Linear and Nonlinear Optical Properties of All-cis and All-trans Poly(p-phenylenevinylene).

Poly(p-phenylenevinylene) (PPV) is a staple of the family of conjugated polymers with desirable optoelectronic properties for applications including light-emitting diodes (LEDs) and photovoltaic devices. Although the significant impact of olefin geometry on the steady-state optical properties of PPVs has been extensively studied, PPVs with precise stereochemistry have yet to be investigated using nonlinear optical spectroscopy for quantum sensing, as well as light harvesting for biological applications. Herein, we report our investigation of the influence of olefin stereochemistry on both linear and nonlinear optical properties through the synthesis of all-cis and all-trans PPV copolymers. We performed two-photon absorption (TPA) using a classical and entangled light source and compared both classical TPA and entangled two-photon absorption (ETPA) cross sections of these stereodefined PPVs. Whereas the TPA cross section of the all-trans PPV was expectedly higher than that of all-cis PPV, presumably because of the larger transition dipole moment, the opposite trend was measured via ETPA, with the all-cis PPV exhibiting the highest ETPA cross section. DFT calculations suggest that this difference might stem from the interaction of entangled photons with lower-lying electronic states in the all-cis PPV variant. Additionally, we explored the photoinduced processes for both cis and trans PPVs through time-resolved fluorescence upconversion and femtosecond transient absorption techniques. This study revealed that the sensitivity of PPVs in two-photon absorption varies with classical versus quantum light and can be modulated through the control of the geometry of the repeating alkenes, which is a key stepping stone toward their use in quantum sensing, bioimaging, and the design of polymer-based light-harvesting systems.

In vitro and in silico analysis of L. donovani enoyl acyl carrier protein reductase - A possible drug target.

The emergence of increased resistance to the available drugs has created a situation that demands to find out more specific molecular drug targets for Leishmaniasis. The enoyl acyl carrier protein reductase (ENR), a regulatory enzyme in type II fatty acid synthesis, was confirmed as a novel drug target and triclosan as its specific inhibitor in many microorganisms. In this study, the triclosan was tested for the leishmanicidal property against Leishmania donovani (L. donovani) and the results of in vitro and ex vivo drug assays on promastigotes and amastigotes showed that triclosan possessed antileishmanial activity with a half minimal inhibitory concentration (IC50) of 30 µM. Consequently, adopting in silico approach, we have tested the triclosan's ability to bind with the L. donovani enoyl acyl carrier protein reductase (LdENR). The 3D structure of LdENR was modelled, triclosan and cofactors were docked in LdENR model and molecular dynamic simulations were performed to observe the protein-ligands interactions, stability, compactness and binding energy calculation of the ligands-LdENR complexes. The observation showed that triclosan stably interacted with LdENR in presence of both the cofactors (NADPH and NADH), however, simulation results favor NADH as a preferred co-factor for LdENR. These results support that the reduction of L. donovani growth in the in vitro and ex vivo drug assays may be due to the interaction of triclosan with LdENR, which should be confirmed through enzymatic assays. The results of this study suggest that LdENR could be a potential drug target and triclosan as a lead for Leishmaniasis.Communicated by Ramaswamy H. Sarma.

Validation of NAD synthase inhibitors for inhibiting the cell viability of Leishmania donovani: In silico and in vitro approach.

NAD (nicotinamide adenine dinucleotide) synthase catalyses the biochemical synthesis of NAD, from nicotinic acid adenine dinucleotide (NAAD). NAD may be synthesized through the de novo pathways and/or the salvage pathways in cells. However, in Leishmania parasite, the synthesis of NAD solely depends on the salvage pathways. NAD synthetase is widely explored as a drug target in various microorganisms. In Bacillus anthracis, a group of sulphonamides 5599, 5617 and 5824 and complex amide 5833 were reported to have activity at micromolar range against NAD synthetase. Hence, in the present study, the same group of sulphonamides and complex amide were validated through in silico and in vitro studies for its efficiency towards Leishmania donovani NAD synthase. In silico study revealed the ligands 5824 and 5833 to have better docking score. Molecular dynamics simulation for a duration of 50 ns of all the ligand-protein complexes suggested that the complexes with the ligands 5824 and 5833 were stable and interacting. In vitro and ex vivo studies have shown that 5824 and 5833 inhibit the cell viability of the organism at a lower concentration than 5599 and 5617. Hence, with further in vivo validation, 5824 (or its synthetic analogues) and 5833 could be the choice that may work synergistically with other potential drugs in treating drug-resistant cases of leishmaniasis. Communicated by Ramaswamy H. Sarma.

Excitation-dependent electron exchange energy and electron transfer dynamics in a series of covalently tethered N,N-bis(4'-tert-butylbiphenyl-4-yl)aniline - [C60] fullerene dyads via varying π-conjugated spacers.

