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High reflectivity and long-term stability in multilayer mirrors (MLMs) are crucial for extreme ultraviolet (EUV) photolithography. The conventional base stack to reflect 13.5 nm light is a Mo/Si multilayer, which offers a maximum theoretical reflectivity of 75%. In practice, however, the efficiency of the mirror is strongly affected by intermixing between Mo and Si. Diffusion barriers have therefore been adopted, which nevertheless do not provide a perfect solution. In this work, we propose to suppress intermixing in Mo/Si MLMs by substituting the pure Si with a Si compound that can ensure higher thermodynamic stability, while simultaneously providing comparably high EUV theoretical reflectivity, with the net effect of increasing both reflectivity and lifetime. Our theoretical calculations show that rubidium silicide is the most promising material for this purpose. We estimate the optical and thermodynamic properties for each phase of rubidium silicide, and we show that Mo/Rb12Si17 provides the highest theoretical reflectivity, while Mo/RbSi is the most thermodynamically stable. The suppression of intermixing in Mo/RbSi MLMs should lead to a maximum reflectivity at least 2% higher than the best Mo/Si MLMs, integrated with diffusion barriers. The proposed Mo/RbSi MLM solution has the potential to increase the total EUV lithography throughput by ~50%.

The majority of the proteins involved in processing of DNA double-strand breaks (DSBs) accumulate at the damage sites. Real-time imaging and analysis of these processes, triggered by the so-called microirradiation using UV lasers or heavy particle beams, yielded valuable insights into the underlying DSB repair mechanisms. To study the temporal organization of DSB repair responses triggered by a more clinically-relevant DNA damaging agent, we developed a system coined X-ray multi-microbeam microscope (XM3), capable of simultaneous high dose-rate (micro)irradiation of large numbers of cells with ultra-soft X-rays and imaging of the ensuing cellular responses. Using this setup, we analyzed the changes in real-time kinetics of MRE11, MDC1, RNF8, RNF168 and 53BP1—proteins involved in the signaling axis of mammalian DSB repair—in response to X-ray and UV laser-induced DNA damage, in non-cancerous and cancer cells and in the presence or absence of a photosensitizer. Our results reveal, for the first time, the kinetics of DSB signaling triggered by X-ray microirradiation and establish XM3 as a powerful platform for real-time analysis of cellular DSB repair responses.

Carbon contamination on extreme ultraviolet (EUV) optics has been observed in EUV lithography. In this paper, we performed in situ monitoring of the build-up and removal of carbon contamination on Mo/Si EUV multilayers by measuring the secondary electron yield as a function of primary electron energy. An electron beam with an energy of 2 keV was used to simulate the EUV radiation induced carbon contamination. For a clean EUV multilayer, the maximum secondary electron yield is about 1.5 electrons per primary electron at a primary electron energy of 467 eV. The maximum yield reduced to about 1.05 at a primary electron energy of 322 eV when the surface was covered by a non-uniform carbon layer with a maximum thickness of 7.7 nm. By analyzing the change in the maximum secondary electron yield with the final carbon layer thickness, the limit of detection was estimated to be less than 0.1 nm. (C) 2010 Elsevier B. V. All rights reserved.

The photo-induced oxidation of NADH and its model compound, 1-benzyl-1,4–dihydronicotinamide (BNAH), by N,N′-dimethyl-4,4′-bipyridinium dichloride (methyl viologen: MV++) was studied in homogeneous and in micellar solution. In the overall, slightly endoergonic, reaction: NADH + 2 MV+ + + hv NAD+ + 2 MV +• + H+ the dihydropyridines lose a hydride equivalent via a stepwise e,H+,e sequence in which only the initial one-electron transfer requires photostimulation. It is shown that the singlet-excited state of the dihydropyridine is the photoreactive intermediate in this initial electron transfer process and that the overall quantum yield depends critically on the ratio of the rates for ionic dissociation and back-electron transfer in the encounter complex between this excited state and MV+ + molecules. This ratio can be influenced dramatically by changing the solvent medium. In aqueous solution (for NADH) and especially in anionic micellar solution (for BNAH) strong ground state association between MV++ and the dihydropyridines occurs. Evidence for this association is provided by the appearance of a long-wavelength charge transfer absorption, as well as by the large deviations from a Stern-Volmer behaviour observed in the quenching of the dihydropyridine fluorescence by MV++. The ground state complexation appears to hamper ionic dissociation of the excited complex. This is evident from the low quantum yield for photooxidation of such systems, upon excitation of the dihydropyridine chromophore (365 nm), and the complete absence of such oxidation upon their excitation within the charge transfer absorption (546 nm). The data obtained are shown to allow for a complete thermodynamic analysis of the Gibbs free-energy changes involved in loss of a hydride equivalent from a 1,4-dihydropyridine via a stepwise e,H+,e mechanism. The consequences of this analysis for the mechanism of reactions in which 1,4-dihydropyridines transfer a hydride equivalent to various substrates under thermal conditions are discussed.

