Structural and Optical Properties of New 2-Phenylamino-5-nitro-4-methylopyridine and 2-Phenylamino-5-nitro-6-methylpyridine Isomers

Two new 2-<i>N</i>-phenylamino-5-nitropyridine-4-methyl (2PA5N4MP) and 2-<i>N</i>-phenylamino-5-nitropyridine-6-methyl (2PA5N6MP) isomers were synthesized and comprehensively characterized by single-crystal X-ray diffraction, IR/Raman spectroscopy, UV-Vis absorption, and photoluminescence measurements. DFT and TD-DFT calculations were also carried out to support the experimental results. The X-ray analysis revealed significant structural differences: 2PA5N6MP adopted an almost planar conformation (pyridine-phenyl dihedral ~3°), whereas 2PA5N4MP was markedly twisted (~45°), leading to distinct hydrogen-bonding motifs (N-H⋯N dimers vs. N-H⋯O interactions). These geometric disparities influenced their electronic properties: 2PA5N6MP exhibited a narrower HOMO-LUMO gap (≈2.45 eV) than 2PA5N4MP (≈3.77 eV), which was consistent with a pronounced bathochromic shift in absorption. Both isomers showed broad UV-Vis absorption (200-520 nm), but the 6-methyl derivative featured an additional low-energy charge-transfer band around 460 nm (with a maximum at ~500 nm) compared to ~355 nm in the 4-methyl isomer. Likewise, their photoluminescence spectra differed as follows: 2PA5N4MP emitted in the violet-blue region (bands at ~415 and 450 nm), whereas 2PA5N6MP had an extra orange band peaking at ~560 nm (in addition to a ~450 nm band). The red-shifted 560 nm emission of 2PA5N6MP was attributed to intersystem crossing into triplet states, in line with TD-DFT predictions. Furthermore, both isomers readily formed complexes with Eu³⁺ ions, and the Eu³⁺ chelates exhibited the characteristic red f-f emissions (⁵D₀ → ⁷F transitions ~590-700 nm), demonstrating efficient sensitization of Eu³⁺ luminescence. Overall, the position of the methyl substituent strongly modulates the compounds' optical behavior, and these isomers show promise as tunable organic dyes and effective ligands for luminescent lanthanide complexes.