Synthesis and photophysical properties of a new carbazole-based acidochromic molecular switch

A reversible color-changing and fluorescence molecular switch was designed and synthesized based on carbazole fluorophores and amino group receptors. The crab shape molecule 1 is not soluble in most organic solvents and show slow solubility in acetic acid and DMSO (1 mg/ml in DMSO). The solution of 1 in DMSO performed as single on/off switch triggered by acidic protons. The colorless solution in basic and neutral conditions turned to yellow upon addition of a small amount of acid together with quench in fluorescence emission. The solid state of 1 has a dual-mode fluorescence switching property. The creamy white powder of 1 has an emission lambda(max) at 398 nm but when exposed to acidic vapors or suspended into acidic media the color of the powder changed to yellow and its emission lambda(max) appeared at 516 nm. Monte-Carlo simulation has been performed to search the conformational space of the neutral and acidic forms. The frontier molecular orbitals of the most stable conformers has been calculated using DFT/B3LYP method using 6-31 g (d) basis set in attempt to explain the difference in electronic transition of 1 and its porotonated form.