Palladium-Catalyzed Arylations towards 3,6-Diaryl-1,3a,6a-triazapentalenes and Evaluation of Their Fluorescence Properties.

Methyl 4-(1,3a,6a-triazapentalen-3-yl)benzoate (TAP1) shows interesting properties as a small molecule fluorophore. In the search for post-functionalization methods, palladium-catalyzed arylation reactions were demonstrated. Direct CH arylation reactions of TAP1 with various aryl halides resulted in 3,6-diaryltriazapentalenes TAP4, although mostly in poor yields. Bromination of TAP1 followed by Suzuki coupling, on the other hand, requires a more delicate procedure, but gave arylated products with the same regiochemistry (TAP4) in moderate to good yields. The structure of 6-phenyltriazapentalene TAP4a was confirmed by crystallographic analysis. In addition, the effect of the C6 arylation on the fluorescent properties of 3-aryl-1,3a,6a-triazapentalenes was studied in dichloromethane at room temperature and in 2-methyltetrahydrofuran at 77 K, while the photophysical properties of two saponified derivatives were measured in acetonitrile.

Synthesis and Spectroscopic Properties of 1,2,3-Triazole BOPAHY Dyes and Their Water-Soluble Triazolium Salts.

Water-soluble BOPAHY fluorophores have not yet been reported. The potential of 1,2,3-triazolium salts for the formation of water-soluble chromophores is explored. 1,2,3-Triazole-substituted pyrroles were synthesized in a metal-free pathway and alkylated to obtain water-soluble 1,2,3-triazolium BOPAHY dyes. High fluorescence quantum yields were observed for triazole-bridged BOPAHY dyes in DCM and moderate fluorescence quantum yields for 1,2,3-triazolium-bridged BOPAHY chromophores in DCM and water. The fluorescence of the freely rotatable 1,2,3-triazolium-linked BOPAHYs is partially quenched in water.

New Metal-Free Route towards Imidazole-Substituted Uridine.

Nucleosides with a bi(hetero)aryl nucleobase have unique potential applications as antiviral drugs and molecular probes. The need for transition metal catalysis to synthesize these nucleosides from pre-functionalized building blocks and the use of nucleobase protection groups results in expensive and tedious syntheses. Herein we report that 5-imidazolyl-uracil can be obtained by scalable Van Leusen imidazole synthesis and regioselectively introduced on ribose to obtain the desired nucleoside in a 5 step synthesis (total yield 55 %). The 5-imidazolyl moiety leads to improved fluorescence properties. The only side-product formed was characterized by 2D-NMR and X-ray crystallography and could be suppressed during synthesis in favor of the desired product.

Bay-Substituted Thiaza[5]helicenes: Synthesis and Implications on Structural and Spectroscopic Properties.

A series of bay-substituted thiaza[5]helicenes was synthesized to investigate the effect of different substituents on the properties of these helicenes. These thiaza[5]helicenes with different substituents were prepared in a straightforward manner through indole- and benzo[b]thiophene synthesis, palladium-catalyzed Suzuki coupling, oxidative cyclization, and functional group interconversion reactions. We investigated the impact of these different bay area substituents compared to the unsubstituted thiaza[5]helicene on the structural parameters and studied the steady-state electronic spectroscopy of these thiaza[5]helicenes in toluene and acetonitrile. We found that different functional groups influence the solid state structure and spectroscopic properties, but a single substituent in the bay area of a thiaza[5]helicene was not enough to prevent enantiomerization at room temperature.

Impact of the Keto-Enol Tautomeric Equilibrium on the BODIPY Chromophore.

An intramolecular tautomeric fluorescent BODIPY sensor has been designed and synthesized. The obtained BODIPY dye is a combination of the 4-bora- 3a, 4a-diaza- s-indacene core and a diketone fragment. The study of conformational equilibria in the ground and excited states has been completed for a broad range of solvent polarity by steady state and NMR methods as well as by DFT and TD-DFT calculations. The interpretation of the unique emission observed in hydrogen bond accepting solvents upon the excitation of the fluorescent dye in the S0-S2 transition has been accomplished. The Jablonski diagram has been analyzed for the observed processes in the BODIPY dye studied on the basis of DFT and TD-DFT calculations.

