1 Introduction
Organic semiconductor materials have attracted much attention due to their rich structure, tunable energy level, large area of solution processing devices, thin and flexible, and are widely used in luminescence, laser, storage, computing, energy and other fields
[1⇓⇓~4]. steric design, rather than donor-acceptor (p-n/D-a) design, is an indispensable and fundamental method to regulate the molecular structure, conformation, arrangement, assembly behavior and film morphology of organic semiconductors, and to affect the construction of multi-dimensional organic semiconductor materials. Among the various spatial design methods, molecular tension is one of the special regulation methods, which can eventually form a new class of high-tension organic semiconductor materials
[5]. Due to the angular tension (E
angle), twist/bend tension (E
torsion/bend) and steric tension (E
steric) generated in the molecule, the highly strained molecule has a special p orbital arrangement reorganization and configuration change, which has an impact on its optoelectronic, mechanical and other properties and shows different properties, so the intramolecular tension has become an important method to regulate the molecular properties. At present, it has attracted wide attention in the fields of synthetic chemistry, nanochemistry, organometallic chemistry, supramolecular chemistry and organic electronics
[6][7][8][9][10]. The effects of tension on the properties of materials range from molecular topology to aggregation morphology. High tension makes some molecular structures change from two-dimensional plane to three-dimensional structure, which can not only affect the structure of functional groups and electronic arrangement in molecules microscopically, but also realize the precise regulation of physical and chemical properties and photoelectric properties of materials macroscopically.
In recent years, a series of breakthroughs have been made in the study of high-tensile organic semiconductors, and the molecular species of high-tensile organic semiconductors have increased dramatically, such as carbon nanorings, carbon nanoribbons, fused rings, cage-like molecules, catenanes and other twisted molecules, which make it possible to explore high-tensile organic semiconductor materials
[11,12][13,14][15,16][17,18][19][20⇓⇓~23]. In order to further clarify the role and value of tension in organic semiconductor materials, this paper first describes the classification, theoretical and experimental quantification, and visualization of molecular tension. The ways of introducing molecular tension into organic semiconductor materials are summarized, and the effects of molecular tension on the photoelectric properties of materials are emphasized. Finally, the application of this kind of materials in the field of organic electronics is summarized, the existing problems of this kind of materials are analyzed, and the future research focus of this material is prospected.
2 Types, Quantification and Visualization of Molecular Tension
2.1 Type of molecular tension
According to the theory of molecular mechanics, when the distance between atoms or molecular groups in a molecule is too close, the space repulsion force is molecular tension. As far as a molecule dominated by carbon and hydrogen is concerned, according to the influence of molecular tension on molecular structure, it is mainly divided into stretching tension, twisting tension, bending tension and space tension, as shown in Figure 1. Therefore, the sum of the tensions within the molecule was calculated as :Estrain=Estretch+Etorsion/bend+Esteric+……. Here, the stretching energy is the energy corresponding to the change of the relatively ideal bond length, the twisting tension energy is the energy change caused by the bond rotation, and the bending tension is the corresponding energy caused by the bond or the bending in the corner plane. For steric tension, it mainly refers to the non-bonding interaction of atoms or groups with interaction, which is not directly connected, usually refers to the interaction distance of four bonds. If the non-bonded atoms or groups are too close, steric tension can also occur in the most stable configuration of the molecule.
As far as the practical situation is concerned, the molecular tension is composed of the tensions of the above multiple primitives, and many other types of tensions can be considered as the combination of the basic tensions. For example, the angular tension is a compound tension, which is caused by the difference between the real bond angle and the ideal geometric angle. When a bond in a molecule is distorted, it will bring about changes in bond angle and bond length at the same time, thus producing angular tension, distortion tension or spatial tension. All kinds of molecular tension can lead to changes in bond length or bond angle or dihedral angle to a certain extent. These tensions are interdependent and interactive. Another example is ring strain (RSE), which is the change of bond length, bond angle and dihedral angle in a molecule through cyclization, resulting in angular strain, twist/bend strain and space strain, and the sum of these strains is called ring strain
[24].
