Open AccessCCS ChemistryRESEARCH ARTICLES4 Oct 2023

Highly Fluorescent Semiconducting Two-Dimensional Conjugated Polymer Films Achieved by Side-Chain Engineering Showing Large Exciton Diffusion Length

    CCS Chem. 2023, 5, 2366–2377

    Semiconducting two-dimensional conjugated polymers (2DCPs) with strong fluorescence emission have great potential for various optoelectronic applications. However, it is enormously challenging to achieve this goal due to the significant compact interlayer π–π stacking-induced quenching effect in these systems. In this work, we found that highly fluorescent semiconducting 2DCPs can be prepared through an effective side-chain engineering approach in which interlayer spacers are introduced to reduce the fluorescence quenching effect. The obtained two truxene-based 2DCP films that, along with –C6H13 and –C12H25 alkyl side chains as interlayer spacers both demonstrate superior fluorescence properties with a high photoluminescence quantum yield of 5.6% and 14.6%, respectively. These are among the highest values currently reported for 2DCP films. Moreover, an ultralong isotropic quasi-two-dimensional exciton diffusion length constrained in the plane with its highest value approaching 110 nm was revealed by the transient photoluminescence microscopy technique, suggesting that the π-conjugated structure in these truxene-based 2DCP films has effectively been extended. This work can enable a broad exploration of highly fluorescent semiconducting 2DCP films for more deeply fundamental properties and optoelectronic device applications.

    Introduction

    In the 1970s, the discovery of conducting polymers challenged the traditional concept that polymers could only be used as insulators and opened the new research field of organic electronics.1 Since then, thousands of organic optoelectronic materials, including small molecules, polymers, oligomers, and even mesopolymers with unique one-dimensional conjugated structure characterization, have been successfully designed and synthesized, which greatly contribute to the rapid development of organic electronics.25 Until now, the properties of organic optoelectronic materials and devices have been improved considerably, and they demonstrate great potential for various fields, including organic field-effect transistors and circuits,68 organic light-emitting diodes and flexible displays,9,10 organic photovoltaics and energy applications,11 bioelectronics, and other related fields.12 Even so, there are also some bottlenecks for the further development of this field, one of which is how to break through the theoretical limits of optoelectronic performance that is determined by the finite one-dimensional conjugation size and weak intermolecular interactions of current organic optoelectronic materials.13,14

    Two-dimensional conjugated polymers (2DCPs) that have potentially extended conjugation dimensions provide a promising alternative approach to overcoming the above-mentioned bottlenecks and also would produce new photophysical properties.1517 Thus, ever since the concept of two-dimensional polymers was proposed,18 they have been widely applied in various fields of electrochemical energy,19 gas separation and storage,20 and spintronic devices.21 In addition, the synthesis of such 2DCPs with the promise of integrating superior optoelectronic properties from different aspects of design-appropriate functional precursors,22,23 optimizing the preparation method for resulting high crystallinity,2426 has increasingly attracted interest. Currently, some advances have been achieved in this field; however, the optoelectronic properties of most 2DCPs are still relatively poor and far from the initial expectations.27,28 Moreover, the development of multifunctional optoelectronic 2DCPs also faces great challenges. For instance, the highly fluorescent semiconducting 2DCPs that have great potential for chemical sensing,2932 semiconductor devices,33 photodetectors,34,35 and organic light-emitting diodes36 are hard to prepare since the compact interlayer π–π stacking in these systems can induce the significant quenching effect, leading to low or even no fluorescence efficiency.29,37 Currently, several strategies, such as incorporating aggregation-induced emission active units,38,39 eliminating photoinduced electron transfer process,40,41 introducing chromophores,36,42 blocking the nonradiative energy decay pathway through limiting rotational/vibrational relaxation,43,44 and so on,29,30,45,46 have been developed to design and synthesize fluorescent 2DCPs. However, the resulting fluorescent 2DCPs are generally in the powder state and have poor quantum yield and semiconducting characteristics which to some extent hinder their potential applications in related fundamental property studies and organic optoelectronic devices.43

