Publications

A molecule featuring two distinct cooperatively grown J-aggregates is investigated. Interestingly, when cooling a hot monomer solution, the thermodynamically less stable J1 is exclusively formed even at a particularly slowed temperature dropping rate, which transforms to the more stable J2 at room temperature with very slow kinetics. This observation is ascribed to the differed nucleus sizes of J1 and J2 . During the cooling process, smaller J1 nuclei are formed first at a higher temperature, favored by the entropy effect. At intermediate temperatures, the elongation of J1 out-competes the nucleation of J2 . Then, below the elongation temperature of J2 , the formation of this thermodynamically stable aggregate is hindered kinetically, due to the depletion of monomer by the slow dissociation of J1 . Additional evidence proving the larger nucleus size of J2 is also identified with the varied-temperature spectral analyses and mathematic simulations.

Polycyclic aromatic hydrocarbon (PAH) structures with suitable electron-withdrawing groups are useful building blocks for developing optical and electron-transporting materials. Here, we report the application of double benzannulation processes to synthesize PAH diimides with enlarged π-frameworks featuring a central anthracene moiety. The processes are realized by copper-catalyzed [4+2] cycloaddition of ethynyl-substituted aromatic dicarobximide to 2,5-bis(phenylethynyl)terephthalaldehyde, followed by intramolecular photocyclization or direct arylation via Heck cross coupling. Specifically, A central symmetric benzo[1,2- k :4,5- k ']-bis(fluoranthene)-3,4,12,13-tetracarboxyl diimide (BFDI) is acquired, with a single crystal revealing its completely planar polycyclic skeleton. Such a shape-persistent PAH expectedly exhibits a tendency to stack face-to-face and forms J-aggregates. Moreover, BFDI can be expediently difunctionalized site-selectively in reactive 9 and 10 positions of the anthracene unit and applied to prepare conjugated polymers. When coupled with 1,4-diketopyrrolo[3,4- c ]-pyrrole (DPP) via thiophene and dithiophene linkers, two significantly broadened absorption bands extended to the near-infrared regime appear, evidencing the effective π-conjugative extension ability of BFDI unit.

Abstract In this paper, a difluoro-monobromo end group is designed and synthesized, which is then used to construct a novel polymer acceptor (named PY2F-T) yielding high-performance all-polymer solar cells with 15.22% efficiency. The fluorination strategy can increase the intramolecular charge transfer and interchain packing of the previous PY-T based acceptor, and significantly improve photon harvesting and charge mobility of the resulting polymer acceptor. In addition, detailed morphology investigations reveal that the PY2F-T-based blend shows smaller domain spacing and higher domain purity, which significantly suppress charge recombination as supported by time-resolved techniques. These polymer properties enable simultaneously enhanced JSC and FF of the PY2F-T-based devices, eventually delivering device efficiencies of over 15%, significantly outperforming that of the devices based on the non-fluorinated PY-T polymer (13%). More importantly, the PY2F-T-based active layers can be processed under ambient conditions and still achieve a 14.37% efficiency. They can also be processed using non-halogenated solvent o-xylene (no additive) and yield a decent performance of 13.05%. This work demonstrates the success of the fluorination strategy in the design of high-performance polymer acceptors, which provide guidelines for developing new all-PSCs with better efficiencies and stabilities for commercial applications.

Abstract Polymerization sites of small molecule acceptors (SMAs) play vital roles in determining device performance of all-polymer solar cells (all-PSCs). Different from our recent work about fluoro- and bromo- co-modified end group of IC-FBr (a mixture of IC-FBr1 and IC-FBr2), in this paper, we synthesized and purified two regiospecific fluoro- and bromo- substituted end groups (IC-FBr-o & IC-FBr-m), which were then employed to construct two regio-regular polymer acceptors named PYF-T-o and PYF-T-m, respectively. In comparison with its isomeric counterparts named PYF-T-m with different conjugated coupling sites, PYF-T-o exhibits stronger and bathochromic absorption to achieve better photon harvesting. Meanwhile, PYF-T-o adopts more ordered inter-chain packing and suitable phase separation after blending with the donor polymer PM6, which resulted in suppressed charge recombination and efficient charge transport. Strikingly, we observed a dramatic performance difference between the two isomeric polymer acceptors PYF-T-o and PYF-T-m. While devices based on PM6:PYF-T-o can yield power conversion efficiency (PCE) of 15.2?%, devices based on PM6:PYF-T-m only show poor efficiencies of 1.4?%. This work demonstrates the success of configuration-unique fluorinated end groups in designing high-performance regular polymer acceptors, which provides guidelines towards developing all-PSCs with better efficiencies.

