Longer Excited State Lifetime Research Articles

Effect of free rotation in polypyridinic ligands of Ru(ii) complexes applied in light-emitting electrochemical cells

In the present work we report the synthesis and the electrochemical, photoluminescent and electroluminescent properties of two new Ru(II) complexes described by the general formula [Ru(phen)2X](2+), where phen is 1,10-phenanthroline. The X ligand consists of a 2,2'-bipyridine (bpy) unit substituted with two phenyl rings connected to the bpy core through a saturated (Lhydro = 4,4'-diphenylethyl-2,2'-bipyridine) or a conjugated (LH = 4,4'-bis(α-styrene)-2,2'-bipyridine) carbon-carbon bridge. The photoluminescent spectra indicate that, both in solution and solid state, the complex bearing the aliphatic substitution bridges exhibits a higher quantum yield and a longer excited state lifetime than the fully conjugated complex. The new complexes were used in light-emitting electrochemical cells (LECs) showing red emission for the complex with the Lhydro ligand and no light emission for the complex incorporating the LH ligand. This and the photophysical properties make it plausible that for these complexes the degree of freedom increases with aliphatic substitution. As a consequence, the negative effect of the auto-quenching processes taking place in solid LEC devices due to the close molecular packing is limited. When compared with the archetype [Ru(phen)3](2+) complex, the complex with aliphatic substitution shows better performance in the device supporting the beneficial effect of the bulky substitution.

Enhanced emission of ultra-small-sized LaF3:RE3+ (RE = Eu, Tb) nanoparticles through 1,2,4,5-benzenetetracarboxylic acid sensitization

Uniform, ultra-small-sized and well-water-dispersible LaF(3) nanoparticles doped with trivalent rare earth (RE) ions (Eu(3+) or Tb(3+)) have been synthesized by a simple, low temperature synthesis route. The nanoparticles, with sizes of about 3.2 nm (for those doped with Eu(3+)) and 3.0 nm (for those doped with Tb(3+)), are roughly spherical and monodisperse. 1,2,4,5-Benzenetetracarboxylic acid (labeled as BA) as sensitizer has been bonded to the surface of the nanoparticles, which can sensitize the emission of RE(3+) in the LaF(3) nanoparticles. The BA-LaF(3):RE(3+) (RE = Eu or Tb) nanoparticles have a broad absorption band in the UV domain, and show enhanced luminescence of RE(3+) based on an energy transfer from BA ligands to RE(3+) ions (i.e. the so-called "antenna effect"). Due to the dual protection of organic ligands (BA) and inorganic matrices (LaF(3)), BA-LaF(3):RE(3+) (RE = Eu or Tb) nanoparticles have longer excited state lifetimes than LaF(3):RE(3+) (RE = Eu or Tb) nanoparticles as well as lanthanide coordination polymers of BA.

Characterization and emissive property of microfibers doped with fluorine functionalized iridium complex

In this paper, we used two fluorine-containing ligands of 2-(2-fluorophenyl)benzo[d]thiazole (F-BT) and 1,1,1,5,5,5-hexafluoropentane-2,4-dione (acac-F6) to construct an Ir complex of Ir(acac-F6)(F-BT) 2. The single crystal of Ir(acac-F6)(F-BT) 2 is observed, which confirms its identity. Ir(acac-F6)(F-BT) 2 single crystal belongs to monoclinic system with two molecules in each unit cell. Theoretical calculation is performed on Ir(acac-F6)(F-BT) 2 to investigate its electronic nature, which suggests that the first electronic transition owns a mixed character of metal-to-ligand-charge-transfer and ligand-to-ligand-charge-transfer. We select a polymer matrix of poly(vinylpyrrolidone) (PVP) to explore the photophysical property of Ir(acac-F6)(F-BT) 2 within microfibers. Results indicate that the photophysical performances of Ir(acac-F6)(F-BT) 2 within microfibers are improved compared with those of Ir(acac-F6)(F-BT) 2 powder, including microfiber sample's higher emission energy, longer excited state lifetime, and better photostability.

