chiral phosphoric acid with a 2,2’-binaphthol core was prepared that displays two thioxanthone moieties at the 3,3’-position as light-harvesting antennas. Despite its relatively low triplet energy, the phosphoric acid was found to be an efficient catalyst for the enantioselective intermolecular [2+2] photocycloaddition of β-carboxyl-substituted cyclic enones (e.r. up to 93:7). Binding of the carboxylic acid to the sensitizer is suggested by NMR studies and by DFT calculations to occur by means of two hydrogen bonds. The binding event not only enables an enantioface differentiation but also modulates the triplet energy of the substrates.     

One‐Component Thioxanthone Acetic Acid Derivative Photoinitiator for Free Radical Polymerization

Acetic acid–based thioxanthone (TXCH₂COOH) was synthesized and characterized and used as a photoinitiator for free radical photopolymerization of methyl methacrylate (MMA) in the absence and presence of a tertiary amine (MDEA) in different solvents. Different absorption properties were observed depending on the solvent. Fluorescence and phosphorescence experiments were also carried out successfully. The fluorescence quantum yield was found to be 0.09 and the phosphorescence lifetime was calculated as 138 ms at 77 K. The photoinitiator undergoes efficient intersystem crossing into the triplet state and the lowest triplet state possesses π–π* configuration. Laser flash photolysis experiments show that transient absorption of TXCH₂COOH is similar to the parent thioxanthone and the triplet lifetime was calculated as 2.3 μs at 630 nm.     

The Primary Steps in Excited‐State Hydrogen Transfer: The Phototautomerization of o‐Nitrobenzyl Derivatives

The quantum yield for the release of leaving groups from o‐nitrobenzyl “caged” compounds varies greatly with the nature of these leaving groups, for reasons that have never been well understood. We found that the barriers for the primary hydrogen‐atom transfer step and the efficient nonradiative processes on the excited singlet and triplet surfaces determine the quantum yields. The excited‐state barriers decrease when the exothermicity of the photoreaction increases, in accord with Bell–Evans–Polanyi principle, a tool that has never been applied to a nonadiabatic photoreaction. We further introduce a simple ground‐state predictor, the radical‐stabilization energy, which correlates with the computed excited‐state barriers and reaction energies, and that might be used to design new and more efficient photochemical processes.     

Triplet reactivity and regio-/stereoselectivity in the macrocyclization of diastereomeric ketoprofen-quencher conjugates via remote hydrogen abstractions

Intramolecular excited triplet state interactions in diastereomeric compounds composed of a benzophenone chromophore (ketoprofen) and various hydrogen donor moieties (tetrahydrofuran, isopropylbenzene) have been investigated by laser flash photolysis. The rate constants for hydrogen abstraction by excited triplet benzophenone are in the order of 10(4)-10(5) s(-1), with the highest reactivity for the tetrahydrofuran residue. A remarkable diastereodifferentiation, expressed in the triplet lifetimes of the carbonyl chromophore (e.g., 1.6 versus 2.7 micros), has been found for these compounds. With an alkylaromatic moiety as donor, related effects have been observed, albeit strongly dependent on the length of the spacer. The reactivity trend for the initial hydrogen transfer step is paralleled by the quantum yields of the overall photoreaction. The biradicals, formed via remote hydrogen abstraction, undergo intramolecular recombination to macrocyclic ring systems. The new photoproducts have been isolated and characterized by NMR spectroscopy. The stereochemistry of the macrocycles, which contain up to four asymmetric carbons, has been unambiguously assigned on the basis of single-crystal structures and/or NOE effects. Interestingly, a highly regio- and stereoselective macrocyclization has been found for the ketoprofen-tetrahydrofuran conjugates, where hydrogen abstraction from the less substituted carbon is exclusive; cisoid ring junction is always preferred over the transoid junction. The photoreaction is less regioselective for compounds with an isopropylbenzene residue. The reactivity and selectivity trends have been rationalized by DFT (B3LYP/6-31G*) calculations.     

2,5-dihydroxybenzyl and (1,4-dihydroxy-2-naphthyl)methyl, novel reductively armed photocages for the hydroxyl moiety

Irradiation of alcohols, phenols, and carboxylic acids "caged" with the 2,5-dihydroxybenzyl group or its naphthalene analogue results in the efficient release of the substrate. The initial byproduct of the photoreaction, 4-hydroxyquinone-2-methide, undergoes rapid tautomerization into methyl p-quinone. The UV spectrum of the latter is different from that of the caging chromophore, thus permitting selective irradiation of the starting material in the presence of photochemical products. These photoremovable protecting groups can be armed in situ by the reduction of photochemically inert p-quinone precursors.     

