Structural insights and molecular advancements in fluorene-based electroluminescent materials: A mini-review

The phenomenon of organic electroluminescence was discovered relatively recently, and foundational theories regarding material luminescence have not yet been fully developed. This situation leads to various uncertainties in the research of organic electroluminescent materials. Polymer materials, characterized by their polydispersity, often exhibit poor reproducibility in research results and can occasionally yield contradictory findings. In contrast, small-molecule organic electroluminescent materials have well-defined structures, and the direct orthogonal relationship between their properties and structures makes experimental control easier. Additionally, small-molecule luminescent materials offer irreplaceable advantages in terms of solubility and processability compared to polymers, making the synthesis of various structural fluorene-based small-molecule luminescent materials a crucial strategy. As summarized in Table 1, Tsutsui et al. [14] synthesized luminescent materials with alternating fluorene and alkyne units via the Sonogashira reaction between fluorene and 2,7-diyne fluorene, catalyzed by Pd/Cu (Fig. 2). This material displays strong blue fluorescence and is soluble; the resulting light-emitting diode has a maximum emission wavelength of 402 nm and a photoluminescent efficiency of 64 %. Furthermore, the maximum emission wavelength can be tuned by altering the conjugation length of the material. The introduction of acetylene groups brings about the relative coplanarity of the molecules, which helps in the efficient transport of electrons on the polymer chain, which improves the luminescence efficiency. Besides, Fluorenone deficiency may lead to molecular aggregation and the formation of exciton associations, which cause fluorenone materials to produce green light emission bands, which seriously affect their blue light purity. The copolymers of fluorene and alkyne alternating synthesized by Sonogashira reaction can effectively inhibit molecular aggregation and improve the blue light purity of polymers.

Wong et al. synthesized two types of small-molecule derivatives of fluorene. The twisted helical structure resulting from the steric hindrance of the phenyl ring at the 9-position of fluorene leads to an amorphous aggregation state that inhibits exciton transfer between molecules, achieving a photoluminescent efficiency of over 90 %. These materials have a glass transition temperature exceeding 200 °C, demonstrating good thermal stability. Further research reveals that both materials exhibit high carrier mobility and bipolar transport characteristics: As indicated in Fig. 3, this material has a transport rate of 10⁻³ cm²/V s for electrons, while another material with structure shown in Fig. 4 displays superior hole mobility, reaching 4 × 10⁻³ cm²/V s, comparable to traditional hole transport materials, such as aromatic amines [15,16].

Dmitrii et al. [17] synthesized a series of heterocyclic small-molecule compounds derived from fluorene. As indicated in Fig. 5, these kinds of chemical compounds feature a typical D-π-A charge transfer structure. Experimental results indicated that replacing S atoms with Se atoms had little effect on the intramolecular charge transfer of the materials. However, shifting these atoms from the 1,3-positions to the 1,2-positions resulted in a redshift of the emission spectrum, suggesting potential applications of these materials in holographic technologies.

Kim et al. [18] synthesized small-molecule blue emitters via the Suzuki reaction between monobrominated spiral fluorene and anthracene. As shown in Fig. 6, this small molecule has a glass transition temperature of 207 °C, demonstrating good thermal stability. The small molecule blue light material generated by the Suzuki reaction has three rigid planes that share one atom, and the rigid structure reduces the interaction between molecules, thereby improving thermal stability. The material is soluble in common organic solvents, and films prepared by thermal evaporation are smooth and free of pinholes. The incorporation of helical fluorene units into the molecule allows it to maintain an amorphous aggregation state, effectively preventing exciton quenching that adversely affects luminescent performance. The material has a bandgap of 2.181 eV; at a driving voltage of 7 V and a brightness of 300 cd/m², the luminous efficiency is 1.22 lm/W; the maximum emission wavelength is approximately 424 nm, with a full width at half maximum (FWHM) of 48 nm, achieving nearly pure blue light emission. Cyclic voltammetry further confirms that the non-planar structure of the material inhibits the recombination of electrons and holes at the interface between the transport and emission layers, facilitating pure blue light emission.

