11H-Pyrido[2,1-b]quinazolin-11-one - A reversible turn-on/off fluorescent probe for solution and gas-phase detection of acids or basic amines

Abstract

The present study details a metal- and solvent-free synthesis of 11H-pyrido[2,1-b]quinazolin-11-one (1) and its photophysical properties as an acid or base-sensitive sensor. The absorption spectra in organic solvents revealed a small blue shift on passing from nonpolar to polar solvents, but when measured in buffered aqueous solutions the spectra were observed to be pH-dependent. Additionally, the emission spectra recorded in different organic solvents revealed only minor differences between the solvents and a moderate fluorescence quantum yield. However, the emission spectrum of 1 revealed a pH-dependent behavior. At pH 7 the emission spectrum was similar to that in acetonitrile, whereas at pH 1 the emission intensity skyrocketed, whilst at pH 14 fluorescence was switched off. The emission from 1 was reversibly switched on and off by control of the pH. The adsorption of 1 on silica disks provided a simple means for sensing gas-phase acids or basic amines. Exposure of the disks to AcOH, HCl, or TFA vapors revealed an acid-dependent increase of the emission spectrum intensity area. Disks exposed to acid vapors (switch on fluorescence) were readily recycled by exposure to ammonia or other volatile amines (switch off fluorescence). The modified silica disks could be cyclically switched on and off multiple times without significant change to the signal efficiency. NMR spectroscopy and TD-DFT calculations were used to investigate the dramatic changes to the photophysical properties of 1 as a consequence of the pH-dependent behavior.