3-Keto-indazole derivatives exhibiting multi-coloured phosphorescence

Abstract

To advance the development of luminescent materials based on indazoles, a class of nitrogen-containing aromatic compounds, it is crucial to establish a reliable synthetic method for their derivatives. Five 1H-indazole derivatives with a ketoaryl group at the 3-position were synthesized by a cyclisation reaction using phenyltriazene derivatives. In solution state, only 3-ketoindazole derivatives bearing 4-diphenylaminophenyl or pyrenyl groups showed fluorescence at room temperature, whereas all 3-ketoindazole derivatives showed blue, green, or red phosphorescence, depending on the substituents, at 80 K. In addition, double luminescence has been observed at 80 K for 3-ketoindazole derivatives bearing 4-diphenylaminophenyl or pyrenyl groups. Furthermore, the 3-ketoindazole derivative did not exhibit room-temperature phosphorescence in either solution or solid state; however, when it was dispersed in a phenylbenzoate matrix at a concentration of 0.1 wt%, room-temperature phosphorescence was successfully produced in a variety of colours. These optical properties were elucidated through theoretical calculations.

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Introduction

Indazoles,¹ nitrogen-containing aromatic compounds have been extensively investigated in medicinal chemistry owing to their diverse biological activities, including anti-inflammatory, anti-cancer, analgesic, and anti-microbial activities.^2–6 Luminescent materials having an indazole skeleton hold potential for applications such as fluorescence probes,⁷ particularly when paired with their inherent biological activities. However, although various derivatives of benzimidazole, an isomer of indazole, have been synthesized and applied to organic electronic materials,^8–10 research on the application of indazole derivatives to optoelectronic materials has been limited. One reason for the slower progress in the study of indazole derivatives compared to benzimidazole derivatives is the limited number of reports on their synthesis. Indazole has two tautomeric forms: 1H-indazole, in which a hydrogen atom is attached to the nitrogen at position 1, and 2H-indazole, which adopts a quinoid structure (Fig. 1a). However, most previous studies have focused on 2H-indazole derivatives with substituents on the nitrogen atom at the 2-position,^7,11–14 and the synthesis and characterization of 1H-indazole derivatives have been less investigated.¹⁵ Furthermore, tautomerism is not exhibited by N-substituted indazoles (Fig. 1a). Therefore, establishing the synthesis of N-unsubstituted 1H-indazole derivatives is crucial for evaluating the potential of indazole as a luminescent material. Recently, we synthesized N-unsubstituted 3-ketoindazole derivatives from 2-ethynylphenyl triazene derivatives,¹⁶ and one of the N-unsubstituted 3-ketoindazole derivatives was found to adopt the 2H form in the crystal, facilitated by hydrogen bonding with H₂O. Some previous studies have also reported the synthesis of 3-ketoindazole derivatives as byproducts of Richter cyclisation and discussed their selectivity.^17–21 In these studies, 3-ketoindazole derivatives were prepared from azonium salts. However, in our approach, they were synthesized under mild conditions without the use of strong acids, allowing the introduction of various functional groups at the 3-position. Additionally, we focused on the phosphorescent emission of diarylketone structures via inter-system crossing (ISC) and demonstrated this phosphorescence in 3-ketoindazole derivatives for the first time. To attain room-temperature phosphorescence (RTP)^22,23 and facilitate its application in thermally activated delayed fluorescence (TADF) materials,^24,25 the singlet–triplet splitting energies (ΔE_ST), ISC rate constants (k_ISC), and phosphorescence decay rate constants (k_p) should be adjusted depending on the substituents. In particular, dual-luminescence systems that emit fluorescence and multi-component phosphorescence simultaneously act as single-molecule white-light-emission materials,²⁶ which are anticipated for use in next-generation solid-state materials and displays.^27,28 In addition, oxygen-quenched phosphorescence was well-suited for highly sensitive and non-invasive detection of hypoxia cancer cells.^29,30 In this study, by optimizing the conditions for cyclisation reactions, we successfully introduced phenyl (Ind-Ph), 4-bromophenyl (Ind-BP), 4-methoxyphenyl (Ind-MP), triphenylamine (Ind-TPA), and pyrenyl (Ind-Pyr) groups through carbonyl linkages at the 3-position of indazole (Fig. 1b). Furthermore, by varying the type of ketoaryl group introduced, a range of phosphorescent colours and dual emissions were achieved, contributing to the expansion of the application possibilities of indazole-based molecules.

