Orange-Red Fluorescent (Partially Rigidified) Donor-π-(rigidified)-Acceptor System - Computational Studies.

D-π-A chromophore derived coumarins are studied using "DFT and TD-DFT" to compute vertical excitation as well as NLO properties using "global hybrid" (GH) functionals B3LYP and BHandHLYP and "range separated hybrid" (RSH) functionals CAM B3LY', wB97, wB97X, and wB97XD with basis set 6-311++G(d,p) and "correlation consistence polarized valence double and triple zeta" cc-pVDZ and cc-pVTZ respectively in the gas phase and two solvents, N,N-Dimethylformamide (DMF) and ethyl acetate (EA). The trends in absorption and emission values calculated by TD-DFT using all the above mentioned functional and basis sets were studied and it was observed that the trends seen in the computed parameters using B3LYP, BHandHLYP and CAM B3LYP are in good agreement with the trends in experimental values. DFT calculations were performed to determine "static dipole moment" (µ), "linear polarizability" (α), "first order hyperpolarizability" (ß0), "second order hyperpolarizability" (γ). We have calculated the mean average errors in dipole moment, linear polarizability, first and second hyperpolarizability and vertical excitation. We have observed large values of 'first order hyperpolarizability' (301-938 × 10^-30 e.s.u) and 'second order hyperpolarizability' (684-2498 × 10^-34) and they can act as good nonlinear optical materials. Also, vibrational contribution indicates the red shifted absorption and emission in 2c. They show higher values of electrophilicity index which indicates the stability and reactivity of molecules.

Design and Synthesis of Coumarin-Imidazole Hybrid Chromophores: Solvatochromism, Acidochromism and Nonlinear Optical Properties.

A set of linear and asymmetric coumarin-imidazole hybrid compounds having a N,N-diethylamine at 7-position and imidazole at 3-position on the coumarin were synthesized. Insertion of thiophene π-spacer between coumarin and imidazole moieties (5b, 5d) leads to redshifted absorption and emission compared to 5a and 5c. All the compounds show a noticeable response to trifluoroacetic acid with a redshifted absorption and an increase in emission intensity by twofold. The ratio of ground and excited state acidity constant was calculated using Förster energy cycle, and the ratios were found to be 0.25, 0.96, 0.52 and 1.87, respectively, for 5a-5d. Due to the thiophene π-spacer, elongation of π-conjugation in 5b and 5d leads to high values of polarizability (α), first-order hyperpolarizability (ß) and second-order hyperpolarizability (γ). Compound 5b exhibits a high value (895 GM) of two-photon absorption cross section (σ2PA), measured using two-level model.

Fluorescent 7-Substituted Coumarin Dyes: Solvatochromism and NLO Studies.

The effect of three substituents N,N-diethylamine, carbazole and diphenylamine at the 7 position of coumarin on linear and nonlinear optical properties are studied using absorption and emission solvatochromism, and DFT. By varying the substituent 53 nm red shift is achieved in emission. The polarity plots with regression close to unity revealed good charge transfer in the system. Solvent polarizability and dipolarity are mainly responsible for solvatochromic shift as proved by multilinear regression analysis. General Mulliken Hush analysis shows diphenylamine substituent leads to more charge separation in compound 6c. The hyperpolarizabilities are evaluated by quantum mechanical calculations. Structure of the compounds are optimized at B3LYP/6-31G(d) level and NLO computations are done using range separated hybrid functionals with large basis sets. Second order hyperpolarizability (γ) found 589.27 × 10-36, 841.29 × 10-36 and 1043.00 × 10-36 e.s.u for the compounds 6a, 6b and 6c respectively.

Viscosity-active D-π-A chromophores derived from benzo[b]thiophen-3(2H)-one 1,1-dioxide (BTD): Synthesis, photophysical, and NLO properties.

Donor-π-Acceptor (D-π-A) compounds comprising of benzo[b]thiophen-3(2H)-one 1,1-dioxide (BTD) as acceptor with dibenzofuran, carbazole, triphenylamine, and N-methyl diphenylamine moieties as donors were synthesized for aggregation, viscosity induced emission enhancement and nonlinear optical studies. Compounds 3a-3d exhibited solid state emission. The compounds 3a-3d are viscosity sensitive in a solution of MeOH: PEG-400 and showed 10, 23, 14, and 25 fold viscosity induced enhanced emission. The compounds 3a, 3c, and 3d are aggregation-induced emission enhancement (AIEE) active while 3b quenches the fluorescence on aggregation. The quantum yield of 3a, 3c, and 3d in acetonitrile are 0.041, 0.002 and 0.002 which are enhanced in the aggregate state to 0.31, 0.009, and 0.22 respectively. Solvent-dependent parameters like dipole moment (µ), static polarizability (α), and hyperpolarizability (ß and γ) were determined spectroscopically and using Density Functional Theory (DFT) calculations. First and second order hyperpolarizability increase as donor strength increases and the trend is found as 3a < 3b < 3c < 3d. Two-photon absorption (2PA) cross sections were calculated by the spectroscopic method, and large 2PA was observed 484.39 GM for compound 3c.

A combined experiment and computation study of the fused polycyclic benzimidazole derivatives.

Novel fused polycyclic benzimidazole derivatives were synthesized from dimethyl 4,5-diaminophthalate/dimethyl 3,4-diaminophthalate with aromatic anhydride (a-b) by condensation. The UV-Visible absorption and fluorescence emission spectra of the dyes were studied in solvents of differing polarity. The dyes were characterized by the spectral analysis. Density Functional Theory computations have been used to derive more understanding of structural, molecular, electronic and photophysical parameters of the push-pull dyes. The computed absorption wavelength values were observed to be in good agreement with the experimental results. The second order hyperpolarizability (ß(o)) values were computed by Density Functional Theory and found to be in the range of 67.16 × 10(-31) to 107.76 × 10(-31) e.s.u.