The pH-dependent fluorescence of pyridylmethyl-4-amino-1,8-naphthalimides

Regioselective photoinduced electron transfer (PET) has been previously observed in aminoalkyl-4-amino-1,8-naphthalimide 'fluorophore-spacer-receptor' systems. PET from the amine to the fluorophore was only observed when the electron entered the fluorophore in the region of its 4-NH group. This has received two related but distinct explanations. The first is a directing effect of the molecular-scale electric field of internal charge transfer (ICT) excited state of the fluorophore. The second is a peculiarity of 4-amino-1,8- naphthalimides in possessing a node at the imide nitrogen in its frontier orbitals. The six isomeric pyridylmethyl-4-amino-1,8-naphthalimides 1-3 and 4-6 are configured as 'fluorophore-spacer-receptor' systems in order to test the relative importance of these two explanations. The two regioisomeric sets are designed such that PET is thermodynamically feasible when they are protonated, which should lead to fluorescence quenching by protons. In practice, the proton-induced fluorescence quenching is moderate and more clearly observed in the set 1-3. This evidence points to the PET-accelerating effect of the molecular-scale electric field being mitigated by the presence of the node at the imide nitrogen in the frontier orbitals. Compound 4 also shows this proton-induced fluorescence quenching effect but to a still smaller extent. In this instance, the PET is hampered by the electric field alone. Compounds 5 and 6 show an excited state intramolecular proton transfer (ESIPT) involving water-mediated hydrogen-bonded rings, which dominates over any residual PET to produce proton-induced fluorescence enhancement. Again, the effect is moderate. The two mechanisms of PET regioselectivity can now be understood to operate additively in the aminoalkyl-4-amino-1,8-naphthalimides and subtractively in the protonated pyridylmethyl-4-amino-1,8-naphthalimides.