We propose a strategy for the design of coordination complexes capable of sensing volatile organic compounds (VOCs) through integrating two distinct sensing mechanisms on the base of a single platform. The first mechanism, emission switching via ligand-enabled, VOC-induced structural distortion, relies on the incorporation of a flexible structural component into the ligand architecture. This design enables sensing of weakly coordinating VOC molecules through the reversible distortion of the molecular and crystal structure. The second mechanism, emission quenching via VOC-driven labile ligand displacement, relies on the tuning of the ligand coordination strength: it should be sufficient for stable complex formation yet allow for dissociation in the presence of VOCs with stronger electron-donating properties. To illustrate this strategy, we report the synthesis of mononuclear copper(i) complexes, [CuL(PPh3)2](PF6)& centerdot;nSolv and [CuL(XantPhos)](PF6)& centerdot;nSolv (where L is LH or LMe; LH = 2-(benzylthio)-4-(1H-benzotriazol-1-yl)pyrimidine, LMe = 2-(benzylthio)-4-(1H-benzotriazol-1-yl)-6-methylpyrimidine), which exhibit a visually detectable luminescence response to MeCN and CH2Cl2 vapours. For these complexes, the response to MeCN is associated with the reversible decoordination of L, while the response to CH2Cl2 stems from the reversible uptake of solvent molecules into the crystal lattice. The successful integration of these two orthogonal sensing pathways within a single family of compounds renders them unique examples of rationally designed bifunctional copper(i)-based VOC sensors.

Dinuclear ESIPT-active complexes, [Zn2(HL)2X4] (X = Cl, Br, I), based on 2-(1H-pyrazol-1-yl)-4-(2-hydroxyphenyl)pyrimidine (HL), have been synthesized. Along with an ESIPT site with a short O-H⋯N hydrogen bond (O⋯N distance: 2.6 Å), the HL ligand molecule features an additional N,N-chelating site as a prerequisite for metal coordination. This dual-site architecture of the ligand molecule has become a key precondition for the design of ESIPT-active metal complexes, as it ensures the coordination of metal ions to the N,N-chelating site while preserving the integrity of the ESIPT site during complex formation. In the solid state, the ESIPT-active zinc(II) complexes exhibit dual excitation-wavelength-dependent emission as a result of the interplay of a phosphorescence band in the orange region and a fluorescence band in the green region. According to TDDFT calculations, the dependence of photoluminescence on excitation energy stems from the coexistence of two minima on the potential energy surface of the S0 state, corresponding to enol (E) and keto (K) forms. Excitation with high-energy photons (λex = 300-400 nm) facilitates population of the S1 state in the enol form, i.e., S0E → S1, followed by intersystem crossing S1 → T1 and subsequent phosphorescence T1E → S0. Conversely, low-energy photons (λex > 420 nm) selectively excite molecules in the keto form, S0K → S1, leading to S1K → S0 fluorescence.

A series of mononuclear copper(I) complexes, [CuL(PPh3)Br], [CuL(PPh3)I] and [CuL(PPh3)(2)](PF6) (L = 4-(3,5-dimethyl-1H-pyrazol-1-yl)-2-(pyridin-2-yl)pyrimidine), was synthesized. Free L exhibits bright fluorescence in the blue region (PLQY 29 %). The complexes demonstrate luminescence in the yellow-to-red region. Considerable emission quenching in [CuL(PPh3)Br] and [CuL(PPh3)I] (PLQY < 1 %) is contributed by the electron-poor heterocycles in the ligand structure, which favour the overstabilization of LUMO in the T-1 state, the decrease of HOMO-LUMO and S-0-T-1 gaps and, as a consequence, the increase of the ratio of non-radiative events. Instead, the introduction of the second PPh3 molecule in the coordination sphere of Cu+ ion on going from [CuL(PPh3)Br] and [CuL(PPh3)I] to [CuL(PPh3)(2)](PF6) causes the widening of the HOMO-LUMO and S-0-T-1 gaps, which in turn leads to the increase of PLQY up to 9 %.

The greater the degree of merging of CuLHal monomers into the [Cu 2 L 2 Hal 2 ] dimers, the higher the PLQY.

