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A twisted π-conjugated framework does not necessarily promote efficacious ISC, energy matching of S1/Tn excited states offers a more promising route to triplet enhancement.

A boron dipyrromethene (BODIPY) derivative bearing a

A small series of boranil complexes has been studied by fluorescence spectroscopy. Weakly fluorescent in most organic solvents at room temperature, the target compounds display bright emission in the crystalline phase. X-ray diffraction patterns obtained for single crystals indicate a distorted tetrahedral geometry around the O-B-N center with the boron atom being displaced from the plane of the heterobicyclic ring. Consideration of the various bond lengths in comparison with those of reference compounds indicates that the ancillary phenyl ring, bearing different para-substituents, does not make a prominent contribution to the molecular dipole moment in the solid state. Absorption and fluorescence spectra recorded for the crystals remain remarkably similar to those for liquid solutions and display large Stokes shifts. Proximity broadening is observed in one case. The nitrophenyl derivative exhibits additional absorption and emission bands unique to the solid state and could be indicative of an intermolecular charge-transfer transition. The optical properties are discussed in terms of the crystal packing diagrams.

The syntheses and crystal structures of sterically crowded mono- and bichromophoric BODIPY-based dyes are reported. The "monomeric" compound is weakly fluorescent in the liquid phase due to fast internal conversion associated with rotation of aryl rings at the boron atom. The side-by-side "dimer" exhibits weak excitonic coupling between the dipyrrin units and is much more emissive in fluid solution. Solid samples of both molecular entities are strongly fluorescent under near-UV illumination. Thus, the mono-chromophore exhibits dual fluorescence from what appears to be a mixture of crystalline and possibly amorphous (or interfacial regions) distributions. The bi-chromophore packs in the crystal as pairs of chromophores with each unit being provided by a different molecule. This leads to excitonic splitting and the formation of a strong H-band in the absorption spectrum. Fluorescence occurs from the corresponding J-species and also from what appears to be an aggregated state associated with interfacial areas. Both bulk and interface-bound states show relatively long-lived fluorescence while the crystal structures indicate the likelihood for fast electronic energy migration between molecules.

Light-induced blinking, an inherent feature of many forms of super-resolution microscopy, has been linked to transient reduction of the fluorescent cyanine dye used as an imaging agent. There is, however, only scant literature information related to one-electron reduced cyanine dyes, especially in an aqueous environment. Here, we examine a small series of cyanine dyes, possessing disparate π-conjugation lengths, under selective reducing or oxidizing conditions. The experiment allows recording of both differential absorption spectra and decay kinetics of the resultant one-electron reduced or oxidized transient species in water. Relative to the ground state, absorption transitions for the various radicals are weak and somewhat broadened but do allow correlation with the π-conjugation length. In all cases, absorption maxima lie to the blue of the main ground-state transition. Under anaerobic conditions, the transient species decay on the microsecond to millisecond time scale, with the mean lifetime depending on molecular structure, radiation dose, and dye concentration. The experimental absorption spectra recorded for the one-electron reduced radicals and the presumed dimer cation radical compare well to spectra obtained from time-dependent density functional theory calculations. The results allow conclusions to be drawn regarding the plausibility of the reduced species being responsible for light-induced blinking in direct stochastic optical reconstruction microscopy.

A covalently linked bichromophore, embracing 6,13-bis(triisopropylsilylethinyl)pentacene (TIPS-pentacene) terminals bridged by a rigid fluorene spacer, generates a relatively high yield (i.e., 65 ± 6%) of the spin-correlated, triplet biexciton upon illumination in toluene. Under the same conditions, the extent of fluorescence quenching relative to the parent TIPS-pentacene approaches 97% and is insensitive to temperature. The biexciton, having overall singlet spin multiplicity, undergoes internal conversion in competition to spin decorrelation. These latter processes occur on the relatively slow time scale of a hundred or so nanoseconds, possibly reflecting the restricted level of electronic communication between the terminals. Spin decorrelation leads to evolution of an independent triplet pair with an overall quantum yield of 0.50 ± 0.06 and a lifetime of 8 ± 2 μs in deaerated toluene. Photoacoustic calorimetry (PAC) indicates three separate enthalpy changes: a very fast step associated with intramolecular singlet exciton fission to form the correlated triplet biexciton, a fast step essentially reflecting spin decorrelation, and a slow step associated with relaxation of the independent triplet pair. Analysis of the PAC data, in conjunction with the transient absorption results, establishes excitation energies for both spin-correlated and independent triplet pairs. Polar solvent enhances both fluorescence quenching and triplet formation at the expense of radiationless decay while temperature effects have been recorded for all important intermediate species. acceptedVersion

