Synthesis and luminescence of indium(III) complexes with (1H-pyrazol-1-yl)pyridazines

Abstract

New deep-blue-emitting materials are crucial for the development of OLED technology because of the most useful iridium-containing complexes, which are applied as blue emitters. However, they suffer from degradation during operation and do not show the required color characteristics. One potential methodology for the design of blue emitters is the synthesis of indium(III) complexes, due to the absence of metal-associated transitions. The coordination of an organic ligand to In³⁺ may result in enhanced ligand fluorescence (CHEF effect), and these emitters may also be phosphorescent. The reaction of InCl₃ with 3-chloro-6-(1H-pyrazol-1-yl)pyridazine (L^H) and 3-chloro-6-(3,5-dimethyl-1H-pyrazol-1-yl)pyridazine (L^Me) afforded two new complexes, namely, [In(L^H)(H₂O)Cl₃] (1) and [In(L^Me)₂Cl₂][InCl₄] (2). The crystal structures of compounds 1 and 2 were determined by X-ray diffraction analysis. Complex 1 is octahedral neutral with one coordinated L^H ligand. Complex 2 has an ionic structure, which consists of a octahedral complex cation {In(L₂)₂Cl₂}⁺ containing two coordinated L^Me ligands and one tetrahedral complex anion {InCl₄}⁻. The UV-Vis spectra of complexes 1 and ₂ are similar and consist of a main intense signal at 258 nm for 1 and 261 nm for 2 and a less intense signal at 286 nm for 1 and 309 nm for 2. Quantum chemical calculations were performed using the TD-DFT method for assigning electron transitions. It was shown that the most intense transition in complex 1 was mainly intraligand (ILCT). The main transition in complex 2 was a mixed interligand (LL'CT) and intraligand (ILCT) characteristic. The photoluminescent properties of 1 and 2 were investigated in both solid state and solution at room temperature. In the solid state, the indium complexes demonstrated excitation-dependent emission, that is, the complexes displayed blue fluorescence at λ_max = 375 nm upon 340 nm excitation, but upon 420 nm excitation, fluorescence bands can be detected in the green region at 500–510 nm.

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Introduction

The history of exploring the chemistry of luminescent coordination compounds of group 13 elements began with the discovery of bright electroluminescent (EL) tris(8-oxyquinolinate) aluminum(III) (Alq₃) by Dr Tang in 1987, which actually became the first organic emitter for the OLED industry.¹ At the same time, as a fluorescence emitter with an IQE limitation of 25%, it lost its significance over time. Subsequently, a significant number of luminescent complexes of aluminium(III) with modified 8-oxyquinoline were also obtained and studied. The aim of this research was to improve the photoluminescent (PL) properties of aluminium(III) complexes.² This class of compounds expanded down group 13, and a considerable number of luminescent gallium(III) and indium(III) complexes were synthesized and investigated. The first to be obtained were In(III) and Ga(III) complexes with 8-oxyquinoline and some of its derivatives. The electroluminescent properties of these complexes have been exhaustively documented.^3–12 In the majority of studies, indium(III) complexes have been observed to exhibit fluorescence within the blue or green spectral regions,^13–15 and researchers continue to study indium(III) 8-oxyquinolinates and their derivatives. For instance, in a previous study,³ the OLEDs were examined using Inq₃, Gaq₃ and Alq₃ complexes as the emission layer. The devices exhibited predominantly green emission, with EL maxima ranging from 530 to 550 nm. Furthermore, it was observed that the efficiency decreased with the increase in atomic radius. In general, the photo- and electroluminescent properties of the Inq₃ complex have been the subject of extensive research to date,^5,16,17 as have indium(III) complexes with 8-oxyquinoline derivatives.¹⁸

Despite the fact that the In³⁺ ion is oxophilic, luminescent indium(III) complexes are known to have approximately equal extents with nitrogen-donor chelate ligands (NN),^14,19–26 including phthalocyanines.^27,28 Furthermore, there are a sufficient number of works in which mixed-ligand complexes with N- and O-donor ligands and NO-ligand complexes are synthesized.^29–33 Indium complex compounds with dipyrines demonstrated green fluorescence with a maximum at approximately 500 nm in solution with a quantum yield of approximately 7%.²⁵ Intriguingly, the introduction of additional substituents into the pyrrole cycle, without altering the central fragment [In(NN)₃], results in a red shift in emission to the 600–650 nm region with quantum yields in solution of about 30–40%.¹⁹ Impressive emission properties are also observed for indium(III) complexes with salen ligands,^29,34 for example, in a previous study,²⁹ an indium(III) complex with bright blue luminescence demonstrated a high quantum yield of 31.3%. It has been demonstrated that dialdimeminate complexes of indium(III) exhibit fluorescence (λ_max = 550–560 nm) with quantum yields up to 30%. It is noteworthy that in this work, the quantum efficiency of some indium(III) complexes exceeds this parameter for their analogs in the subgroup–aluminum(III) and gallium(III) complexes.¹⁴ In continuation of this work, the authors made fine tuning of diketiminate ligands It was demonstrated that the indium(III) complex exhibited phosphorescence, in contrast to the aluminum(III) and gallium(III) complexes.³⁵