2.2 Quantification of the magnitude of molecular tension
In order to measure the magnitude of molecular tension, various experimental and theoretical methods have been established. For example, tension can be determined by combustion analysis experiments. Because the experimental method needs to burn the existing tension semiconductor materials, which has certain limitations, the most commonly used method to measure the tension is through theoretical calculation, especially for the tension quantification of unknown compounds. According to the literature, the following virtual reactions are mainly designed to obtain the tension by calculating the energy difference between the reactant and the product: (1) isogyric reaction
[25]; (2) isodesmic reaction
[26]; (3) homodesmotic reaction
[27,28]. Specifically, the isogyric reaction is based on keeping the number of electron pairs on both sides of the reaction the same; The isodesmic reaction can be assigned to a subset of the isogyric reaction, in which the type and number of bonds involved on both sides of the reaction equation remain the same; The homodesmotic reaction starts from the orbital hybridization mode, and the type and number of hybridized carbon and carbon-hydrogen bonds on both sides of the equation remain the same. In order to further visualize the differences among the three calculation methods, taking n-hexane as an example, the reaction equation for calculating the tension is designed according to the reaction principles of isogyric, isodesmic and homodesmotic, as shown in Figure 2. It can be seen from the figure that the homodesmotic reaction method is more accurate than the above two methods in calculating the tension because it fully considers the information such as the types of various bonds. According to the concept of homodesmotic reaction, a series of other reactions for calculating tension have been derived, such as hyperhomodesmotic, semihomodesmotic, quasihomodesmotic, isogeitonic, isoplesiotic, homoplesiotic and s-homodesmoic reactions
[29].
In addition to the above methods, Haddon et al. Invented the π-orbital axial vector angle (POAV) analysis method to measure the molecular tension of curved surface tension organic semiconductors
[30⇓⇓~33]. The POAV method is mainly used to quantitatively measure the bending degree of a carbon atom and a non-planar or bent conjugated system by establishing a vector with the same angle with the adjacent σ bond, and the tension is measured by the bending degree. As shown in fig. 3, this method is also known as the POVA1 method
[33]. In an ideal case, the Θ
σπ of the sp
2 carbon center is 90 °, and the Θ
σπ of the sp
3 hybrid carbon with a tetrahedral structure is 109.5 °.
2.3 Visualization of molecular tension
The molecular tension can be treated as van der Waals force. Van der Waals force is a weak interaction force, which can be analyzed and visualized by existing theoretical calculation methods. According to the quantum chemical model, this force can be studied by reduced density gradient (RDG) analysis, charge density, electrostatic potential, electron localization function (ELF), and atom in molecule (AIM) topological analysis
[34][35][36][37][38]. Among them, the RDG analysis method proposed by Yang Weitao has been widely adopted
[34]. Where the RDG function is defined as:
Where ▽ is the gradient operator and | ▽ ρ (R) | is the modulus of the electron density gradient. The weak interaction region can be visualized by calculating the RDG function values in the "weak interaction region" and "near the chemical bond". The values of RDG function and | ▽ ρ (R) | in this region are relatively small and difficult to distinguish, but the values of ρ (R) are different. Therefore, by using the RDG function and the ρ (R) function in combination, it is possible to determine that weak interactions are involved in the molecular region. Combined with the sign(λ
2) function to determine its type, where the sign(λ
2) function is the second largest eigenvalue λ
2 sign of the electron density Hessian matrix, the location, type and strength of the weak interaction can be shown by projecting the sign(λ
2)ρ function onto the RDG isosurface. Both the NCIPLOT and Multiwfn programs were designed to visualize weak interactions using this principle
[34][39]. RDG vs.sign(λ
2)ρ scatter plots were calculated by this software, and different weak interactions, including tension, were visualized by VMD software. Fig. 4 is a lattice diagram (fig. 4B) and a weak interaction visualization diagram (fig. 4C) of the azafluorene derivative (1) optimized based on density functional theory
[40]. In Figure 4C, the green region represents intramolecular π-π stacking (black circles), the blue region represents weak attraction between molecules (red circles), and the red region represents intramolecular tension (yellow circles).
It is known in Section 2.2 that the magnitude of the tensile energy of a molecule is usually quantified by the difference in energy between the tensile molecule and its corresponding non-tensile polymer or monomer. Recently, Jasti et al. Fragmented the molecule step by step to make it gradually become a non-tensile structure, and confirmed the total tensile energy by comparing the coordinates of each step of the molecule and calculating the local tensile energy
[41]. The total intramolecular strain is calculated as:
Where F is the estimated force magnitude given to each coordinate by the algorithm, and △ X is the change value of the coordinate. The StrainViz software developed by them can accurately visualize the local tension of molecules. As shown in Fig. 5A, B, trans-bicyclo [6.1.0] nonene (2) and trans-cyclooctene (3), the difference in total tension energy between the two is very small (1.5 kcal·mol
-1), but the reaction rate of 2 is 160 times that of 3, which is more reactive
[41]. It can be seen from the figure that when the cyclopropyl structure is added to the ring, the rigidity of the ring is increased, and the distribution of tension is changed, so that the molecular tension is more concentrated on the olefin, and the reactivity of the molecule is enhanced.
