UV-Induced Formation of Long-Lived Organic Radicals in Pyrazinium-Based Salts via Bromide-to-Cation Electron Transfer

The photochromic behavior of a series of bromide salts prepared from 3,6-bis(bromomethyl)durene and nitrogen-containing heterocycles─2-methylpyrazine (1), 2,6-dimethylpyrazine (2), 3-methylpyridine (3), and 3,5-lutidine (4)─was studied. Only salts 1 and 2 showed color changes when exposed to UV light (365 nm): compound 1 changed from white to brown, and compound 2 shifted from pale yellow to deep yellow, indicating photochromism. Electron spin resonance (ESR) measurements confirmed that the color changes result from the formation of organic radicals caused by light. Remarkably, the colored states persisted for over a month, indicating the formation of highly stable radical species. Single-crystal X-ray analysis revealed shorter Br–···pyrazinium distances in compound 2 (3.439 Å) compared to compound 1 (3.507 Å), which correlates with the 6-fold faster photoresponse of compound 2 estimated from UV–vis measurements. Additionally, C–H···π interactions were observed in compound 1, while π–π stacking was noted in compound 2, suggesting that these interactions may help stabilize radicals. Density functional theory (DFT) calculations revealed that the photochromism results from electron transfer from the bromide anion to the pyrazinium cation. Density of states (DOS) analysis showed that pyrazinium cations (1, 2) have smaller band gaps than their pyridinium counterparts (3, 4), explaining the enhanced photochromic behavior. These findings provide valuable insights into designing molecules to improve radical stability in photoresponsive materials.