A molecular spring for vision

Light absorption by the visual pigment rhodopsin leads to vision via a complex signal transduction pathway that is initiated by the ultrafast and highly efficient photoreaction of its chromophore, the retinal protonated Schiff base (RPSB). Here, we investigate this reaction in real time by means of unrestrained molecular dynamics simulations of the protein in a membrane mimetic environment, treating the chromophore at the density functional theory level. We demonstrate that a highly strained all-trans RPSB is formed starting from the 11-cis configuration (dark state) within approximately 100 fs by a minor rearrangement of the nuclei under preservation of the saltbridge with Glu113 and virtually no deformation of the binding pocket. Hence, the initial step of vision can be understood as the compression of a molecular spring by a minor change of the nuclear coordinates. This spring can then release its strain by altering the protein environment.     

π+π Cycloaddition between electron deficient π-systems and photoreaction of β,γ-enones: a novel stereoselective entry into angular triquinanes

Cycloaddition of electron deficient partners such as cyclohexa-2,4-dienone and acrylates leading to an efficient synthesis of complex bicyclo[2.2.2]octenones having a β,γ-enone chromophore, and their transformation to angular triquinanes via photoreaction, is described.     

Remote Sensitized Photoisomerization via Through‐Bond Triplet–Triplet Energy Transfer Mediated by a Salt Bridge in a Supramolecular Dyad

A supramolecular dyad, BP‐(amidinium‐carboxylate)‐NBD is constructed, in which benzophenone (BP) and norbornadiene (NBD) are connected via an amidinium‐carboxylate salt bridge. The photophysical and photochemical properties of the assembled BP‐(amidinium‐carboxylate)‐NBD dyad are examined. The phosphorescence of the BP chromophore is efficiently quenched by the NBD group in BP‐(amidinium‐carboxylate)‐NBD via the salt bridge. Time‐resolved spectroscopy measurements indicate that the lifetime of the BP triplet state in BP‐(amidinium‐carboxylate)‐NBD is shortened due to the quenching by the NBD group. Selective excitation of the BP chromophore results in isomerization of the NBD group to quadricyclane (QC). All of these observations suggest that the triplet–triplet energy transfer occurs efficiently in the BP‐(amidinium‐carboxylate)‐NBD salt bridge system. The triplet–triplet energy transfer process proceeds with efficiencies of approximately 0.87, 0.98 and the rate constants 1.8×10³ s^?1, and 1.3×10⁷ s^?1 at 77 K and room temperature, respectively. The mechanism for the triplet–triplet energy transfer is proposed to proceed via a “through‐bond” electron exchange process, and the non‐covalent bonds amidinium‐carboxylate salt bridge can mediate the triplet–triplet energy transfer process effectively for photochemical conversion.     

Oxidative dearomatization and retro-Diels-Alder/Diels-Alder cascade: Synthesis and photoreactions of azepane annulated bicyclo[2.2.2]octenone

Synthesis of azepane annulated bicyclo[2.2.2]octenone via oxidative dearomatization and intramolecular π⁴s?+?π²s cycloaddition has been described. Manipulation of the adduct led to tricyclic compounds having a β,γ-enone chromophore whose photoreaction upon triplet and singlet excitation furnished polycyclic systems having azepane ring systems.     

Accessing the triplet excited state in perylenediimides

Here, we present a strategy designed to permit access to the PDI triplet manifold that preserves the desirable colorfastness and visible light-absorption properties associated with these dyes. To this end, three new Pt(II) complexes each bearing two PDI moieties tethered to the metal center via acetylide linkages emanating from one of the PDI bay positions have been synthesized, structurally characterized, and thoroughly examined by nanosecond laser flash photolysis. Upon ligation, the bright singlet-state fluorescence of the PDI chromophore is quantitatively quenched, and no long wavelength photoluminescence is observed from the Pt(II)-PDI complexes in deaerated solutions. In each of the Pt-PDI chromophores, quantitatively similar transient absorption difference spectra were obtained; the only distinguishing characteristic is in their single-exponential lifetimes (tau = 246 ns, 1.0 micros, and 710 ns). Triplet-state sensitization experiments of "free" PDI-CCH using thioxanthone confirmed the PDI triplet state assignments in each of the Pt-PDI structures.     

