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Through-Space Conjugation: A Thriving Alternative for Optoelectronic Materials

2019; Chinese Chemical Society; Volume: 1; Issue: 2 Linguagem: Inglês

10.31635/ccschem.019.20180020

2096-5745

Jinshi Li, Pingchuan Shen, Zujin Zhao, Ben Zhong Tang,

Organic Electronics and Photovoltaics

Open AccessCCS ChemistryMINI REVIEW1 Jun 2019Through-Space Conjugation: A Thriving Alternative for Optoelectronic Materials Jinshi Li, Pingchuan Shen, Zujin Zhao* and Ben Zhong Tang Jinshi Li State Key Laboratory of Luminescent Materials and Devices, Center for Aggregation-Induced Emission, South China University of Technology, Guangzhou 510640 (China) , Pingchuan Shen State Key Laboratory of Luminescent Materials and Devices, Center for Aggregation-Induced Emission, South China University of Technology, Guangzhou 510640 (China) , Zujin Zhao* State Key Laboratory of Luminescent Materials and Devices, Center for Aggregation-Induced Emission, South China University of Technology, Guangzhou 510640 (China) and Ben Zhong Tang State Key Laboratory of Luminescent Materials and Devices, Center for Aggregation-Induced Emission, South China University of Technology, Guangzhou 510640 (China) Department of Chemistry, Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration and Reconstruction, The Hong Kong University of Science & Technology, Kowloon, Hong Kong (China) https://doi.org/10.31635/ccschem.019.20180020 SectionsAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Efficient electronic coupling is the key to constructing optoelectronic functional π systems. Generally, the delocalization of π electrons must comply with the framework constructed by covalent bonds (typically σ bonds), representing classic through-bond conjugation. However, through-space conjugation offers an alternative that achieves spatial electron communication with closely stacked π systems instead of covalent bonds thus enabling multidimensional energy and charge transport. Because of the ever-accelerating advances of through-space conjugation studies, researchers are inspired greatly by the beauty of through-space conjugated systems and their potential in high-tech applications. In this mini review, we introduce some representative and newly developed π systems having the through-space conjugation feature. In addition to discussing the profound impacts of through-space conjugation on the luminescence properties and charge transport, we will review some impressive findings of distinctive molecules with attractive characteristics, such as aggregation-induced emission, thermally activated delayed fluorescence, bipolar charge transport, and multichannel. These achievements may bring about new breakthroughs of theory, materials, and devices in the fields of organic electronics and molecular electronics. Download figure Download PowerPoint Introduction Through-space conjugation, one of the most significant properties of π electron delocalization, can lead to non-covalent inter-ring interaction among closely face-to-face overlapped aromatic rings. Unlike traditional through-bond conjugation in which the π electrons are delocalized along with a framework built with σ bonds, through-space conjugation possesses more flexibility and possibilities due to its noncovalent structure and spatial delocalization of π electrons. Fascinated by its novel mechanism and potential applications in optoelectronics1–3 and bioscience,4 numerous researchers have devoted themselves to deep exploration of through-space conjugated molecules for decades. The very first report on [2.2]paracyclophane ([2.2]pCp) in 19495 symbolized the inception of through-space conjugation research. [2.2]pCp enables π electrons to delocalize between two close parallel phenyl rings as the central distances between two stacked phenyl rings is approximately 3.1 Å (Figure 1a), a typical through-space conjugated conformation.6,7 The special structure of [2.2]pCp leads to distinctive optical and electronic properties. Based on experimental investigation and theoretical calculation, researchers have gained much information on the unique features of [2.2]pCp and its derivatives. For instance, the charge delocalization and energy-transfer dynamics in electronically coupled π systems have been thoroughly elucidated. Theoretical results indicate that the energy band gap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of [2.2]pCp (3.72 eV) is about 1.4 eV smaller than that of benzene (5.15 eV), demonstrating [2.2]pCp has a more effective and expansive π-conjugated structure than benzene because of the strong through-space conjugation.8 By characterizing π–π interaction and quantifying through-space charge transfer, it is found that the quenching