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State-of-the-art Y6-type molecular acceptors exhibit nanosecond excited-state lifetimes despite their low optical gaps (~1.4 eV), thus allowing organic solar cells (OSCs) to achieve highly efficient charge generation with extended near-infrared (NIR) absorption range (up to ~1000 nm). However, the precise molecular-level mechanism that enables low-energy excited states in Y6-type acceptors to achieve nanosecond lifetimes has remained elusive. Here, we demonstrate that the distinct packing of Y6 molecules in film leads to a strong intermolecular charge-transfer (iCT) character of the lowest excited state in Y6 aggregates, which is absent in other low-gap acceptors such as ITIC. Due to strong electronic couplings between the adjacent Y6 molecules, the iCT-exciton energies are greatly reduced by up to ~0.25 eV with respect to excitons formed in separated molecules. Importantly, despite their low energies, the iCT excitons have reduced non-adiabatic electron-vibration couplings with the electronic ground state, thus suppressing non-radiative recombination and allowing Y6 to overcome the well-known energy gap law. Our results reveal the fundamental relationship between molecular packing and nanosecond excited-state lifetimes in NIR-absorbing Y6-type acceptors underlying the outstanding performance of Y6-based OSCs.

We investigated the influence of two passivating molecules containing a P═O group on the performance of quasi-2D Dion–Jacobson halide perovskite light-emitting diodes, namely, triphenylphosphine oxide (TPPO) and diphenyl-4- triphenylsilylphenyl phosphine oxide (TSPO1). We found that both passivating molecules lead to increased efficiency compared to control devices, while they had opposite effects on device lifetime, with a decrease observed for TPPO and an increase observed for TSPO1. The two passivating molecules resulted in differences in energy-level alignment, electron injection, film morphology and crystallinity, and ion migration during operation. While TPPO resulted in improved photoluminescence decay times, overall higher maximum external quantum efficiency (EQE) and device lifetime were obtained for TSPO1 compared to TPPO (14.4% vs 12.4% EQE, 341 min vs 42 min T50).

Dion-Jacobson (DJ) perovskites have been demonstrated to be highly promising for improved lifetime and stability of solar cells, but their applications in light emitting diodes (LEDs) have been scarce. A large number of potential spacer ligands have not been explored, offering potential to achieve significant improvements in spectral regions where DJ perovskite-based LEDs lag behind other perovskite counterparts. Therefore, we investigated the effect of the choice of spacer cation on the properties and green light emitting diode applications of DJ perovskites. We found that optimized mixing of the spacer cations results in perovskite films with improved morphology and a change in phase distribution, leading to enhanced funneling and more efficient light emission. Devices containing mixed spacer cations 1, 6-hexanediammonium (HDA) and 1, 10- decanediammonium (DDA) exhibited significant enhancement of the external quantum efficiency (EQE), with a maximum value for DDA0.75HDA0.25 of 9.41% significantly higher than that obtained for pure HDA (1.23%) and DDA (7.28%), as well as increased lifetime with T50 of 70 min for optimized composition, compared to 4 min for pure HDA and 60 min for pure DDA. The maximum EQE can be further increased by triphenylphosphine oxide passivation to 12.85%.

Racemic low-conjugated non-emissive THPs 1–3 can form highly emissive RS- and RR/SS-packing polymorphs with mixed through-bond and through-space conjugation., Racemic C6-unsubstituted tetrahydropyrimidines (THPs) are a series of fluorophores with a strong aggregation-induced emission (AIE) effect. However, they do not possess the structural features of conventional AIE compounds. In order to understand their AIE mechanism, here, the influences of the molecular packing mode and the conformation on the optical properties of THPs were investigated using seven crystalline polymorphs of three THPs (1–3). The racemic THPs 1–3 have low-conjugated and highly flexible molecular structures, and hence show practically no emission in different organic solvents. However, the fluorescence quantum yields of their polymorphs are up to 93%, and the maximum excitation (λ ex) and emission (λ em) wavelengths of the polymorphs are long at 409 and 484 nm, respectively. Single-crystal structures and theoretical calculation of the HOMOs and LUMOs based on the molecular conformations of these polymorphs indicate that the polymorphs with the shortest λ ex and λ em values possess a RS-packing mode (R- and S-enantiomers self-assemble as paired anti-parallel lines) and a more twisted conformation without through-space conjugation between the dicarboxylates, but the polymorphs with longer λ ex and λ em values adopt a RR/SS-packing mode (R- and S-enantiomers self-assemble as unpaired zigzag lines) and a less twisted conformation with through-space conjugation between the dicarboxylates. The molecular conformations of 1–3 in all these polymorphs are stereo and more twisted than those in solution. Although 1–3 are poorly conjugated, the radiative rate constants (k r) of their polymorphs are as large as conventional fluorophores (0.41–1.03 × 108 s–1) because of improved electronic conjugation by both through-bond and through-space interactions. Based on the obtained results, it can be deduced that the strong AIE arises not only from the restriction of intramolecular motion but also from enhanced electronic coupling and radiatively-favored inter-crossed local excitation (LE) and intramolecular charge transfer (ICT) excitation states. The abnormal molecular structures, easily-controllable self-assembly of the R- and S-enantiomers, and the strong AIE effect make THPs very useful fluorophores for applications and theoretical research.

