2026 Volume 9 Issue 4

Open Access

INEOS OPEN, 2026, 9 (4), 101–114  Journal of Nesmeyanov Institute of Organoelement Compounds of the Russian Academy of Sciences Download PDF DOI: 10.32931/io2619r

Star-Shaped Organic Molecules with a Substituted Benzene Core for Optoelectronics

N. S. Revizorova, D. V. Kupriyanova, A. I. Kovalev,
M. A. Lebedeva, O. A. Filippov, and I. A. Khotina*

Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, ul. Vavilova 28, str. 1, Moscow, 119334 Russia
 

Corresponding author: I. A. Khotina, e-mail: khotina@ineos.ac.ru
Received 10 February 2026; accepted 1 April 2026

Abstract

This review highlights monodisperse branched oligomers with a substituted benzene core, located between small molecules and branched polymers, the so-called star-shaped molecules. They represent a new type of materials for organic optoelectronic devices. A core ensures the attachment of linear conjugated oligomers radiating from it like spokes. Cores vary from simple atoms to conjugated aromatic units and can provide a high level of symmetry to the entire structure. Such star-shaped compounds can combine the advantages of both conjugated polymers and small molecules and could be used in electronic devices.

Key words: star-shaped compounds, substituted benzene core, organic optoelectronic devices.

 

Introduction

Interest in organic materials is exceptionally high due to the development of various types of electronic devices based on them, some of which are now firmly established in the market. Historically, the main type of materials used in organic semiconductor applications has been conjugated polymers. They have been featured primarily in organic light-emitting diodes (OLEDs) [1–12], organic solar cells [13–22], and organic thin-film transistors [23–27].

Until now, in addition to the materials based on conjugated polymers obtained for organic electronics, the materials based on small molecules have also been developed [28–32]. Fabricating devices based on organic small molecules typically requires expensive high-temperature vacuum deposition. Furthermore, high-efficiency OLED displays manufactured by the deposition method are limited for use in large-area flexible displays [9]. Small molecules typically have good solubility, so applying additional layers when creating a device using a solution technology can be challenging.

In recent years, macromolecules with branched structures have attracted attention in optoelectronics [33–40].

The physical properties of branched high-molecular-weight compounds can differ significantly from their simple, linear conjugated analogs. First, they typically have good solubility, which is necessary for processing from solution. Second, such structures may contain functional groups at the periphery, and, as a result, they have improved luminescence and charge-transport properties. Furthermore, aggregation quenching in the solid state may be not characteristic of them. The potential for practical application of such polymers is increased by the simple (usually single-step) production method [36]. However, ensuring the stability of properties of polydisperse polymer structures appears to be an unsolvable problem.

Monodisperse branched oligomers, which lie between small molecules and polymers, the so-called star-shaped molecules, are attracting increasing attention. They are emerging as a new type of materials for organic optoelectronic devices. Their core provides the unification of conjugated linear branches radiating from it like spokes [41–48]. The cores range from simple atoms to conjugated aromatic units and can provide a high level of symmetry to the entire structure [49, 50].

Such star-shaped compounds can combine the advantages of both conjugated polymers and small molecules, since they have well-defined molecular structures [48]. In addition, star-shaped conjugated molecules, being a component of conjugated branched or cross-linked polymers, can form the basis of a three-dimensional polymer structure leading to the formation of graphene-like carbon materials [51, 52].

The branched molecular configuration of monodisperse star-shaped molecules can provide good electroluminescent stability owing to lower aggregation in the solid state, as in the case of branched polymers. A star-shaped molecule can simultaneously contain both donor and acceptor groups [49, 53, 54]. This can lead to broad absorption, a decrease in the band gap, easily tunable fluorescence, and good nonlinear optical properties [49]. Furthermore, precise HOMO/LUMO energy levels can be achieved in such materials. In addition, star-shaped compounds are characterized by high thermal stability, good solubility, and film-forming properties [48–58].

The creation of monodisperse star-shaped systems involves synthetic techniques not only for creating the core, but also the cross-coupling reactions commonly used in the synthesis of linear π-conjugated oligomers and polymers, catalyzed by Pd(0) complexes, which are necessary for obtaining peripheral fragments [59].

Star-shaped compounds with a 1,3,5-trisubstituted benzene core

A planar 1,3,5-trisubstituted benzene core provides the overall two-dimensional geometry of the entire molecule if the branches are conjugated linear structures. There are numerous examples of the creation of new trisubstituted benzene cores using, for example, trimerization of oligomers with a terminal acetyl functionality to obtain star-shaped structures with three arms [45, 60, 61]. The resulting compounds with a 1,3,5-triphenylbenzene core are excellent building blocks [62] for the preparation of dendritic molecules with a wide band gap and a high triplet energy level owing to the C3 structural symmetry [63, 64].

The above parameters are necessary to achieve efficient blue emission. Numerous derivatives of 1,3,5-triphenylbenzene have been reviewed [59, 65–67].

