Dynamics of Anthracene Excimer Formation within a Water-Soluble Nanocavity at Room Temperature.

Excited anthracene is well-known to photodimerize and not to exhibit excimer emission in isotropic organic solvents. Anthracene (AN) forms two types of supramolecular host-guest complexes (2:1 and 2:2, H:G) with the synthetic host octa acid in aqueous medium. Excitation of the 2:2 complex results in intense excimer emission, as reported previously, while the 2:1 complex, as expected, yields only monomer emission. This study includes confirming of host-guest complexation by NMR, probing the host-guest structure by molecular dynamics simulation, following the dynamics AN molecules in the excited state by ultrafast time-resolved experiments, and mapping of the excited surface through quantum chemical calculations (QM/MM-TDDFT method). Importantly, time-resolved emission experiments revealed the excimer emission maximum to be time dependent. This observation is unique and is not in line with the textbook examples of time-independent monomer-excimer emission maxima of aromatics in solution. The presence of at least one intermediate between the monomer and the excimer is inferred from time-resolved area normalized emission spectra. Potential energy curves calculated for the ground and excited states of two adjacent anthracene molecules via the QM/MM-TDDFT method support the model proposed on the basis of time-resolved experiments. The results presented here on the excited-state behavior of a well-investigated aromatic molecule, namely the parent anthracene, establish that the behavior of a molecule drastically changes under confinement. The results presented here have implications on the behavior of molecules in biological systems.

Competing pathways for photoremovable protecting groups: the effects of solvent, oxygen and encapsulation.

Extending the applications of Photoremovable Protecting Groups (PPGs) to "cage" phenols has generally met with unusually complex PPG byproducts. In this study, we demonstrate that the p-hydroxyphenacyl (pHP) cage for both simple and complex phenolics, including tyrosine, dispenses free phenols. With the simpler unsubstituted phenols, the reaction is governed by their Brønsted Leaving Group ability. On the other hand, the byproducts of the cage vary with these phenols. For the more acidic phenols the cage byproduct follows the Favorskii rearrangement to form p-hydroxyphenylacetic acid whereas for the weaker phenols other reactions such as reduction and hydrolysis begin to emerge. When the photolysis is conducted in octa acid (OA) containers, non-Favorskii, unrearranged fragments of the cage and other byproducts arise.

Remote electron and energy transfer sensitized photoisomerization of encapsulated stilbenes.

Excited state chemistry and physics of molecules, in addition to their inherent electronic and steric features, depend on their immediate microenvironments. This study explores the influence of an organic capsule, slightly larger than the reactant molecule itself, on the excited state chemistry of the encapsulated molecule. Results presented here show that the confined molecule, in fact, is not isolated and can be manipulated from outside even without direct physical interaction. Examples where communication between a confined molecule and a free molecule present outside is brought about through electronic and energy transfer processes are presented. Geometric isomerization of octa acid encapsulated stilbenes induced by energy and electron transfer by cationic sensitizers that attach themselves to the anionic capsule is examined. The fact that isomerization occurs when the sensitizer present outside is excited illustrates that the reactant and sensitizer are communicating across the molecular wall of the capsule. Ability to remotely activate a confined molecule opens up new opportunities to bring about reactions of confined radical ions and triplet excited molecules generated via long distance energy and electron transfer processes.

Marcus Relationship Maintained During Ultrafast Electron Transfer Across a Supramolecular Capsular Wall.

Photoinduced electron transfer across an organic capsular wall between excited donors and ground-state acceptors is established to occur with rate constants varying in the range 0.32-4.0 × 1011 s-1 in aqueous buffer solution. The donor is encapsulated within an anionic supramolecular capsular host, and the cationic acceptor remains closer to the donor separated by the organic frame through Coulombic attraction. Such an arrangement results in electron transfer proceeding without diffusion. Free energy of the reaction (ΔG°) and the rate of electron transfer show Marcus relation with inversion. From the plot, λ and Vel were estimated to be 1.918 and 0.0058 eV, respectively. Given that the donor remains within the nonpolar solvent-free confined space, and there is not much change in the environment around the acceptor, the observed λ is believed to be because of "internal" reorganization rather than "solvent" reorganization. A similarity exists between the capsular assembly investigated here and glass and crystals at low temperature where the medium is rigid. The estimated electronic coupling (Vel) implies the existence of interaction between the donor and the acceptor through the capsular wall. Existence of such an interaction is also suggested by 1H NMR spectra. Results of this study suggest that molecules present within a confined space could be activated from outside. This provides an opportunity to probe the reactivity and dynamics of radical ions within an organic capsule.

Room-Temperature Phosphorescence from Encapsulated Pyrene Induced by Xenon.

Phosphorescence from pyrene especially at room temperature is uncommon. This emission was recorded utilizing a supramolecular organic host and the effect due to the heavy atom. Poor intersystem crossing from S1 to T1, small radiative rate constant from T1, and large rate constant for oxygen quenching hinder the phosphorescence of aromatic molecules at room temperature in solution. In this study, these limitations are overcome by encapsulating a pyrene molecule within a water-soluble capsule (octa acid, OA) and purging with xenon. While OA suppressed oxygen quenching, xenon enabled the intersystem crossing from S1 to T1 and radiative process from T1 to S0 through the well-known heavy atom effect. The close interaction facilitated between the pyrene and the heavy atom perturber xenon in the three-component supramolecular assembly (OA, pyrene, and xenon) resulted in phosphorescence from pyrene. Computational modeling and NMR studies supported the postulate that pyrene and more than one molecule of xenon are present within a confined space of the OA capsule.

Supramolecular photochemistry of encapsulated caged ortho-nitrobenzyl triggers.

ortho-Nitrobenzyl (oNB) triggers have been extensively used to release various molecules of interest. However, the toxicity and reactivity of the spent chromophore, o-nitrosobenzaldehyde, remains an unaddressed difficulty. In this study we have applied the well-established supramolecular photochemical concepts to retain the spent trigger o-nitrosobenzaldehyde within the organic capsule after release of water-soluble acids and alcohols. The sequestering power of organic capsules for spent chromophores during photorelease from ortho-nitrobenzyl esters, ethers and alcohols is demonstrated with several examples.

Space Constrained Stereoselective Geometric Isomerization of 1,2-Diphenylcyclopropane and Stilbenes in an Aqueous Medium.

Confinement provided by the reaction space alters the photostationary state isomer distribution during the geometric isomerization of excited 1,2-diphenylcyclopropane and stilbenes. The selectivity in 1,2-diphenylcyclopropane is suggested to result from the supramolecular steric hindrance exerted by the medium for the rotational motion. The alteration in the selectivity between a dimethyl and n-propyl substituted stilbenes is attributed to the medium influence on the location of the transition state on the ground state surface.

Implication of the Whitefly, Bemisia tabaci, Collagen Protein in Begomoviruses Acquisition and Transmission.

Begomoviruses are the largest group of plant viruses transmitted exclusively by the whitefly, Bemisia tabaci (Gennadius), in a persistent, circulative, and nonpropagative manner. Begomoviruses in association with B. tabaci cause enormous loss to world agricultural crops. Transmission, retention, and circulation of begomovirus in B. tabaci are facilitated by its interaction with several proteins of the insect and its endosymbionts. However, very few such proteins have been identified from B. tabaci that are involved in this specific interaction. Here, we have performed yeast two-hybrid assay between B. tabaci complementary DNA expression library and the coat protein (CP) of tomato leaf curl New Delhi virus (ToLCNDV) and cotton leaf curl Rajasthan virus (CLCuV). Collagen was the common protein found to be interacting with both of the viruses. The collagen protein was found to be localized in gut layers of B. tabaci. Additionally, pull-down and dot-blot assays confirmed the association of endogenous collagen with ToLCNDV CP. Immunolocalization analysis also showed colocalization of ToLCNDV particles and collagen within insect gut. Finally, B. tabaci fed on anticollagen antibody and exhibited ∼46% reduction in ToLCNDV transmission, suggesting a supportive role for collagen in virus transmission.

