Photophysical properties of 1,4,5,8-tetraazaphenanthrene

1,4,5,8-Tetraazaphenanthrene (TAP) has close-lying n,π* and π,π* singlet states and therefore shows very weak fluorescence (Φf = 0.001); laser flash photolysis indicated that it undergoes efficient intersystem crossing (Φisc = 0.95). The intensity of fluorescence of TAP increases on addition of sodium acetate because N-Na cation coordination elevates the lowest excited 1(n,π*) state to a higher energy. Because of mixing between the lowest excited 1(n,π*) state and 1(π,π*), TAP shows the heavy-atom effect, which is generally not observed in the pure 1(n,π*) state. Since the phosphorescence lifetime is relatively long (about 0.5 – 0.8 s) and the phosphorescence is negatively polarized, the lowest triplet state has the π,π* configuration. The photoreactivity of TAP is compared with the photophysical parameters of azaphenanthrenes.