Abstract
A series of four water-soluble benzazole dyes that emit fluorescence by the excited state intramolecular proton transfer (ESIPT) mechanism were structurally characterized by Fourier transform infrared spectroscopy (FTIR), 1H and 13C nuclear magnetic resonance (NMR) attached proton test (APT) and mass spectrometry. Their photophysical properties were systematically studied by UV-absorption and fluorescence emission. Some photophysical parameters were obtained by semi-empirical PM3 and ZINDO methods, and related to experimental photophysical data. Changes in the absorption and fluorescence emission spectra as well as conformational equilibrium between different species were investigated in solvents of different polarities and under different pH conditions. These benzazole dyes emit dual fluorescence emission in the blue-orange region with a Stokes shift between 2617-12337 cm-1. In general, these dyes are potentially interesting for studies of biological systems in an aqueous environment due to the presence of groups that increase aqueous solubility and reactivity with biomolecules. The present dyes were successfully used as new probes by means of direct staining of larvae. The obtained results indicate that these fluorescent dyes permit a quick, easy and selective visualization of larvae.
Highlights
IntroductionHeterocyclic benzazole compounds are of great synthetic interest because of their versatility and a variety of applications, such as the detection of latent fingermarks,[1] their utility as laser dyes,[2] UV photostabilizers[3,4,5] and optical filters[6,7] and, in studies on biological systems.[8,9,10,11,12] The latter is largely influenced by their pharmacological activities, i.e., antiviral,[13] antibiotic,[14] antifungal,[15] anticancer,[16] antiparasitic,[17] antimicrobial[18] and anti-Parkinson’s disease.[19]
When the enol conformer (E1) is irradiated by UV light, it absorbs energy and gives rise to the excited enol conformer (E1*), which is rapidly converted to the excited keto tautomer (K1*), known as the excited state intramolecular proton transfer (ESIPT) form, by intramolecular proton transfer
The UV-Vis absorption and fluorescence emission spectra of the dyes HB-6, HB-7, HB-9 and HB-11 were performed in solution (10-5 mol L-1) using solvents with different polarities
Summary
Heterocyclic benzazole compounds are of great synthetic interest because of their versatility and a variety of applications, such as the detection of latent fingermarks,[1] their utility as laser dyes,[2] UV photostabilizers[3,4,5] and optical filters[6,7] and, in studies on biological systems.[8,9,10,11,12] The latter is largely influenced by their pharmacological activities, i.e., antiviral,[13] antibiotic,[14] antifungal,[15] anticancer,[16] antiparasitic,[17] antimicrobial[18] and anti-Parkinson’s disease.[19]. Molecules that exhibit ESIPT exist exclusively in the enol conformer, which is stabilized by intramolecular hydrogen bonding. When the enol conformer (E1) is irradiated by UV light, it absorbs energy and gives rise to the excited enol conformer (E1*), which is rapidly converted to the excited keto tautomer (K1*), known as the ESIPT form, by intramolecular proton transfer. The keto tautomer (K1) is less stable than the enol conformer (E1), so it is regenerated into the initial enol conformer (E1)
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