Excitation-Dependent Multiple Fluorescence of a Substituted 2-(2′-Hydroxyphenyl)benzoxazole

Abstract

Excitation-dependent multiple fluorescence of a 2-(2′hydroxyphenyl)benzoxazole (HBO) derivative ( 1) is described. Compound 1 contains the structure of a charge-transfer (CT) 4hydroxyphenylvinylenebipy fluorophore and an excited-state intramolecular proton transfer capable (ESIPT-capable) HBO component that intersect at the hydroxyphenyl moiety. Therefore, both CT and ESIPT pathways, while spatially mostly separated, are available to the excited state of 1. The ESIPT process offers two emissive isomeric structures (enol and keto) of 1 in the excited state, while the susceptibility of 1 to a base adds another option to tune the composite emission color. In addition to the ground-state acid−base equilibrium that can be harnessed for the control of emission color by excitation energy, compound 1 exhibits excitation-dependent emission that is attributed to solventaffected ground-state structural changes. Therefore, depending on the medium and excitation wavelength, the emission from the enol, keto, and anion forms could occur simultaneously, which are in the color ranges of blue, green, and orange/red, respectively. A composite color of white with CIE coordinates of (0.33, 0.33) can be materialized through judicious choices of medium and excitation wavelength.

Structural Features of a Family of Coumarin–Enamine Fluorescent Chemodosimeters for Ion Pairs

“…Modest chemical shifts were observed for the other carbon atoms in the six-membered ring system, as shown in Figure 5. The same trend was seen for compound 7 and [Zn (7) 2 ] (Figures S28 and S43, respectively).…”

“…Interestingly, compound 3 did not show appreciable hydrolysis, presumably because of the lack of nitrogen atoms in the ring system. 50 The 1 H and 13 C NMR assignments of the [Zn(3) 2 ] and [Zn (7) the free compound 3 and the coordination complex. The most significant change in the 1 H NMR spectrum is the loss of the NH signal.…”

“…The solid was filtered and washed with cold MeOH. The yields for [Zn (7) 2 ] and [Zn(3) 2 ] were 85.9 mg (0.19 mmol, 52%) and 100.2 mg (0.14 mmol, 63%), respectively. 1D and 2D NMR spectra, IR spectra, and X-ray tables for [Zn (3) The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.inorgchem.1c01734.…”

“…This often leads to large changes in the optical response of these molecules, such as large Stokes shifts in luminescence emission (>200 nm), as well as new absorbance and emission signals. Several fluorophores incorporate the cyclic RAHB motif, including hydroxy- and amino-substituted 2-arylbenzoxazole (HBO), , 2-arylbenzothiazole (HBT), and anthraquinone (AQ) − as well as 3-hydroxyflavone (3HF) derivatives − (Figure ). In these compounds, the enol tautomer ( E ) has a distinct UV–vis absorption from the keto tautomer ( K ).…”

Hydroxyaromatic Fluorophores

“…BIPYVHBO was constructed as a fusion of the ESIPT-capable HBO and an internal charge transfer (ICT)-type stilbenoid (Figure 10). 27 Deprotonation of BIPYVHBO would amplify the ICT of the stilbenoid component. The presence of the bipyvinyl substituent on the 5′-position of HBO shifts both the enol (when solvated) and keto (when intramolecularly hydrogen bonded) emission of HBO to cyan and green regions, respectively, while deprotonation moves the emission maximum to almost 600 nm.…”