domain: chim phys

text:
Armands Sebris

Matas Guzauskas

Malek Mahmoudi

Dmytro Volyniuk

Juozas V Grazulevicius

Anatoly Mishnev

Irina Novosjolova

Māris Turks

Gediminas Jonusauskas

Kaspars Traskovskis
email: kaspars.traskovskis@rtu.lv

Ma ¯ris Turks

Homoleptic purine-based NHC iridium

come

Introduction

The last two decades have witnessed significant advances in organic light emitting diode (OLED) technology with the most prominent beneficiary of this being the display industry, which has subsequently delivered innovative products with unmatched image quality and previously inaccessible functionality, such as transparency and flexibility. [START_REF] Hong | A Brief History of OLEDs---Emitter Development and Industry Milestones[END_REF][START_REF] Chen | Liquid crystal display and organic light-emitting diode display: present status and future perspectives[END_REF][START_REF] Park | Efficient Large-Area Transparent OLEDs Based on a Laminated Top Electrode with an Embedded Auxiliary Mesh[END_REF][START_REF] Park | World's first large size 77-inch transparent flexible OLED display[END_REF] Phosphorescent cyclometalated iridium(III) complexes are the leading commercialized light emitting OLED material class due to the ability of providing efficient light emission from the triplet excited states, characterized with large photoluminescence quantum yield values (F PL ) and few microsecond long emission lifetimes (t). [START_REF] Lamansky | Highly Phosphorescent Bis-Cyclometalated Iridium Complexes: Synthesis, Photophysical Characterization, and Use in Organic Light Emitting Diodes[END_REF][START_REF] Ulbricht | Recent Developments in the Application of Phosphorescent Iridium(III) Complex Systems[END_REF][START_REF] Yersin | The triplet state of organo-transition metal compounds. Triplet harvesting and singlet harvesting for efficient OLEDs[END_REF][START_REF] Li | Rational design of phosphorescent iridium(III) complexes for emission color tunability and their applications in OLEDs[END_REF] Despite the fact that iridium(III) based emitters are extensively used for providing light in green and red spectral regions, the development of blue phosphorescent materials is still an unresolved challenge. [START_REF] Monkman | Why Do We Still Need a Stable Long Lifetime Deep Blue OLED Emitter?[END_REF] The main culprit of the existing structures is insufficient chemical stability, which results in short operational lifetimes of the blue OLEDs.

A widespread strategy towards attaining blue emitting iridium(III) complexes involves an attachment of electron accepting groups at the cyclometalating ligands. The introduction of fluorine atoms is one of the commonly used approaches, as seen in the quintessential blue emitter FIrpic. [START_REF] Adachi | Endothermic energy transfer: A mechanism for generating very efficient high-energy phosphorescent emission in organic materials[END_REF] However, the experimental observations show that a detachment of fluorine atoms in the excited states of molecules is a common in-device degradation pathway of such materials. [START_REF] Seifert | Chemical degradation mechanisms of highly efficient blue phosphorescent emitters used for organic light emitting diodes[END_REF] Besides intraligand bond cleavage, metal-ligand dissociation is the other widely encountered degradation mechanism of iridium(III) phosphors. This is related to the high energies of the blue-emitting metalto-ligand charge transfer triplet levels ( 3 MLCT), which place them in close energetic proximity to the metal centered ( 3 MC) excited states. [START_REF] Jacquemin | The short device lifetimes of blue PhOLEDs: insights into the photostability of blue Ir(III) complexes[END_REF] Upon the population of MC levels, the geometries of complexes transform through a partial dissociation of ligands. Such a process may not only lead to a complete ligand dissociation, but is also associated with a severe drop in F PL values because [START_REF] Park | Efficient Large-Area Transparent OLEDs Based on a Laminated Top Electrode with an Embedded Auxiliary Mesh[END_REF] MC levels are considered to be nonemissive. [START_REF] Jacquemin | The short device lifetimes of blue PhOLEDs: insights into the photostability of blue Ir(III) complexes[END_REF] One of the proposed structural strategies towards making iridium(III) complexes more resilient is a substitution of the relatively weak metal-nitrogen (Ir-N:) bonds with stronger metal-carbene (Ir-C:) analogues. Due to sturdier ligand bounding [START_REF] Park | Efficient Large-Area Transparent OLEDs Based on a Laminated Top Electrode with an Embedded Auxiliary Mesh[END_REF] MC states in the corresponding complexes being less accessible and, even if populated, a reverse 3 MC -3 MLCT pathway in the excited state configuration may be possible for the mentioned emitters. [START_REF] Jacquemin | The short device lifetimes of blue PhOLEDs: insights into the photostability of blue Ir(III) complexes[END_REF][START_REF] Zhou | Nonradiative Decay and Stability of N -Heterocyclic Carbene Iridium(III) Complexes[END_REF][START_REF] Arroliga-Rocha | Facial and Meridional Isomers of Tris(bidentate) Ir(III) Complexes: Unravelling Their Different Excited State Reactivity[END_REF] Following this concept numerous successful blue-emitting iridium(III) molecular designs have been developed, where the chemical composition of the emitters involves the use of N-heterocyclic carbene (NHC) building blocks. The first structural examples of such emitters featured ligands bearing imidazol-2-yl and benzimidazol-2-yl, [START_REF] Sajoto | Blue and Near-UV Phosphorescence from Iridium Complexes with Cyclometalated Pyrazolyl or N -Heterocyclic Carbene Ligands[END_REF][START_REF] Pal | High-Efficiency Deep-Blue-Emitting Organic Light-Emitting Diodes Based on Iridium(III) Carbene Complexes[END_REF] or pyridylimidazol-2-yl [START_REF] Lee | Deep blue phosphorescent organic light-emitting diodes with very high brightness and efficiency[END_REF] NHC fragments. However, the disadvantage of these early NHC-based emissive materials is their high triplet energies, which complicate OLED preparation due to the lack of suitable charge transporting host materials. The introduction of an additional nitrogen atom to the pyridylimidazole ring system stabilizes the LUMO level of the corresponding compounds and redshifts their emission, resulting in more robust devices, as seen in the case of pyrazinoimidazol-2-yl ligand containing emitters. [START_REF] Maheshwaran | High Efficiency Deep-Blue Phosphorescent Organic Light-Emitting Diodes with CIE x, y (r 0.15) and Low Efficiency Roll-Off by Employing a High Triplet Energy Bipolar Host Material[END_REF][START_REF] Jung | A bipolar host based high triplet energy electroplex for an over 10 000 h lifetime in pure blue phosphorescent organic light-emitting diodes[END_REF][START_REF] Idris | Blue Emissive fac/mer -Iridium (III) NHC Carbene Complexes and their Application in OLEDs[END_REF] Very recently another closely related NHC-type ligand was introduced in the form of 7,9dihydro-8H-purin-8-ylidene. Structural examples f-2tBu, m-2tBu, [START_REF] Jin | Iridium(III) Phosphors-Bearing Functional 9-Phenyl-7,9-dihydro-8H-purin-8ylidene Chelates and Blue Hyperphosphorescent OLED Devices[END_REF] f-CF 3 and f-PhCF 3 [START_REF] Qin | Facially Coordinated, Trisbidentate Purin-8-ylidene Ir(III) Complexes for Blue Electrophosphorescence and Hyperluminescence[END_REF] (Fig. 1) exhibit blue emission with exceptional F PL values and good OLED performance. A common structural characteristic of the mentioned compounds is the presence of bulky alkyl substituents at the aromatic rings of the cyclometalating ligands. Bulky groups are known to provide some benefit in the form of preventing close contact between the emitter molecules in the emissive layers of OLEDs, thus minimizing the extent of such negative molecular aggregation-induced effects such as luminescence concentration quenching and triplet-triplet annihilation, [START_REF] Kim | Green phosphorescent homoleptic iridium(III) complexes for highly efficient organic lightemitting diodes[END_REF][START_REF] Mackenzie | Bulky Iridium NHC Complexes for Bright, Efficient Deep-Blue OLEDs[END_REF] yet, there are several drawbacks. Alkyl substituents possess weak electron donating properties, which are known to redshift the emission wavelength and reduce the F PL of the corresponding complexes. [START_REF] Kim | Influence of bulky substituents on the photophysical properties of homoleptic iridium(III) complexes[END_REF] Bulky groups may also block charge carrier transport to emitter molecules, reducing the overall electroluminescence performance. [START_REF] Ruduss | Effects of steric encumbrance of iridium(III) complex core on performance of solution-processed organic light emitting diodes[END_REF][START_REF] Zhang | Bulky, dendronized iridium complexes and their photoluminescence[END_REF] Another disadvantage is the increased molecular mass of the compounds, which negatively affects their thermal stability and may facilitate partial material degradation during the vacuum deposition stage of OLED preparation.

