Retheesh Krishnan

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.

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.

Origin of stretched-exponential photoluminescence relaxation in size-separated silicon nanocrystals

A detailed understanding of the photoluminescence (PL) from silicon nanocrystals (SiNCs) is convoluted by the complexity of the decay mechanism, including a stretched-exponential relaxation and the presence of both nanosecond and microsecond time scales. In this publication, we analyze the microsecond PL decay of size-resolved SiNC fractions in both full-spectrum (FS) and spectrally resolved (SR) configurations, where the stretching exponent and lifetime are used to deduce a probability distribution function (PDF) of decay rates. For the PL decay measured at peak emission, we find a systematic shift and narrowing of the PDF in comparison to the FS measurements. In a similar fashion, we resolve the PL lifetime of the ‘blue’, ‘peak’, and ‘red’ regions of the spectrum and map PL decays of different photon energy onto their corresponding location in the PDF. A general trend is observed where higher and lower photon energies are correlated with shorter and longer lifetimes, respectively, which we relate to the...

Total Syntheses of the Isomeric Aglain Natural Products Foveoglin A and Perviridisin B via Selective ESIPT 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.

Synthesis of silicon quantum dots using cyclohexasilane (Si6H12)

We report a novel, ambient pressure, continuous gas-phase, synthesis of Si-QDs by direct pyrolysis of cyclohexasilane (CHS, Si6H12), a liquid hydrosilane. We also report that using a low activation energy precursor such as Si6H12 enabled effective gas-phase reduction of the hydrosilane leading to better nucleation of Si nanoparticles than other hydrosilanes. By suitably designing the reactor length and its temperature profile, the structure of the Si-QDs are rendered amorphous or crystalline. The as-synthesized Si-QDs were thermally hydrosilylated with 1-dodecene and their properties examined. High resolution transmission electron microscopic (HRTEM) analysis revealed that the nano-crystalline Si-QDs (Si-NCs) have an average size of 2.0 nm. The amorphous Si-QDs (a-Si-QDs) and the Si-NCs samples exhibited UV-Vis absorptions below 300 and 375 nm, respectively. The a-Si-QDs and Si-NCs excited at 300 nm showed PL emissions at ∼370 nm and 374 nm, respectively, while the latter showed additional characteristic emissions at 403, 425 and 457 nm. The quantum yields were determined to be 9 and 13%, respectively, with PL relaxation life times of 6.5 and 13 ns, respectively.

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

Hierarchical self-assemblies of β-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 β-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 β-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 β-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 β-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.