As indicated in Table 2, Setagesh et al. [20] synthesized a fluorene-based monomer with a novel chromophore, which was then polymerized via a Yamamoto coupling reaction catalyzed by a nickel complex to produce a fluorene homopolymer with a biphenyl side group (Fig. 7).

As indicated in Fig. 7, the large side group in this homopolymer prevents the broadening of the emission range and enhances the color purity of the emitted blue light. The device operates with a driving voltage of less than 4 V, confirming that the side group does not cause significant disruption to the conjugated structure of the fluorene units. Additionally, incorporating highly fluorescent small molecule dyes into the polyfluorene system creates a host-guest energy transfer system. By tuning Forster energy transfer, the emission wavelength of the material can be adjusted, a method that has become effective for modifying emission wavelengths [21]. Researchers have produced a luminescent material with aromatic amide side groups by partially side-chain functionalizing the polyfluorene backbone. As presented in Fig. 8, this structure enables the maximum emission wavelength of the material to shift from 558 nm to 675 nm [22].

Compared to fluorene homopolymers, researchers have delved deeper into the study of fluorene copolymers. On one hand, the flexibility in selecting fluorene copolymer monomers allows for a broader range of adjustments in the material’s highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), thereby regulating the maximum emission wavelength, luminescence efficiency, color purity, and charge carrier transport capabilities to better meet practical application requirements. On the other hand, the limitation in reactive sites may be associated with mutual impacts between the benzene rings on fluorene. The comprehensive luminescent performance of a material is influenced by various factors; solely modifying fluorene by introducing different side groups cannot address all factors simultaneously. For instance, while introducing groups to enhance fluorene solubility and charge carrier transport properties, there may be a risk of disrupting the material’s conjugated structure, leading to diminished luminescent performance or even inhibition of emission. Therefore, solely introducing different side groups to polyfluorene serves a limited "modifying" role; by copolymerizing fluorene with other monomers in binary or ternary systems, it is possible to enhance or broaden the overall luminescent performance of fluorene, leading to the development of superior fluorene-based electroluminescent materials. DOW has extensively researched organic electroluminescent materials, leading to the development of a range of fluorene-based copolymeric luminescent materials and the filing of numerous patents. They have developed luminescent materials through the copolymerization of fluorene with various monomers which were show in Fig. 9. By adjusting the copolymerization ratios of different monomers with fluorene, properties such as solubility, maximum emission wavelength, molecular weight, and thermal stability of the copolymer can be optimized to better meet practical application requirements [23].

The maximum emission wavelength of organic conjugated materials is dictated by their conjugation length; as the conjugation length decreases, the emission wavelength shifts towards the blue. Thus, by adjusting the conjugation length of fluorene materials, high-purity blue light emission can be attained, effectively preventing the red shift commonly observed in conventional fluorene materials. Incorporating a certain proportion of aliphatic hydrocarbons, fatty ethers, or siloxane chains into the main chains of specific conjugated polymers can improve the saturation color purity of their blue light emission. Researchers have successfully achieved this by introducing a specific amount of aliphatic hydrocarbons into the main chain of trapezoidal polymers, which structure is shown in Fig. 10 [24]. The structure of pure trapezoidal polymers is relatively ordered, with molecules exhibiting enhanced planar configurations and higher crystallinity. This allows for easier exciton migration between molecular chains, resulting in the final material displaying two absorption peaks at 461 nm and 600 nm. Introducing non-conjugated aliphatic hydrocarbons allows for the adjustment of the material’s conjugation length, which can partially inhibit the emergence of the latter emission peak and reduce the full width at half maximum (FWHM) of the emission spectrum. Moreover, these approaches offer valuable insights for investigating the electroluminescent mechanisms of organic conjugated materials.

Fluorene, when used as a blue light material, tends to facilitate exciton formation following annealing or electrical current treatment. Miller from BM [25] developed a fluorene-terminated blue light material by incorporating triphenylene into the main chain of polyfluorene. Experiments have demonstrated that the terminal groups play a critical role in the electroluminescence of this material. This compound, after being annealed at 200 °C for 3 days, continues to emit blue light stably. Miller attributes this stability to the ability of triphenylene to trap excitons, thus preventing their migration and subsequent deactivation. Research also indicates that materials with intramolecular cross-linking structures can enhance the overall electroluminescent performance. Inaoka et al. [26] synthesized polyfluorene electroluminescent materials with a novel structure. As indicated in Fig. 11, the fluorene groups on the side chains enhance the polymer’s affinity with other compounds, facilitating the fabrication of multilayer devices. The fluorene side groups reduce the π-bond overlap in the polymer, thereby inhibiting intermolecular exciton migration.

