Wavelengths of optical bands of higher acenes with N hexagons from Bettinger et al. (2016) and Shen et al. (2018): full symbols, theoretical values with f-values; open symbols, experimental Ar-matrix data taken from Tönshoff & Bettinger (2010), except for undecacene (N = 11; Bettinger in prep.). See the symbol explanation in the right column, with the names of the excited molecular levels and the labels “exp” for measured wavelengths and “th” for theoretical estimates. See text (Sect. 4.3) for estimates of order of magnitude of matrix shifts. The short horizontal blue lines on the left show the wavelength of the C₂ DIBs (Thorburn et al. 2003), and the main ζ DIBs (Fig. 4) are marked by short horizontal red lines on the right. The expected extrapolation to longer wavelengths shows that similar strong bands of higher polyacenes, N ~ 12–16, should extend through the whole main spectral domain of DIBs from ~5000 to ~7000 Å (Fig. 3).