Colorful Azulene and Its Equally Colorful Derivatives

A simple pictorial method using HOMO, LUMO, and LUMO+1 of azulene coupled with interaction between the electrons in the singly occupied orbitals is introduced to show how the S 2 -S 0 energy gap relative to the S 1 -S 0 gap can be changed by a choice of substituents. The result is changing the characteristic blue color of azulene to an array of other colors: emerald green for 1,3-difluoroazulene, magenta for azulene-1-carboxaldehyde, and red for azulene-1,3-dicarboxaldehyde.