A symmetrical molecular array has been synthesized comprising a central zinc(II) 5,10,15,20-tetraphenylporphyrin with identical boron dipyrromethene (BODIPY) units appended at each of the meso sites. Excitation of any subunit causes a cascade of electronic energy-transfer steps, ultimately leading to the BODIPY triplet-excited state in high yield. Coincidentally, the triplet energy levels of the zinc(II) porphyrin and BODIPY appendage are closely balanced such that an equilibrium is established at both 77 K and room temperature. This fast equilibration allows global distribution of the triplet exciton around the array, leading to a significantly increased capture volume for bimolecular quenching and a substantial increase in the rate constant for bimolecular triplet-triplet annihilation. The corresponding free-base porphyrin analogue does not favor triplet exciton decentralization because of the large disparity in the electronic energy levels.

A boron dipyrromethene (BODIPY) derivative has been synthesized whereby a phenoxyl ring attached at the 3-position is bound through the oxygen atom to the boron center. This compound is structurally distorted, with the molecular surface being curved, and undergoes further geometrical perturbation at the excited singlet state level. Fluorescence is readily observed in solution at ambient temperature, with the quantum yield rising with increasing viscosity of the surrounding solvent. Dual-exponential decay kinetics are observed, corresponding to E-type delayed fluorescence. In solution, the emission yield falls with increasing temperature, but the opposite situation is found for the same compound dispersed in an amorphous sugar. These results are considered in terms of two radiationless decay channels. A viscosity-dependent avenue allows the fluorophore to function as a conventional fluorescent rotor for tracking changes in local rheology. A temperature-dependent channel leads to trapping within a new conformation, which is weakly coupled to the ground state but is able to repopulate the emitting state on a relatively slow time scale. Analysis of the experimental data allows estimation of some of the key kinetic parameters as a function of temperature.

Two Chromophore-Quencher Conjugates (CQCs) have been synthesized by covalent attachment of the anti-oxidant dibutylated-hydroxytoluene (BHT) to a pyrrole-BF2 chromophore (BOPHY) in an effort to protect the latter against photofading. In fluid solution, light-induced intramolecular charge transfer is favoured in polar solvents and helps to inhibit photo-bleaching of the chromophore. The rate of photo-fading, which scales with the number of BHT residues, is zero-order in polar solvents but shows a linear dependence on the number of absorbed photons. The zero-order rate constant shows an inverse correlation with the fluorescence quantum yield measured in the same solvent. Photo-bleaching in benzonitrile involves autocatalysis while reaction in cyclohexane shows an unexpected stoichiometry. NMR spectroscopy indicates initial damage takes place at the BHT unit and allows identification of a reactive hydroperoxide as being the primary product. In the presence of an adventitious substrate, this hydroperoxide is a photocatalyst for amide formation under mild conditions.

This paper provides an overview of modular open systems approach (MOSA) design principles and open business practices in Department of Defense (DoD) programs, as well as selected examples of MOSA implementations by the Military Departments. The National Defense Authorization Act (NDAA) for Fiscal Year (FY) 2017 (Public Law 114-328) Section 805(a) requires DoD to implement MOSA, and an increasing body of evidence indicates MOSA can enable outcomes that merit consideration and advancement. Most DoD programs had already incorporated MOSA to some extent before the NDAA required it, but the formal acknowledgment in law further emphasizes the currency of the approach. MOSA has multiple, similar definitions. For the purposes of this paper, MOSA is defined as a method to design systems with highly cohesive, loosely coupled, and severable modules that DoD can compete separately and acquire from independent vendors. MOSA is an approach rather than a technical or warfighting requirement. The approach allows the Department to flexibly acquire full capabilities and individual components – including systems, subsystems, and software – and create end item systems and services that can respond to changing threats and missions, allow for technology advances, upgrade and sustain when necessary, and maintain appropriate security assurances. Through this paper, the Office of the Deputy Assistant Secretary of Defense for Systems Engineering (ODASD(SE)) discusses considerations for implementing MOSA, provides three examples of specific implementations by the Air Force, Army, and Navy, and discusses the vision for the future of MOSA application in DoD. The information is based on a literature review of published DoD reports and public documents, interviews with DoD Services discussing lessons, the Services’ written responses to the authors’ requests of exemplars, and review and comments from stakeholders across DoD. The authors concluded that the DoD Services and Agencies’ initiatives contribute to the MOSA body of knowledge and practice.

