Optical and electron paramagnetic resonance studies of the lowest excited triplet states of 1-phenyl-4-(4-pyridyl)butadiyne and its protonated cation

The lowest excited triplet (T1) states of 1-phenyl-4-(4-pyridyl)butadiyne (4-PPB) and its protonated cation (4-PPBH+) have been studied through measurements of the phosphorescence, time-resolved EPR and parallel-polarized EPR spectra. Unusually large zero-field splittings were observed for 4-PPB and 4-PPBH+. The replacement of CH by a nitrogen atom appears to have little effect on the zero-field splittings and anisotropic sublevel populating rates of the T1 state of 1,4-diphenylbutadiyne. The sublevel preferentially populated by intersystem crossing is Tz in both 4-PPB and 4-PPBH+ (the z-axis is parallel to the in-plane long axis). This is the first report of EPR of a triplet aza-polyyne.