We report a two-step synthesis for N-aryl- and N-alkyl-substituted 4-piperidones, in which the N substituent can easily be varied. A number of intramolecular donor-acceptor systems was synthesized from these piperidones by conversion of the carbonyl functionality. The influence of the N-aryl donor on the electronic absorption and fluorescence spectra was investigated systematically. It was concluded that some systems can be used as efficient fluorescent probes with a high sensitivity for solvent polarity.

The mechanism of exciplex formation of two donor-bridge–acceptor compounds, both containing a strong donor - acceptor pair interconnected by a piperidine ring, was studied in nonpolar solvents as a function of temperature. Fluorescence spectra at low temperature in methylcyclohexane show that in both compounds the exciplex is formed via the harpooning mechanism, i.e. long-range charge separation leading to an extended charge-transfer state, followed by Coulomb-induced folding to the exciplex state. The kinetic link between these two emissive states could be established by studying their transient fluorescence curves. Furthermore, the activation energy of the folding process was estimated from the decay rates of the extended charge-transfer state in various solvents. An interpretation of the observed activation energies in terms of Kramers theory is presented, which gives an indication of the relative importance of internal steric effects and solvent viscosity on the dynamics of the folding process.

The incorporation of fullerene-C60 and of C60 adducts into Triton X-100 (TX100) micelles and into liposomes made of phosphatidyl-ethanolamine from Escherichia coli has been studied. Incorporation curves and UV-Vis absorption spectra for the liposomes are presented. Flash photolysis experiments in TX100 micelles in the presence of the electron donors N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD) and cytochrome c resulted in the observation of oxidized transient species.

The condensation of amines of the general type H2N-(CH2)n-D-(CH2)m-NH2 (D is an aromatic system with electron donor properties) with 1,2:4,5-benzene-tetracarboxylic dianhydride or with 1,8:4,5-naphthalene-tetracarboxylic dianhydride under high dilution conditions is shown to lead to the formation of electron donor(D)-acceptor(A) cyclophanes of the type containing the 1,2:4,5-benzene-tetracarboxylic diimide or the 1,8:4,5-naphthalene-tetracarboxylic diimide group as an electron acceptor. The fragmentation pattern of the cyclophanes upon electron impact is studied and found to agree completely with the proposed structures.

Synthesis of enamines 2a-f, derived from α-tetralones 1a, 1b, is described. Steric effects in the formation of these enamines and in the spectral properties of the corresponding 2-substituted derivatives are discussed.

A kinetic study of the reduction of the 10-methyl-9-phenylacridinium ions (AcPh⊕) by 1-benzyl-1,4-dihydronicotinamide (PyH2) is presented. The primary kinetic isotope effect (kH/kD = 4.5) of this reaction equals the product isotope partitioning observed upon reduction with mono C-4 deuterated 1-benzyl-1,4-dihydronicotinamide (PyHD). Furthermore, the rate constant for the reduction decreases upon increasing the solvent polarity. This behaviour contrasts with that reported for the reduction of the 10-methylacridinium ion (AcH⊕) where acceleration due to increased solvent polarity and significant discrepancies between the kinetic isotope effect and the product isotope partitioning have been observed. Reinvestigation of the latter process by means of kinetic measurements and via a competition experiment, involving reduction with equal amounts of PyH2 and the dideuterated species (PyD2), shows that the kinetic data and especially the kinetic isotope effect data are easily subject to experimental error. Thus it is shown that an intolerable inaccuracy is introduced if the value of kH/kD is determined by comparing the rate of reduction with PyH2 with that of PyHD rather than by comparing those of PyH2 and PyD2. Furthermore, the kinetic data in protic solvents for AcH⊕, but not for AcPh⊕, are found to be disturbed by a reversible side reaction. This reaction is thought to involve formation of a covalent adduct by attack of PyH2 at the electrophilic C-9 position of AcH⊕. The less electrophilic nature of this position in AcPh⊕ prevents such adduct formation and leads to uncomplicated kinetic behaviour in all the solvent systems investigated.