Acid-Sensitive BODIPY Dyes: Synthesis through Pd-Catalyzed Direct C(sp3 )-H Arylation and Photophysics.

A novel palladium-catalyzed direct C(sp3 )-H arylation of the methyl group at the 8-position of BODIPY by bromoarenes was established. A deprotonative cross-coupling process was supposed to be involved in the reaction. This approach allowed us to attach electron-donating/withdrawing, halogen substituted aryls and a heteroaryl with a yield running from 55 to 99 %. Novel pH sensors, which in the absence of acid showed the occurrence of photoinduced electron transfer, were synthesized by attaching dimethylaniline to the methyl at the C8-position of BODIPY. The reference compounds with dimethylaniline directly attached to the C8-position were also synthesized and besides photoinduced electron transfer also showed a charge-transfer emission. Their photophysical properties were investigated by steady-state fluorescence, time-correlated single-photon counting and femtosecond fluorescence up-conversion. Time-dependent density functional (TD-DFT) electronic-structure calculations on the properties of the excited states corresponding to local excitation of the BODIPY core and to charge transfer were conducted. Upon addition of trifluoroacetic acid in toluene and ethanol, the partial fluorescence intensity recovery was at least an order of magnitude more efficient with the newly synthesized sensors compared to the traditional reference sensors. The improved sensitivity of these novel BODIPY-based pH sensors was attributed to less efficient proton-coupled electron transfer of the protonated species.

Investigation of the Qx -Qy Equilibrium in a Metal-Free Phthalocyanine.

Phthalocyanines (Pcs) have attracted a lot of interest as small molecules for organic electronics. However, some excited-state properties of metal-free phthalocyanines, as for example, the dynamics of the transition between the nondegenerate Qx and Qy states in a metal-free phthalocyanine, have not been fully established. This effect results in a blue-shifted shoulder with low intensity in the Pc fluorescence spectrum. This shoulder was suggested to be related to emission from the more energetic Qy state. By using ultrafast femtosecond transient absorption, we have found a clear equilibrium between the Qx and Qy state of metal-free phthalocyanines in solution.

On the stability of surface-confined nanoporous molecular networks.

Self-assembly of molecular building blocks into two-dimensional nanoporous networks has been a topic of broad interest for many years. However, various factors govern the specific outcome of the self-assembly process, and understanding and controlling these are key to successful creation. In this work, the self-assembly of two alkylated dehydrobenzo[12]annulene building blocks was compared at the liquid-solid interface. It turned out that only a small chemical modification within the building blocks resulted in enhanced domain sizes and stability of the porous packing relative to the dense linear packing. Applying a thermodynamic model for phase transition revealed some key aspects for network formation.

Tip-induced chemical manipulation of metal porphyrins at a liquid/solid interface.

Changing abruptly the potential between a scanning tunneling microscope tip and a graphite substrate induces "high-conductance" spots at the molecular level in a monolayer formed by a manganese chloride-porphyrin molecule. These events are attributed to the pulse-induced formation of µ-oxo-porphyrin dimers. The pulse voltage must pass a certain threshold for dimer formation, and pulse polarity determines the yield.

Identifiability of models for time-resolved fluorescence with underlying distributions of rate constants.

The deterministic identifiability analysis of photophysical models for the kinetics of excited-state processes, assuming errorless time-resolved fluorescence data, can verify whether the model parameters can be determined unambiguously. In this work, we have investigated the identifiability of several uncommon models for time-resolved fluorescence with underlying distributions of rate constants which lead to non-exponential decays. The mathematical functions used here for the description of non-exponential fluorescence decays are the stretched exponential or Kohlrausch function, the Becquerel function, the Förster type energy transfer function, decay functions associated with exponential, Gaussian and uniform distributions of rate constants, a decay function with extreme sub-exponential behavior, the Mittag-Leffler function and Heaviside's function. It is shown that all the models are uniquely identifiable, which means that for each specific model there exists a single parameter set that describes its associated fluorescence δ-response function.