    Herein, we propose an effective side-chain engineering strategy for highly fluorescent semiconducting 2DCPs (Figure 1). The basic design principle is consciously incorporating a spacer at the central building unit to produce an enlarged interlayer stacking distance, thus effectively eliminating the aggregation-induced fluorescence quenching effect in common 2DCPs. Moreover, such 2DCPs with an enlarged interlayer distance and in-plane extended π-conjugation structure would also enable a preferably effective two-dimensional charge transport and exciton diffusion process in the constrained conjugated plane. To achieve this goal, in our study, two triaxially symmetrical, strong fluorescence emissive truxene-based molecules with –C6H13 and –C12H25 alkyl side chains are separately synthesized as the building components (abbreviated as C6-truxene monomer and C12-truxene monomer, separately) for the preparation of fluorescent 2DCPs. A larger interlayer stacking distance with a value of 17.02 and 32.20 Å is estimated for C6-truxene and C12-truxene-based 2DCPs which results in the obvious improvement of the fluorescence property with photoluminescence quantum yield (PLQY) in film of 5.6% and 14.9%, respectively. Furthermore, ultralong exciton diffusion lengths with the highest value approaching 110 nm are also revealed by the transient photoluminescence microscopy (TPLM) technique,47 which are much higher than those of common organic semiconductor materials ( Supporting Information Table S1), suggesting the highly extended π-conjugated structures in these obtained 2DCP systems, enabling an effective charge transport process. This work opens a new avenue for a broad exploration of highly fluorescent semiconducting 2DCP films for more deep fundamental properties and related optoelectronic device applications.

    Figure 1

    Figure 1 | Schematic diagram of side-chain engineering for designing fluorescent 2DCPs.

    Experimental Methods

    C6-truxene 2DCP synthesis

    CuCl (0.050mmol, purchased from J&K Scientific, Beijing, China) was added to a solution of C6-truxene monomer (11.9 mg, 0.013 mmol, the 1H and 13C nuclear magnetic resonance (NMR) spectroscopy are shown in Supporting Information Figures S10 and S12) in acetone/tetrahydrofuran (THF)/N,N,N′,N′-tetramethylethylenediamine (TMEDA) (10 mL∶10 mL∶1 mL). Then the mixture was stirred in the air for 96 h. After that, the product was washed by CH3OH/NH3·H2O (40 mL/1 mL) and acetone twice. Finally, the C6-truxene films were successfully prepared. The prepared C6-truxene films are freestanding and can be easily transferred onto the surface of Si/SiO2 wafers, copper grids, and other substrates. The samples were rinsed with ammonia water, methanol, and then acetone three times to remove residual catalyst and precursors from the surface of truxene-2DCP films for further characterization.

    C12-truxene 2DCP synthesis

    CuCl (0.050mmol, purchased from J&K Scientific, Beijing, China) was added to a solution of C12-truxene monomer (18.5 mg, 0.013 mmol, the 1H and 13C NMR spectroscopy are shown in Supporting Information Figures S11 and S13) in acetone/THF/TMEDA (10 mL∶10 mL∶1 mL). Then the mixture was stirred in the air for 96 h. Following that, the product was washed by CH3OH/NH3·H2O (40 mL/1 mL) and acetone twice. Finally, the C12-truxene films were successfully prepared. The prepared C12-truxene films are freestanding and can be easily transferred onto the surface of Si/SiO2 wafers, copper grids, and other substrates. To remove residual catalyst and precursors from the surface of truxene-2DCP films for further characterization, the samples were rinsed three times with ammonia water, methanol, and then acetone.