In the current research for the synthesis of potential luminescent indium(III) complexes, we selected two nitrogen-donor ligands, 3-chloro-6-(1H-pyrazol-1-yl)pyridazine (L^H) and 3-chloro-6-(3,5-dimethyl-1H-pyrazol-1-yl)pyridazine (L^Me), because they (i) can adopt a chelate coordination mode, (ii) are easily synthesized using commercially available reagents, and (iii) are simply modified to develop the work in further studies to investigate the influence of substituents in ligand scaffolds on emission properties. Moreover, the (1H-pyrazol-1-yl)pyridazine ligands have not been previously used for the synthesis of indium(III) complexes to date. However, manganese(II) complexes based on L^Me have been obtained, which exhibit emission in the green region in the solid state.³⁶ Furthermore, rhenium(I) carbonyl complexes with L^H demonstrate emission in the red region in a dichloromethane solution.³⁷

In the present study, novel indium(III) complexes with (1H-pyrazol-1-yl)pyridazines derivatives were synthesized. The luminescence properties of the compounds in both solution and solid state were investigated. The behaviour of the complexes in solution was studied by UV and NMR spectroscopy.

Experimental section

Materials and methods

All commercially available reagents, InCl₃·2.2H₂O (Sigma-Aldrich, 98%), 3,6-dichloropyridazine (98%, abcr), hydrazine hydrate (98%), acetylacetone (98%), 1,1,3,3-tetramethoxypropane (99%, abcr), dimethyl sulfoxide-d₆ (purity >99.9%), chloroform-d (purity >99.9%), were used as purchased. Organic solvents (acetone ((CH₃)₂CO), ethanol (C₂H₅OH), and hexane (C₆H₁₄)) were dried by standard methods. Acetonitrile solvent (MeCN) for electronic and fluorescence spectroscopy was of HPLC purity grade and commercially available. 3-Chloro-6-(3,5-dimethyl-1H-pyrazol-1-yl)pyridazine (L^Me) was prepared in two steps according to the procedures described in the literature: the first step is the reaction of 3,6-dichloropyridazine with hydrazine hydrate³⁸ and the second step is the interaction of 3-chloro-6-hydrazinylpyridazine hydrate with acetylacetone.³⁹ To synthesize 3-chloro-6-(1H-pyrazol-1-yl)pyridazine (L^H), a reaction between 3-chloro-6-hydrazinylpyridazine and tetramethoxypropane was carried out in acidic media, following the previously reported procedure with slight modifications.⁴⁰ The chemical composition and purity of 3-chloro-6-hydrazinylpyridazine hydrate, L^H and L^Me were confirmed by elemental analysis data and ¹H NMR spectroscopy. According to the X-ray powder analysis, the phase of prepared L^H is the same as the published one (CCDC number 788398)⁴¹ (Fig. S1).

Physical measurements

Elemental C, H, and N analysis was performed using a Vario MICRO cube instrument following a standard procedure. IR spectra were recorded in the 4000–400 cm⁻¹ range using a PerkinElmer System 2000 FTIR spectrometer (KBr pellets). Electron absorption spectra of the solutions 1, 2 and ligands (L^H, L^Me) in acetonitrile were recorded using a Cary 60 spectrometer (Agilent Technologies) and an SF-2000 spectrophotometer. ¹H NMR spectra (500 MHz) were acquired using a Bruker Avance-500 spectrometer with a 5 mm PABBO-PLUS probe at room temperature. The chemical shifts were given in parts per million (ppm) from tetramethylsilane. Thermal analysis was performed using a NETZSCH TG 209 F1 analyzer (Iris Thermo Microbalance), with Al₂O₃ powder as a standard. The experiments were conducted in an open alumina crucible under a helium stream at a heating rate of 10 °C min⁻¹. Photoluminescence (PL) and excitation spectra of complexes and L^H in the solid state and MeCN solutions were recorded using a Horiba Fluorolog 3 spectrofluorometer equipped with steady (450 W) and pulsed xenon lamps as light sources, a cooled detector, and double grating excitation and emission monochromators. The photoluminescence and excitation spectra were corrected for the source intensity (lamp and grating) and to the spectral response to emission (detector and grating) using standard correction curves. The photoluminescence decay kinetics for the complexes and L^H in the solid state at room temperature was recorded by time-correlated single photon counting using a NanoLED pulsed light source and a NanoLED-C2 controller. Excitation and emission (registration) wavelengths for decay kinetic measurements for each compound are presented in Table 2. The quantum yields (YQ) of complexes and L^H were measured using a Fluorolog 3 spectrofluorometer equipped with a quantum sphere (Quanta-φ), and maxima in excitation spectra were used as excitation wavelengths for YQ measurements.

