Effect of protonation and hydrogen bonding on the fluorescent properties and exciplex formation of N-(4-pyridyl)-1,2-naphthalimide

Abstract

The effects of fluorinated hydroxy compounds and naphthalene on the fluorescence of N-(4-pyridyl)-1,2-naphthalimide (PyNI) have been studied in toluene. The interaction of the pyridyl moiety of PyNI with hexafluoro-2-propanol (HFIP) gave rise to a hydrogen-bonded complex, whereas a more stable, hydrogen-bonded ion pair was formed with trifluoroacetic acid (TFA). Time-resolved fluorescence measurements demonstrate that hydrogen bonding with HFIP is a reversible process, even in the excited state, and revealed the rate constants of the various energy dissipation processes. The fluorescence yield enhancement of about one order of magnitude upon the 1 ∶ 1 binding of PyNI to HFIP or TFA is primarily attributed to the deceleration of the internal conversion, and the fluorescence proved to be the dominant deactivation pathway of the singlet excited complexes. Both PyNI and its TFA complex produced fluorescent exciplexes with naphthalene. Protonation of PyNI markedly decreases the energy of the exciplex, leading to faster radiationless energy dissipation as well as to slow dissociation into an excited PyNI–TFA complex and ground-state naphthalene.