3 Molecular Design Strategy of High Strain Conjugated Aromatic Hydrocarbon
Organic semiconductor material systems are mainly π conjugated molecular systems, so this section mainly takes the introduction of tension into conjugated aromatic molecular systems as an example to illustrate the method of introducing tension into organic semiconductor systems. According to the different ways of introduction, it is mainly divided into the following four methods:
3.1 Introduction of non-six-ring structure
This kind of molecule is mainly aimed at condensed ring molecules. When more non-six-ring structures are introduced into the conjugated system of this kind of molecule, the non-planarization of the molecule increases, resulting in tension. For example, in [6] cycloalkene (4), the structure of this kind of compound meets the requirements of bond length and bond angle of sp
2 carbon, the molecular structure is two-dimensional planar, and the tension between benzene rings can be ignored
[42]. However, when thiophene is introduced into [6] cycloalkene, the structure of the compound is distorted due to the presence of tension (fig. 6, 5)
[43]. When it becomes [5] cycloalkene (6), because of the five-membered ring at the bottom, the tension makes it bowl-shaped, also known as bowl-alkene. By POVA analysis, the maximum Θ
σπ of the middle six carbons is 8.2 °
[44]; [7] The structure of cyclene (7) is twisted into a boat shape due to the seven rings at the bottom, showing a shallow saddle shape
[45]; [8] The bottom of cycloalkene (8) is an eight-circle ring, which shows a deep saddle structure
[46]. The tension of these molecules is mainly related to angle tension, ring tension and twist tension. In contrast to [6] ringene, [7] ringene (C
2) has a tensile energy of 58.3 kcal·mol
-1,[8] and ringene (D
6h) has a tensile energy of 109.7 kcal·mol
-1.
3.2 Introducing large sterically hindered group
In the fjord or crowded region of the molecule, the introduction of large steric hindrance groups can force it to produce greater steric tension. For example, biphenyl compounds, through the introduction of bulky groups at the 2,2 '-position and 6,6' -position of biphenyl, increase the intramolecular dihedral angle and make the molecule chiral
[47]. In compounds 9 – 11, the hydrogen atoms on the anthracene group and the substituents on the benzene ring are overcrowded, resulting in strong 1,3 and 1,4-interatomic van der Waals repulsion, which leads to the distortion of the anthracene skeleton and exhibits a non-planar configuration. With the increase of the number of substituents and fused rings, the degree of twist of the molecule increases, and the total twist angle in molecule 11 is as high as 144 °, as shown in fig. 7
[48,49].
3.3 Construction of large π conjugate structure
This kind of molecule is mainly aimed at condensed ring molecules. When more non-six-ring structures are introduced into the conjugated system of this kind of molecule, the non-planarization of the molecule increases, resulting in tension. As in the large π-conjugated structure of [6] ringene (4), the intramolecular atoms are overcrowded, creating tension. Another example is helicene compounds, with the increase of molecular conjugation system, the molecule changes from the initial planar structure to the three-dimensional helix configuration, resulting in axial chirality according to the different helix directions
[50,51]. In 2015, Kamikawa et al. Synthesized double-helicene molecules 12 and 13 by using the palladium-catalyzed coupling method, as shown in Fig. 8. From the single crystal structure, it can be observed that with the increase of the number of rings, the intramolecular tension of helicene increases, and the angle between the relevant planes in the molecule changes from 41.8 ° to 47.8 °, and the degree of non-planarity increases
[52].
3.4 Constructing novel topological conjugate structures
In recent years, various research groups have forcibly constructed various molecules with topological conjugated structures such as rings and cages through chemical bonds, which has sharply increased the molecular tension within these molecules and shown some special photoelectric properties, which have been widely concerned. Different from the windmill type fluorene lattice C1-1 (tension energy ≤2.29 kcal·mol
-1) and well-shaped fluorene lattice GSF (tension energy ≤7.40 kcal·mol
-1) designed by the lattice method, the macrocyclization can often lead to an increase in tension energy, such as the phenyl-series p-cyclophenylene ( [n] CPPs) materials and their 3D cage-like molecules
[53][54][11].