In this work, after developing a successful synthetic pathway to unsubstituted 7-methyl-9-phenyl-9H-purin-7-ium ligand precursor, we present two stereoisomers of the corresponding homoleptic complex, mer-PhP and fac-PhP (Fig. 1). The compounds show strong emission in the blue spectral region with F PL values up to 0.99. Comprehensive photophysical and theoretical characterization of light emission processes reveal striking differences in emission characteristics between the two investigated stereoisomers in terms of emission efficiency, emissive rate constants and response to the surrounding medium. The compounds were successfully integrated in blue Fig. 1 Chemical structures of known [START_REF] Jin | Iridium(III) Phosphors-Bearing Functional 9-Phenyl-7,9-dihydro-8H-purin-8ylidene Chelates and Blue Hyperphosphorescent OLED Devices[END_REF][START_REF] Qin | Facially Coordinated, Trisbidentate Purin-8-ylidene Ir(III) Complexes for Blue Electrophosphorescence and Hyperluminescence[END_REF] and newly synthesized 7,9-dihydro-8H-purin-8-ylidene based iridium(III) complexes.

OLEDs with mer-PhP exhibiting the best performance with an external quantum efficiency (EQE) of 16.1% and emission wavelength maximum of 467 nm.

Results and discussion

Synthesis

For the initial attempts towards carbene precursor 5 (Scheme 1) methylation of the N(7) position of purine ring was selected. However, methylation of 9-phenylpurine and 9-phenyladenine using methyl iodide occurred selectively at the N(1) position, similar to reactions for structure analogs reported in the literature. [START_REF] Laxer | Molecular recognition of adenosine deaminase: 15N NMR studies, Nucleosides[END_REF][START_REF] Vik | Antimicrobial and cytotoxic activity of agelasine and agelasimine analogs, Bioorganic[END_REF][START_REF] Les ˇkovskis | Applications of Purine Ring Opening in the Synthesis of Imidazole, Pyrimidine, and New Purine Derivatives[END_REF] Attempts to produce the desired product using 6-chloro-9-phenylpurine and 6-cyano-9-phenylpurine [START_REF] Traskovskis | All-organic fast intersystem crossing assisted exciplexes exhibiting submicrosecond thermally activated delayed fluorescence[END_REF] as starting materials were unsuccessful. Steric hindrance in the pyrimidine ring next to the N(1) position was suggested as a means to achieve N(7) alkylation in some literature sources. [START_REF] Jin | Iridium(III) Phosphors-Bearing Functional 9-Phenyl-7,9-dihydro-8H-purin-8ylidene Chelates and Blue Hyperphosphorescent OLED Devices[END_REF][START_REF] Qin | Facially Coordinated, Trisbidentate Purin-8-ylidene Ir(III) Complexes for Blue Electrophosphorescence and Hyperluminescence[END_REF] However, we did not pursue this approach as it would have resulted in large alkyl groups on the end product with little benefit to the photophysical properties. Also 6-hydroxy and 6-alkoxyimino purine derivatives have been reported to undergo selective N(7) alkylation, [START_REF] Vik | Antimicrobial and cytotoxic activity of agelasine and agelasimine analogs, Bioorganic[END_REF][START_REF] Senthilvelan | Highly regioselective methylation of inosine nucleotide: an efficient synthesis of 7-methylinosine nucleotide, Nucleosides[END_REF] but this approach would require multiple functionalization steps on rather sensitive pyrimidoimidazolium derivatives, which are highly prone to hydrolysis towards structures of type 6. To limit the aforementioned issues, we designed a de novo synthetic pathway towards ligand precursor 5 with imidazolium cation formation as the last step -right before the iridium complex formation.

