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

Abstract

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...

Polymeric AIE-based nanoprobes for biomedical applications: recent advances and perspectives

“…These polymeric micelles are formed through self-assembly of amphiphilic block copolymers in aqueous solution. [133][134][135][136][137][138][139][140][141] In 2010, Jen et al reported a non-covalent method for the coencapsulation of two AIE dyes (1,1,2,3,4,5-hexaphenylsilole (HPS) and/or bis(4-(N-(1-naphthyl) phenylamino)-phenyl)fumaronitrile (NPAFN)) into three amphiphilic block copolymers (Fig. 1).…”

Poly(l-lactide)s with tetraphenylethylene: role of polymer chain packing in aggregation-induced emission behavior of tetraphenylethylene

“…50–55 Because of its intrinsic properties, many fluorescent molecules have been incorporated into PLLA to afford unique functionalities. 22,56–62 Hsiao et al showed that the crystallization of PLLA chains determines the AIE properties of 1,2-bis(2,4-dihydroxybenzylidene)hydrazine after incorporation into the backbone of PLLA. 57 Zhao et al incorporated TPE into the PLLA chains by living chain transfer polymerization and demonstrated that these polymers exhibit unique emission properties in the aggregated states.…”

“…22,56–62 Hsiao et al showed that the crystallization of PLLA chains determines the AIE properties of 1,2-bis(2,4-dihydroxybenzylidene)hydrazine after incorporation into the backbone of PLLA. 57 Zhao et al incorporated TPE into the PLLA chains by living chain transfer polymerization and demonstrated that these polymers exhibit unique emission properties in the aggregated states. 22 They have demonstrated that star-shaped PLLA (SSPLLA) was non-luminescent in good solvents like tetrahydrofuran (THF) and emits strongly when a poor solvent (water) was added due to aggregate formation.…”

Star-Shaped Poly(l-lactide) with a Dipyridamole Core: Role of Polymer Chain Packing on Induced Circular Dichroism and Photophysical Properties of Dipyridamole

“…The incorporation of chromophores into the insulating thermoplastic polymer by the chemical method can be categorized into the following groups: chromophores as part of the backbone of polymer chains, − polymer chains end-capped with chromophores, , and chromophores as core molecules in the star-shaped polymer. − The major advantage of these methods is the easy fabrication of thin solid films and devices by spin-casting or inkjet printing under mild conditions. Recently, a few saturated polymers containing chromophores are prepared and tested for their photophysical properties. ,, Besides these achievements, the use of biodegradable and biocompatible polymers attracted the scientific and industrial interest because these materials can find applications as the bioprobes of real-time imaging.…”