Chemistry

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Shiv Nadar University

Achieving synthetic control over light-driven molecular dynamics is essential for designing complex molecule-based devices. Here we design a novel coumarin-imidazole conjugate (1) whose excited state structural dynamics are primarily controlled by a distant intramolecular H-bonding interaction within the backbone. The coumarin conjugate is based on a 1,2,4,5-aryl substituted imidazole framework (aryl = -Ph and -PhOH) covalently connected to the coumarin moiety via a C-N bond. A carefully positioned OH group in the aryl part of the imidazole fragment resulted in achieving two dissimilar O-H⋯N and O-H⋯O distal intramolecular hydrogen bonding interactions. NMR studies in conjunction with density functional theory (DFT) at the B3LYP/6-311G(d,p) level of theory show the existence of two ground state conformers with a rotational barrier of 6.12 kcal mol-1. Due to the presence of conformational isomers of 1, the local excited state dynamics of the parent coumarin get biased towards a long-lived fluorescence state with diminished non-radiative decay channels. Time-resolved emission studies show an ∼4-5 times increase in the excited state lifetime in 1 when compared to coumarin-imidazole conjugates, 2 and 3, without the OH group. Solvent dependent studies show that solvent polarity, the H-bond donating ability and viscosity dictate the conformational distribution in the ground state and the dynamical evolution to the final emissive state. Our studies highlight the importance of rotamerism around the C1-C4 single bond, which leads to rigidification along the coumarin-imidazole backbone through a combination of distal H-bonding and solvent interactions. The concept of new emission signaling pathways caused by conformational switching between two states offers a new paradigm to introduce functional allostery in macromolecular backbones. © 2018 the Owner Societies.

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Physical Chemistry Chemical Physics

Conformational switching: Via an intramolecular H-bond modulates the fluorescence lifetime in a novel coumarin-imidazole conjugate

The emergence of single-component organic dual light emitters holds great promise for white light-emitting diodes (WLEDs) and biological detection due to the involvement of broad emission covering visible spectrum. Here we show experimental studies on dual emission of carbazole-quinoline conjugate (CQ) that exhibits both thermally activated delayed fluorescence (TADF) via reverse intersystem crossing (rISC) from the higher-lying triplet state (T2) to the singlet state (S1) and room-temperature phosphorescence (RTP) from the lowest triplet state (T1) due to low energy gap between T2 and S1, and energetic proximity of T1 with T2. We found in thermal effect that the intensity of the dual features is enhanced with increasing temperatures up to 100 °C, which can be explained by a thermal-induced structural change (TISC) mechanism that compensates the emission losses due to nonradiative transitions at elevated temperatures. This property, in addition to its enhanced TADF and phosphorescence decay rates (∼107 s-1and 101 s-1) at 100 °C, would have great promise for high-efficiency LEDs. © 2018 American Chemical Society.

Journal of Physical Chemistry Letters

Dual Emission through Thermally Activated Delayed Fluorescence and Room-Temperature Phosphorescence, and Their Thermal Enhancement via Solid-State Structural Change in a Carbazole-Quinoline Conjugate

Despite significant progress in the synthesis of covalent organic frameworks (COFs), reports on the precise construction of template-free nano- and microstructures of such materials have been rare. In the quest for dye-containing porous materials, a novel conjugated framework DPP-TAPP-COF with an enhanced absorption capability up to $\lambda$=800 nm has been synthesized by utilizing reversible imine condensations between 5,10,15,20-tetrakis(4-aminophenyl)porphyrin (TAPP) and a diketopyrrolopyrrole (DPP) dialdehyde derivative. Surprisingly, the obtained COF exhibited spontaneous aggregation into hollow microtubular assemblies with outer and inner tube diameters of around 300 and 90 nm, respectively. A detailed mechanistic investigation revealed the time-dependent transformation of initial sheet-like agglomerates into the tubular microstructures.

