In contrast to the consistent red-shift in absorption, the fluorescence emission of seven derivatives showed different wavelength shift behavior according to their substituents type. HBOE and HBOA which possess only an electron accepting substituent at 6-position exhibited red-shifted K* emission by 11 nm and 31 nm compared to that of non-substituted HBO (λem = 480 nm). On the other hand, the other derivatives which have an electron-donating substituent at 4′-position exhibited strongly blue-shifted K* emission (Figure 7b).
This peculiar spectral tuning behavior of HBO derivatives can be explained in terms of π-electron density of the frontier orbital diagrams of the E and K forms. In contrary to HBQ case, the HOMO and LUMO of HBO derivatives were delocalized over the whole molecule in both E and K form. However, π-electron density at 4′-position of the K form in its HOMO is extremely lower than that of the E form (Figure 7c). Consequently, the negative inductive effect of donating group was likely to contribute to the lowering of HOMO level rather than mesomeric effect, resulting in the blue-shift of K* emission.