To investigate the capability of pi-pi stacking motifs to enable spin-spin coupling, we designed and synthesized three pairs of regio-isomers featuring two radical moieties joined by a [2.2]paracyclophane (CP) unit. By fusing indeno units to CP, two partially stacked fluorene radicals are covalently linked, exhibiting evident antiferromagnetic (AFM) coupling regardless of the orientation of two spins. Remarkably, while possessing high diradical indices of 0.8 and 0.9, the two molecules demonstrate good air stability by virtue of their singlet ground state. Single crystals help unravel the structural basis of their AFM coupling behaviors. When two radical centers are arranged at the pseudometa-positions around CP, the face-to-face stacked phenylene rings intrinsically confer orbital interactions that promote AFM coupling. On the other hand, if two radicals are directed in the pseudopara-orientation, significant orbital overlapping is observed between the radical centers (i.e., C9 of fluorene) and the aromatic carbons laid on the side, rendering AFM coupling between the two spins. In contrast, when two fluorene radicals are tethered to CP via C9 through a single C-C bond, ferromagnetic (FM) coupling is manifested by both diradical isomers featuring pseudometa- and pseudopara-connectivity. With minimal spin distributed on CP and thus limited contribution from pi-pi stacking, their spin-spin coupling properties are more similar to a pair of nitroxide diradical analogues, in which the two spins are dominantly coupled via through-space interactions. From these results, important conclusions are elucidated such as that although through-space interactions may confer FM coupling, with weakened strength shown by PAH radicals due to their lower polarity, face-to-face stacked p-frameworks tend to induce AFM coupling, because favorable orbital interactions are readily achieved by PAH systems hosting delocalized spins that are capable of adopting varied stacking motifs.  

Luminophores with tunable emission properties are appealing due to various applications. Among those properties, thermally activated delayed fluorescence (TADF) has been attracting enormous research interests. Herein, we synthesized a 9,9'-spirobifluorene based homo-conjugated molecule 1, which connects a diphenylamino moiety as electron donor and a naphthalimide group as electron acceptor via 2,2'-positions of spirofluorene. Compound 1 displays dual emission behaviour with both blue and orange fluorescence. The one orange fluorescence around 555 nmshows sensitivity to oxygen and a prolonged lifetime of 284 ns in degassed toluene. Such characteristics imply TADF nature for this emission from a charge-transfer excited state. The other emission at 440 nm with blue colour displayed resistance to oxygen quenching and a normal fluorescence lifetime of 1.5 ns. Compared with control molecule, this emission band is assigned as conventional fluorescence from a localized excited state. In addition, dual emission property allows molecule 1 to be modulated to emit white photoluminescence in thin film with a CIE color coordinate of (0.25, 0.33).

Over the past years, perylenediimide (PDI)-based polymers have emerged as one of the widely studied polymer acceptors applicable to all-polymer solar cells (PSCs) due to their outstanding photovoltaic properties. Covalently fused PDI units, such as naphthodiperylenetetraimide (NDP), are proven beneficial to increasing the regularity of polymer backbones and enhancing the molecular packing in blend films, thus optimizing the active-layer morphology and improving the device performance. However, most investigated PDI polymers commonly demonstrated low open-circuit voltage (V-oc) in solar cells due to their low-lying lowest unoccupied molecular orbital (LUMO), which greatly limited the power-conversion efficiencies (PCEs) of their devices. Herein, we design and synthesize two new polymer acceptors (PTP-TT and PTP-Th) using thiophene-fused dimeric PDI (i.e., PTP) as the key building block. Both polymers exhibit much elevated LUMO levels at ca. -3.8 eV and achieve higher V-oc in devices compared with NDP-derived polymers. In particular, PTP-TT exhibits stronger light-absorption ability than PTP-Th and a presumably more planar backbone conformation, which are favorable for molecular packing and charge carrier transport in the active layer. Using PTB7-Th as the donor, PTP-TT-based devices achieve the best PCE of 7.04%, with a V-oc of 0.86 V, a short-circuit current density of 14.96 mA/cm(2), and a fill factor of 54%. The current results demonstrate that fusing PDIs with a proper electron-rich moiety can synergistically elevate the LUMO level and optimize the backbone regularity of polymer acceptors to obtain desirable efficiencies of PSCs.