ESIPT-based materials (ESIPT = Excited State Intramolecular Proton Transfer) find diverse applications in optoelectronics and biomedicine owing to the peculiarities of their luminescence properties. Here, an ESIPTcapable compound 2-(3,5-dimethyl-1H-pyrazol-1-yl)-4-(2-hydroxyphenyl)pyrimidine (HL4,2,Me) featuring a short O-H & sdot;& sdot;& sdot;N intramolecular hydrogen bond and two N,N-sites for metal binding has been synthesized. HL4,2,Me is the first reported molecule which can act as an ESIPT molecular switch triggered by metal ion coordination without its deprotonation. Drastic changes in the HL4,2,Me conformation in the [Zn(HL4,2,Me)X2] (X = Cl, Br, I) complexes significantly alter the photoluminescence response compared to the free ligand. In the solid state, HL4,2,Me exhibits barrierless ESIPT and large Stokes-shifted yellow-orange emission due to the interplay of antiKasha S2 -> S0 fluorescence and Kasha-like T1 -> S0 phosphorescence radiative channels. The violation of Kasha's rule for HL4,2,Me is justified by an extraordinarily large S2 - S1 energy gap (ca. 0.9 eV), slowing down the rate of S2 -> S1 internal conversion. The photoluminescence behavior of the ESIPT-incapable zinc(II) coordination compounds strongly depends on the halide anion: the chlorido complex exhibits only fluorescence, the bromido complex displays a minor phosphorescence channel in addition to a major fluorescence channel, while the iodido complex exhibits predominantly phosphorescence. As a result, the emission color of the [Zn(HL4,2,Me)X2] complexes changes gradually from blue for X = Cl to orange for X = I, providing a platform for the fine-tuning of emission by the halide anion.

Dinuclear zinc(II) complexes [Zn2L2X2]center dot nSolv (X = Cl, nSolv = 1.5MeOH; X = Br, I, nSolv = CH2Cl2 center dot 0.5MeOH) and ionic solids [H2L]X center dot H2O (X = Cl, Br) on the base of 2-(3,5-dimethyl-1H-pyrazol-1-yl)-4-(2-hydroxyphenyl) pyrimidine (HL) were synthesized. In these compounds, excited state intramolecular proton transfer in the ligand core is switched off as a result of complexation and protonation. In the structure of zinc(II) complexes, mononuclear {ZnLX} units are combined into non-centrosymmetric dinuclear [Zn2L2X2] molecules due to the bridging function of the deprotonated hydroxyphenyl group. The crystal structures of [H2L]X center dot H2O are stabilized by the formation of supramolecular clusters with N-H center dot center dot center dot O and O-H center dot center dot center dot Cl hydrogen bonds between [H2L]& thorn; cations, H2O molecules and X-anions. Whereas the ionic compounds with the protonated ligand, [H2L]X center dot H2O, exhibit only weak fluorescence in the blue region, the zinc(II) complexes, [Zn2L2X2]center dot nSolv, demonstrate bright fluorescence in the green region whose efficiency increases with a decrease in the size of the halogen atom (PLQY 7 %, 10 % and 21 % for X = I, Br and Cl, respectively), reflecting reduced non-radiative decay through triplet-state quenching as the heavy atom effect weakens.

2-Azido-4-chloro-6-phenylpyrimidine-5-carbaldehyde was synthesized by the Vilsmeier–Haack reaction. In DMSO, it transforms into 7-oxo-5-phenyl-4,7-dihydrotetrazolo[1,5-a]pyrimidine-6-carbaldehyde and then undergoes isomerization recyclization into 6-benzoyltetrazolo[1,5-a]pyrimidin-7(4H)-one. The structures of the obtained compounds were determined by NMR spectroscopy and X-ray diffraction.

Azide-tetrazole tautomerism in a series of substituted 2-azidopyrimidines were studied by NMR spectroscopy methods. The structures of the tautomers existing in a solution were determined. By varying the temperature, the activation parameters of the studied tautomerism were determined from the integrated signal intensity in the NOESY/EXSY spectra. It was found that the phenyl substituents accelerate the tautomeric transformations.

The alkylation of ethyl 2-aryl-1-hydroxy-4-methyl-1H-imidazole-5-carboxylates with substituted benzyl halides resulted in the selective formation of O-alkoxy derivatives. N-Alkylation products (1-alkylimidazole 3-oxides) were obtained by the condensation reaction from acyclic starting compounds. In the presence of the 2-hydroxyphenyl substituent in the 2-position of the imidazole ring, selective monoalkylation involving the hydroxyl substituent on the imidazole nitrogen takes place.

