Naphthoquinone colouring matters. Part 1. Synthesis and electronic absorption spectra of 1,4-naphthoquinone derivatives with electron-donating groups in the quinonoid ring

Abstract

The syntheses of several yellow to blue 1,4-naphthoquinone derivatives with electron-donating groups (NHMe, NHPh, NMe₂, and OMe) in the quinonoid ring are described. The visible absorption spectra are discussed in terms of the PPP SCF–MO model. The first π→π* transition of the 2-donor derivatives involves electron-density migration from the donor atom to the quinone system. Both electron-donating and -withdrawing groups in the 3-position give rise to bathochromic shifts, and it is concluded that for the former substituents the effect is principally electronic, whereas for electron-withdrawing groups the shifts are caused by steric distortion of the quinonoid ring. Electron-withdrawing groups in the 5- or 8-position of the benzenoid ring also cause bathochromic displacements of the visible band.