Methylated compound 2 was synthesized from compound 1 using established literature protocols. [START_REF] Read | Synthesis and biological evaluation of pyrimidine analogs of antimycobacterial purines[END_REF] An aminophenyl group was introduced in the symmetrical compound 2 via S N Ar reaction and compound 3 was obtained as the major product. According to our observations, 70 1C was the optimal temperature for this reaction, since only minimal amounts of disubstituted product were observed and the reaction proceeded at a reasonable rate. Electronic effects, such as additional electrondonating groups in the pyrimidine ring, hindered the formation of the disubstituted product in further S N Ar reactions. [START_REF] Sebris | Synthesis of 7-Arylpurines from Substituted Pyrimidines[END_REF] Pd catalyzed hydro dehalogenation with formic acid additive yielded compound 4 in high yield. Imidazolium ring closure towards compound 5 was achieved in anhydrous acidic conditions using orthoformate, although an amount of compound 6 and 8-oxopurine derivative were formed as byproducts. Based on previous observations, [START_REF] Cola | Novel transition metal complexes comprising symmetric tetradentate ligands[END_REF] the optimal conditions for the synthesis of iridium complex mer-PhP involved the use of AgOAc without the addition of extra bases. Alternatively, Ag 2 O and a base additive (triethylamine) were also tested, [START_REF] Idris | Blue Emissive fac/mer -Iridium (III) NHC Carbene Complexes and their Application in OLEDs[END_REF] but compound mer-PhP was not observed. In our hands, the complex formation with ligand precursor 5 provided exclusively merisomer, while other reports regarding a few purine iridium complexes often produced a mixture of both mer-and facisomers requiring flash chromatography for separation. [START_REF] Jin | Iridium(III) Phosphors-Bearing Functional 9-Phenyl-7,9-dihydro-8H-purin-8ylidene Chelates and Blue Hyperphosphorescent OLED Devices[END_REF][START_REF] Qin | Facially Coordinated, Trisbidentate Purin-8-ylidene Ir(III) Complexes for Blue Electrophosphorescence and Hyperluminescence[END_REF] Next, fac-isomer fac-PhP was achieved through trifluoroacetic acid-catalyzed isomerization similarly to a previously described methodology. [START_REF] Osiak | Twist it! the aciddependent isomerization of homoleptic carbenic iridium(III) complexes[END_REF] However, we had to add additional DCM (EtOAc:DCM = 2 : 1) to address issues related to solubility. The isomerization reaction eventually slowed down, and we did not continue it to reach full conversion as we also noticed minor simultaneous degradation.

Both iridium complexes were characterized by mass spectrometry, 1 H and 13 C NMR spectra. As expected for the mer-isomer all 3 ligands possessed NMR signals with different chemical shifts, while for the fac-isomer all 3 ligands had identical chemical shifts, owing to the symmetrical nature of the complex. The description of synthetic procedures and compound characterization data are provided in the ESI. †

Structure

Single crystals of mer-PhP and fac-PhP were obtained by slow evaporation of a DCM/MeOH mixture. X-ray crystallographic measurements of these samples confirmed the assumed structural composition of the two cyclometalated complexes, revealing isomers with either mer-or fac-configuration (Fig. 2). Although the octahedral configuration of fac-PhP predetermines a structure with C 3 symmetry, the geometry of the compound in crystal form deviates from the ideal case, as illustrated by slight alternations in carbon-metal bond length across three phenylpurine fragments. Distinctive differences can be seen in Ir-C bonding, whether bond-forming carbon is either carbenic (C1, C3, C5) or comes from a phenyl ring Scheme 1 The synthetic route towards purine-iridium complexes mer-PhP and fac-PhP.

(C2, C4, C6). In the first case the bonds are notably shorter (2.013-2.028 Å vs. 2.081-2.093 Å). This observation can be reasoned with the presumed backbonding between the carbenic carbon and iridium, where d p electrons of the metal interact with an empty p orbital of the carbon, leading to shorter and more stable covalent interactions. [START_REF] Yang | Stable luminescent iridium(III) complexes with bis(N-heterocyclic carbene) ligands: photostability, excited state properties, visible-light-driven radical cyclization and CO 2 reduction, and cellular imaging[END_REF] The structure of mer-PhP assumes C 1 symmetry. Bond lengths here exhibit a greater variance due to the octahedral structure featuring transpositioned Ir-C bond pairs of either carbenic-carbenic (C3 and C5, 2.017, 2.018 Å), carbenic-phenyl (C1 and C4, 2.028, 2.081 Å) or phenyl-phenyl (C2 and C6, 2.112, 2.106 Å) origin. It can be seen that in the last case the metal-ligand interatomic distance is the longest. Similar observations have been made for several mer configured cyclometalated Ir(III) complexes and are attributed to the destabilizing trans effect of phenyl-type ligands. [START_REF] Tamayo | Synthesis and Characterization of Facial and Meridional Triscyclometalated Iridium(III) Complexes[END_REF] Compared to the previously reported derivatives of purinylidene Ir(III) complexes, the structural characteristics of mer-PhP and fac-PhP are comparable, although we note a slight shortening of metal-ligand bonds, which might be indicative of higher chemical stability of the compounds. [START_REF] Jin | Iridium(III) Phosphors-Bearing Functional 9-Phenyl-7,9-dihydro-8H-purin-8ylidene Chelates and Blue Hyperphosphorescent OLED Devices[END_REF][START_REF] Qin | Facially Coordinated, Trisbidentate Purin-8-ylidene Ir(III) Complexes for Blue Electrophosphorescence and Hyperluminescence[END_REF]

Thermal properties

The thermal properties of the synthesized compounds including their thermal stability and morphological properties were studied through thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) under a nitrogen stream. The results of thermal analysis are shown and summarized in Fig. S1 and Table S3 (ESI †). As shown in Fig. S1a (ESI †), mer-PhP possesses exceptional thermal stability with a five percent weight-loss temperature (T d-5% ) of 480 1C which is higher than T d-5% of 402 1C for fac-PhP. To the best of our knowledge, mer-isomer exhibits the highest thermal stability among reported NHC-based iridium(III) emitters (Table S4, ESI †). The residue mass of the mer-PhP and fac-PhP after heating to 800 1C is 23.5% and 22.1, respectively. DSC measurements show that the investigated mer-PhP sample is an amorphous solid (Fig. S1b, ESI †) since temperature scans between À20 1C and 450 1C with a heating rate of 10 1C min À1 show no peaks associated with melting or crystallization. Similar solid state phase behavior can be observed for fac-PhP.

