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Four new N, N-diethylaniline comprising D-π-A and D-A′-π-A type dyes were synthesized and characterized by 1H NMR, 13C NMR, and elemental analysis. The dyes show positive and negative solvatochromism in absorption and emission, especially the compounds (E)− 2-Cyano-3-(5-((E)− 1-cyano-2-(4-(diethylamino)− 2-(hexyloxy) phenyl) vinyl) thiophen-2-yl) acrylic acid (D1) and (E)− 2-Cyano-3-(5-((E)− 4-(diethylamino)− 2-(hexyloxy) styryl) thiophen-2-yl) acrylic acid (D2) having cyanoacrylic acid as an acceptor group show negative emission solvatochromism, and (E)− 2-((5-(1-Cyano-2-(4-(diethylamino)− 2-(hexyloxy) phenyl) vinyl) thiophen-2-yl) methylene) malononitrile (B1) and (E)− 2-((5-(4-(Diethylamino)− 2-(hexyloxy) styryl) thiophen-2-yl) methylene) malononitrile (B2) show positive emission solvatochromism. The D-A′-π-A type dyes (B1 and D1) show viscosity sensitive emission intensity enhancement (6-fold) in far red (FR) to near infra-red (NIR) region in the electromagnetic spectrum. The charge transfer characteristics were established through solvatochromism, polarity graphs, bond length alternation (BLA), bond order alternation (BOA), natural bond order (NBO) analysis and HOMO-LUMO energy gap. Kamlet-Taft and Catalan parameter results confirmed that dipolarity and polarizability factor are responsible for solvatochromism. These dyes exhibit higher first order hyperpolarizability than the urea. Fluorescence lifetimes are more in D-A′-π-A system compared to the D-π-A system. Due to the higher non-linear optical characters, these dyes can be considered as promising candidates for organic NLOphores.

A series of five azo acid dyes were synthesized using different diazotized aromatic amine sulfonic and carboxylic acids followed by coupling with 2H-pyrido[1,2-a]pyrimidine-2,4(3H)-dione. The dyes were characterized by 1H-NMR, 13CNMR, FTIR, and elemental analysis. They were applied on nylon, silk, and wool. Their fastness properties were evaluated and color on the fabric was assessed. Yellow, brown, and crimson dyeings with good fastness properties were obtained. The dyes are water soluble and showed absorption from 380 nm to 550 nm. Acid dyes from diazo component 4- aminonaphthalene-1-sulfonic acid (naphthionic acid) and 4-amino-5-hydroxynaphthalene-2,7-disulfonic acid (H-Acid) were found to be more red shifted as compared to the other dyes. More red shifted absorption maximum was observed in acidic pH than in alkaline pH. The trends in vertical excitations obtained from Time Dependent Density Functional Theory calculations are in good agreement with the experimental absorptions.

Four novel fluorescent whitening agents based on 4-methyl-2H,5H-pyrano derivatives of 2H-pyrido[1,2- a ]pyrimidine-2,4(3H)-dione were synthesized using a simple two step route. The structures were characterized by spectroscopy techniques such as, 1 H, 13 C NMR, and HRMS. Photophysical properties in various microenvironments were evaluated experimentally and correlated with optical absorption and fluorescence emission using global hybrid (B3LYP) and long range separated functional (CAM-B3LYP) with the Pople's 6–31G(d) basis set and in combination with polarizable continuum model. The experimental and theoretical photophysical properties were found in good agreement with each other. The excited state calculations revealed that the excitation values were in the range of 350–370 nm and emission values were in the range of 400–425 nm suggesting their potential application as fluorescent whitening agents. Dyes 4a-d were applied to polyester fabric and their performance was evaluated by means of measuring the degree of whiteness, the degree of brightness, color values, and reflectance spectrum in comparison with a standard fluorescent whitening agent, Hostalux ESR. Interestingly our synthesized optical whitener, 4a showed pronounced enhancement in the whitening properties over the Hostalux ESR.

Density Functional Theory and Time Dependent Density Functional Theory computations reveal experimental trends accurately and provide an insight into the origin of red shifted optical spectra, large Stokes shift, nonlinear optical responses and quantitative description of the singlet-triplet energy gap of recently synthesized highly fluorescent membrane permeable benz[c,d]indole N-heteroarene BF2 rigidified BBN and BBC dyes. Our results benchmark the importance of the DFT functionals and polarization functions using the accurate basis set for the calculations of excitation energies, band gaps, dipole moment, non-linear optical properties and geometrical parameters. The anomalous increase in dipole moment in the excited state and pronounced geometric distortion are found to be responsible for the increased Stokes shift in BBN dyes. The resultant decrease in the HOMO–LUMO energy gap was found to be responsible for the red shift in BBC dyes. Enhanced nonlinear optical properties and inverse relationship between the Stokes shift and static hyperpolarizability were found in these dyes. Their possible potential therapeutic use as a photosensitizer in photodynamic therapy is proposed on the basis of vertical triplet energies resulting in the range of 0.78 to 0.88 eV. The singlet–triplet energetic gaps suggest that the simple variation of the N-heteroarenes or their post functionalization can lead to the efficient generation of singlet oxygen. Further, we compared these new class of dyes with conventional aza-BODIPY and BODIPY dyes, thereby confirming their use as alternative promising candidates for nonlinear optics, triplet sensitizers, and imaging.

