Two-photon-induced intramolecular excited-state proton transfer process and nonlinear optical properties of HBI in ethanol solution

NLOphoric imidazole-fused fluorescent anthraquinone dyes

The Z-scan and DFT techniques were explored to investigate the non-linear optical properties of anthraquinone fused imidazole-based D-π-A dyes. With the help of UV-visible spectral analysis, pH study, CV analysis, HOMO-LUMO interaction, MEP plots, and quinoidal character the influence of intramolecular charge transfer characteristics and H-bonding process on electronic and photophysical studies of anthraquinone derivatives were understood. The dyes 2-(4-(diethylamino)-2-hydroxyphenyl)-3H-anthra[1,2-d]imidazole-6,11-dione (AQ1) and 2-(2-hydroxynaphthalen-1-yl)-1H-anthra[1,2-d]imidazole-6,11-dione (AQ2) displayed a single emission with pronounced Stokes shift and thermal stability (upto 290 °C). The dye AQ1 exhibited strong charge transfer character which is explained by the ICT process leading to high nonlinear susceptibility χ⁽³⁾ in AQ1 3.43 × 10^-13 e.s.u relative to the dye AQ2 which has only ESIPT core. But, the dye AQ2 6.27 J cm^-2 showed better optical limiting value. The NLO properties of AQ1 and AQ2 were computed by DFT functionals based on Hartree Fork (HF) percentage exchange. The dye AQ1 exhibits noticeable NLO properties. The global hybrid functionals with HF composition beyond 50% (BHHLYP, M06-HF) and the long rang corrected functionals (CAM-B3LYP, ωB97, and ωB97X) demonstrated comparable NLO properties relative to B3LYP and PBE0. It was observed that the combined effect of ICT and the H-bonding cores enhance the NLO properties. The experimental findings (Z-scan) were successfully correlated with theoretical (DFT) results.

Red Emitting Hydroxybenzazole (HBX) Based Azo Dyes: Linear and Non Linear Optical Properties, Optical Limiting, Z Scan Analysis with DFT Assessments

Herein, we report the hydroxybenzazole (HBX) containing azo dyes for "linear and non-linear optical" (NLO) applications. These bi-heterocyclic dyes have HBX scaffold (decorated with ESIPT core) and connected to another thiazole moietiy through azo bond. In DMF and DMSO, dyes are "emissive in yellow-red region" and "large Stokes shift" in the range of 62-121 nm were observed. "Nonlinear absorptive coefficient" (β), "nonlinear refractive index" (ƞ₂), "third order non-linear optical susceptibility" (χ³) in DMSO, ethanol and methanol were calculated using simple and effective "Z-scan technique" having "Nd: YAG laser" at 532 nm wavelength. 4.46 × 10^-13 (e.s.u.) was the highest (χ³) was observed in DMSO among all the dyes. Optical Limiting (OL) values are in the range of 7.61-19.06 J cm^-2 in solvents. Thermo Gravimetric Analysis (TGA) supports that, these compounds are useful for numerous high-temperature practices in the construction of electronic as well as optical devices. Band gap was calculated by CV as well as by DFT in acetonitrile. The same trend was observed when these HOMO-LUMO gaps were correlated in between CV and DFT. To gain more insights into structural parameters, molecular geometries were optimized at "B3LYP-6-311 + G (d,p)" level of theory. Further, "Molecular Electrostatic Potential" (MEP), "Frontier Molecular Orbitals" (FMO) were presented using "Density Functional Theory (DFT)". Global hybrid functional (B3LYP, BHandHLYP) and range separated hybrid functionals (RSH) i.e. CAM-B3LYP, ωB97, ωB97X, and ωB97XD were used to calculate linear and NLO properties. Graphical Abstract.

Al3+ selective coumarin based reversible chemosensor: application in living cell imaging and as integrated molecular logic gate

The new coumarin based fluorescent ‘turn-on’ chemosensor (H₂L) efficiently detects Al³⁺ over other metal ions. It is efficient in detecting Al³⁺ in the intracellular region of HeLa cells and also exhibits an INHIBIT logic gate with Al³⁺ and EDTA as chemical inputs.

Designing two-photon fluorescent probes based on the target-induced enhancement of the absorption cross-section

We have reported a new strategy for two-photon probe design for the simultaneous target-induced enhancement of the two-photon absorption cross-section and quantum yield.

A TDDFT study on the excited-state intramolecular proton transfer (ESIPT): excited-state equilibrium induced by electron density swing

One important issue of current interest is the excited-state equilibrium for some ESITP dyes. However, so far, the information about the driving forces for excited-state equilibrium is very limited. In this work, the time-dependent density functional theory (TDDFT) method was employed to investigate the nature of the excited-state intramolecular proton transfer (ESIPT). The geometric structures, vibrational frequencies, frontier molecular orbitals (MOs) and the potential-energy curves for 1-hydroxy-11H-benzo[b]fluoren-11-one (HHBF) in the ground and the first singlet excited state were calculated. Analysis of the results shows that the intramolecular hydrogen bond of HHBF is strengthened from E to E*. Moreover, it is found that electron density swing between the proton acceptor and donor provides the driving forces for the forward and backward ESIPT, enabling the excited-state equilibrium to be established. Furthermore, we proposed that the photoexcitation and the interchange of position for electron-donating and electron-withdrawing groups are the main reasons for the electron density swing. The potential-energy curves suggest that the forward ESIPT and backward ESIPT may happen on the similar timescale, which is faster than the fluorescence decay of both E* and K* forms.