X-Ray diffraction analysis

Single-crystal X-ray diffraction (XRD) data for 1 and 2 were acquired using a Bruker D8 Venture diffractometer with a CMOS PHOTON III detector and an IμS 3.0 microfocus source (collimating Montel mirrors). All of the experiments were conducted at 150 K using Mo Kα (λ = 0.71073 Å) radiation. Absorption correction was applied by SADABS.⁴² Structures were solved by dual space algorithm with SHELXT-2018/2⁴³ refined by full-matrix least-squares treatment against |F|² with SHELXL-2019/2⁴⁴ using ShelXle GUI.⁴⁵ Atomic displacement parameters for non-hydrogen atoms were refined anisotropically. The positions of hydrogen atoms were calculated corresponding to their geometrical conditions and refined in the riding model. In the structure of complex 2, disordering of the InCl₄ fragment was observed over two positions with occupancies of 0.95/0.05.

The crystallographic data and details of diffraction experiments for complexes 1 and 2 are presented in Table S1 and S2. The crystallographic data were deposited in the Cambridge Crystallographic Data Centre under the deposition codes CCDC 2489745 (1) and CCDC 2489746 (2).

X-ray powder diffraction patterns of L^H were recorded using a Tongda TD-3700 instrument (Dandong Tongda Science and Technology, Dandong, China; CuKα radiation; λ = 1.54178 A, Ni filter, linear Dectris Mythen2 1D detector, 2θ angle range 3°–40°, step 0.0286°, accumulation 1 s per point). The samples for the experiments were prepared as follows: polycrystals were ground in an agate mortar in the presence of heptane; the resulting suspension was applied to the polished side of a standard quartz or glass cuvette; after drying the heptane, the sample formed a thin even layer (∼100 µm thick).

Quantum chemical calculations

Quantum chemical calculations were performed using the ORCA 6.0.0 software package.^46,47 For complex 1, cation of complex 2 and L^H, the geometry in the gas phase was optimized (Tables S3 nad S4), The XRD data were taken as an initial point.⁴¹ Finding the minimum point on the potential energy surface was confirmed by the absence of imaginary vibrational frequencies. Calculations were conducted within the framework of density functional theory (DFT), employing the non-empirical PBE functional⁴⁸ in conjunction with empirical corrections for dispersion interactions, D4.⁴⁹ To describe all atoms, the def2-TZVP basis sets were used. The effect of the solvent was taken into account using the CPCM model.⁵⁰ Excitation calculations were performed by the TD-DFT methodology as implemented in ORCA.⁵¹

Synthesis of [In(L^H)(H₂O)Cl₃] (1)

InCl₃·2.2H₂O (85 mg, 326 µmol) was dissolved in 40 ml of ethanol. Then, ligand L^H (59 mg, 326 µmol) was added to the resulting solution. As a result, a colorless solution was obtained, which was refluxed for 15 hours. After cooling, the colorless solution was evaporated to dryness, washed several times with diethyl ether and air dried. A white solid was obtained (127 mg, yield 93%). Colorless crystals suitable for X-ray analysis were obtained by layering hexane on a solution of the complex in acetone. Found for C₇H₇Cl₄InN₄O: C 20.5, H 2.1, N 13.2; calculate for C₇H₇Cl₄InN₄O: C 20.1, H 1.7, N 13.4. IR (KBr, cm⁻¹): 3543 (br. w), 3313 (br. m), 3155 (w), 3122 (m), 3095 (w), 2991 (w), 2972 (w), 2924 (w), 1830 (w), 1788 (w), 1743 (w), 1647 (w), 1587 (m), 1566 (m), 1552 (w), 1525 (m), 1510 (w), 1471 (vs), 1404 (vs), 1371 (s), 1265 (m), 1209 (s), 1167 (s), 1138 (w), 1105 (m), 1067 (s), 1022 (m), 953 (s), 914 (w), 866 (m), 829 (s), 775 (s), 638 (m), 596 (m), 542 (m), 498 (m), 442 (w). ¹H NMR (CD₃CN, δ, ppm): 8.69 (br. s, 1H), 8.35 (br. s, 1H), 8.31 (d, 1H), 8.13 (d, 1H), 6.96 (br. s, 1H).