Purpose: Radiographic parameters of mandibular trabecular bone structure between 67 subjects having hypodontia and those without were studied on digital panoramic radiographs. Materials and Methods: Three regions of interest (ROI) were defined: the ascending ramus, apical of the mandibular molar and mesial of the first mandibular molar. The effects of the presence of hypodontia and the ROI on the mandibular trabecular bone structure were tested for statistical significance by means of multivariate analysis. Results: Radiographic parameters of trabecular bone architecture were found to differ between various regions of the mandible (p = 0.000), but not between the group of hypodontia subjects and their controls (p = 0.23). There was no interaction effect between the ROIs and the two groups (p = 0.79). For people having hypodontia, some directional parameters of trabecular bone have a reverse correlation with the number of missing teeth. The fractal dimension and the number and perimeter of white segments in the binarized image correlate positively with the number of congenitally missing teeth. Conclusions: A limited number of parameters of radiographic mandibular trabecular bone structure correlate with the number of missing teeth. However, a markable difference in radiographic parameters of mandibular trabecular bone structure between hypodontia and non- hypodontia subjects could not be demonstrated.

Localized induction of DNA damage is a valuable tool for studying cellular DNA damage responses. In recent decades, methods have been developed to generate DNA damage using radiation of various types, including photons and charged particles. Here we describe a simple ultrasoft X-ray multi-microbeam system for high dose-rate, localized induction of DNA strand breaks in cells at spatially and geometrically adjustable sites. Our system can be combined with fixed- and live-cell microscopy to study responses of cells to DNA damage.

Femtosecond transient absorption spectroscopy was employed to determine quantitatively the ultrafast S 1 -T 1 intersystem crossing in a 2-substituted 9,10-anthraquinone derivative (3), k isc = 2.5 × 10 12 s -1 . Notwithstanding this rapid process, photoexcitation of dyad 1 is followed by competition between intersystem crossing and intramolecular charge separation, the latter leading to a short-lived (2 ps) singlet charge-transfer (CT) state. The local triplet state itself undergoes slower charge separation to populate a relatively long-lived (130 ns) triplet CT state. An earlier report about the formation of an extremely long-lived CT state (>900 μs) in 1 was found to be erroneous and was related to the sacrificial photo-oxidation of the dimethylsulfoxide solvent used in that study. Finally, some important criteria have been formulated for future experimental validation of "unusually long-lived" CT states.

This review presents some of the efforts that have been made over the last decades to produce systems in which photo-excitation leads to one or more intramolecular electron transfer events ultimately resulting in a charge-transfer (CT) excited state with a relatively long lifetime. This process is generally considered as a mimic of natural photosynthesis and is not only of relevance in relation to solar energy conversion but also in relation to perspectives such as molecular information storage, molecular electronics, and molecular photonics. A long-lived CT state in general may be considered as a weakly coupled radical (ion)pair and in this review we focus especially on the consequences of the eventual electron spin correlation in that radical (ion)pair. If substantial spin–spin interaction is still present, such as in compact dyads, CT states can be assigned pure singlet or triplet configurations (1CT, 3CT) and as we demonstrate this configuration has significant influence on the CT lifetime because charge recombination from 3CT is spin forbidden. For small spin–spin interaction such as is typical for CT states in which the radical sites are further removed from each other – e.g., in triads, tetrads, etc., – rapid interconversion of 1CT and 3CT becomes possible especially via a hyperfine interaction (HFI) driven mechanism. This HFI driven mechanism is strongly influenced by external magnetic fields, which allows sensitive detection of the actual spin–spin interaction via magnetic field effects on the electron transfer kinetics, as well as via time-resolved EPR and field-dependent CIDNP. Examples of such studies on artificial multichromophoric electron transfer systems are presented and the results are discussed.

Photoinduced electron transfer is a widely applied method to convert photon energy into a useful (electro)chemical potential, both in nature and in artificial devices. There is a continuing effort to develop molecular systems in which the charge-transfer state, populated by photoinduced electron transfer, survives sufficiently long to tap the energy stored in it. In general this has been found to require the construction of rather complex molecular systems, but more recently a few approaches have been reported that allow the use of much more simple and relatively small electron donor-acceptor dyads for this purpose. The most successful examples of such systems seem to be those that apply "electron spin control" to slow down the spontaneous decay of the charge-transfer state, and these are reviewed in this minireview, with a discussion of the underlying principles and a critical evaluation of some of the claims made with regard to using a pronounced "inverted-region effect" as an alternative method to prolong the lifetime of charge-transfer states.