of resonant photoluminescence processes is stronger in [2.2]pCp than in [4.4]paracyclophane ([4.4]pCp), which has a larger inter-ring separation of 4.0 Å.9 There is much faster electron delocalization in [2.2]pCp because of the better through-space coupling. As a result, photoexcited energy transfer occurs efficiently from donor-to-acceptor π-electron systems with energy transfer efficiency over 99.9% and a rate constant of about 1012 s−1 when [2.2]pCp is applied to polymers as a skeleton to construct single-molecular wires.10,11 Another representative characteristic of [2.2]pCp and its derivatives is their remarkable photophysical properties. The presence of a shoulder in the UV–vis spectrum arising from the lower energy level compared with isolated analogue is observed in both oligomers, such as pg-CP(PV2)2 and pg-CP(PV3)2,12 with [2.2]pCp cores and polymers such as pg-poly(PE3)13 (Figure 1a) with a through-space conjugated [2.2]pCp backbone. Their photoluminescence spectra usually display a broad, structureless, and largely redshifted peak, consistent with the excited states of these compounds undergoing rapid energy transfer to relax and forming a lower energy "phane" electronic state. In a transoid–cisoid diad, pg-poly(PV3) (Figure 1b) has been shown to experience an energy transfer process along the well-stacked [2.2]pCp skeleton,14 resulting in a larger Stokes shift than the unstacked model. This kind of energy transfer can be limited by a break of through-space conjugation due to different conformation of the monomers. Other than the energy transfer via a through-space pathway in ladder-like stacked structures, it is also found that energy transfer can be achieved in linear structures linked via through-space interaction (Figure 1b).15pp-Poly(F-CP-TDZ) possesses a donor and acceptor π-conjugated system that are linked by through-space conjugated [2.2]pCp in a single-polymer main chain. Spectroscopic results demonstrate Förster-type intramolecular energy transfer from the donor to the acceptor, as the photoluminescence spectrum of the donor overlaps with the absorption spectrum of the charge-transfer band of the acceptor. Figure 1 | (a) Chemical structure, crystal structure, and HOMO and LUMO orbital distribution of [2.2]pCp and chemical structures of [2.2]pCp derivatives. (b) Cisoid and transoid conformations of chromophoric tier in the stacked polymer, pg-poly(PV3), and chemical structure of pp-poly(F-CP-TDZ) with a schematic figure of charge transfer within the polymer. Reprinted with permission from ref. 8 Copyright 2012 AIP Publishing; ref. 12 Copyright 2012 Wiley-VCH; ref. 13 Copyright 2010 American Chemical Society; ref. 14 Copyright 2012 Royal Society of Chemistry; and ref. 15 Copyright 2011 Royal Society of Chemistry. Download figure Download PowerPoint Because of their intriguing photoluminescence and charge transport properties, through-space conjugated molecules have been adopted to build various functional materials with desirable multidimensional energy and charge-transfer capabilities. As through-space conjugation has high sensitivity to geometric and electronic structures,16 the moieties generally need to overcome steric hindrance to form a favorable face-to-face geometry with an inter-ring separation of less than 3.5 Å.17 Based on this general principle, many molecules with diverse optical and electronic properties have been designed by exploiting through-space conjugation, such as carbazole hexaphenylbenzene (CzHPB),18 naphthalene-1,8∶4,5-bis(dicarboximide) dimer ((−)-2NDI),19o-phenylene hexamer (oP6(H)6),20 1,8-triarylamino naphthylene (N-1,8TAA),21 paracyclophane analogue (CP[(NBP)2F]),22 etc. Active studies on these molecules have propelled further development of through-space conjugation theory and has shown them to be highly promising for a series of optoelectronic materials and devices. In this mini review, we focus on the latest advances of through-space conjugated molecules. Emphases will be placed on introducing two major kinds of performance: photoluminescence and charge transport, which we will give through-space conjugated systems a promising future in optoelectronic devices. We present some important findings aided by through-space conjugation, such as aggregation-induced emission (AIE), thermally activated delayed fluorescence (TADF), and intramolecular energy and charge-transfer dynamics. Based on molecular design and characterization, we then unveil the structure–property correlation, fundamental implications, and feasible applications of the newly emerging through-space conjugated foldamers bearing a tetraphenylethene (TPE) core and elucidate their great potential in bipolar charge transport and multichannel conductance. These insights should