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.

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.

Tuning ESIPT-coupled luminescence by expanding π-conjugation of a proton acceptor moiety in ESIPT-capable zinc(II) complexes with 1-hydroxy-1H-imidazole-based ligands.

The emission of ESIPT-fluorophores is known to be sensitive to various external and internal stimuli and can be fine-tuned through substitution in the proton-donating and proton-accepting groups. The incorporation of metal ions in the molecules of ESIPT fluorophores without their deprotonation is an emerging area of research in coordination chemistry which provides chemists with a new factor affecting the ESIPT reaction and ESIPT-coupled luminescence. In this paper we present 1-hydroxy-5-methyl-4-(pyridin-2-yl)-2-(quinolin-2-yl)-1H-imidazole (HLq) as a new ESIPT-capable ligand. Due to the spatial separation of metal binding and ESIPT sites this ligand can coordinate metal ions without being deprotonated. The reactions of ZnHal2 with HLq afford ESIPT-capable [Zn(HLq)Hal2] (Hal = Cl, Br, I) complexes. In the solid state HLq and [Zn(HLq)Hal2] luminesce in the orange region (λmax = 600-650 nm). The coordination of HLq by Zn2+ ions leads to the increase in the photoluminescence quantum yield due to the chelation-enhanced fluorescence effect. The ESIPT process is barrierless in the S1 state, leading to the only possible fluorescence channel in the tautomeric form (T), S1T → S0T. The emission of [Zn(HLq)Hal2] in the solid state is blue-shifted as compared with HLq due to the stabilization of the ground state and destabilization of the excited state. In CH2Cl2 solutions, the compounds demonstrate dual emission in the UV (λmax = 358 nm) and green (λmax = 530 nm) regions. This dual emission is associated with two radiative deactivation channels in the normal (N) and tautomeric (T) forms, S1N → S0N and S1T → S0T, originating from two minima on the excited state potential energy surfaces. High energy barriers for the GSIPT process allow the trapping of molecules in the minimum of the tautomeric form, S0T, resulting in the possibility of the S0T → S1T photoexcitation and extraordinarily small Stokes shifts in the solid state. Finally, the π-system of quinolin-2-yl group facilitates the delocalization of the positive charge in the proton-accepting part of the molecule and promotes the ESIPT reaction.

N-Hydroxy-N-oxide photoinduced tautomerization and excitation wavelength dependent luminescence of ESIPT-capable zinc(II) complexes with a rationally designed 1-hydroxy-2,4-di(pyridin-2-yl)-1H-imidazole ESIPT-ligand.

The ability of 1-hydroxy-1H-imidazoles to undergo proton transfer processes and to exist in N-hydroxy and N-oxide tautomeric forms can be used in coordination chemistry for the design of ESIPT-capable complexes. A series of ESIPT-capable zinc(II) complexes [Zn(HL)Hal2] (Hal = Cl, Br, I) with a rationally designed ESIPT-ligand 1-hydroxy-5-methyl-2,4-di(pyridin-2-yl)-1H-imidazole (HL) featuring spatially separated metal binding and ESIPT sites have been synthesized and characterized. Crystals of these compounds consist of a mixture of two isomers of [Zn(HL)Hal2]. Only a major isomer has a short intramolecular hydrogen bond O-H⋯N as a pre-requisite for ESIPT. In the solid state, the complexes [Zn(HL)Hal2] demonstrate temperature- and excitation wavelength dependent fluorescence in the cyan region due to the interplay of two intraligand fluorescence channels with excited state lifetimes spanning from 0.2 to 4.3 ns. The coordination of HL by Zn2+ ions results in an increase in the photoluminescence efficiency, and the photoluminescence quantum yields (PLQYs) of the complexes reach 12% at λex = 300 nm and 27% at λex = 400 nm in comparison with the PLQY of free HL of ca. 2%. Quantum chemical calculations indicate that N-hydroxy-N-oxide phototautomerization is both thermodynamically and kinetically favourable in the S1 state for [Zn(HL)Hal2]. The proton transfer induces considerable geometrical reorganizations and therefore results in large Stokes shifts of ca. 230 nm. In contrast, auxiliary ESIPT-incapable complexes [ZnL2][Zn(OAc)2]2·2H2O and [ZnL2][ZnCl2]2·4H2O with the deprotonated ligand exhibit excitation wavelength independent emission in the violet region with the Stokes shift reduced to ca. 130 nm.

Near-infrared emitting copper(I) complexes with a pyrazolylpyrimidine ligand: exploring relaxation pathways.