Photophysical properties

The results of photophysical measurements for mer-PhP and fac-PhP are summarized in Table 1. UV-Vis measurements of the complexes were carried out for DCM solution. Both compounds show strong absorbance bands in the 250-330 nm range, attributed to the ligand centered p-p* electronic transitions (Fig. 3a andb). The region of 330-425 nm is covered by weaker absorbing spectral features associated with the d-p* type singlet and triplet metal-to-ligand charge transfer states ( 1 MLCT and 3 MLCT). [START_REF] Hay | Theoretical Studies of the Ground and Excited Electronic States in Cyclometalated Phenylpyridine Ir(III) Complexes Using Density Functional Theory[END_REF] Notably, MLCT bands for fac-PhP are more intensive than for mer isomer, a case that is typically encountered for structural isomers of cyclometalated Ir(III) complexes. [START_REF] Idris | Blue Emissive fac/mer -Iridium (III) NHC Carbene Complexes and their Application in OLEDs[END_REF][START_REF] Jin | Iridium(III) Phosphors-Bearing Functional 9-Phenyl-7,9-dihydro-8H-purin-8ylidene Chelates and Blue Hyperphosphorescent OLED Devices[END_REF][START_REF] Qin | Facially Coordinated, Tris -bidentate Purin-8-ylidene Ir(III) Complexes for Blue Electrophosphorescence and Hyperluminescence[END_REF] Another common observation is a redshift of the absorption onset in the case of mer-PhP. Photoluminescence (PL) characteristics of the compounds were investigated using DCM and TOL solutions as well as films of 5 wt% solid solutions in PMMA and bulk PS. In all cases the compounds show a broad and featureless PL spectral profile. In DCM solution the PL band maximum is at 540 nm for mer-PhP and 466 nm for fac-PhP, while in less polar TOL both compounds possess blueshifted emission with PL maxima at 492 and 436 nm, respectively. The change of medium also influences the width of the PL bands, as in DCM the full width at half maximum (FWHM) is substantially increased (by 56 nm for mer-PhP and by 28 nm for fac-PhP). It is also apparent that in both solvents mer-PhP shows significantly larger FWHM in comparison to its stereoisomer. For iridium(III) complexes PL band broadening is indicative of a greater deviation between ground (S 0 ) and excited triplet (T 1 ) state geometries and suggests increased conformational uncertainty of the corresponding structures. [START_REF] Li | Thermally Activated Delayed Fluorescence Carbonyl Derivatives for Organic Light-Emitting Diodes with Extremely Narrow Full Width at Half-Maximum[END_REF][START_REF] Cao | Intramolecular Cyclization: A Convenient Strategy to Realize Efficient BT.2020 Blue Multi-Resonance Emitter for Organic Light-Emitting Diodes[END_REF] To support this assumption time-wavelength resolved maps of mer-PhP emission for TOL solution covering the first 50 ns after excitation was obtained at room temperature and at 77 K (Fig. 4). Striking difference can be seen between the two measurements, as under room temperature conditions dynamic red displacement of the emission band with a peak maxima shift from 470 to 520 nm can be seen. In the frozen sample the emission wavelength is almost static and resides at about 470 nm. It can be concluded that for mer-PhP a transfer between two local minima on the T 1 state potential energy surface becomes possible upon thermal activation or during the transfer from rigid to liquid surrounding medium. The role of the surrounding matrix rigidity is also underlined by PL measurements for PMMA and PS polymer guest-host systems, where mer-PhP shows emission wavelength values that are comparable to the frozen TOL sample (Table 1).

In DCM solutions the compounds show relatively low photoluminescence quantum yields (F PL ). The values of 0.12 and 0.36 were observed for DCM solutions of mer-PhP and fac-PhP respectively. In TOL solutions the emission efficiency substantially increases for mer-PhP reaching a value of 0.40, while for its isomer it drops to 0.22. In the first case the observation can be reasoned with the energy gap law, where a blueshift in emission leads to higher F PL . [START_REF] Englman | The energy gap law for radiationless transitions in large molecules[END_REF] The lower quantum yield for fac-PhP can be explained by the poor solubility of the compound in TOL, which may lead to aggregation induced emission quenching. The poorer solubility of fac-isomer can be related to a higher polarity of the complex, which is indicated by the calculated dipole moment parameter for the molecule: 6.9 D vs. 4.4 D for the mer-counterpart. The F PL substantially increases for the films of the solid solutions in PMMA, where fac-isomer shows the value of 0.77, while mer-has a value of 0.99. Again, this points to substantial differences in the degree of complex conformational freedom between DCM and the rigid solid-state host, as in liquid solution molecular geometries associated with rapid non-radiative pathways may be accessible, while in films these conformations are not attainable due to restricted structural motion. [START_REF] Sajoto | Temperature Dependence of Blue Phosphorescent Cyclometalated Ir(III) Complexes[END_REF] Notably, the non-radiative relaxation can also be induced in a solid host by heating (see Fig. S2,S3 and Table S5 and the associated discussion in the ESI †).

PL kinetics of the compounds were investigated by employing time-resolved photoluminescence measurements (Fig. 3c,d and Fig. S4,ESI †). In deoxygenated DCM the compounds show monoexponential PL decays with lifetimes (t) of 353 and 532 ns for mer-PhP and fac-PhP respectively. These values fall in a typical emission lifetime range observed for cyclometalated Ir(III) complexes and are indicative of a rapid phosphorescence form T 1 state enabled by a heavy atom effect. [START_REF] Lamansky | Highly Phosphorescent Bis-Cyclometalated Iridium Complexes: Synthesis, Photophysical Characterization, and Use in Organic Light Emitting Diodes[END_REF][START_REF] Yersin | The triplet state of organo-transition metal compounds. Triplet harvesting and singlet harvesting for efficient OLEDs[END_REF] Temperature dependent PL decay measurements were performed for the films of doped PMMA, covering the temperature range 300-77 K (Fig. 3c,d and Table S6, ESI †). While the PL decay profile for mer-PhP retains its monoexponential character, the film sample of fac-PhP requires a double exponential fit. This particular behavior of fac-PhP can be presumably explained by a partial aggregation of the more polar emitter molecules or specific polymer-complex interactions, which gives rise to an additional population of emissive species. Notably, in a less polar PS host fac-PhP exhibits monoexponential decay. This points to a polymer-complex interaction being the reason for double exponential PL decay in PMMA due to the presence of a highly polar ester group, which can facilitate dipolar interactions between guest and host molecules and alter the emissive properties of the emitter. Another notable observation for the examined film samples is a substantial difference in radiative rate constants (k r ) between the two compounds. mer-PhP shows about twofold higher radiative rate constant in comparison to fac-PhP. With cooling, both compounds exhibit a slight PL lifetime increase. This is a characteristic behavior of phosphorescent iridium(III) complexes and is related to considerable zero-field splitting between the sublevels of the triplet state and the associated thermal activation of the emissive process. [START_REF] Sajoto | Temperature Dependence of Blue Phosphorescent Cyclometalated Ir(III) Complexes[END_REF]

Computational analysis

A series of DFT calculations were performed. Geometry optimizations were carried at the PBE0, LACVP** level. The calculated ground-state structures are in good agreement with the experimental X-ray data (Table S7, ESI †). Ground state molecular orbital distribution and TD-DFT excitation calculations were performed (see Fig. S5 and the associated discussion in the ESI †).

