Phosphorescence emission and polarization of aminopyridines

The phosphorescence quantum yield and lifetime of 2‐, 3‐, and 4‐aminopyridine have been measured at 77°K in four solvent systems using 285 and 254 nm excitation. Luminescence polarization measurements in EPA at 77°K provide evidence that the lowest triplet in these molecules is π,π*. Of the three compounds studied only 4‐aminopyridine exhibits vibronic activity in its phosphorescence spectrum and an out‐of‐plane and an in‐plane vibration are readily discernible in its phosphorescence spectrum. No phosphorescence could be detected in methylcyclohexane for the three molecules, whereas it was measurable from EPA, acetonitrile and dichloromethane glasses. The observation suggests that intersystem crossing in these molecules may be strongly dependent upon the nature of the solvent. The O–O band in the phosphorescence spectrum is negatively polarized for all three molecules and the coupling scheme appears to be ${}^1{\mathrm{n,\pi *\leftrightarrow \hspace{0.17em}}}_{\mathrm{s.o.}}{\hspace{0.17em}}^3\mathrm{\pi ,\pi *}$ . 4‐Aminopyridine shows evidence for second order vibronic spin‐orbit coupling and the mixing scheme appears to be ${}^3{\mathrm{\pi ,\pi *\leftrightarrow }}_{\mathrm{vib}}{\hspace{0.17em}}^3{\mathrm{n\hspace{0.17em}\pi *\leftrightarrow }}_{\mathrm{vib}}^1\mathrm{\pi ,\pi *}$ .