Fig. 1 Chemical structures of (a) N-substituted and N-unsubstituted indazole with their tautomer and (b) 3-(arylcarbonyl)-1H-indazole derivatives.

Experimental

General procedures

Starting materials were purchased from Kanto Chemical, TCI, and Sigma-Aldrich and used without further purification unless otherwise stated. All reactions were performed under a dry-nitrogen atmosphere unless otherwise noted. ¹H and ¹³C nuclear magnetic resonance (NMR) spectra used in the characterization of products were recorded on a JEOL ECZ-600 (¹H: 600 MHz, ¹³C: 150 MHz) spectrometer with chemical shifts (in ppm) relative to tetramethylsilane (¹H) and the solvent (¹³C) as references. Ultraviolet (UV)-visible absorption spectra were recorded on JASCO V-750ST spectrometer using a 10 mm quartz cell. Fluorescence spectra were recorded on JASCO FP-8600 fluorescence spectrometer. PL decay time was detected using a Hamamatsu Photonics C11367 Quantaurus-Tau fluorescence lifetime spectrometer with an excitation wavelength of 340 nm. The temperature dependence of PL spectra and PL decay time were measured using a liquid-nitrogen cryostat system (UNISOKU CoolSpeK). All solution samples were measured using degassed solvents. Solid samples were measured in air. Purifications with preparative gel permeation chromatography (GPC) were carried out on a Japan analytical industry LC-5060 system using tandem JAIGEL 2H and 2.5H columns (CHCl₃ as an eluent, flow rate = 10 mL min⁻¹) equipped with an UV detector monitored at 254 nm. High-resolution electrospray ionization time-of-flight mass spectrometry (ESI-TOFMS) was conducted using Waters Synapt G2 HDMS + Acquity or Bruker micrOTOFQ II ESI mass spectrometers in the positive ion mode. TLC analyses were carried out on aluminum sheets coated with silica gel 60 (Merck 5554). Column chromatography was performed on Mightysil Si₆₀ (Kanto Chemical).

General synthesis of Ind-Ar

Trimethylsilyl chloride (TMSCl) (3 equiv.) was added to a solution of TAz-Ar (0.200 mmol) in DMF (3.50 mL) and a few drops of water. The mixture was stirred in a sealed tube at 60 °C for 24 h. The resulting mixture was extracted with CH₂Cl₂ and washed with water. The organic phase was dried over Na₂SO₄, and the solvents were removed under reduced pressure. The residue was purified by silica gel column chromatography to afford Ind-Ar.

Method for single-crystal X-ray analysis

Crystallographic data are summarized in Table S1.† A single crystal of Ind-TPA was obtained from ethyl acetate/hexane by vapor diffusion. The crystal was a green plate of approximate dimensions 0.50 mm × 0.50 mm × 0.20 mm. Data was collected at 150 K on or a Saturn724R diffractometer with a Mo Ka source. The structures were refined by a full-matrix least-squares method by using a SHELXL 2014.³¹ In each structure, the non-hydrogen atoms were refined anisotropically. CIF files (CCDC 2365317†).

Computational method

The calculations were performed using the Gaussian 16 program.³² Unless stated, all structures are confirmed to be minimum-energy structures with no imaginary frequencies.

