Complexation of 6-hydroxyquinoline with trimethylamine in polar and non-polar solvents

Selectively probing ferric ions in aqueous environments using protonated and neutral forms of 7-azaindole as a multiparametric chemosensor

7-azaindole (7AI) dimer is a model molecule for DNA study and understanding the mutagenic behavior based on the excited-state proton transfer process in hydrogen-bonded networks. The neutral and protonated forms of 7AI monomer with significant fluorescence (FL) intensity fit the fluorescent sensor strategy to recognize selective metal ions. Out of several metal ions (Fe³⁺, Al³⁺, Fe²⁺, Pb²⁺, Ba²⁺, Ni²⁺, Zn²⁺, Mg²⁺, Ca²⁺, Cu²⁺, Hg²⁺ and Cd²⁺), the absorption, fluorescence and fluorescence lifetime of 7AI in the aqueous medium are selectively sensitive to the ferric (Fe³⁺) ions. The absolute value of absorption intensity increases linearly with concentration of a particular metal ions. FL intensity of both the forms of 7AI decreases gradually with Fe³⁺ ions and trails the linear Stern-Volmer relation. The formation of non-fluorescent complexes was confirmed with Benesi-Hildebrand and Job plots, along with FL and FL decays. The FL lifetime of the protonated form of 7AI, which is 0.83 ± 0.01 ns, is nearly constant with Fe³⁺ ions concentrations, confirming the static quenching mechanism. The limit of detection (LoD) of Fe³⁺ ions over the long range of 16-363 µM for the neutral and protonated forms of 7AI is 0.46 ± 0.02 and 0.49 ± 0.02 µM, respectively, estimated using FL spectra. Additionally, the linear plot of absorbance with Fe³⁺ ions of both the forms of 7AI can also act as a calibration curve with very close LoDs, as obtained by FL spectra. Thus, the multi-parameters-based probe for detecting the Fe³⁺ ions over long-range in real aqueous environments with operational, high sensitivity, fast response (< 5 s), and good selectivity (over 12 metal ions) is undoubtedly a superior approach over other methods.

Complexes of a naphthalimide photoacid with organic bases, and their excited-state dynamics in polar aprotic organic solvents

Complex formation and intermolecular excited-state proton transfer (ESPT) between a dihydroxy-1,8-naphthalimide photoacid and organic bases are investigated in polar aprotic solvents. First, quantum chemical calculations are used to explore the acid-base and spectroscopic properties and to identify energetically favorable complexes. The two hydroxyl groups of the photoacid enable stepwise formation of 1 : 1 and 1 : 2 complexes. Weak bases exhibit only hydrogen-bonding interactions whereas strong bases are able to deprotonate one of the hydroxyl groups resulting in strong negative cooperativity (K1≫ 4K2) in the formation of the 1 : 2 complex. Time-resolved fluorescence studies of the complexes provide strong indications of a three-step dissociation process. The species involved in the model are: a hydrogen-bonded complex, a hydrogen-bonded ion pair, a solvent separated ion pair, and a free ion pair.

TDDFT study of the polarity controlled ion-pair separation in an excited-state proton transfer reaction

6-Hydroxyquinoline (6HQ) is an ideal photoacid system for exploring excited-state proton transfer (ESPT) reactions. We have previously (Mahata et al. (2002)) shown that the ESPT reaction between 6HQ and trimethylamine (TMA) leads to an "unusual" emission in the 440-450 nm range, containing two decay components (∼5 ns and ∼12 ns). The observed results suggest the presence of a contact ion-pair and a solvent separated ion-pair. In this work, density functional theory (DFT) and time-dependent density functional theory (TD-DFT) have been employed to study the nature of the contact ion-pair formed between 6HQ and TMA and to determine why the decay component ∼12 ns is absent in a non-polar solvent. Calculations of the hydrogen-bonded complexes formed between 6HQ and TMA and its ESPT reaction product, namely 6HQ-TMA and 6HQ-TMA-PT, respectively, have been carried out, both in the electronic ground and excited states. Moreover, by using the CPCM model, different dielectric constants have been introduced into the calculations. On increasing the dielectric constant, the hydrogen bond in 6HQ-TMA-PT becomes weaker and the hydrogen bond length becomes larger; this effectively facilitates the proton transfer reaction and formation of separated ion-pair. Thus, the separation and diffusion of the contact ion-pair can be controlled by changing the polarity of the surroundings.

Development of fluorescent sensing of anions under excited-state intermolecular proton transfer signaling mechanism

[reaction: see text] A substantially red-shifted fluorescence emission in 3-hydroxyl-2-naphthanilide in acetonitrile was developed and drastically enhanced upon addition of anions such as F(-), AcO(-), and H(2)PO(4)(-), with the enhancement depending on anion basicity. Excited-state intermolecular proton transfer in the sensor-anion hydrogen-bonding complex was suggested to be the signaling mechanism.