In order to describe the phosphorescence process TD-DFT calculations were performed at optimized T 1 geometries. NTO distributions for the S 0 -T 1 transition are fairly similar to the lowest energy singlet excitation and indicate an MLCT character. It should be noted that the geometries of the compounds undergo substantial transformations, evident by the metalligand bond length changes (Table S7, ESI †). For fac-PhP this causes a loss of C 3 symmetry, and as a result, the electron transition in the T 1 state for both complexes proceeds predominantly between the metal d orbital and one of the attached purine rings. Interestingly, in the case of mer-PhP the ligand, which is involved in charge transfer and hosts through a MLCT process excited electron, shifts further away from the central metal atom due to a significant elongation of metal-carbene bond Ir-C1 (Ir-C1 changes from 2.04 to 2.15 Å, while Ir-C2 changes from 2.11 to 2.10 Å). This is a contrasting behavior to some conventional iridium(III) emitters, e.g. Ir(ppy) 3 , for which an opposite trend is observed -in the T 1 state the electronhosting ligand moves closer to the metal. [START_REF] Li | Theoretical Study of Phosphorescence of Iridium Complexes with Fluorine-Substituted Phenylpyridine Ligands[END_REF][START_REF] Jansson | Timedependent density functional calculations of phosphorescence parameters for fac-tris(2-phenylpyridine) iridium[END_REF] fac-PhP demonstrates an in-between case, as for the electron-hosting ligand the length of the metal-purine bond increases, while the phenyl-metal bond shortens (Ir-C1 changes from 2.01 to 2.06 Å, while Ir-C2 changes from 2.10 to 2.06 Å). Since an increased interatom distance can be related with a lower bonding strength and higher susceptibility to structural transformations, the predicted T 1 geometry of mer-PhP may explain the previously described observations regarding its PL behavior, where the compound exhibited a dynamic shift in PL wavelength and high sensitivity to the surrounding medium polarity and viscosity. Partial or full dissociation of ligands is one of the proposed structural transformation pathways of excited cyclometalated iridium(III) complex molecules and would be more pronounced in the case of mer-isomer. [START_REF] De Moraes | Analysis of chemical degradation mechanism within sky blue phosphorescent organic light emitting diodes by laserdesorption/ionization time-of-flight mass spectrometry[END_REF][START_REF] De Moraes | Role of oxygenbonds in the degradation process of phosphorescent organic light emitting diodes[END_REF] The calculated T 1 energies of mer-PhP and fac-PhP are 2.60 SOC-TD-DFT calculations were performed to better characterize the emissive triplet excited state of the synthesized complexes (Fig. 5). For mer-PhP two of the three triplet sublevels (T I and T III ) show fast radiative rates. In contrast, only T III possesses a comparable emissive k r value for fac-PhP. A three level Boltzmann relationship was applied to calculate the total emissive rate of the complexes at 300 K temperature. [START_REF] Mori | Predicting phosphorescent lifetimes and zero-field splitting of organometallic complexes with time-dependent density functional theory including spin-orbit coupling[END_REF] The obtained values of 1.41 Â 10 6 and 5.49 Â 10 5 s À1 for mer-PhP and fac-PhP, respectively, are in close agreement with the experimental observations in PMMA film samples. A significant increase in k r for mer-isomer can be, at least partly, explained by the slightly larger MLCT related HOMO-to-LUMO electron transition contribution towards the S 0 -T 1 excitation, amounting to 95% (in comparison to 89% in the case of fac-isomer, Table S8, ESI †). Another notable observation is the increased predicted zero-field splitting in the case of fac-PhP. This provides an explanation of the more pronounced coolinginduced PL lifetime increase in the case of fac-isomer (Fig. 3d).

Charge-transporting properties

For the investigation of the charge-transporting properties of vacuum-deposited layers of mer-PhP and fac-PhP we used the time-of-flight (TOF) method at room temperature. [START_REF] Bratina | Characterisation of charge carrier transport in thin organic semiconductor layers by time-offlight photocurrent measurements[END_REF] The ITO/mer-PhP or fac-PhP samples were used for the measurements of the TOF photocurrent. The layer thicknesses (d) were 1043 nm for mer-PhP and 1475 nm for fac-PhP. The photocurrent charge carrier transients were measured under different electric fields (E) by applying a positive or negative voltage (V) to ITO. During the measurements carried out by applying a positive voltage at ITO, it was possible to detect the transit times (t tr ) for holes (Fig. 6a andb). The transition times for electrons were not observed (Fig. S6,ESI †). This implies that mer-PhP and fac-PhP only transport holes. The hole mobility (m h ) values were calculated using the formula m h = d 2 /(V  t tr ). [START_REF] Bratina | Characterisation of charge carrier transport in thin organic semiconductor layers by time-offlight photocurrent measurements[END_REF] They are plotted as a function of the electric field in Fig. 6c. The electric field hole mobility dependences of mer-PhP and fac-PhP were well-fitted by the formula m h = m 0  exp (b  E 1/2 ) expressing the Poole-Frenkel electric field dependence. [START_REF] Connell | Theory of Poole-Frenkel conduction in low-mobility semiconductors[END_REF] The values of zero-electric-field mobilities (m 0 ) for mer-PhP and fac-PhP were obtained through the Poole-Frenkel fitting. The obtained values of m 0 were found to be of 2.95  10 À6 cm 2 V À1 s À1 and 2  10 À4 cm 2 V À1 s À1 , respectively. Field dependence parameters (b) were also obtained through the same process. The values of b were found to be 3.77  10 À3 cm 1/2 V À1/2 and 1.22  10 À3 cm 1/2 V À1/2 for mer-PhP and fac-PhP, respectively. The TOF measurements demonstrate that changing the isomers of homoleptic purine-based NHC iridium(III) complexes has a strong effect on the hole-transporting properties Fig. 5 Triplet sublevel configuration of the T 1 state of the synthesized complexes. Energy differences and radiative rates for individual sublevels are given. The total emissive rate is calculated at a temperature of 300 K. of the tested complexes. Meanwhile, the different hole-transporting properties of mer-PhP and fac-PhP may result in varied electronic device output parameters, as discussed in the following section.