This review highlights further investigations into fluorene-based electroluminescent materials, revealing that research has combined fluorene with thiophene, nitrogen-containing quaternary ammonium salts, or aromatic rings with silicon heteroatoms, as well as polymers featuring p-i-n (positive-interfacial-negative) block structures in their main chains. These materials have shown significant improvements in photonic quantum efficiency, color purity of the emission wavelength, solubility, thermal stability, and tunability of the material’s orbital band gaps [[30], [31], [32], [33], [34]]. A notable example is the copolymerization of helical fluorene with fluorene to produce electroluminescent materials featuring a helical fluorene structure [35]. As the structure shown in Fig. 12, the inclusion of helical fluorene endows the material with excellent thermal stability. When R is octyl, the resulting polymer exhibits good solubility in common organic solvents and favorable film-forming properties. Although the material requires a voltage of 12 V to emit light, it produces high-purity blue light with a photoluminescent efficiency ranging from 42 % to 48 % in film form, and a maximum external quantum efficiency of 0.45 %. Research indicates that the steric hindrance introduced by helical fluorene disrupts the material’s crystallinity, reduces exciton migration deactivation between chains, and thereby enhances the material’s luminescent efficiency.

Wu et al. [36] synthesized fluorene copolymers with oxadiazole side groups. As indicated in Fig. 13, the three-dimensional structure unit with oxadiazole enhances the material’s thermal stability and ensures that the conjugated structure of polyfluorene main chain is not distorted due to steric hindrance, preventing interchain π overlap and exciton deactivation between chains, thereby improving the saturated color purity of blue light emission. Additionally, the transfer of charge carriers from oxadiazole to the main chain can improve the electron affinity of polyfluorene to some extent. This material achieves an external quantum efficiency of 0.52 % at a driving voltage of 714 V, with a brightness of 537 cd/m².

Ultimately, the fabrication of fluorene-based electroluminescent materials containing metal-organic complexes represents a viable method to enhance material performance. According to the spin conservation principle in quantum chemistry, only 25 % of the excitons generated through light excitation are capable of emitting light. By incorporating coordination compounds with heavy metal ions, the strong spin-orbit coupling between the ions and ligands can enable nearly 100 % luminescence of the generated excitons. As one of the primary colors in full-color displays, red has led to the development of a variety of red-emitting materials. However, these materials often produce some mixed light along with red light, with the half-width of the emission wavelength typically ranging from 50 to 200 nm, which does not effectively achieve pure red emission. Pei et al. [37] introduced bipyridine ligands into the side chains of fluorene and phenol copolymers, coordinating them with Eu³⁺, which resulted in a series of luminescent polymer materials featuring various ligands, which is shown in Fig. 14. This class of polymer complexes exhibits good solubility and excellent processing properties; compared to non-coordinated compounds, the thermal stability of the materials has been significantly enhanced. After fabricating light-emitting diode devices, although the driving voltage can reach 15 V, selecting the appropriate ligands allows for the emission of pure red light at 630 nm, with a maximum half-width of approximately 5 nm, marking a notable breakthrough (Table 3).

As summarized in Table 4, Loy et al. [39] synthesized three types of hole transport materials with 2,7-dibromofluorene as the core and a series of aromatic amines serving as the hole transport layer, which structures are presented in Fig. 15. In comparison to similar branched materials with biphenyl as the core, these materials demonstrate the introduction of branched structures changes the intermolecular forces of the material, enhancing thermal stability. The reduced ionization potential of fluorene facilitates hole injection, thereby lowering the energy barrier between the anode and the organic semiconductor layer, which leads to favorable results in device applications.