Foreword to the 39th International Conference on Coordination Chemistry Bioinorganic Research Front

This Research Front features papers selected from the 39th International Conference on Coordination Chemistry held in Adelaide (24–30 July 2010). This meeting, the premier inorganic coordination chemistry conference worldwide, had not been held in Australia since 1989. In 2010 it coincided with the RACI Inorganic Chemistry Divisional (IC’10) conference. This internationalmeeting featured excellent contributions across the various themes of the conference including materials chemistry, metals in synthesis, solar energy conversion, metals in catalysis, supramolecular chemistry, and metals in biological systems. It is from this last theme that papers have been chosen for this Research Front, and these span a wide range of topics within the ‘bioinorganic’ area. Inorganic chemistry taken literally in its original meaning from the days of ‘Vital Force Theory’, excludes the chemistry of life. Nearly 200 years after Friedrich Wohler rescued inorganic chemistry from biological irrelevance the roles of metals in biological systems are now, more than ever, the focus of intense research in biological chemistry. Within this Research Front we have two reviews. One a personal perspective by David Parker (Durham) on the development of fluorescent complexes for bio-imaging. The long fluorescent lifetimes of lanthanoid excited states enable time resolution to be used in fluorescence imaging with essentially zero background interference from any other fluorescent molecules in the sample. A second review is fromZhiguang Xiao and Tony Wedd (Melbourne) on the topic of metallo-oxidases, a fascinating family of enzymes that catalyze the oxidation of important low valent metals such as Cu(I), Fe(II), and Mn(II) bymolecular oxygen. These reactions are critical to maintaining balanced levels of redox active metals within cells and to their transport and uptake by other metalloproteins. The full papers in this Research Front are equally comprehensive in their coverage of contemporary biological inorganic chemistry. The emergence of organometallic complexes in medicinal chemistry has challenged traditional views on metal-based drugs and their interactions with biomolecules (see also the Research Front published in Aust. J. Chem. 2010, 63, issue 11). In this vein, a paper by Gilles Gasser (Zurich), Nils Meztler-Nolte (Bochum), and coworkers report new peptide nucleic acid containing ligands and their Re and Tc carbonyl complexes, the latter being of great interest in radio-imaging. The stability of these complexes and ease of formation is investigated and discussed. Water solubility can be challenge in developing ligands and complexes that exhibit suitable pharmacological properties. Paul Donnelly (Melbourne) and coworkers have developed new water soluble bisthiosemicarbazone ligands that strongly bind Cu, a suitable isotope for positron emission tomography diagnostic imaging. The effects on cellular uptake and the redox properties of their Cu(II) complexes are discussed. The use of synchrotron radiation as a tool for imaging metals within cells is covered in two different papers using distinct techniques. Trevor Hambley (Sydney) and coworkers, using X-ray absorption near-edge spectroscopy, have developed a way of extracting information about the relative proportions of Pt(II) and Pt(IV) within cells. Pt(II) complexes comprise the most thoroughly investigated metal-based anticancer drugs to date but their side effects, linked to reactions with other biomolecules, have led to new approaches including the use of more chemically inert Pt(IV) analogues. The present paper shows how the relative proportions of the Pt(II) and Pt(IV) forms of some complexes change over time within cells. Lou Rendina (Sydney), Hugh Harris (Adelaide), and colleagues have used synchrotron X-ray fluorescence (XRF) spectroscopy to identify the cellular location of a DNA-intercalating Pt-terpyridyl complex bearing a carborane thiolate ligand; a potential target in boron neutron capture therapy. Rather than localising within the nucleus, the bifunctional complex is found to be distributed throughout the cell both within and outside the nucleus. Addition of the Pt complex also has significant effects on the concentrations of other elements within the cell (alsomeasured with XRF) with an increase in Cl, K, and Cu being found while Fe levels decreased. The purple acid phosphatase (PAP) family are an interesting group of enzymes that require two metal ions in proximity, one in a trivalent and the other in a divalent oxidation state for activity. Lawrie Gahan and coworkers report a new binucleating ligand and its Fe(III)/Zn(II) complex that mimics the PAP active site found in the plant enzyme in structure and in function. This ligand also bears an olefinic substituent for future copolymerization of the complex. Only a relatively small selection of the work presented at ICCC39 was able to be included in this Research Front but the quality of these contributions, and the contributors, herein is representative of the meeting. I was fortunate to be a PhD student in 1989 when ICCC last came to Australia. It was a long wait of 21 years to see the conference return to Australia and then from a different perspective as one of the organizing committee. If this pattern is to be repeated I will have retired CSIRO PUBLISHING