The impact of isomerism of pyrimidine-based ligands and their rhodium(III) complexes with regard to their structures and properties was investigated. Two isomeric ligands, 4-(3,5-dimethyl-1H-pyrazol-1-yl)-2,5-diphenylpyrimidine (HL2,5) and 4-(3,5-dimethyl-1H-pyrazol-1-yl)-2,6-diphenylpyrimidine (HL2,6), were synthesized. The ligands differ by the degree of steric bulk: the molecular structure of HL2,5 is more distorted due to presence of pyrazolyl and phenyl groups in the neighbouring positions 4 and 5 of the pyrimidine ring. The complexation of HL2,5 and HL2,6 with RhCl3 leads to the sp2 C-H bond activation, resulting in the isolation of two complexes, [RhL2,5( Solv)Cl2]center dot nEtOH and [RhL2,6( Solv)Cl2]center dot nEtOH (Solv = H2O, EtOH), with the deprotonated forms of the pyrazolylpyrimidine molecules which coordinate the Rh3+ ion as N<^>N<^>C-tridentate ligands. According to DFT modelling, the mechanism of the deprotonation involves (i) the C-H bond breaking in the 2-phenyl group followed by the coordination of the C atom to the Rh atom, (ii) the protonation of coordinated chlorido ligand, (iii) the ejection of the HCl molecule and (iv) the coordination of the H2O molecule. The ligand isomerism has an impact on emission properties and cytotoxicity of the complexes. Although the excited states of the complexes effectively deactivate through S0/T1 and S0/S1 crossings associated with the cleavage of the weak H2O ligands upon excitation, the [RhL2,5( Solv)Cl2]center dot nEtOH complex appeared to be emissive in the solid state, while [RhL2,6( Solv) Cl2]center dot nEtOH is non-emissive at all. The complexes show significant cytotoxic activity against cancerous HepG2 and Hep2 cell lines, with the [RhL2,6( Solv)Cl2]center dot nEtOH complex being more active than its isomer [RhL2,5( Solv)Cl2]center dot nEtOH. On the other hand, noticeable cytotoxicity of the latter against HepG2 is supplemented by its non-toxicity against non-cancerous MRC-5 cells.

The corresponding amides and hydrazides were obtained by the reaction of 1-hydroxy-1H-imidazole-2-carboxylates with amines and hydrazine. In the reaction of an ester of 1-methoxy-4-methyl-5-(thiophen-2-yl)-1H-imidazole-2-carboxylic acid with NBS, bromination occurred at the thiophene ring, whereas in the reaction with 1,3-dibromo-5,5-dimethylhydantoin it took place at both the methyl group and the thiophene ring; its nitration led to mononitro- or dinitrothiophene derivatives. The reaction of 1-hydroxy-1H-imidazole-2-carboxylates with chloroacetone in Me2CO in the presence of Et3N resulted in the formation of reduced compounds.

ESIPT-capable 2-(2-hydroxyphenyl)-4-(1 H -pyrazol-1-yl)pyrimidines exhibit dual emission associated with interplay of phosphorescence and fluorescence.

A rare example of pyrimidine-based ESIPT-capable compounds, 2-(2-hydroxyphenyl)-4-(1H-pyrazol-1-yl)-6-methylpyrimidine (HLH), was synthesized (ESIPT─excited state intramolecular proton transfer). Its reactions with zinc(II) salts under basic or acidic conditions afforded a dinuclear [Zn2LH2Cl2] complex and an ionic (H2LH)4[ZnCl4]2·3H2O solid. Another ionic solid, (H2LH)Br, was obtained from the solution of HLH acidified with HBr. In both ionic solids, the H+ ion protonates the same pyrimidinic N atom that accepts the O-H···N intramolecular hydrogen bond in the structure of free HLH, which breaks this hydrogen bond and switches off ESIPT in these compounds. This series of compounds which includes neutral HLH molecules and ionic (LH)- and (H2LH)+ species allowed us to elucidate the impact of protonation and coordination coupled deprotonation of HLH on the photoluminescence response and on altering the emission mechanism. The neutral HLH compound exhibits yellow emission as a result of the coexistence of two radiative decay channels: (i) T1 → S0 phosphorescence of the enol form and (ii) anti-Kasha S2 → S0 fluorescence of the keto form, which if feasible due to the large S2-S1 energy gap. However, owing to the efficient nonradiative decay through an energetically favorable conical intersection, the photoluminescence quantum yield of HLH is low. Protonation or deprotonation of the HLH ligand results in the significant blue-shift of the emission bands by more than 100 nm and boosts the quantum efficiency up to ca. 20% in the case of [Zn2LH2Cl2] and (H2LH)4[ZnCl4]2·3H2O. Despite both (H2LH)4[ZnCl4]2·3H2O and (H2LH)Br have the same (H2LH)+ cation in the structures, their emission properties differ significantly, whereas (H2LH)Br shows dual emission associated with two radiative decay channels: (i) S1 → S0 fluorescence and (ii) T1 → S0 phosphorescence, (H2LH)4[ZnCl4]2·3H2O exhibits only fluorescence. This difference in the emission properties can be associated with the external heavy atom effect in (H2LH)Br, which leads to faster intersystem crossing in this compound. Finally, a huge increase in the intensity of the phosphorescence of (H2LH)Br on cooling leads to pronounced luminescence thermochromism (violet emission at 300 K, sky-blue emission at 77 K).