Results and discussion

Synthesis

Phenyltriazene derivatives bearing ethynylbenzene (TAz-Ph), 4-ethynylmethoxybenzene (TAz-MP), 4-ethynyltriphenylamine (TAz-TPA), and ethynylpyrene (TAz-Pyr) were synthesized by Sonogashira coupling using 1-(2-iodophenyl)-3,3-diethyltriazene as starting materials for the ketoindazole derivatives. TAz-BP was prepared by Sonogashira coupling using 1-(2-ethynylphenyl)-3,3-diethyltriazene and 4-iodobromobenzene. The resulting five phenyltriazene derivatives were used to synthesize ketoindazole derivatives via cyclisation. According to a previous study, the cyclisation of TAz-Ph was examined in the presence of 3 equiv. of TMSCl and 4 equiv. of NaI in MeCN/CCl₄ at 40 °C.¹⁶ However, in this reaction, 1-iodo-2-(2-phenylethynyl)benzene was obtained instead of the corresponding ketoindazole derivative (Ind-Ph) (Scheme 1 and Table 1, run 1). This may be due to the lower electron density of the triple-bond moiety of TAz-Ph compared to that of the phenyltriazene derivatives with electron-rich arylethynyl substituents, resulting in lower reactivity of the triple bond. The substitution of excess iodide ions with azonium ions preferentially occurs over cyclisation. Therefore, to avoid the iodine substitution reactions, we optimized the NaI-free reaction using TAz-Ph. Based on our previous experiments, it is evident that the reaction did not proceed at 25 °C when only TMSCl was used.¹⁶ Cyclisation requires forming azonium salts by the reaction of triazene with TMS, and the results indicated that the reactivity of TMSCl is lower than that of trimethylsilyl iodide (TMSI). Therefore, the reaction was carried out at 60 °C with the addition of 1 or 3 equiv. of TMSCl (Table 1, run 2, 3). As a result, a small amount of the target compound Ind-Ph was obtained when using 3 equiv. of TMSCl (run 3). However, in runs 2 and 3, chlorocinnoline and cinnolinone derivatives were obtained as the main products, respectively. These derivatives are obtained by a similar cyclisation reaction when an electron-deficient aryl group is substituted on acetylene.^17,20,33,34 Next, when the reaction solvent was changed to DMF, the yield of the target Ind-Ph improved to 19% (Table 1, run 4). According to these results, the other phenyltriazene derivatives were also reacted with 3 equiv. of TMSCl in DMF at 60 °C to yield Ind-BP, Ind-MP, Ind-TPA, and Ind-Pyr in 35%, 61%, 85%, and 43% yields, respectively (Table 1, runs 5–8). Previous research has suggested that these cyclisation reactions progress via an intermediate in which TMS is attached to triazene.¹⁶ Therefore, we investigated the differences in the yields of cyclisation reactions depending on the substituent by calculating the molecular orbitals of the intermediates using density functional theory (DFT) calculations at B3LYP/6-31+G(d). As a result, the azonium salts with electron-rich substituents such as TAz-TPA and TAz-Pyr showed the HOMO localized on the α-carbon (Fig. 2). On the other hand, the HOMO exhibited greater localisation on the α-carbon in the order TAz-MP > TAz-BP > TAz-Ph. This trend can be likely attributed to the influence of substituents via the mesomeric effect. Therefore, it is thought that the yield of indazole derivatives is low because the formation of indazole and cinnoline is competitive in the derivatives with Ph and BP. All ketoindazole derivatives were characterized using ¹H and ¹³C NMR spectroscopy and ESI-TOF-MS.

Scheme 1 Synthesis of 3-(arylcarbonyl)-1H-indazole derivatives using triazene derivatives.

Fig. 2 Frontier orbitals (HOMO) of TMS adducts for (a) TAz-Ph, (b) TAz-BP, (c) TAz-MP, (d) TAz-TPA, and (e) TAz-Pyr (B3LYP/6-31+G(d)) (isovalue = 0.07).

Table 1 Summary of reaction conditions for Scheme 1

Run	Ar	TMSCl (equiv.)	NaI (equiv.)	Solvent	Temp. (°C)	Yield (%)
a 1-Iodo-2-(2-phenylethynyl)benzene was obtained.
1	Ph	3	4	MeCN/CCl4/H2O	40	—^a
2	Ph	1	—	MeCN/CCl4/H2O	60	—
3	Ph	3	—	MeCN/CCl4/H2O	60	Trace
4	Ph	3	—	DMF/H2O	60	19
5	BP	3	—	DMF/H2O	60	35
6	MP	3	—	DMF/H2O	60	61
7	TPA	3	—	DMF/H2O	60	85
8	Pyr	3	—	DMF/H2O	60	43