A productive approach to the formation of 1,3,5-triphenylbenzenes is the Lewis acid-catalyzed cyclocondensation of acetylarylenes, or trifluoromethanesulfonic acid in the presence of an alcohol, or compounds that release hydrogen chloride in the presence of an alcohol [45, 61, 63], as well as the cyclotrimerization of monoacetylarylenes in the presence of orthoformic ether and gaseous hydrogen chloride [39, 40, 60, 68]. The yields of the target products can vary from 40 to 80%, depending on the substituent in the benzene ring. A series of star-shaped oligophenylenes was developed using this method [69–71].

The photophysical characteristics of the resulting star-shaped compounds were explored to evaluate the influence of the 1,3,5-trisubstituted benzene ring environment and the number of benzene rings in the substituents on their photophysical properties [60].

The UV-Vis absorption spectra of cyclotrimers 1–5 (Fig. 1) are shown in Fig. 2, and the absorption maxima and molar extinction coefficients are summarized in Table 1. As can be seen from the spectra, the absorption bands were located in the UV region (250–320 nm). UV absorption is primarily due to the aromatic structure. The higher the number of benzene rings in the cyclotrimer molecules, the higher the molar extinction coefficient of the UV band. It is worth noting that compound 4, which should be considered a second-generation dendrimer relative to 1, had a structured spectrum.

Figure 1. Examples of the star-shaped structures.

Figure 2. Absorption spectra of aromatic cyclotrimers 1–5 in acetonitrile. Spectra are normalized to a concentration of 10·10^–6 M.

Table 1. Absorption maxima, extinction coefficients, fluorescence maxima and quantum yields of cyclotrimers (l_ex = 290 nm) [60]

Compound	lmax (abs), nm	e, M–1·cm–1	lmax, (PL), eV (nm)	PLQY, %
1	253	0,52.105	3,52 (352)	29
2	288	1,10.105	3,42 (363)	43
3	268	0,79.105	3,47 (357)	20
4	263; 290	1,38.105	3,44 (360)	72
5	308	1,58.105	3,40 (365)	92
6	254	–	3,42 (363)	63

This indicates significant steric hindrances within the molecule during a transition from the ground state to the excited one. All compounds, especially 2, 4, and 5, have high molar extinction coefficients, i.e., higher than 1^.10⁵ M^–1cm^–1. The absorption maximum of compound 2 bearing diphenyl groups is red-shifted compared to those of 1 and 3 (Table 1), presumably, due to a longer conjugation chain. The absorption maximum for aromatic planar cyclotrimer 5 containing acetylene groups is also red-shifted compared to the spectra of other cyclotrimers. This can be explained by the appearance of conjugated diphenylacetylene groups in the branches of the substituents (Fig. 3) [68].

Figure 3. PL spectra of model compounds M1, M2 and 5 in CH₂Cl₂.

In general, as can be seen from the above spectra, the absorption band position and absorption intensity depend on the number of benzene rings in the substituents.

The photoluminescence spectra of cyclotrimers exhibit blue emission with a very slight difference between the maxima, which are located in the range of 352–365 nm, sometimes with a shoulder at 370 nm. These spectra are typical for aromatic compounds.

The luminescence quantum yield (QY) of 1,3,5-triphenylbenzene (1) was 29% (Table 1), which is in good agreement with QY = 27% determined for 1 in Ref. [72]. The hexaphenylbenzene molecule, despite its symmetry, had a QY of less than 0.5% due to a different π-system than 1, since in this molecule all the side benzene rings are perpendicular to the central benzene core and there is no overlap of the π-orbitals. For 1,2,3,4-tetraphenylbenzene (asymmetrically substituted terphenyl), QY was equal to 0.5%. Only p-terphenyl with a conjugated structure had a higher QY (84% [72], 93% [73]).

Apparently, this quite a high value of QY for 1 was due to the conjugation of adjacent benzene rings at the m-positions and is close to the value for m-terphenyl (23%) [72]. The optical properties of phenylacetylene stars with a trisubstituted benzene ring offer that such a molecule can be largely considered as a set of independent chromophores linked through meta-substitution in the central phenyl ring of 1,3,5-triphenylbenzene. However, the existence of meta-conjugation during emission in phenylacetylene dendrimers was shown, for example, in Ref. [74]. Meta-conjugation can also occur in phenylene dendrimers.

When examining star-shaped compounds 1–6, it was observed that the presence of a higher number of benzene rings surrounding the central 1,3,5-trisubstituted core leads to an increase in the luminescence quantum yields. The only exception was the compound with phenylacetylene groups (5) [60], for which the QY is very high, namely, 92%. Another exception is compound 3 with oxygen bridges, in which the conjugation chain is interrupted. Its QY is lower than that for 1 and amounts to 20%. The quantum yield for compound 6 with hexaphenyl-substituted benzene groups at the periphery equal to 63% is lower than that for compound 4 with 1,3,5-triphenyl-substituted benzene branches (QY = 72%). This difference may be due to the weak luminescence efficiency of hexaphenylbenzene itself, where the peripheral phenyl substituents are not coplanar to the central core, and conjugation in hexaphenylbenzene is practically absent.