be meaningful for researchers to understand and design new molecules with specific optoelectronic functions. Aggregation-Induced Emission AIE luminogens (AIEgens) refer to luminogenic molecules that are nonluminescent or weakly luminescent when molecularly dissolved in solvents but emit intensely in the aggregated state.23 These intriguing luminescent materials are of great importance for practical applications in optoelectronic devices and bioimaging because of their high solid-state photoluminescence quantum yields (PLQYs). Generally, when AIEgens aggregate, the free intramolecular motions such as rotation and vibration are restricted by spatial constraint and collective forces from weak intermolecular interactions, and thus the nonradiative decay channels can be blocked.24 Instead, the radiative decay of the excited state will dominate, which enables AIEgens to emit efficiently. Normally, the through-bond conjugated framework forms the structural basis of AIEgens by the development of electronic delocalization among the building blocks through covalent bonds. But AIEgens often require a highly twisted conformation in which the aromatic rings can spatially interact with each other and effective through-space conjugation is possible. The propeller-shaped TPE is a classic through-bond conjugated AIEgen.25 However, 1,1,2,2-tetraphenylethane (s-TPE)26 without a typical π-conjugated structure is also reported to be AIE-active (Figure 2a). It exhibits a weak emission peak in nonvisible region (297 nm) in pure tetrahydrofuran (THF) solution, associated with the emission of individual phenyl ring. But a strong, long-wavelength emission peaking at 460 nm, accompanied by a greatly decreased emission intensity at 297 nm, is observed in THF–water mixture (water fraction of 90%) caused by the formation of s-TPE aggregates due to its hydrophobic nature. Although it is weakly through-bond conjugated, it can emit strong visible light at 467 nm with a high solid-state PLQY of up to 69% under 280 nm excitation, which is an unexpected and unique phenomenon. As shown in Figure 2a, the four phenyl rings in s-TPE are linked by C–C single bonds, and thus they freely rotate in dilute solutions, which causes nonradiatively deactivation of the excited state. In the aggregated state, the rotation of these phenyl rings is restricted, and the phenyl rings can form a fixed and well-stacked conformation in which the π electron clouds overlaps and results in a spatial delocalization effect. In that case, the band gap of s-TPE is severely narrowed, thus leading to a largely redshifted emission. Namely, the through-space conjugation of s-TPE is promoted in the aggregated state and contributes significantly to its AIE property and strong long-wavelength emission. Figure 2 | (a) Through-space conjugation among phenyl rings in s-TPE and photoluminescence spectra of s-TPE in THF/water mixture with different water fractions (fw). Inset: Chemical structure and fluorescent photos of s-TPE solid, taken under 365 nm UV light irradiation. (b) Schematic illustration of the unconventional clusteroluminogens. Reprinted with permission from ref. 26 Copyright 2017 American Chemical Society and refs. 28 and 29 Copyright 2018 American Chemical Society. Download figure Download PowerPoint Similar through-space conjugation-related AIE phenomenon is observed in other AIE systems. For example, an unorthodox luminogen with poor through-bond conjugation, the racemic C6-unsubstitued tetrahydropyrimidine (THP),27 is highly emissive in the crystalline state with a high PLQY of 93%, which is in sharp contrast to its nonfluorescent property in the solution state. The efficient through-space conjugation formed among closely-aligned, electron-rich aromatic rings and heteroatoms in crystals is rationalized to account for the interesting AIE effect and strong solid-state emission. Along with these interesting discoveries, a new concept of clusteroluminogen28,29 is proposed and has drawn considerable attention. As illustrated in Figure 2b, strong through-space conjugation and multiple lone-pair electronic interactions within a single molecule enable formation of a through-space conjugated luminescent material. The chain entanglement and intra/interchain interactions in polymers will further enhance electronic communication among heteroatoms and aromatic rings, which facilitates the molecular clustering, conformational rigidity and, finally, strong photoluminescence. Therefore, clusteroluminogens can be considered as a new kind of nonconventional chromophores and may serve as light-emitting materials for optoelectronic devices and biotechnology. These inspiring achievements can provide us some insights into through-space conjugation and clustering-induced