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Single-crystal X-ray diffraction

Ind-TPA was successfully obtained as a single crystal from ethyl acetate/hexane by vapor diffusion, and its exact structure was revealed by single-crystal X-ray diffraction. Ind-TPA was crystalized in the triclinic P1̄ space group. The indazole moiety adopted the 1H-form, and the dihedral angle between the phenyl group and the indazole adjacent to the carbonyl was 23.5°. Interestingly, an intermolecular hydrogen-bonding network between the NH of the indazole and the O of the carbonyl group was observed with an N(–H)⋯O distance of 2.77 Å along the b axis (Fig. 3).

Fig. 3 Molecular packing structure of Ind-TPA.

Optical properties

UV–vis absorption, photoluminescence (PL), and phosphorescence spectra of all compounds were measured in both solid and solution states. In their absorption spectra in CH₂Cl₂ (10⁻⁵ M) at room temperature, Ind-Ph, Ind-BP, Ind-MP, Ind-TPA, and Ind-Pyr exhibited lowest energy absorption bands at 307, 313, 312, 380, and 366 nm, respectively (Fig. 4a). Ind-Ph, Ind-BP, and Ind-MP showed similar spectra, whereas red shifts were observed in the absorption spectra of Ind-TPA and Ind-Pyr, in which extended π-conjugated systems were introduced as substituents. Ind-TPA and Ind-Pyr exhibited featureless fluorescence bands at 523 and 447 nm, respectively (Fig. 4b). In contrast, other derivatives exhibited weak or no fluorescence peaks. Furthermore, when the fluorescence spectra of Ind-TPA were measured in different solvents, a red shift in the maximum fluorescence wavelength was observed as solvent polarity increased. Conversely, no solvatochromism was observed in the absorption spectrum, suggesting that Ind-TPA was more polarized in the excited state than in the ground state.

Fig. 4 (a) UV/vis absorption and (b) fluorescence spectra of Ind-Ph (blue), Ind-BP (purple), Ind-MP (green), Ind-TPA (orange), and Ind-Pyr (red) in CH₂Cl₂ (10⁻⁵ M) at 298 K. The fluorescence spectra were obtained by excitation at the absorption maximum.

Next, the optical properties of the derivatives were investigated at low temperature using 2-methyltetrahydrofuran (2-MTHF) as solvent. Ind-Ph, Ind-BP, and Ind-MP showed negligible emission at room temperature but exhibited strong emission in the long-wavelength region at 80 K (Fig. 5a). Ind-Ph, Ind-BP, and Ind-MP showed emissions at 479, 472, and 468 nm, respectively, which originated from phosphorescence, as indicated by their long emission lifetimes (τ_ave = 3.075, 1.310, and 2.800 s for Ind-Ph, Ind-BP, and Ind-MP, respectively) (Fig. 5a and Table S2†). As shown in Fig. 5b, light blue phosphorescence can be observed with the naked eye in the second timescale. At 80 K, multiple emission peaks were observed for Ind-TPA at 434 and 531 nm and for Ind-Pyr at 404, 619, and 809 nm (Fig. 5a). Both Ind-TPA and Ind-Pyr emitted light blue or light green emission under UV-lamp irradiation, whereas Ind-TPA showed green emission and Ind-Pyr showed red emission after the UV lamps were turned off (Fig. 5b). To further understand these dual emissions, the emission lifetimes of Ind-TPA and Ind-Pyr were measured in the short-wavelength and long-wavelength regions. The emission lifetime of Ind-TPA was monitored at 434 nm, and decay of the three components was confirmed, with an average lifetime (τ_ave) of 5.789 s (Table S2†). Moreover, the emission of Ind-TPA at 531 nm was monitored to measure its emission lifetime, and the decay of the three components was confirmed, with a τ_ave of 3.337 s (Table S2†). These results suggest that the phosphorescence emission is attributable to the T₁ and T_n states, because all emission showed long-lived luminescence. In contrast, the emission lifetime of Ind-Pyr at 404 nm was estimated to be τ_ave = 25.28 ns, indicating that the short-lived emission was derived from the fluorescence emission. Meanwhile, the emission of Ind-Pyr at long-wavelength region was monitored, and its τ_ave was estimated to be 0.4876 s, indicating that the emission was derived from phosphorescence emission.