Supramolecular-Surface Photochemistry: Assembly and Photochemistry of Host-Guest Capsules on Silica Surface.

Host cavitands and organic guest molecules independently adsorbed on silica particles when mixed and shaken in the presence of a few drops of water underwent intra- and interparticle migration to form capsular complexes that were not formed either in water or organic solvents. Importance of cavitand migration and tumbling on silica surface was established by demonstrating that covalently linked cavitands do not form capsular complexes. The encapsulated guests exhibited selective photochemistry as they do within an organic capsule in solution.

Competitive Binding of Organic Dyes between Cucurbiturils and Octa Acid.

Employing six cationic water-soluble organic dye molecules as probes, we have attempted to qualitatively understand the factors that govern the attraction between such molecules and the anionic water-soluble host, octa acid (OA). Examination of the competitive host-guest complexation between cucurbit[8]uril (CB[8]) and OA using absorption and emission spectroscopy revealed that the dye molecules included within CB[8] could be "pulled out" by OA. However, an order of magnitude higher concentration of OA was required to shift the equilibrium toward OA, suggesting that attraction between the anionic host OA and the cationic dye molecules such as cresyl violet perchlorate and methylene blue is weaker than the hydrophobic and cation-dipolar interaction between these dye molecules and CB[8]. The importance of Coulombic attraction between OA and dye molecules is also revealed by monomer-to-dimer conversion upon addition of OA to an aqueous solution of monomeric dye molecules. Under conditions where the dye-to-OA ratio is high, freely dissolved monomeric dye molecules are attracted to the exterior of OA and aggregate as dimers on the exterior wall of OA. On the other hand, at high ratios of OA to dye molecules, the dye molecules adsorb as monomers on the exterior of OA. Thus, the monomer-to-dimer ratio in aqueous solution can be controlled by adjusting the ratio of dye to OA molecules. The results presented are of value in qualitatively understanding the relative binding properties of ionic guests with ionic hosts. Studies are qualitative in nature, and further detailed quantitative studies planned for the future are likely to provide deeper understanding of the interaction between water-soluble dye molecules, OA, and CB.

Ultrafast Electron Transfer across a Nanocapsular Wall: Coumarins as Donors, Viologen as Acceptor, and Octa Acid Capsule as the Mediator.

Results of our study on ultrafast electron transfer (eT) dynamics from coumarins (coumarin-1, coumarin-480, and coumarin-153) incarcerated within octa acid (OA) capsules as electron donors to methyl viologen dissolved in water as acceptor are presented. Upon photoexcitation, coumarin inside the OA capsule transfers an electron to the acceptor electrostatically attached to the capsule leading to a long-lived radical-ion pair separated by the OA capsular wall. This charge-separated state returns to the neutral ground state via back electron transfer on the nanosecond time scale. This system allows for ultrafast electron transfer processes through a molecular wall from the apolar capsular interior to the highly polar (aqueous) environment on the femtosecond time scale. Employing femtosecond transient absorption spectroscopy, distinct rates of both forward (1-25 ps) and backward eT (700-1200 ps) processes were measured. Further understanding of the energetics is provided using Rehm-Weller analysis for the investigated photoinduced eT reactions. The results provide the rates of the eT across a molecular wall, akin to an isotropic solution, depending on the standard free energy of the reaction. The insights from this work could be utilized in the future design of efficient electron transfer processes across interfaces separating apolar and polar environments.

Volume Conserving Geometric Isomerization of Encapsulated Azobenzenes in Ground and Excited States and as Radical Ion.

To probe the role of the supramolecular steric effects and free volume on photoreactions, geometric isomerization of neutral azobenzenes (ABs) and their radical ions, generated by electron transfer with gold nanoparticles, included within an octa acid capsule, was investigated. A comparison of the isomerization of ABs that proceed by volume conserving pyramidalization and stilbene analogues that proceed by volume demanding one bond flip has indicated the differing influence of 4-alkyl groups on these two processes.