Thus, the resulting data determined the choice of 1,3,5-triphenylbenzene as a core of star-shaped aromatic structures with the subsequent introduction of conjugated branches to the periphery of the core.

Cyclotrimers 2 [69] and 5 [71] were studied by X-ray diffraction. The bond lengths in the central benzene ring of molecule 2 (1.395(3) Å) correspond to the standard length of an aromatic bond (1.394(5) Å). The C_Ph–C_Ph bond lengths reach 1.480 Å for 2. These values are close or slightly shorter than the same bonds in 1,3,5-triphenylbenzene (1) (1.487 Å) [75].

The bond lengths in the terminal phenyl groups, as well as the bond lengths between the terminal units and the preceding moieties, are somewhat shorter than the standard values, probably due to the thermal motion of the molecule.

The triphenylbenzene fragment, in accordance with its 32-symmetry [76], has a propeller conformation. In general, for molecule 2, most bond lengths and angles are close to the corresponding values in their low-molecular analogs, i.e., the geometric parameters do not change with increasing molecule sizes.

Figure 4 depicts the structure of molecule 5 in projection onto its mean plane with atom numbering.

Figure 4. Projection of the crystal structure of compound 5 along the x-axis. Both types
of "channels" filled with chloroform molecules in a clathrate manner are shown [71].

Compound 5 crystallizes from chloroform as a crystal solvate that is unstable in air under normal conditions. There are two symmetrically independent molecules in the crystal, 1A and 1B, which have similar conformations and a geometry close to the standard (Fig. 4). In both molecules, the central 1,3,5-tris-p-phenylenebenzene fragment has a non-planar asymmetric conformation: the dihedral angles between the planes of the 1,3,5-substituted central and adjacent 1,4-substituted benzene rings vary within the range of 20.9–37.5°, and one of the p-phenylene rings in each molecule is turned in the direction opposite to the inclination of the other two p-phenylene rings.

The mutual orientation of the benzene rings in each of the tolane residues varies within a wider range: the dihedral angles between the planes of the cycles linked by acetylene groups lie in the range from 0.8 to 47.3°.

The structure of the crystals of compound 5 is characterized by significant "heterogeneity": on the one hand, the molecules of 5 are packed into fairly compact stacks along the x-axis, and on the other hand, the mutual arrangement of these stacks is very loose—there are "channels" between them (also along the x-axis) filled with solvate chloroform molecules (Fig. 4). Obviously, this packing pattern is caused by significant anisometry and conformational rigidity of molecules of 5.

Molecules 1A and 1B are arranged in stacks, alternating and approximately parallel to each other. However, in reality, the stacks are not uniformly arranged. The manner in which the molecules are superimposed on each other in the stacks is shown in Fig. 5, where the shortest distances between the acetylene groups of adjacent molecules are also indicated. The uneven packing of molecules of compound 5 limits the arrangement of acetylene groups along the stack axes, which would potentially lead to the formation of polyene chains as a result of topochemical polymerization, similar to what occurs, for example, in diacetylene crystals [77]. The main factor limiting the proximity of acetylene groups to each other in crystals is the mutual orientation of the benzene rings in the contacting tolane residues. The comparative conformational rigidity of molecules of 5 is apparently the main obstacle to the realization of close packing in the crystal.

Figure 5. Projection of a fragment of the stack (5), including the basic molecules 1A (double lines), as well as the molecule 1B' symmetrically derived from 1B by translation along the x-axis (black lines), onto the midplane of the molecule 1A. The dashed lines show the shortest distances between the acetylene groups of neighboring molecules [71].

However, the structure of compound 5 in principle enable the possibility of a relatively close arrangement of triple bonds of adjacent molecules, which favors their possible topochemical interaction (cross-linking) under sufficiently strong external stimuli (heating, irradiation).

Star-shaped compounds with a 1,3,5-triphenylbenzene core and bulky substituents at the periphery were also considered. They were, for example, the derivatives with acenaphthyl (7) and acenaphthynyl (8) groups (Fig. 6) [78].

Figure 6. Structures of 1,3,5-tris(5-acenaphthenyl)benzene (7) and 1,3,5-tris(5-acenaphthylenyl)benzene (8).

Compounds 9 and 10 (Fig. 7), containing o-carboranyl groups, were also obtained and their crystal structures were studied [69, 70].

Figure 7. Structures of 1,3,5-tris[4-(C-o-carboranylmethyl)phenyl]benzene (9) and 1,3,5-tris[4'-(C-o-carboranyl)biphenyl-4-yl]benzene (10).

The molecules have an asymmetric conformation, likely due to intramolecular interactions resulting from possible bending of the molecular branches, as well as the presence of inclusion molecules—chloroform.

