Quantum Computational and Spectroscopic Investigation of 3-aminosalicylic Acid.

Quantum chemical calculations of 3-aminosalicylic acid (3ASA) (monomer and dimer forms) have been performed using DFT and TD-DFT theories with B3LYP/6-311 G(d,p) functional level in the ground and excited states. Using TD-DFT with IEF-PCM model, the electronic spectra of 3ASA in solvents were computed and correlated with the experimental data. The theoretically calculated absorption and emission maxima of 3ASA (monomer) are observed in the range of 343 - 347 nm (S0 → S1 transition) and 429 - 448 nm (S1 → S0 transition), respectively. The natural bond orbital (NBO) analysis shows that charge transfer interaction contributes significantly to stabilize the molecular system. In the case of dimer, hydrogen bonding plays a dominant role in stabilizing the molecular framework. Additionally, the obtained nonlinear optical (NLO) properties: polarizability (13.86 × 10-24 e.s.u for monomer and 29.46 × 10-24 e.s.u. for dimer), first-order hyperpolarizability (4.21 × 10-30 e.s.u for monomer and 0.18 × 10-30 e.s.u for dimer) and second-order hyperpolarizability (7.44 × 10-36 e.s.u. for monomer and 14.32 × 10-36 e.s.u. for dimer) were found to be larger than those of standard organic compounds suggesting that 3ASA has a significant NLO character for optoelectronic applications. The NLO properties of dimer may differ from monomer due to dimerization. Further, the radiative lifetime, light harvesting efficiency and band gap energy were calculated, and proposed that 3ASA may be useful in photovoltaics and wide bandgap power devices. HIGHLIGHTS: • DFT and TD-DFT theories were employed to calculate structural and molecular properties of 3ASA (monomer and dimer) in ground and excited states. • HOMO-LUMO study shows monomer and dimer of 3ASA are good reactive. • NBO analysis reflects that charge transfer interactions stabilized the 3ASA molecule. • Electronic absorption/emission spectra in solvents calculated by IEF-PCM/TD-DFT method correlate with experimental results. • Calculated NLO parameters suggested that 3ASA is a potential candidate for NLO material.

Excited-state properties of 6-methoxyflavone in the presence of halide ions in aqueous media.

The present work investigated the influence of different halides on the excited state dynamics of 6-methoxyflavone (6MF) in an aqueous solution with steady-state and time-resolved techniques. On successive addition of I-and Br-ions, the fluorescence of 6MF quenched significantly, whereas the respective ions do not change the maximum fluorescence band. Fluorescence of 6MF was quenched 66% by I-ions and 34% by Br-ions. In a pure aqueous medium, both the H-bonded: CT and protonated species of 6MF participate in the quenching of fluorescence. The quenching process was categorized by Stern-Volmer (S-V) and Lehrer equations. Quenching parameters such as KSV, KSV-Land kqwere higher for I-ions than Br-ions. The decrease in fluorescence intensity and a reduction in fluorescence lifetime suggested the dynamic nature of quenching by I-ions following the electron transfer mechanism. Fluorescence quenching of 6MF has also been observed in the acidic medium in the presence of different halides. Thus, the study reveals that 6MF is responsive towards I-ions in a wide range of pH, specifically in a purely aqueous environment (pH∼7), hence important for sensing/detection applications.

Effect of halide ions on the fluorescence properties of 3-aminoquinoline in aqueous medium.

Fluorescence (FL) quenching of 3-aminoquinoline (3AQ) by halide ions Cl - Br - and I - has been explored in an aqueous acidic medium using the steady-state and time-domain FL measurement techniques. The halide ions showed no significant change in the absorption spectra of 3AQ in an aqueous acidic medium. The FL intensity was strongly quenched by I - ions and the order of FL quenching by halide ions was I - > Br - > Cl - . The decrease in FL lifetime along with the reduction in FL intensity of 3AQ suggested the dynamic nature of quenching. The obtained K SV values were 328 M - 1 for I - ions and 119 M - 1 for Br - ions and the k q values were ~ 1.66 × 10 10 M - 1 s - 1 and 6.02 × 10 9 M - 1 s - 1 , respectively. The observations suggested that the likely governing mechanism for FL quenching may be an electron transfer process and the involvement of the heavy atom effects.

Solvatochromism and estimation of ground and excited state dipole moments of 6-aminoquinoline.

