Hydrogen-Bonded Thiol Undergoes Unconventional Excited-State Intramolecular Proton-Transfer Reactions.

The chapter on the thiol-related hydrogen bond (H-bond) and its excited-state intramolecular proton-transfer (ESIPT) reaction was recently opened where compound 4'-diethylamino-3-mercaptoflavone (3NTF) undergoes ESIPT in both cyclohexane solution and solid, giving a 710 nm tautomer emission with an anomalously large Stokes shift of 12,230 cm-1. Considering the thiol H-bond to be unconventional compared to the conventional Pauling-type -OH or -NH H-bond, it is thus essential and timely to probe its fundamental difference between their ESIPT. However, thiol-associated ESIPT tends to be nonemissive due to the dominant nπ* character of the tautomeric lowest excited state. Herein, based on the 3-mercaptoflavone scaffold and π-elongation concept, a new series of 4'-substituted-7-diethylamino-3-mercaptoflavones, NTFs, was designed and synthesized with varied H-bond strength and 690-720 nm tautomeric emission upon ultraviolet (UV) excitation in cyclohexane. The order of their H-bonding strength was experimentally determined to be N-NTF < O-NTF < H-NTF < F-NTF, while the rate of -SH ESIPT measured by fluorescence upconversion was F-NTF (398 fs)-1 < H-NTF (232 fs)-1 < O-NTF (123 fs)-1 < N-NTF (101 fs)-1 in toluene. Unexpectedly, the strongest H-bonded F-NTF gives the slowest ESIPT, which does not conform to the traditional ESIPT model. The results are rationalized by the trend of carbonyl oxygen basicity rather than -SH acidity. Namely, the thiol acidity relevant to the H-bond strength plays a minor role in the driving force of ESIPT. Instead, the proton-accepting strength governs ESIPT. That is to say, the noncanonical thiol H-bonding system undergoes an unconventional type of ESIPT.

Chapter Open for the Excited-State Intramolecular Thiol Proton Transfer in the Room-Temperature Solution.

We report here, for the first time, the experimental observation on the excited-state intramolecular proton transfer (ESIPT) reaction of the thiol proton in room-temperature solution. This phenomenon is demonstrated by a derivative of 3-thiolflavone (3TF), namely, 2-(4-(diethylamino)phenyl)-3-mercapto-4H-chromen-4-one (3NTF), which possesses an -S-H···O═ intramolecular H-bond (denoted by the dashed line) and has an S1 absorption at 383 nm. Upon photoexcitation, 3NTF exhibits a distinctly red emission maximized at 710 nm in cyclohexane with an anomalously large Stokes shift of 12 230 cm-1. Upon methylation on the thiol group, 3MeNTF, lacking the thiol proton, exhibits a normal Stokes-shifted emission at 472 nm. These, in combination with the computational approaches, lead to the conclusion of thiol-type ESIPT unambiguously. Further time-resolved study renders an unresolvable (<180 fs) ESIPT rate for 3NTF, followed by a tautomer emission lifetime of 120 ps. In sharp contrast to 3NTF, both 3TF and 3-mercapto-2-(4-(trifluoromethyl)phenyl)-4H-chromen-4-one (3FTF) are non-emissive. Detailed computational approaches indicate that all studied thiols undergo thermally favorable ESIPT. However, once forming the proton-transferred tautomer, the lone-pair electrons on the sulfur atom brings non-negligible nπ* contribution to the S1' state (prime indicates the proton-transferred tautomer), for which the relaxation is dominated by the non-radiative deactivation. For 3NTF, the extension of π-electron delocalization by the diethylamino electron-donating group endows the S1' state primarily in the ππ* configuration, exhibiting the prominent tautomer emission. The results open a new chapter in the field of ESIPT, covering the non-canonical sulfur intramolecular H-bond and its associated ESIPT at ambient temperature.

SimCP3-An Advanced Homologue of SimCP2 as a Solution-Processed Small Molecular Host Material for Blue Phosphorescence Organic Light-Emitting Diodes.

We have overcome the synthetic difficulty of 9,9',9'',9''',9'''',9'''''-((phenylsilanetriyl)tris(benzene-5,3,1-triyl))hexakis(9H-carbazole) (SimCP3) an advanced homologue of previously known SimCP2 as a solution-processed, high triplet gap energy host material for a blue phosphorescence dopant. A series of organic light-emitting diodes based on blue phosphorescence dopant iridium (III) bis(4,6-difluorophenylpyridinato)picolate, FIrpic, were fabricated and tested to demonstrate the validity of solution-processed SimCP3 in the device fabrication.