Study of absorption spectrum and dynamics evaluation of the indocyanine‐green first singlet excited state

AbstractExcited state absorption and excited state dynamics of indocyanine‐green (ICG) dissolved in dymethyl sulfoxide were measured using white‐light continuum Z‐scan (WLCZScan) and white‐light continuum pump–probe (WLCPP) techniques. The excited state absorption spectrum, obtained through Z‐scan measurements, revealed saturable absorption (SA) for wavelengths longer than 630 nm, while reverse saturable absorption (RSA) appeared, as indicated by a band at approximately 570 nm. Both processes were modeled by a three‐energy‐level diagram, from which the excited state cross‐section values were determined. SA and RSA were also observed in pump–probe experiments, with a recovery time in the hundreds of picoseconds time scale due to the long lifetime of the first excited state of ICG. Such results contribute to the understanding of ICG optical properties, allowing application in photonics and medicine. Copyright © 2010 John Wiley & Sons, Ltd.

Mode-specific intermolecular vibrational energy transfer. I. Phenyl selenocyanate and deuterated chloroform mixture

Vibrational energy transfer from the first excited state (2252 cm−1) of the C–D stretch of deuterated chloroform (DCCl3) to the 0-1 transition (2155 cm−1) of the CN stretch of phenyl selenocyanate (C6H5SeCN) in their 1:1 liquid mixture was observed with a pump/probe two-color two dimensional infrared spectroscopic technique. The mode-specific energy transfer can occur mainly because of the long vibrational lifetime of the CN stretch first excited state (∼300 ps) and the relatively strong hydrogen-bond between the C–D and CN (calculated H-bond formation energy in gas phase ∼−5.4 kcal/mol). The mode-specific energy transfer is relatively low efficient (only ∼2%), which is mainly because of the relatively short vibrational lifetime (∼9 ps) of the C–D stretch first excited state and the big donor/acceptor energy mismatch (97 cm−1) and the slow transfer kinetics (1/kCD→CN=330 ps).

Unusually Long Lifetime of Excited Charge-Transfer State of All-Inorganic Binuclear TiOMnII Unit Anchored on Silica Nanopore Surface

The lifetime and back electron transfer kinetics of an all-inorganic, oxo-bridged heterobinuclear TiIVOMnII group covalently anchored on a silica nanopore surface was investigated by transient optical absorption spectroscopy. Mesoporous silica particles of type SBA-15 loaded with TiOMn sites (1 wt %) were suspended in an index matching liquid for performing spectroscopy in transmission mode. Upon excitation of the TiIVOMnII → TiIIIOMnIII metal-to-metal charge-transfer transition (MMCT) by a visible laser pulse of 8 ns duration, a transient bleach was observed whose intensity versus pump wavelength dependence agreed with the MMCT absorption profile. The decay kinetics is well described by a superposition of first-order rates with a mean time constant of 1.8 ± 0.3 μs (room temperature). The dispersion of γ = 2 ± 0.2 (Albery model) is attributed to variations in the local silica coordination environment reflecting the disordered, amorphous nature of the silica nanopore surface. The result constitutes the first observation of the electron transfer kinetics of an all-inorganic heterobinuclear group. It is proposed that the microsecond lifetime, unusually long for such as small charge transfer chromophore, originates from strong polarization of the local and remote silica environment upon light-triggered electron transfer from Mn to Ti. This results in a substantial reorganization barrier for back electron transfer. The long lifetime makes oxo-bridged heterobinuclear units anchored on silica surfaces efficient visible light photocatalysts and suitable as charge-transfer chromophores for driving multielectron catalysts in artificial photosynthetic systems.

