Faculty Publications
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Item Ultralow thermal conductivity and thermally-deactivated electrical transport in a 1D silver array with alternating δ-bonds(Royal Society of Chemistry, 2024) Hassan, N.; Nagaraja, S.; Saha, S.; Tarafder, K.; Ballav, N.We report the synthesis of a (TMA)AgBr2 (TMA = tetramethylammonium) crystal, which comprises inorganic anionic chains of -(AgBr2)∝- stabilized by columnar stacks of organic TMA cations with a periodic arrangement of shorter and longer Ag(i)⋯Ag(i) bonds, even though all the Ag(i) ions are chemically equivalent. The presence of two chemically non-equivalent bridging Br ions is attributed to the primary cause of such an unusual arrangement, as clearly visualized in the charge density plot of (TMA)AgBr2 extracted from the theoretical calculations based on density functional theory. Remarkably, we identified from the orbital-projected density of states the existence of alternate δ-like bonding involving dxy orbitals of 4d10 Ag(i), which was attributed to the cause for ultralow thermal conductivity and thermally-deactivated electrical transport in (TMA)AgBr2. Barring the energetics, our observations on the existence of a δ-bond will shed new light in understanding the nature of metal-metal chemical bonding and its unprecedented implications. © 2024 The Royal Society of Chemistry.Item Thermally-driven conformational twist in organic azobenzene linker activates molecular doping effect in thin films of lanthanide MOFs(Royal Society of Chemistry, 2025) Bhoi, U.; Kalyani, M.; Ananthram, K.S.; Saha, S.; Acharya, A.; Hassan, N.; Raj, M.; Tarafder, K.; Ballav, N.Azobenzene-based photo-switchable molecules have shown significant potential in stimuli-responsive systems, especially when incorporated into metal–organic frameworks (MOFs). This study reports thin films of lanthanide-based metal–organic frameworks (Ln-MOFs) with 4,4?-azobenzene dicarboxylic acid (H2ADA) as the organic linker – Tb-ADA, Eu-ADA, and Gd-ADA – using an electrodeposition method. Upon heating to 400 K, a reversible structural transition was observed via variable temperature grazing-incidence X-ray diffraction (GIXRD) and Raman spectroscopy, not due to trans–cis isomerization but rather a thermally-induced conformational twist of the ADA linker. Density functional theory (DFT) combined with molecular dynamics (MD) simulations supports this interpretation, revealing high-energy atropisomeric states stabilized by MOF confinement. Molecular doping of these films with 7,7,8,8-tetracyanoquinodimethane (TCNQ) significantly enhanced their electrical conductivity, increasing by two orders of magnitude at 400 K. This enhancement is attributed to improved ?–? stacking and charge-transfer interactions facilitated by the conformational twist. Temperature-dependent X-ray photoelectron spectroscopy (XPS) confirmed redox activity in TCNQ@Tb-ADA films, showing reversible conversion between Tb(iii) and Tb(iv), with back electron transfer at 400 K restoring Tb(iii). These findings introduce a new mechanism of thermally-driven conformational switching in MOFs and open avenues for developing responsive electronic materials based on azobenzene linkers. This journal is © The Royal Society of Chemistry, 2025Item Rotational Flexibility in Dication Drives Ambient Temperature Ferroelectricity in an Organic–Inorganic Hybrid Halide(John Wiley and Sons Inc, 2025) Hassan, N.; Panday, R.; Chandru, P.G.; Ananthram, K.S.; Jose, T.M.; Bhoi, U.; Sieradzki, A.; Zar?ba, J.K.; Boomishankar, R.; Tarafder, K.; Ballav, N.Organic–inorganic hybrid halides (OIHHs) have gained attention as potential ferroelectric materials due to structure-property synergy of the organic and inorganic constituents. This study introduces an unusual Ag(I)-based ternary OIHH, (4,4?-bpy)Ag2Br4, featuring rotational flexibility in the organic dication to induce asymmetry into the structure. The compound crystallizes in a monoclinic crystal system with a non-centrosymmetric polar P21 space group at room-temperature and undergoes a structural phase transition to a centrosymmetric phase (P21/c) at Curie temperature (Tc) of 330 K which was further supported by differential scanning calorimetry (DSC), second harmonic generation (SHG) signals, dielectric anomaly, current-voltage (I–V) profiles, and X-ray photoelectron spectroscopy (XPS) data. Ferroelectricity is confirmed through polarization–electric field (P–E) hysteresis loops and piezoresponse force microscopy (PFM), exhibiting switchable polar domains. Density functional theory (DFT) calculations revealed electronic structures of the ferroelectric and paraelectric phases, identified the (?-AgBr2)nn? inorganic anionic chain contributing to the net polarization, and in general, complemented the experimental results. Comparative studies with structurally analogous Ag(I)-based OIHHs lacking dication rotational freedom endorse the critical role of organic flexibility in driving ferroelectricity. This study provides insights into the role of organic dications in controlling ferroelectric behavior and offers a promising pathway for developing coinage metal-based OIHH ferroelectric materials. © 2025 Wiley-VCH GmbH.