Taking the most strained [5] CPP (14) molecule as an example, this kind of molecule still maintains the structure of benzene under the action of high strain (116.9 kcal·mol
-1)
[55]. Relative to benzene, the benzene ring in [5] CPP has an A-B carbon-carbon bond length of 1.3838 Å and an B-B bond length of 1.4040 Å, and the carbon-carbon single bond length connecting benzene and the benzene group (1.5454 Å) is 0.0505 Å shorter than that of the ordinary single bond. Due to the existence of tension, the benzene group in the planar structure exists in a boat-shaped structure, and the angle between the carbon connected by the benzene ring and the molecular plane reaches 15.6 °, as shown in Figure 9.
Through decades of development, a variety of groups, such as thiophene, bipyridine, pyrene, fluorene, 1,8-diazapyrene and benzocynate, have been introduced into the CPP system, and a series of high-tension biphenyl nanocyclic organic semiconductor materials have been synthesized, expanding the π-electron system
[11,56⇓⇓ ~59]. In addition, a class of "8" configuration of Booth doubly-knotted line molecules capped by nanocycles has been reported recently
[23,60]. Itami et al. Recently used silicon atoms to cleverly connect segments of two nanorings to construct catenanes and trefoil knot molecules composed of benzene rings with high tension
[19].
4 Ffect of molecular strain on structural and optoelectronic properties of organic semiconductor material
The effect of intramolecular tension on the molecular configuration can be seen intuitively from the single crystal structure. The change of configuration affects the electron arrangement in the molecule, which is macroscopically manifested as the change of structure and properties. For example, as the diameter of the ring becomes smaller and smaller, the tension becomes larger, which shows significant tension-dependent photoelectric properties, and the maximum absorption peak of the emission spectrum has a blue shift. In this section, we focus on the tensile conjugated aromatic [n] CPPs material system as an example to summarize the effects of tension on the structure and optoelectronic properties of materials.
4.1 Chemical shift effect
For [n] CPPs, as the molecular diameter becomes smaller, the twist angle between the phenyl groups gradually decreases, the bending angle of the phenyl groups and the molecular tension gradually increase, and the chemical shift of the hydrogen in the benzene ring gradually shifts upfield (Table 1)
[61][62]. However, the hydrogen chemical shifts in [5] CPP (14) and [6] CPP (15) are 0.37 and 0.15 ppm larger than those in [7] CPP (16), respectively. Through the single crystal structure study, it is found that [5] CPP (14) and [6] CPP (15) have very high strain, so the angle between the phenyl groups becomes very small, which causes the hydrogen atom on the benzene ring to be in the deshielding region and makes its displacement move to the lower field
[63,64]. At the same time, this phenomenon can also be explained by the results of Wong's independent chemical shift calculation for this series of materials
[65].
4.2 Electronic structure effect
In general, the electronic structure of a molecule is affected by the length of the chain, the twist angle of the group in the molecule, and the bend angle of the group itself. For CPP materials, due to the existence of intramolecular tension, the lowest unoccupied molecular orbital (LUMO) energy level becomes higher and higher with the length of the molecular chain, and the highest occupied molecular orbital (HOMO) energy level becomes lower and lower, resulting in a larger and larger molecular gap. The energy level of LUMO + 1 is lower and lower, and the energy level of HOMO-1 is higher and higher, as shown in fig. 10
[59,60,62]. This rule is just opposite to the change trend of molecular orbital energy levels of the corresponding linear compounds with the increase of chain length.
In order to further understand the effect of tension on the electronic structure of CPP, in 2012, Itami et al conducted a systematic study on the effect of bending and twisting on the electronic structure of molecules by combining theory with experiment
[62]. They used even [n] CPPs (n = 6, 8, 10, 12) molecules as model molecules, and calculated the energy levels of the phenyl groups of each molecule under different bending degrees and changed the twist angle between the phenyl groups of [12] CPP by fixing the twist angle between the phenyl groups in each molecule at 31 °, to study the effects of bending tension and twist tension on the energy levels of the molecules, respectively. The results are shown in Figure 11
[62]. In CPP, the HOMO is mainly distributed in the phenyl group, while the LUMO is mainly on the carbon-carbon bond between the connecting benzene rings, which leads to the HOMO being more sensitive to the bending of the benzene rings, while the LUMO is more affected by the angle change between the benzene rings.