Colorant: A substance which is used to impart colour to another material! [...]

Most quantitative fluorescence measurements report emission quantum yields by referring the integrated fluorescence profile to that of a well-known standard compound measured under carefully controlled conditions. This simple protocol works well provided an appropriate standard fluorophore is available and that the experimental conditions used for reference and unknown are closely comparable. Commercial fluorescence spectrophotometers tend to perform very well at wavelengths between 250 and 650 nm but are less responsive at longer wavelengths. There are no recognized emission standards for the far-red region. We now report fluorescence quantum yields for a series of commercially available cyanine dyes in methanol solution at room temperature. The compounds are selected to span the wavelength region from 600 to 850 nm, with absolute emission quantum yields being determined by thermal blooming spectrometry. Calibration of the instrument is made by reference to aluminium(iii) phthalocyanine tetrasulfonate and aza-BODIPY in methanol.

The recently introduced BOPHY (i.e., symmetrical bis(pyrrole-BF2)) chromophore is a highly attractive building block for constructing linear molecules with extended π-conjugation running along the molecular backbone. In order to assess the level of electronic communication between the terminal groups in this type of supermolecule, we have examined selective protonation and oxidation of the bis-(N,N-dimethylanilino-styryl) derivative. The amino N atoms are separated by 22.6 A via a conjugated pathway but there is no indication of through-bond electronic coupling. Instead, the disparate pKa values and also reduction potentials found for one-electron oxidation of each aniline group are well explained in terms of very long-range electrostatic interactions. It is necessary, however, to allow for the effects of ion-pairing. As an interesting aside, it is shown that protonation can be achieved in a cast PMMA film by using a suitable photo-acid generator.

In this paper we report the synthesis of a series of double-tailed BODIPYs bearing twin alkoxy groups with long linear alkyl side chains (C10, C12, C14, C16) attached to the meso positioned phenyl unit. The compounds were fully characterized and exhibited the typical BODIPY strong absorbance in the visible region and emission maxima around 530 nm, with high fluorescence quantum yields (ca. 0.8) and small Stokes shifts. The goal was to improve the lipophilicity of the BODIPYs without disturbing their excellent fluorescent properties. All synthetized BODIPYs were tested in vivo as fluorescent sensors for lipids and one of them (BD-C12) showed great potential to act as a lipid fluorescent probe.

The photochemical stability of two symmetrical pyrrole-BF2 (BOPHY) dyes has been assessed under continuous exposure to white light. The parent dye, lacking conjugated substituents, is highly stable when illuminated in a nonpolar film for prolonged periods but autocatalysis is observed such that the rate of degradation escalates with increasing exposure time. The rate of photofading is faster in solution and involves intermediate formation of a coloured product. The nature of the solvent exerts a small effect on the rate of bleaching. Under the same conditions, a BOPHY derivative equipped with both extended π-conjugation and a high internal dipole moment displays a pronounced solvent effect. Here, the rate of photofading varies over a factor of 2 000-fold on changing the solvent from cyclohexane to toluene. Most of the kinetic data can be explained in terms of polar and neutral resonance forms but, on the basis of NMR spectroscopy, it is concluded that toluene forms a weak complex with the dye that provides protection against photochemical damage.

The process of singlet-exciton fission (SEF) has attracted much attention of late. One of the most popular SEF compounds is TIPS-pentacene (TIPS-P, where TIPS = triisopropylsilylethynyl) but, despite its extensive use as both a reference and building block, its photophysical properties are not so well established. In particular, the triplet state excitation energy remains uncertain. Here, we report quantitative data and spectral characterization for excited-singlet and -triplet states in dilute solution. The triplet energy is determined to be 7940 &plusmn, 1200 cm&minus, 1 on the basis of sensitization studies using time-resolved photoacoustic calorimetry. The triplet quantum yield at the limit of low concentration and low laser intensity is only ca. 1%. Self-quenching occurs at high solute concentration where the fluorescence yield and lifetime decrease markedly relative to dilute solution but we were unable to detect excimer emission by steady-state spectroscopy. Short-lived fluorescence, free from excimer emission or phosphorescence, occurs for crystals of TIPS-P, most likely from amorphous domains.