Synthesis of [In(L^Me)₂Cl₂][InCl₄] (2)

The synthesis procedure of 2 is similar to that described for complex 1. A total quantity of indium trichloride (81 mg, 311 µmol) and L^Me (65 mg, 311 µmol) were taken. A white solid was obtained (131 mg, yield 98%). Crystals that are suitable for X-ray structural analysis were obtained by analogy with complex 1. Found for C₁₈H₁₈Cl₈In₂N₈: C 25.3, H 2.2, N 13.1; calculate for C₁₈H₁₈Cl₈In₂N₈: C 25.2, H 2.1, N 13.1. IR (KBr, cm⁻¹): 3165 (w), 3116 (w), 3072 (w), 1652 (vw), 1585 (s), 1547 (w), 1472 (s), 1427 (vs), 1383 (m), 1366 (m), 1273 (m), 1220 (m), 1171 (m), 1134 (s), 1067 (m), 1035 (w), 995 (m), 835 (s), 825 (s), 799 (s), 621 (w), 594 (w), 536 (w), 519 (m). ¹H NMR (CD₃CN, δ, ppm): 8.23 (d, 1H), 8.04 (d, 1H), 6.63 (br. s, 1H), 2.68–2.63 (s, 6H).

Results and discussion

The reactions of indium trichloride with L^H and L^Me result in the formation of two indium complexes [In(L^H)(H₂O)Cl₃] (1) and [In(L^Me)₂Cl₂](InCl₄) (2) (Scheme 1). The complexes were characterized by elemental analysis, IR spectroscopy, and UV spectroscopy. Crystals suitable for X-ray diffraction analysis were obtained, and the crystal structures of complexes 1 and 2 were determined. The molecular structures of 1 and 2 are shown in Fig. 1. Table 1 presents a summary of the main bond lengths. The structure of complex 1 can be described as follows: the central In atom is octahedrally surrounded by three chlorine atoms, two nitrogen atoms of the pyrazolyl–pyridazine ligand and one oxygen atom of water molecule. The position of the chloride ligands indicates the implementation of the fac-geometry. The In–Cl1 bond length (2.4604(6) Å) is noticeably longer than those of In–Cl2 (2.4065(6) Å) and In–Cl3 (2.4056(8) Å). Furthermore, the In–N1 (2.258(2) Å) and In–N2 (2.350(2) Å) distances differ significantly. It is important to note that the pyrazole and pyridazine rings are coplanar, as demonstrated by the angle between their planes being 3.15°. However, in the crystal structure of L^H, this torsion angle is 177.99° 41. Furthermore, the crystal packing of complex 1 exhibits intermolecular H-bonds between the hydrogen atoms of the coordinated water molecule and the chlorine atoms of the coordination sphere, with a range of 2.371(3)–2.632(4) Å (O–Cl distances in the range of 3.131(2)–3.269(2) Å) (Fig. S2). This leads to the formation of pseudo-layers arranged in the (1,0,1) band.

Scheme 1 Synthesis of the complexes 1 and 2.

Fig. 1 The molecular structure of 1 (left) and 2 (right).

Table 1 Selected bond lengths in complexes 1 and 2

1		2
Bond type	Distance, Å	Bond type	Distance, Å
In–Cl1	2.4604(6)	In–Cl1	2.381(1)
In–Cl2	2.4065(6)	In–Cl2	2.403(1)
In–Cl3	2.4056(8)	In–N1	2.277(3)
In–N1	2.258(2)	In–N2	2.295(3)
In–N2	2.350(2)	In–N1′	2.275(3)
In–O1	2.266(2)	In–N2′	2.314(3)

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Conversely, complex 2 has an ionic structure consisting of cationic part [In(L^Me)₂Cl₂]⁺ with a charge of +1 and a tetrahedral anion, InCl₄⁻. The structure of the cation part of 2 can be described as follows: the central atom In is octahedrally surrounded by two chlorine atoms and four nitrogen atoms from two pyrazolyl–pyridazine ligands. The positioning of the chloride ligands is indicative of the occurrence of the cis isomer in the cation of 2. In contrast to the complex 1, the distances between the In–Cl1 (2.381(1) Å) and In–Cl2 (2.403(1) Å) bonds do not differ significantly due to the symmetrical structure of the cation part. The lengths of In–N1 (2.277(3) Å), In–N2 (2.295(3) Å), In–N1′ (2.275(3) Å) and In–N2′ (2.314(3) Å) bonds also demonstrate closer proximity to each other than was observed in the complex 1. However, a significant difference of 17.57° is apparent in the planes of the pyrazole and pyridazine rings in one of the ligand fragments (N1N2). The formation of the cationic complex [In(L^Me)₂Cl₂]⁺ with the InCl⁴⁻ counterion, as opposed to Cl⁻, can be rationalized by two principal factors. First, the small chloride ion is insufficient for the effective crystallization of the large, bulky complex 2. Second, the pronounced Lewis acidity of indium(III) favors the formation of stable anionic species such as InCl₄⁻.

