Dual emission of ESIPT-capable 2-(2-hydroxyphenyl)-4-(1H-pyrazol-1-yl)pyrimidines: interplay of fluorescence and phosphorescence

Abstract

ESIPT-capable pyrimidine-based compounds featuring short O–H···N intramolecular hydrogen bonds, 2-(2-hydroxyphenyl)-4-(3,5-dimethyl-1H-pyrazol-1-yl)-6-methylpyrimidine (HL1) and 2-(2-hydroxyphenyl)-4-(3,5-diphenyl-1H-pyrazol-1-yl)-6-methylpyrimidine (HL2), were synthesized by the condensation of 4-hydrazinyl-2-(2-hydroxyphenyl)-6-methylpyrimidine with acetylacetone and dibenzoylmethane. In solution, HL1 and HL2...

Complexes on the Base of a Proton Transfer Capable Pyrimidine Derivative: How Protonation and Deprotonation Switch Emission Mechanisms

“…According to the analysis of frontier molecular orbitals, T 1 E → S 0 is a LUMO (π*) → HOMO (π) transition of mixed CT + LE character (Figure h). Noteworthy, the emission mechanisms proposed here for the HL H ligand (i.e., Kasha-like T 1 E → S 0 phosphorescence of the enol form + anti-Kasha S 2 K → S 0 fluorescence of the keto form) are the same for its structurally similar congeners HL Me and HL Ph (Chart ), which were previously reported by us …”

“…Kinetic barriers between the minima of the normal and tautomeric forms in the excited state can partially or fully suppress the proton transfer process and lead to the emission of the normal form. If the barriers separating the normal and tautomeric forms in the excited state are surmountable, ESIPT-capable compounds can show dual emissions associated with the fluorescence of both forms. − Along with singlet-to-singlet transitions, these molecules can be converted to the ground state through triplet excited states and triplet-to-singlet transitions such as phosphorescence (including room temperature phosphorescence) ,,− and thermally activated delayed fluorescence. − Some ESIPT-capable compounds exhibit anti-Kasha emissions, which can be observed in molecules wherein the first and the second excited states are separated by large energy gaps. − …”

“…Azole and benzazole ESIPT-capable derivatives are workhorses in the area of ESIPT studies. − The most common molecular pattern of these compounds which is widely relied upon in the design of ESIPT dyes includes the proton-accepting azole/benzazole moiety combined with the proton-donating 2-hydroxyphenyl group or its analogues introduced in the α position to the azolic N atom. In contrast, pyrimidine derivatives are relatively rarely used for the design of ESIPT-fluorophores in comparison with azole and benzazole ones. ,− However, being combined with suitable proton-donating groups, they can serve as proton-accepting moieties. ,, Moreover, introducing additional donor groups in the pyrimidine core can lead to more complex molecular architectures of pyrimidine-based compounds suitable for binding metal ions and providing numerous sites for protonation, including isomeric ones. ,, In turn, the synthesis of isomeric ESIPT-capable compounds allows researchers to shed more light on the impact of structural factors on ESIPT and on relationships between ESIPT and luminescence. − The coordination of conventional dyes and ESIPT emitters to metal ions with d electronic configuration such as Zn 2+ is known to enhance the quantum efficiency of emission. − Earlier we demonstrated that emissions of 4-(1 H -pyrazol-1-yl)-6-(2-hydroxyphenyl)­pyrimidines and 2-(2-hydroxyphenyl)-4-(1 H -pyrazol-1-yl)­pyrimidines, which belong to two different isomeric families (Chart ), share such a common feature as dual emission associated with singlet-to-singlet and triplet-to-singlet transitions, which is contributed by anti-Kasha fluorescence of the tautomeric form. ,, However, their coordination behavior toward Zn 2+ ions appeared to be quite different, whereas the 4-(1 H -pyrazol-1-yl)-6-(2-hydroxyphenyl)­pyrimidine derivative can bind Zn 2+ ions through the N,N-site of the molecule, which produces multicolor emission of the complex, 2-(2-hydroxyphenyl)-4-(3,5-dimethyl-1 H -pyrazol-1-yl)­pyrimidine derivatives cannot do this. We associated this with steric effects imposed by methyl and phenyl substituents introduced in positions 3 and 5 of the pyrazolyl group.…”

Pd(II)- and Pt(II)-Assisted P–C Activation/Cyclization Reactions with a Luminescent α-Aminophosphine

“…Compound 1 features dual-band emission with maxima at 450 and 610 nm. The first band is attributed to conventional fluorescence, while the second one can be the result of excited-state intramolecular proton transfer (ESIPT) or the excited-state charge transfer process. ,, The latter transition appears to be the common process for derivatives containing {NH-Pbt} moieties. ,,, The relative intensity of the bands in 1 negligibly changes in the excitation wavelength range of 250–400 nm (Figure S12, SI). The UV–vis absorption spectrum of compound 5 resembles that for 1 with the exception for a less-pronounced long wavelength band in the former.…”

Luminescence of ESIPT-capable zinc(ii) complexes with a 1-hydroxy-1H-imidazole-based ligand: exploring the impact of substitution in the proton-donating moiety

“…However, if kinetic barriers are high enough to impede the ESIPT photoreaction, the N-form of the molecule can emit itself and can produce dual emission associated with the emission of both forms of the molecule. [21][22][23][24][25][26][27][28][29][30][31] Furthermore, the emission of ESIPT-dyes often involves triplet excited states, giving rise to phosphorescence (including room temperature phosphorescence) and thermally-activated delayed fluorescence. [32][33][34][35][36][37][38][39][40][41] This plethora of emissive excited states makes ESIPT-capable compounds an appealing platform for the design of multicolor luminescent materials.…”