Luminescence Properties of Eu2+‐Doped Glass Ceramics Containing SrF2 Nanocrystals

The Eu2+‐doped glasses and glass ceramics containing SrF2 nanocrystals were prepared and their luminescence properties were investigated. The formation of SrF2 nanocrystals in glass ceramics was confirmed by X‐ray diffraction and transmission electron microscopy. The melting in a reducing atmosphere and then the crystallization of glass resulted in the reduction of almost all Eu3+ ions to Eu2+ ions in the glass ceramics. The Eu2+‐doped glass ceramics containing SrF2 nanocrystals exhibited a much stronger broad blue emission band and a longer lifetime of excited state than the glasses.

Role of Solvent Protic Character on the Aggregation Behavior of Polybenzimidazole in Solution

The aggregation behavior of poly(4,4'-diphenylether-5,5'-bibenzimidazole) (OPBI) in polar aprotic (dimethyl acetamide, DMAc) and protic (formic acid, FA) solvents is studied as a function of the polymer concentration and solution temperature. The effects of solvent protic character on the aggregation behavior of OPBI are elucidated. The photophysical studies suggest that the OPBI chains form aggregated structures in both DMAc and FA solutions when the OPBI concentration is increased. The dependences of the emission spectra on the polymer concentrations in two solvents are not similar in nature, indicating that in both of the solvents the aggregations are intermolecular processes, though their mechanisms are different owing to the polyelectrolytic nature of OPBI in FA medium. The triexponential decay profiles obtained from the time-resolved fluorescence study for the concentrated solutions (both in DMAc and FA) display a negative fractional coefficient and longer excited state lifetime, providing support for the aggregations at higher concentration. The temperature dependence emission spectra suggest that the aggregations in both of the solvents destabilize with increasing temperature. The higher activation energy of aggregation (E(A)) in DMAc (5.62 KJ/mol) compared with that in FA (3.07 kJ/mol) reveals that the aggregation formation pathways are different in two solvents and stronger aggregates are formed in the former solvent. The dilute solution viscometry (DSV) studies demonstrate that the OPBI chains adapt a bigger extended conformation in FA compared with DMAc owing to the stronger intramolecular chain repulsion in FA arising due to the polyelectrolyte nature of OPBI in this solvent. A conformation transition of OPBI chains from compact collapsed to extended conformer is observed in DMAc solvent with increasing concentration, whereas any such transition is absent in FA medium. Transmission electron microscope (TEM) images and circular dichroism (CD) spectra are also in agreement with the presence of a conformational transition in DMAc and the absence of it in FA. The temperature dependent DSV studies further support the disruption of aggregated structure with increasing temperature in both of the solvents. DSV studies exhibit that the deaggregation is driven by a conformation transition (extended to compact collapsed) in DMAc, whereas in FA the disruption happens without conformational transition.

Deuterium isotope effect on femtosecond solvation dynamics in methyl β-cyclodextrins

Deuterium isotope effect on the solvation dynamics and fluorescence anisotropy decay of coumarin 153 (C153) bound to dimethyl beta-cyclodextrin (DMB) and trimethyl beta-cyclodextrin (TMB) is studied using femtosecond upconversion. In D(2)O, there is a marked increase in the steady state emission quantum yield and fluorescence lifetime of C153 bound to DMB and TMB. This suggests strong coupling between C153 and D(2)O inside the cyclodextrin cavity. In D(2)O, average solvation time of C153 in DMB is about 1.7 times slower compared to that in water. For TMB in D(2)O, solvation is 1.5 times slower. The deuterium isotope effect on solvation dynamics at long time arises mainly from the longer excited state lifetime. The longest components of solvation dynamics are ascribed to self-diffusion of C153 out of the cyclodextrin cavity. The nearly 1.5 times slower anisotropy decay of C153 bound to DMB and TMB in D(2)O (compared to H(2)O) is attributed to higher viscosity of D(2)O.