The IR spectra of complexes 1 and 2 show characteristic C–H stretching bands in the 2924–3095 cm⁻¹ and 3072–3165 cm⁻¹ regions, respectively. The bands associated with C=N and C=C stretching vibrations were observed within the 953–1647 cm⁻¹ range for 1 and the 995–1652 cm⁻¹ range for 2. Vibrational bands belonging to coordinated water molecules (ν(O–H)) are observed within the range 3474–3543 cm⁻¹ for complex 1. However, these vibration bands are not detected in the spectrum of complex 2 (Fig. S3).

The ¹H NMR spectra of complexes 1 and 2 (Fig. S4) revealed the characteristic signals of the coordinated L^H and L^Me ligands. In the spectra of complexes 1, broad singlets at 8.69 and 8.35 ppm, as well as doublets at 8.31 and 8.13 and a broad singlet at 6.96 ppm for 1, are observed. A set of signals in the form of doublets at 8.23 and 8.04 ppm, as well as a broad singlet at 6.63 ppm, are observed in the spectra of complex 2. In the spectra of complexes 2, the presence of methyl substituents is indicated by the appearance of two singlets at 2.68 and 2.63 ppm. The spectra of the free ligands differ from those of the complexes. It is evident that certain signals undergo a shift towards a more intense field, as evidenced by the ¹H NMR spectrum of L^H. Specifically, signals at 8.25 ppm (8.35 ppm in 1), 7.89 and 7.82 ppm (8.31 and 8.13 ppm in 1), and 6.64 ppm (6.96 ppm in 1) are observed. In the spectrum of L^Me, a comparable signal shift is evident. The signals at 8.12 ppm (8.23 ppm in 2), 7.76 ppm (8.04 ppm in 2), 6.17 ppm (6.63 ppm in 2), 2.68 ppm (2.68 in 2) and 2.27 ppm (2.63 ppm in 2) are detected.

It is important to note that the NMR spectra of the complexes in MeCN solutions with concentrations of 10⁻³ M and 10⁻⁵ M are identical, confirming the stability of the complexes in solutions at dilutions. This is crucial for the further investigation of the photophysical properties of the complexes in acetonitrile solutions with concentrations of approximately 10⁻⁵ M.

Thermogravimetric analysis (TGA) was employed to evaluate the thermal stability of the synthesized complexes 1 and 2. The corresponding TGA curves, recorded over a temperature range of 25 to 568 °C, are presented in Fig. S5. Complex 1 is stable up to approximately 100 °C. Its thermal decomposition occurs in multiple steps. The first two closely spaced stages, observed between 100 and 180 °C, are apparently associated with the release of HCl. The main decomposition stage, exhibiting the most significant mass loss and a strong exothermic effect, occurs between 360 and 568 °C and is attributed to the cleavage of the L^H ligand. The residual mass at 568.1 °C was 65.84%. In contrast, complex 2 is significantly less stable, with its decomposition commencing at approximately 30 °C. The first mass loss step, observed between 27 and 90 °C, is attributed to the elimination of HCl. The main decomposition stage, occurring between 310 and 568 °C, corresponds to the cleavage of the L^Me ligand. A residual mass at 567.7 °C was 62.63%.

The absorption spectrum of L^H in the MeCN solution exhibits a major band at 260 nm and a weak shoulder at 285–318 nm (Fig. S6). The photophysical data of ligands and indium(III) complexes are summarized in Table 2. In Fig. S6 (left), the absorption spectrum of 1 is presented in comparison with the spectrum of free ligand L^H. The difference between the absorption spectra of L^H and 1 is in more intense shoulder at 282–325 nm. Due to the fact that absorption band at 260 nm in the spectra of 1 and L^H is caused by intraligand π–π* transition, the energy of the transition does not change (Fig. 2 (left), see TD-DFT below and major contributions of transitions in Table S5). At the same time, the optical density at 282–325 nm in the spectra of L^H and 1 is notably different from one another, which is caused by the fact that n–π* transitions localized on coordinated chloride ions are mixed with intraligand π–π* transitions. The absorption spectra of 2 demonstrate a quite similar major band at 260 nm and a shoulder at 289–337 nm (Fig. 2 (right) and Fig. S6 (right)), but according to TD-DFT calculations, the nature of the transitions is different (see below).

