Fluorescent 1,8-naphthalimide dyes as photochemical sensors for the detection of transition metal ion concentrations in non-aqueous solutions.

摘要

Designing molecules that combine a fluorophore and receptor in a modular fashion is a growing field used for chemical sensing. The photoactive supramolecule N-butyl-4(2'-aninoethyl)amino-1,8-naphthalimide is shown to be capable of considerable tuning to satisfy the requirements of a versatile sensing system. In this research it is shown that efficient through-space intramolecular photoinduced electron transfer (PET) between the fluorophore and amino group (used here as a receptor for the metals and protons) is responsible for the photoreactive signaling pathway. There exists an intramolecular charge transfer reaction leading from a locally excited (LE) state to a charge transfer state in the singlet excited state which causes the fluorescence to exhibit dual emission. The normal or planar conformation (LE) occurs at the shorter wavelength due to coupling of the lone pair of electrons on the proximal amine with the electrons on the p-orbital of the aromatic naphthalimide. The other conformation exists as the longer wavelength, or anomalous band, is described as the twisted intramolecular charge transfer (TICT) state, arising from the decoupling of the electron pairs.; The phenomenon of molecular fluorescence possesses many features that make it suitable for real-time monitoring and responding to atomic and molecular species. It is shown in this work that this naphthalinide is an efficient system where measurable spectral changes in the locally excited state and TICT state are dependent upon transition metal complexation, pH, and temperature. It is observed that a fluorescence enhancement of the LE state occurs at acidic pH, high temperatures, or in the presence of complexing metal ions, whereas the TICT state predominates at basic pH levels, lower temperatures or the absence of complexing metal ions. Because these environmental changes affect the naphthalimide's equilibrium between the TICT excited state and LE excited state, the ratio of the TICT and LE excited states can be used to measure physical properties of the naphthalimide such as the metal binding affinity and pKa values. The photophysical properties of this sensor and its fluorescence response toward the changing environment of metal complexation, pH and temperature are reported here.