    Results and Discussion

    Adopting monomers with excellent luminescent properties is the central premise for the preparation of luminescent 2DCPs.41,48 Here, a building block of 10,15-dihydro-5H-diindeno[1,2-a;1′,2′-c]fluorene (truxene, Figure 2a) with triple symmetric C≡C connecting knots was selected as our research target due to its intrinsically strong fluorescence feature49 and ideal fused-conjugated molecular structure,50,51 beneficial for 2D extended π-conjugation in 2DCPs. To reduce the aggregation-induced quenching effect while ensuring polymerization efficiency, a large interlayer distance in 2DCPs for high fluorescence was obtained by consciously incorporating the long alkyl side chains, such as the –C6H13 group and the –C12H25 group, into the truxene central unit. Accordingly, two new truxene-based monomers of 2,7,12-triethynyl-5,5,10,10,15,15-hexahexyl-10,15-dihydro-5H-diindeno[1,2-a:1',2'-c]fluorene (C6-truxene monomer) and 5,5,10,10,15,15-hexadodecyl-2,7,12-triethynyl-10,15-dihydro-5H-diindeno[1,2-a:1',2'-c]fluorene (C12-truxene monomer) were successfully synthesized ( Supporting Information Figure S1a,b). Simulated results regarding the side-chain effect on the molecular structures (as shown in Figure 2b,c) clearly reveal that the side chains are preferably perpendicular to the main building block plane in the individual truxene molecule, giving a relatively large axial length of 17.04 Å for the C6-truxene molecule and 32.03 Å for the C12-truxene molecule, respectively, which were used as the effective spacers for modulating interfacial distances in the following 2DCPs. Rhabdolith crystals and sphaerocrystals were obtained for C6-truxene and C12-truxene monomers, respectively, by optimizing the solvent evaporation process, which demonstrated strong blue emission under ultraviolet (UV) light (Figure 2d,e) and a typical layer-by-layer growth model as characterized by X-ray diffraction (XRD). A layer distance of 18.9 Å for the C6-truxene monomer and 33.4 Å for the C12-truxene monomer (Figure 2f) was obtained, which was quite consistent with the simulated results. Particularly, the in-plane molecular arrangement of C6-truxene monomer was confirmed by the high-resolution atomic force microscopy (HRAFM) image (Figure 2g). Accordingly, the lattice parameters were estimated as 16.39 and 18.86 Å, respectively, through the corresponding Fourier transform pattern (Figure 2g, inset), which are quite consistent with that of crystal lattices based on single crystal data. Furthermore, high-quality single-crystal data were also obtained for the C6-truxene monomer based on its large single crystals. Based on the single-crystal data (CCDC: 2190942, Supporting Information Table S3), more detailed structure packing showed that C6-truxene monomer belonged to the P 21 21 21 space group with crystal parameters of a = 16.44 Å, b = 18.64 Å, c = 18.94 Å, and α = 90.00°, β = 90.00°, γ = 90.00°. Additionally, it adopted a typical AB-stacking mode, in which two adjacent molecules are oriented in opposite directions in the ab crystal plane (Figure 2h). Along the c-axis, the interlayer distance for C6-truxene monomers in single crystal was estimated to be around 18.94 Å (Figure 2i), which was very close to the simulated axial length, allowing for a complete alienation between layers and further weakening of the interlayer interactions. Unfortunately, we failed to produce high-quality crystals suitable for single-crystal analysis of C12-truxene monomer after many attempts, which were mainly attributed to its long alkyl chain groups. Even though, when compared to C6-truxene monomer, the interlayer distance of C12-truxene monomer was assumed to be further enhanced to around 32.03 Å according to the simulation results. These features provided a good prerequisite for further modulating their corresponding interfacial distances in 2DCPs.

    Figure 2

    Figure 2 | (a) Chemical structures of truxene-based monomer with different alkyl side-chain groups of –C6H13 and –C12H25, respectively. Simulated axial length of (b) C6-truxene monomer and (c) C12-truxene monomer with side chain extending perpendicular to the main conjugated plane. UV fluorescent image of (d) C6-truxene monomer and (e) C12-truxene monomer. (f) XRD pattern of C6-truxene monomer (blue line) and C12-truxene monomer (red line). (g) HRAFM image of the surface of C6-truxene monomer single crystal and its corresponding Fourier transform image (inset). The a and c axes are labeled with white arrows. According to the results of single-crystal analysis, the molecular packing structure of C6-truxene monomer single crystal, respectively, observed perpendicular to (h) ab crystal plane and (i) ac crystal plane.