Bo et al. [41] directly linked the blue light-emitting fluorene oligomers to the porphyrin core via chemical bonds, resulting in a branched star-shaped porphyrin oligomer. As indicated in Fig. 16, the blue light energy absorbed by the fluorene arms is rapidly transferred to the porphyrin core via these chemical bonds, leading to the emission of saturated red light. Comparative experiments demonstrated that when the length of the fluorene arms exceeds a certain threshold, saturated red light can be achieved, enhancing its suitability for device fabrication. The incorporation of fluorene arms significantly improves the solubility of the porphyrin, and the steric effects of the fluorene rings prevent material aggregation in the film, resulting in a photonic quantum efficiency that exceeds twice that of common porphyrin derivatives.

Trisanthene, a highly symmetric polycyclic aromatic hydrocarbon compound, stands as an ideal candidate for synthesizing liquid crystal materials, fullerene derivatives, and asymmetric catalytic materials. With appropriate chemical modifications to trisanthene, serving as a structural building block for novel organic conjugated functional materials, distinct optoelectronic behaviors are expected to emerge. Pei et al. [42] synthesized the highest molecular weight branched electroluminescent material among similar molecules using benzene as the core. As shown in Fig. 17, this compound exhibits solubility in common organic solvents, with maximum absorption and emission wavelengths in tetrahydrofuran solution at 310 nm and 330 nm, making it a promising candidate as a blue light material. The substantial torsional angles between the aromatic ring planes within these branched molecules result in a remarkably slow increase in effective conjugation length with an increase in the number of aromatic rings, allowing for precise tuning of the optoelectronic properties of this conjugated material through chemical modifications.

In 2004, Bo et al. synthesized a homopolymer with alkoxy-substituted helical fluorene units in the side chains. Compared to linear fluorene polymers, this material exhibits a 5 % weight loss temperature of approximately 400 °C, demonstrating excellent thermal stability and good solubility in tetrahydrofuran. Experimental results confirm that the helical structure of this material provides unique advantages in addressing certain defects [45]. Building on the bis-helical fluorene structure, a six-membered helical structure has also been introduced at the 9-position of the fluorene unit. In 2004, Shu et al. also synthesized polyfluorene with cyclohexyl substituents at the C-9 position. The presence of a rigid aliphatic structure inhibited the aggregation of molecular chains, resulting in an increased glass transition temperature and subsequently enhancing both the spectral stability and thermal stability of polyfluorene [46]. In 2004, Kim et al. introduced a helical anthracene structure at the 9-position of fluorene, resulting in a polymer. Due to the rigid structure of the helical anthracene, no glass transition temperature (Tg) was observed below the decomposition temperature (367 °C). This material was then used to fabricate a light-emitting device with the following layer structure: ITO/PEDOT (60 nm)/PEHSAF (70 nm)/LiF (5 nm)/Ca (10 nm)/Ag (150 nm), achieving a maximum luminous brightness of 1600 cd/m² and color coordinates of (0.17, 0.12) [47]. In 2005, Kim et al. synthesized polyfluorene A, which contains nitrogen atoms in its helical structure, and polyfluorene B, which contains oxygen atoms. These polymers emit pure blue light, and their thermal stability has been enhanced. The Tg of polyfluorene B is 205 °C, while no Tg was observed for polyfluorene A below the decomposition temperature. Research indicates that heteroatoms, even when incorporated into the helical structure, can significantly influence the energy levels of the polymer; the HOMO level of polyfluorene A is elevated, enhancing its hole transport capability [48]. In 2004, Shu et al. synthesized an alkoxy-substituted helical [fluorene 9,9-oxyanthracene] homopolyfluorene. Due to the presence of long-chain alkoxy groups, the Tg of this polymer decreased to 149 °C compared to previous products, but it remains significantly higher than that of polyalkyl. The luminous brightness of the device ITO/PEDOT/PSFX/TPBI/Mg:Ag/Ag exceeded 1000 cd/m², with an EQE of 1.33 % [49]. In 2006, Kim further advanced their previous research by synthesizing copolymer 33 of fluorene, incorporating a bulky triphenylamine structure. This polymer exhibits a thermal decomposition temperature exceeding 389 °C and a glass transition temperature ranging from 112 to 207 °C. The above is summarised in Table 5. By adjusting the content of the spiral triphenylamine monomer, the energy levels of the polymer can be fine-tuned. All polymers in this series emit blue light, leading to an enhancement in hole transport performance [50].