emission, but deciphering their underlying mechanism is still in its infancy and requires more scientific effort. Thermally Activated Delayed Fluorescence TADF emitters have recently drawn increasing interests because of their theoretical 100% exciton utilization,30 which enables them to compete with phosphorescent materials in comparable electroluminescence (EL) efficiency but at a greatly reduced cost. For TADF emitters, one of the vital issues is how to simultaneously achieve a small singlet–triplet energy splitting (ΔEST) to trigger up conversion from the triplet state (T1) to the singlet state (S1) by efficient reverse intersystem crossing process and a large enough transition dipole moment to guarantee a high PLQY.31–33 Most TADF emitters have a highly twisted noncoplanar connection between electron donors and acceptors to acheive effective separation of HOMO and LUMO and thus a small ΔEST. However, in this situation, the transition dipole moment is also decreased, leading to a weak oscillator strength and a low PLQY. To address this issue, a twisted structural model of an electron acceptor and donor linked by an aryl backbone has recently been proposed in which through-space conjugation between donor and acceptor groups is utilized to activate the reverse intersystem crossing process without causing a large loss of PLQY. The nearly perpendicular linkage with large torsion angles can minimize the overlap of HOMO on electron donor and LUMO on electron acceptor, resulting in a small ΔEST, whereas the closely parallel arrangement of donor and acceptor ensures strong through-space conjugation, which can improve electronic coupling to enhance PLQY. Therefore, these TADF molecules built on a through-space conjugated skeleton can achieve nearly 100% internal quantum efficiency. Figure 3a illustrates a typical through-space conjugated TADF emitter, cis-Bz-PCP-TPA,34 based on a [2.2]pCp skeleton with stacked donor–acceptor groups. The through-space charge transfer from donor to acceptor is detected by the appearance of a high-intensity absorption band at 311 nm. The cis-Bz-PCP-TPA has a small ΔEST of 0.13 eV and a prominent blue delayed fluorescence at 480 nm with a lifetime of 1.8 μs, thus demonstrating its TADF character. Figure 3 | (a) Chemical structures of through-space conjugated TADF emitters. (b) Crystal structures of B-oTC and XPT, and photoluminescence transient spectra of P-Ac50-TRZ50 in toluene under nitrogen and air at 298 K. Reprinted with permission from ref. 34 Copyright 2018 Royal Society of Chemistry; ref. 35 Copyright 2017 Wiley-VCH; and refs. 36–44 Copyright 2017 American Chemical Society. Download figure Download PowerPoint Two additional typical TADF emitters based on through-space conjugated skeletons are B-oTC35 and XPT,36 which have even better EL performance. In both emitters, the donor groups are nearly parallel to the acceptor groups with short distances of 2.8–3.4 Å (Figure 3b), leading to intramolecular donor–acceptor interactions. The combined through-bond and through-space charge-transfer paths simultaneously allow small ΔEST values and large transition dipole, bringing about strong TADF property and high emission efficiencies. Microsecond delayed lifetimes and high PLQYs in films (94% and 65% for B-oTc and XPT, respectively) have been obtained. Moreover, it has been observed that the through-space conjugated donor–acceptor structure results in further restriction of intramolecular vibration and rotation in the aggregated state and thus enhanced emission, which can be considered as aggregation-induced delayed fluorescence (AIDF). When applying these two emitters to organic light-emitting diodes as light-emitting layers, both devices show good EL performance. The device using XPT as emissive dopant radiates green light at 584 nm with a maximum external quantum efficiency (EQE) of 10%, which exceeds the theoretical limit of a simple fluorescent OLED. More importantly, the nondoped OLED using B-oTc neat film as light-emitting layer can achieve blue light at 474 nm with a remarkable EQE of 19.1%. This result demonstrates the feasibility of nondoped OLEDs with brilliant performance by applying AIDF materials, and many other breakthroughs have been achieved in different AIDF systems.37–44 Unlike the molecules discussed above with π-conjugated backbones that allow the coexistence of through-space and through-bond conjugation, a new type of polymer, P-Ac(1-x)-TRZx,45 comprised of a saturated hydrocarbon backbone and aromatic electron donors and acceptors as pendants (Figure 3a), possesses only through-space conjugation yet exhibits an interesting blue TADF property. The saturated hydrocarbon backbone avoids the strong electronic coupling between donors and