Cumulative intensity of the combination band (CB) region (1600–2000 cm⁻¹, black) and the ratio to the 0–1600 cm⁻¹ region (blue) as well as to the CH out-of-plane (OOPs) bands (green) as a function of the number of aromatic rings in polyacenes (and benzene for the single ring species). The green trace corresponds to a linear fit to the CBs/OOPs ratio data with a slope of 0.0698 ± 0.0062 and an intercept of 0.070 ± 0.021.