Angewandte Chemie - International Edition

Microtubular Self-Assembly of Covalent Organic Frameworks

Achieving red phosphorescence from purely organic system is a challenging feat due to the predominant thermal nonradiative decay pathways of the excited electrons. Here, we design single-component charge-transfer (CT) complexes based on phenoxazine-quinoline conjugates (PQ1-PQ3), in which the phenoxazine ring is covalently attached to the quinolinyl fragment via a C-N bond. These conjugates in concentration-dependent absorption studies show a new low-energy CT absorption band along with the parent Ï€-Ï€∗ band with binding constants of up to 102 M-1 due to self-association via intermolecular CT (I2CT). Steady-state emission and phosphorescence decay transient measurements of all of the conjugates in solutions, thin films, and crystals reveal the signature of I2CT that leads to orange-red phosphorescence (ORP) at ambient conditions. Theoretical calculations show the existence of dimer with stronger I2CT characteristics and reduced energy gap between the lowest singlet (S1) and triplet (T1) states (Î"EST = 0.05-0.14 eV), which is in line with emission measurements. These conjugates are used for solid-state dichloromethane vapor sensors, and PQ3 can be transformed into oxygen sensor. These studies give an insight into the ORP properties and provide a rational strategy for the design of single-component self-CT complexes with ORP feature at ambient conditions. © 2018 American Chemical Society.

Journal of Physical Chemistry C

Roomerature Orange-Red Phosphorescence by Way of Intermolecular Charge Transfer in Single-Component Phenoxazine-Quinoline Conjugates and Chemical Sensing

Diabetes a non-communicable disease occurs either due to the lack of insulin or the inability of the human body to recognize it. The recent data indicated an increase in the trend of people diagnosed with type 2 diabetes mainly due to unhealthy life style. Here in we report a new class of oxindole derivatives 6a-k via scaffold hopping of known α-glucosidase inhibitors 1–4. When molecular docking was performed against a homology model of α-glucosidase the resulting compound 6d revealed binding interactions comparable to 1–4. The compounds were accessed through a unique condensation-ring opening protocol of pyridofuranone building blocks. Overall the compounds exhibited decent binding to the yeast α-glucosidase, where the most potent compound 6h, inhibited the enzyme with IC 50 of 0.6 µM. This was nearly threefold improvement from the original known compounds 1–4, selected to design the newer analogs. The reaction kinetics of 6h indicated competitive inhibition. © 2018 Elsevier Ltd

Bioorganic and Medicinal Chemistry

Design, synthesis and in vitro study of densely functionalized oxindoles as potent α-glucosidase inhibitors

Formulae for the static electronic polarizability and hyperpolarizability are derived in terms of moments of the ground-state electron density matrix by applying the Unsöld approximation and a generalization of the Fermi-Amaldi approximation. The latter formula for the hyperpolarizability appears to be new. The formulae manifestly transform correctly under rotations, and they are observed to be essentially cumulant expressions. Consequently, they are additive over different regions. The properties of the formula are discussed in relation to others that have been proposed in order to clarify inconsistencies. The formulae are then tested against coupled-perturbed Hartree-Fock results for a set of 40 donor-π-acceptor systems. For the polarizability, the correlation is reasonable; therefore, electron density matrix moments from theory or experiment may be used to predict polarizabilities. By constrast, the results for the hyperpolarizabilities are poor, not even within one or two orders of magnitude. The formula for the two- and three-particle density matrices obtained as a side result in this work may be interesting for density functional theories. © 2018 CSIRO.

Australian Journal of Chemistry

Is it Reasonable to Obtain Information on the Polarizability and Hyperpolarizability only from the Electron Density?