Density functional theory and time-dependent density functional theory computations were used to understand the electronic and photophysical parameters of NIR β-thiophene-fused BF2-azadipyrromethene dyes. The computed data are in good agreement with those obtained experimentally and they provide insights into the origin of red shifted optical spectra compared to the parent aza-BODIPY, low Stokes shift, non-linear optical responses and quantitative description of the singlet–triplet energy gap. The resultant decrease in the HOMO − LUMO energy gap is responsible for the red shift. The possible use as non-linear optical materials is supported by large enhancement in the non-linear optical properties. On the basis of vertical triplet energies, their possible potential therapeutic use as a photosensitizer in photodynamic therapy is proposed. The singlet–triplet energetic gaps suggest that the β-thiophene-fused BF2-azadipyrromethene dyes can act as a sensitizer to produce an efficient generation of singlet oxygen. Their optimal use as an efficient singlet fission materials has been proposed on the basis of excitation energies in the ground, lowest singlet and triplet excited states.

Density Functional Theory and Time Dependent Density Functional Theory computations reveal experimental trends accurately and provide an insight into the origin of red shifted optical spectra, large Stokes shift, nonlinear optical responses and quantitative description of the singlet-triplet energy gap of recently synthesized highly fluorescent membrane permeable benz[c,d]indole N-heteroarene BF

Density functional theory and time-dependent density functional theory computations were used to understand the electronic and photophysical parameters of NIR β-thiophene-fused BF

Novel carbazole based styryl derivatives (6a-6c) having styryl group at third position and a methoxy substitution were synthesized by condensing 4-methoxy-9-methyl-9H-carbazole-3-carbaldehyde 3 and different active methylene derivatives (5a-5c). Evaluated photophysical properties of these synthesized novel chromophores, studied the effect of solvent polarity on absorption, emission and quantum yield of these styryl derivatives. DFT and TD-DFT computations are carried out to study structural, molecular, electronic and photophysical parameters of dyes. The ratio of ground state to excited state dipole moment was calculated using Bakhshiev and Kawski-Chamma-Viallet correlations.

3-Ethoxy-N,N-diethylaniline substituted aza-BODIPY dye (7) was designed and synthesized that could recognize Cu (II) ions through sequential binding interactions. The visible changes in the absorption and enhancement in fluorescence intensity with the addition of Cu (II) ions makes this dye as a selective and sensitive probe for on-site visual detection and analysis. The presence of 3-ethoxy-N,N-diethylaniline group at the peripheral positions shifted its absorption to the near-infrared region, ca. 640–890 nm of the electromagnetic spectrum. This dye showed a gradual shift in the absorption maximum towards red with the increase in polarity of the medium. The triplet excited state properties of this dye suggest that it shows negligible singlet oxygen generation efficiency. The dye possesses excellent photostability, low fluorescence quantum yields, sharp absorption bands in the near-infrared region (NIR) and exhibits selective interactions with Cu(II) ions. The limit of detection was found to be as low as 350 nm and selectivity was not interfered by other metal ions and anions, thereby demonstrating its use as a novel probe for the detection and analysis of copper ions through visual absorption changes and ‘turn on’ fluorescence intensity.

9 p. Four new N-ethylcarbazole-linked aza-boron-dipyrromethene (aza-BODIPY) dyes (8 a,b and 9 a,b) were synthesized and characterized. The presence of the N-ethylcarbazole moiety shifts their absorption and fluorescence spectra to the near-infrared region, λ≈650–730 nm, of the electromagnetic spectrum. These dyes possess strong molar absorptivity in the range of 3–4×104 m−1 cm−1 with low fluorescence quantum yields. The triplet excited state and singlet oxygen generation of these dyes were enhanced upon iodination at the core position. The core-iodinated dyes 9 a,b showed excellent triplet quantum yields of about 90 and 75 %, with singlet oxygen generation efficiency of about 70 and 60 % relative to that of the parent dyes. Derivatives 8 a,b showed dual absorption profiles, in contrast to dyes 9 a,b, which had the characteristic absorption band of aza-BODIPY dyes. DFT calculations revealed that the electron density was spread over the iodine and dipyrromethene plane of 9 a,b, whereas in 8 a,b the electron density was distributed on the carbazole group and dipyrromethene plane of aza-BODIPY. The uniqueness of these aza-BODIPY systems is that they exhibit efficient triplet-state quantum yields, high singlet oxygen generation yields, and good photostability. Furthermore, the photoacoustic (PA) characteristics of these aza-BODIPY dyes was explored, and efficient PA signals for 8 a were observed relative to blood serum with in vitro deep-tissue imaging, thereby confirming its use as a promising PA contrast agent. MOE (Min. of Education, S’pore) NMRC (Natl Medical Research Council, S’pore) MOH (Min. of Health, S’pore) Accepted Version