Delayed electron-hole pair recombination in iron(III)-oxo metal-organic frameworks.

The photodynamic properties of a series of Fe(III)-MOFs have been examined via redox reactions with N,N,N',N'-tetramethyl-p-phenylenediamine as an electron donor and methyl viologen as an electron acceptor. Furthermore, photogeneration of long-lived species in MIL-88B(Fe) has been proven via transient absorption spectroscopy.

8-HaloBODIPYs and their 8-(C, N, O, S) substituted analogues: solvent dependent UV-vis spectroscopy, variable temperature NMR, crystal structure determination, and quantum chemical calculations.

The UV-vis electronic absorption and fluorescence emission properties of 8-halogenated (Cl, Br, I) difluoroboron dipyrrin (or 8-haloBODIPY) dyes and their 8-(C, N, O, S) substituted analogues are reported. The nature of the meso-substituent has a significant influence on the spectral band positions, the fluorescence quantum yields, and lifetimes. As a function of the solvent, the spectral maxima of all the investigated dyes are located within a limited wavelength range. The spectra of 8-haloBODIPYs display the narrow absorption and fluorescence emission bands and the generally quite small Stokes shifts characteristic of classic difluoroboron dipyrrins. Conversely, fluorophores with 8-phenylamino (7), 8-benzylamino (8), 8-methoxy (9), and 8-phenoxy (10) groups emit in the blue range of the visible spectrum and generally have larger Stokes shifts than common BODIPYs, whereas 8-(2-phenylethynyl)BODIPY (6) has red-shifted spectra compared to ordinary BODIPY dyes. Fluorescence lifetimes for 6, 8, and 10 have been measured for a large set of solvents and the solvent effect on their absorption and emission maxima has been analyzed using the generalized Catalán solvent scales. Restricted rotation about the C8-N bond in 7 and 8 has been observed via temperature dependent (1)H NMR spectroscopy, whereas for 10 the rotation about the C8-O bond is not hindered. The crystal structure of 8 demonstrates that the short C8-N bond has a significant double character and that this N atom exhibits a trigonal planar geometry. The crystal structure of 10 shows a short C8-O bond and an intramolecular C-H···π interaction. Quantum-chemical calculations have been performed to assess the effect of the meso-substituent on the spectroscopic properties.

Temperature-induced structural phase transitions in a two-dimensional self-assembled network.

Two-dimensional (2D) supramolecular self-assembly at liquid-solid interfaces is a thermodynamically complex process producing a variety of structures. The formation of multiple network morphologies from the same molecular building blocks is a common occurrence. We use scanning tunnelling microscopy (STM) to investigate a structural phase transition between a densely packed and a porous phase of an alkylated dehydrobenzo[12]annulene (DBA) derivative physisorbed at a solvent-graphite interface. The influence of temperature and concentration are studied and the results combined using a thermodynamic model to measure enthalpy and entropy changes associated with the transition. These experimental results are compared to corresponding values obtained from simulations and theoretical calculations. This comparison highlights the importance of considering the solvent when modeling porous self-assembled networks. The results also demonstrate the power of using structural phase transitions to study the thermodynamics of these systems and will have implications for the development of predictive models for 2D self-assembly.

Diazadithia[7]helicenes: Synthetic Exploration, Solid-State Structure, and Properties.

Diazadithia[7]helicenes were synthesized from the readily available building block ethyl 7-chloro-8-formylthieno[3,2-f]quinoline-2-carboxylate by a Wittig reaction-photocyclization strategy. The helicene core was functionalized by nucleophilic aromatic substitution with a variety of nucleophiles (e.g., O-, N-, and C-centered) and palladium-catalyzed reactions such as Suzuki coupling and Buchwald-Hartwig amination. Racemization studies confirmed that the enantiopure forms of these [7]helicenes are conformationally stable compared to their lower analogues. The solid-state structures of the novel diazadithia[7]helicenes were determined by single-crystal X-ray diffraction. The crystal structures of these azathia[7]helicenes show columnar stacking in antiparallel fashion. The HOMO-LUMO gaps of the new compounds were determined on the basis of electrochemical and optical measurements.

