and S23). Compounds 2a and 2b show similar absorption spectra in solution (Figure ) displaying lowest energy maxima centered around 538 and 539 nm, respectively. The simple replacement of a phenyl ring by and a naphthalene unit in dye 4 leads to a significant red-shift of the lowest energy maximum to 562 nm (Figure ). These three molecules have absorption spectra featuring a vibronic progression of their lowest energy transition similar to that of regular PDIs. This is not the case for O-doped BACD 6 since an intense transition is observed at 387 nm associated with an ε exceeding 10 6 M -1 cm -1 followed by a poorly defined absorption pattern ending in a sharper band at 526 nm (Figure ). Compared to mono-annulated derivatives 2a and 2b, BACD 6 displays a slightly blue-shifted and reduced absorption at low energy. The dimerized species 9 shows a similar absorption spectrum with a more well-defined pattern at low energy and a maximum at 524 nm. Solid-state absorption spectra were recorded by spin-coating 10 mg/mL CHCl 3 solutions of the dyes in on glass substrates, except for AzaBPDI 4 because of its poor solubility and processability preventing the formation of a homogeneous film. Compared to their absorption in solution, mono-annulated dyes 2a and 2b