An observational trend between the I_6.2/I_11.3 ratio as a function of the luminosity at 1.3 mm (L₁₃₀₀/L_∗). The red dots show the objects studied in this paper. The gray line gives the best fit to the data (Eq. (1)). The parameters of all the objects are listed in Table 1. For high mm luminosities, the I_6.2/I_11.3 ratio is lowest (tracing neutral PAHs), while the I_6.2/I_11.3 ratio is highest (tracing ionized PAHs) for low mm values.