Tuning the fluorescence emission of DADQ based molecular solids by dielectric environment variation

Abstract

Enhanced fluorescence emission in supramolecular aggregates and molecular solids is of significant interest from fundamental photophysics and functional materials perspectives. Fluorescence response tuning in these materials is mostly realized through the modification of the molecular structure or supramolecular assembly patterns. We show that the fluorescence emission of diaminodicyanoquinodimethane (DADQ) based molecular solids can be fine-tuned by short exposure to weak acid vapor, and the effects are reversible. Contrary to the earlier observations in similar contexts, no significant molecular or crystal structure changes are observed. Optical spectroscopy, mass spectrometry and NMR investigations of representative systems indicate that the fluorescence variation results from the weak protonation that leads to subtle changes in the dielectric environment of the molecules. This idea is supported by computational modelling of the electronic ground and excited states of a DADQ molecule including the standard solvation effect; this can be visualized as the solid state analogue of conventional solvatochromism. Dielectric measurement by impedance spectroscopy and surface potential measurement by Kelvin probe force microscopy provide experimental verification of this general concept. The model developed in this study should prove useful for a broader understanding of fluorescence emission tuning in molecular solids under mild perturbations.