Chiral nematic liquid crystals (N*LCs) are a kind of smart material that can be obtained directly from cholesteric LCs or from nematic LCs by adding a chiral dopant. As a 1D photonic crystal, [ 1 ] the helical and periodic structure of a N*LC leads to unique optical properties, including selective reflection, circular dichroism (CD), optical rotation, and so on. [ 2–5 ] The most important property of N*LCs is the selective reflection of circularly polarized (CP) light. When N*LCs are sandwiched between two substrates with planar anchoring surfaces, CP light with the same handedness as the helix will be selectively reflected whereas light with opposite handedness will be transmitted.

Self-assembling of luminescent molecules into one-dimensional nanostructures is of particular interest in fabricating nanoscale electronic and photonic devices. Herein, we report the rational design and synthesis of a chiral fluorescent tetraphenylethylene derivative containing L-leucine methyl ester moiety (TPE-Leu). In solution, TPE-Leu is non-emissive and CD silent, but becomes highly emissive and CD active upon aggregation, exhibiting aggregation-induced emission (AIE) and chirality (AIC). Upon evaporation of its solution, TPE-Leu readily self-assembles into helical fluorescent micro/nanofibers, which show circularly polarized luminescence (CPL) and have the CPL dissymmetry factors in the range of 0.02–0.07. This molecular design combines the AIE effect, chirality, and self-assembling capability together, and is highly efficient in constructing novel functional micro/nanomaterials with well-defined structures and enhanced emission.

Femtosecond time-resolved reflectivity was used to investigate below-band-gap (3.1 eV) carrier dynamics in a nominally undoped GaN epilayer under high excitation. A 2.5-ps rising process can be observed in the transient trace. This shot rising time results from the hot phonon effects which can cause a delayed energy relaxation of the initial photocarriers toward the band edge. From the density dependence of the carrier dynamics, the Mott density was estimated to be 1.51-1.56×1019 cm-3. Below the Mott density, the initial probed carrier dynamics was explained to the effect of acoustic phonon-assisted tunnelling for localized states, where a significant excitation density dependence of the tunnelling probability was observed due to the optically induced bandtail extension to lower energies. Above the Mott density, the measured carrier dynamics reflected the relaxation of an electron-hole plasma, in which a distinct fast decay component of 2.3 ps was observed due to the onset of nonlinear relaxation processes such Auger recombination.

In this work, a heteroatom-containing luminogen (TPE-Py) with multi-functionalities was synthesized in a reasonable yield by melding a pyridinium unit with tetraphenylethene through vinyl functionality. TPE-Py is weakly emissive in solution but becomes a strong emitter when aggregated as nanoparticle suspensions in poor solvents or in the solid state, displaying a phenomenon of aggregation-induced emission. Crystallization generally weakens and red-shifts the light emission. The crystalline aggregates of TPE-Py, however, emit stronger and bluer light than their amorphous counterparts. The solid-state emission of TPE-Py can be reversibly switched between green and yellow color by grinding–fuming and grinding–heating processes with a high contrast due to the transformation from the crystalline to the amorphous state and vice versa. The large Stokes shift and well-ordered molecular arrangement of the crystalline microrods of TPE-Py make it promising as an optical waveguide material with a low optical loss coefficient of ∼0.032 dB μm−1. TPE-Py works as a good fluorescent visualizer for specific staining of mitochondria in living cells with a high photostability, thanks to its hydrophobic and cationic features.

Tetraphenylethene-containing diyne (1), named 1,2-bis(4-ethynylphenyl)-1,2-diphenylethene, was synthesized and polymerized by TaBr5 catalyst, affording the hyperbranched polymer hb-P1 with a high molecular weight (Mw up to 157[thin space (1/6-em)]800) in a nearly quantitative yield. The cyclotrimerization of 1-(4-ethynylphenyl)-1,2,2-triphenylethene was also carried out to give 1,3,5- and 1,2,4-tris[4-(1,2,2-triphenylvinyl)phenyl]benzene (1,3,5-2 and 1,2,4-2) that served as model compounds for structural characterization and properties investigation. The structures of hb-P1 and 2 are characterized by MS, IR, and NMR spectroscopies with satisfactory results. All the molecules and polymer are completely soluble in common organic solvents such as chloroform, toluene and THF. They show high thermal stability with degradation temperatures up to 443 and 446 °C, respectively, in nitrogen and air. Whereas their solutions are almost non-emissive or weakly fluorescent, their aggregates in poor solvents and solid powders emit intensely with fluorescence quantum yields up to 81%, demonstrating a novel phenomenon of aggregation-induced or enhanced emission. The unreacted triple bonds on the periphery of hb-P1 allow its thin film to cross-link under UV irradiation, generating well-resolved photopatterns. The hyperbranched polymer can limit harsh laser pulses. Its emission can be quenched efficiently by picric acid in solution and aggregated states with large quenching constants, suggesting that it is a promising fluorescent chemosensor for explosives detection.