Delineation of G-Quadruplex Alkylation Sites Mediated by 3,6-Bis(1-methyl-4-vinylpyridinium iodide)carbazole-Aniline Mustard Conjugates.

A new G-quadruplex (G-4)-directing alkylating agent BMVC-C3M was designed and synthesized to integrate 3,6-bis(1-methyl-4-vinylpyridinium iodide)carbazole (BMVC) with aniline mustard. Various telomeric G-4 structures (hybrid-2 type and antiparallel) and an oncogene promoter, c-MYC (parallel), were constructed to react with BMVC-C3M, yielding 35 % alkylation yield toward G-4 DNA over other DNA categories (<6 %) and high specificity under competition conditions. Analysis of the intact alkylation adducts by electrospray ionization mass spectroscopy (ESI-MS) revealed the stepwise DNA alkylation mechanism of aniline mustard for the first time. Furthermore, the monoalkylation sites and intrastrand cross-linking sites were determined and found to be dependent on G-4 topology based on the results of footprinting analysis in combination with mass spectroscopic techniques and in silico modeling. The results indicated that BMVC-C3M preferentially alkylated at A15 (H26), G12 (H24), and G2 (c-MYC), respectively, as monoalkylated adducts and formed A15-C3M-A21 (H26), G12-C3M-G4 (H24), and G2-C3M-G4/G17 (c-MYC), respectively, as cross-linked dialkylated adducts. Collectively, the stability and site-selective cross-linking capacity of BMVC-C3M provides a credible tool for the structural and functional characterization of G-4 DNAs in biological systems.

Pregnenolone activates CLIP-170 to promote microtubule growth and cell migration.

Pregnenolone (P5) is a neurosteroid that improves memory and neurological recovery. It is also required for zebrafish embryonic development. However, its mode of action is unclear. Here we show that P5 promotes cell migration and microtubule polymerization by binding a microtubule plus end-tracking protein, cytoplasmic linker protein 1 (CLIP-170). We captured CLIP-170 from zebrafish embryonic extract using a P5 photoaffinity probe conjugated to diaminobenzophenone. P5 interacted with CLIP-170 at its coiled-coil domain and changed it into an extended conformation. This increased CLIP-170 interaction with microtubules, dynactin subunit p150(Glued) and LIS1; it also promoted CLIP-170-dependent microtubule polymerization. CLIP-170 was essential for P5 to promote microtubule abundance and zebrafish epiboly cell migration during embryogenesis, and overexpression of the P5-binding region of CLIP-170 delayed this migration. P5 also sustained migration directionality of cultured mammalian cells. Our results show that P5 activates CLIP-170 to promote microtubule polymerization and cell migration.

A new strategy for intracellular delivery of enzyme using mesoporous silica nanoparticles: superoxide dismutase.

We developed mesoporous silica nanoparticle (MSN) as a multifunctional vehicle for enzyme delivery. Enhanced transmembrane delivery of a superoxide dismutase (SOD) enzyme embedded in MSN was demonstrated. Conjugation of the cell-penetrating peptide derived from the human immunodeficiency virus 1 (HIV) transactivator protein (TAT) to mesoporous silica nanoparticle is shown to be an effective way to enhance transmembrane delivery of nanoparticles for intracellular and molecular therapy. Cu,Zn-superoxide dismutase (SOD) is a key antioxidant enzyme that detoxifies intracellular reactive oxygen species, ROS, thereby protecting cells from oxidative damage. In this study, we fused a human Cu,Zn-SOD gene with TAT in a bacterial expression vector to produce a genetic in-frame His-tagged TAT-SOD fusion protein. The His-tagged TAT-SOD fusion protein was expressed in E. coli using IPTG induction and purified using FMSN-Ni-NTA. The purified TAT-SOD was conjugated to FITC-MSN forming FMSN-TAT-SOD. The effectiveness of FMSN-TAT-SOD as an agent against ROS was investigated, which included the level of ROS and apoptosis after free radicals induction and functional recovery after ROS damage. Confocal microscopy on live unfixed cells and flow cytometry analysis showed characteristic nonendosomal distribution of FMSN-TAT-SOD. Results suggested that FMSN-TAT-SOD may provide a strategy for the therapeutic delivery of antioxidant enzymes that protect cells from ROS damage.

