Studies of the solvatochromic emission properties of N-aroylurea derivatives I: Influence of the substitution pattern.

The influence of the substituent of the N-aroylurea functionality on the solvatochromic properties of this class of compounds was investigated with eight examples. The absorption spectra of these compounds exhibit the characteristic spectroscopic properties of the corresponding arene fragment and are only slightly dependent on the solvent. In contrast, all investigated aroylurea derivatives exhibit a strong solvatochromism with a good linear correlation between the emission energy and the acceptor numbers (AN) of the solvents; that is, the emission maximum shifts bathochromically (Δλ = 50-93 nm) with increasing AN. Furthermore, in media with increasing viscosity, as established in glycerol or ethanol solutions with decreasing temperature, the emission maxima are significantly shifted to shorter wavelengths and the full width at half maximum (FWHM) changes. All experimental data point to two emitting states, namely the locally excited (LE) state and the charge-transfer (CT) state. Thus, after initial photoexcitation to the LE state an internal charge transfer (ICT) takes place due to the donor-acceptor interplay between the arene unit and the N-acylureido functionality, mainly assisted by the intramolecular hydrogen bond between the terminal NH group and the aryl-substituted carbonyl functionality, hence interconverting the latter to a stronger acceptor. In the polarized CT state the acylurea unit develops a negative charge, which, after solvent relaxation, is stabilized by solvents with high acceptor number. Time-resolved emission spectroscopy revealed additional conformational changes in the excited state. Two emissive species were identified at room temperature, whose lifetimes depend strongly on the chemical environment. In addition, time-resolved emission spectra (TRES) showed red-shifted emission bands at longer delays after the excitation pulse in polar solvents. These findings are rationalized by the presence of two different emitting rotational conformers.

Effect of the structure of bile salt aggregates on the binding of aromatic guests and the accessibility of anions.

The binding of naphthalene (Np), 1-ethylnaphthalene (EtNp), acenaphthene (AcN), and 1-naphthyl-1-ethanol (NpOH) as guests to the aggregates of sodium cholate (NaCh), taurocholate (NaTC), deoxycholate (NaDC), and deoxytaurocholate (NaTDC) was studied with the objective of determining how the structure of the bile salts affects the binding dynamics of guests and quenchers with the bile salt aggregates. Time-resolved and steady-state fluorescence experiments were used to determine the binding efficiency of the guests with the aggregates and were also employed to investigate the quenching of the singlet excited state of the guests by iodide anions. Quenching studies of the triplet excited states using laser flash photolysis were employed to determine the accessibility to the aggregate of nitrite anions, used as quenchers, and the dissociation rate constants of the guests from the bile salt aggregates. The binding efficiency of the guests to NaDC and NaTDC is higher than for NaCh and NaTC, and the protection efficiency is also higher for NaDC and NaTDC, in line with the larger aggregates formed for the latter bile salts. The formation of aggregates is in part driven by the structure of the guest, where an increased protection efficiency and residence time can be achieved by the introduction of short alkyl substituents (AcN or EtNp vs Np). NpOH was shown to be located in a very different environment in all four bile salts when compared to AcN, EtNp, and Np, suggesting that hydrogen bonding plays an important role in the formation of the aggregate around NpOH.

Pyrene binding to persistent micelles formed from a dendro-calixarene.

The formation of micelles of an amphiphilic dendro-calixarene (1) was studied using pyrene as a guest molecule. Steady-state fluorescence experiments were performed, which showed that pyrene is readily solubilized in the micelles and senses an environment with a moderate polarity. Time-resolved fluorescence measurements showed that pyrene in the micelles have two lifetimes, suggesting some compartmentalization of the guest within the micelles. Pyrene bound to the micelles is completely protected from the interaction with anionic species in the aqueous phase, due to the repulsion between the anions and the negatively charged micelles. Compared to conventional micelles, such as those formed with sodium dodecyl sulfate, micelles of 1 form at much lower concentrations of monomer.

Dual fluorescence of 2-methoxyanthracene derivatives.