Indigo, an emblematic violet dye used for thousands of years to colour fabric, is resistant to fading on exposure to sunlight. Prior work has indicated that indigo is reactive towards both hydroperoxyl radicals and superoxide anions in solution. In order to promote photobleaching of indigo, we have utilised a BOPHY-based (BOPHY = aryl fused symmetrical pyrrole-BF2 complex) chromophore known to form both superoxide ions and a stable alkyl hydroperoxide under illumination in aerated solution. Selective irradiation of the photocatalyst causes relatively fast fading of indigo, with the rate increasing gently with increasing concentration of indigo. Molecular oxygen and light are essential for effective bleaching. One molecule of photocatalyst can bleach more than 40 molecules of indigo. An active component of the photocatalyst is a butylated hydroxytoluene (BHT) residue which itself quenches the triplet excited state of indigo. This provides an ancillary mechanism for effecting photofading of indigo but, because the triplet is formed in very low yield, this route is less practical.

Cyanine dyes, as used in super-resolution fluorescence microscopy, undergo light-induced "blinking", enabling localization of fluorophores with spatial resolution beyond the optical diffraction limit. Despite a plethora of studies, the molecular origins of this blinking are not well understood. Here, we examine the photophysical properties of a bio-conjugate cyanine dye (AF-647), used extensively in dSTORM imaging. In the absence of a potent sacrificial reductant, light-induced electron transfer and intermediates formed via the metastable, triplet excited state are considered unlikely to play a significant role in the blinking events. Instead, it is found that, under conditions appropriate to dSTORM microscopy, AF-647 undergoes reversible photo-induced isomerization to at least two long-lived dark species. These photo-isomers are characterized spectroscopically and their interconversion probed by computational means. The first-formed isomer is light sensitive and transforms to a longer-lived species in modest yield that could be involved in dSTORM related blinking. Permanent photobleaching of AF-647 occurs with very low quantum yield and is partially suppressed by the anaerobic redox buffer.

It is now more than 100 years since the original proposal that a combination of sunlight and appropriate catalysts could provide both renewable energy and a means to transform carbon dioxide. It is almost 50 years since Calvin introduced the concept of an artificial leaf. Yet still, there is no large-scale industrial process that makes economic use of renewable energy to split water and produce a viable fuel on a scale worthy of major financial investment. Research into the fundamental mechanisms of applicable catalysts continues unabated, making use of operando techniques, and new procedures for atomic layer dispersion to maximise the rate of reaction at discrete catalytic centres. Here, we look at developments over the last few years concerning catalysts for water oxidation, carbon dioxide transformation and nitrogen fixation. The emphasis is not on mechanistic understanding but practical significance, thereby questioning where we stand in terms of exploiting contemporary research at the pilot-scale level.

Tin(IV) catalysis allows isolation of a boron dipyrromethene derivative bearing a solitary strap around the boron center. The conditions favor internal cyclization without contamination by side products and provide high yields of product in good purity. A phenolate-based strap imposes chirality and causes geometrical distortion of the dipyrrin. Relatively strong fluorescence is observed for single crystals, evaporated films, and adsorbed layers. Single-crystal absorption and emission spectra resemble those observed from solution with contributions from a dimer.

A small series of boron dipyrromethene (BODIPY) dyes has been synthesized whereby the boron atom is constrained in a five-membered ring formed from either o-dihydroxypyridine or o-aminophenol. In the latter case, the amino group has been converted into the corresponding amide derivative so as to curtail the possibility for light-induced charge transfer from strap to BODIPY. These compounds are weakly emissive in fluid solution but cleavage of the strap, by treatment with a photoacid generator, restores strong fluorescence. Surprisingly, the same compounds remain weakly fluorescent in a rigid glass at 80 K where light-induced charge transfer is most unlikely. In fluid solution, the fluorescence quantum yield increases with increasing temperature due to a thermally activated step but does not correlate with the thermodynamics for intramolecular charge transfer. It is proposed that the strap causes rupture of the potential energy surface for the excited state, creating traps that provide new routes by which the wave packet can return to the ground state. Access to the trap from the excited state is reversible, leading to the delayed emission. Analysis of the temperature dependent emission intensities allows estimation of the kinetic parameters associated with entering and leaving the trap.

The O-doped aromatic polycycle peri-xanthenoxanthene (PXX) was prepared from 1,1′-bi(2-naphthol) by means of an oxidative ring closure reaction. In solution, PXX is strongly emissive and displays well-structured absorption and fluorescence spectra but low-temperature phosphorescence is not detected. There is little change in geometry on excitation but, in most solvents, nonradiative decay of the excited singlet state is promoted by slight twisting of the molecular backbone. This route is partially blocked in viscous solvents and at low temperature. In the liquid phase under low friction, the rate of nonradiative decay correlates with the inverse viscosity of the solvent. Nonradiative deactivation of the excited singlet state is activated in solution while the Huang–Rhys factor is sensitive to temperature changes in a glassy matrix. Assuming the twisting movement is equivalent to translational motion about the ether linkages, Kramers’ theory allows estimation of the potential curvature at the apex of the barrier.