Reaction-based and single fluorescent emitter decorated ratiometric nanoprobe to detect hydrogen peroxide.

A novel reaction-based cross-linked polymeric nanoprobe with a self-calibrating ratiometric fluorescence readout to selectively detect H2O2 is reported. The polymeric nanoprobe is fabricated by using hydrophobic H2O2-reactive boronic ester groups, crosslinker units, and environmentally sensitive 3-hydroxyflavone fluorophores through a miniemulsion polymerization. On treatment with H2O2, the boronic esters in the polymer are cleaved to form hydrophilic alcohols and subsequently lead to a hydrophobic-hydrophilic transition. Covalently linked 3-hydroxyflavones manifest the change in polarity as a ratiometric transition from green to blue, accompanied by a 500-fold increase in volume. Furthermore, this nanoprobe has been used for ratiometric sensing of glucose by monitoring the H2O2 generated during the oxidation of glucose by glucose oxidase, and thus successfully distinguished between normal and pathological levels of glucose.

Alkaline earth metal ion induced coil-helix-coil transition of lysine-coumarin-azacrown hybrid foldamers with OFF-OFF-ON fluorescence switching.

A new metal-ion-responsive and fluorescent foldamer, OPLM(8), composed of eight lysine-coumarin-azacrown units, has been designed and synthesized. The flexible OPLM(8) can be forced into a well-defined helix structure only upon the addition of alkaline earth metal ions. The structural change is based on the crown ether moieties being positioned in the requisite arrangement along the peptide chain, that is, at i, i+4 spacing, such that the alkaline earth metal ions can mediate the formation of four sandwich complexes between them. Moreover, varying the chelator-to-metal-ion ratio from 2:1 to 1:1 resulted in disassembly of the sandwich complexes leading to collapse of the helical structure to a random coil. These metal-ion-induced structural transitions could not only be monitored by the CD amplitude change but also easily probed by unique "OFF-OFF-ON" fluorescence intensity changes from 0.7-fold to 14-fold as the structure changed from the folded helix to a random coil. To further verify that the helix formation was indeed induced by metal-ion complexation, two kinds of control octamers with only four metal-ion chelators on the side chains were studied. One, which was capable of forming two sandwich complexes between the i and i+4 residues, displayed a negative Cotton couplet with the magnitude of its A value close to half that of OPLM(8), and the second had four metal-ion chelators positioned in the same turn, and hence was incapable of forming intramolecular metal complexes and showed different induced CD signals. Collectively, the photospectroscopic data and the results of the control studies suggest that alkaline earth metal ions can efficiently promote the flexible octamer OPLM(8) into a well-organized helix by the formation of sandwich complexes between substituents at an i, i+4 spacing.

Pregnenolone stabilizes microtubules and promotes zebrafish embryonic cell movement.

Embryonic cell movement is essential for morphogenesis and the establishment of body shapes, but little is known about its mechanism. Here we report that pregnenolone, which is produced from cholesterol by the steroidogenic enzyme Cyp11a1 (cholesterol side-chain cleavage enzyme, P450scc), functions in promoting cell migration during epiboly. Epiboly is a process in which embryonic cells spread from the animal pole to cover the underlying yolk. During epiboly, cyp11a1 is expressed in an extra-embryonic yolk syncytial layer. Reducing cyp11a1 expression in zebrafish using antisense morpholino oligonucleotides did not perturb cell fates, but caused epibolic delay. This epibolic defect was partially rescued by the injection of cyp11a1 RNA or the supplementation of pregnenolone. We show that the epibolic delay is accompanied by a decrease in the level of polymerized microtubules, and that pregnenolone can rescue this microtubule defect. Our results indicate that pregnenolone preserves microtubule abundance and promotes cell movement during epiboly.

Tuning intramolecular charge transfer and spin-orbit coupling of AIE-active type-I photosensitizers for photodynamic therapy.

