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

Two new 2-N-phenylamino-5-nitropyridine—4-methyl (2PA5N4MP) and 2-N-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.