Mononuclear copper(I) complexes [CuL2]I (1), [CuL2]2[Cu2I4]·2MeCN (2) and [CuL2]PF6 (3) with a new chelating pyrazolylpyrimidine ligand, 2-(3,5-dimethyl-1H-pyrazol-1-yl)-4,6-diphenylpyrimidine (L), were synthesized. In the structures of complex cations [CuL2]+, Cu+ ions coordinate two L molecules (N,N-chelating coordination). Extended π-systems of the L molecules in [CuL2]+ favor the formation of paired π-π stacking intramolecular interactions between the pyrimidine and phenyl rings leading to significant distortions of tetrahedral coordination cores, CuN4. The free ligand L demonstrates dual excitation wavelength dependent luminescence in the UV and violet regions, which is attributed to S1 → S0 fluorescence and T1 → S0 phosphorescence with intraligand charge transfer character. The complexes 1-3 demonstrate T1 → S0 phosphorescence in the near-infrared region. Theoretical investigations point to its ligand-to-metal charge transfer (LMCT) origin. Large Stokes shifts of emission (ca. 200 nm) are the result of notable planarizations of CuN4 cores in the T1 state as compared to the S0 state. Spin-orbit coupling computations revealed that the most effective intersystem crossing channels for [CuL2]+ appear in high-lying excited states, while the S1 → T1 transition is unfavourable according to El-Sayed's rule and the energy gap law. Electron-vibration coupling calculations showed that the C-C and C-N stretching vibrations of the pyrimidine and phenyl moieties, the asymmetric Cu-N stretching vibrations and the wagging motions of phenyl rings contribute the most to the non-radiative deactivation of L and [CuL2]+.

A 1-Hydroxy-1H-imidazole ESIPT Emitter Demonstrating anti-Kasha Fluorescence and Direct Excitation of a Tautomeric Form.

The ability of 1-hydroxy-1H-imidazoles to exist in the form of two prototropic tautomers, the N-hydroxy and the N-oxide forms, can be utilized in the design of new types of ESIPT-fluorophores (ESIPT=excited state intramolecular proton transfer). Here we report the first example of 1-hydroxy-1H-imidazole-based ESIPT-fluorophores, 1-hydroxy-5-methyl-2,4-di(pyridin-2-yl)-1H-imidazole (HL), featuring a short intramolecular hydrogen bond O-H⋅⋅⋅N (O⋅⋅⋅N 2.56 Å) as a pre-requisite for ESIPT. The emission of HL originates from the anti-Kasha S2 →S0 fluorescence in the N-oxide form as a result of a large S2 -S1 energy gap slowing down the S2 →S1 internal conversion. Due to an energy barrier between the N-hydroxy and N-oxide forms in the ground state, the HL molecules can be trapped and photoexcited in the N-oxide form leading to the Stokes shift of ca. 60 nm which is the smallest among known ESIPT-fluorophores.

Coordination-induced emission enhancement in copper(I) iodide coordination polymers supported by 2-(alkylsulfanyl)pyrimidines.

First examples of copper(i) complexes with 2-(alkylsulfanyl)pyrimidine ligands have been synthesized. Reactions of copper(i) iodide with 2-(methylsulfanyl)pyrimidine (L1) in various metal-to-ligand molar ratios in MeCN afford a ladder-type coordination polymer [Cu2L1I2]n with polymeric chains built from double-stranded (Cu2I2)n ribbons supported on both sides by µ2-N,S-L1 molecules. Although the second ligand, 2-(ethylsulfanyl)pyrimidine (L2), differs from L1 only by a methylene group, its reactions with copper(i) iodide in MeCN yield not only a congenerous coordination polymer, [Cu2L2I2]n, but also [CuL2I]n, in which a similar (Cu2I2)n ribbon is decorated by N-monodentate L2 molecules. Absorption spectra of all compounds represent an interplay of metal + iodine-to-ligand charge transfer (XMLCT) and ligand-centered (LC) and cluster-centered (CC) transitions, while the emission occurs from the excited states of XMLCT nature. The luminescence of [Cu2L1I2]n and [Cu2L2I2]n is blue-shifted and greatly enhanced in comparison with that of [CuL2I]n (quantum yields: 89% and 68% for [Cu2L1I2]n and [Cu2L2I2]nvs. 23% for [CuL2I]n at 77 K), which can be associated with a more rigid µ2-N,S coordination of 2-(alkylsulfanyl)pyrimidine ligands in [Cu2L1I2]n and [Cu2L2I2]n leading to a less distorted T1 state.

Hysteretic spin crossover in isomeric iron(ii) complexes.