Development of photosensitizers (PSs) featuring type-I reactive oxygen species (ROS) with aggregation-induced emission (AIE) properties is a judicious approach to overcome the deficit of conventional photodynamic therapy (PDT). However, it remains a challenge to design AIE-active type-I ROS PSs using a simple theranostic scaffold paired with a delicate balance between intramolecular charge transfer (ICT) and large spin-orbit coupling (SOC) features to facilitate intersystem crossing (ISC) and hence to intensify triplet excitons for type-I ROS generation as well as to improve optical properties for the desired biomedical applications. In this work, a rationally designed series of PSs based on C-6-substituted tetraphenylethylene-fused benzothiazole-coumarin scaffolds, named TPE-nCUMs, were synthesized via a fused-ring-electron-acceptor (FREA) strategy, endowed with AIE properties in aqueous solution and thus self-monitoring type-I ROS generation under white-light irradiation to study the effects of diverse ICT and SOC potentials on their photochemical and optical properties. Both experimental and theoretical results revealed that the concomitantly increasing strengths of both ICT and SOC features promote type-I ROS generation by TPE-nCUMs. The key role of the SOC-promoting carbonyl group towards the ROS generation ability of TPE-nCUMs was then examined. Among TPE-nCUMs, gem-2OMe-TPE-2CUM displayed highly efficient type-I ROS generation. Importantly, gem-OMe-TPE-1CUM acts as a fluorescent indicator in HeLa cells (in vitro), revealing its excellent diffusion capability in both lysosomal and mitochondrial organelles with low dark toxicity, high cytotoxicity under white-light and remarkable PDT efficiency. Our study has thus elucidated a rationally designed strategy at the molecular level to fine-tune ICT and SOC features for the advance of AIE-active type-I ROS PSs, opening a new avenue for cancer treatment and image-guided therapy.

Specifically and reversibly immobilizing proteins/enzymes to nitriolotriacetic-acid-modified mesoporous silicas through histidine tags for purification or catalysis.

Six nitriolotriacetic-acid-modified ordered mesoporous silicas (NTA-OMPSs) with different pore sizes and surface features for specific and reversible protein immobilization were fabricated and characterized. Specific immobilization of a genetically engineered undecaprenyl pyrophosphate synthase (UPPs) from cell lysate and a chemically modified His-tagged horseradish peroxidase (HRP) in these Ni-NTA-OMPSs through histidine coordination to the nickelated NTA was demonstrated and confirmed by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry and sodium dodecyl sulfate polyacrylamide gel electrophoresis. Negligible leakage of these enzymes over a wide range of acidic conditions was observed. Moreover, histidine tags with different lengths (His6, His4, His3, and His2) applied to HRP were evaluated to find the minimum length for effective complexation. Enzymatic assessment studies indicated that the pore size of the OMPSs has minimal influence on the enzymatic activity, whereas chemical entities such as unreacted mercapto groups tailored on the interior surfaces of the OMPSs played certain roles in inhibiting the enzymatic activity and stability. On MCF-S-NTA, SBA-S-NTA, and film-S-NTA, which contained unreacted mercaptopropyl groups on the interior surface, immobilized His-tagged HRP showed lower catalytic activity and stability than on MCF-NTA, film-NTA, and SBA-NTA. Selective hydroxylation of optically pure L-tyrosine to (S)-2-amino-3-(3,4-dihydroxyphenyl)propanoic acid (L-DOPA) by the immobilized HRP was also demonstrated.

Highly efficient control of thrombin activity by multivalent nanoparticles.

We have demonstrated that the incorporation of sulfated galactose acid (sulf-Gal) into thrombin-binding-aptamer (TBA)-conjugated gold nanoparticles (TBA-AuNPs) enables highly effective inhibition of thrombin activity toward fibrinogen. AuNP bioconjugates (TBA(15)/TBA(29)/sulf-Gal-AuNPs) were prepared from 13 nm AuNPs, 15-mer thrombin-binding aptamer (TBA(15)), 29-mer thrombin-binding aptamer (TBA(29)), and sulf-Gal. The numbers of TBA and sulf-Gal molecules per AuNP proved to have a strong impact on inhibitory potency. The best results were observed for 15-TBA(15)/TBA(29)/sulf-Gal-AuNPs (with 15 TBA(15) and 15 TBA(29) molecules per AuNP), which, because of their particularly flexible conformation and multivalency, exhibited ultrahigh binding affinity toward thrombin (K(d)=3.4×10(-12) M) and thus extremely high anticoagulant (inhibitory) potency. Compared to the case without inhibitors (the "normal" value), their measured thrombin clotting time (TCT) was 91 times longer, whereas for TBA(15) alone it was only 7.2 times longer. Their anticoagulant activity was suppressed by TBA-complementary-sequence (cTBA)-modified AuNPs (cTBA(15)/cTBA(29)-AuNPs) at a rate that was 20 times faster than that of free cTBA(15)/cTBA(29). Thus, easily prepared, low-cost, multivalent AuNPs show great potential for biomedical control of blood clotting.