4.3 Influence of absorption and emission spectra
For non-strained molecules, the emission spectrum is gradually red-shifted with the increase of the length of the conjugated chain, and then stabilized in a certain band. However, for strained organic semiconductor molecules, the existence of strain makes their spectra, especially the emission spectra, change with the growth of the chain, which is different from that of the corresponding straight-chain compounds. Taking [n] CPPs series materials as an example, with the decrease of n, the intramolecular tension increases, and the maximum absorption spectral band of CPP does not change significantly, but the absorption intensity gradually decreases. A second absorption peak (about 410 nm) due to the theoretically forbidden HOMO → LUMO transition can be observed in the absorption spectrum of CPPs with ring strain ≥73.40 kcal·mol
-1. With the decrease of n and the increase of tension, the position of the maximum emission peak of the emission spectrum is red-shifted and the intensity is gradually weakened (Table 1)
[11]. When n was equal to 5 (ring tension 117.20 kcal·mol
-1) and 6 (ring tension 97.23 kcal·mol
-1), essentially no emitted light was observed (Table 1 and Fig. 12)
[63⇓~65].
Wong theoretically calculated the S
0→S
1 transition density matrix of CPP and the corresponding linear chain polyphenylene, and found that for the linear chain molecule, the electron-hole pairs generated by photoexcitation are mainly distributed in the middle of the molecule, away from the edge. For CPP molecules, the electron-hole pairs generated by photoexcitation are delocalized in the whole ring system
[66].
Jasti et al., Itami et al. And Yamago et al. Have also carried out experimental and theoretical studies on the special spectral characteristics of CPP
[65⇓⇓⇓⇓~70]. Among them, Itami and Irle's views are more widely accepted. They suggested that the doubly degenerate vibronic vibrations during the excitation of the molecule from S
0 to S
2 and S
3 caused the Jahn-Teller deformation of the molecule, and its structure changed from a circular to an elliptical configuration. The blue shift of the strongest emission peak observed in the experiment with the decrease of the molecular conjugation system can be explained by molecular dynamics simulation of the excitation potential energy surface: in CPP with small n, there is a larger vibration amplitude, resulting in a larger Stokes shift; In CPP molecules with larger n, the structure is more rigid and therefore exhibits a smaller Stokes shift (dynamic "blue shift"). In the same way, for CPPs with smaller n, the symmetry rule is broken to a greater extent, and the participation of the S
1state in their fluorescence spectrum contributes to the "static blue shift"
[68].
Similarly, tension-dependent photoelectric phenomena can also be observed in other molecular systems. As shown in Fig. 13, the structures of 26 and 27 are very similar, but due to the introduction of tert-butyl into the pyrene inner ring of 26, the molecular tension is increased, resulting in a different degree of bending of the molecule from that of 27, which further makes the absorption (30 nm difference) and emission spectra (53 nm difference) of the two different. According to the calculation of NICS, it is found that the increase of tension and the change of structure lead to the aromaticity of the pyrene inner ring in 27 molecules being larger than that in 26
[71].
4.4 Influence of Raman spectrum
Raman spectroscopy is a technique that uses scattered light to obtain molecular vibration information, thus providing molecular information such as molecular structure and electronic environment. Because of its high sensitivity and convenience, Raman spectroscopy has become a new analytical technique for studying molecular structure.
In 2013, Fujitsuka, Majima and Yamago were the first to characterize and analyze the Raman spectra of CPP (15-21) with even n in [6, 6]
[72]. Subsequently, Chen et al. And Casado et al. Further discussed the effect of tension on the Raman spectrum, and the types of bonds in the related CPPs are shown in Fig. 14
[73⇓⇓~76]. Based on the above references, it can be seen that in CPP, the peaks of 1564~1594 cm
-1 in the high frequency region are mainly from C
ortho-C
ortho and C
ipso-C
ipso bond stretching vibrations, which are defined as G
1 and G
2 peaks, respectively. The peak at 1496~1511 cm
-1 is considered to be caused by the stretching vibration of the C
ipso-C
ortho bond and is defined as the G
io peak. With the increase of tension, the molecular structure of CPP changes, which leads to the weakening of C
ipso-C
ipso bond and the strengthening of C
ipso-C
ortho bond, thus shifting the G
1 and G
2 peaks to lower frequencies and the G
io peak to higher frequencies. In the low frequency region, the peaks in the 497~501 cm
-1 region are mainly caused by the out-of-plane distortion of the benzene ring, while the peaks in the 153~250 cm
-1 region are caused by the ring breathing of the nanorings, and they gradually move to the high frequency region with the increase of tension.