The fluorescence lifetime and quantum yield are seen to depend in an unexpected manner on the nature of the solvent for a pair of tripartite molecules composed of two identical boron dipyrromethene (BODIPY) residues attached to a 1,10-phenanthroline core. A key feature of these molecular architectures concerns the presence of an amide linkage that connects the BODIPY dye to the heterocyclic platform. The secondary amide derivative is more sensitive to environmental change than is the corresponding tertiary amide. In general, increasing solvent polarity, as measured by the static dielectric constant, above a critical threshold tends to reduce fluorescence but certain hydrogen bond accepting solvents exhibit anomolous behaviour. Fluorescence quenching is believed to arise from light-induced charge transfer between the two BODIPY dyes, but thermodynamic arguments alone do not explain the experimental findings. Molecular modelling is used to argue that the conformation changes in strongly polar media in such a way as to facilitate improved rates of light-induced charge transfer. These solvent-induced changes, however, differ remarkably for the two types of amide.

The optical properties are compared for two boron dipyrromethene (BODIPY) dyes that differ by virtue of the substituent at the meso-site, namely, aza-N versus C-methine atoms. Both compounds are equipped with aryl rings at the 3- and 5-positions of the dipyrrin backbone, which help to extend the degree of π-delocalization. The aza-BODIPY dye absorbs and fluoresces at much lower energy than does the conventional BODIPY dye, with red shifts of about 100 nm being observed in fluid solution, but with comparable fluorescence yield and lifetime. Hydrogen bonding donors, such as alcohols, attach to the aza-N atom and promote nonradiative decay without affecting the properties of the conventional dye. Triplet formation is ineffective in the absence of a spin-orbit coupler. Quantum chemical calculations indicate that the electronegative aza-N atom lowers the energy of the LUMO while having little effect on the corresponding HOMO energy. The resultant decrease in the HOMO-LUMO energy gap is primarily responsible for the red shift. The HOMO-LUMO energy gap is also affected by the dihedral angle subtended by the aryl rings, but this is insensitive to the geometry around the central 6-membered ring. The aza-N atom, by virtue of restricting spatial overlap between the HOMO and LUMO, decreases the energy gap between excited-singlet and -triplet states.

Funding: University of Newcastle. A pair of complementary molecular dyads have been synthesized around a 1,2-diaminocyclohexyl spacer that itself undergoes ring inversion. Despite these conformational exchange processes, the donor and acceptor occupy quite restricted spatial regions, and they are not interchangeable. The donor and acceptor pair comprise disparate boron dipyrromethene dyes selected to display favorable electronic energy transfer (EET). Steady-state emission spectroscopy confirms that through-space EET from donor to acceptor is almost quantitative, aided by the relatively short separations. Ultrafast time-resolved fluorescence spectroscopy has allowed determination of the rates of EET for both dyads. Surprisingly, in view of the close proximity of donor and acceptor (center-to-center separations less than 20 A), the EET dynamics are well-accounted for in terms of the computed molecular conformations and conventional Forster theory. One dyad appears as a single family of conformations, but EET for the second dyad corresponds to dual-exponential kinetics. In this latter case, an intramolecular hydrogen bond helps stabilize an open geometry, wherein EET is relatively slow. Postprint