The target donor-acceptor compound forms an acridinium-like, locally excited (LE) singlet state on illumination with blue or near-UV light. This LE state undergoes rapid charge transfer from the acridinium ion to the orthogonally sited mesityl group in polar solution. The resultant charge-transfer (CT) state fluoresces in modest yield and decays on the nanosecond time scale. The LE and CT states reside in thermal equilibrium at ambient temperature; decay of both states is weakly activated in fluid solution, but decay of the CT state is activationless in a glassy matrix. Analysis of the fluorescence spectrum allows precise location of the relevant energy levels. Intersystem crossing competes with radiative and nonradiative decay of the CT state such that an acridinium-like, locally excited triplet state is formed in both fluid solution and a glassy matrix. Phosphorescence spectra position the triplet energy well below that of the CT state. The triplet decays via first-order kinetics with a lifetime of ca. 30 micros at room temperature in the absence of oxygen but survives for ca. 5 ms in an ethanol glass at 77 K. The quantum yield for formation of the LE triplet state is 0.38 but increases by a factor of 2.3-fold in the presence of iodomethane. The triplet reacts with molecular oxygen to produce singlet molecular oxygen in high quantum yield. In sharp contradiction to a recent literature report, there is no spectroscopic evidence to indicate the presence of an unusually long-lived CT state.

In the recent literature a simple 9-aryl-acridinium ion was claimed to undergo an intramolecular, photoinduced charge shift to produce an extremely long-lived and very high energy charge-transfer state. The possible consequences of this observation are discussed and the tenability of the claims made is investigated via time resolved spectroscopy of a closely related system with spectroscopic characteristics allowing more solid identification of the actual photophysical events taking place. From the results obtained it appears likely that the long-lived species observed earlier in solution cannot be charge transfer in nature but must instead be identified as the lowest triplet state of the acridinium chromophore.

A novel perylene imide and oligo-pentaphenyl bisfluorene containing molecule is shown to undergo electron transfer to form an emissive charge transfer state in di-benzyl ether and THF. At the single molecule level in a PMMA film, fluorescence spectra characteristic of both emissive states (locally excited and charge transfer) are observed with 44% of the molecules studied showing switching between the two states. These results demonstrate that charge transfer fluorescence from single molecules can be used to report on the properties and dynamics of a molecule’s immediate surroundings or nano-environment.

The combination of transient absorption and fluorescence experiments performed on a femtosecond to nanosecond time scale was used to characterize the electron-transfer process in mono- (N1P1) and multichromophoric (N1P3) triphenylamine core dendrimers carrying one and three peryleneimide electron acceptor chromophores, respectively. Comparison of the monochromophoric N1P1 to the multichromophoric N1P3 allowed us to investigate the influence of the number of chromophores upon the electron transfer kinetics. The solvent effect on the electron transfer process was investigated by comparing the results obtained in the highly polar solvent benzonitrile with those in the less polar solvents as diethyl ether, 2-methyltetrahydrofuran, tetrahydrofuran, and toluene. By means of fluorescence anisotropy and transient absorption anisotropy experiments, the occurrence of energy hopping in the multichromophoric N1P3 has been demonstrated. Furthermore singlet−singlet annihilation, observed earlier in a model compound wit...

A long-lived intramolecular charge-separated state in an electron-donor-acceptor molecule is characterized by time-resolved visible and infrared absorption spectroscopy. Bands that can be assigned to the negatively charged acceptor chromophore can be clearly observed in the time-resolved IR spectrum. In contrast, the electronic absorption spectrum of the radical anion lacks characteristic absorption bands. The IR spectra can be interpreted on the basis of harmonic vibrational frequencies and intensities calculated using B3LYP ab initio methods for model systems of the separate electron donor and acceptor units. This result indicates that the charge-separated state, in spite of the small distance between electron donor and acceptor, can be represented well as an intramolecular radical ion pair.

A new family of bidentate phosphine ligands based on the biscarbazole backbone has been synthesized and applied in the ruthenium- and rhodium-catalyzed asymmetric hydrogenations of methyl acetoacetate and dimethyl itaconate. The nitrogen atoms in these BICAP ligands allow facile introduction of substituents providing structurally similar, but electronically different ligands, which were used to fine-tune these reactions to 98% and 55% ee, respectively.