Correction: Photoinduced electron transfer across an organic molecular wall: octa acid encapsulated ESIPT dyes as electron donors.

Correction for 'Photoinduced electron transfer across an organic molecular wall: octa acid encapsulated ESIPT dyes as electron donors' by Fabiano S. Santos et al., Photochem. Photobiol. Sci., 2017, 16, 840-844.

Photoinduced electron transfer across an organic molecular wall: octa acid encapsulated ESIPT dyes as electron donors.

Efficient photoinduced electron transfer from proton transfer dyes encapsulated within water soluble supramolecular host octa acid to electron acceptors present outside the capsule was observed in aqueous solution. 4,4'-Dimethylviologen dichloride was found to be the best acceptor compared to N-methylpyridinium iodide, most likely due to its better binding with the exterior of the host octa acid walls [corrected].

Melding Caged Compounds with Supramolecular Containers: Photogeneration and Miscreant Behavior of the Coumarylmethyl Carbocation.

By merging well-established concepts of supramolecular chemistry, protecting group strategy, and photochemistry, we have solubilized in water hydrophobic organic molecules consisting of a photoactive protecting group and masked carboxylic acids, released the desired acid, and confined a reactive carbocation intermediate within a capsule. Confinement of the photogenerated carbocation brought out the latent radical-like behavior. This observation is consistent with the recent theoretical prediction of the 7-(diethylamino)coumarinyl-4-methyl carbocation having a triplet diradical ground-state electronic contribution.

Photorelease of Incarcerated Caged Acids from Hydrophobic Coumaryl Esters into Aqueous Solution.

Photolysis of aqueous solutions of carboxylic acid esters of 7-(methoxycoumaryl)-4-methanol included within the capsule made up of two molecules of octaacid released the acids in water. The trigger 7-(methoxycoumaryl)-4-methyl chromophore remains within octaacid either as the alcohol or as an adduct with the host octaacid through a hydrogen abstraction process. The method established here offers a procedure to release hydrophobic acid molecules in water at will in a timely manner with light. In addition, the system offers an unanticipated opportunity to probe the mechanistic dichotomy of a diradicaloid intermediate expressing both radical and ionic behavior when generated by coumarylmethyl ester photolysis in a hydrophobic environment.

Eusynnada, a resurrected genus of Ochyromerina (Coleoptera: Curculionidae: Curculioninae: Tychiini), with description of a second species from India and Thailand.

Eusynnada is resurrected as a valid genus of Ochyromerina and includes the type species E. plaxoides Heller and a related second new species, E. canariumi sp. nov., from Andaman Island (India) and Thailand.

Descriptions of three new species of Strattis Pascoe, 1883 (Coleoptera: Curculionidae: Cryptorhynchinae) from the Indian subcontinent.

The cryptorhynchine genus Strattis Pascoe originally described from Sri Lanka is recorded from India with the descriptions of three new species: S. pascoei sp. nov., S. maculatus sp. nov., and S. srilankaiensis sp. nov. from the Indian subcontinent. Strattis biguttatus is designated as type species for the genus. A key to species is provided. The character of a presumable stridulatory structure between the meta-femur and the first abdominal ventrite is discussed.

Adenylate cyclase in Arthrospira platensis responds to light through transcription.

Cyclic 3',5' adenosine monophosphate (cAMP) is a ubiquitous signaling molecule, but its role in higher plants was in doubt due to its very low concentration. In this study we wanted to look at the flux of cAMP in response to light in algae, considered to be the more primitive form of photosynthetic organisms. While it did not fluctuate very much in the tested green algae, in the cyanobacterium Arthrospira platensis its level was closely linked to exposure to light. The expression from cyaC, the major isoform of adenylate cyclase was strongly influenced by exposure of the cells to light. There was about 300 fold enhancement of cyaC transcripts in cells exposed to light compared to the transcripts in cells in the dark. Although post-translational regulation of adenylate cyclase activity has been widely known, our studies suggest that transcriptional control could also be an important aspect of its regulation in A. platensis.