Fluorescent substrate analog for monitoring chain elongation by undecaprenyl pyrophosphate synthase in real time.

Farnesyl pyrophosphate (FPP) is a common substrate for a variety of prenyltransferases for synthesizing isoprenoid compounds. In this study, (2E,6E)-8-O-(N-methyl-2-aminobenzoyl)-3,7-dimethyl-2,6-octandien-1-pyrophosphate (MANT-O-GPP), a fluorescent analog of FPP, was synthesized and demonstrated as a satisfactory substrate for Escherichia coli undecaprenyl pyrophosphate synthase (UPPS) with a K(m) of 1.5 µM and a k(cat) of 1.2s(-1) based on [(14)C]IPP consumption. Interesting, we found that its emission fluorescence intensity at 420 nm increased remarkably during chain elongation, thereby useful for real-time monitoring kinetics of UPPS to yield a K(m) of 1.1 µM and a k(cat) of 1.0 s(-1), consistent with those measured using radiolabeled substrate. Using this assay, the IC(50) of a known UPPS inhibitor farnesyl thiopyrophosphate (FsPP) was confirmed. Our studies provide a convenient and environmentally friendly alternative for kinetics and inhibition studies on UPPS drug target.

Tailoring C-6-Substituted Coumarin Scaffolds for Novel Photophysical Properties and Stimuli-Responsive Chromism.

A judicious strategy was utilized to envision the substantial regio-positional effects of substituents on the photophysical properties of the 2H-chromen-2-one-3-benzothiazole scaffold-based push-pull framework, named 6-X-CUMs. Among them, 6-NEt2-CUM reveals prominent excited-state intramolecular charge transfer with a large change of dipole moment (Δµ ∼ 18.23 D), hence displaying remarkable emission solvatochromism from the green (536 nm in cyclohexane) to far-red region (714 nm in dimethyl sulfoxide) and a high-temperature sensitivity (-0.23 nm °C-1). These, together with unique basicity and acido-/vaporchromism upon acidification elucidated by NMR and photospectroscopic studies, show stark contrast to the conventional 7-NEt2-CUM. The new series of these tailored 6-X-CUMs represents a new dimension in tailoring the photophysical properties for the development of a promising class of multistimuli-responsive materials.

Synthesis of fluorescent carbohydrate-protected Au nanodots for detection of Concanavalin A and Escherichia coli.

This study describes a novel, simple, and convenient method for the preparation of water-soluble biofunctional Au nanodots (Au NDs) for the detection of Concanavalin A (Con A) and Escherichia coli (E. coli). First, 2.9 nm Au nanoparticles (Au NPs) were prepared through reduction of HAuCl(4).3H(2)O with tetrakis(hydroxymethyl)phosphonium chloride (THPC), which acts as both a reducing and capping agent. Addition of 11-mercapto-3,6,9-trioxaundecyl-alpha-D-mannopyranoside (Man-SH) onto the surfaces of the as-prepared Au NPs yielded the fluorescent mannose-protected Au nanodots (Man-Au NDs) with the size and quantum yield (QY) of 1.8 (+/-0.3) nm and 8.6%, respectively. This QY is higher than those of the best currently available water-soluble, alkanethiol-protected Au nanoclusters. Our fluorescent Man-Au NDs are easily purified and by multivalent interactions are capable of sensing, under optimal conditions, Con A with high sensitivity (LOD = 75 pM) and remarkable selectivity over other proteins and lectins. To the best of our knowledge, this approach provided the lowest LOD value for Con A when compared to the other nanomaterials-based detecting method. Furthermore, we have also developed a new method for fluorescence detection of E. coli using these water-soluble Man-Au NDs. Incubation with E. coli revealed that the Man-Au NDs bind to the bacteria, yielding brightly fluorescent cell clusters. The relationship between the fluorescence signal and the E. coli concentration was linear from 1.00 x 10(6) to 5.00 x 10(7) cells/mL (R(2) = 0.96), with the LOD of E. coli being 7.20 x 10(5) cells/mL.

Simple Molecular-Engineering Approach for Enhancing Orientation and Outcoupling Efficiency of Thermally Activated Delayed Fluorescent Emitters without Red-Shifting Emission.