Triplet-sensitized generation of a long-lived intramolecular charge-separated excited state is described in an electron donor-acceptor molecule with a short distance between the donor and the acceptor. Time-resolved UV-Vis optical absorption spectroscopy shows that the lifetime of this triplet excited state is 1.4 micros in acetonitrile at 298 K, i.e. five orders of magnitude greater than that of the corresponding singlet charge-separated state. In slightly less polar alkane nitrile solvents, the local and CT triplet states coexist, which allows determination of their relative energies.

Investigation of the photophysical and photochemical processes that occur in single molecules by means of fluorescence microscopy has rapidly developed over the last decade.[1,2] We have focused on the photophysics of synthetic multichromophore systems and have shown evidence for intramolecular (excitation) energy migration, that is, energy hopping[3,4] and directional F%rster energy transfer.[5] Photoinduced electron transfer is another fundamental physical process, but it is more difficult to monitor by single-molecule spectroscopy. Electron transfer is usually an efficient mechanism for fluorescence quenching which leaves no signal to be detected at the single-molecule level. In some special cases, chargetransfer fluorescence may be observed after photoinduced electron transfer,[6] but its quantum yield is usually quite low and therefore less suited for single-molecule detection. Despite this difficulty some reports on forward electron transfer in single molecules exist in the literature. Interfacial electron transfer from a single molecule of a cresyl violet dye to the indium–tin oxide surface on which it was physisorbed was shown by Lu and Xie.[7] Weaker fluorescence and a decreased fluorescence lifetime with respect to the same molecule on a glass surface demonstrated the occurrence of electron transfer. In a similar way electron transfer between a guanine base and a dye linked to a DNA molecule could also be monitored, and conformational changes of DNA molecules were observed.[8–10] Intramolecular electron transfer in a synthetic system was recently demonstrated by Zang et al.[11] in a compound in which a perylenediimide dye that serves as the electron acceptor upon excitation is connected to an aniline donor. On a sufficiently polar substrate, no fluorescence from single molecules could be observed, but after protonation of the amine by treatment of the sample with hydrogen chloride, the molecules could be visualized. This experiment nicely dem

A highly flexible molecular tweezer, (aqua)(2,6-bis[(dimethylamino)methyl]-4-{3-[(1-pyrenyloxy)methyl]-5-(trimethylsilyl)benzyloxy}phenyl)palladium(II) tetrafluoroborate (1a), constructed from a para-hydroxy-functionalized NCN-pincer palladium(II) complex {NCN−OH = [2,6-(Me2NCH2)2C6H2-4-OH]}, a pyrenoxy unit and a xylyl spacer, was synthesized. X-ray analysis of the structure of 1-[3-({3,5-bis[(dimethylamino)methyl]-4-(trimethylsilyl)phenyloxy}methyl)-5bromobenzyloxy]pyrene (10), a precursor ligand of 1a, showed a completely flattened conformation in the solid state, imposed by favorable intramolecular π-stacking interactions. The molecular structure of 1a in solution proved to be fluxional on the NMR timescale. UV/Vis and (time-resolved) fluorescence spectroscopy revealed that, in spite of the high flexibility, the catalyst and pyrenoxy units are in close proximity because of cation−π interactions. This was further supported by conformer searches using molecular mechanics (MM2). The binding affinity of picric acid towards the pyrenoxy moiety was studied by 1H NMR titrations, revealing association constants Ka in the order of 101 to 102M−1. Tweezer 1a shows small rate enhancements in aldol condensations of aromatic aldehydes with methyl isocyanoacetate compared to (aqua)(2,6-bis[(dimethylamino)methyl]4-{[3-(trimethylsilyl)benzyloxy}phenyl)palladium(II) tetrafluoroborate (2), which lacks the pyrenoxy unit. The tweezer was immobilized on a G2-carbosilane dendrimer, generating the dendritic (aqua)(G2-carbosilane)palladium complex 1b, showing a decreased catalytic activity compared to 1a. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)

The light generating mechanism of a series of light emitting diodes with electron donor-bridge-acceptor systems (D-b-A) as the emitting species was examined by constructing model diodes based on small organic molecules (OLEDs) as well as on molecularly doped electroactive (poly-N-vinylcarbazole, PVK) and insulating (polystyrene, PS) polymers (PLEDs). The direct electrogeneration of an intramolecular charge-transfer (CT) fluorescence of the donor-bridge-acceptor systems occurred readily in OLED devices with a D-b-A system as the emissive layer. In diodes with PS as the host matrix, hole-injection and electron-injection occurred directly in the D-b-A molecules residing close to the anode and the cathode, respectively. In the PVK diodes, hole-injection occurred primarily into PVK and the positive charge carrier was subsequently trapped on the D-b-A molecule, whereas electron-injection at the cathode side occurred directly into the D-b-A molecules. Charge-hopping between neighboring molecules then occurred until a hole and electron resided on the same molecule, which is equivalent to the formation of the CT excited state, and which finally relaxed by intramolecular charge recombination under the emission of CT fluorescence.