    Through the Glaser–Hay coupling process, the precursors C6-truxene monomer and C12-truxene monomer, which have alkynyl (–C≡CH) as linking knots, can be bonded to produce the corresponding 2DCPs. Traditional Glaser–Hay coupling reaction was carried out in the bulk phase, generally resulting in powder-state products. To obtain film-state 2DCPs, an improved synthetic strategy of Glaser–Hay coupling reaction (the synthetic route is illustrated in Supporting Information Figure S2) at a dynamic air/liquid interface was developed. Figure 3a depicts the essential advancements of this strategy with details in the following: (1) efficient dehydrogenation reactions occur in the liquid phase under the assistance of the CuCl-TMEDA catalyst; (2) dehydrogenated monomers are continuously supplied towards the interface from the liquid phase since the dehydrogenation is ongoing; (3) the diffusion and flux of reactants are determined by their concentration gradient; and (4) oxygen participates in the polymerization in a confined space, induced by the air/liquid interface to form film-state 2DCPs. As a result, instead of generating 2DCP powder, 2DCP films were prepared effectively thanks to the deliberately designed interface and gentle reaction procedure. The 2DCPs prepared by the C6-truxene monomer was named the C6-truxene 2DCP. Meanwhile, the 2DCPs prepared by the C12-truxene monomer was denoted as C12-truxene 2DCP (as shown in Figure 3b). Before the Glaser–Hay coupling reaction, the mixed solution exhibited a homogeneous transparent dark green color. As the reaction proceeded, the truxene-2DCP films precipitated onto the liquid surface and adhere to the walls of the reaction glass bottle.

    Figure 3

    Figure 3 | (a) Dynamic air/liquid interface Glaser–Hay coupling reaction for preparing truxene-based 2DCP films. (b) The proposed molecular structure of truxene-based 2DCPs. Optical microscopic images and UV fluorescent images of the as-prepared (c and d) C6-truxene 2DCP films and (e and f) C12-truxene 2DCP films. TEM and AFM images of (g and h) C6-truxene 2DCP films and (i and j) C12-truxene 2DCP films.

    Optical and fluorescent microscopic images were obtained to examine the morphologies and optical properties of the C6-truxene monomer, the C12-truxene monomer, and the corresponding 2DCP films. From the optical microscopic and UV fluorescent images of monomers (as shown in Figure 2d,e), it is clear that under UV light irradiation, the powder-state C6-truxene monomer emitted blue fluorescence as well as C12-truxene monomer. Differently, C6-truxene 2DCP film and C12-truxene 2DCP film exhibited a green fluorescent emission feature under the same UV light (as shown in Figure 3cf), suggesting the highly extended 2D π-conjugation structure. Moreover, the 2DCP films also showed remarkable stability in several organic solvents, including acetone, methanol, and ammonia water, further demonstrating their solid networks and high degree of polymerization. The intrinsic and surface morphologies of the C6-truxene 2DCP film (Figure 3g,h) and the C12-truxene 2DCP film (Figure 3i,j) were further characterized using transmission electron microscopy (TEM) and atomic force microscopy (AFM). Well-defined film features with thicknesses of 30.4 and 31.6 nm and roughness of 1.3 and 2.0 nm, respectively, were obtained (Figure 3h,j), showing a certain polycrystalline property as evidenced by the out-of-plane one-dimensional grazing incident X-ray diffraction (1D GIXRD) results ( Supporting Information Figure S3).

    After investigating the morphology and the fundamental fluorescence emission properties of the obtained 2DCP films, the chemical structures of obtained 2DCP films were further evaluated. The vibrational frequencies calculated by density functional theory (DFT)-based calculation on the B3LYP/6-31g(d,p) level52 using Gaussian 16 and GaussView 6 programs for the monomer were assigned to Raman and infrared (IR) spectra, as shown in Supporting Information Figure S4. The calculated vibrational frequencies were scaled by a factor of 0.961 to fit the experimental data.53 As a result, the Raman spectra of the C6-truxene monomer ( Supporting Information Figure S4a) and the C12-truxene monomer ( Supporting Information Figure S4b) exhibited two distinct main peaks located at 1594 and 2135 cm−1, and can be attributed to the conjugated structure (1594 cm−1) and alkynyl groups (2135 cm−1) in-plane stretching vibration mode (as shown in Supporting Information Figure S4c,d), respectively. Similarly, the in-plane stretching vibration characteristic peaks of the alkynyl groups (2135 cm−1) were also observed in the IR spectra of C6-truxene monomer ( Supporting Information Figure S4e) and C12-truxene monomer ( Supporting Information Figure S4f). Different from Raman spectra, the multipeaks from 2880 to 3025 cm−1 in the both monomers’ IR spectra were derived from the vibration of hydrogen atoms in the alkyl chains, and the sharp peak at 3354 cm−1 resulted from the in-plane stretching vibration of the hydrogen atoms in alkynyl groups ( Supporting Information Figure S4g,h).

