AIE-related emission of polypeptide containing an AIE-active terminal is correlated with secondary structures (α-helix, β-sheet and random coil) of the peptide chains.
Fluorescent 1,2-bis(2,4-dihydroxybenzylidene)hydrazine (CN 4 OH) 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, CN 4 OH 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.
IntroductionIn 2001, Tang's group 1,2 discovered that the propeller-shaped molecule of 1-methyl-1,2,3,4,5-pentaphenylsilole (MPPS) emits strongly in the aggregated solution and solid film states despite being non-emissive in a dilute solution. This concentration-dependent emission behaviour was designated as aggregation-induced emission (AIE). With beneficial emission efficiency in the solid film state, lots of organic and polymeric fluorophores with AIE or aggregation-enhanced emission (AEE) properties 3-8 have been prepared and characterized. Although an AIE process may mechanistically be associated with pathways like J-aggregate formation (JAF), 9 twisted intramolecular charge transfer (TICT), 10-12 and excited-state intramolecular proton transfer (ESIPT), 13 the intrinsic mechanism is actually due to the effective restriction on the intramolecular motions of the AIE-active fluorophores. A study 14 on the AIEactive fluorophores of different molecular shapes has concluded that restrictions of intramolecular rotations and vibrations (RIR and RIV) are the main causes for the AIE phenomenon observed in the propeller-shaped and shell-like fluorophores, respectively. Similar to the first discovered silole derivative of MPPS, 1,2 most of the AIE-active fluorophores are propeller-like molecules. In the aggregated state of MPPS, the sluggish molecular rotation 15,16 of aromatic rings reduces the possibilities of non-radiative decay pathways and results in an enhanced emission.Crystallization is an important factor contributing to the intense solid state emission in view of the fact that fluorophores with crystallization-enhanced emission (CEE) 17-22 properties are highly-emissive in the solid crystalline state but are weakly emissive when they are in the am...
ARTICLE
RSC Adv.As restricted intramolecular rotation (RIR) is the main mechanism responsible for the aggregationinduced or -enhanced emission (AIE or AEE), we thereby use rigid polyrotaxane to impose effective RIR for an ionic water-soluble, AEE-active luminogen of sodium 4,4',4'',4'''-(thiophene-2,3,4,5tetrayl)tetrabenzenesulfonate (TPS). Jeffamine-included β-cyclodextrin (JA-included β-CD) with cationic ammonium terminals was used as rigid component to complex with sulfonate anions of TPS, resulting in iTP-CD-JA complex for characterization. Comparison is also made on complex of iTP-JA prepared from the complexation of TPS with the flexible JA chains. Without the incorporation of rigid β-CD rings, the iTP-JA complex is inferior to the iTP-CD-JA complex in the emission efficiency. The role of rigid template in imposing effective RIR on AEE-active luminogen is thus demonstrated in this study.ARTICLE 2 | RSC Adv., 2014 RSC Adv.
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