Jose Antonio Paixão

For the rational design of solid-state organic luminophores, achieving predictable control over excimer emission remains a central challenge, as it is governed by subtle variations in molecular packing. Here, we demonstrate that excimer emission in fluorene-based molecular solids is a sterically programmable excited-state pathway that can be selectively enabled or suppressed through molecular design. A systematic series of fluorene derivatives bearing symmetric (2,7-dmF, 2,7-dtbF, and 3,6-dtbF) and asymmetric (2-tbF) alkyl substituents was investigated to elucidate how steric effects control supramolecular organization and excited-state behavior. Combined photophysical characterization, single-crystal X-ray diffraction, and DFT/TDDFT calculations establish a direct structure-packing-property relationship governing emission pathway. Planar fluorene and 2,7-dimethyl derivatives adopt antiparallel π–π stacked arrangements that stabilize excimer states, resulting in intense solid-state emission with high photoluminescence quantum yields (up to ∼60%) and nanosecond-scale lifetimes (up to ∼12 ns), accompanied by delayed components indicative of packing-controlled excimer formation. In contrast, tert-butyl substitution disrupts cofacial π–π interactions, suppressing excimer formation and yielding predominantly monomer-like emission. This steric control is further validated in fluorene: diamantane mixtures, where diamantane acts as a rigid three-dimensional spacer that increases intermolecular separation and inhibits excimer formation. Complementary dimer-based electronic structure calculations and charge-density analyses provide molecular-level insight into how steric constraints modulate intermolecular electronic coupling and excited-state character. These findings establish excimer emission as a packing-controlled and designable excited-state phenomenon and provide general design principles for tuning solid-state luminescence through steric engineering of molecular assemblies.

Double perovskite oxides have emerged as promising multifunctional materials for sustainable energy applications due to their tunable structural, electronic, magnetic, and electrocatalytic properties. In this research study, the cation-disordered double perovskite TbNdFeCoO6 (TNFCO) was successfully synthesized by a conventional solid-state reaction method and systematically investigated through structural, vibrational, electronic, magnetic, and electrochemical characterization. Rietveld refinement of X-ray diffraction data confirmed the formation of a single-phase orthorhombic structure with Pnma symmetry, accompanied by significant octahedral distortion and B-site cation disorder. Raman spectroscopy revealed lattice distortions consistent with the structural analysis, while X-ray photoelectron spectroscopy (XPS) confirmed the coexistence of mixed oxidation states and the presence of oxygen vacancies. Magnetic measurements revealed complex magnetic behavior characterized by ferrimagnetic correlations at low temperatures and a positive Curie–Weiss temperature (θcw = 89.08 K), indicating dominant ferromagnetic interactions associated with competing Fe–O–Co, Fe–O–Fe, and Co–O–Co exchange pathways. The electrocatalytic hydrogen evolution reaction (HER) performance was evaluated in 1.0 M NaOH, where the TNFCO-modified electrode exhibited an overpotential of 673 mV at 10 mA·cm–2, representing a 151 mV improvement compared with bare glassy carbon, together with a Tafel slope of 221 mV·dec–1 and excellent long-term stability over 50 h of continuous operation. Electrochemical impedance spectroscopy yielded a charge-transfer resistance of 626 Ω, while double-layer capacitance measurements provided a capacitance value of 45.5 μF, corresponding to an electrochemically active surface area of 1.13 cm2. The enhanced HER activity is attributed to the synergistic effects of cation disorder, mixed-valence states, oxygen vacancies, and magnetic exchange interactions that promote electron delocalization and facilitate charge transfer. These findings establish TbNdFeCoO6 as a promising multifunctional perovskite electrocatalyst for hydrogen production under alkaline conditions.

This investigation focused on isoniazid (INH)—saccharin (SAC) salts. One hydrate and one anhydrous INH-SAC salt form were synthesized and characterized spectroscopically by Raman and infrared spectroscopy. Solvent (methanol, acetone, acetonitrile)-assisted synthesis in the presence of water, or in water, resulted in production of the monohydrated form of the salt (MH: (INH+H)⁺/(SAC–H)⁻.H₂O). The anhydrous form (A: (INH+H)⁺/(SAC–H)⁻) was obtained using the same synthesis method but in the absence of water or, together with the hydrate, in the presence of traces of water. Differential scanning calorimetry studies revealed that the hydrate can be converted into the anhydrous form of the salt upon heating, with the latter melting at a T_m (onset) of 131.7 ± 0.5 °C. Melting was followed by a reaction between isoniazid and saccharin leading to saccharin ring opening and formation of a new covalent hydrazide–amide derivative, via nucleophilic acyl substitution at the saccharin carbonyl. The newly formed adduct, 2-[2-(pyridine-4-carbonyl)hydrazine-1-carbonyl] benzene-1-sulfonamide, melts at T_m (onset) = 204.4 ± 0.5 °C. The crystal structures of the hydrate and of the anhydrous form were determined by single-crystal X-ray diffraction, and the dominant intermolecular interactions in the crystalline INH-SAC salts were evaluated using Hirshfeld surface analysis. To complement the experimental results, density functional theory (DFT) calculations were performed both on relevant isolated structural units and on the two salts, employing fully periodic DFT methods.

This study reports on the synthesis, structural elucidation, and property investigation of the hybrid organic–inorganic compound (C ₆ H ₉ N ₂ ) ₂ CdI ₄ , which is composed of 2-amino-5-picolinium cations and discrete cadmium tetraiodide anions.

A concise synthetic route to diverse acyclic and heterocyclic compounds has been developed by combining hetero‐Diels–Alder reactions, DBU‐promoted ring openings, and Lewis base‐catalyzed annulations. Nitrosoalkenes reacted with furan derivatives to form 4a,7a‐dihydro‐4 H ‐furo[2,3‐ e ][1,2]oxazines, which underwent DBU‐promoted rearrangements to 6 H ‐1,2‐oxazines and 2‐acetylpyridines. Subsequent DABCO‐mediated reaction of 6 H ‐1,2‐oxazines with allenoates afforded conjugated polyenes and dihydropyrans with excellent regio‐ and stereoselectivity. This chemistry applied to bis‐furans led to novel furan hybrids and furan derivatives bearing 6 H ‐oxazine, pyridine, polyene, dihydropyran, and 4 H‐ pyran motifs—recognized as privileged structures in medicinal chemistry—through the selective modification of one of the two furan units.