Computational Analysis of Structural and Photophysical Properties of a 1,8-Naphthalimide Derivative in Aqueous Solution

N-[2-(Trimethylammonium)ethyl]-1,8-naphthalimide (TENI) is a promising, versatile anion sensor that can detect the electronegativity of a target anion depending on the fluorescence intensity. In TENI, an emission from a lower-energy state than the first excited state of a single TENI is observed experimentally, and its character has remained unknown. Here, we have investigated the origin of the lower-energy peaks in the fluorescence spectrum of TENI using molecular dynamics (MD) simulations and time-dependent density functional theory (TD-DFT) calculations with three functionals. The computation reveals that TENIs form a structural ensemble in solution, and that stacked TENI dimers exhibit red-shifted fluorescence compared to a single TENI. The formation of excimer results in the longer-wavelength fluorescence that can be corroborated by changes in frontier orbital energies due to the intermolecular interaction. Our work will provide valuable knowledge for the development of new anion sensors.