Charge-transfer character of excitons in poly[2,7-(9,9-di-n-octylfluorene)(1−x)-co-4,7-(2,1,3-benzothiadiazole)(x)

Quantum-chemical calculations performed on poly[2,7-(9,9-di-n-octylfluorene)(1-x)-co-4,7-(2,1,3-benzothiadiazole)(x)] copolymers (0 < or = x < or = 0.5) show that the lowest unoccupied molecular orbital is always highly localized on the benzothiadiazole (BT) units while the highest occupied molecular orbital is delocalized over the whole chain. Chains with a low BT content are characterized by a reduced oscillator strength of the lowest optical transition and by an increased charge-transfer character of the exciton. These results are supported experimentally by a blueshift of the lowest energy absorption band upon reduction in the BT ratio, lower photoluminescence efficiency, longer excited state lifetimes, and greater solvent dependence of the emission properties.

Detection of Single Photoluminescent Diamond Nanoparticles in Cells and Study of the Internalization Pathway

Diamond nanoparticles are promising photoluminescent probes for tracking intracellular processes, due to embedded, perfectly photostable color centers. In this work, the spontaneous internalization of such nanoparticles (diameter 25 nm) in HeLa cancer cells is investigated by confocal microscopy and time-resolved techniques. Nanoparticles are observed inside the cell cytoplasm at the single-particle and single-color-center level, assessed by time-correlation intensity measurements. Improvement of the nanoparticle signal-to-noise ratio inside the cell is achieved using a pulsed-excitation laser and time-resolved detection taking advantage of the long radiative lifetime of the color-center excited state as compared to cell autofluorescence. The internalization pathways are also investigated, with endosomal marking and colocalization analyses. The low colocalization ratio observed proves that nanodiamonds are not trapped in endosomes, a promising result in prospect of drug delivery by these nanoparticles. Low cytotoxicity of these nanoparticles in this cell line is also shown.

Electrochemical, Spectroelectrochemical, and Molecular Quadratic and Cubic Nonlinear Optical Properties of Alkynylruthenium Dendrimers1

A combination of cyclic voltammetry (CV), UV-vis-NIR spectroscopy and spectroelectrochemistry, hyper-Rayleigh scattering (HRS) [including depolarization studies], Z-scan and degenerate four-wave mixing (DFWM) [including studies employing an optically transparent thin-layer electrochemical (OTTLE) cell to effect electrochemical switching of nonlinearity], pump-probe, and electroabsorption (EA) measurements have been used to comprehensively investigate the electronic, linear optical, and nonlinear optical (NLO) properties of nanoscopic pi-delocalizable electron-rich alkynylruthenium dendrimers, their precursor dendrons, and their linear analogues. CV, UV-vis-NIR spectroscopy, and UV-vis-NIR spectroelectrochemistry reveal that the reversible metal-centered oxidation processes in these complexes are accompanied by strong linear optical changes, "switching on" low-energy absorption bands, the frequency of which is tunable by ligand replacement. HRS studies at 1064 nm employing nanosecond pulses reveal large nonlinearities for these formally octupolar dendrimers; depolarization measurements are consistent with lack of coplanarity upon pi-framework extension through the metal. EA studies at 350-800 nm in a poly(methyl methacrylate) matrix are consistent with the important transitions having a charge-transfer exciton character that increases markedly on introduction of peripheral polarizing substituent. Time-resolved pump-probe studies employing 55 ps, 527 nm pulses reveal absorption saturation, the longest excited-state lifetime being observed for the dendrimer. Z-scan studies at 800 nm employing femtosecond pulses reveal strong two-photon absorption that increases significantly on progression from linear complex to zero- and then first-generation dendrimer with no loss of optical transparency. Both refractive and absorptive nonlinearity for selected alkynylruthenium dendrimers have been reversibly "switched" by employing the Z-scan technique at 800 and 1180 nm and 100-150 fs pulses, together with a specially modified OTTLE cell, complementary femtosecond time-resolved DFWM and transient absorption studies at 800 nm suggesting that the NLO effects originate in picosecond time scale processes.