Purely organic biluminescent materials are of great interest due to the involvement of both singlet and long-lived triplet emissions, which have been rarely reported in bioimaging and organic light-emitting diodes. We show two molecules 3,4,5,6-tetraphenyloxy-phthalonitrile (POP) and 3,4,5,6-tetrakis-p-tolyloxy-phthalonitrile (TOP), in which POP was found to exhibit fluorescence and persistent room-temperature green phosphorescence (pRTGP) in the amorphous powder and crystal states. Both POP and TOP show aggregation-induced emission in a tetrahydrofuran-water mixture. We found in single-crystal X-ray analysis that intra- and intermolecular lp(O)···π interactions along with π(C = C)···π(C≡N), hydrogen bond (H-B), and C-H···π interactions induce a head-to-tail slipped-stack arrangement in POP. In addition, the X-ray structure of TOP with a slipped-stack arrangement induced by only π(C=C)···π(C≡N) and H-B interactions shows dim afterglow only in crystals. These indicate that more noncovalent interactions found in POP may reinforce relatively efficient intersystem crossing that leads to pRTGP. Given the unique green afterglow feature in amorphous powder of POP, document security protection application is achievable. Copyright © 2018 American Chemical Society.

Biluminescence via Fluorescence and Persistent Phosphorescence in Amorphous Organic Donor(D4)-Acceptor(A) Conjugates and Application in Data Security Protection

We report on the preparation of hexa-functional cardanol (renewable phenolic compound) benzoxazine with a phosphazene core (CPN) for use as a greener eco-friendly halogen-free flame retardant reactive additive for the formation of sustainable polyphosphazene polybenzoxazine networks for flame resistant applications. The structure and purity of the monomer was confirmed by Fourier transform infrared (FTIR), nuclear magnetic resonance (1H-, 13C-, 31P NMR) and gel permeation chromatography studies. The CPN monomer showed good compatibility with benzoxazine monomer (CPN0) as suggested by the cocuring studies. The thermal properties of the copolymer can be directly tuned by altering the composition of the monomer blend. The occurrence of phosphazene-phosphazane thermal rearrangement is also suggested for the thermal behavior (thermogravimetry analysis) at higher loading of CPN in the monomer feed ratio. An improvement in mechanical properties of the copolymer with increase in glass transition temperature was confirmed by enhancement in cross-link density as compared to neat polybenzoxazine. The reactive nature and presence of phosphazene core improved both the smoke density rating, vertical burning rating and led to higher limiting oxygen index. The FTIR and scanning electron microscopy studies of residual char supported the formation of functionalities and morphologies favorable to support the flame resistance behavior of polymer by incorporation of reactive benzoxazine with phosphazene core. Finally, we demonstrate that incorporation of cardanol phosphazene network has good compatibility with the polybenzoxazine phenolic thermosets with improvement in flame retardancy. The higher cardanol (65.7\%) and phosphorus content (3.4\%) and reactive nature of synthesized compound is attractive as a sustainable additive with the scope of their utilization with other polymeric resins. © 2017 American Chemical Society.

ACS Sustainable Chemistry and Engineering

Eco-Friendly Halogen-Free Flame Retardant Cardanol Polyphosphazene Polybenzoxazine Networks

A family of pharmacophore acetylenic indole-3-carboxaldehyde (3) and acetylenic indole schiff bases (5 a-5 i) bearing indole framework were synthesized to afford structurally diverse moieties. Acetylenic indole-3-carboxaldehyde was coupled with various amines to furnish the novel scaffolds 5 a-5 i. These acetylenic indole Schiff bases (AISB) were synthesized in shorter times under microwave irradiation in comparison to classical reaction conditions. AISB were experimentally characterized by using IR, 1H NMR, 13C NMR, X ray crystallographic and mass spectrometric methods. The synthesized compounds showed promising anti-bacterial and anti-fungal properties when tested in vitro against six bacterial and seven fungal strains. Among these, hybrids 5 a, 5 c, 5 e and 5 g were found to be even more potent than the standard drugs and were more influential against the bacterial than the fungal strains. These more potent compounds were then evaluated for their cytotoxicity by MTT assay on Hek293 and HeLa cells. The synergistic effect of bis-acetylenic compound 5 g with kanamycin and amphotericin B has also been scrutinized. Further, the structural and electronic properties of the molecules were investigated theoretically by performing density functional theory (DFT). © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim