Publications

All-polymer solar cells with 7.57% power conversion efficiency are achieved via a new perylenediimide-based polymeric acceptor. Furthermore, the device processed in ambient air without encapsulation can still reach a high power conversion efficiency (PCE) of 7.49%, which is a significant economic advantage from an industrial processing perspective. These results represent the highest PCE achieved from perylenediimide-based polymers.

Efficient visible-to-UV photon upconversion via triplet triplet annihilation (TTA) is accomplished in polyurethane (PU) films by developing new, powerful photo sensitizers fully functional in the solid-state matrix. These rationally designed triplet sensitizers feature a bichromophoric scaffold comprising a tris-cyclometalated indium(111) complex covalently tethered to a suitable organic small molecule. The very rapid intramolecular triplet energy transfer from the former to the latter is pivotal for achieving the potent sensitizing ability, because this process out-competes the radiative and nonradiative decays inherent to the metal complex and produces long-lived triplet excitons localized with the acceptor moiety readily available for intermolecular transfer and TTA. Nonetheless, compared to the solution state, the molecular diffusion is greatly limited in solid matrices, which even creates difficulty for the Dexter-type intramolecular energy transfer. This is proven by the experimental results showing that the sensitizing performance of the bichromophoric molecules strongly depends on the spatial distance separating the donor (D) and acceptor (A) units and that incorporating a longer linker between the D and A evidently curbs the TTA upconversion efficiency in PU films. Using a rationally optimized sensitizer structure in combination-with 2,7-di-teit-butylpyrene as the annihilator/emitter, the doped poly-urethane (PU) films demonstrate effective visible-to-UV upconverted.emission signal under noncoherentlight irradiation, attaining an upconversion quantum yield of 2.6%. Such quantum efficiency is the highest value so far reported for the visible-to-UV TTA systems in solid matrices.

Herein, we report an all-polymer solar cell with a PCE of over 5% fabricated with non-halogenated solvent. Our method of polymer side-chain engineering using polystyrene enhanced the solubility of polymers in toluene. The phase separation size of the polymer-polymer blend was controlled by tuning the additive concentration. Three different additives were employed and studied. To the best of our knowledge, this is the highest performing all-polymer solar cell fabricated with both non halogenated solvent and non-halogenated additive, which highlights its potential toward environmentally friendly manufacturing of all polymer organic solar cells.

The folding and aggregation behavior of a pair of oligo(phenylene ethynylene) (OPE) foldamers are investigated by means of UV/Vis absorption and circular dichroism spectroscopy. With identical OPE backbones, two foldamers, 1 with alkyl side groups and 2 with triethylene glycol side chains, manifest similar helical conformations in solutions in n-hexane and methanol, respectively. However, disparate and competing folding and aggregation processes are observed in alternative solvents. In cyclohexane, oligomer 1 initially adopts the helical conformation, but the self-aggregation of unfolded chains, as a minor component, gradually drives the folding-unfolding transition eventually to the unfolded aggregate state completely. In contrast, in aqueous solution (CH3OH/H2O) both folded and unfolded oligomer 2 appear to undergo self-association; aggregates of the folded chains are thermodynamically more stable. In solutions with a high H2O content, self-aggregation among unfolded oligomers is kinetically favored; these oligomers very slowly transform into aggregates of helical structures with greater thermodynamic stability. The folded-unfolded conformational switch thus takes place with the free (nonaggregated) molecules, and the very slow folding transition is due to the low concentration of molecularly dispersed oligomers.

The self-assembly behaviors of two series of monodispersed oligomers consisting of perylenediimide (PDI) linked by ethynylene and butadiynylene spacers are investigated in solutions. In spite of the very similar chemical structures, the two sets of oligomers manifest completely different optical properties upon self-aggregation, implying differed aggregate structures. While the oligomers containing butadiynylene spacers form H-aggregates, those featuring ethynylene linkers display J-aggregation characteristics. Thermodynamic analysis revealed that the self-association constants of both series of oligomers increase with the number of PDI units in the backbones. Oligomers containing the same number of PDI units but different spacers display nearly identical enthalpy changes. According to the molecular exciton theory, the observed H-and J-aggregates are suggested to comprise similar packing motifs with slightly varied slipping angles, giving rise to greatly disparate optical properties.

The aim of this review article is to introduce recent studies on an emergent class of singlet oxygen photosensitizers of potential applications to the photodynamic therapy, with a primary focus on the cyclometalated transition-metal complexes. Singlet oxygen photosensitization performances of various cyclometalated Ir and Pt scaffolds are reviewed, and the general photo-physical properties of relevant systems and the mechanisms of singlet oxygen production via photo-sensitization are also briefly discussed. Thus far, investigations of singlet oxygen sensitization by such Ir and Pt complexes are mainly carried out in organic solvents and under non-physiological conditions, while some research efforts have been made at examining the feasibility of applying pertinent cyclometalated complexes to photodynamic therapy.

The photophysics of a series of bichromophoric molecules featuring an intramolecular triplet energy transfer between a triscyclometalated iridium(III) complex and covalently linked organic group are studied. By systematically varying the energy gap (0.1-0.3 eV) between the donor (metal complex) and acceptor (pyrene unit), reversible triplet energy transfer processes with equilibrium constant K ranging from ca. 500 to 40 000 are established. Unique photophysical consequences of such large K values are observed. Because of the highly imbalanced forward and backward energy transfer rates, triplet excitons dominantly populate the acceptor moiety in the steady state, giving rise to ultralong luminescence lifetimes up to 1-4 ms. Because the triscyclometalated Ir and triplet pyrene groups both impart relatively small nonradiative energy loss, decent phosphorescence quantum yields (Phi = 0.1-0.6) are attained in spite of the exceptionally prolonged excited states. By virtue of such precious combination of long-lived triplet state and high Phi, these bichromophoric molecules can serve as highly sensitive luminescent sensors for detecting trace amount of O-2 and as potent photosensitizers for producing singlet oxygen even under low-oxygen content conditions.

Heavy-atom-free triplet photosensitizers are developed by harnessing the thermally active triplet state of carbazolyl dicyanobenzene (CDCB) derivatives and applied to realize visible-to-ultraviolet photon upconversion (UC) via triplet-triplet annihilation (TTA). Demonstrating an annihilator-appending strategy, the designed sensitizers effectively realize TTA UC in polyurethane films with 2,7-di-tert-butylpyrene (DBP) as the annihilator/emitter. The covalently tethered DBP to CDCB is proven critical for achieving the superior sensitizing and UC performance in the solid matrix, essentially by suppressing the reverse ISC and more effectively transferring triplet excitons to free emitters.

A novel hetero-polycyclic aromatic compound manifesting strong near-infrared (NIR) absorption as well as high-performance n-type semiconducting properties is developed. With an exceptionally low LUMO level at -4.7 eV, this NIR dye (lambda(max) approximate to 1100 nm, epsilon approximate to 105 mol(-1) L cm(-1)) exhibits adequate stability under ambient conditions, with electron mobility up to 0.96 cm(2) V-1 s(-1) measured in solution-processed organic field-effect transistors. A special metal-free C-C coupling serves as a pivotal step in constructing the polycyclic pi-framework of this low-bandgap chromophore, by fusing an electron-deficient naphthalenediimide moiety with an electron-donating naphthalenediamine. Such a rare combination of extraordinary optical and semiconductive attributes is quite valuable for organic small molecules, and promising for unique applications in the opto-electronic field.

In the present investigation, we reported the fabrication of host networks formed by two newly prepared phenanthrene-butadiynylene macrocycles (PBMs) at the liquid-solid interface. Size, shape and concentration controlled experiments have been performed to investigate the PBMs/coronene (COR) host-guest system with the structural polymorphism phenomenon. Initially, PBM1 could form a regular linear network structure and PBM2 form a well-ordered nanoporous network structure. When the COR molecules were introduced, the self-assembled structure of PBM1 remained unchanged, while COR could be entrapped into the cavities of the PBM2 nanoporous network, and the co-assembly of the PBM2/COR host-guest systems underwent a structural transformation with the increase of concentration of COR. Scanning tunneling microscopy (STM) measurements and density functional theory (DFT) calculations are utilized to reveal the formation mechanism of the molecular nanoarrays controlled by the solution concentration.

Ordered two-dimensional arrays of silver adatoms with tunable metal atom density stabilized by 1,4-diethynyl-2,5-dimethylbenzene, a terminal alkyne, were prepared on Ag(111) and scrutinized by scanning tunneling microscopy and density functional theory calculations. Stabilization of the adatom arrays was attributed to the substrate-mediated electron localizations of the Ag adatom and terminal alkynyl in the molecule.

The nanostructures of a series of conjugated oligo(p-phenylene-ethynylene)s (OPE) adsorbed on a surface were thoroughly studied using scanning tunneling microscopy (STM). These oligomers have different backbone lengths and side chains. As a result, various nanostructures displaying periodic linear patterns at a single molecule level were obtained. Based on careful measurements on the STM images in combination with density functional theory (DFT) calculations, it could be found that the vertical and parallel distances between neighboring oligomers were responsible for the specific arrangement of the backbone and side chains. The results showed that these molecular designs strongly affect their self-assembled structure, which is important to clarify the structure-property relationship in the nanoscience field.

Using readily available aryl glyoxylic acids and arylene diacetic acids as starting materials, a series of polycyclic aromatic molecules bearing two phthalimide functional groups are synthesized via Perkin condensation followed by intramolecular cyclization reactions. Two different cyclization methods, photo-oxidation and Heck cross-coupling, are employed, both of which effectively accomplish the transformations from diaryl maleic anhydride or maleimide to polycyclic aromatic phthalimide functionality. The photocyclization protocol conveniently allows direct bridging of two plain aromatic C-H sites linked by a maleic anhydride group and uniquely produces the more twisted polycyclic framework as the major product, whereas the Heck coupling approach can typically afford more extended polycyclic skeletons. Thionation reactions are then carried out for the obtained polycyclic diimide molecules using Lawesson's reagent. For all isolated stable products, partial thionation occurs. The prepared polycyclic diimide compounds possess relatively low LUMO levels, and thionation further decreases the LUMO energy of the molecules by 0.2-0.3 eV. Electron-transporting properties are characterized by using solution-processed OFET devices, and an electron mobility of 0.054 cm(2) V-1 s(-1) is demonstrated by a selected compound. Such semiconducting performance promises great potentials of this class of compounds as useful electron-accepting and transporting building blocks in developing various new semi-conductive materials.