Nagaoka et al. evaluated the nodal plane model with the experimental results of hydroxyphenyl benzazole derivatives.47 They synthesized two hydroxynaphthyl derivatives of HBO which has a hydroxyl substituent at different position of naphthalene ring (2H3NBO, 1H2NBO, Scheme 5b). Although both 2H3NBO and 1H2NBO have a hydroxynaphthyl group, their emission properties were very different. 2H3NBO exhibited typical dual fluorescence in hexane solution: The short¬-wavelength emission at 450 nm originated from its E* form and the long-wavelength emission at 670 nm originated from K* form of 2H3NBO. A much larger Stokes' shift of the K* emission of 2H3NBO than that of HBO was attributed to the effect of extended conjugation length of its naphthalene ring. In contrast, 1H2NBO exhibited a K* emission at 470 nm in hexane solution, whose Stokes' shift was rather similar to that of HBO.
The difference in the Stokes' shift of the K* emission between these two molecules can be explained in terms of nodal plane model (Scheme 5b). In 1H2NBO, the nodal plane lies on only one of the two aromatic rings of the naphthalene moiety of the K form. As a result, the extended aromatic conjugation length of the additional aromatic ring has no impact on the stabilization of the excited states. On the contrary, in 2H3NBO, the K form has a nodal plane through both aromatic rings of the naphthalene moiety, thus the energy of the K form of 2H3NBO is significantly lowered than that of 1H2NBO, resulting in a very large Stokes' shift of 2H3NBO.