Alkylaromatic 1-hydroxyamino-2-oximes reacted at the hydroxyamino group with glyoxylic acid hydrate, providing 4-aryl(hetaryl)-1-hydroxy-2,5-dihydro-1H-imidazole-2-carboxylic acid 3-oxides that were converted upon heating into 5-aryl(hetaryl)-1-hydroxy-1H-imidazoles containing a hydrogen atom at the heterocycle position 2.

The rational design of ESIPT-capable metal complexes (ESIPT - Excited State Intramolecular Proton Transfer) requires two sites, namely, an ESIPT site and a metal binding site, to be spatially separated into the ligand core. Ligands featuring such sites are able to bind metal ions without being deprotonated upon their coordination. The use of ESIPT-capable ligands for the synthesis of metal complexes paves the way toward the exploration of ESIPT in the field of coordination chemistry. In this study, we present a new ESIPT-capable ligand on the base of 1-hydroxy-1H-imidazole, 1-hydroxy-5-methyl-4-[(2,2'-bipyridin)-6-yl]-2-(pyridin-2-yl)-1H-imidazole (HLb), and a series of ESIPT-capable zinc(II) halido complexes, [Zn(HLb)X2] (X = Cl, Br, I). Due to the incorporation of a (2,2'-bipyridin)-6-yl group at position 4 of the imidazole cycle, HLb acts as an N,N,N-chelating ligand. In the solid state, HLb and [Zn(HLb)X2] emit in the yellow region of the spectrum with excited state lifetimes in the nanosecond domain. Chelation-induced emission enhancement (CHEF) effect in zinc(II) complexes leads to an increase in the photoluminescence quantum yield (PLQY) for these compounds in comparison with free HLb ligand. The ESIPT process in HLb and [Zn(HLb)X2] is barrierless. The emission of [Zn(HLb)X2] is associated with the S1T → S0 transition in the tautomeric form (T-form). In contrast, due to (i) the dark nature of the S1 state and the bright nature of the S2 state and (ii) the large S1-S2 energy gap, HLb shows weak S2T → S0 fluorescence, in violation of Kasha's rule. Finally, the analysis of atomic charges in a series of ESIPT-capable 1-hydroxy-1H-imidazoles and their zinc(II) complexes allowed us to reveal the influence of expanding π-conjugation in the proton-donating and proton-accepting moieties on the stabilization/destabilization of the T-form and on the position of the emission band.

1H-Imidazole derivatives establish one of the iconic classes of ESIPT-capable compounds (ESIPT = excited state intramolecular proton transfer). This work presents the synthesis of 1-hydroxy-4-(2-hydroxyphenyl)-5-methyl-2-(pyridin-2-yl)-1H-imidazole (LOH,OH) as the first example of ESIPT-capable imidazole derivatives wherein the imidazole moiety simultaneously acts as a proton acceptor and a proton donor. The reaction of LOH,OH with chloroacetone leads to the selective reduction of the imidazolic OH group (whereas the phenolic OH group remains unaffected) and to the isolation of 4-(2-hydroxyphenyl)-5-methyl-2-(pyridin-2-yl)-1H-imidazole (LH,OH), a monohydroxy congener of LOH,OH. Both LOH,OH and LH,OH demonstrate luminescence in the solid state. The number of OH···N proton transfer sites in these compounds (one for LH,OH and two for LOH,OH) strongly affects the luminescence mechanism and color of the emission: LH,OH emits in the light green region, whereas LOH,OH luminesces in the orange region. According to joint experimental and theoretical studies, the main emission pathway of both compounds is associated with T1 → S0 phosphorescence and not related to ESIPT. At the same time, LOH,OH also exhibits S1 → S0 fluorescence associated with ESIPT with one proton transferred from the hydroxyimidazole moiety to the pyridine moiety, which is not possible for LH,OH due to the absence of the hydroxy group in the imidazole moiety.

Routes were proposed for the synthesis of new pyrimidine-derived polycyclic nitrogen heterocycles – 4-(1 H -pyrazol-1-yl)-2-(pyridin- 2-yl)pyrimidines with various donor substituents, isomeric 2-(1 H -pyrazol-1-yl)-4-(pyridin-2-yl)pyrimidines, 2-(pyrazin-2-yl)-4-(1 H pyrazol- 1-yl)pyrimidines, and 4-(1 H -pyrazol-1-yl)-2-(pyrimidin-2-yl)pyrimidines – which are of interest as N , N , N -tridentate ligands for the synthesis of transition metal coordination compounds. Experimental and theoretical studies of the electronic structure of the resulting compounds were carried out.