acceptors to achieve blue emission and a small ΔEST. On the other hand, the efficient through-space conjugation between donor and acceptor is conducive to charge transport and light emission. Therefore, polymer P-Ac50-TRZ50 shows typical TADF features of a long-delayed fluorescence lifetime of 1173.0 ns in toluene under nitrogen (Figure 3b), a blue emission peak at 498 nm, and a high PLQY of 60% in film. The solution-processed OLED based on P-Ac95-TRZ05 can perform efficiently, affording a strong blue EL emission at 472 nm with a high EQE of 12.1% and a low efficiency roll-off of 4.9% (at 1000 cd m−2). The achievement of a high EQE mainly arises from extracting light from both triplet and singlet excitons involving through-space charge transfer. Expanding the study of molecular design based on through-space conjugation may be the key to further improve the performance of blue TADF emitters. Energy and Charge Transfer Energy and charge transfer often occur simultaneously between electron donors and acceptors that are in close proximity.46 The through-space intervalence charge-transfer (IVCT) phenomenon originating from mixed valency has been found in metal–organic frameworks (MOFs), such as [Zn2(BPPTzTz)2(tdc)2]n (Figure 4a),47 for which both experiment and theory have confirmed the presence of two closely aligned BPPTzTz ligands with a short interchain distance of 3.80 Å. Visual inspection of the molecular orbitals reveals that two kinds of energy transitions occur originating from a singly occupied molecular orbital localized on one BPPTzTz unit to LUMO+1 localized on the adjacent ligand or to LUMO+2 localized on the same ligand. Both transitions display substantial IVCT character (Figure 4b). According to the IVCT between BPPTzTz•− and BPPTzTz°, the reduction of one ligand induces a resonance effect in the other that decreases the stacking offset and increases the overall donor–acceptor orbital overlap, thereby facilitating the IVCT process. The relative intensity of the underlying IVCT transitions is also highly dependent on the distances between ligands. Specifically, the IVCT behavior intensifies with the shorter distances between the stacked ligands. This interesting through-space, mixed-valence interaction can lead to long-range delocalization throughout the entire framework (Figure 2c), which is essential for the conductivity in MOFs.48–50 Figure 4 | (a) Chemical structures of the cofacial pair of BPPTzTz ligands showing reduction to the mixed-valence state, which facilitates the through-space IVCT interaction. (b) Molecular orbitals involved in the transitions in the near-infrared region for the cofacial mixed-valence dimer (BPPTzTz0/•−)2 extracted from the crystal structure of [Zn2(BPPTzTz)2(tdc)2]n. (c) Crystal structure of [Zn2(BPPTzTz)2(tdc)2]n showing the cofacial alignment of the BPPTzTz ligands marked by red rectangle. Reprinted with permission from ref. 47 Copyright 2018 American Chemical Society. Download figure Download PowerPoint Similarly, a multichromophoric tetracationic cyclophane DAPPBox4+ exhibiting efficient intramolecular energy and electron transfer is reported by Gong et al.51 The asymmetric, rigid, and box-like cyclophane is comprised of an ExBIPY2+ unit and a DAPP2+ unit linked together by two p-xylylenes (Figure 5a). Two types of through-space interactions occur in DAPPBox4+, which has been confirmed via crystallography analysis and spectroscopy measurement.52 The first type is intramolecular through-space conjugation between ExBIPY2+ and DAPP2+, and the second type is intermolecular through-space conjugation between two closely aligned ExBIPY2+ units. An apparent redshift is observed in the absorption maximum of DAPPBox4+ in comparison to corresponding methylated compounds (Figure 5b),53,54 thus verifying the existence of through-space electronic interaction. Efficient energy transfer within the cyclophane can be expected from the emission spectrum dominated by green emission at 517 nm, under excitation at 339 nm of DAPPBox4+ (Figure 5c). The quenching of ExBIPY2+ emission at 380 nm, accompanied by the enhancement of DAPP2+ emission at 510 nm, suggests quantitative energy transfer from ExBIPY2+ to DAPP2+. The emission spectrum of DAPPBox4+ is not a simple collection of multiple emission bands from different subunits, but a structureless broad band associated with the DAPP2+ unit, which confirms the existence of through-space energy transfer. Ultrafast intramolecular charge transfer happens from DAPP2+ to ExBIPY2+ to yield DAPP3+•–ExBIPY+• radical ion pair in 1.5 ps, verified by the appearance of the 1150 nm absorption originating from a DAPP3+•–ExBIPY+• radical ion pair (Figure 5d). These results demonstrate the positive effect of through-space conjugation