Table 2 Photophysical data for L^H, L^Me, 1, and 2 in the MeCN solution and solid state at room temperature

	In MeCN solution				Solid state (300 K)
	Absorption spectra λmax (nm)	Excitation spectra λmax (nm)	Emission spectra λmax (nm)	Life times, τ (ns)	Excitation spectra λmax (nm)	Emission spectra λmax (nm)	Life times, τ (ns)
L^H	260, 285–318 (sh.)	260 (sh.), 300	375	6.0 ns (90%), 2.0 ns (10%)	∼300–320	360	1.6 ns (44%), 10 ns (56%)
				λ ex = 300 nm, λem = 375 nm	360	440	λ ex = 300 nm, λem = 375 nm
							2.7 ns (76%), 12 ns (24%)
							λ ex = 350 nm, λem = 425 nm
1	260, 282–325	250, 300	325, 375	6.4 ns (89%), 1.2 ns (11%)	340, 370	375, 470	1.7 ns
				λ ex = 300 nm, λem = 325 nm	370, 420	510	λ ex = 300 nm, λem = 375 nm
				2.1 ns (92%), 8.5 ns (8%)			6.7 ns (89%), 57 ns (11%)
				λ ex = 300 nm, λem = 400 nm			λ em = 350 nm, λem = 500 nm
L^Me36	260, ∼300 (sh.)	270	390	2.7 ns	∼300–320	375	6.0 ns
				λ ex = 300 nm, λem = 390 nm	360	450	λ ex = 300 nm, λem = 375 nm
							6.5 ns
							λ ex = 350 nm, λper = 450 nm
2	260, 289–337 (sh.)	260, 290–300 (sh.)	375	6.4 ns (94%), 2.6 ns (6%)	∼300–330	375	19 ns (9%), 1.4 ns (91%)
				λ ex = 300 nm, λem = 375 nm	350, 380	440	λ ex = 300 nm, λem = 375 nm
							16 ns (10%), 3.8 ns (90%)
							λ BO36 = 350 nm, λper = 440 nm
						500	—

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Fig. 2 Experimental and calculated (TD-DFT) electronic absorption spectra of 1 (left) and 2 (right) in acetonitrile.

To explain the observed electronic transitions, quantum chemical calculations were performed using the TD-DFT method for the complex 1, cation part of complex 2 and L^H using XRD data as an initial point (Table S5). The optimized geometry agrees well with the X-ray diffraction data. Further geometry optimization in acetonitrile did not lead to a significant change in the structure of complexes 1, 2 and L^H. A comparison of the calculated and experimental spectra for the complexes is shown in Fig. 2, for L^H is presented in Fig. S6.

For L^H, the most intense transition (f = 0.603) with an energy of 4.88 eV (254 nm) is primarily attributed to the electron transfer from the HOMO−1 (highest occupied molecular orbital) to the LUMO (lowest unoccupied molecular orbital). The second most intense transition (f = 0.101) with an energy of 4.72 eV (263 nm) is primarily attributed to the electron transfer from the HOMO to the LUMO+1. The less intense transitions (f = 0.052) and (f = 0.074) with energies of 4.37 eV (284 nm) and 5.79 eV (214 nm), respectively, are clearly characterized by electron transfer from HOMO to LUMO and from HOMO−3 to LUMO, respectively.

TD-DFT calculations indicate that for complex 1, the most intense transition (f = 0.272) with an energy of 4.81 eV (258 nm) is predominantly related to the intraligand charge transfer (ILCT). Furthermore, a substantial contribution is attributed to the transition (f = 0.089), exhibiting an energy of 4.34 eV (286 nm), which is predominantly associated with the intraligand π–π* transition between the frontier orbitals. The transition (f = 0.113) with an energy of 4.91 eV (252 nm) is of a mixed nature and consists of intraligand and interligand (LClCT) π–π* transitions between pyrazolyl–pyridazine and chloride ligands (Table S5). High-energy transitions are facilitated by the transfer of electrons from lower occupied molecular orbitals to the lowest unoccupied molecular orbital or the next most energetically favourable unoccupied orbital (LUMO+1).

In the case of complex 2, the most intense transition (f = 0.602) with an energy of 4.75 eV (261 nm) has a mixed character, since both the charge transfer between the pyrazolyl–pyridazine ligands (LL′CT, where L is the first coordinated L^Me molecule in cation part of 2 and L′ is the second coordinated L^Me) and the intraligand charge transfer (ILCT) make a comparable contribution to it. In a similar manner, within the transition (f = 0.178) with an energy of 4.71 eV (263 nm), charge transfer can occur both between different and between the same pyrazolyl–pyridazine. The transition (f = 0.103) with an energy of 4.01 eV (309 nm) is also combined, since it occurs due to the π–π* transition between the frontier orbitals localized on the two pyrazolyl–pyridazine ligands (Table S5). Higher energy transitions are achieved by electron transfer from lower occupied molecular orbitals to LUMO or LUMO+1.

