A Naphtho-p-quinodimethane Exhibiting Baird's (Anti)Aromaticity, Broken Symmetry, and Attractive Photoluminescence.

We report a novel reductive desulfurization reaction involving π-acidic naphthalene diimides (NDI) 1 using thionating agents such as Lawesson's reagent. Along with the expected thionated NDI derivatives 2-6, new heterocyclic naphtho-p-quinodimethane compounds 7 depicting broken/reduced symmetry were successfully isolated and fully characterized. Empirical studies and theoretical modeling suggest that 7 was formed via a six-membered ring oxathiaphosphenine intermediate rather than the usual four-membered ring oxathiaphosphetane of 2-6. Aside from the reduced symmetry in 7 as confirmed by single-crystal XRD analysis, we established that the ground state UV-vis absorption of 7 is red-shifted in comparison to the parent NDI 1. This result was expected in the case of thionated polycyclic diimides. However, unusual low energy transitions originate from Baird 4nπ aromaticity of compounds 7 in lieu of the intrinsic Hückel (4n + 2)π aromaticity as encountered in NDI 1. Moreover, complementary theoretical modeling results also corroborate this change in aromaticity of 7. Consequently, photophysical investigations show that, compared to parent NDI 1, 7 can easily access and emit from its T1 state with a phosphorescence 3(7a)* lifetime of τP = 395 µs at 77 K indicative of the formation of the corresponding "aromatic triplet" species according to the Baird's rule of aromaticity.

New perspectives for triplet-triplet annihilation based photon upconversion using all-organic energy donor & acceptor chromophores.

It is recognized that metal organic complexes that serve as sensitizers can present various degrees of challenges viz. synthesis and stability for photonic applications such as triplet-triplet annihilation based photon upconversion (TTA-PUC). Presently, researchers, including our group, are turning their attention toward purely organic triplet sensitizers, which can be handled more easily for photon management science. In this review, we surveyed recently developed all-organic chromophoric systems that were devised and used for TTA-PUC research. Knowing that TTA-PUC research has mainly been focused on the design and synthesis of the triplet sensitizers, we detailed the underlying photophysics and thermodynamics that served as the starting point for the synthesis of the purely organic chromophores in question. Accordingly, this review details triplet sensitizers that operate on (i) spin-orbit coupling or heavy atom effect, (ii) Baird-type aromaticity and antiaromaticity, (iii) open-shell characteristics or doublet excited state and (iv) thermally activated delayed fluorescence.

Lamellar Peptide-Cadmium-Doped Zinc Oxide Nanohybrids That Emit White Light.

A variety of hybrid nanostructures have been developed that emit white light. Two different white-light-emitting systems are reported. These are cadmium-doped zinc oxide nanosheets and complex lamellar nanostructures that consist of alternating inorganic cadmium-doped zinc oxide domains with the self-assembled aromatic-capped peptide BPI-FF-OH (BPI: benzo[ghi]perylene monoimide, F: d-phenylalanine). An electrochemical method is employed to synthesize cadmium-doped zinc oxide nanosheets and lamellar organic/cadmium-doped zinc oxide nanoflakes on a gallium-doped ZnO/p-Si (111) substrate. External structural features and internal structural ordering of wurtzite cadmium-doped zinc oxide and lamellar organic/cadmium-doped zinc oxide nanohybrids are characterized by small-angle X-ray scattering, XRD, field-emission SEM, energy-dispersive X-ray spectroscopy, secondary-ion mass spectrometry, ellipsometry, and photoluminescence spectroscopy. Cadmium-doped zinc oxide nanosheets and lamellar organic/cadmium-doped zinc oxide hybrids emit white light with a broad emission covering the visible spectrum from λ=415 to 700 nm. Characteristic white-light emissions of both materials were well characterized by photoluminescence studies. The white-light luminescence is attributed to cadmium doping in the zinc oxide crystal and the presence of the dipeptide-functionalized BPI fluorophore in the lamellar nanohybrid.

Electrodeposited Lamellar Photoconductor Nanohybrids Driven by Peptide Self-Assembly.

Aromatic organic molecules serve as optoelectronic materials owing to their intrinsic optical and electronic properties. Herein, self-assembled lamellar nanostructures as photoconductor hybrids, which are obtained from naphthalene-2-methoxycarbonyl (Nmoc)-capped peptide amphiphiles, are described. Hybrid nanostructures are constructed in a controlled manner by an electrochemical deposition technique in combination with the inorganic Zn(OH)2 phase. Inorganic Zn(OH)2 layers turn into semiconductor ZnO layers upon annealing at 150 °C and lamellar nanostructures are formed in a periodic manner. Synergistic effects of hydrogen bonding and π-π stacking interactions of aromatic peptide amphiphiles are the driving force for the formation of self-assembled lamellar nanostructures. Morphological, structural, and optical studies of such lamellar hybrid nanostructures are reported. Photoconduction of these hybrid nanostructures is also examined in detail.

Peptide-Nanofiber-Supported Palladium Nanoparticles as an Efficient Catalyst for the Removal of N-Terminus Protecting Groups.

Sonication-induced tryptophan- and tyrosine-based peptide bolaamphiphile nanofibers have been used to synthesize and stabilize Pd nanoparticles under physiological conditions. The peptide bolaamphiphile self-assembly process has been thoroughly studied by using several spectroscopic and microscopic techniques. The stiffness of the soft hydrogel matrix was measured by an oscillatory rheological experiment. FTIR and circular dichroism (CD) experiments revealed a hydrogen-bonded ß-sheet conformation of peptide bolaamphiphile molecules in a gel-phase medium. The π-π stacking interactions also played a crucial role in the self-assembly process, which was confirmed by fluorescence spectroscopy. Electron (SEM and TEM) and atomic force microscopy (AFM) studies showed that the peptide bolaamphiphile molecules self-assemble into nanofibrillar structures. Pd nanoparticles were synthesized in the hydrogel matrix in which redox-active tryptophan and tyrosine residues reduce the metal ions to metal nanoparticles. The size of the Pd nanoparticles are in the range of 3-9 nm, and are stabilized by peptide nanofibers. The peptide-nanofiber-supported Pd nanoparticles have shown effective catalytic activity for the removal of N-terminus protecting groups of amino acids and peptides.