Both naphthalene and perylene derivatives bearing six-membered-ring dicarboximide groups are proven valuable modules for preparing organic electron-transporting semiconductors. However, obtaining their analogue with pyrene appeared challenging, as all previous endeavors resulted in 5- instead of 6-membered dicarboximide rings. Here, we report the first successful synthesis of pyrene-1,5,6,10-tetracarboxyl diimide (PyDI), a centro-symmetric structure comprising two 6-membered dicarboximide groups. Moreover, this molecule undergoes regioselective dihalogenation to afford a 3,8-dibromo-substituted derivative, which may conveniently be incorporated into conjugated polymers via cross-coupling with suitable comonomers. Two representative conjugated polymers, PyDI-T and PyDI-TT, are thus prepared, composed of alternating PyDI and thiophene or bithiophene units. OFETs are then fabricated using these polymers under solution-processing conditions. An optimal electron mobility of 0.335 cm(2) V(−1) s(−1) is achieved by PyDI-TT, which clearly demonstrates the potential of 1,5,6,10-PyDI as a potent building block for developing new high-performance electronic-transporting materials.

Reducing the voltage loss (Vloss) is a critical factor in optimizing the open-circuit voltage (Voc) and overall power-conversion efficiency (PCE) of polymer solar cells. In the current work, by designing a novel electron-accepting unit of coronenediimide (CDI) and using it as the main functional building block, a new polymer acceptor CDI-V is developed and applied to fabricate all-polymer solar cells. Compared with the perylenediimide-based polymer acceptors we previously reported, the current CDI-V polymer possesses a noticeably elevated lowest unoccupied molecular orbital (LUMO). Thereby, by virtue of the enlarged energy gap between the donor HOMO and acceptor LUMO, a high Voc value of 1.05 V is achieved by the all-polymer photovolatic device, along with an impressively low Vloss of 0.55 V. As remarkably, in spite of an extremely small LUMO level offset of 0.01 eV exhibited by the donor and acceptor polymers, effective charge separation still takes place in the all-polymer device, as evidenced by a proper short-circuit current (Jsc) of 9.5 mA·cm2 and a decent PCE of 4.63%.

Abstract The mechanism of aggregation-induced emission, which overcomes the common aggregation-caused quenching problem in organic optoelectronics, is revealed by monitoring the real time structural evolution and dynamics of electronic excited state with frequency and polarization resolved ultrafast UV/IR spectroscopy and theoretical calculations. The formation of Woodward?Hoffmann cyclic intermediates upon ultraviolet excitation is observed in dilute solutions of tetraphenylethylene and its derivatives but not in their respective solid. The ultrafast cyclization provides an efficient nonradiative relaxation pathway through crossing a conical intersection. Without such a reaction mechanism, the electronic excitation is preserved in the molecular solids and the molecule fluoresces efficiently, aided by the very slow intermolecular charge and energy transfers due to the well separated molecular packing arrangement. The mechanisms can be general for tuning the properties of chromophores in different phases for various important applications.

Abstract Short-wavelength infrared (SWIR) photodetection and visualization has profound impacts on different applications. In this work, a large-area organic SWIR photodetector (PD) that is sensitive to SWIR light over a wavelength range from 1000 to 1600 nm and a SWIR visualization device that is capable of upconverting SWIR to visible light are developed. The organic SWIR PD, comprising an organic SWIR sensitive blend of a near-infrared polymer and a nonfullerene n-type small molecule SWIR dye, demonstrates an excellent capability for real-time heart rate monitoring, offering an attractive opportunity for portable and wearable healthcare gadgets. The SWIR-to-visible upconversion device is also demonstrated by monolithic integration of an organic SWIR PD and a perovskite light-emitting diode, converting SWIR (1050 nm) to visible light (516 nm). The most important attribute of the SWIR visualizing device is its solution fabrication capability for large-area SWIR detection and visualization at a low cost. The results are very encouraging, revealing the exciting large-area SWIR photodetection and visualization for a plethora of applications in environmental pollution, surveillance, bioimaging, medical, automotive, food, and wellness monitoring.