    Based on the above analysis, it can be seen that the alkyl chains had a negligible contribution to Raman activity. Therefore, the models of monomer and dimer without alkyl chains were constructed (corresponding chemical structure is presented in Supporting Information Figure S5) to calculate the Raman spectra of these two structures, and the calculated vibrational frequencies were scaled by a factor of 0.961. Similar to the C6-truxene monomer and the C12-truxene monomer, the in-plane stretching vibration characteristic Raman peaks of conjugated structures and alkynyl groups of both monomer ( Supporting Information Figure S5a) and dimer ( Supporting Information Figure S5b) were all located at 1594 and 2135 cm−1 (as shown in Figure 4a). In contrast, the dimer produces a new peak at 2220 cm−1 which can be attributed to the butadiyn structure in-plane stretching vibration (Figure 4b). To examine the feasibility of the experimental method we used to prepare truxene-2DCP film, the monomers as well as the truxene-2DCP films were experimentally characterized by Raman and IR spectroscopy. As shown in Figure 4c,d, compared with the –C≡C– Raman peaks within C6-truxene monomer (located at 2103 cm−1) and C12-truxene monomer (located at 2105 cm−1), the two truxene-2DCP films show distinct characteristic Raman peaks of –C≡C–C≡C– stretching (for C6-truxene 2DCP located at 2208 cm−1 and C12-truxene 2DCP located at 2210 cm−1) after the Glaser–Hay coupling reaction. Additionally, both of the monomers and polymer films show the characteristic Raman peaks of the conjugated vibrations near 1600 cm−1. The IR spectra of monomer and polymer films are shown in Figure 4e,f. From the characterization results, the peaks of alkynyl and alkynyl hydrogen are clearly exhibited in the IR spectra of the C6-truxene monomer and the C12-truxene monomer. However, in the C6-truxene 2DCP and C12-truxene 2DCP after the Glaser–Hay coupling reaction, these two peaks disappeared significantly, demonstrating the fully polymerized conjugated structures. Furthermore, X-ray photoelectron spectroscopy was also performed on truxene-2DCP films to characterize the elemental composition and their chemical states ( Supporting Information Figure S6a), The result shows that the truxene-2DCP films consist of elemental carbon while the appearance of elemental silicon and oxygen can be attributed to Si/SiO2 substrate. Deconvolution and curve fitting of the C 1s core level spectra ( Supporting Information Figure S6b,c) display the major fractions of C–C (near 285.0 eV), C=C (near 284.3 eV), and C≡C (near 286.0 eV). These observed signals agree well with the theoretical simulation results, and these characterizations demonstrate that the films taken out from the air/liquid interface are identical with the expected molecular structure of truxene-2DCP.

    Figure 4

    Figure 4 | Simulated Raman spectra of (a) monomer and dimer without alkyl chains and (b) its corresponding stretching vibration structure of the dimer. Experimental Raman spectra of (c) C6-truxene monomer, C6-truxene 2DCP and (d) C12-truxene monomer, C12-truxene 2DCP. Experimental IR spectra of (e) C6-truxene monomer, C6-truxene 2DCP, and (f) C12-truxene monomer, C12-truxene 2DCP.

