Phosphorescence emission and polarization of hydroxypyridines

Phosphorescence quantum yields and lifetimes of 2-, 3-, and 4- hydroxypyridine and 2-methoxypyridine have been measured at 77 ° K in EPA and alkaline EPA using 285 and 254 nm excitation. The phosphorescence polarization is negative with the exception of 4-hydroxypyridine in alkaline EPA, and lowest triplet state is in all cases π,π*. The lack of phosphorescence for 2-, 3-, and 4-hydroxypyridine in a methylcyclohexane glass is attributed to a solvent dependent intersystem crossing process. The O–O band for the lowest triplet of 4-hydroxypyridine and 2-methoxypyridine in EPA appears at 29.9 and 26.7×103 cm−1, respectively. The primary spin–orbit coupling process 1n,π* 3 π,π*; however, in 4-hydroxypyridine there is evidence of second order vibronic spin–orbit coupling which involves an out of plane vibration (∼828 cm−1). In the latter case the mixing scheme appears to be 3π,π* 3n,π* 1π,π*. The negative phosphorescence polarization observed for 3-hydroxypyridine in alkaline EPA when excited into S1 and S2 indicates that its transition moment is out of plane polarized.