Karlsson, Joshua/0000-0001-7281-1257; Harriman, Anthony/0000-0003-0679-2232 WOS: 000448083300023 PubMed: 29957960 Exposing TIPS-pentacene in deaerated benzene to ionizing radiation generates a mixture of singlet- and triplet-excited states of the solute. The singlet undergoes radiative decay without spin conversion whereas the triplet undergoes radiationless decay on the microsecond time scale. The concentration of each species was established by dosimetry. The excited-singlet state is not observed on the nanosecond-time scale for a related fluorene-bridged bis(pentacene), but the triplet is present in high concentration. Failure to detect the excited-singlet state is attributed to fast intramolecular singlet-exciton fission (iSEF) which is found to produce two triplet species. A short-lived intermediate (tau(T) = 145 ns) is identified as the species (T_T) having both pentacene units present as triplet states. The second transient is longer lived (tau(T) = 7.5 mu s) and is assigned to the corresponding species (T_G) with a single pentacene promoted to the triplet level. Dosimetry is used to conclude that iSEF partitions overwhelmingly in favor of T_G (70%) relative to T_T (25%). The total triplet yield from iSEF, therefore, is ca. 120% in this system, where the pentacene terminals are weakly coupled. Newcastle University; TUBITAK (The Scientific and Technological Research Council of Turkey)Turkiye Bilimsel ve Teknolojik Arastirma Kurumu (TUBITAK); Ministry of Education, Culture, Sports, Science and Technology of the Japanese GovernmentMinistry of Education, Culture, Sports, Science and Technology, Japan (MEXT) [25220806]; RSC We thank Newcastle University for financial support. A.A. thanks TUBITAK (The Scientific and Technological Research Council of Turkey) for the award of a research fellowship and J.G.K.K. acknowledges the award of a travel scholarship from the RSC. T.M. thanks the Ministry of Education, Culture, Sports, Science and Technology of the Japanese Government for partly support by Grant-in-Aid for Scientific Research (Projects 25220806 and others).

General discussion on natural and artificial photosynthesis.

Palladium catalysed coupling of the 2-iodoBODIPY 3 with a range of anilines and a primary alkylamine succeeds in generating the corresponding 2-aminoBODIPYs. These 2-aminoBODIPY derivatives are non-emissive and quantum chemical calculations and electrochemistry are consistent with charge transfer from the amine substituent. Attenuation of this charge transfer pathway by conversion of the 1,2-phenylenediamine derivative 9 into the corresponding benzimidazolone 10 restores the fluorescence and has been used as the basis for a fluorescence sensor for phosgene.

International audience; It is noted that, for a small series of 3,5-diacetyl-1,4-dihydrolutidine (DDL) derivatives and the corresponding Hantzsch esters, the presence of methyl groups at the 2,6-positions serves to extinguish fluorescence in solution but not in the solid state. Emission is weakly activated and affected by changes in solvent polarity. The latter situation arises because the optical transition involves intramolecular charge transfer. Calculations, both semiempirical and DFT, indicate that, in all cases, rotation of the carbonyl function is facile and that the dihydropyridine ring is planar. These calculations also indicate that the 2,6-methyl groups do not affect the generic structure of the molecule. It is proposed that illumination increases the molecular dipole moment and pushes electron density toward the carbonyl oxygen atom. Proton transfer can now occur from one of the methyl groups, leading to formation of a relatively low-energy, neutral intermediate, followed by a second proton transfer step that forms the enol. Reaction profiles computed for the ground-state species indicate that this route is highly favored relative to hydrogen transfer from the 4-position. The barriers for light-induced proton transfer are greatly reduced relative to the ground-state process but such large-scale structural transformations are hindered in the solid state. A rigid analogue that cannot form an enol is highly emissive in solution, supporting the conclusion that proton transfer is in competition to fluorescence in solution. ■ INTRODUCTION Formaldehyde is one of the more harmful air pollutants 1,2 due to its relatively high atmospheric concentration in urban environments and its deleterious impact on human health. Indeed, it has been classified as carcinogenic by IARC. 3 Such concerns mean that there is a need for reliable but straightforward analytical protocols to enable the routine monitoring of formaldehyde vapors; in practical terms, such analyses are most conveniently performed after extraction of gaseous formaldehyde into water. 1,4−7 In seeking to develop sensitive ways to monitor the presence of formaldehyde in aqueous solution, we were drawn to the in situ formation of fluorescent 3,5-diacetyl-1,4-dihydrolutidine (DDL) derivatives. 8−16 Although this earlier work focused mostly on identification of water-soluble DDL derivatives, it was noted that the fluorescence properties of these compounds were highly sensitive to the nature of substituents in close proximity to the ring nitrogen atom. We set out now to examine the origin of this observation. In so doing we note that these DDL derivatives, being members of the 1,4-dihydropyridine family, are closely related to the so-called Hantzsch esters, which are already known to display solvent-dependent fluorescence spectral properties. 17 Both DDL and Hantzsch esters (Chart 1) can be formed by reactions of formaldehyde under rather mild conditions and might be adapted as fluorescent markers for the presence of this analyte in solution. 18 To aid the mechanistic studies, a small series of 1,4-dihydropyridine derivatives has been prepared and fully characterized. Included within this series is a rigid derivative that inhibits internal rotation of the carbonyl functions. It is evident that the Hantzsch esters exhibit the same relationship between fluorescence quantum yield and molecular topology as observed for the DDL derivatives. Specifically, the presence of methyl groups at the 2,6-positions extinguishes emission in solution but not in the solid state. There is no obvious structural reason for this observation.