Spectroscopic Characterization of Thiacarbocyanine Dye Molecules Adsorbed on Hexagonal Boron Nitride: a Time-Resolved Study.

Physisorption on hexagonal boron nitride (hBN) gained interest over the years thanks to its properties (chemically and thermally stable, insulating properties, etc.) and similarities to the well-known graphene. A recent study showed flat-on adsorption of several cationic thiacarbocyanine dyes on hBN with a tendency to form weakly coupled H- or I-type aggregates, while a zwitterionic thiacarbocyanine dye rather led to a tilted adsorption. With this in-depth time-resolved study using the TC-SPC technique, we confirm the results proven by adsorption isotherms, atomic force microscopy, and stationary state spectroscopy combined with molecular mechanics simulations and estimation of the corresponding exciton interaction. The absence of a systematic trend for the dependence of the decay times, normalized amplitudes of the decay components, and contribution of different components to the stationary emission spectra upon the emission wavelength observed for all studied dyes and coverages suggests the occurrence of a single emitting species. At low coverage levels, the non-mono-exponential character of the decays was attributed to adsorption on different sites characterized by different intramolecular rotational freedom or energy transfer to nonfluorescent traps or a combination of both. The difference between the decay rates of the four dyes reflects a different density of the nonfluorescent traps. Although the decay time of the unquenched dyes was in the order of magnitude of that of dye monomers in a rigid environment, it is also compatible with weakly coupled aggregates such as proposed earlier based on the stationary spectra. Hence, the adsorption leads to a rigid environment of the dyes, blocking internal conversion. Increasing the concentration of the dye solution from which the adsorption on hBN occurs increases not only the coverage of the hBN surface but also the extent of energy transfer to nonfluorescent traps. For TDC (5,5-dichloro-3-3'-diethyl-9-ethyl-thiacarbocyanine) and TD2 (3-3'-diethyl-9-ethyl-thiacarbocyanine), besides direct energy transfer to traps, exciton hopping between dye dimers followed by energy transfer to these traps occurs, which resulted in a decreasing decay time of the longest decaying component. For all dyes, it was also possible to analyze the fluorescence decays as a stretched exponential as would be expected for energy transfer to randomly distributed traps in a two-dimensional (2D) geometry. This analysis yielded a fluorescence decay time of the unquenched dyes similar to the longest decay time obtained by analysis of the fluorescence decays as a sum of three of four exponentials.

On the nucleation and fast reaction kinetics of 2D polymerisation with a 2-in-1 monomer.

We report on the fast reaction kinetics of an imine based 2D polymer (2DP) formed from a single monomer carrying both aldehyde and amine groups. Our results point towards a direct monomer-to-crystalline polymer transition without an amorphous intermediate.

Origin of the polychromatic photoluminescence of zeolite confined Ag clusters: temperature- and co-cation-dependent luminescence.

Zeolite confined silver clusters (AgCLs) have attracted extensive attention due to their remarkable luminescent properties, but the elucidation of the underlying photophysical processes and especially the excited-state dynamics remains a challenge. Herein, we investigate the bright photoluminescence of AgCLs confined in Linde Type A zeolites (LTA) by systematically varying the temperature (298-77 K) and co-cation composition (Li/Na) and examining their respective influence on the steady-state and time-resolved photoluminescence. The observed polychromatic emission of the tetrahedral Ag4(H2O) n 2+ clusters ranges from orange to violet and three distinct emitting species are identified, corresponding to three long-lived triplet states populated consecutively and separated by a small energy barrier. These long-lived species are at the origin of the polychromatic luminescence with high photoluminescence quantum yields. Furthermore, the Li-content dependence of decay times points to the importance of guest-host-guest interactions in tuning the luminescent properties with a 43% decrease of the dominating decay time by increasing Li content. Based on our findings, a simplified model for the photophysical kinetics is proposed that identifies the excited-state processes. The results outlined here pave the way for a rational design of confined metal clusters in various frames and inspire the specified applications of Ag-zeolites.

