Center for Excellence in Basic Sciences

The search for molecules that undergo efficient singlet (exciton) fission (SF) to generate two triplet excitons following absorption of a single photon and create the possibility of generation of a pair of charge carriers has been very intense during the last two decades. Tetracene and its derivatives have shown great potential to become efficient SF materials. Under this consideration, earlier workers predicted the potentiality of naphtho-[2,1,8-qra]-tetracene (NpTc), which nearly satisfies the energetic requirement of SF, to work as an efficient SF material. But its detailed photophysics and dynamics of the SF process are yet to be explored. This work provides a comprehensive account of a detailed investigation of the photophysical properties of the excited states of the molecule in polycrystalline mediums by using ultrafast fluorescence and transient absorption (TA) spectroscopic techniques to resolve the intricate mechanism of the SF process. Generation of a pair of free triplet (T1) excitons in the SF process has been found to follow a mechanism of two sequential and one parallel step after the population of the singlet (S1) exciton state. Two sequential steps consist of the ultrafast decay of the S1 exciton populating the spin-correlated interacting triplet pair state (CTP1), in which two T1 excitons reside on two molecules at neighboring sites in the crystal. In turn, the CTP1 undergoes the spatial separation of two triplets to create a spin-correlated but noninteracting spin correlated triplet pair, the CTP2 state, in which the triplet excitons are spatially separated beyond their neighboring sites. In the last step of the SF process, the disappearance of the CTP2 state follows a parallel step, in which two T1 excitons either get separated into two free T1 excitons or undergo triplet fusion to generate back one S1 exciton and one molecule in the S0 state. The overall lifetime of completion of the SF process (or generation of two free T1 excitons) in 45 nm thin films is about 31 ps. However, the SF rate decreases upon increasing the thickness of the film, for which one of the possible reasons may be attributed to reduction in intermolecular electronic coupling energy in thicker films.

ABSTRACTThis study reports the development of a 3‐step strategy that is both cost‐effective and quick to screen marine organisms and validate the presence of extracellular nucleases. The assay plates (M9 or Luria Broth [LB] media with 500 mM salt) were overlaid with a thin layer of top agar containing Toluidine Blue as the indicator and salmon sperm DNA as the substrate. Primary screening of halophiles was based on their zone of clearance. Secondary screening of the isolates involved assaying the supernatants using a well‐diffusion assay. The isolates were further screened and validated by ammonium sulfate fractionation of the cell‐free supernatants to enrich the secreted nuclease. The three‐step method narrowed down nine potential isolates from ∼500 bacterial colonies, of which SH1 demonstrated nuclease activity, discernibly due to a secreted extracellular enzyme(s). Further characterization of this enriched nuclease(s) showed that it is likely made up of multiple peptides/subunits, acts as an endo‐ as well as exonuclease, degrades both DNA and RNA, is Mg+2 dependent, has a wide range of salt tolerance from 80–1500 mM, is optimally active at 37°C, and is stable against reducing agents. This validates the screening strategy thus opening doors to further bioengineering of novel nucleases from other extremophiles.

Electron-neutral collisions play an important role in the physics of Hall thrusters, providing the necessary ionization crucial for thrust generation. In this work, we have analyzed the effect of electron-neutral collisions on the high-frequency instabilities in Hall thruster plasmas. Numerical studies of Hall thruster plasmas have shown enhanced anomalous transport and rapid radial heating due to high-frequency oscillations driven by the Electron Cyclotron Drift Instability (ECDI) and Modified Two-Stream Instability (MTSI). Although numerical simulations demonstrate these instabilities, the complex nonlinear interactions associated with these instabilities and the effect of collisions on these instabilities are yet to be understood completely. In this work, we present the results of our computational studies on the effect of collisions on ECDI/MTSI using an in-house 2D particle-in-cell-Monte Carlo Collision code, primarily focusing on the spectral behavior of these instabilities. Our findings indicate that electron-neutral collisions certainly enhance the MTSI mode at the expense of the ECDI mode, causing an inverse-cascade to long wavelength modes. In addition, collisions enhance the mobility of electrons along the axial direction, accompanied by a stronger radial-azimuthal electron temperature anisotropy.

Metal-organic frameworks (MOFs) are an excellent platform for photochemical CO2 reduction into valuable chemicals. Herein, we report the synthesis and photocatalytic behavior of Ru@MOF-808, a Zr-based MOF that was post-synthetically modified with a Ru-polypyridyl complex. The post-synthetic modification was achieved using solvent-assisted incorporation of bipyridine-carboxylate ligands onto the nodes of the MOF-808, followed by the coordination of the Ru(II)-terpiridine moiety. A thorough characterization including 1H-NMR, diffuse reflectance UV/Vis spectroscopy, X-ray absorption spectroscopy and gas adsorption studies, combined with DFT calculations, provide strong support for efficient incorporation of the molecular Ru-complex at the loading of one Ru center per node. In the presence of a strong sacrificial reductant BIH(1,3-Dimethyl-2-phenylbenzimidazoline), Ru@MOF-808 was found to catalyze photochemical reduction of CO2 into a mixture of CO and formate ion. When compared to the homogeneous model catalyst Ru(tpy)(bpy)₂⁺, Ru@MOF-808 was found to exhibit higher formate yields. To explain these formate enhancements, we propose a mechanism that involves the CO2 capture at the MOF nodes to form Zr-bicarbonate species, which further react in a hydride transfer reaction with photo-generated Ru-H donor, thereby outperforms molecular catalyst in HCOO- production. Overall, the results presented in this work indicate the potential of Zr-based MOFs in integrating CO2 capture with its photochemical conversion to desired products.

An effective design strategy for creating imidazo[1,2- a ]pyridine-derived luminogens is proposed, offering a dependable avenue for building TADF-DSE materials for applications in OLEDs and cell imaging.