Two mononuclear iron(ii) complexes with isomeric N,N,N-tridentate pyrimidine-based ligands were synthesized. Both complexes show reproducible hysteretic spin crossover. Low spin state to high spin state switching is cooperative due to autocatalysis.

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

Manipulating the relaxation pathways of excited states and understanding mechanisms of photochemical reactions present important challenges in chemistry. Here we report a unique zinc(II) complex exhibiting unprecedented interplay between the excitation-wavelength-dependent emission, thermally activated delayed fluorescence (TADF) and excited state intramolecular proton transfer (ESIPT). The ESIPT process in the complex is favoured by a short intramolecular OH⋅⋅⋅N hydrogen bond. Synergy between the excitation-wavelength-dependent emission and ESIPT arises due to heavy zinc atom favouring intersystem crossing (isc). Reverse intersystem crossing (risc) and TADF are favoured by a narrow singlet-triplet gap, ΔEST ≈10 kJ mol-1 . These results provide the first insight into how a proton-transfer system can be modified to show a synergy between the excitation-wavelength-dependent emission, ESIPT and TADF. This strategy offers new perspectives for designing ESIPT and TADF emitters exhibiting tunable excitation-wavelength-dependent luminescence.

Temperature- and excitation wavelength-dependent emission in a manganese(ii) complex.

A mononuclear manganese(ii) complex with a chelating 4-(3,5-diphenyl-1H-pyrazol-1-yl)-6-(piperidin-1-yl)pyrimidine ligand (L), [MnL2Cl2]·H2O, shows intriguing excitation wavelength-dependent emission. Depending on the excitation wavelength, the complex demonstrates three emission bands with the maxima at 380 nm, 440 nm and 495 nm. The 380 nm and 440 nm emissions originate from the π → π* and n → π* ligand-centered transitions. The long-wave 495 nm emission with microsecond lifetimes is related to the d-d transitions and/or metal-to-ligand and halogen-to-ligand charge transfer. The emission behavior of this complex is strongly temperature-dependent: upon cooling from 300 K down to 77 K, the intensity of emission considerably increases. The enhancement of the luminescence upon cooling is accompanied by the appearance of the vibrational structure. This complex is the first example of manganese(ii) complexes demonstrating excitation wavelength-dependent emission.

Prototypical iron(ii) complex with 4-amino-1,2,4-triazole reinvestigated: an unexpected impact of water on spin transition.

The magnetic and thermodynamic properties of the prototypical 1D polymeric complex Fe(ATrz)3(NO3)2·H2O (ATrz = 4-amino-1,2,4-triazole) were reinvestigated to gain an insight into the impact of water molecules on the spin transition. Variations in the outerspheric water molecule content in the complex induce drastic and unpredictable changes in its spin crossover regimes. Under vacuum the complex loses water molecules and shows a wide (ca. 30 K) and reproducible hysteresis loop, Tc↑ = 337-345 K and Tc↓ = 316-313 K. In sealed ampoules the complex Fe(ATrz)3(NO3)2·H2O shows a narrow hysteresis (ca. 1-4 K), Tc↑ = 326-329 K and Tc↓ = 326-324 K. After adsorption of water the complex Fe(ATrz)3(NO3)2·nH2O (n = 1.25-1.6) demonstrates a narrow two-step spin transition. In all these cases the kinetics of the LS → HS and HS → LS transitions has decelerating non-cooperative character. For the system Fe(ATrz)3(NO3)2·nH2O (n = 3.6-16.6) wide hysteresis (ca. 5-20 K) re-appears near room temperature (Tc↑ = 319-321 K and Tc↓ = 300-315 K). Surprisingly, the kinetics of the HS → LS spin transition for the systems with high water content switches from decelerating to sigmoidal (cooperative). The activation energy of the LS → HS transition was estimated for the first time for iron(ii) spin crossover complexes with 1,2,4-triazoles (ca. 1000-2000 kJ mol-1). The systems Fe(ATrz)3(NO3)2 and Fe(ATrz)3(NO3)2·nH2O show compensation effects (ΔH - ΔS, ln A - Ea). A correlation between the Tonset↑, the ΔH values and the water content in the complex is observed: the highest ΔH values (27-29 kJ mol-1) and the lowest Tonset↑ values (317-320 K) correspond to the samples with high water content, whereas the lowest ΔH values (19-23 kJ mol-1) and the highest Tonset↑ values (337-345 K) correspond to water-free samples, Fe(ATrz)3(NO3)2. Our results provide the first experimental evidence that the presence of water (and even air humidity) produces dramatic changes in the spin crossover behavior of the prototypical 1D polymeric complex Fe(ATrz)3(NO3)2·H2O (ATrz = 4-amino-1,2,4-triazole).

