Jin-Long Hong

Aggregation-enhanced emission in fluorophores containing pyridine and triphenylamine terminals: restricted molecular rotation and hydrogen-bond interaction

Restriction on molecular rotation of fluorophores reduces non-radiative decay channels and promotes strong fluorescence due to aggregation-enhanced emission (AEE) behavior. To evaluate the important role of restricted molecular rotation on AEE behavior, tetraphenylthiophene (TP) derivatives with two pyridine (Py) or two triphenylamine (TPA) terminals were synthesized and characterized to be AEE-active fluorophores. Because of the efficient hindered molecular rotation of the larger TPA terminals, TP-2TPA emitted with higher emission efficiency than TP-2Py with smaller Py terminals. In addition, Py and TPA terminals can serve as hydrogen-bond (H-bond) accepting groups to bind with H-bond donating hydroxyl groups in poly(vinyl phenol) (PVPh) and poly(vinyl alcohol) (PVA) to further reinforce rotational restriction on the TP-2Py and TP-2PTA fluorophores. TP-2Py and TP-2PTA were therefore blended with PVPh and PVA and the emissive properties of the resultant blends were characterized and compared with the unblended TP-2Py and TP-2PTA to emphasize the role of H-bond on restricted molecular rotation.

Complexing AIEE-active tetraphenylthiophene fluorophore to poly(N-isopropyl acrylamide): fluorescence responses toward acid, base and metal ions

In this article, a multiple-responsive polymer micelles system was constructed by using an ionic bond as the link to connect the hydrophobic tetraphenylthiophene (TP) fluorophores having aggregation-induced emission enhancement (AIEE), and the hydrophilic poly(N-isopropyl acrylamide) (PNIPAM). The susceptibility of the ionic ammonium-sulfonate (Am-Sul) bond towards metal ions, acid and base triggered the AIEE-operative fluorescence (FL) response. To exercise the idea, PNIPAM with a sulfonate terminal was primarily prepared to react with an ammonium-functionalized TP derivative to generate a polymer complex of TP-PNIPAM. When in water, the polymer complex TP-PNIPAM formed micelles with the aggregated TP core interconnecting the hydrophilic PNIPAM shell by the ionic Am-Sul bonds. With the operative AIEE effect, the aggregated TP core in the micelles fluoresced strongly but upon the additions of metal ions, acid and base, the ionic bonds dissociated to result in the collapse of the micelles and the corresponding emission quenching. A novel fluorogenic sensor capable of responding to multi-stimuli was therefore constructed.

Unexpected fluorescence from maleimide-containing polyhedral oligomeric silsesquioxanes: nanoparticle and sequence distribution analyses of polystyrene-based alternating copolymers

In this study, we synthesized unusual fluorescent polyhedral oligomeric silsesquioxane (POSS)-containing polymers lacking any common fluorescent units (e.g., phenyl or heterocyclic rings): a poly(maleimide isobutyl POSS) [poly(MIPOSS)] homopolymer and poly(styrene-alt-maleimide isobutyl POSS) [poly(S-alt-MIPOSS] and poly(4-acetoxystyrene-alt-maleimide isobutyl POSS) [poly(AS-alt-MIPOSS)] alternating copolymers, through free radical polymerization, and a poly(4-hydroxystyrene-alt-maleimide isobutyl POSS) [poly(HS-alt-MIPOSS)] alternating copolymer, through acetoxy hydrazinolysis of poly(AS-alt-MIPOSS). We used 1H, 13C, and 29Si nuclear magnetic resonance spectroscopy, Fourier transform infrared (FTIR) spectroscopy, and MALDI-TOF mass spectrometry to examine the chemical structures and sequence distributions of these POSS-containing polymers. The FTIR spectra revealed the existence of specific intermolecular interactions, namely dipole–dipole interactions between the CO groups in poly(MIPOSS) and poly(AS-alt-MIPOSS) and intermolecular hydrogen bonding between the CO groups of the MIPOSS units and the OH groups of the HS units in poly(HS-alt-MIPOSS). Differential scanning calorimetry and thermogravimetric analyses revealed that the incorporation of MIPOSS units could enhance the thermal stability, but decrease the glass transition temperatures, of these alternating copolymers. The photoluminescence emission of poly(MIPOSS) was greater than those of the POSS-containing alternating copolymers, presumably because of the formers crystallinity and clustering of locked CO groups of POSS units.