Scanning tunneling microscopy-induced reversible phase transformation in the two-dimensional crystal of a positively charged discotic polycyclic aromatic hydrocarbon.

We report on the observation and manipulation of a two-dimensional crystal formed by a positively charged discotic polycyclic aromatic hydrocarbon at the liquid-solid interface. Using scanning tunneling microscopy (STM) as a tool, the supramolecular scaffolds of charged molecules could be switched between dissimilar polymorphs of different molecular densities. The observed phase transformation was found to be driven by electrical parameters such as magnitude of change of the substrate bias and voltage pulses applied to the STM tip. We conclude that the electrical manipulation of these charged molecules is a result of the creation of large local electric fields that interact with the adsorbed ionic molecules and thus cause molecular rearrangement.

Morpholinecarbonyl-Rhodamine 110 based substrates for the determination of protease activity with accurate kinetic parameters.

Commonly used fluorogenic substrate analogues for the detection of protease activity contain two enzyme-cleavable bonds conjugated to the fluorophore. Enzymatic cleavage follows a two-step reaction with a monoamide intermediate. This intermediate shows fluorescence at the same wavelength as the final product complicating the kinetic analysis of fluorescence-based assays. Fluorogenic substrate analogues for α-chymotrypsin with one cleavable peptide bond have been prepared from morpholinecarbonyl-Rhodamine 110 (MC-Rh110). A comparison of their kinetic properties with the corresponding (peptide)(2)-Rh110 derivatives revealed that these frequently used double-substituted substrate analogues yield only apparent K(m) and k(cat) values that are quite different from the kinetic parameters obtained from the monosubstituted MC-Rh110 based substrate analogues. Although both the monoamide intermediate and MC-Rh110 are monosubstituted Rhodamine 110 derivatives, they show different spectroscopic properties. The data from the spectroscopic analysis clearly show that these properties are directly related to the electron structure of the fluorophore and not to the previously proposed equilibrium between the lactone form and the open ionic form of the fluorophore. This knowledge about the determinants of the spectroscopic properties of monosubstituted Rhodamine 110 introduces a way for a more systematic development of new fluorogenic protease substrate analogues.

Oligo(p-phenylene ethynylene)-BODIPY derivatives: synthesis, energy transfer, and quantum-chemical calculations.

The synthesis and energy-transfer properties of a series of oligo(p-phenylene ethynylene)-BODIPY (OPEB) cassettes are reported. A series of oligo(p-phenylene ethynylene)s (OPEs) with different conjugated chain lengths as energy donor subunit in the energy-transfer system were capped at both ends with BODIPY chromophores as energy-acceptor subunits. The effect of the conjugated chain of OPEs on energy transfer in the OPEB cassettes was investigated by UV/Vis and fluorescence spectroscopy and modeling. With increasing number n of phenyl acetylene units (n=1-7), the absorption and emission maxima of OPEn are bathochromically shifted. In the OPEBn analogues, the absorption maximum assigned to the BODIPY moieties is independent of the length of the OPE spacer. However, the relative absorption intensity of the BODIPY band decreases when the number of phenyl acetylene units is increased. The emission spectra of OPEBn are dominated by a band peaking at 613 nm, corresponding to emission of the BODIPY moieties, regardless of whether excitation is at 420 or 550 nm. Furthermore, a very small band is observed with a maximum between 450 and 500 nm, and its intensity relative to that of the BODIPY emission increases with increasing n, that is, the excited state of OPE subunits is efficiently quenched in OPEBn by energy transfer to the BODIPY moieties. Energy transfer (ET) from OPEn to BODIPY in OPEBn is very efficient (all Φ(ET) values are greater than 98 %) and only slightly decreases with increasing length of the OPE units. These results are supported by theoretical studies that show very high energy transfer efficiency (Φ(ET) >75 %) from the OPE spacer to the BODIPY end-groups for chains with up to 15-20 units.