Comparative Study of UVA Filtering Compounds 2-Methyl/2-Amino Pyrimidine Derivatives of Avobenzone under Acidic Conditions

This study presents a comparative investigation of two avobenzone derivatives, 2-methyl pyrimidine (2-MPDA) and 2-amino pyrimidine (2-APDA), focusing on their ultraviolet A (UVA) filtering activity under acidic conditions. Both compounds structurally mimic the locked enol form of avobenzone and display pH-dependent spectral properties, including enhanced UVA absorption and intense visible fluorescence in acidic media. Photodegradation behavior was assessed using UV spectroscopy under sunlight exposure, revealing that 2-MPDA remains photostable, while 2-APDA undergoes degradation. Upon acidification, both derivatives exhibit photodegradation, with 2-MPDA degrading less and 2-APDA more than avobenzone under identical sunlight conditions. Time-dependent density functional theory (TD-DFT) calculations of the UV spectra for potential protonated species, corroborated by experimental data, confirmed the formation of singly protonated 2-MPDA and 2-APDA in acidic media. To address limitations of conventional ¹H NMR analysis, fluorinated analogues (2-MPDA-F and 2-APDA-F) were synthesized and characterized, enabling detailed structural elucidation via ¹⁹F-NMR spectroscopy. The combined use of ¹⁹F-NMR and computational methods facilitated the identification of key structural and tautomeric forms in the presence of trifluoroacetic acid (TFA), which were not easily observed by using ¹H NMR techniques. Comparative results indicate that 2-MPDA offers superior UVA filtering efficiency and stronger fluorescence under acidic conditions compared to 2-APDA, highlighting its potential for use in fluorescence-based UVA-protective materials. Furthermore, the unique amino-to-imino tautomerization observed in 2-APDA, but not in 2-MPDA, along with protonation at the pyrimidine nitrogen and proton exchange reactions with TFA, appear to play crucial roles in the photodegradation mechanisms of both compounds.