The crystal structures of five N-arylpiperidin-4-one derivatives 2P2, 3P2, 5P2, 1P3, and 2P3 are presented (Fig. 2 and Tables 1–5) and discussed together with the derivatives 1P2 and 4P2 published previously. In all but one structure, 1P2, the aryl group is in an equatorial position. The piperidine ring adopts a normal chair conformation. In 1P2, the piperidine ring central CC bonds are significantly elongated, which is consistent with the idea that through-bond interaction is more pronounced in the axial conformation. Through-bond interaction also influences the pyramidalization at the piperidine C(4)-atom in such a way that a strong interaction is directing the ethylene C-atom C(9) into the axial direction.

The crystal structures of seven N-aryltropan-3-one (=8-aryl-8-azabicyclo[3.2.1]octan-3-one) derivatives 1T1, 2T1, 2T2, 3T2, 5T2, 2T3, and 3T3 are presented (Fig. 2 and Tables 1–5) and discussed together with the derivatives 1T2 and 4T2 published previously. The piperidine ring adopts a chair conformation. In all structures, the aryl group is in the axial position, with the plane through the aryl C-atoms nearly perpendicular to the mirror plane of the piperidine ring. The through-bond interaction between the piperidine ring N-atom (one-electron donor) and the substituted exocyclic CC bond (acceptor) not only elongates the central CC bonds of the piperidine ring but also increases the pyrimidalization at C(4) of the piperidine ring. Flattening of the C(2)–C(6) part of the piperidine ring decreases the through-bond interaction.

The nonradiative decay pathways are studied of the 1- cyano- 4- cyclohexylidenenaphthalene chromophore and electron donor acceptor compounds containing the same chromophore. It is argued that the rapid decay at room temperature of the locally excited singlet state of the electron acceptor chromophore involves twisting of the double bond via a low barrier, and a polar transition state. In non- polar solvents the charge transfer excited states also decay mostly via a thermally activated decay process, possibly via a similar pathway as the locally excited state of the acceptor unit. In more polar solvents, decay via direct charge recombination to the ground state obeys the energy gap law, but in contrast to the prediction of semiclassical Marcus theory an activation energy of 1 - 2 kcal mol(-1) is found.

International audience; Although luminescent complexes of lanthanide ions and organic ligands have been studied intensively, relatively little attention has been paid to the natural (or ‘radiative’) lifetime of the lanthanide centered luminescent state in these systems. Here, the applicability of the well-known Judd–Ofelt theory to the emissive properties of Eu3+ complexes is investigated. Moreover, it is demonstrated experimentally that the radiative lifetime of the 5D0 excited state of Eu3+ can be calculated directly from its corrected emission spectrum, without using Judd–Ofelt theory. We also discuss briefly the possibility of finding the natural lifetimes of lanthanide ions other than Eu3+.

In electron donor–acceptor systems containing a nonrigid bridge, geometrical relaxations can occur after photoinduced charge separation under the influence of attractive Coulomb forces (harpooning mechanism). The investigated systems contained an aniline moiety as donor and a substituted naphthalene as acceptor, either one or both being substituted with large aliphatic wings. Two different types of large amplitude motions were identified: a rotation around a single bond leading from the fully extended charge transfer (ECT) conformer to an intermediate charge transfer (ICT) conformer, and the folding of the bridging piperidyl ring forming a compact charge transfer (CCT) state. To elucidate the effects of solvent viscosity and permittivity on these two relaxation processes, the time profiles of the fluorescence from the CCT state were recorded as functions of temperature (283

Efficient intramolecular triplet-triplet energy transfer from an N,N-dialkylanilino donor to an alkene acceptor was observed in 4-(4-(propan-2-ylidene)cyclohexyl)-N,N-dimethylaniline (2) and 1-phenyl-4-cyclohexylidenepiperidine (4) using nanosecond transient absorption spectroscopy and time-resolved microwave conductivity. This enables the unequivocal assignment of the absorption spectrum of the first singlet excited state of the N,N-dialkylanilino chromophore in 4-cyclohexyl-N,N-dimethylaniline (1), 2, 1-phenyl-4-cyclohexylpiperidine (3) and 4. It has a maximum positioned between 600 and 635 nm.