Fig. 5 (a) Emission spectra of Ind-Ph (blue), Ind-BP (purple), Ind-MP (green), Ind-TPA (orange), and Ind-Pyr (red) in 2-MTHF at 80 K (*: artifact); (b) photographs in 2-MTHF under UV (365 nm) irradiation and after switching off the UV lamp at 80 K for (i) Ind-Ph, (ii) Ind-BP, (iii) Ind-MP, (iv) Ind-TPA, and (v) Ind-Pyr.

Theoretical calculation

To investigate the stability of the tautomers, structural optimization was performed using DFT calculations for all compounds at the B3LYP/6-31+G(d) level of theory.³² Consequently, we found that the 1H forms of all derivatives were stable relative to their 2H forms above 5 kcal mol⁻¹. Furthermore, in Ind-Ph, we evaluated the stability of the 1H and 2H forms in various solvents, including CHCl₃, acetone, and DMSO. The 1H form was more stable by at least 4.3 kcal mol⁻¹, irrespective of the solvent (Table S4†). Additionally, ¹H NMR measurements in each solvent revealed the presence of a single tautomer, indicating that only the 1H form existed, regardless of solvent polarity (Fig. S12†). The dihedral angles θ between the indazole and aryl groups adjacent to the carbonyl group were calculated to be θ = 30.0°, 26.5°, 25.2°, 25.5°, and 47.5° for Ind-Ph, Ind-BP, Ind-MP, Ind-TPA, and Ind-Pyr, respectively (Fig. S27†). These results indicate that the unsubstituted phenyl group provided the largest dihedral angle, excluding the bulky pyrenyl group. Similarly, structural optimizations of the S₁ and T₁ states were performed, and the 1H form was found to be the most stable tautomer for all derivatives. The dihedral angles θ of the S₁ states were calculated to be 0.1°, 0.0°, 0.7°, 1.5°, and 75.8° for Ind-Ph, Ind-BP, Ind-MP, Ind-TPA, and Ind-Pyr, respectively (Fig. S28†), with improved planarity for all derivatives except Ind-Pyr, which showed a large dihedral angle. The dihedral angles θ for the T₁ states were calculated to be 0.0°, 0.0°, 28.1°, 0.2°, and 39.3° for Ind-Ph, Ind-BP, Ind-MP, Ind-TPA, and Ind-Pyr, respectively, and Ind-MP and Ind-Pyr showed twisted structures (Fig. S29†).

To investigate the absorption bands in detail, time-dependent (TD)-DFT calculations were performed using the TDA-B3LYP/6-31+G(d) basis set based on the optimized B3LYP/6-31+G(d) structure. The lowest energy absorption bands of Ind-TPA and Ind-Pyr were attributed to intramolecular charge transfer (ICT) from the donor TPA or the pyrene moiety to the acceptor carbonyl moiety. In contrast, for the other derivatives, the HOMO is localized at the indazole moiety, which possessed a relatively low electron density, indicating that ICT was inactive.