In 2019, our research group carried out theoretical and experimental studies on the Raman spectra of the synthesized all-fluorenyl high-tension nanorings [4] CF (28, tension energy is 79.8 kcal·mol
-1) crystal, and found that all the chemical bonds corresponding to the [4] CF (28) vibration mode are involved in molecular vibration by comparing with [4] LF (29).The [4] LF is mainly concentrated on the two fluorene units in the middle, and the amplitude of the bond decreases gradually from the center to both sides, resulting in the enhancement of the [4] CF (28) Raman shift peak relative to the [4] LF (29)
[77].
5 Applications of Tensile Organic Semiconductors in Optoelectronics
5.1 Organic light emitting diode (OLED)
In OLED devices, the performance of the device is affected by the intrinsic optoelectronic properties of the luminescent material in the luminescent layer. In the high-tension organic semiconductor molecule, the existence of high tension makes the molecule have a three-dimensional topological structure, reduces the stacking effect between chains, and improves the luminescence stability.
For the first time, our group used the all-fluorenyl high-tensile organic semiconductor material [4] CF (28) as a single-component light-emitting layer, and prepared the first OLED device based on the tensile semiconductor by spin coating. As shown in Figure 15, when the starting voltage of the device is 5.38 V, the EL spectrum has only one emission peak at 508 nm, which is 4 nm different from its fluorescence spectrum, showing ultra-stable luminescence performance. The device has a maximum luminance of 878 cd·cm
-2 and a maximum luminous efficiency of 0.83 cd · A at 10 V
-1[78].
5.2 Organic field effect transistor (OFET)
In OFET devices, the mobility is affected by the way the crystal of the material is packed
[79,80]. In strained organic semiconductor molecules, the distorted structure makes the molecules themselves easily interact to produce a special face-face packing mode.
Tao and Hsu et al. Applied high-tensile organic semiconductor materials TBN (30), TBT (31), BH (32), and TBO (33) to single-crystal FET devices
[81]. The single crystal structure shows that these materials have high tension and distorted configuration. The crystal structure of TBO (33) has a large area of face-on-face π-electron stacking and exhibits a lower mobility (0.32 cm
2·v
-1·s
-1 on average), while TBT, which has the smallest twist angle in the fjord region, has the largest mobility, 0.64 cm
2·v
-1·s
-1 on average. The molecular formulas of the relevant materials are shown in Fig. 16.
Bao Zhenan et al. Recently doped [6] CPP (15) into the polymer (DPP-TT) in different proportions, and used its unique three-dimensional hoop barrel structure to change the volume of the additive between the chains, regulate the dynamic movement of the main chain, reduce the long-range crystalline order, and enhance the flexibility of the film. The flexible field effect transistor based on 5 wt% [6] CPP/DPP-TT film can still maintain its horizontal and vertical mobilities at 0.14 cm
2·V
-1·s
-1 and 0.53 cm
2·V
-1·s, respectively, after 1000 cycles under 25% stress
-1[82].
In order to further understand the charge transport performance of this kind of material, Yamago et al. First used the SCLC method to test the electron mobility of the spin-coated [10] CPP film to be 4.5×10
-6cm
2·V
-1·s
-1[83]. Then Poriel et al. Used the same method to test the hole mobility of [8] CPP deposited by vacuum evaporation and the all-carbazole nanorings [4] CCs film synthesized by them, and found that the hole mobility (1.37/2.78×10
-4cm
2·V
-1·s
-1) of [4] CCs was three orders of magnitude higher than that of [8]CPP(1.21×10
-7cm
2·V
-1·s
-1)
[84].
5.3 Organic Photovoltaic Cell (OPV)
In OPV devices, the host-guest interface has a great influence on the performance of solar cells. The distorted structure of tension organic semiconductor molecules improves the interaction with guest materials and improves the device performance.
