Towards Upcycling Biomass-Derived Crosslinked Polymers with Light.

Photodegradable, recyclable, and renewable, crosslinked polymers from bioresources show promise towards developing a sustainable strategy to address the issue of plastics degradability and recyclability. Photo processes are not widely exploited for upcycling polymers in spite of the potential to have spatial and temporal control of the degradation in addition to being a green process. In this report we highlight a methodology in which biomass-derived crosslinked polymers can be programmed to degrade at ≈300 nm with ≈60 % recovery of the monomer. The recovered monomer was recycled back to the crosslinked polymer.

Self-Assembled Helical Arrays for the Stabilization of the Triplet State.

Room-temperature phosphorescence of metal and heavy atom-free organic molecules has emerged as an area of great potential in recent years. A rational design played a critical role in controlling the molecular ordering to impart efficient intersystem crossing and stabilize the triplet state to achieve room-temperature ultralong phosphorescence. However, in most cases, the strategies to strengthen phosphorescence efficiency have resulted in a reduced lifetime, and the available nearly degenerate singlet-triplet energy levels impart a natural competition between delayed fluorescence and phosphorescence, with the former one having the advantage. Herein, an organic helical assembly supports the exhibition of an ultralong phosphorescence lifetime. In contrary to other molecules, 3,6-phenylmethanone functionalized 9-hexylcarbazole exhibits a remarkable improvement in phosphorescence lifetime (>4.1 s) and quantum yield (11 %) owing to an efficient molecular packing in the crystal state. A right-handed helical molecular array act as a trap and exhibits triplet exciton migration to support the exceptionally longer phosphorescence lifetime.

Effect of N-Alkyl Substituents on the Hierarchical Self-Assembly of ß-Cyclodextrin-Linked Pyrene-Pyromellitic Diimide Charge-Transfer Complexes.

Hierarchical self-assemblies of ß-cyclodextrin-linked pyrene and N-alkyl derivatives of pyromellitic diimides are studied in detail. The charge-transfer interaction between pyrene and pyromellitic diimide is augmented by ß-cyclodextrin-pyromellitic diimide binding interactions in these cases. When the alkyl group is adamantyl, a 1:1 complex was formed with a very high association constant (Ka =1.82×106 m-1 ). Here, the charge-transfer interaction is reinforced by inclusion binding of the adamantyl group in the ß-cyclodextrin cavity leading to the formation of 2D sheets, which undergo twisting to give twisted fibres. When the alkyl group is tert-butyl, a 1:2 complex was formed with a high association constant (Ka =2.91×104 m-1 ). A detailed analysis showed that the tert-butyl pyromellitic diimide undergoes both inclusion and rim-binding interactions with the ß-cyclodextrin. The charge-transfer complex further self-assembled into chiral nanostructures as evident from SEM, TEM and AFM analysis. In the case of N-methyl-substituted pyromellitic diimide, the interaction with ß-cyclodextrin-linked pyrene was only through rim binding, which resulted in the formation of a weak charge-transfer complex with Ka =4.2×103 m-1 . Formation of a hierarchical assembly was not observed in this case. A rational mechanism for the self-assembly, which relies on the strength of the cyclodextrin-pyromellitic diimide complexation is presented.

Total Syntheses of the Isomeric Aglain Natural Products Foveoglin†A and Perviridisin†B: Selective Excited-State Intramolecular Proton-Transfer Photocycloaddition.

Selective excited-state intramolecular proton-transfer (ESIPT) photocycloaddition of 3-hydroxyflavones with trans, trans-1,4-diphenyl-1,3-butadiene is described. Using this methodology, total syntheses of the natural products (±)-foveoglin A and (±)-perviridisin B were accomplished. Enantioselective ESIPT photocycloaddition using TADDOLs as chiral hydrogen-bonding additives provided access to (+)-foveoglin A. Mechanistic studies have revealed the possibility for a photoinduced electron-transfer (PET) pathway.

Metal-Free Visible Light-Mediated Photocatalysis: Controlling Intramolecular [2 + 2] Photocycloaddition of Enones through Axial Chirality.

Atropisomeric enone-imides and enone-amides featuring N-CAryl bond rotation were evaluated for intramolecular [2 + 2] photocycloaddition. Straight addition product was observed over cross-addition product with good control over reactivity. The atropselectivity was found to be dependent on the substituent on the aryl ring. Substitution-dependent atropselectivity was rationalized on the basis of a divergent mechanistic pathway.

Programmed photodegradation of polymeric/oligomeric materials derived from renewable bioresources.

Renewable polymeric materials derived from biomass with built-in phototriggers were synthesized and evaluated for degradation under irradiation of UV light. Complete decomposition of the polymeric materials was observed with recovery of the monomer that was used to resynthesize the polymers.