Mixed-valence copper(I,II) complexes with 4-(1H-pyrazol-1-yl)-6-R-pyrimidines: from ionic structures to coordination polymers.

Two pyrimidine-based ligands, 4-(3,5-diphenyl-1H-pyrazol-1-yl)-6-(morpholino)pyrimidine () and 4-(3,5-diphenyl-1H-pyrazol-1-yl)-6-phenoxypyrimidine (), and a series of mixed-valence copper(i,ii) halide complexes, [Cu(L(2))2Br]2[Cu2Br4] (), [Cu(L(2))2Cl][CuCl2] (), and [Cu2L(3)Br3]n (), have been synthesized. The complex [Cu(L(2))2Br]2[Cu2Br4] was prepared by the reaction of with CuBr2 in a 1 : 1 molar ratio in MeCN. Its chlorido-analogue, the complex [Cu(L(2))2Cl][CuCl2], was synthesized by the reaction between , CuCl2 and CuCl in a 2 : 1 : 1 molar ratio in MeCN. The ligand acts as a chelating one. In the structures of the complexes [Cu(L(2))2Br]2[Cu2Br4] and [Cu(L(2))2Cl][CuCl2] the Cu(2+) ion is in the cationic part of the complex whereas the Cu(+) ion is located in the anionic part. The best way to synthesize the mixed-valence 1D coordination polymer [Cu2L(3)Br3]n is to react CuBr2 with in a 2 : 1 molar ratio in the MeCN/CHCl3 mixture on heating. In the structure of [Cu2L(3)Br3]n the ligand shows chelating/bridging tridentate coordination. This is the first example of the tridentate coordination of 4-(1H-pyrazol-1-yl)-6-R-pyrimidines. The striking difference between the coordination behavior of and (chelating bidentate vs. chelating/bridging coordination) is related with the possibility of rotation of the 6-phenoxy group around the C-O bond which makes the N(1) pyrimidine atom less sterically hindered, enabling it to participate in metal ion binding. Importantly, all copper ions in [Cu2L(3)Br3]n show similar tetrahedral environments, CuNBr3 and CuN2Br2, which is extremely rare for mixed-valence copper(i,ii) compounds. The ligands and show blue emission which is quenched upon their coordination to copper ions. The 1D coordination polymer [Cu2L(3)Br3]n shows high thermal stability and unusual solvent-occlusion properties. The role of the substituents favoring the formation of the mixed-valence copper(i,ii) complexes with 4-(1H-pyrazol-1-yl)-6-R-pyrimidines is discussed.

Halide impact on emission of mononuclear copper(I) complexes with pyrazolylpyrimidine and triphenylphosphine.

A series of mononuclear heteroleptic copper(I) halide complexes, [CuL(PPh3)X] (X = Cl, Br, I), based on 4-(3,5-diphenyl-1H-pyrazol-1-yl)-6-(piperidin-1-yl)pyrimidine (L) and triphenylphosphine, have been synthesized by reaction between CuX (X = Cl, Br, I), L and PPh3 in a molar ratio of 1/1/1 in MeCN solutions. The copper atom, showing the distorted tetrahedral environment, is bound by the N,N-chelating ligand L, triphenylphosphine and a halide ion. The complexes [CuL(PPh3)Cl] and [CuL(PPh3)Br] are isostructural. In CH2Cl2 solutions, L and the complexes [CuL(PPh3)X] (X = Cl, Br, I) display a luminescence band with λ(max) = 377 nm and a lifetime of 1.9 ns (ligand-based luminescence (LL*)). However, the complex [CuL(PPh3)I] has an additional weak luminescence band with λ(max) = 681 nm and a lifetime of 96 ns of (3)MLCT origin. In the solid state, L shows the splitting of the luminescence band to λ(max) = 365 and 384 nm and a slight increase of the lifetime to 2.66 ns. Solid samples of the complexes [CuL(PPh3)X] demonstrate (3)MLCT luminescence bands at 620 nm (X = Cl), 605 nm (X = Br) and 559 nm (X = I) with lifetimes in the range 3.6-11.2 µs, whereas the LL* band (377 nm) is absent. Quantum yields and rate constants of radiative and nonradiative processes were determined in CH2Cl2 solutions and in the solid state for all complexes. The luminescence quantum yield and lifetimes for the solid samples increase in the order [CuL(PPh3)Cl] < [CuL(PPh3)Br] < [CuL(PPh3)I]. This is due to the increase of radiative decay and simultaneous suppression of nonradiative decay. The complex [CuL(PPh3)I] shows a high quantum yield of 29.4% and an excited state lifetime of 11.2 µs.