Color Changes of a Full‐Color Emissive ESIPT Fluorophore in Response to Recognition of Certain Acids and Their Conjugate Base Anions

Abstract

2-(1,3-Benzothiazol-2-yl)-4-methoxy-6-(1,4,5-triphenyl-1H-imidazol-2-yl)phenol (BTImP) is an excited-state intramolecular proton transfer (ESIPT) fluorophore, containing an acid-stimuli-responsive intramolecular hydrogen bond (H-bond) that can switch from the central phenolic proton to the imidazole (Im) or benzothiazole (BT) nitrogen atoms. Here, we demonstrate that BTImP shows full-color (red, green, blue, and white) emission upon the addition of different concentrations of HClO or, with time, after the addition of HBF . It also shows thermally dependent color changes from pink through white to blue in a narrow temperature range of 25-60 °C. H and N NMR measurements suggest that, after the green fluorescent BTImP is protonated at its Im nitrogen atom, a conjugate base anion coordinates to the imidazolium (HIm ) proton, forming two types of complexes with different coordination states. One state shows a significantly Stokes-shifted red emission resulting from ESIPT at the BT side, whereas the other shows a typical Stokes-shifted blue emission, probably caused by interaction of the anion with the phenolic proton, which breaks the H-bond on the BT side. BF and ClO are effective in forming such a blue emitter, whereas Cl and PF are not; this behavior depends on whether the anion can fit into the bidentate binding site consisting of HIm and the phenolic hydroxy group.

Polarity‐Induced Excited‐State Intramolecular Proton Transfer (ESIPT) in a Pair of Supramolecular Isomeric Multifunctional Dynamic Metal–Organic Frameworks

“…Upon excitation, thet wo tautomeric forms give rise to two broad peaks. [13] If the ease of protont ransfer in an ESIPT-driven system can be controlled by externals timuli, then the emission wavelengthr egion can be effectively changed, [4,14,15] and by virtue of this fact, the system coulde asily serve for all the aforesaid purposes.I nt his context, solvent polarity may be as mart way to control the proton-transfer process, [4,14] and in this way, such as olvatochromic ESIPT system could be utilized as polarity-induced fluorescentm olecular switches. [12] Besides all these smart applications, ESIPT systemsr esponsive to externals timuli can also be useful as fluorescentm oleculars witches, [13] which can be patternedin the fabrication of optical devices related to data recording, photodisplays, and light isolators(or light filters).…”

“…[1][2][3] For theser easons, materials that exhibit the ESIPT phenomenon have been successfully utilized in different optoelectronic applications, for example, in solar concentrators, [5] UV stabilizers, [6] photopatterning, [7] opticalm emory devices, [8] fluorescencei maging, [9] white-light emitters, [10,11] and smarto pticalw riters. [15] Moreover, these polarity-sensitive materials could also be useful to chemically determine the polarityo fa nu nknown solvents ystem. [13] If the ease of protont ransfer in an ESIPT-driven system can be controlled by externals timuli, then the emission wavelengthr egion can be effectively changed, [4,14,15] and by virtue of this fact, the system coulde asily serve for all the aforesaid purposes.I nt his context, solvent polarity may be as mart way to control the proton-transfer process, [4,14] and in this way, such as olvatochromic ESIPT system could be utilized as polarity-induced fluorescentm olecular switches.…”

“…[12] Besides all these smart applications, ESIPT systemsr esponsive to externals timuli can also be useful as fluorescentm oleculars witches, [13] which can be patternedin the fabrication of optical devices related to data recording, photodisplays, and light isolators(or light filters). [16] To date, ac onsiderable research on the ESIPT process has been carriedo ut with respect to real-time spectroscopic behavior [1][2][3][13][14][15] andt heoretical understanding [1][2][3][4]17] to identify the structure-property relationshipso fE SIPT systems. [15] Moreover, these polarity-sensitive materials could also be useful to chemically determine the polarityo fa nu nknown solvents ystem.…”

“…[13] If the ease of protont ransfer in an ESIPT-driven system can be controlled by externals timuli, then the emission wavelengthr egion can be effectively changed, [4,14,15] and by virtue of this fact, the system coulde asily serve for all the aforesaid purposes.I nt his context, solvent polarity may be as mart way to control the proton-transfer process, [4,14] and in this way, such as olvatochromic ESIPT system could be utilized as polarity-induced fluorescentm olecular switches. [4,14,15] Many naturally occurring organic molecules as well as some synthetic derivatives have been found to be excellent candidates for ESIPT study, [17,18] although such materials have limited practicala pplicationsd ue to their low emission efficiency,s tability,c oncentration quenching, and solution-statep roperties. [16] To date, ac onsiderable research on the ESIPT process has been carriedo ut with respect to real-time spectroscopic behavior [1][2][3][13][14][15] andt heoretical understanding [1][2][3][4]17] to identify the structure-property relationshipso fE SIPT systems.…”

Excitation‐Wavelength‐Dependent Emission and Delayed Fluorescence in a Proton‐Transfer System

“…It includes photoexcitation of am olecule in the enol form, S 0 e !S 1 e ,afast enol-to-keto S 1 e !S 1 k ESIPT (k ESIPT % 10 12 s À1 )a nd, finally,t he S 1 k !S 0 k emission in the keto form followedb yf ast ground state intramolecular protont ransfer (GSIPT), S 0 k !S 0 e . [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] The ESIPT leads to significant spatials eparation of HOMO and LUMO which reduces the singlet-triplet (S 1 -T 1 )e nergy separation, DE ST . [17] Importantly,s mall DE ST values ( 0.2 eV or 19 kJ mol À1 )f acilitatet hermally activated delayedf luorescence (TADF).…”