With the aim to achieve air-stable polyradical species manifesting strong spin coupling, synthetic endeavors are made toward triradical molecules featuring a truxene-triyl skeleton. Commonly used steric-hindering side groups such as 2,4,6-trichlorophenyl and 9-anthracenyl are both found to be incompetent at stabilizing the targeted truxene triradical, which appears to be elusive from isolation and characterization. Nonetheless, singlecrystal structures of adducts formed by relevant radicals are obtained, which strongly suggests the transient existence of the designed triradicals. Finally, a truxene triradical comprising 1-anthracenyl along with two 9-anthracenyl substituents is successfully isolated and found to exhibit decent stability in air. We propose that because of the smaller dihedral angle assumed by 1-anthracenyl with respect to the plane of truxene-triyl, more effective pi-conjugation allows the spin density to be more widely delocalized and distributed to the anthracenyl side groups. Thus, higher stability is gained by the triradical molecule.

By harnessing a highly efficient metal-catalyzed tandem cycloaddition reaction as the key benzannulation step, a series of cyclopolyarene nanorings of varied sizes are obtained from poly(arylene-butadiynylene) macrocyclic precursors, which can be synthesized relatively conveniently. Interestingly, due to the nonparallel bond connectivity of the repeat unit, unique Mobius topology is manifested by the cyclopolyarene nanorings composed of an odd number of repeat units, whereas cylindrical tubular structures with radial conjugation are formed with those consisting of an even number of repeat units.

By integrating azulene with a quinoid moiety, a novel non-alternant polycyclic aromatic hydrocarbon molecule BCHF1 exhibiting manifold zwitterionic, quinoidal and diradical behaviors is designed and synthesized. Its zwitterionic feature is evidenced by the changes shown by the 1H-NMR and absorption spectra when the molecule undergoes reversible protonation and deprotonation reactions at varied pH. The diradical facet, manifesting a small singlet–triplet energy gap (ΔES–T), is characterized with a paramagnetic resonance signal detected by the EPR spectroscopy at room temperature. As the diradical properties are not observed in the protonated form, BCHF1+H+, a pH-controlled reversible magnetic switching behavior is illustrated by monitoring the on and off cycles of EPR signals upon successively adding bases and acids to a solution or exposing a thin film of BCHF1+H+ to base vapor followed by acid vapor.

Dissymmetric reactions, which enable differentiated functionalization of equivalent sites within one molecule, have many potential applications in synthetic chemistry and materials science, but they are very challenging to achieve. Here, the dissymmetric reaction of 1,4-dibromo-2,5-diethynylbenzene (2Br-DEB) on Ag(111) is realized by using a stepwise activation strategy, leading to an ordered two-dimensional organometallic network containing both alkynyl–silver–alkynyl and alkynyl–silver–phenyl nodes. Scanning tunneling microscopy and density functional theory calculations are employed to explore the stepwise conversion of 2Br-DEB, which starts from the H-passivation of one Br-substituted site at 300 K in accompaniment with an intermolecular reaction to form one-dimensional organometallic chains containing alkynyl–silver–alkynyl nodes. Afterwards, the other equivalent Br-substituted site undergoes metalation reaction at 320–450 K, resulting in transformation of the chains into the binodal networks. These findings exemplify the achievement of the dissymmetric reaction and its practical application for controlled fabrications of complicated yet ordered nanostructures on a surface.

The efficient ternary all-polymer solar cells (PSCs) are designed and fabricated, using a polymer acceptor of NDP-V-C7 and analogue co-donors containing a chlorinated polymer PBClT and classical PTB7-Th. PBClT and PTB7-Th possess very similar chemical structure and matched energy levels to form the cascade of the co-donors. Meanwhile, benefiting from those analogous polymer structures, there is little influence of the morphology in blend film compared to their pristine polymer films. The binary PBClT:NDP-V-C7 devices exhibit a high open-circuit voltage (Voc) due to the deep HOMO level of PBClT. The Voc of all-PSCs could be finely manipulated by adjusting the content of PBClT in blend film. The ternary all-PSCs have the more balanced charge mobility and prolonged carrier lifetime compared to the binary devices. The PBClT also help improve the miscibility of ternary blend and suppress crystallization in films, bringing about favorable morphology with appropriate orientation and surface roughness in blend film. With the optimal processing, the champion ternary all-PSCs obtain a high PCE of 9.03%, which is about 10% enhancement compared to that of binary device. The results indicate that the ternary approach using analogue co-donors is a practical method to enhance the performance of all-PSCs.