The spectral behaviour of 6AQ was investigated using fluorescence spectroscopy in several polar and non-polar solvents. Both the absorption and fluorescence spectra displayed solvatochromism. The Stokes shift increased significantly with increasing solvent polarity and signifies a more polar excited state with possible change in the excited state (ES) geometry. The involvement of π→π∗ transition was observed. The ground state (GS) and excited state (ES) dipole moments were determined by the solvatochromic shift method using Bilot-Kawaski, Lippert-Mataga, Kawski-Chamma-Viallet, and Reichardt equations. The experimental value of GS dipole moment matches closely with the theoretical value computed using DFT/B3LYP/6-311G(d,p). The ES dipole moment is higher than the GS dipole moment. Besides, the solvatochromic study reveals that the ES of 6AQ is more polarized than the GS due to intramolecular charge transfer (ICT), possibly aided by a change in the geometry of the molecule in the ES. The influence of the non-specific and specific interactions in the photophysical properties of the titled molecule was analyzed using the Catalan scale. The study shows that 6AQ has reasonable band-gap energy and good CIE chromaticity coordinate in the blue region close to the national television standard committee system (NTSC) for the ideal blue CIE coordinate. Therefore, future research into 6AQ as a source of light-emitting diodes and fluorescent sensors may have potential applications in the field of optoelectronics.

Structural, Electronic and NLO Properties of 6-aminoquinoline: A DFT/TD-DFT Study.

A computational study based on the DFT/TD-DFT approach was performed to explore various properties of 6-aminoquinoline (6AQ). The geometrical parameters, molecular orbitals (MOs), electronic spectra, electrostatic potential, molecular surface, reactivity parameters and thermodynamic properties of 6AQ were explored. The absorption and emission spectra of 6AQ in solvents have been estimated by TD-DFT coupled with the PCM model and correlated with the available experimental results. Depending on the solvents, the computed absorption maxima of 6AQ were noticed between 327 nm - 340 nm and ascribed to [Formula: see text] transition. The simulated emission maxima were obtained between 389 to 407 nm and ascribed to [Formula: see text] transition. On increasing the solvent polarity, both the emission and absorption maxima showed a bathochromic shift. The LUMO and HOMO were localized on the entire molecule. It was observed that the lowest excited state is possibly the [Formula: see text] charge-transfer (CT) state. The natural bonding orbital (NBO) study points out that ICT plays a significant role in stabilizing the molecular system. Moreover, the NLO (nonlinear optical) properties (polarizability, first-order hyperpolarizability and dipole moment) were computed using different hybrid functionals. The estimated values indicate that 6AQ can be considered a desirable molecule for further studies of the NLO applications.

Reinvestigation of the photophysics of 3-aminobenzoic acid in neat and mixed binary solvents.

The present study elucidates the reinvestigation of the photophysical behavior of 3-aminobenzoic acid (3ABA) in solvents of different polarities using the steady-state spectroscopic techniques. Kamlet-Taft and Catalan solvatochromic models have been used to analyze the solvatochromic changes in neat solvents. The hydrogen bond donating ability of the solvent was found to be the main parameter affecting the spectral behavior of 3ABA. The solvatochromic characteristics of 3ABA have also been examined in binary solvent mixtures viz. acetonitrile (ACN)-methanol (MeOH) and benzene (BEN)-MeOH using the concept of preferential solvation. The preferential solvation of 3ABA shows unusual behavior for BEN-MeOH binary mixture and described unnoticed sigmoidal behavior in the ground state and synergistic impact in the excited state. Besides, the 3ABA was studied theoretically by quantum chemical calculations using (HF) Hartree-Fock and (DFT/B3LYP) density functional theories and its electronic absorption bands have been assigned by time-dependent density functional theory (TD-DFT). The effect of solvents on 3ABA was considered using a IEF-PCM-TDDFT (integral equation formalism of the polarizable continuum model- TDDFT) method. Thus, the theoretical results were found to be closer to the experimental results.

Flavones Fluorescence-Based Dual Response Chemosensor for Metal Ions in Aqueous Media and Fluorescence Recovery.

Responsiveness of sensing materials 3-hydroxyflavone (3HF) towards metal ions in aqueous medium has been explored following photoexcitation. 3HF exhibited both colorimetric and fluorescence (FL) turn-off response towards Cu2+ and Fe2+ with high sensitivity and selectivity. Meanwhile, the distinct colour change and the rapid quenching of FL intensity provide naked-eye detection. On successive addition of Cu2+and Fe2+ ions, FL of 3HF was "turned off," whereas there is no change in wavelength of FL bands. Quenching efficiencies for Cu2+and Fe2+ ions are 88% and 49%, respectively. The detection limit of the sensor towards Cu2+ and Fe2+ was 1.54 µM and 1.98 µM, respectively. The binding strategy between 3HF and metal ions (Cu2+, Fe2+) and the nature of quenching have been explored with the Benesi-Hildebrand and Stern-Volmer plots, respectively. The FL of 3HF significantly quenched in the presence of Cu2+ ions, and then recovered upon addition of HCl, providing the possibility of constructing a sensitive Cu2+-HCl off-on fluorescent probe. Moreover, the proposed simple, quick response and visual test strip-based chemosensor could be used for the detection of Fe2+ and Cu2+ ions.