A new fluorescent sensor based on 1H-pyrazolo[3,4-b]quinoline skeleton. Part 2.

A novel fluorescent dye bis-(pyridin-2-yl-methyl)-(1,3,4-triphenyl-1H-pyrazolo[3,4-b]quinolin-6-ylmethyl)-amine (P1) has been synthesized and investigated by means of steady state and time-resolved fluorescence techniques. This compound acts as sensor for fluorescence detection of small inorganic cations (lithium, sodium, barium, magnesium, calcium, and zinc) in highly polar solvents such as acetonitrile. The mechanism which allows application of this compound as sensor is an electron transfer from the electron-donative part of molecule (amine) to the acceptor part (pyrazoloquinoline derivative), which is retarded upon complexation of the electro-donative part by inorganic cations. The binding constants are strongly dependent on the charge density of the analyzed cations. The 2/1 complexes of P1 with Zn(++) and Mg(++) cations posses large binding constants. Moreover, in the presence of these cations a significant bathochromic shift of fluorescence is observed. The most probable explanation of such behaviour is the formation of intramolecular excimer. This is partially supported by the quantum chemical calculations.

Exploring reversible quenching of fluorescence from a pyrazolo[3,4-b]quinoline derivative by protonation.

Pyrazolo[3,4-b]quinoline derivatives are reported to be highly efficient organic fluorescent materials suitable for applications in light-emitting devices. Although their fluorescence remains stable in organic solvents or in aqueous solution even in the presence of H(2)O, halide salts (LiCl), alkali (NaOH) and weak acid (acetic acid), it suffers an efficient quenching process in the presence of protic acid (HCl) in aqueous or ethanolic solution. This quenching process is accompanied by a change in the UV spectrum, but it is reversible and can be fully recovered. Both steady-state and transient fluorescence spectra of 1-phenyl-3,4-dimethyl-1H-pyrazolo-[3,4-b]quinoline (PAQ5) during quenching are measured and analyzed. It is found that a combined dynamic and static quenching mechanism is responsible for the quenching processes. The ground-state proton-transfer complex [PAQ5H(+)] is responsible for static quenching. It changes linearly with proton concentration [H(+)] with a bimolecular association constant K(S)=1.95 M(-1) controlled by the equilibrium dissociation of HCl in ethanol. A dynamic quenching constant K(D)=22.4 M(-1) is obtained by fitting to the Stern-Volmer equation, with a bimolecular dynamic quenching rate constant k(d)=1.03x10(9) s(-1) M(-1) under ambient conditions. A change in electron distribution is simulated and explains the experiment results.

Investigation of the photoisomerisation process in four p-benzoxazoyl-substituted stilbenes.

Fluorescence properties and trans-cis photoisomerisation of the benzoxazole derivatives 2-[4-(E)-(styryl)phenyl]benzoxazole (I), 2-{4-[(E)-2-(4-methoxyphenyl)vinyl]phenyl}benzoxazole (II), {4-[(E)-2-(4-benzoxazol-2-yl-phenyl)vinyl]phenyl}dimethylamine (III) and {4-[(E)-2-(4-benzoxazol-2-yl-phenyl)vinyl]phenyl}diphenylamine (IV) have been investigated in solvents of different polarities. It was found that these compounds exhibit efficient fluorescence with quantum yields and lifetimes strongly dependent on solvent polarity, although only compounds III and IV possess a significant charge transfer character in solvents of medium and high polarities. In addition, the photoisomerisation quantum efficiency depends strongly on the substitution of the phenyl ring in the electron donor moiety. A strong dependence of the quantum efficiency of the photoisomerisation on solvent was established. That quantity depends linearly on the non-radiative quantum yield of the deactivation of the excited singlet state for all investigated compounds. These results are consistent with a singlet state mechanism of the photoprocess. For compounds III and IV, with strong electron donors (N,N-dimethylaniline and triphenylamine), the molecule in the excited state trans configuration is more stabilized by solvent polarity than in the perpendicular form which causes more efficient isomerisation in nonpolar solvents. For compounds I and II the energy of the perpendicular configuration decreases more rapidly than that of the trans configuration when solvent polarity increases. In this case the energy barrier decreases with increasing solvent polarity. This makes the photoisomerisation process easier in polar solvents.

New fluorescent sensors based on 1H-pyrazolo[3,4-b] quinoline skeleton.

Novel fluorescing dyes 1,3,4-triphenyl-6-(1,4,7,10-tetraoxa-13-aza-cyclopentadec-13-ylmethyl)-1H-pyrazolo [3,4-b]quinoline (K1) and 2-[(2-hydroxyethyl)-(1,3,4-triphenyl-1H-pyrazolo[3,4-b]quinolin-6-ylmethyl)-amino] ethanol (L1) have been synthesized and investigated by the means of steady state and time-resolved fluorescence techniques. These compounds act as sensors for the fluorescence detection of small inorganic cations (lithium, sodium, barium, magnesium and calcium) in solvents of different polarities (THF and acetonitrile). The mechanism, which allows application of these compounds as sensors, is an electron transfer from the electro-donative part of molecule to the acceptor part (fluorophore), which is retarded upon complexation of the electro-donative part by inorganic cations. We found that crown ether-containing compound is very sensitive to the addition of any investigated ions but amino alcohol-containing one exhibits better selectivity to the addition of two-valued cations. Two kinds of the complexes (LM(+) and L(2)M(+)) were found in the investigated systems. In addition, the dyes may be used as fluorescence indicators in solvents of lower polarity like tetrahydrofuran.