Complexation of Fluorescent Tetraphenylthiophene-Derived Ammonium Chloride to Poly(N-isopropylacrylamide) with Sulfonate Terminal: Aggregation-Induced Emission, Critical Micelle Concentration, and Lower Critical Solution Temperature

Amphiphilic polymers with hydrophilic poly(N-isopropylacylamide) (PNIPAM) shell connecting hydrophobic tetraphenylthiophene (TP) core, which has the novel aggregation-induced emission (AIE) property, by ionic bonds were prepared to explore the AIE-operative emission responses toward critical micelle concentration (CMC) and lower critical solution temperature (LCST). To exercise the idea, ammonium-functionalized TP2NH(3)(+) and sulfonate-terminated PNIPAM were separately prepared and mixed in different molar ratios to yield three amphiphilic TP-PNIPAMn complexes for the evaluations of CMC and LCST by fluorescence responses. The nonemissive dilute aqueous solutions of TP-PNIPAMn became fluorescent when increasing concentrations above CMC. Heating micelles solution to temperatures above LCSTs causes further enhancement on the emission intensity. The fluorescence responses are explained by the extent of aggregation in the micelles and in the globules formed at room temperature and at high temperatures, respectively.

Enhanced emission of a pyridine-based luminogen by hydrogen-bonding to organic and polymeric phenols

Fluorescent bis(pyridinylvinyl)anthracene (An2Py) with two pyridine terminals was synthesized and used to prepare miscible blends with hydroxyl-containing components (organic bisphenol A (BPA) and polymeric poly(vinyl phenol) (PVPh)) through the facile intermolecular hydrogen-bond (H-bond) interactions between the pyridine and the hydroxyl functions. Before blending, the solution of An2Py already emits appreciably due to its aggregation-induced emission enhancement (AIEE) behavior; after blending with the hydroxyl components, the fluorescence can be further intensified due to the restricted molecular rotation, which leads to the blockage of non-radiative decay channels, imposed by the H-bond interactions. The role of the H-bonding on the restricted molecular rotation of the An2Py/BPA (and the An2Py/PVPh) blends was characterized by solution 1H NMR and solid infrared spectroscopy. The effectiveness of the organic BPA and the polymeric PVP as molecular anchors to lock the free rotation of the An2Py luminogen were compared and discussed in this study.

Strong emission of 2,4,6-triphenylpyridine-functionalized polytyrosine and hydrogen-bonding interactions with poly(4-vinylpyridine)

In this paper 2,4,6-triphenyl pyridine-functionalized polytyrosine (Pyridine-PTyr) was successfully synthesized by living ring-opening polymerization where 2,6-bis(4-aminophenyl)-4-phenylpyridine (Pyridine-NH2) was the initiator. The photo-physical characteristics of Pyridine-NH2 and Pyridine-PTyr were elucidated via UV-vis absorption and photoluminescence spectra, revealing that unlike Pyridine-PTyr, Pyridine-NH2 shows solvatochromic effects in solvents with different polarities. Additionally, Pyridine-NH2 exhibited aggregation-caused quenching (ACQ) phenomena; however, it became an aggregation-induced emission (AIE) material after attachment to the rigid-rod conformation of polytyrosine. Based on differential scanning calorimetry results, we observed that after blending Pyridine-PTyr with P4VP a single glass transition temperature due to their miscibility through the intermolecular hydrogen bonding of the phenolic OH groups in the PTyr backbone and pyridine ring in P4VP was revealed, as indicated by IR spectroscopy. Obviously, the emission intensity of Pyridine-PTyr decreased after blending with P4VP with a hypsochromic shift from 536 to 489 nm, presumably due to the release of the restricted intramolecular rotation of the triphenyl pyridine unit in the center of the polymer and the polymer chains of Pyridine-PTyr became separated random coils based on WAXD results.