Partial flattening of the spatially extended molecular scaffold has been employed as an effective tactic to improve the device performance of a perylenediimide (PDI)-based small-molecule acceptor because the less twisted yet not completely planar molecular geometry is anticipated to improve the molecular packing and thereby attain a more suitable balance between the carrier transport ability and phase domain size. A small-molecule acceptor BF-PDI comprising four alpha-substituted PDI units attached around a 9,9'-bifluorenylidene (BF) central moiety is designed and studied in polymer solar cells. The BF group is deemed a ring-fused analogue of the tetraphenylethylene (TPE) unit. Due to the less twisted and better conjugated BF skeleton, BF-PDI displays more delocalized lowest unoccupied molecular orbital. By virtue of both the electronic and steric effects, BF-PDI is suggested to bring about superior intermolecular stacking and donor-acceptor phase separation morphology in blend films. Indeed, the experimental results show that BF-PDI displays improved charge transport ability and a higher power-conversion efficiency of 8.05% than that of TPE-PDI. Grazing-incidence wide-angle X-ray diffraction and resonant soft X-ray scattering confirm the more compact and ordered molecular packing as well as smaller domain sizes in the P3TEA/BF-PDI blend.

Ternary solar cells have been proven to be an effective way to increase the power conversion efficiency (PCE) of organic solar cells (OSCs). Up to now, research effort has mostly focused on fullerene derivatives and acceptor–donor–acceptor (A–D–A) type non-fullerene acceptor-based ternary solar cells, while perylene diimide (PDI)-based ternary devices have been rarely studied. In this contribution, we introduced a new type of ternary solar cell based on a PDI-based small-molecule acceptor (PBI-Por) and a polymer donor (PTB7-Th) with a third PDI-based polymer acceptor (PDI-V). The introduction of PDI-V into the ternary blends not only broadens the absorption of blend films but also increases the electron mobilities. As a result, a high efficiency of 9.43% was obtained for the ternary OSC, which is 20% higher than that of the binary OSC. Detailed studies indicate that PDI-V showed good compatibility with PBI-Por in the blend films, which demonstrates a promising way to fabricate high-performance PDI-based OSCs.

The photodynamic properties involving both intra-and intermolecular triplet energy transfers (ET) of a bichromophoric photosensitizer having a tris-cyclometalated Ir(III) tethered with a pyrene derivative are studied. Due to the triplet energy gap of the two chromophores, a reversible intramolecular triplet ET equilibrium is quickly established upon photoexcitation, with the triplet exciton mainly residing on the acceptor side in the photostationary state. By virtue of the very small decay rate of triplet pyrene, a considerably extended triplet lifetime (2 ms) is observed. Next, the intermolecular triplet-triplet ET properties are investigated. Using steady-state and time-resolved spectroscopy, the ET rate constants from the Ir complex and pyrene unit in the sensitizer to an external triplet acceptor (unattached, free pyrene derivative) in solution are found to be around 10(9) s(-1) and 10(8) M-1 s(-1), respectively. In spite of a lower ET rate constant, the tethered pyrene serves as the main intermolecular ET channel because of the large, favorable intramolecular ET equilibrium (K similar to 10(3)). Importantly, this cascade ET process, from Ir complex to linked pyrene, and then to free pyrene, offers an overall improved ET efficiency than a direct ET from Ir complex to free pyrene, by virtue of the much smaller spontaneous decay rate compared to that of the metal complex. Finally, the more efficient ET ability is demonstrated experimentally by applying the molecule as sensitizer in a triplet-triplet annihilation upconversion. The bichromophoric sensitizer achieved upconverted emission intensity S times higher than a monochromophoric Ir-complex analogue.