Photochemistry of visual pigment chromophore models by ab initio molecular dynamics

Ab initio excited-state molecular dynamics calculations have been performed to study the effect of methyl substitution and chromophore distortion on the photoreaction of different four-double-bond retinal model chromophores. Randomly distributed starting geometries were generated by zero-point energy sampling; after Franck-Condon excitation the reaction was followed on the S1 surface. For determining the photoproduct and its configuration, a simplified approach--torsion angle following--is discussed and applied. We find that chromophore distortion significantly affects the outcome of the photoreaction: with dihedral angles taken from the rhodopsin-embedded 11-cis-retinal chromophore, the reaction rate of the model chromophore is increased by a factor of 3 compared to that of the relaxed chromophore. Also, the reaction proceeds in a completely stereoselective manner involving only the cis double bond and with a minimum quantum yield of 72%. Bond torsion is more effective than methyl substitution for fast and selective photochemistry, which is in agreement with photophysical measurements on rhodopsin analogues. We conclude that apart from the geometric distortions caused by the protein pocket it is not necessary to postulate other specific interactions between the protein and the chromophore to effect the selective and ultrafast photoreaction in rhodopsin.     

Thioxanthone–carbazole as a visible light photoinitiator for free radical polymerization

A thioxanthone (TX) derivative with the additional carbazole chromophore, namely thioxanthone‐carbazole (TX‐C) was synthesized and characterized. The photophysical properties and its efficiency to polymerize methyl methacrylate both in the presence and absence of N,N‐dimethylaniline (DMA) as coinitiator was investigated and compared with that of the commercially available TX. TX‐C was found to display better photophysical properties and in both cases initiate polymerization more efficiently. Detailed real‐time Fourier transform infrared studies revealed that high polymerization rates can be obtained when TX‐C in conjunction with DMA was used. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010     

含二茂铁基偶氮染料的光稳定性研究

利用UV-Vis吸收光谱仪和光化学反应器研究了含二茂铁基偶氮染料及相应的偶氮染料溶液的光降解动力学.在溶液中,它们的光降解反应遵循零级动力学模型.与相应的偶氮染料相比,含二茂铁基的偶氮染料溶液具有较好的光稳定性.这表明,在其分子内存在着有效的从偶氮染料母体到二茂铁基团的分子内三重态-三重态能量传递,上述过程降低了偶氮染料激发三重态的生成,避免了染料的过早降解,提高了其稳定性.     

Simultaneous detection of two triplets: a time-resolved resonance Raman study

Solvents are known to affect the triplet state structure and reactivity. In this paper, we have employed time-resolved resonance Raman (TR3) spectroscopy to understand solvent-induced subtle structural changes in the lowest excited triplet state of thioxanthone. Density functional theory (DFT) combined with the self-consistent reaction field (SCRF) implicit solvation model has been used to calculate the vibrational frequencies in the solvents. Here, we report a unique observation of the coexistence of two triplets, which has been substantiated by the probe wavelength-dependent Raman experiments. The coexistence of two triplets has been further supported by photoreduction experiments carried out at various temperatures.     

Photochemistry of thioxanthones—III. Spectroscopic and flash photolysis study on hydroxy and methoxy derivatives

The spectroscopic properties of 9 oil soluble hydroxy and methoxy thioxanthone derivatives have been examined in various solvents and the data compared to their photopolymerization efficiency and flash photolysis behaviour in solution. Absorption maxima, extinction coefficients, fluorescence and phosphorescence spectra and quantum yields have been measured. Generally, most of the compounds exhibit low fluorescence and high phosphorescence quantum yields except 1-substituted derivatives where intra-molecular hydrogen bonding is involved. These observations are consistent with the high photoreactivity of the molecules occurring via the lowest excited triplet state. Photopolymerization rates of n-butyl methacrylate, using N-diethylmethylamine as co-initiator, correlate to some extent with the absorption maxima and extinction coefficients of the thioxanthones. Transient formation on micro-second flash photolysis is associated with the ketyl radical formed by the lowest excited triplet state of the thioxanthones abstracting a hydrogen atom from the solvent. In the presence of a tertiary amine, a new longer wavelength transient absorption is produced and is assigned to a radical-anion formed by the lowest excited triplet state of the thioxanthones abstracting an electron from the amine. A correlation was observed between the transient absorption due to the radical-anion and the ionisation potential of various amines. Flash photolysis studies in acid and base media confirmed the identity of the radical and radical-ion species. Intra-molecular hydrogen bonding in the α-position to the carbonyl group deactivates both the lowest excited singlet and triplet states of thioxanthone but has little effect on polymerization efficiency. The latter is associated with competition of the carbonyl group with the amine co-initiator for hydrogen bonding and consequent electron abstraction to give an active radical-anion. This is confirmed using micro-second flash photolysis.     

