Abstract
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 (1)H NMR analysis, fluorinated analogues (2-MPDA-F and 2-APDA-F) were synthesized and characterized, enabling detailed structural elucidation via (19)F-NMR spectroscopy. The combined use of (19)F-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 (1)H 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.