Consequently, quantum chemical modelling validates the occurrence of a high-energy electron π–π* transition, attributable to inter- or intraligand charge transfer, and the same conclusion about the nature of the transitions has been demonstrated for [ReL^H(CO)₃Br] in a previous study³⁷ and for a similar organic molecule, 2-bromo-5-(1H-pyrazol-1-yl)pyrazine, in a previous study.⁵²

Photophysical data for L^Me in the MeCN solution and solid state have been published in our work.³⁶ However, no spectroscopic data have been reported for L^H. Upon excitation at 260 nm, a weak fluorescence band (λ_max = 375 nm) due to the S₁ → S₀ relaxation pathway is observed. Nevertheless, the more intense maximum in the excitation spectrum is the band at 300 nm. Thus, using excitation at 300 nm results in more intense fluorescence with the same maximum and vibrational structures (the major life time component is 6.0 ns; Fig. S8 and S9).

The solid-state emission spectrum of L^H is comparable to the published spectrum of L^Me, and the PL spectrum of L^H depends on the excitation wavelength (Fig. 3, left). Upon excitation at 300–320 nm, the solution of L^H displays a fluorescence band at 360 nm with two lifetimes of 1.6 ns and 10 ns, but upon excitation at 360 nm, a more intense red-shifted broad emission band at 440 nm can be detected with longer life times (2.7 and 12 ns). Taking into account that spectrum of L^H shows only one emission band at 375 nm in the solution, and it seems that the second emission band in the solid state (λ_max = 440 nm) with a longer life time is due to the relaxation of excimers according to the following likely mechanism 2L^H + hν₁ → (L^H)₂* → 2L^H + hv₂.

Fig. 3 Emission and excitation spectra of L^H (left) and 1 (right) in the solid state.

The fluorescence of 1 and 2 was recorded in an acetonitrile solution with a concentration of approximately 10⁻⁵ M (Fig. S8, right and Fig. S9, right). It is important to note that the discussed indium(III) complexes exist in MeCN solutions with concentrations of 10⁻³–10⁻⁵ M, as confirmed by ¹H NMR spectroscopy. The emission spectrum of 1 depends on the excitation energy: the excitation of 1 at 300 nm in an acetonitrile medium gives a fluorescence band at 320 nm and excitation at 330 nm results in the appearance of a band with maximum at 375 nm. It seems that in the solution of 1, two fluorescent complex forms with different life times (6.4 and 2.1 ns) exist. The excitation and absorption spectra of 2 are close to each other and demonstrate two maxima at 260 and 289–337 nm, and the fluorescence spectrum of 2 is slightly blue-shifted compared with the fluorescence spectrum of L^Me (Fig. S7, right). Taking into account the differences in emission and excitation spectra in combination with the NMR data of 2, it may be assumed that the emission band at 375 nm in the spectrum of 2 is caused by the relaxation of the complex form and not free L^Me molecules.

In Fig. 3 and 4, the emission and excitation spectra of L^H and indium(III) complexes in the solid state at room temperature are depicted. The complexes demonstrate blue or light green fluorescence, and the CIE 1931 diagram and coordinates for the complexes are shown in Fig. 4 and Table S6.⁵³ Complex 1 exhibits excitation-dependent emission in the solid state too, and in the spectrum, there are two fluorescence bands with maxima at 375 and 510 nm, but the maxima of these ones have been red-shifted compared with the spectra in solution. Upon excitation at 340 nm, the band of fluorescence (τ = 1.7 ns) with maxima at 375 is observed. We suppose that this band is not caused by the impurity of L^H (for L^Hλ_max = 360 nm with the same life time), because the purity of 1 was confirmed by ¹H NMR. It might be assumed that the emission (λ_max = 375 nm) occurs from an excited state localized on a coordinated ligand (intraligand transitions). Moreover, the excitation spectrum of fluorescence band at 375 nm of 1 (green dotted line, Fig. 3 left) and the excitation spectrum of L^H (black dotted line, Fig. 3 right) are different, and the excitation spectrum of the complex has two maxima – at 340 and 370 nm. Upon excitation of 360–370 nm, a broad complicated band is observed with maximum at 470 nm, and this band consists of two components – 375 nm and 500–510 nm. The long-wavelength band has own excitation spectra and longer life time 6.7 ns. It can be assumed that this fluorescence band appears due to the relaxation of the pseudo-layers of complex molecules (see structure part, Fig. S2).

Fig. 4 CIE 1931 diagram and coordinates for the indium(III) complexes in the solid state (left). Emission and excitation spectra of 2 in the solid state (right).

