Exploring unique fluorescence characteristics of novel star-shaped molecules containing NBD fluorophores via different linkages: Distinct results with an aromatic ether linkage

Abstract

This paper presents a novel design of three star-shaped molecules linked with 4-chloro-7-nitrobenzofurazan (NBD-Cl), highlighting their unique properties for fluorescence applications. NBD-Cl is a substance that is non-fluorescent until it is bound to different functional groups. The three molecules in the shape of a star were created by connecting various equivalents of NBD-Cl to three different cores. The three molecules are referred to as S-1 (NBD-SR), S-2 (NBD-NHAr), and S-3 (NBD-OAr), which are based on thioether, amine, and ether cores, respectively. In addition, a fourth compound A was studied, where A is a single linear branch which is a precursor to S-3. The chemical structure and physical and spectroscopic properties of these molecules were studied and characterized using ¹H NMR, ¹³C NMR, and IR spectroscopy. UV–visible and luminescence spectrometry techniques were used to explore the unique electronic characteristics of all of the molecules using solvents of differing polarity. The star-shaped molecule S-1 showed a distinct, blue-shifted UV–vis absorption spectrum compared to NBD-Cl and the other compounds, with a wavelength range between 410 and 415 nm. Meanwhile, its emission peaks were like those of the other compounds, falling between 530 and 545 nm. This molecule exhibited the highest Stokes shift, ranging between 110 and 137 nm in different solvents, which is higher than the other molecules being studied. The Stokes shift values for compounds A, S-2, and S-3 range between 60 and 100 nm and the spectra display a red-shifted UV–vis absorption spectrum compared to that of S-1. This study reveals that the fluorescence emission wavelength of S-2 (NBD-NHAr) is the longest among the fluorescence emissions. Interestingly, the fluorescence emission of compounds A and S-3 (NBD-OAr) was unexpectedly higher than S-1, though still lower than S-2. This outcome was surprising as it deviates from previous reports, which indicated that NBD-OR derivatives generally lack significant fluorescence. The observed fluorescence enhancement in compounds A and S-3 suggests a unique interaction between the NBD moiety and the aromatic group, possibly due to electronic effects or structural configuration, leading to a more efficient fluorescence pathway. Additionally, there was a noticeable decrease in relative fluorescence as the solvent’s dielectric constant increased, accompanied by a redshift, with slight deviations in the pattern for some molecules.