Quantum computing on long-lived donor states of Li in Si

We predict a gigantically long lifetime of the first excited state of an interstitial lithium donor in silicon. The nature of this effect roots in the anomalous level structure of the {\em 1s} Li manifold under external stress. Namely, the coupling between the lowest two states of the opposite parity is very weak and occurs via intervalley phonon transitions only. We propose to use these states under the controlled ac and dc stress to process quantum information. We find an unusual form of the elastic-dipole interaction between %the electronic transitions in different donors. This interaction scales with the inter-donor distance $R$ as $R^{-3}$ or $R^{-5}$ for the transitions between the states of the same or opposite parity, respectively. The long-range $R^{-3}$ interaction provides a high fidelity mechanism for 2-qubit operations.

Synthesis and Optical Properties of Simple Amine-Containing Conjugated Polymers

Conjugated polymers (CPs) containing amino groups have been synthesized, and their optical properties in both solution and thin film have been studied. New monomers required for the synthesis of these polymers have been readily prepared via efficient synthetic routes. These monomers have been successfully polymerized with a variety of comonomers. The spectral positions of the absorption and emission spectra correlate with the degree of electron density on the polymer chain. Polymers containing N-alkylcarbazole units display similar optical properties in solution to most CPs. Polymers with dialkylamino groups, however, display very different optical properties, including broadened absorbance and emission spectra, larger Stokes shifts, and longer excited state lifetimes. These results are consistent with a significant difference between the molecular geometries of the absorbing and emitting states. The solid state emission of most of the polymers is sufficient to warrant them for consideration as fluorescent sensing materials.

Photoinduced electron transfer between metal-coordinated cyclodextrin assemblies and viologens.

Two novel tris(bipyridine)ruthenium(II) complexes bearing two and six beta-cyclodextrin binding sites on their ligands have been synthesised and characterised. Complex 1, bearing two cyclodextrins, adopts a conformation in aqueous solution where parts of the aromatic ligands are self-included into the cyclodextrin moieties. This results in a loss of symmetry of the complex and gives rise to a much more complicated 1H NMR spectrum than expected. Photophysical studies indicate that the appended cyclodextrins protect the luminescent ruthenium core from quenching by oxygen, which results in longer excited state lifetimes and higher emission quantum yields compared with the reference compound, the unsubstituted ruthenium tris(bipyridine). Inclusion of suitable guests such as dialkyl-viologens leads to a quenching of the luminescence of the central unit. In these supramolecular donor-acceptor dyads an efficient photoinduced electron transfer from the excited ruthenium moiety (the donor) to the viologen unit (the acceptor) is observed. The alkyl chain length of the acceptor plays an important role on the binding properties; when it exceeds a certain limit the binding becomes strong enough for electron transfer to occur. Interestingly, a viologen with only one long alkyl tail instead of two shows no efficient quenching; this indicates that cooperative interactions between two cyclodextrins binding one viologen are essential to raise the binding constant of the supramolecular dyad.

Reappearance of fine structure as a probe of lifetime broadening mechanisms in the 4f(N) --> 4f(N-1)5d excitation spectra of Tb3+, Er3+, and Tm3+ in CaF2 and LiYF4.

High-energy transitions in the 4f(N) -->4f(N-1)5d excitation spectra of lanthanide ions in host crystals are usually broadened due to the short excited-state lifetimes, whereas low-energy transitions, with longer excited-state lifetimes, may show fine structure. We report the surprising observation that for some materials fine structure is observed not only for the low-energy excitation bands but also for some high-energy transitions. The excited states that display fine structure are those for which the 5d electron is in the lowest crystal-field level but the 4f(N-1) core is in a highly excited state, indicating that the broadening depends only on the energy of the 5d electron and not on the total energy of the 4f(N-1)5d excited state.