Overall, X-ray structural analysis of the cyclotrimers showed that the central 1,3,5-triphenylbenzene ring of all compounds is fairly planar relative to the adjacent benzene rings, while the branch structure is loose and allows the insertion of inclusion molecules.

An even bulkier substituent, namely, fullerene can be introduced into the peripheral benzene rings. Thus, compounds 2 and 11 were obtained (Fig. 8) [76].

Figure 8. Cyclotrimer with fullerene groups (11).

Using the cyclocondensation of acetylarylenes, Cremer and Bäuerle [53] synthesized star-shaped oligophenylenes 12, 13 (Fig. 9) as a new class of efficient hole-type molecular materials for application in OLEDs, which was demonstrated for multilayer devices with an emitting layer of a star-shaped compound and AlQ₃ (tris(8-hydroxyquinoline)aluminum) as an electron transport layer.

Possessing high p-type conductivity and low HOMO, they effectively prevented the formation of an exciplex, often observed between the p-type and electron transport layers. The best performance was achieved for fluoro-substituted derivative 12, which provided efficient blue–violet emission from OLEDs with various amorphous molecular emitters [53].

Figure 9. Compounds 12, 13.

A group of similar hole-blocking molecules 2, 12, 14, and 15, with a 1,3,5-triphenylbenzene core was synthesized [45] (Fig. 10). They are characterized by high oxidation potentials and a large HOMO–LUMO band gap. It was shown that these materials can be used in organic EL devices in hole-blocking layers. This use of 2, 12, and 15 enabled efficient blue-violet emission from amorphous molecular materials. The devices demonstrated high performance at low turn-on voltages of 3 to 5 V and high external quantum efficiencies of 1.08 to 1.95%. However, no significant differences were observed when studying the photophysical properties of these compounds.

Figure 10. Star-shaped molecules with a 1,3,5-triphenylbenzene core.

The only difference observed was that the absorption maximum of the fluorene-containing oligomers appeared to be bathochromically shifted by 20–30 nm compared to the absorption maxima of oligomers without fluorene groups. This is explained by the occurrence of a higher degree of conjugation in the fluorene-containing branches.

It is known that conjugated fluorene-based oligomers can emit deep blue light with high efficiency in both solutions and films. Several star-shaped compounds with a 1,3,5-triphenylbenzene core and fluorene-containing branches were described in Ref. [55] (Fig. 11).

Figure 11. Star-shaped compounds with π-conjugated oligofluorene branches and a benzene core.

The work [64] reports the synthesis of three star-shaped molecules based on 1,3,5-triphenylbenzene, with terminal alkylated fluorene fragments. The fluorophores in each molecule are linked through various bridging groups containing single, double, and triple bonds (Fig. 12).

Figure 12. Cyclotrimers with dialkylfluorene fragments.

All the compounds under consideration have C3 symmetry, achieved through the use of a 1,3,5-triphenylbenzene core linked to dialkylfluorene fragments (Fig. 12) [64].

As expected, a bathochromic shift of the absorption band is observed upon the introduction of π-conjugated bridges in 18 and 19. Thus, the absorption spectra of the above-mentioned branched molecules are red-shifted at 36 and 11 nm (for 10^–4 M THF solutions) and 48 and 11 nm (for thin solid films), respectively, compared to the absorption spectra of 17. Moreover, the absorption maxima of the spectra of 18 exhibit bathochromic shifts compared to those of 19 (by 25 and 37 nm for THF solutions and thin films, respectively), indicating a more efficient conjugation in the dendritic molecule with the ethynyl-containing linker. The higher energy absorption of 19 compared to 18 can apparently be attributed to the more electronegative nature of the triple bond between 1,3,5-triphenylbenzene and fluorene fragments 19 relative to the double bond present in bond 18.

Star-shaped derivatives of 1,3,5-triphenylbenzene showed relatively high QYPL (0.40, 0.54, 0.46 for 17, 18, 19, respectively). The highest QY value of 0.54 was observed for 18, in which the chromophores are linked through an ethynyl-containing linker. The lowest QY value was observed for 17 (QY = 0.40), in which the chromophores are linked by single bonds. Apparently, an increase in the photoluminescence efficiency in the range 17 < 19 < 18 can be explained by the enhancement of conjugation between the fluorene and 1,3,5-triphenylbenzene units.

Using the Wittig–Horner reaction, propeller-shaped star-shaped conjugated molecules with a triphenylbenzene core were synthesized [79] that bear three oligomeric phenylene-vinylene branches with either electron-donating or electron-withdrawing peripheral groups (20) (n = 1, 2; R = D (donor) or A (acceptor)) (Fig. 13). The authors found that these octopolar derivatives exhibit a solvatochromic behavior in absorption and, especially, in emission, which indicates intramolecular charge transfer occurring between the center and the periphery of the molecules [79].

Figure 13. Star-shaped molecules with a triphenylbenzene core bearing three phenylene-vinylene branches.

