Colorless, Transparent, Dye-Doped Polymer Films Exhibiting Tunable Luminescence Color: Controlling the Dual-Color Luminescence of 2‑(2′-Hydroxyphenyl)imidazo[1,2‑<i>a</i>]pyridine Derivatives with the Surrounding Matrix

Colorless, transparent, polymer films including 2-(2′-hydroxyphenyl)­imidazo­[1,2-<i>a</i>]­pyridine (HPIP) derivatives, <b>1</b> and <b>2</b>, are prepared by a spin-coating method. The observed emission spectra upon photoexcitation of these polymer films were composed of dual emission bands: the normal luminescence at 370–410 nm (purple) and the greatly Stokes-shifted emission at 520–580 nm (yellow) assigned as the excited-state intramolecular proton transfer (ESIPT) luminescence. Relative intensity of the two emissions varied according to the polymer matrices, resulting in change in the luminescent color of the dye-doped polymer films. Particularly, the luminescence properties of 6-cyano HPIP, <b>2</b>, are highly susceptible to the surrounding environment, and therefore successfully tuned to produce a wide range of colors, from purple to orange, by changing its concentration within and the type of the polymer matrix. This observation can be ascribed to the formation of a relatively weak intramolecular hydrogen bond resulting from the electron-withdrawing 6-cyano group. Thus, we demonstrate large variations in emission color can be achieved using interactions of the single component with the surrounding matrix. These results offer promise as a convenient and effective method for a wide-range tuning the luminescence colors of dye-doped polymer films

Tuning of Excited-State Intramolecular Proton Transfer (ESIPT) Fluorescence of Imidazo[1,2‑<i>a</i>]pyridine in Rigid Matrices by Substitution Effect

2-(2′-Hydroxyphenyl)­imidazo­[1,2-<i>a</i>]­pyridine (HPIP, <b>1</b>) and its derivatives are synthesized, and their fluorescence properties are studied. Although all the compounds show faint dual emission (Φ ≈ 0.01), which is assigned to the normal and excited-state intramolecular proton transfer (ESIPT) fluorescence in a fluid solution, they generally display efficient ESIPT fluorescence (Φ up to 0.6) in a polymer matrix. The introduction of electron-donating and electron-withdrawing groups into the phenyl ring causes blue and red shifts of the ESIPT fluorescence emission band, respectively. On the other hand, the introduction of such groups into the imidazopyridine part results in fluorescence shifts in the opposite directions. The results of ab initio quantum chemical calculations of the intramolecular proton-transferred (IPT) state are well in line with the ESIPT fluorescence energies. The plots of the calculated highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels against the Hammett substituent constants (σ) show good linearity with different slopes, which can rationalize the effect of the substituent and its position on the IPT state. Therefore, we have developed a series of HPIPs as new ESIPT fluorescent compounds and demonstrate that ESIPT fluorescence properties would be rationally tuned using quantum chemical methods