Protonation effect on the electronic structure of small PAHs: Acenaphthylene and Acenaphthene

► Two lowest electronic excited states of protonated ACYN/ACN lie in the visible region. ► The strong red shift effect on the S1 ← S0 transition observed upon to protonation. ► CT is the most important character of S1-S0 transition in protonated ACYN and ACN.