Electrophosphorescence

Electrophosphorescent properties of the complexes were studied by selecting the following initial PhOLED structure: ITO/ MoO 3 (0.3 nm)/NPB(36 nm)/TCTA(4 nm)/mCP(4 nm)/emitting layer (EML)(24 nm)/TSPO1(4 nm)/TPBi(40 nm)/LiF/Al. Due to the exceptional thermal stability of the compounds, the devices could be fabricated using a vacuum deposition technique. Commonly used commercial materials, i.e., molybdenum oxide (MoO 3 ), N,N 0 -di(1-naphthyl)-N,N 0 -diphenyl-(1,1 0 -biphenyl)-4,4 0diamine (NPB), tris(4-carbazoyl-9-ylphenyl)amine (TCTA), 1,3bis(9-carbazolyl)benzene (mCP), diphenyl-4-triphenylsilylphenylphosphineoxide (TSPO1), 2,2 0 ,2 00 -(1,3,5-benzinetriyl)tris(1-phenyl-1-H-benzimi-dazole) (TPBi) and lithium fluoride (LiF) were selected as functional materials for multi-layered PhOLED. The selected materials were used for deposition of the hole and electron injecting layers (MoO 3 and LiF), hole and electron transporting layers (NBB/TCTA and TPBi), exciton, hole and electron blocking layers (mCP and TSPO1). Taking into account its higher solid-state F PL value, mer-PhP was initially selected as the emitter. The devices named A, B and C incorporated EMLs with 1, 5 or 10 wt% of emitter molecularly dispersed in the mCP host material. For benchmarking purposes a reference device (R) with 5 wt% of conventional blue emitter bis[2-(4,6-difluorophenyl)pyridinato-C2,N](picolinato)iridium (FIrpic) [START_REF] Adachi | Endothermic energy transfer: A mechanism for generating very efficient high-energy phosphorescent emission in organic materials[END_REF] was fabricated. The obtained output characteristics of the devices (Table 2) show that the optimal concentration of mer-PhP is 5 wt%.

Further optimization was attempted by preparing devices D-G. We investigated how the nature of hosts (DPEPO and BCPO) affects the electroluminescent properties of mer-PhP and fac-PhP. The architecture of the devices was as follows: ITO/HAT-CN(10 nm)/TAPC(32 nm)/TCTA(10 nm)/EML (24 nm)/ TSPO1 (4 nm)/TPBi(40 nm)/LiF/Al, where 1,4,5,8,9,11-hexaazatriphenylenehexacarbonitrile (HATCN) is the hole-injecting material, TCTA and 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) are the hole-transporting materials, and bis [2-(diphenylphosphino)phenyl]ether oxide (DPEPO), or bis-4-(N-carbazolyl)phenyl)phenylphosphine oxide (BCPO) are EML hosts. The selection of hosts was mainly reasoned with their triplet levels, charge-injecting and charge-transporting properties, which fit well with that of mer-PhP. For example, triplet levels (2.9 eV for mCP, [START_REF] Holmes | Blue organic electrophosphorescence using exothermic host-guest energy transfer[END_REF] 2.99 eV for DPEPO [START_REF] Xu | Application of Chelate Phosphine Oxide Ligand in Eu III Complex with Mezzo Triplet Energy Level, Highly Efficient Photoluminescent, and Electroluminescent Performances[END_REF] and 3.01 eV for BCPO [START_REF] Chou | A Highly Efficient Universal Bipolar Host for Blue, Green, and Red Phosphorescent OLEDs[END_REF] ) are close to that of mer-PhP (3.02 eV, Fig. S7,ESI †). Equilibrium energy diagrams of the investigated devices are given in Fig. 7. The ionisation potentials (IP PE ) of the emitters were determined by photoelectron emission spectroscopy in air, yielding the values of 5.60 and 5.81 eV observed for mer-PhP and fac-PhP respectively (Fig. S8, ESI †). The electron affinity (EA PE ) of 2.46 and 2.77 eV was calculated for mer-PhP and fac-PhP respectively by the formula EA PE = IP PE À E g . Here E g is the optical band gap taken from the edge of the absorption spectra. For mer-PhP and fac-PhP the E g values were found to be of 3.14 and 3.04 eV, respectively (Fig. 3a andb). MoO 3 /NPB/ TCTA/mCP and HAT-CN/TAPC/TCTA architectures were used to provide good hole transport and injection into EML by minimising the energy barrier between the anode and HOMO of the guest and host. With the same logic the layers of TSPO1/TPBi/ LiF were used for efficient and barrier-free electron transport from the cathode. Consequently, there were no interlayer energy barriers higher than 0.3 eV (Fig. 7). In addition, high triplet levels of mCP (2.9 eV), [START_REF] Holmes | Blue organic electrophosphorescence using exothermic host-guest energy transfer[END_REF] TCTA (2.8 eV) [START_REF] Reineke | White organic light-emitting diodes with fluorescent tube efficiency[END_REF] and TSPO1 (3.36 eV) [START_REF] Jeon | External Quantum Efficiency Above 20% in Deep Blue Phosphorescent Organic Light-Emitting Diodes[END_REF] were meant to block triplets within the EML and to localize their emissive relaxation.