From theoretical calculations, energy diagrams of the S₁ and T₁ states for each derivative were calculated (Fig. 6). For Ind-Ph, Ind-BP, and Ind-MP, the S₁ and T₁ states are dominated by (n,π*) and (π,π*) transitions, respectively. According to the El-Sayed rule,³⁵ efficient spin–orbital coupling occurs during the transition from a singlet state with an electronic configuration such as (n,π*) or (π,π*) to a triplet state such as (π,π*) or (n, π*), respectively. This efficiency arises from the effective orbital overlap facilitated by the operation of the orbital angular momentum operator. Consequently, similar to other N-heterocyclic compounds containing carbonyl groups,^24,36,37 the ISC from the S₁ to T₁ states is allowed in Ind-Ph, Ind-BP, and Ind-MP. In addition, ΔE_ST for Ind-Ph, Ind-BP, and Ind-MP were calculated to be 0.55, 0.56, and 0.57 eV, respectively. In contrast, for Ind-TPA and Ind-Pyr, the introduction of the extended π-conjugated system decreased the energy levels of the (π,π*) transitions, resulting in both the S₁ and T₁ states being dominated by (π,π*) transitions. This result suggests that the ISC from S₁ to T₁ is forbidden. The S₂ and T₂ states were attributed to (n,π*) transitions. According to the El-Sayed rule, the ISC from S₁ to T₂ is allowed. For Ind-TPA, the ΔE_ST was estimated as 0.02 eV. Simultaneously, the phosphorescence from the T_nvia ISC from the S₂ or S_n states was also detected, which was considered to confirm the dual emission of phosphorescence. In contrast, for Ind-Pyr, multiple emissions were observed, including fluorescence from the S₁, phosphorescence from the T₁via ISC from the S₂ to the T₁ state, and phosphorescence from the T_n state. The ΔE_ST was calculated to be 0.07 eV for Ind-Pyr.

Fig. 6 Energy diagrams and electronic transition characters calculated at the TDA-B3LYP/6-31+G(d,p) level for (a) Ind-Ph, (b) Ind-BP, (c) Ind-MP, (d) Ind-TPA, and (e) Ind-Pyr. H: highest occupied molecular orbital, L: lowest unoccupied molecular orbital.

Room-temperature phosphorescence

The ketoindazole derivatives were dispersed in phenylbenzoate (PhB), and their phosphorescence behavior was investigated at room temperature. Recently, RTP was observed when benzophenone derivatives were dispersed in PhB, which suppressed the nonradiative inactivation from the T₁ state.^38,39 Each derivative was dispersed in PhB at a concentration of 0.1 wt%, and their emission spectra were measured. All derivatives showed phosphorescent emissions at room temperature (Fig. 7). As in the solution state, phosphorescence was observed only for Ind-Ph–PhB, Ind-BP–PhB, and Ind-MP–PhB, whereas dual emissions derived from fluorescence and/or phosphorescence were observed for Ind-TPA–PhB and Ind-Pyr–PhB.

Fig. 7 Photographs of (a) Ind-Ph–PhB, (b) Ind-BP–PhB, (c) Ind-MP–PhB, (d) Ind-TPA–PhB and (e) Ind-Pyr–PhB under UV (365 nm) irradiation and after switching off the UV lamp at RT.

Conclusions

Five ketoindazole derivatives were synthesized via cyclisation using triazene derivatives. The phosphorescence emissions of all obtained ketoindazole derivatives were observed at low temperatures, and blue, green, or red phosphorescence emissions were successfully produced depending on the substituent group. Furthermore, RTP was successfully produced by dispersing the ketoindazole derivatives in phenylbenzoate. The combination of the metal coordination ability and pharmacological activity of indazoles, along with their phosphorescence properties, is expected to enable applications in organic electronics and bioimaging materials in the future. Further modifications of ketoindazole derivatives are underway for the development of novel RTP materials.

Author contributions

TM: formal analysis, investigation, visualization, writing – original draft; RK: investigation, writing – review and editing; TC: resources, writing – review and editing; SO: resources, writing – review and editing; RY: conceptualization, formal analysis, funding acquisition, investigation, project administration, resources, supervision, visualization, writing – original draft, and writing – review and editing.

Data availability

Data for this article, including synthetic procedures, analytical data, optical properties, and theoretical calculations, are available at https://doi.org/10.1039/x0xx00000x.

The data supporting this article have been included as part of the ESI.†

Crystallographic data for Ind-TPA has been deposited at the CCDC 2365317.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by JSPS KAKENHI Grant Numbers JP21K14606 and JP24K08523 and Iketani Science and Technology Foundation. The computation was performed using the Research Center for Computational Science, Okazaki, Japan (Project: 23-IMS-C098 and 24-IMS-C093).

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Footnote

† Electronic supplementary information (ESI) available: Synthetic procedures, analytical data, optical properties, and theoretical calculations. CCDC 2365317. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d5ob00108k

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