    To investigate the fluorescent semiconducting property of truxene-2DCP films, exciton diffusion characterization was further perfomed. Obviously, after the polymerization, obvious bathochromic shifts in both UV–vis absorption (Figure 5a, and the state of the sample used on UV–vis absorption characterization is shown in Supporting Information Figure S7) and solid-state fluorescence spectra (Figure 5b) were observed compared with the spectra of their monomers. For the cases of C6-truxene monomer and C12-truxene monomer, their redshifts after polymerization are almost identical, specifically, about 70 nm for UV–vis absorption, and about 105 nm for solid-state fluorescence spectra, respectively. These results confirm the effectively extended delocalization of π-electrons in the resulting 2DCPs networks. Semiconducting bandgaps of the 2DCPs can also be calculated from the tangent of UV–vis spectra, 2.97 eV for C6-truxene 2DCP and 2.95 eV C12-truxene 2DCP, respectively. A strong green fluorescence emission can be observed in 2DCPs (Figure 5b), which is largely different from the blue fluorescence emission given by monomer single crystal. As shown in Figure 5c, two types of monomers have similar PLQY, up to 33.2% for C6-truxene monomer and up to 37.6% for C12-truxene monomer in methanol solution. In contrast, a significant difference was observed for 2DCP films in which C6-truxene 2DCPs performed a PLQY of 5.6% whereas C12-truxene 2DCPs showed a PLQY of 14.9%. This phenomenon can be explained by the alky chain-induced weaker interlayer interaction. The prepared 2DCPs tended to form a layer-by-layer morphology because of the prolonged epitaxial growth of monomers along two in-plane directions. The axial lengths of polymers were simulated as shown in Figure 5d and Supporting Information Figure S8. The presence of alkyl chains significantly increases the distance along the direction perpendicular to the conjugation plane of the polymers, further decreasing the interlayer fluorescence quenching. The C6-truxene 2DCP and C12-truxene 2DCP films, respectively, have exceptionally high PLQY values of 5.6% and 14.9%, which are amongst of the highest PLQY values for 2DCPs reported to date (Figure 5e and Supporting Information Table S2) and also the first-reported fluorescent 2DCP samples in the film state.

    Figure 5

    Figure 5 | (a) UV–vis absorption, (b) solid-state fluorescence spectra, and (c) PLQY of C6-truxene monomer, C6-truxene 2DCP and C12-truxene monomer, C12-truxene 2DCP. (d) Schematic diagram of simulated interlayer structure and interlayer distance of C6-truxene 2DCP and C12-truxene 2DCP. (e) A summary map of PLQY and maximum emission peak reported in the literature based on solid-state 2DCPs.

    Exciton diffusion length in organic semiconductors is the average distance that an exciton can migrate during its lifetime, which plays an essential role in the performance of optoelectronic devices.54 The ability of exciton diffusion is strongly dependent on molecular stacking, the degree of crystallinity, and the density of energetic traps. Many strategies, such as increasing the structural order and elimination of exciton quenching defects, have been reported for increasing the exciton diffusion length,55 Among the organic semiconductors, 2DCPs are expected to exhibit distinct long-range in-plane exciton diffusion with their remarkable extended conjugated molecular structure and stacking order. The exciton diffusion length can be determined by different methods, including exciton quenching at a surface or an interface, exciton–exciton annihilation, microwave conductivity, and spatial transient optical techniques.56 Among them, TPLM is one of the representative optical techniques for characterizing the in-plane exciton diffusion properties with the unique merits of noncontact and in situ testing.57 Therefore, the exciton diffusion of C6-truxene 2DCP and C12-truxene 2DCP films was investigated based on TPLM technique as illustrated in Figure 6a. The π-conjugated structure was successfully extended according to the highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) topologies of the dimer without alkyl chains, as shown in Figure 6b, which is beneficial for the exciton diffusion in the constrained conjugated plane. The solid-state fluorescence spectra of C6-truxene 2DCP and C12-truxene 2DCP films, which were measured at a total of 20 different places and display features consistent with Figure 5b, are presented in Supporting Information Figure S9a,b. The typical spatial and temporal evolutions of the fluorescence intensity distribution that is normalized at each time slice of the C6-truxene 2DCP and C12-truxene 2DCP films are shown in Figure 6c,f, respectively. To further analyze the exciton diffusion property, the time-dependent mean-square distribution (MSD) was derived from a Gaussian fitting of the emission profile at different decay times (Figure 6d,g). The MSD curve was linearly fitted with a similar slope at different detecting positions (Figure 6d,g), indicating the isotropic transport property along in-plane directions. According to the MSD model of MSD = 2Dt, the exciton diffusion coefficient (D) was extracted as 0.05 and 0.04 cm2/s for C6-truxene 2DCP and C12-truxene 2DCP films, respectively.41 Furthermore, the singlet exciton diffusion length can be extracted based on the relationship with the fluorescence lifetime (τ) and diffusion coefficient: L D = 2 D τ . Accordingly, the exciton diffusion lengths were obtained for C6-truxene 2DCP and C12-truxene 2DCP films (Figure 6e,h and Supporting Information Figure S9c,d). The average diffusion lengths for C6-truxene 2DCP and C12-truxene 2DCP films are 83.35 and 71.91 nm, respectively. The small distribution of the exciton diffusion length at 20 random positions for each sample further confirmed the isotropic transport property along in-plane directions. More excitingly, the maximum exciton diffusion lengths that approach 110 and 90 nm were achieved for C6-truxene 2DCP and C12-truxene 2DCP films, which are among the highest values for organic semiconductors ( Supporting Information Table S1). These values may be further increased by improving the crystallinity of 2DCP films. The large exciton diffusion length observed in these 2DCP films suggests their potentially decent carrier transport properties, demonstrating their great promise for constructing high performance organic optoelectronic devices.