Femtosecond time-resolved fluorescence and transient absorption studies are reported for three newly synthesized covalently linked N,N-bis(4'-tert-butylbiphenyl-4-yl)aniline (BBA) and pyrrolidinofullerenes (C60)-based donor-π conjugated bridge-acceptor dyads (D-B-A) as functions of the bridge length (7.1, 9.5 and 11.2 Å for Dyad-1, Dyad-2 and Dyad-3), dielectric constants of the medium and pump wavelengths. In polar solvent, ultrafast fluorescence quenching (kEET ≥ 2 × 1012 s-1) of the BBA moiety upon excitation of the BBA moiety (320 nm) is observed in the dyads and is assigned to a mechanism involving electron exchange energy transfer (EET) from 1BBA* to C60 followed by electron transfer from BBA to 1C60*. Cohesive rise and decay dynamics of conjugated BBA˙+-C60˙- anion pairs confirm the involvement of a distance independent adiabatic charge-separation (CS) process (kCS ≥ 2.2 × 1011 s-1) with near unity quantum efficiency (φCS ≥ 99.7%) and a distance-dependent non-adiabatic charge-recombination (CR) process [kCR ∼ (1010-108) s-1]. In contrast, excitation of the C60 moiety (λex = 430 to 700 nm) illustrates photoinduced electron transfer from BBA to 1C60*, involving non-adiabatic (diabatic) and distance-dependent CS (kCS in the range of 0.59-1.78 × 1011 s-1) with 98.86-99.6% (Dyad-3-Dyad-1) quantum efficiency and a CR process with kCR values [kCR ∼ (1010-108) s-1] up to three orders greater than kCS of the respective dyads. Both the processes, CS and CR, upon C60 excitation and the CR process upon BBA excitation show distance dependent rate constants with exponential factor ß ≤ 0.5 Å-1, and electron transfer is concluded to occur through a covalently linked conjugated π bridge. Global and target analysis of fsTA data reveal the occurrence of two closely lying CS states, thermally hot (CShot) and thermally relaxed (CSeq) states, and two CR processes with two orders of different rate constants. Careful analysis of the kinetic and thermodynamic data allowed us to estimate the total reorganization energy and electronic coupling matrix (V), which decrease exponentially with distance. These novel features of the distance independent adiabatic CS process and the distance-dependent diabatic CR process upon donor excitation are due to extending the π-conjugation between BBA and C60. The demonstrated results may provide a benchmark in the design of light-harvesting molecular devices where ultrafast CS processes and long-lived CS states are essential requirements.

Photoinduced bimolecular electron transfer from aromatic amines to pentafluorophenyl porphyrin combined with ultrafast charge recombination persistence with Marcus inverted region.

The dynamics of photoinduced bimolecular reductive electron transfer between meso-tetrakis(pentafluorophenyl)porphyrin (H2F20TPP), an acceptor (A), and five aromatic amines (donor (D)) with varying oxidation potentials (aniline (AN), N-methylaniline (MAN), N-ethylaniline (EAN), N,N-dimethylaniline (DMAN) and N,N-diethylaniline (DEAN)) in dichloromethane (DCM) as a solvent as well as in neat donor solvents were investigated by employing nanosecond to femtosecond time-resolved fluorescence spectroscopy and femtosecond time-resolved transient absorption spectroscopy upon S2 excitation of H2F20TPP. Systematic studies of time-resolved fluorescence quenching dependent on the donor concentration in the concentration range of 0.01-2 M and finally in neat donor solvents broadly enabled us to determine the electron transfer dynamics in three regimes of electron transfer: stationary or diffusion-controlled electron transfer, non-stationary electron transfer and intrinsic or ultrafast electron transfer. Depending upon the electron-donating ability of the studied donors, intrinsic electron transfer was found to occur in the time domain of ∼1-9 ps and diffusion-controlled ET dynamics was observed in the time domain of 200-500 ps, whereas the maximum limit of non-stationary electron transfer could be observed in the time domain of 15-50 ps. Femtosecond transient absorption studies together with global and target analysis helped to identify the spectral signature of the (H2F20TPP˙-) radical anion as the product of ET. To the best of our knowledge, this is the first ever evidence that shows the spectra of an anion as the product of ET for any porphyrin-based electron transfer dynamics. However, transient absorption measurements confirm that intrinsic ET occurs in the Qy state, whereas diffusion-controlled ET occurs in the hot Qx as well as in the thermal equilibrium Qx state. The most remarkable fact derived from the measurements of transient absorption was that the rate of the forward electron transfer (CS) is exactly the same as the rate of the backward electron transfer (CR) for all three regimes of ET. The thermodynamic driving force for CR was found to lie in the range of the total reorganization energy for the studied systems and hence falls in the Marcus optimal region, and the CR process is barrierless. The dependence on the driving force of the combination of forward and reverse electron transfer exhibited a bell-shaped curve for all three regimes of electron transfer, even though the rate of intrinsic ET is higher by a factor of ∼102 than that of diffusion-controlled ET. These results unambiguously favour the Marcus theory, in particular the controversial Marcus inverted region, of outer-sphere electron transfer.