Loo et al. Synthesized cHBC derivatives cTBFDBC (34) and cTBTDBC (35), and the intramolecular tension made their structures non-planar and bent, thus improving the interface with C
60 materials and increasing the fill factor of bent materials in solar cells
[85]. The structural formulas of the related molecules are shown in Fig. 17.
Nuckolls et al. Designed strained macrocycle cPBPB (36) and cP
4(37) with different electronic structures, which were introduced into OPV devices as n-type optoelectronic materials, as shown in Fig. 18. It is the first time to compare the performance of strained cyclic compounds with that of acyclic compounds in OPV devices. It is found that the films doped with strained macrocycle and electron donor polymer show higher electron mobility and good morphology. The highest photoelectric conversion efficiency (PCE) of OPV devices based on 36 and 37 is 3.5% and 3.6%, while the highest PCE of the corresponding linear molecular devices is only 1.4% and 1.8%
[86].
5.4 Optical Detector (OPD)
In OPD, charge defects in the material improve its ability to transport charge carriers, but limit the sensitivity of the device, while high dark current determines its noise current level and sensitivity.
Due to the cyclic conjugated structure of the material 37, the charge defects in the material are reduced while good electron transmission performance is maintained, and the strong absorption in the visible light region reduces the dark current in the film and improves the sensitivity of the device. As shown in Fig. 19, the photodetector prepared by Nuckolls et al. By mixing 37 with a ring structure similar to C
60 and PTB7-Th material has a detectivity of ~10
14Jones near zero bias voltage, which is comparable to the performance of the current organic photodetector based on C
60[87].
Recently, they have synthesized a series of tensile organic semiconductor materials by changing the electronic structure of the materials in the ring and using the above preparation methods. Its three-dimensional structure, electron enrichment on the concave-convex surface of the molecule and low LUMO energy level make it an n-type organic semiconductor, which is applied to OFETs, OPVs and OPDs, and its performance is better than that of the corresponding straight-chain molecules
[88].
6 Conclusion and prospect
Tension organic semiconductor materials have special electronic structure and optoelectronic properties due to the high tension in the molecule, which is expected to develop new control ideas. According to the four basic elements of organic semiconductor design: electronic structure, steric hindrance, conformational topology and supramolecular weak interaction, the size of intramolecular tension can be effectively adjusted by regulating molecular steric hindrance and topology, thereby changing the electronic behavior. However, due to its high tension, the material is difficult to prepare and synthesize, and the reaction yield is low, which limits its development and application to a certain extent. We summarize the basic tension types and compound tension, and review the quantitative description methods of tension. So far, macrocyclization and crowding steric hindrance strategies and curved surface structure design have become important methods to introduce molecular tension, while tension molecules represented by CPP have been reported, and tension-dependent optoelectronic properties have been found, which provides a basis for the exploration of organic tension semiconductors.
Considering the important role of stress and strain engineering in the fields of inorganic semiconductors and two-dimensional materials, there is still a huge space for the development of organic tension semiconductors in the future. According to the development history of this kind of materials, the future research mainly focuses on the following three directions: (1) expanding the synthesis methods: using high atom economy reaction, template method or precursor method to synthesize this kind of materials to improve the yield and output of synthesis; (2) New tensile organic semiconductor materials: design and synthesize organic tensile materials with unit structures of different electronic properties, and expand their material systems. And (3) expanding the application field of the material: the material can be applied to photoelectric flexible devices such as OLED, memory and the like by utilizing the single-molecule property and the characteristic of stable luminescence. The nanoscale pore structure of the material can adsorb small molecular compounds and gas molecules, and can be used in the fields of detection, sensing and the like. One of the purposes of future research is to design multifunctional and high-performance organic semiconductor materials through molecular tension, fully tap the special photoelectric properties of such materials, further grasp the influence of molecular tension on material properties, and make such materials widely used. At the same time, this strategy provides new ideas and theoretical basis for the design of multifunctional and high-performance organic semiconductor materials, and lays the foundation for their application in flexible electronics.
National Natural Science Foundation of China(61905121)
National Natural Science Foundation of China(61605090)
Natural Science Foundation of Jiangsu Province(BK20190734)
Natural Science Research Project of Universities in Jiangsu Province(20KJB150038)
Open Project from State Key Laboratory of Supramolecular Structure and Materials at Jilin University(sklssm202108)
Nanjing Vocational University of Industry Technology Start-up Fund(YK21-02-07)
Nanjing University of Posts and Telecommunications Start-up Fund(NY219157)
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