The inclusion of a tetraphenylbenzene (4Ph) unit in thermally activated delayed fluorescence emitters is demonstrated as a novel strategy for greatly enhancing the horizontally oriented alignment of the emitters without shifting the emission spectrum to longer wavelengths. Doping of blue-emitting 4PhOXDDMAC or greenish-blue-emitting 4PhOXDPXZ into o-DiCbzBz host layers yielded much higher degrees of horizontally oriented alignment for the emitter (up to 92%) compared to those when the 4Ph unit was excluded (69 and 75%, respectively). The enhanced alignment results in high outcoupling efficiencies of 24 and 35% in organic light-emitting diodes based on 4PhOXDDMAC and 4PhOXDPXZ, respectively, and boosts the external quantum efficiencies to values (8.8 and 29.2%, respectively) that are higher than what would be expected for randomly oriented emitters (outcoupling efficiency of 20%). These enhancements are achieved while avoiding the redshift that often occurs using the common strategy of increasing molecular length and, thereby, conjugation, to increase orientation.

Substrate and product specificities of cis-type undecaprenyl pyrophosphate synthase.

UPPS (undecaprenyl pyrophosphate synthase) catalyses consecutive condensation reactions of FPP (farnesyl pyrophosphate) with eight isopentenyl pyrophosphates to generate C55 UPP, which serves as a lipid carrier for bacterial peptidoglycan biosynthesis. We reported the co-crystal structure of Escherichia coli UPPS in complex with FPP. Its phosphate head-group is bound to positively charged arginine residues and the hydrocarbon moiety interacts with hydrophobic amino acids including L85, L88 and F89, located on the alpha3 helix of UPPS. We now show that the monophosphate analogue of FPP binds UPPS with an eight times lower affinity (K(d)=4.4 microM) compared with the pyrophosphate analogue, a result of a larger dissociation rate constant (k(off)=192 s(-1)). Farnesol (1 mM) lacking the pyrophosphate does not inhibit the UPPS reaction. GGPP (geranylgeranyl pyrophosphate) containing a larger C20 hydrocarbon tail is an equally good substrate (K(m)=0.3 microM and kcat=2.1 s(-1)) compared with FPP. The shorter C10 GPP (geranyl pyrophosphate) displays a 90-fold larger K(m) value (36.0+/-0.1 microM) but similar kcat value (1.7+/-0.1 s(-1)) compared with FPP. Replacement of L85, L88 or F89 with Ala increases FPP and GGPP K(m) values by the same amount, indicating that these amino acids are important for substrate binding, but do not determine substrate specificity. With GGPP as a substrate, UPPS still catalyses eight isopentenyl pyrophosphate condensation reactions to synthesize C60 product. Computer modelling suggests that the upper portion of the active-site tunnel, where cis double bonds of the product reside, may be critical for determining the final product chain length.

Gold nanoparticle-based competitive colorimetric assay for detection of protein-protein interactions.

A gold nanoparticle-based competitive colorimetric assay uses the ensemble of Concanavalin (ConA) and mannopyranoside-encapsulated gold nanoparticles (Man-GNPs) to identify the binding partners for ConA and the binding constants are determined based on the wavelength shifts.

A novel phosphate chemosensor utilizing anion-induced fluorescence change.

[structure: see text] The neutral receptor N,N'-bis[3,5-di[(1-pyrenylmethyl)carbamoyl]benzyl] pyridine-2,6-dicarbamide (2) provides a pseudo-tetrahedron cleft and multiple hydrogen bondings to form a 1:1 complex with phosphate ion in a highly selective manner, by comparison with other anions (F(-), Cl(-), Br(-), SCN(-), AcO(-), NO(3)(-), ClO(4)(-), and HSO(4)(-)). The binding strength can be inferred from the emission intensity ratio of the pyrene monomer (lambda(max) 377 nm) to the excimer (lambda(max) 477 nm). Fluorescence titration, X-ray analysis, and NMR studies support a proposed complexation model.

A novel photochromic system of 4,5-dialkenylthiophenes constructed by the smarium diiodide promoted coupling reactions of thiophene-2-carboxylate with aryl ketones.

[reaction: see text] The SmI2-promoted coupling reaction of ethyl thiophene-2-carboxylate with aryl ketones (2 equiv), followed by acid-catalyzed dehydration and oxidative aromatization, gave dialkenylthiophenes 1b-d, which underwent electrocyclizations upon irradiation with 300-nm light in CH3CN solution to give the corresponding closed-ring species with absorption lambda(max) approximately 425 nm. The interconversion between dialkenylthiophenes and their corresponding closed-ring species constitutes a novel photochromic system bearing an ester group for potential uses in linkage and wavelength tuning.