Enhancement of this effect by using strong acceptor (or donor) end groups and increasing the core–periphery distance leads to increased molecular nonlinearities. Using these approaches, high hyperpolarizability was achieved while maintaining transparency in the visible region, thus demonstrating a trade-off between transparency and nonlinearity (||b|| = 810×10-30  at λ_max= 377 nm).

Acetylene dendrimers are a special class of compounds with promising properties in electronics and photonics (Fig. 14). The rigid acetylene moieties ensure a constant shape and size. Moore and co-workers made fundamental contributions to the field of acetylene dendrimers, in particular, phenylacetylene dendrimers, in the 1990s (21). Their synthetic approaches, based on convergent and divergent strategies, led to the synthesis of bulk acetylene-containing dendrimers that have found applications in the fields of light-harvesting antennas, nonlinear optical materials, and organic light-emitting diodes [80–93].

Figure 14. Phenylene-acetylene dendrimers as light-harvesting antennas.

Xu et al. [81] showed that phenylacetylene monodendrons are capable of directed excitation energy transfer to the luminophore at the focal point. Thus, if charge recombination occurs in the dendritic segment, energy transfer to the luminescent core is expected.

Many of Moore's dendrimers contained side arms with acetylene groups. By controlling the length of the conjugated phenylacetylene arms, it was possible to ensure efficient energy transfer from the dendritic portion of the macromolecule to one chromophore core, where the absorbed energy was converted into radiation at a different wavelength. These conjugated structures exhibit UV absorption in the range of 250–350 nm, the intensity of which increases with the number of dendrimer generations. The dendrimer emits light at 350–450 nm. When perylene groups, which have a narrower band gap, are introduced into the dendrimer, the phenylacetylene-containing units act as peripheral energy donors, while the perylene serves as a central energy acceptor. Excitation of the dendrimer (Fig. 14) at 312 nm results in emission at 450–600 nm, which corresponds to emission from only the perylene fragment. This is accompanied by almost complete quenching of the luminescence of the phenylacetylene branches.

In a series of dendrimers described by Moore et al. [83], featuring asymmetric 1,3,5-triaryl-substituted benzene groups, emission occurs primarily from the perylene or phenylacetylene chromophore. The order of their QY values appeared to be inverse to the dendrimer generations, although the energy transfer efficiency increases with increasing generation, i.e., the phenylacetylene environment of the chromophore group acts as an antenna.

However, the synthesis was based on the use of an AB2-type monomer. The synthesis yields a wide variety of compounds, where, when isolating the macromolecule shown in Fig. 14, it may be present in the majority, but not be the only product of the synthesis.

In subsequent works, star-shaped phenyleneacetylenes with a more defined structure were synthesized by various authors.

Star-shaped molecules with a 1,3,5-substituted benzene core and naphthyl- or phenylethynyl branches of different lengths were obtained by the cross-coupling between 1,3,5-triiodobenzene and conjugated oligomers with terminal ethynyl groups (Scheme 1) [84]. The resulting starting compounds (n = 0–2) showed typical trends in photophysical properties with increasing arm lengths, i.e., an increase in the luminescence quantum yield (0.16, 0.48, 0.80) with increasing length of the linear conjugated arm, as well as bathochromic shifts in both the absorption maxima (λ_abs = 308, 351, 366 nm) and emission maxima (λ_PL = 353, 356, 377 nm) [84].

Scheme 1. Synthesis of star-shaped compounds with phenylacetylene branches.

The synthesis of dendron structures with extended naphthylethynyl and phenylethynyl chains was developed to improve fluorescence properties. The terminal 5-(N,N-dimethylamino)naphthyl-1-ethynyl moiety, linked to conjugated phenylethynyl moieties, exhibits a constant quantum yield with increasing number of phenylethynyl moieties (Scheme 1).

The synthesis of star-shaped oligo(phenyleneethynylenes) with a 1,3,5-trisubstituted benzene core and ethynyl groups in the branches and at the periphery was described in Refs. [85–88]. Narayanan et al. [89] reported the synthesis of cyclotrimer 23 and related compounds with different alkyl substituents and studied their fluorescence and absorption spectra (Fig. 15).

Figure 15. Compound 23 with a trisubstituted benzene core and ethynyl groups in the branches and at the periphery.

This field was further developed by Xu et al. [87, 88]. In the mentioned works, branched dodecaines 1,3,5-{(4-t-Bu)[2,3-(RC≡C)₂]Ph(C≡C–C≡C)}₃Ph (R = H (24), Ph (25)), including those with a propeller structure, were synthesized and characterized (Fig. 16).

Figure 16. Branched dodecaines 24 and 25 with a trisubstituted benzene ring.

These molecules hold great promise for further branching. Steady-state spectroscopy was used to compare their electron absorption and luminescence characteristics. Simultaneously, the molecular configuration of dodecaine 24 was described using density functional theory (DFT), and the maximum absorption was confirmed by subsequent TD-DFT calculations. Electrochemiluminescence (ECL) emission was also studied. The results showed that π-extended dodecaine 25, with a longer conjugation chain and a more distorted trigonal plane, exhibits strong and stable ECL emission, indicating its potential application in ECL and luminescence.