Amorphous and crystalline blends from polytyrosine and pyridine-functionalized anthracene: hydrogen-bond interactions, conformations, intramolecular charge transfer and aggregation-induced emission

A new pyridine-terminated fluorophore of (E)-4-(2-(anthracen-9-yl)vinyl)pyridine (AnPy) with intramolecular charge transfer (ICT) and aggregation-induced emission (AIE) properties was synthesized and was blended with different amounts of polytyrosine (PTyr) through preferable hydrogen-bond (H-bond) interactions. In blends of a low AnPy content, the rigid PTyr peptide chains serve as templates to H-bond to AnPys, imposing rotational restriction and reinforcing the AIE-related emission intensity of AnPys, resulting in amorphous blends with the observed glass transitions dependent on the composition of the blends. In contrast, when large amounts of AnPys were added, excess AnPys will form new crystals, in between the amorphous regions, constituted of the near parallel dimers of AnPys. With the hampered molecular rotation, the parallel dimers of AnPys in the highly AnPy-loaded blends emit strongly with intensity much higher than those for the amorphous blends. In this study, conformations of the blends and the degree of restricted molecular rotation were assessed in order to correlate with the AIE-related fluorescence behaviour.

Crystallization-enhanced emission through hydrogen-bond interactions in blends containing hydroxyl-functionalized azine and poly(4-vinyl pyridine)

An organic azine derivative of CN4OH, containing both para- and ortho-hydroxyl (o- and p-OH) groups, is a fluorescent material with an emission efficiency dependent on the degree of crystallinity. With inherent hydroxyl groups, CN4OH can be homogeneously blended with different amounts of poly(4-vinyl pyridine) (PVP) through intermolecular hydrogen-bond (H-bond) interactions. With the incorporation of one and two molar equivalents of PVP, the solid CN4OH/PVP (4/1) and (2/1) blends emit strongly with intensity higher than pure CN4OH. Nevertheless, a further increase of the PVP content considerably reduced the crystallinity and the emission efficiency of the blend. Initially, PVP was preferably H-bonded to the p-OHs of CN4OH, resulting in the beneficial crystallization-enhanced emission (CEE); nevertheless, the PVP added in the later step started to bond to the o-OHs of CN4OH, reducing the crystallinity and the CEE-related fluorescence. With appropriate H-bond interactions, the CN4OH/PVP (2/1) blend emits with a high quantum yield (ΦF) of 88%, in contrast to the low ΦF of 15% for pure CN4OH.

Luminescent polymers and blends with hydrogen bond interactions

Organic luminogens with aggregation-induced emission (AIE) properties have unusual emission behaviour in that their non- or weakly-emissive solutions can be tuned to emit strongly in the solution aggregated and solid states. As restricted molecular rotation of luminogens is the main mechanism leading to AIE activity, hydrogen bond (H bond) interactions are therefore used in organic and polymeric luminogens to impose efficient rotational restriction and to intensify the emission intensity of the AIE-active luminogens. Luminogenic polymers containing H-bonding groups and luminogenic blends consisting of H-bond donors and acceptors are therefore described in this review to evaluate the relationship between H bond interactions, rotational restriction and AIE-related emission intensity.

Crystallization-promoted emission enhancement of poly(L-lactide) containing a fluorescent salicylideneazine center with aggregation-enhanced emission properties

Fluorescent 1,2-bis(2,4-dihydroxybenzylidene)hydrazine (CN4OH) with aggregation-enhanced emission (AEE) properties was used as the initiator to induce the ring-opening polymerization (ROP) of L-lactide, resulting in polymer CN-PLLA(n)s containing an AEE-active CN center. With both pairs of p- and o-hydroxyl (OH) groups, CN4OH initiates an ROP of L-lactide solely with the p-OH groups and the resulting CN-PLLA(n)s are highly-emissive due to the rotational restriction imposed by the remaining p-OHs of the central CN unit. A study on the solid emission of CN-PLLA(n)s reveals that crystallization of the neighbouring PLLA chains, instead of the fluorescent CN center itself, determines the AEE activity, e.g. the emission of crystalline CN-PLLA(n)s is much higher in emission intensity than amorphous CN-PLLA(n)s. As restricted molecular rotation is the main mechanism leading to AEE activity, effective rotational restriction imposed by crystalline polylactide chains is responsible for the high emission of crystalline CN-PLLA(n)s, in contrast to the weak emission of amorphous CN-PLLAs. The emission promotion of the fluorescent CN center by the neighbouring polylactide chains is designated as crystallization-promoted emission enhancement (CPEE) and is the focus of the study.