The ability of an unconstrained boron dipyrromethene dye to report on changes in local viscosity is improved by appending a single aryl ring at the lower rim of the dipyrrin core. Recovering the symmetry by attaching an identical aryl ring on the opposite side of the lower rim greatly diminishes the sensory activity, as does blocking rotation of the meso-aryl group. On the basis of viscosity- and temperature-dependence studies, together with quantum chemical calculations, it is proposed that a single aryl ring at the 3-position extends the molecular surface area that undergoes structural distortion during internal rotation. The substitution pattern at the lower rim also affects the harmonic frequencies at the bottom of the potential well and at the top of the barrier. These effects can be correlated with the separation of the H1,H7 hydrogen atoms.

This essay is based on research leading to the identification of catalysts capable of the selective oxidation of water to molecular oxygen. The real need for such materials relates to the large-scale photochemical dissociation of water into its constituent elements, as opposed, for example, to the electrochemical decomposition of water at macroscopic electrodes. Combining the catalyst with the essential components needed for efficacious photochemistry brings special challenges, as does the ultimate need to scale up the system by a massive amount. Nature has developed a highly successful process for O2 evolution under ambient illumination that makes use of a cubane tetramanganese cluster having a closely associated calcium cation in attendance. This catalyst is surprisingly delicate and it is debatable as to whether we could adapt such a system for use with artificial photosystems. Historically the latter have used colloidal metal oxides, and consideration is given here as to which materials might offer the most promising catalytic performance. Moving towards heterogeneous systems has more practical meaning, but the same materials come to mind. Recent attention to cobalt-based catalysts is highlighted as a possible breakthrough that might lead to interesting electrochemical systems when combined with wind turbines. Molecular catalysts provide interesting opportunities for photochemical O2 evolution but suffer from problems of scale-up. This field has witnessed the most important progress over the past decade or so but still needs urgent attention if advanced materials are to be identified in a timely manner. Finally, consideration is given to the actual status of the field in specific terms of developing an effective artificial photosynthetic apparatus. Moving progressively from using sacrificial redox agents as a simple means to isolate the oxidative photochemical cycle towards full water cleavage will stimulate the development of demonstration models suitable for public display. The reward should be increased investment.

Photophysical properties of the strongly fluorescent dye, 1H,3H-isochromeno[6,5,4-mna]xanthene-1,3-dione (CXD), are reported. This highly planar, rigid dye absorbs at around 420 nm, which is ideal for excitation with a blue laser diode, and is extremely stable towards prolonged illumination. Under near-UV excitation, the dye readily sensitises free-radical polymerisation, forming a plastic film with excellent optical transparency. Weakly structured emission is observed with a small Stokes' shift and remains essentially insensitive to changes in solvent polarity. For example, in tetrahydrofuran the fluorescence quantum yield is 0.96 while the excited-singlet state lifetime is 7.4 ns. Quantum chemical calculations provide further insight into the electronic nature of the dye in solution.

International audience; An artificial light-harvesting array, comprising 21 discrete chromophores arranged in a rational manner, has been synthesized and characterized fully. The design strategy follows a convergent approach that leads to a molecular-scale funnel, having an effective chromophore concentration of 0.6 M condensed into ca. 55 nm(3), able to direct the excitation energy to a focal point. A cascade of electronic energy-transfer steps occurs from the rim to the focal point, with the rate slowing down as the exciton moves toward its ultimate target. Situated midway along each branch of the V-shaped array, two chromophoric relays differ only slightly in terms of their excitation energies, and this situation facilitates reverse energy transfer. Thus, the excitation energy becomes spread around the array, a situation reminiscent of a giant holding pattern for the photon that can sample many different chromophores before being trapped by the terminal acceptor. At high photon flux under conditions of relatively slow off-load to a device, such as a solar cell, electronic energy transfer encounters one or more barriers that hinder forward progress of the exciton and thereby delays arrival of the second photon. Preliminary studies have addressed the ability of the array to function as a sensitizer for amorphous silicon solar cells.