Sensitised excitation of the DMN[3]DCME donor[bridge]acceptor system allows population of a triplet CT state with a microsecond lifetime in polar solvents. This is more than five orders of magnitude longer than the lifetime of the corresponding singlet CT state which is populated exclusively upon direct excitation. The picture indicates the spin-forbidden nature of the charge recombination between the acceptor (red) to donor (green) moieties.

The photophysical properties of a bicyclohexylidene (1DA) and a bicyclohexyl (2DA) substituted with an anilino electron donor and a dicyanoethylene electron acceptor have been studied. Quenching of local donor emission is observed for these compounds as well as quenching of the "pseudo-local" acceptor emission. Transient absorption spectra show dialkylanilino-type radical-cation and dicyanoethylene-type radical-anion absorptions. These results show that intramolecular charge separation takes place in 1DA and 2DA. This was corroborated by time-resolved microwave conductivity measurements from which large excited-state dipole moments were found for both 1DA and 2DA. Time-resolved fluorescence spectroscopy revealed that in the charge-separated state in cyclohexane for 2DA, molecular folding takes place on a nanosecond timescale. For 1DA in cyclohexane, either charge separation takes place in a (fully) folded conformation or very rapid (subnanosecond timescale) folding takes place subsequent to charge separation. In addition to this difference in conformational behavior, the presence of the exocyclic double bond between the cyclohexyl-type rings results in efficient quenching of the anilino donor triplet state and acceleration of the charge recombination rate by a factor of 20.

Trifunctional electron donor−donor−acceptor molecules are described in which photoinduced charge separation, D2−D1−A* → D2−D1+−A-, is followed by a charge migration step D2−D1+−A- (CS1) → D2+−D1−A- (CS2), leading to a relatively long-lived charge-separated state. The rate of the charge migration process could be determined in a range of solvents of low polarity. In benzene and dioxane, reversibility of the process allowed the determination of the free energy difference between CS1 and CS2. The relative energy of the CS2 state is much lower than expected from simple electrostatic models. An increase of the charge migration rate was found with increasing solvent polarity within a series of alkyl ethers or alkyl acetates. However, an apparent preferential stabilization of the CS1 state in acetates relative to ethers leads to discontinuities in the solvatochromic shift behavior of the CT fluorescence from the CS1 state, and in the increase of the charge migration rate as a function of dielectric constant. In ...

The electron transfer kinetics of several completely rigid dyads and triads which contain N,N-dimethylaniline (DMA) and dimethoxynaphthalene (DMN) as possible donors and the dicyanovinyl group (DCV) as acceptor were studied by means of (sub)picosecond time-resolved transient absorption spectroscopy. In the dyads DMN[n]DCV, the rate of charge separation decreases exponentially with the number of σ-bonds n in the bridge, the ‘‘damping factor’’ being 0.8 per bond in tetrahydrofuran solvent. In the triads DMA[4]DMN[8]DCV, the primary electron transfer from DMN to DCV occurs in solvents of low and medium polarity within 10 ps in both isomers (syn and anti). The rates of the secondary electron transfer step (formation of the fully charge separated state, DMA+[4]DMN[8]DCV−) and the following deactivation processes depend, however, strongly on the conformation. In acetonitrile, the primary electron transfer involves the two donor groups yielding preferentially DMA+[4]DMN−[8]DCV. In the anti-conformer this state is fairly long-lived; in the syn-conformer, however, it decays rapidly, in part to locally excited triplet states.

The rate of photoinduced charge separation is measured as a function of solvent for four donor (D)−bridge−acceptor (A) systems: DMN[10]DCV, DMN[10nb]DCV, DMN[10cy]DCV, and DMAN[10cy]DCV. In the first three members of this series, the D/A pair is kept constant and contains the strong dimethoxynaphthalene (DMN) donor which enables detection of electron transfer over a wide range of solvent polarity. In the fourth member, DMN is substituted by a dimethoxyanthracene (DMAN) unit, which decreases the driving force for photoinduced charge separation by about 0.58 eV and thereby limits the occurrence of electron transfer to polar solvents. In all systems the bridge is held at a length of 10 σ bonds. The configuration of the bridge is, however, varied to increase its bending in the series, which leads to center-to-center D/A distances decreasing from 13.4 A in the first system to 9.54 A in the second, and 7.50 A in the latter two. In DMN[10nb]DCV, the rate of intramolecular charge separation over 9.54 A is always...

