Publications

Treating diethynyl-substituted perylenediimides with ICI successfully induced an annulation reaction and generated a series of coronenediimide derivatives. Instead of the expected iodine-substituted cyclization product, chlorine-substituted analogues were produced. The mechanism of this annulation reaction thus necessarily involved a chlorine addition step prior to the aromatic substitution reaction. With facile subsequent transformations, various tetraaryl coronenediimides could be obtained via the annulated chloro-substituted coronenediimide.

As a representative folding system that features a conjugated backbone, a series of monodispersed (o-phenyleneethynylene)-alt-(p-phenyleneethynylene) (PE) oligomers of varied chain length and different side chains were studied. Molecules with the same backbone but different side-chain structures were shown to exhibit similar helical conformations in respectively suitable solvents. Specifically, oligomers with dodecyloxy side chains folded into the helical structure in apolar aliphatic solvents, whereas an analogous oligomer with tri(ethylene glycol) (Tg) side chains adopted the same conformation in polar solvents. The fact that the oligomers with the same backbone manifested a similar folded conformation independent of side chains and the nature of the solvent confirmed the concept that the driving force for folding was the intramolecular aromatic stacking and solvophobic interactions. Although all were capable of inducing folding, different solvents were shown to bestow slightly varied folding stability. The chain-length dependence study revealed a nonlinear correlation between the folding stability with backbone chain length. A critical size of approximately 10 PE units was identified for the system, beyond which folding occurred. This observation corroborated the helical nature of the folded structure. Remarkably, based on the absorption and emission spectra, the effective conjugation length of the system extended more effectively under the folded state than under random conformations. Moreover, as evidenced by the optical spectra and dynamic light-scattering studies, intermolecular association took place among the helical oligomers with Tg side chains in aqueous solution. The demonstrated ability of such a conjugated foldamer in self-assembling into hierarchical supramolecular structures promises application potential for the system.

A series of dihydro- and tetrahydro-tetraazaacene diimides containing 6 or 7 laterally fused six-membered rings were synthesized. Halochromic and redox-switchable vis-NIR optical characteristics were exhibited. Quinonoid tautomers of dihydrotetraazaacene derivatives were obtained and characterized, which exhibited adequate stability and existed in equilibrium with the more commonly observed benzenoid tautomer.

The photophysical properties of a series of tris-cyclometalated h(Ill) complexes bearing oligofluorene-substituted 2-phenylpyridine (ppy) and/ or 1-phenylisoquinoline (piq) ligands were studied at both room temperature and 77 K, for delineating the oligomer-substitution effects on the photophysics in such metal-complex-containing conjugated oligomers/polymers. Unique temperature dependence was observed with the triplet excited-state lifetime of the studied oligomers. Molecules having one of the three ppy ligands substituted with an oligofluorenyl group at varied positions exhibited two distinct types of phosphorescing behaviors. When the oligoflurene group was coupled to ppy in a conjugative fashion (i.e., at 5- or 4'- position), the complexes appeared to emit from a (MLCT)-M-3-dominated state perturbed by LC transition, as evidenced by the relatively short lifetimes of phosphorescence as well as hypsochromic shift upon lowering the temperature. Surprisingly, even shorter triplet lifetimes were detected at 77 K for such oligomers. When the oligofluorenyl was tethered to the phenyl ring of ppy meta to pyridine, emission properties were consistent with a (LC)-L-3-dominated state, mixed with a certain MLCT component. Uniquely, for these oligomers an evident bathochromic shift of emission with a significantly retarded radiative decay rate was observed at 77 K. Furthermore, when a piq ligand was incorporated, red phosphorescence characteristic of Ir-piq-based (MLCT)-M-3 transition emerged, disregarding the substation position of the oligofluorene. All these different photophysical behaviors, particularly their unique temperature dependence, were explained by considering an energy transfer process between different triplet states, with dominant MLCT and LC characteristics. In complexes having all ppy-derived ligands, these two states were of similar but different energy. While one played a more important role than the other, both were contributing to the phosphorescence emission. The temperature dependence of the photophysics reflected the equilibrium shifting process. When the (MLCT)-M-3-dominated state was lower in energy, faster radiative decay and shorter lifetimes were manifested upon lowering the temperature, as a result of more favored (MLCT)-M-3-dominated state. Whereas if the (LC)-L-3-dominated state was more stable, slower radiative decay emerged at decreased temperature due to further a reduced MLCT contribution. The bathochromic shift was also a result of equilibrium shifting to the state of lower energy. When the piq ligand was engaged, the emission was governed by the (MLCT)-M-3 state of the Ir-piq moiety, which had much lower energy compared to the triplet states localized in oligofluorenyl ppy. DFT calculations substantiated the above hypothesis by identifying separate molecular orbitals possessing mixed but imbalanced MLCT and LC components.

A series of triangular, shape-persistent arylene-ethynylene macrocycles (AEMs) of related structures were synthesized and studied, with a focus on their mesomorphic behavior in correlation with their chemical structure. Generally, these discotic molecules decorated with flexible side chains demonstrated a propensity to form thermotropic liquid-crystalline (LC) phases. Characterized by differential scanning calorimetry (DSC), polarized optical microscopy (POM), and X-ray diffraction (XRD), four of the eight investigated macrocycles manifested thermodynamically stable mesophases, featuring discotic nematic or columnar structures. Longer alkyl side chains were found more conducive to mesophases, and the alkoxycarbonyl functionality was a more effective side-chain linkage at inducing and stabilizing the LC states than the alkoxy side group. The size and structure of the cyclic aromatic backbone influenced both the occurrence and type of mesophase exhibited.