Ultrafast energy relaxation in bacteriorhodopsin studied by time-integrated fluorescence

Time-integrated fluorescence experiments on native bacteriorhodopsin and on its non-isomerizing form bR5.12 are reported. The experimental set-up was designed such as to observe emission exclusively from the excited state intermediate I-460. We obtain the first systematic investigation of the fluorescence spectra as a function of the excitation wavelength tuned throughout the entire absorption band of bR. An important finding is that the position of the fluorescence maximum does not show a systematic shift when the excitation wavelength is shortened. For excitation with high excess energy, we observe a broadening of the blue wing of the bR fluorescence, indicating incomplete vibrational energy relaxation on the time scale of the lifetime of I-460. Due to a much longer excited state lifetime, vibrational energy relaxation is more effective in bR5.12 and the fluorescence spectra are much less dependent on excitation wavelength. The results are placed in the general framework of thermalization between the retinal chromophore and the protein environment, and are compared with information obtained by femtosecond experiments.

Excimer Formation in Oligo[2,5-bis(hexadecyloxy)-1,4-phenylene]s Followed by Fluorescence Spectroscopy

Using the steady-state and time-resolved fluorescence spectroscopy, the behavior of "hairy-rod" oligo- and poly[2,5-bis(hexadecyloxy)-1,4-phenylene]s in tetrahydrofuran solutions was investigated. The materials were prepared by the Yamamoto coupling reaction using zinc as a reducing metal, the nickel(II)/triphenylphosphine complex as a catalyst, and 2,2'-bipyridine as a coligand. The appropriate oligomer fractions were separated by fractional precipitation and characterized by GPC and end group analysis. The fluorescence quantum yield of oligomers and polymers increased with their increasing conjugation length. The fluorescence emission spectra of polymers and longer oligomers exhibited one emission maximum at 390 nm with a single-exponential decay and fluorescence lifetimes (τ) around 1 ns. The substitution in positions 2 and 5 forces the adjacent backbone benzene units out of the plane, which results in twist angles 60-80°, and the bulky substituents exclude the cofacial sandwich-type configuration necessary for excimer formation. However, with shorter oligomers, another emission band at 460 nm appeared. Fluorescence decays at 460 nm were found to be double-exponential with longer excited-state lifetimes [e.g. τ1 = 6.9 ns (76%), τ2 = 2.4 ns (24%)]. With shorter oligomers (dimer, trimer), we assume a sandwich-type configuration with sufficiently close interchain distance and hence the excimer can form. Hydrophobic interactions of long aliphatic side chains in a polar medium play an important role in the excimer formation.

Efficient photocycloaddition of phenanthroquinones with simple olefins

A series of substituted phenanthroquinones (PQ’s) was prepared, their phosphorescence spectra and decay rate constants, cyclic voltammograms, and photoreaction in the presence of tetramethylethylene (TME) were investigated. With 450 nm irradiation their triplet excited state cyclized with TME to give the corresponding dioxenes, 2′,2′,3′,3′-tetramethyl-1′,4′-dioxa-1′,2′,3′,4′-tetrahydrotriphenylene, cleanly and much faster than that with 300 nm irradiation. The pattern of the reaction was effected weakly in random fashion by the substituent at the 3-position of PQ’s. The photocycloaddition, as exemplified by PQ itself, is so efficient that it occurred even by room light and during optical spectroscopic recordings; its limiting quantum yield in benzene is established to be unity with 450 nm irradiation. It owes the unusual efficiency to a long lifetime of the PQ triplet excited state with the n– ∗ π configuration, and to the successful cyclization in every diffusion controlled collision as implicated by TME quenching of PQ phosphorescence intensity in CCl 4 solution. The triplet excited state reaction was supported by competitive quenching of the dioxene formation by triplet quenchers as well as by oxygen. These PQ’s do not fluoresce in solution, but show phosphorescence in solid solution with lifetimes of about 10 ms and in CCl 4 solution with lifetimes in the order of 100 μs in the room temperature range. PQ’s with 3-cycano, 3-chloro, 3-methoxy, and without substitutions photocyclized to cyclohexene to give the corresponding dioxenes with small stereochemical scrambles, that was interpreted that the extent of electron transfer in the 1,6-diradical stage is small. Similar photocycloadditions to isobutene gave a 1:1 mixture of two regio-isomers, this was taken as evidence for a direct radical attack on olefins to give 1,6-diradical intermediates.