Photophysical Investigations on Photoinitiators with Covalently Linked Thioxanthone Sensitizer Moieties

The photophysical properties of three photoinitiators with a covalently linked thioxanthone sensitizer unit absorbing up to 410 nm were investigated by laser‐flash photolysis and CIDNP spectroscopy. These complementary techniques revealed two competing reaction pathways of the molecular dyads 1 – 3 : i) triplet‐energy transfer from the sensitizer to the morpholine moiety followed by α‐cleavage to yield a radical pair, which initiates radical polymerization, and ii) bimolecular electron transfer from the morpholine to the thioxanthone subunit followed by proton transfer. The relative efficiency of these routes is determined by the triplet energy of the photoinitiator moiety relative to that of the sensitizer.     

Primary processes in the photosensitized polymerization of cationic monomers

A detailed time-resolved laser spectroscopy investigation has been carried out on the electron transfer reactions of substituted thioxanthone derivatives with diphenyliodonium (Ph-I⁺) salts having different metal halide counterions (MX^?_n). Quenching of thioxanthones' triplet state has been followed under various conditions, by changing the number and nature of substituents on the thioxanthone skeletone, using anion with different nucleophilicity and employing different solvents, namely methanol and acetonitrile. A Photosensitization mechanism is proposed involving an electron transfer from thioxanthone to diphenyliodonium salt. The absorption spectra of the thioxanthone's excited state and the formed new transient are recorded and the rate constants of the excited state processes are measured. The triplet state of thioxanthone derivatives has been quenched by cationically polymerizable monomers and the quantum yield of the major processes has been evaluated. Photolytic experiments have been performed to measure the extent of acid formation. Form photopolymerization experiments using different photoinitiating systems, the rate of polymerization and percentage of monomer conversion have been determined. Both the reactivity in the excited states and the nucleophilicity of the anions affect the efficiency of the photopolymerization reaction.     

Wavelength-selective photodenitrogenation of azoalkanes to high-spin polyradicals with cyclopentane-1,3-diyl spin-carrying units and their photobleaching: EPR/UV spectroscopy and product studies of the matrix-isolated species

The photolysis of the mono-, bis-, and trisazoalkanes 1, 2, and 3 in a toluene matrix at 77 K has been studied by EPR and UV spectroscopy. The purpose was to find the optimal conditions for the generation of the corresponding organic high-spin polyradicals (the triplet diradicals D-1, D-2, and D-3, the tetraradicals T-2 and T-3, and the hexaradical H-3) all with localized cyclopentane-1,3-diyl spin-carrying units, connected by m-phenylene (except D-1) as ferromagnetic coupler. Irradiation of these azoalkanes at 333, 351, or 364 nm gave different polyradical compositions. This observed wavelength dependence is due to the secondary photoreaction (photobleaching) of the polyradical intermediate. The photobleaching process has been examined in detail for the triplet diradical D-1, for which pi,pi excitation affords the cyclopentenes 5 instead of the housane 4 (the usual product of the diradical D-1 on warm-up of the matrix). The pi,pi-excited diradical D-1 fragments into a pair of allyl and methyl radicals (the latter was observed by EPR spectroscopy of a photobleached sample), and recombination affords the cyclopentene. Similar photochemical events are proposed for the photobleaching of the tetraradical T-2 and hexaradical H-3, derived from the respective azoalkanes 2 and 3. Thus, photobleaching of the polyradicals competes effectively with their photogeneration from the azoalkane. This unavoidable event is the consequence of spectral overlap between the cumyl-radical pi,pi chromophore of the polyradical and the n,pi chromophore of the azoalkane at the wavelength (364 nm), at which the latter is photoactive for the required extrusion of molecular nitrogen.