Sun et al. [85] showed that the emission intensity of polyyne 27 was significantly lower than that of polyyne 26, indicating that the electron withdrawing effect of three fluorine atoms affects the photoluminescence more significantly than the absorption effect in the UV-Vis region of the spectrum (Fig. 17). The relative photoluminescence quantum yield (QY) of polyyne 26 was 83.0%, which was significantly higher than that of polyyne 27 (21.3%). Apparently, fluorine atoms introduced into the central benzene ring can lead to a significant decrease in the photoluminescence quantum yield. Subsequently, three-dimensional (3D) electrochemiluminescence was investigated. For polyyne 26, the maximum ECL emission corresponded to a wavelength of 590 nm. The performed investigations demonstrated that these two dendritic polyynes have potential applications in the field of electrochemiluminescent sensors. However, in this class of dendrimers, both the periphery and the core gradually become crowded due to the invasion of one of the building block branches into the core. Such steric crowding undoubtedly imposes serious limitations on the synthesis of higher generations of dendrimers and their stability.

Figure 17. Branched polyynes 26 and 27.

Oligomers with a hexasubstituted benzene core

In recent years, hexasubstituted benzene derivatives, in particular, hexaphenylbenzene (HPB) have been actively studied for applications in optoelectronics. HPB has a nonplanar structure due to the perpendicular arrangement of the phenylene substituents relative to the central benzene core. Therefore, these molecules have a low degree of aggregation, leading to amorphous packing in the solid state, which is important for optoelectronic devices. Indeed, the absorption and emission parameters in solution and in the solid state typically differ little for such structures. Owing to their electron-donating ability, HPB molecules can be modified with appropriate acceptor moieties to obtain donor-acceptor systems. Such molecules can be used as hole- or electron-transfer materials in organic electronic devices, as they have a sufficiently wide band gap and high HOMO energy levels.

The hexaphenylbenzene ring is formed by cyclotrimerization of disubstituted acetylenes in the presence of Co₂(CO)₈ [90] or by the Diels–Alder reaction of disubstituted acetylenes with tetrasubstituted cyclopentadienones [90–92].

The first studies on the electronic properties of hexaarylbenzene derivatives were carried out in the laboratory of K. Müllen. One of their reports [93] demonstrated the difference in the properties of star-shaped compounds based on hexaphenylbenzene and compounds representing their linear branches (Fig. 18, Table 2).

Figure 18. Star-shaped compounds with a hexaphenylbenzene core and compounds representing their linear branches.

Table 2. Long-wavelength absorption and emission maxima, extinction coefficient and fluorescence quantum
yield of alkyl-substituted p-terphenyl (29), p-quaterphenyl (31) and star-shaped molecules 28 and 30

Comp.	λmax (abs), nm	Ε, L·mol–1·cm–1	λmax (em), nm	QY
28	305	2.5·104	360–380	0.66
29	285	3.6·104	332, 346, 364	0.99
30	316	3.2·104	374, 385, 425	0.93
31	299	4.7·104	354, 367, 390	0.96

Compounds 28 and 30 feature excellent solubility. The expected twisting of the terphenyl units at an angle of approximately 65° to the central benzene ring in 28 prevents close packing of the molecules in the solid state, and this explains the good solubility of 28. The question arises as to whether the terphenyl units are electron-bonded.

Comparison of the cyclic voltammetric parameters of 28 with those of p-terphenyl shows that 28 essentially contains redox-active p-terphenyl units. However, the absorption and emission spectra of 28 and 30 relative to the corresponding model compounds 29 and 31 indicate that chromophores 28 and 30 are not completely independent chromophores (Table 2) and that there is a relationship between the branches of these compounds and the core, or the branches interact with each other.

It can be seen that the molecular extinction coefficients of 28 and 30 are exceptionally high due to the density of chromophores. Star-shaped oligomers 28 and 30 fluoresce in solution and in the solid state. The similarity of the fluorescence spectra of the film and solution indicates the absence of aggregate formation in the solid state. Good solubility, low tendency to crystallization, and high fluorescence quantum yields make 28 and 30 promising materials for LEDs [93].

Constable et al. [94] synthesized star-shaped structures with a hexaphenylbenzene core containing up to thirty alkyne functional groups (Scheme 2). The reaction of compound 32 with Co₂(CO)₈ led to the formation of cobalt dendrimer 33 containing Co atoms, which is interesting from the viewpoint of potential catalytic applications.

Scheme 2. Metallostars and metallodendrimers based on hexaphenylbenzene cores.