A pair of covalently linked molecular dyads is described in which two disparate boron dipyrromethene dyes are separated by a tolane-like spacer. Efficient electronic energy transfer (EET) occurs across the dyad; the mechanism involves important contributions from both Förster-type coulombic interactions and Dexter-type electron exchange processes. The energy acceptor is equipped with long paraffinic chains that favor aggregation at high concentration or at low temperature. The aggregate displays red-shifted absorption and emission spectral profiles, relative to the monomer, such that EET is less efficient because of a weaker overlap integral. The donor unit is insensitive to applied pressure but this is not so for the acceptor, which has extended π-conjugation associated with appended styryl groups. Here, pressure reduces the effective π-conjugation length, leading to a new absorption band at higher energy. With increasing pressure, the overall EET probability falls but this effect is nonlinear and at modest pressure there is only a small recovery of donor fluorescence. This situation likely arises from compensatory phenomena such as restricted rotation and decreased dipole screening by the solvent. However, the probability of EET falls dramatically over the regime where the π-conjugation length is reduced owing to the presumed conformational exchange. It appears that the pressure-induced conformer is a poor energy acceptor.

A series of molecular dyads has been synthesized and fully characterised. These linear, donor–spacer–acceptor compounds comprise terminal dyes selected to exhibit intramolecular electronic energy transfer (EET) along the molecular axis. The spacer is built by accretion of ethynylene–carborane units that give centre-to-centre separation distances of 38, 57, 76, 96, and 115 A respectively along the series. The probability of one-way EET between terminals depends on the length of the spacer but also on temperature and applied pressure. Throughout the series, the derived EET parameters are well explained in terms of through-space interactions but the probability of EET is higher than predicted for the fully extended conformation except in a glassy matrix at low temperature. The implication is that these spacers contract under ambient conditions, with the extent of longitudinal contraction increasing under pressure but decreasing as the temperature is lowered. Longer bridges are more susceptible to such distortion, which is considered to resemble a concertina effect caused by out-of-plane bending of individual subunits. The dynamics of EET can be used to estimate the strain energy associated with molecular contraction, the amount of work done in effecting the structural change and the Young's modulus for the bridge.

A small series of closely spaced, bichromophoric boron dipyrromethene (BODIPY) derivatives has been examined by optical spectroscopy and compared to the corresponding mononuclear dyes. The compounds vary according to the site of attachment and also by the nature of alkyl or aryl substituents incorporated into the dipyrrin backbone. Excitonic coupling splits the lowest-energy absorption transition in each case, but to highly variable degrees. There are also marked changes in the fluorescence quantum yields across the series but much less variation in the excited-state lifetimes. After comparing different models, it is concluded that the ideal dipole approximation gives a crude qualitative representation of the observed splitting of the absorption transition, but the extended dipole approach is not applicable to these systems. Agreement is substantially improved by employing a model that takes into account the dihedral angle between the planes of the two dipyrrin units. The large variation in radiative rate constants, and those for the accompanying nonradiative processes, is accountable in terms of electronic coupling and/or intensity borrowing between the two excitonic states. In all cases, the dihedral angle between the two BODIPY units plays a key role.

This work examines the electronic energy-transfer (EET) processes inherent to a molecular dyad in which aryl polycycles attached to a boron dipyrromethene (Bodipy) dye act as ancillary light harvesters for near-UV photons. The solvent, being methyltetrahydrofuran, is compressed under applied pressure to such an extent that, over the accessible pressure range, there is a 25% decrease in molar volume. This effect serves to increase the effective concentration of the solute and increases fluorescence from Bodipy when this chromophore is excited directly. Illumination into the aryl polycycles, namely pyrene and perylene derivatives, leads to rapid intramolecular EET to Bodipy but fluorescence from these units is partially restored under high pressure. The argument is made that applied pressure restricts torsional motions around the linkages and imposes a near orthogonal geometry for transition dipole moment vectors on the reactants. In turn, this pressure-induced conformational restriction switches off Forster-type EET within the system, leaving the electron-exchange contribution. For the target dyad, the Forster component is ca. 5% for pyrene and ca. 25% for perylene. Such contributions are not inconsistent with calculations made on the basis of Forster theory but modelling is rendered difficult by the absence of accurate information about the nature of the conformational motion. Two possibilities have been considered. In the first case, the appendages remain stiff but pressure reduces the extent of displacement from the lowest-energy position. The results can be accounted for in a quantitative sense on the basis of small deviations from the lowest-energy conformation; the actual amount of displacement needed to explain the pressure effect depends on the method used to compute the Forster rates and ranges from ca. 4° for the ideal dipole approximation to only 0.5° for the extended dipole method. Secondly, pressure is assumed to bend each appendage into a banana-like shape. Again, the full effect of applied pressure can be accounted for by way of minor curvature of the linkage.