on the intra- and intermolecular charge transfer. This kind of constitutionally asymmetric cyclophane is believed to have a promising future in integration into solar energy conversion and organic electronics. Figure 5 | (a) Chemical structure of DAPPBox4+. The energy-transfer and charge-transfer processes of DAPPBox4+ are displayed. (b) Absorption spectra of DAPPBox4+, Me-ExBIPY2+, and Me-DAPP2+ in MeCN at room temperature. (c) Emission spectra of DAPPBox4+, Me-ExBIPY2+, Me-DAPP2+, and a physical mixture of Me-ExBIPY2+ and Me-DAPP2+ in MeCN (1.6 μM) upon excitation at 339 nm. (d) Visible and near-infrared femtosecond spectra with excitation at 330 nm of DAPPBox4+ in MeCN at room temperature. Reprinted with permission from ref. 51 Copyright 2017 American Chemical Society. Download figure Download PowerPoint New Through-Space Conjugated System Because of their unique charge transfer properties, through-space conjugated molecules are equipped with not only fascinating photophysical features such as AIE and TADF but also show promise for charge mobility and electronic communication, which are of great importance for the fabrication of efficient optoelectronic devices. Therefore, numerous systems with novel structures, properties, and applications utilizing through-space conjugated molecules have been investigated. However, the thoroughly studied and well-established through-space conjugated systems are surprisingly rare. To the best of our knowledge, only studies of the [2.2]pCp system are relatively mature and widely used to construct through-space conjugated materials. In our recent work, we have systematically studied a new class of foldamers containing a TPE core and uncovered their intriguing through-space conjugation and optoelectronic functions. The archetypal foldamer, Z-o-BPTPE, has been confirmed by crystallographic analysis to be a cis-isomer. The specific phenyl rings, Φ1 and Φ2, are stacked in a nearly parallel manner, with a plane overlap of about 50% and inter-ring distances of 3.147 and 3.166 Å (Figure 6),55–57 indicating the existence of efficient electronic coupling between the stacked phenyl rings. Compared with the conventional [2.2]pCp, Z-o-BPTPE performs better in photoluminescence and charge transport. Because it has four closely packed phenyl rings that can interact noncovalently, Z-o-BPTPE has better through-space conjugation than [2.2]pCp, which only has two phenyl rings involved in through-space conjugation. Furthermore, through-space and through-bond conjugation can occur in Z-o-BPTPE with vinyl-linked two phenyl fragments simultaneously, while there is only through-space conjugation in [2.2]pCp. Based on this archetypal architecture, we have developed a library of foldamers with interesting properties and studied the structure–property correlation and possible applications. Figure 6 | (a) Schematic figure showing the better conjugation in Z-o-BPTPE compared with [2.2]pCp. (b) Molecular structure and crystal structure of Z-o-BPTPE with the shortest distances between Φ1 and Φ2. (c) Calculated molecular orbitals ranging from HOMO−1 to LUMO+1 of Z-o-BPTPE. Reprinted with permission from refs. 55–57 Copyright 2016 Elsevier. Download figure Download PowerPoint We have expanded our studies on the TPE-based foldamers and constructed more complicated foldamers bearing two through-space conjugated oligo-p-phenylene chains (Figure 7a).58 The crystallographic results confirm their folded structures and cis-conformation, wherein two linear through-bond conjugated oligo-p-phenylene chains are closely aligned in an offset and facing each other in a roughly parallel manner. The shortest distances between two chains are between 3.16 and 3.46 Å, and intense electron clouds of frontier molecular orbitals are observed in the interchain regions, demonstrating efficient through-space conjugation. Absorption spectra reveal that Z-o-BPTPE has two main absorption peaks at around 240 and 300 nm (Figure 7b). The long-wavelength absorption peaks become less distinguishable as tails in the absorption spectra of Z-o-BBPTPE and f-3Ph. These peaks even disappear for f-4Ph, f-5Ph, and f-TPE-PVP, for which the short-wavelength absorption peaks are enhanced and redshifted correspondingly. When the oligo-p-phenylene chains are elongated, the absorption bands of higher energy become stronger and redshifted, whilst the absorption bands of lower energy are relatively weakened. By combining experimental and theoretical results, it is deduced that the enhanced through-space conjugation mainly contributes to the short-wavelength absorptions associated with the high-energy S0–Sn transition, and the weaker through-bond conjugated central TPE unit leads to the small, long-wavelength absorption of the low-en