The attachment of six fluorene units to hexaphenylbenzene and further elongation of the fluorene branches, with the introduction of one to three fluorene groups in each branch, resulted in star-shaped molecules with highly twisted core architectures (Fig. 19) [95, 96]. The propeller-like structure of the new stars appears to provide high efficiency among deep-blue OLED devices. Due to the propeller structure, these molecules are not prone to aggregation. It is noteworthy that the photophysical parameters differ little between the solution and the film, further confirming the absence of aggregation of these star-shaped molecules. As the oligofluorene chain lengthens, the absorption and emission spectra exhibit a bathochromic shift due to the increased degree of conjugation. The absorption and luminescence maxima were observed at 366 nm and 417 nm, respectively.

Figure 19. Propeller-shaped structure of compound 34 with a hexaphenyl-substituted core and fluorene groups at the periphery.

The luminescence quantum yields reached the values up to 92%. Furthermore, the new star-shaped oligofluorenes demonstrated potential as materials for hole transport in OLED displays.

Yu et al. [97] synthesized a star-shaped molecular acceptor based on perylene diimide (PDI) with a hexaphenylbenzene core (Fig. 20). The synthesis was based on the reaction of hexaethynylhexaphenylbenzene with perylene diimide monobromide via the Sonogashira reaction. Compound 35 possesses the highest number of quasi-degenerate LUMO orbitals, indicating its potential as an excellent electron acceptor for organic solar cells. The HOMO and LUMO energies of compound 35 were found to be 5.85 and 3.86 eV, respectively.

Figure 20. Hexaphenylbenzene with perylenediimide groups at the periphery.

To evaluate the electronic properties of 35 as an acceptor, a photovoltaic device with the following architecture was fabricated: ITO/ZnO/electron donor: 35/MoO₃/Ag. A thiophene-containing polymer with suitable HOMO–LUMO energy levels and absorption complementary to 35 was chosen as the electron donor for the fabrication of this solar cell prototype. The device demonstrated the following photovoltaic parameters: Voc 0.86 V, FF 42.5%, and J_sc 9.71 mA·cm². Therefore, this molecule is a non-fullerene acceptor containing up to six perylenediimide units that can absorb sunlight, demonstrating excellent energy conversion efficiency.

The monodisperse, branched macromolecular structure of star-shaped compounds makes them attractive not only for conjugated structures in optoelectronics but also for the production of copies of biologically active molecules.

Weaver et al. [98] obtained conjugated star-shaped macromolecules with a hexaphenylbenzene core as rigid frameworks for the presentation of multiple copies of oligosaccharide units (Fig. 21).

Figure 21. Dendritic framework 36 based on hexaphenylbenzene.

Currently, the main commercial OLED technologies are phosphorescence-based OLED displays. The potential internal quantum efficiency (IQE) of such devices approaches 100%. However, they have a number of obvious drawbacks, such as high cost and toxicity due to heavy metal atoms, as well as the lack of stable, especially blue phosphorescent emitters. In recent years, OLED devices based on the principle of thermally activated delayed fluorescence (TADF) have been actively developed [99]. Since OLED displays require bright and stable emitters, this problem can be solved using purely organic TADF emitters, which represent an alternative approach to efficient emitters, as reflected in a number of publications [100–111], including reviews [106, 107].

TADF [99] is based on a small energy gap between the singlet and triplet states, ΔEST, defined as the gap between the lowest-energy triplet state (T1) and the lowest-energy singlet state (S1). When ΔEST is small enough, thermal conversion from the triplet to the singlet state becomes possible via an inverse intersystem transition back to the singlet state, followed by emission. TADF emitters typically exhibit two types of photoluminescence: fast fluorescence, and delayed fluorescence, which results from an initial intersystem transition to the triplet state followed by recovery of the singlet state. Thus, purely organic TADF emitters can use both singlet and triplet excitations to emit light and achieve 100% internal quantum efficiency [107].

Wang et al. [108] developed a new strategy for the synthesis of hexaarylbenzenes with charge transfer through space with TADF and AIE (aggregation-induced emission) effects for high-efficiency OLEDs obtained using a solution technology.

Two kinds of such molecules were synthesized, containing symmetrically attached acridane (37) or dendritic teracridane (38) groups as donors and symmetrically located triazine groups between them as acceptors at the periphery of the hexaphenylbenzene center (Fig. 22). Spatial π-π interactions between donors and acceptors provide efficient charge transfer emission through space for these molecules. As a result of the physical separation of the donor and acceptor, molecule 37 shows a small ΔEST (energy gap) value of 0.04–0.08 eV, which leads to distinct delayed fluorescence with a lifetime of 3.16–3.54 μs in a solid film.

Figure 22. Hexaphenylbenzene molecules with phenylacridane (37) and phenyltriacridane (38) substituents.

Aromatic compounds substituted with cyano groups (CN) are widely used to create TADF emitters owing to their strong electron-withdrawing properties and high chemical stability. Star-shaped molecules with symmetric donor-acceptor-donor (D–A–D) and asymmetric donor-acceptor-donor* (D–A–D*) molecular structures were obtained to thoroughly study the effect of different substituents on the device characteristics (Fig. 23) [107]. Carbazole groups were most often used as donor groups, and cyano groups were introduced as acceptors.