In agreement with the above assumption, the electroluminescence (EL) in devices A-E mainly originates from mer-PhP (Fig. 8a and Fig. S9, ESI †). EL spectra of devices A and C are supplemented with low-intensity high-energy bands related to the host emission and indicate nonoptimal concentration of the guest. Depending on the device architecture the EL maxima range from 467 to 493 nm. These shifts in EL spectra can be attributed to the solvatochromic response of mer-PhP to the different host materials. The best performing OLED shows stable EL spectra with a maximum at 467 nm without any deviations or host emissions throughout the applied voltage range (Fig. 8a). In contrast, the EL spectra of the reference device R undergo notable voltage-dependent changes (Fig. S9b, ESI †), which is a typical bahaviour for FIrpic-based OLEDs. [START_REF] Lee | Bistriazoles with a Biphenyl Core Derivative as an Electron-Favorable Bipolar Host of Efficient Blue Phosphorescent Organic Light-Emitting Diodes[END_REF] In comparison to the close structural analogue m-2tBu (device L2), [START_REF] Jin | Iridium(III) Phosphors-Bearing Functional 9-Phenyl-7,9-dihydro-8H-purin-8ylidene Chelates and Blue Hyperphosphorescent OLED Devices[END_REF] mer-PhP exhibits notably blue-shifted EL (467 vs. 482 nm). This difference in EL spectra can be explained by the weak electron donating properties of tert-butyl substituents in m-2tBu.

The onsets of electrophosphorescence (at 1 cd m À2 ) for A-E are observed at the voltages of 3.1-5 V, which are relatively low values for blue OLEDs (Fig. 8b, Fig. S10 and Table 2, ESI †). The use of hosts with different HOMO and LUMO and chargetransporting properties allows tunning of the hole-electron balance within the EML of the devices (Fig. S10a,ESI †). This tunning manifests as the difference in current density and brightness-versus-voltage characteristics. Notably, the lowest current density is observed for the best performing device D, indicating the most efficient charge carrier balance in EML. The maximum brightness of 29 520 cd m À2 is attained for device E. This result can apparently be attributed to the highest T g value of the applied host material BCPO (137 1C) [START_REF] Chou | A Highly Efficient Universal Bipolar Host for Blue, Green, and Red Phosphorescent OLEDs[END_REF] in comparison to those of mCP (60 1C) [START_REF] Xiao | Recent Progresses on Materials for Electrophosphorescent Organic Light-Emitting Devices[END_REF] and DPEPO (93 1C). [START_REF] Naqvi | What Controls the Orientation of TADF Emitters?[END_REF] The maximum brightness of device E considerably exceeds that achieved for the reference devices L1-L4 (Table S9, ESI †). The highest external quantum efficiency (EQE) of 16.1% is achieved for device D, using DPEPO as the host material (Fig. 8c and Table 2). Hole-electron pairs recombine on mer-PhP since its HOMO (À5.6 eV) and LUMO (À2.46 eV) values are situated within the band gap of DPEPO with the HOMO (À6.1 eV) and LUMO (À2.0 eV) (Fig. 7). The difference in performance of the different hosts is hard to explain. Most likely it is related to the peculiarities of the charge transporting properties of the corresponding materials. In terms of overall characteristics OLEDs based on emitter mer-PhP are comparable to the reference devices L1-L4 (Table S9, ESI †).

The transient electroluminescence (TREL) signals were recorded for devices D and F, providing additional information on the carrier balance and charge-trapping in EML (Fig. 8d). The TREL signals are characterised by at least two EL components, a dominating microsecond component and a lowerintensity millisecond component. The microsecond component is related to the phosphorescence of mer-PhP. The millisecond component can be associated to the transport of the trapped charge carriers. [START_REF] Kang | Time-Resolved Electroluminescence Study for the Effect of Charge Traps on the Luminescence Properties of Organic Light-Emitting Diodes[END_REF][START_REF] Liu | Transient electroluminescence spikes in small molecular organic lightemitting diodes[END_REF][START_REF] Kim | Study on charge dynamics and recombination process via transient electroluminescence analysis in blue fluorescence organic light-emitting diodes[END_REF] However, the typical signal (spikes) [START_REF] Liu | Transient electroluminescence spikes in small molecular organic lightemitting diodes[END_REF] for detrapped charges with the following recombination at the moment of turn-off is not observed. Thus, the millisecond time component is most likely caused by long-lived triplet excitons of the host molecules before their transfer to the guest and the following emissive relaxation. Overall, the observed TREL signals for D and E are characteristic for devices with a balanced hole and electron ratio in the EML. [START_REF] Cheng | Carrier transport and Recombination Mechanism in Blue phosphorescent organic Light-emitting Diode with Hosts Consisting of Cabazole-and triazole-Moiety[END_REF][START_REF] Liu | The improved performance and mechanism of solution-processed blue PhOLEDs based on double electron transport layers[END_REF] In addition, devices F and G were fabricated using complex fac-PhP as the emitter and employing the previously optimized architecture ITO/HATCN/TAPC/TCTA/fac-PhP (5 wt%):DPEPO or BCPO/TSPO1/TPBi/LiF/Al. The measured characteristics are outlined in Fig. 8 and Fig. S11 and Table 2 (ESI †). In accordance to the previous PL measurements the observed EL maximums are blue-shifted in relation to the mer isomer-based devices. The performance characteristics for fac-PhP are notably lower with the attained EQE value for the best performing device F at only 3.4%. This observation can be partly explained by the high triplet energy level of the emitter (3.22 eV), which is not compatible with that of the applied host material. In addition, the highly contrasting shape of the current density versus external voltage plots for mer-PhPand fac-PhP-based devices indicate significant differences in charge-transporting properties of the emissive layers (Fig. 8b). Both complexes showed the best electroluminescent performance (reaching EQEs of 16.1 and 3.4%, respectively) when DPEPO was used as the host. They demonstrated much lower EQEs when BCPO was used. The high hole mobility of the host and/or guest may lead to an electron and hole disbalance in light-emitting layers of devices. In our case, the electron mobility of 1.96 Â 10 À6 cm 2 V À1 s À1 for DPEPO [START_REF] Zhang | Multiphosphine-Oxide Hosts for Ultralow-Voltage-Driven True-Blue Thermally Activated Delayed Fluorescence Diodes with External Quantum Efficiency beyond 20%[END_REF] is very similar to the hole mobility values of mer-PhP allowing optimal hole-electron balance within the light emitting layer of device D. Device F showed a maximum EQE of only 3.4% despite the relatively high PLQY of the emitter used. This may be attributed to the significant difference between the hole mobility of fac-PhP (2 Â 10 À4 cm 2 V À1 s À1 ) and electron mobility of DPEPO (Fig. 6c).