The time-resolved emission properties of selected anthracene derivatives, namely anthracene (1a), 2-methylanthracene (1b), 2-chloroanthracene (1c), 2-methoxyanthracene (1d), 2-methoxy-6-methylanthracene (1e), 2-(N,N'-dicyclohexylureidocarbonyl)-6-methoxyanthrace ne (1f), 2-(6-methoxyanthr-2-yl)-4,4-dimethyl-2-oxazoline (1g), 2-(6-methoxyanthr-2-yl)-pyridine (1h), and N-cyclohexylanthracene-2-carboxamide (1i) were investigated. In contrast to anthracene (1a), 1b, and 1c, the 2-methoxy-substituted anthracene derivatives 1d-1h exhibit two emission lifetimes. The determination of the lifetimes at different emission wavelengths and additional time-resolved emission spectroscopy (TRES) reveal that the dual emission originates from two different, interconvertible emissive species, with the s-cis and s-trans conformation relative to the exocyclic C2-O bond. The energy difference between the two emissive species is very small (<0.1 eV) both in the ground and in the excited state. The larger energy difference between the conformers in the excited-state is responsible for the interconversion within the singlet excited-state lifetimes of the s-cis into the s-trans conformations leading to coupled decay kinetics. The proposed mechanism for the dual emission was qualitatively supported by theoretical studies on CASSCF and DFT level. In addition, the emission lifetimes of the fluoride- and pH-sensitive fluorescent probes 1f and 1g change upon addition of fluoride or acid, respectively, so that in these cases the detection of the fluorescence lifetime may be used complementary to the steady-state fluorimetric detection of these analytes.

Effect of solvent polarity and viscosity on the guest binding dynamics with bile salt aggregates.

Bile salts form supramolecular aggregates with two binding sites with different properties. The guest binding dynamics to the aggregates and guest protection from species in the aqueous phase were investigated using fluorescence and laser flash photolysis experiments. Sodium cholate, deoxycholate and taurodeoxycholate were used as bile salts and acetonitrile or ethylene glycol were added as co-solvents to water in order to alter the binding properties of 1-ethylnaphthalene and 1-naphthyl-1-ethanol with the aggregates. The binding dynamics are faster and protection efficiencies are lower for guests bound to cholate and in the presence of either co-solvent.

N-Acylureido functionality as acceptor substituent in solvatochromic fluorescence probes: detection of carboxylic acids, alcohols, and fluoride ions.

N,N'-Dicyclohexyl-N-(6'-methoxyanthryl-2-carbonyl)urea (1a) exhibits a strong solvatochromism with respect to its fluorescence properties. The emission maxima of 1a correlate well with the anion-stabilizing properties of the solvent. This feature allows the detection of analytes with high acceptor numbers, such as alcohols and carboxylic acids, and the detection of analytes such as fluoride ions which form strong hydrogen bonds with the amido hydrogen atom.

Modulation with acetonitrile of the dynamics of guest binding to the two distinct binding sites of cholate aggregates.

Bile salt aggregates are supramolecular systems containing two different binding sites. The effect of the addition of acetonitrile on the specificity and dynamics of guest binding to the two binding sites of cholate aggregates was studied. The protection of guests included in the aggregate from interaction with ions in the aqueous phase was evaluated from quenching of the singlet and triplet excited states of guest molecules bound to the cholate aggregates. The dynamics of guest binding to the primary and secondary binding sites of the cholate aggregates were determined at increasing acetonitrile mole fractions. The structure of the aggregates was not significantly altered provided the cholate concentrations were higher than 20 mM and the acetonitrile mole fraction did not exceed 0.033 (9.1% v/v). These results show that acetonitrile can be used to modulate the solubility of guests in the aggregates and to manipulate the residence time of guests in the primary and secondary binding sites.

A new totally flat N(sp(2))C(sp(2))N(sp(2)) pincer palladacycle: synthesis and photoluminescent properties.

The Sonogashira coupling of 2-bromopyridine with 8-quinolinyl-acetylene affords 2-pyridinyl-8-quinolinyl-acetylene (1) in high yields. The chloropalladation of 1 with Li(2)PdCl(4) in methanol at room temperature affords the pincer palladacycle (C(5)H(4)N-2-C=C(Cl)-8-C(9)H(6)kappaN,kappaC,kappaN)PdCl (2) in 63% yield. The X-ray molecular structure of (2) shows that it is totally flat and that it is associated in pairs though pi-stacking between alternate pyridine-quinoline moieties (3.448 A). The pairs are also connected by pi-stacking with an interpair distance of 3.452 A between quinoline-quinoline moieties. A very low fluorescence emission was also revealed by the pincer palladacycle 2 in both solution and the solid state, which has been ascribed to an excimeric emission due to the particular structure (rigid and totally flat) of 2 in the solid state.