Solvent-Dependent Photophysics and Reactivity of Monomeric and Dimeric 4-Amino-1,8-Naphthalimides

Abstract

The solvent-dependent photophysics of two 4-amino-substituted 1,8-naphthalene imides (<b>AIs</b>) were studied using fluorescence spectroscopy and laser flash photolysis. The compounds were functionalized with water-soluble 2,2'(ethylenedioxy) diethylamine groups, yielding a monomer (<b>AI3</b>) and a dimer (<b>AI4</b>). The radiative and nonradiative singlet-state deactivation processes of <b>AI3</b> and <b>AI4</b> were quantified in 10 solvents and at different pH values. The fluorescence quantum yield for the <b>AI4</b> dimer in water was more than 100× lower than in other solvents, or for the monomeric <b>AI3</b>. The enhanced nonradiative decay of aqueous solutions of dimeric <b>AI4</b> is accompanied by biexponential decay kinetics, suggesting equilibration with a dark excited state. An oxygen-quenchable triplet state (T₁) of <b>AI3</b> was produced upon 416 nm excitation in both water and <i>n</i>-octanol. In water, the T₁ state evolved into a long-lived transient that was unreactive toward oxygen or several electron donors. This species was not observed in <i>n</i>-octanol. The transient observed upon 416 nm excitation of <b>AI4</b> in water was extremely weak. However, production of T₁ in both <b>AI3</b> and <b>AI4</b> was evidenced by the photoinduced electron transfer to methyl viologen, albeit in low quantum yield (0.0503 and 0.00778 for <b>AI3</b> and <b>AI4</b>, respectively). The photophysics and reactivity are consistent with the production of an intramolecular charge transfer (ICT) state that is stabilized in water. Significantly, this stabilization enhances nonradiative decay pathways, particularly in the <b>AI4</b> dimer. The results indicate that the photochemistry of these compounds can be environmentally mediated, switching from radical- to triplet-initiated processes.