Conclusions

In this work we have successfully developed a synthetic methodology towards iridium(III) complexes bearing unsubstituted 7,9-dihydro-8H-purin-8-ylidene ligands. The acquired phosphorescent emitters mer-PhP and fac-PhP possess exceptional thermal stability and combine high F PL values of up to 0.99 with sub-microsecond emission lifetimes. Photophysical behavior varies significantly between the investigated mer-and fac-isomers. In polymer films mer-PhP exhibits a higher PL efficiency and significantly faster radiative rate. On the other hand, the compound shows higher sensitivity towards external stimulus, where F PL significantly drops in response to either heating or transfer from rigid to fluid surrounding medium. DFT calculations relate this to weakened metal-ligand bonding in the T 1 excited state of mer-PhP, which may facilitate the probability of non-radiative relaxation pathways. This highlights some additional aspects that need to be taken into consideration during OLED preparation that are often neglected, such as the rigidity of the EML host material and temperature changes in operational devices. In addition, merand fac-isomers exhibit highly contrasting charge-transporting properties with hole mobility values of 2.95 Â 10 À6 cm 2 V À1 s À1 and 2 Â 10 À4 cm 2 V À1 s À1 at an electric field of 4.9 Â 10 5 V cm À1 , resulting in a notable difference in OLED performance. In OLEDs mer-PhP is the best performing emitter, characterized by blue electroluminescence peaking at 467 nm and an external quantum efficiency of 16.1% in this way demonstrating that the presented structural design is well suited for the development of promising blue phosphorescent materials.

Fig. 2 X

 2 Fig. 2 X-ray structures of mer-PhP and fac-PhP. Thermal ellipsoids are drawn at the 50% probability level. Hydrogen atoms and solvent molecules are omitted for clarity.

Fig. 3

 3 Fig. 3 UV-Vis spectra in DCM solution and PL spectra in DCM solution, PMMA films (5 wt%) and in frozen MeTHF glass at 77 K for mer-PhP (a) and fac-PhP (b). Temperature dependent PL decay curves for mer-PhP (c) and fac-PhP (d) measured in PMMA films (5 wt% emitter content).

Fig. 4

 4 Fig. 4 Time-wavelength resolved emission intensity maps of toluene solutions of mer-PhP at a 50 ns timescale at room temperature (a) and 77 K (b).

and 2 .

 2 93 eV (476 and 423 nm), respectively, and correspond well with the experimentally observed phosphorescence band maxima in PMMA films.

Fig. 6

 6 Fig. 6 TOF transients for holes for the layers of mer-PhP (a) and fac-PhP (b) recorded at different voltages applied to ITO. Electric field hole mobility dependencies of mer-PhP and fac-PhP (c). The values of t tr were obtained as demonstrated by the arrows in (a) and (b).

Fig. 7

 7 Fig. 7 Equilibrium energy diagrams of devices A-G and R.

Fig. 8

 8 Fig. 8 EL spectra recorded at different driving voltages (a), plots of current density and brightness versus external voltage (b), plot of EQE versus current density (c), and EL decay curves (d) of mer-PhP-based device D and fac-PhP-based device F.

Table 1

 1 Photophysical properties of mer-PhP and fac-PhP

	Compound	l abs , nm	l em max , nm	F PL	FWHM, nm	t, ns	k r , a 1 Â 10 5 s À1	k nr , b 1 Â 10 5 s À1
	DCM and TOL solution
	mer-PhP DCM	315, 357 (shoulder)	540	0.12	141	353	3.4	24.9
	fac-PhP DCM	308, 336 (shoulder)	466	0.36	94	532	6.8	12.0
	mer-PhP TOL	-	492	0.4	85	790	5.1	7.6
	fac-PhP TOL	-	436	0.22	66	400	5.5	19.5
	PMMA and PS films (5 wt%)
	mer-PhP PMMA	-	466	0.99	94	880	11.3	0.1
	fac-PhP PMMA	-	437	0.77	76	940 (60%), 2226 (40%)	5.3 c	1.6 c
	mer-PhP PS	-	470	-	81	480	-	-
	fac-PhP PS	-	430	-	63	630	-	-

a k r = F PL /t. b k nr = (1 À F PL )/t. c Calculated using intensity averaged PL lifetime.

Table 2

 2 Output electrophosphorescence parameters of devices A-G and R

	Device	EML	l, nm	V ON	a , V	L MAX	b , cd m À2	CE MAX	c , cd A À1	PE MAX d , lm W À1	EQE MAX	e , %
	Structure: ITO/MoO 3 /NPB/TCTA/mCP/EML/TSPO1/TPBi/LiF/Al
	A	mer-PhP (1 wt%):mCP	492	4		870		7.6		3.9	4.3
	B	mer-PhP (5 wt%):mCP	493	4.5		6320		22.3		13.5	11
	C	mer-PhP (10 wt%):mCP	494	5.1		2700		13.1		7.3	6.3
	R	FIrpic (5%):mCP	471; 492	3.9		15 070	32		21.6	14.3
	Structure: ITO/HATCN/TAPC/TCTA/EML/TSPO1/TPBi/LiF/Al
	D	mer-PhP (5 wt%):DPEPO	467	4.2		4310		23.1		14.3	16.1
	E	mer-PhP (5 wt%):BCPO	482	3.1		29 520	17.5		11.9	8.55
	F	fac-PhP (5 wt%):DPEPO	443	4.8		520		3		1.4	3.4
	G	fac-PhP (5 wt%):BCPO	416	4.2		155		0.28		0.12	0.42

a Voltage, at which a luminance of 1 cd m À2 was reached. b Maximal attained luminance. c Maximal current efficiency. d Maximal power efficiency. e Maximal external quantum efficiency.

Published on 02 October 2023. Downloaded by UNIVERSITE DE BORDEAUX on 10/20/2023 8:40:19 AM.View Article Online

Acknowledgements KT acknowledges funding by M-era.Net as part of project Eco-OLED: {Enabling a Commercially Viable Long Lifespan and High-Efficiency Omni-Friendly OLED Lighting Source with G2 and G3 Emittersc, contract No. ES RTD/2020/13. AS, IN, MT, and GJ acknowledge PHC Osmose (project #48351ZC (for France) and #LV-FR/2023/2 (for Latvia)) for financial support of scientific exchanges. IN and MT thank the Latvia-Lithuania-Taiwan joint grant ''Molecular Electronics in functionalized Purines: fundamental Study and applications (MEPS)'' for financial support. This work has received funding from the Research Council of Lithuania (LMTLT), agreement No S-MIP-23-50.

Conflicts of interest

There are no conflicts of interest to declare.