Absorption and fluorescence spectra of fluorophores 4–20, a) absorption spectra of fluorophores 4–11, b) absorption spectra of fluorophores 12–15, c) absorption spectra of fluorophores 16 and 17, d) absorption spectra of fluorophores 18–20, e) fluorescence spectra of fluorophores 4–11, f) emission spectra of fluorophores 12–15, g) emission spectra of fluorophores 16 and 17, h) emission spectra of fluorophores 18 and 19.