The highly solvatochromic probe “Fluoroprobe”, which fluoresces in a wide range of the visible spectrum, was covalently attached to silica and alkylated silica particles to study solvent accessibility and surface micropolarity. A two step attachment method was used which was found to lead to a quite homogeneous distribution of the probes without extensive local clustering. On silica the probe experiences a local dielectric constant of ϵ = 6, while its solvent accessibility is ca. 30%. On alkylated silica this reduces to ϵ = 3.4 and ca. 23% respectively.

The application of fluorogenic probe molecules for in situ monitoring of radiation-induced polymerisation is illustrated by measurements on the polymerisation of methylmethacrylate in a cobalt gamma-ray source. The probe molecules fluoresce only when they are incorporated into growing polymer chains. The fluorescence intensity is therefore a measure of the degree of polymerisation. Changes in the fluorescence spectrum can in addition be used as a monitor of the changes occurring in the rigidity of the medium as polymerisation proceeds. The method is particularly suitable for studying the abrupt changes occurring during the `gel' or `Trommsdorff' phase of polymerisation.

Laser pulse photoexcitation of rigid donor–spacer–acceptor molecules in various solvents leads to chemically induced dynamic nuclear polarization (CIDNP) of the ground state species. This polarization is created during the simultaneous charge recombination of the intermediate ion pairs via singlet and triplet pathways. The different channels yield polarizations with opposite signs and equal magnitudes and are separated in time because the triplet contribution appears delayed. Their time dependence yields the triplet decay parameters. The CIDNP intensities depend strongly on the spacer length and on the dielectric constant of the medium. They are explained in terms of the radical pair theory combined with the known dependence of rate constants on energy gaps, and provide the exchange interaction in the ion pairs.

The fluorescence of four semiflexibly bridged donor−bridge−acceptor systems was studied with a streak camera system. The information-rich streak data make a detailed analysis possible of both the kinetics and the spectral changes that take place. From the analysis, it follows that all compounds studied undergo a photoinduced harpooning process in which long-range electron transfer creates an extended charge-transfer (ECT) species, which converts to a compact charge-transfer (CCT) species due to Coulombic attraction. Both the ECT and CCT species display CT fluorescence. The harpooning process occurs even in solvents of medium polarity (e.g., diethyl ether), whereas it has previously been assumed for related compounds that this process only occurs in the gas phase and in nonpolar alkane solvents.

The likelihood of an infinitely long-lived, charge-shift state being formed by the target compound is re-assessed in light of persistent claims that such chemistry is both viable and observable.

A resonance Raman intensity analysis is performed on the intramolecular charge-transfer molecule 1-aza-adamantane-4-ylidenemalononitrile in acetonitrile solution. We explore the extent to which changes in molecular structure upon charge transfer can be obtained from resonance Raman intensity analysis, and extend the analysis method for charge-transfer excitation to take into account the possible influence of nearby locally excited states. Absolute scattering cross sections are measured at five excitation wavelengths spanning both the charge-transfer band at 324 nm and the lowest locally excited band at 231 nm, and the absorption spectra and resonance Raman intensities are modeled self-consistently to obtain the mode-specific reorganization energies accompanying electronic excitation to both states. Interference effects between the two states are considered but are found to be of minimal importance for this particular charge-transfer molecule. The reorganization parameters in terms of dimensionless normal ...

Six new donor-bridge-acceptor compounds have been synthesized which contain a long n-tetradecyl chain attached to the donor or acceptor moiety, or to both of them. Systems 1, 2, and 3 are analogs of the fluorescent probe molecule Fluoroprobe (4). They contain a rigidly extended 4-methylenepiperidine bridge and show relatively strong charge transfer fluorescence in solvents of low and medium polarity. Systems 1a, 2a, and 3a contain a semiflexible 4-methylpiperidine bridge, obtained after hydrogenation of the exocyclic double bond of 1, 2 and 3, respectively. These systems undergo a conformational change following photoinduced charge separation (harpooning) in nonpolar solvents and probably also in solvents of medium polarity. Both the steady state fluorescence spectra and the fluorescence decay times of the extended charge transfer (ECT) species show that the photoinduced folding process is effectively slowed down by the introduction of the long alkyl tails. This is most pronounced for 1a which has an n-tetradecyl group attached to both donor and acceptor. In solution a small difference in the rate of folding is observed between 2a and 3a, which have a single n-tetradecyl chain attached to the acceptor only and to the donor only, respectively.