The tendency in the fluorescence and excitation spectra of 2 is quite similar to that described above for 1. The emission spectrum of 2 is also dependent on the excitation energy. When excited by short-wavelength light (290–340 nm), a fluorescence band with a maximum at 375 nm is observed (Fig. 4, right). Conversely, when excited by long-wavelength excitation (λ_ex = 420 nm), a weak fluorescence band with a maximum at 500 nm and a longer lifetime is observed. The quantum yields of the investigated compounds were determined using an integrating sphere. Measurements conducted in both solution and the solid-state revealed QYs below 1% in all cases.

Conclusion

With the increasing use of OLED technology in everyday devices, there is a crucial need for new emissive materials, which are of high demand. Despite the fact that a lot of research works devoted for the synthesis and photophysical study of emissive compounds have been published each year, indium complexes are a quite new class of luminescent compounds. The photoluminescence and electroluminescence properties of these complexes have only recently begun to be studied in comparison with aluminium complexes.

The present work is an expansion of the study of this group of emissive materials, with the synthesis and structural characterisation of two new indium(III) complexes based on the chelate N-donor (1H-pyrazol-1-yl)pyridazine ligands (L^H or L^Me). Despite the oxophilicity of the In³⁺ ion, the stability of the complexes with these N-donor ligands has been demonstrated in both solid state and MeCN solution. Therefore, these results establish that readily accessible (1H-pyrazol-1-yl)pyridazine ligands (L^H or L^Me) can be used to synthesize stable indium(III) complexes under mild, aerobic conditions.

It is interesting that some minor changes in the ligand structure, with the substituents in pyrazole rings, lead to sufficient differences in the complex structures. The interaction of indium trichloride with L^H in a ratio of 1 : 1 leads to the formation of an octahedral neutral complex [In(L^H)(H₂O)Cl₃]. In contrast, a similar reaction with L^Me has been observed to yield a cationic bis-chelate {In(L^Me)₂}³⁺ complex. The crystal structure of the latter demonstrates two different coordination environments of the indium atom – octahedral or tetrahedral.

Finally, the results of the study open up new prospects for the synthesis of indium(III) complexes based on various pyrazolyl diazine ligands, and by applying slight modifications of substituents in the mentioned heterocycles, diverse complex structures can be designed. In addition, it was shown that the prepared indium(III) complexes exhibited fluorescence in both the solid state and MeCN solution. The solid-state emission is caused by both intraligand transitions (π → π*) in the coordinated L^H or L^Me molecules and interligand transition (π(L + Cl) → π*(L)), which include the π orbitals of the chlorine ligands. The conclusion is based on the TD-DFT calculations. It is interesting that solid-state emissions depend on the excitation energy. In the spectra of complexes, the presence of blue or green emission bands has been observed. The phenomenon of excitation-dependent emission for organic and coordination compounds has also begun to be discussed in the literature recently. The low emission intensity of the complexes may be due to the negative influence of the chlorine atom in the pyridazine ring of the ligands, but the problem could be easily overcome in the future by replacing the chlorine atom with an electron-donating substituent.

Author contributions

Sedykh E. S.: chemical synthesis and spectroscopic studies; Fomenko I. S.: writing – original draft; Komlyagina V. I.: X-ray diffraction analysis and DFT calculations; Vinogradova K. A.: writing – original draft and photoluminescence studies; Rakhmanova M. I.: photoluminescence studies; and Gushchin A. L.: writing – review and editing.

Conflicts of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information: X-ray experimental data, optimized geometry coordinates for DFT calculations, DFT data, IR, TGA and NMR spectra, emission, excitation and absorption spectra. See DOI: https://doi.org/10.1039/d5nj03873a.

CCDC 2489745 (1) and 2489746 (2) contain the supplementary crystallographic data for this paper.^54a,b

Acknowledgements

The financial support from the Russian Science Foundation (grant 25-13-00200) is acknowledged. The authors thank the Ministry of Science and Higher Education of the Russian Federation and the Centre of Collective Usage of NIIC SB RAS. The authors are grateful to A. A. Shapovalova for recording the IR spectra, A. P. Zubareva and N. N. Komardina for performing the CHN analysis, O. A. Matveeva for powder XRD measurements, N. B. Kompankov and E.K. Sadykov for NMR spectroscopy, and P. E. Plyusnin and K. D. Muyanov for TGA analysis. The authors thank the XRD facility of NIIC SB RAS and personally thank D. V. Kochelakov for the single-crystal X-ray diffraction data collection.

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54. (a) CCDC 2489745: Experimental Crystal Structure Determination, 2025 DOI:10.5517/ccdc.csd.cc2pkscf; (b) CCDC 2489746: Experimental Crystal Structure Determination, 2025 DOI:10.5517/ccdc.csd.cc2pksdg.

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