Benzene-cored luminogens with multiple triarylvinyl units are designed and synthesized. These propeller-shaped molecules are nonemissive when dissolved in good solvents, but become highly emissive when aggregated in poor solvents or in the solid state, showing the novel phenomenon of aggregation-induced emission. Restriction of intramolecular motion is identified as the main cause for this effect. Thanks to their high solid-state fluorescence quantum yields (up to unity) and high thermal and morphological stabilities, light-emitting diodes with the luminogens as emitters give sky-blue to greenish-blue light in high luminance and efficiencies of 10800 cd m−2, 5.8 cd A−1, and 2.7%, respectively. The emissions of the nanoaggregates of the luminogens can be quenched exponentially by picric acid, or selectively by Ru3+, with quenching constants up to 105 and ∼2.0 × 105 L mol−1, respectively, making them highly sensitive (and selective) chemosensors for explosives and metal ions.

Sensors based on fluorescent conjugated polymers (FCPs) have attracted much attention, due to their amplified response and superior sensitivity to analytes, in comparison to their low molar mass congeners.1 Among many potential applications of FCPs, the detection of explosives is of great current interest because of its anti-terrorism implications.2 Much effort has been devoted to the development of linear FCPs with an amplification effect for sensitive detection of explosives. It has been shown that a FCP with a fast rate of exciton migration over a long sequence of its repeat units along the one-dimensional (1D) chain affords a high sensitivity (Fig. 1A).1 The sensing performance of the FCP, however, is often deteriorated by the aggregation of its linear chains. This notorious aggregation-caused quenching (ACQ) effect interferes with the detection of the analytes that induce the polymer chains to aggregate in solutions.

Urban dwelling densities have profound implications on the form of built environment, which in turn incurs significant effects on the patterns of resource use associated with housing construction and operational needs. Hong Kong is a compact, industrialized city with housing at very high densities. This thesis proposes and tests the hypothesis that Hong Kong's housing represents a green model of residential development which is conducive to low per capita consumption. Reducing per capita consumption is a primary greening agenda, especially for industrialized regions. "Wastes" are also increasingly seen as resources. This thesis addresses three aspects of resource use: energy, water (including "wastewater"), and materials (including solid "waste"). Hong Kong's housing development is commonly in the form of estates which are characterized by large site area measured in hectares and towers of 30-40 storeys. The current densities are 2000- 4000 residents per hectare of net site area, plot ratio of 5 and above, 1-2 square metres of communal open space per resident, and 15-20 square metres of gross floor area per resident. In contrast, the corresponding densities of North American single-family dwelling are 50-100 residents per hectare of net site area, plot ratio of less than 1, about 100 square metres of private open space per resident, and 80-100 square metres of floor area per resident. The analysis offered in this thesis shows that Hong Kong's dwelling densification is conducive to efficient centralized systems such as mass transit, sea-water flushing, and inorganic waste segregation — which consequently lead to low per capita consumption of fossil fuels and potable water and high per capita recovery of domestic solid waste. On a building level, the attached, compact dwelling configuration also leads to low per capita energy and material use associated with housing construction. However, very high "building densities" entail physical/spatial constraints on a number of greening practices, such as harnessing of ambient wind for space cooling and on-site recovery of wastewater and organic waste. A direction towards high "occupancy density" in company with moderate "building density" should be considered as a greener housing alternative in long term.

Solar cells incorporating lead halide-based perovskite absorbers can exhibit impressive power conversion efficiencies (PCEs), recently surpassing 15%. Despite rapid developments, achieving precise control over the morphologies of the perovskite films (minimizing pore formation) and enhanced stability and reproducibility of the devices remain challenging, both of which are necessary for further advancements. Here we demonstrate vacuum-assisted thermal annealing as an effective means for controlling the composition and morphology of the CH3NH3PbI3 films formed from the precursors of PbCl2 and CH3NH3I. We identify the critical role played by the byproduct of CH3NH3Cl on the formation and the photovoltaic performance of the perovskite film. By completely removing the byproduct through our vacuum-assisted thermal annealing approach, we are able to produce pure, pore-free planar CH3NH3PbI3 films with high PCE reaching 14.5% in solar cell device. Importantly, the removal of CH3NH3Cl significantly improves the device stability and reproducibility with a standard deviation of only 0.92% in PCE as well as strongly reducing the photocurrent hysteresis.