Figure 23. Star-shaped molecules 39 featuring a donor-acceptor molecular structure.

In order to find the optimal structure of the emitter from the viewpoint of regulating the photophysical properties, the authors studied molecules with symmetrically or asymmetrically located donor and acceptor groups with a clearly defined D–A architecture (Fig. 24).

Figure 24. Hexasubstituted benzene derivatives: benzophenone (40),
benzophenone functionalized with dibenzofuran (41) and dibenzothiophene (42).

Upadhyay and Ray [109] investigated hexaphenyl core substituents, ether-linked benzophenone, and dibenzofuran- or dibenzothiophene-functionalized benzophenone as auxiliary groups in carbazole-phthalonitrile conjugates. These groups affect the aggregation-enhanced thermally activated delayed fluorescence. The films showed high photoluminescence quantum yields (PLQY) of 67% and 70% for 41 and 42, respectively. Compound 40 had a PLQY of 33%, which is explained by strong intermolecular π–π interactions between adjacent carbazole blocks. This implies that compound 40, bearing benzophenone groups, undergoes aggregation-induced quenching, which highlights the critical role of these functionalized groups in enhancing the blue properties of TADF.

Sun et al. [110] obtained two specially developed luminescent materials 43 and 44 and applied them in TADF OLEDs (Fig. 25). These materials differ in that the carbazole group is attached to the hexaphenyl core via a phenylene bridge. The new molecules are capable of lowering the injection barrier, suppressing nonradiative decay, and increasing the efficiency of solid-state photoluminescence. Compound 44 has a high luminescence quantum yield in the film: 14.5%. Its films also exhibit the following electroluminescence characteristics: 45.1 cd·A^–1, 36.0 lm·W^–1. More importantly, the EQE reduction values are extremely small and amount to 3.8% and 3.9% at a luminance of 100 cd·m^–2 and 1000 cd·m^–2, respectively.

Figure 25. Hexasubstituted benzene derivatives with carbazole and nitrile groups.

The multidimensional shielding acceptor structure (Fig. 26) [111] creates significant spatial obstacles for TADF molecules and controls the molecular stacking, resulting in the external quantum efficiency of sky-blue OLED displays reaching 24.1%.

Figure 26. Hexasubstituted benzenes with substituents: carbazole, nitrile and substituted phenylene groups.

The heterodonor system effectively combines charge transfer and locally excited triplet states. The steric effect of rigid, bulky phenyl derivatives and tert-butyl groups significantly reduces intermolecular π–π interactions during stacking, resulting in high PLQY in the solid state. Solution-based OLED displays involving these sky-blue emitters achieved efficiencies exceeding 20% with low efficiency degradation. A device based on compound 47 demonstrated excellent performance, with a peak efficiency of 24.1%, maintaining 23.7% at 100 cd/m² and 15.2% at 1000 cd/m², an outstanding result among OLEDs.

The phenomenon whereby some organic luminophores exhibit stronger light emission in the aggregated or solid state compared to the solution was discussed in detail [112–116]. This is called aggregation-induced emission (AIE). AIE is inherent mainly to monodisperse small molecules, which also include star-shaped aromatic compounds with a substituted benzene ring. It was also noted that emission enhancement upon aggregation (AIEgens) is observed for compounds exhibiting the TADF effect, i.e., in molecules where both donor and acceptor groups are present simultaneously [87, 108, 110]. For example, hexaphenylbenzene 38 with phenyl triacridane groups with a propeller architecture exhibits AIE behavior in THF/water mixed solvents with emission intensity increased by 6–17 times from the solution state to the aggregation state [108]. Compound 44 also exhibits AIE effect [110].

An unusual application of star-shaped organic small molecules is their use in perovskite solar cells, which have great potential for efficient photovoltaics since hole-conducting materials are a key factor in their success (Fig. 27).

Figure 27. Star-shaped molecule 48 based on a hexasubstituted benzene with dithiophene groups.

Martynov et al. [31] presented new star-shaped structures, including those with a hexasubstituted benzene core, which are applicable for increasing the efficiency and stability of perovskite solar cells in the absence of a dopant. Photovoltaic cells based on compound 48 showed an efficiency of 12.6%.

Conclusions

This review presented star-shaped conjugated structures with a substituted benzene core, which exhibit optoelectronic properties. The peripheral branches play a key role in photoluminescence and electroluminescence, while the core serves as a framework for linking photoactive groups. However, conformational effects, intramolecular charge transfer, and energy transfer differ from those of one-dimensional analogs of star-shaped structures. The monodisperse nature of the "stars" and their relatively high molecular weight allow them to be considered nanoobjects and explored in areas of optoelectronics not previously addressed to organic compounds.

Acknowledgements

This work was supported by the Russian Science Foundation (project no. 19-73-20262-P). Access to electronic resources and databases was provided by INEOS RAS with support from the Ministry of Science and Higher Education of the Russian Federation (agreement no. 075-03-2026-024).

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