    Figure 6

    Figure 6 | (a) Schematic diagram of exciton diffusion study in truxene-2DCP films. (b) DFT-calculated (B3LYP/6-31g(d,p) level) HOMO–LUMO topologies of the dimer without alkyl chains. (c) Map of emission intensity as it evolves in space and time. (d) Time evolution of MSD of singlet excitons showing good linearity. (e) Statistical histogram of exciton diffusion distance of C6-truxene 2DCP. (f) Map of emission intensity as it evolves in space and time. (g) Time evolution of the singlet exciton MSD of singlet excitons showing good linearity. (h) Statistical histogram of exciton diffusion distance of C12-truxene 2DCP.

    Conclusion

    Here, we present two strong fluorescent semiconducting 2DCPs based on truxene building blocks via a side-chain engineering approach. By incorporating different side chains of -C6H13 and –C12H25 groups into truxene as the interfacial spacers, obviously increased PLQY with a high value from 5.6% to 14.9% was obtained, which are amongst the highest values reported so far in this field. Through an improved Glaser–Hay reaction, the obtained truxene-based 2DCPs films could be as large as several hundred micrometers with distinct green fluorescence, nearly 100 nm redshift, compared with their corresponding monomer, due to the highly extended 2D conjugated structure. Moreover, TPLM technology reveals an extremely long, isotropic, quasi-two-dimensional exciton diffusion length restricted in the plane with a maximal value approaching 110 nm, suggesting an extended conjugation structure in these truxene-based 2DCP films. This work provides an effective approach for synthesizing highly fluorescent semiconducting 2DCPs films, which will extend the application of such materials in more fields, especially in more deep-fundamental properties in investigation and optoelectronic devices.

    Supporting Information

    Supporting Information is available and includes materials and instrumentations, substrate treatment, computational methods, monomer synthesis, characterization, and NMR spectra.

    Conflict of Interest

    There is no conflict of interest to report.

    Acknowledgments

    This work was financially supported by the Ministry of Science and Technology of China (grant nos. 2018YFA0703200 and 2022YFB3603800), the Natural Science Foundation of China (grant nos. 21875259, 52233010, 51725304, 61890943, and 22021002), the CAS Project for Young Scientists in Basic Research (grant no. YSBR-053), the Youth Innovation Promotion Association of the Chinese Academy of Sciences, the National Program for Support of Top-notch Young Professionals, the Beijing National Laboratory for Molecular Sciences (grant no. BNLMS-CXXM-202012), and the Key Research Program of the Chinese Academy of Sciences (grant no. XDPB13), K.C. Wong Education Foundation (grant no. GJTD-2020-02). The authors acknowledge Hongxiang Li for his help on 1D GIXRD data collection and Zhiwen Liu for his help on HRAFM data collection.

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