V. Ramamurthy

Templation of the Excited-State Chemistry of α-(n-Alkyl) Dibenzyl Ketones: How Guest Packing within a Nanoscale Supramolecular Capsule Influences Photochemistry

Excited-state behavior of eight alpha-alkyl dibenzyl ketones (alkyl = CH3 through n-C8H17) that are capable of undergoing type II and/or type I photoreactions has been explored in isotropic solution and within a water-soluble capsule. The study consisted of two parts: photochemistry that explored the excited-state chemistry and an NMR analysis that revealed the packing of each guest within the capsule. The NMR data (COSY, NOESY, and TOCSY experiments) revealed that ternary complexes between alpha-alkyl dibenzyl ketones and the capsule formed by two cavitands are kinetically stable, and the guests fall into three packing motifs modulated by the length of the alpha-alkyl chain. In essence, the host is acting as an external template to promote the formation of distinct guest conformers. The major products from all eight guests upon irradiation either in hexane or in buffer solution resulted from the well-known Norrish type I reaction. However, within the capsule the excited-state chemistry of the eight ketones was dependent on the alkyl chain length. The first group consisted of alpha-hexyl, alpha-heptyl, and alpha-octyl dibenzyl ketones that yielded large amounts of Norrish type II products within the host, while in solution the major products were from Norrish type I reaction. The second group consists of alpha-butyl and alpha-pentyl dibenzyl ketones that yield equimolar amounts of two rearranged starting ketones within the capsule (combined yield of ca 60%), while in solution no such products were formed. The third group consisted of alpha-methyl, alpha-ethyl, and alpha-propyl dibenzyl ketones that within the capsule yielded only one (not two) rearranged starting ketone in larger amounts (21-35%) while in solution no rearrangement product was obtained. Variation in the photochemistry of the guest within the capsule, with respect to the alpha-alkyl chain length of the guest, highlights the importance of how a small variation in supramolecular structure can influence the selectivity within a confined nanoscale reactor.

Nature of supramolecular complexes controlled by the structure of the guest molecules: formation of octa acid based capsuleplex and cavitandplex.

Factors that govern inclusion of organic molecules within octa acid (OA), a synthetic deep cavity cavitand, have been delineated by examining the complexation behavior of a number of organic molecules with varying dimensions and functionalities with OA. The formation of two types of complexes has been noted: the one which we call cavitandplex is a partially open complex in which a part of the guest molecule remains exposed to water, and the other termed capsuleplex is formed through assembly of two OA molecules. In capsuleplex, the guest is protected from water. Generally, guest molecules that possess ionic head groups form cavitandplex, and all others form capsuleplex. Capsuleplex may contain one or two guest molecules within the capsule. Small organic molecules (<10 A in length) may form both 2:1 and 2:2 capsuleplex, while longer ones (>12 A) preferentially form 2:1 capsuleplex. Extensive 1H NMR experiments have been carried out to characterize host-guest complexes. In the absence of the guest, OA tends to aggregate in water. The extent of aggregation depends on the concentration of OA and the presence of salts in solution. We expect the information obtained from this study to be of great value in predicting the nature of complexes with a given guest and facilitating appropriate guest chosen by researchers.

Steric and electronic effects in capsule-confined green fluorescent protein chromophores

The turn-on of emission in fluorescent protein chromophores sequestered in an octaacid capsule is controlled by stereoelectronic effects described by a linear free energy relationship. The stereochemical effects are governed by both the positions and volumes of the aryl substituents, while the electronic effects, including ortho effects, can be treated with Hammett σ parameters. The use of substituent volumes rather than A values reflects packing of the molecule within the confines of the capsule.

Cavitand octa acid forms a nonpolar capsuleplex dependent on the molecular size and hydrophobicity of the guest

We have been exploring the use of a deep cavity cavitand known by the trivial name octa acid as a photochemical reaction cavity for manipulating photochemical and photophysical properties of organic molecules. In the current study, we have monitored the micropolarity of the interior of the cavitand by recording the fluorescence of five different organic probes. They all indicate that the interior of octa acid capsuleplex (2:1, H/G complex) is nonpolar and does not contain water molecules in spite of the complex being present in water. The nature of the octa acid-probe complex in each case has been characterized by 1H NMR data to be a 2:1 capsuleplex. Photophysical and 1H NMR experiments were employed to probe the factors that control the structure of the complex, 2:2, 2:1, and 1:1. The data we have on hand suggest that the structure of the host/guest complex depends on the size and hydrophobicity of the guest molecule.

Efficient singlet-singlet energy transfer in a novel host-guest assembly composed of an organic cavitand, aromatic molecules, and a clay nanosheet

A supramolecular host-guest assembly composed of a cationic organic cavitand (host), neutral aromatic molecules (guests), and an anionic clay nanosheet has been prepared and demonstrated that in this arrangement efficient singlet-singlet energy transfer could take place. The novelty of this system is the use of a cationic organic cavitand that enabled neutral organic molecules to be placed on an anionic saponite nanosheet. Efficient singlet-singlet energy transfer between neutral pyrene and 2-acetylanthracene enclosed within a cationic organic cavitand (octa amine) arranged on a saponite nanosheet was demonstrated through steady-state and time-resolved emission studies. The high efficiency was realized from the suppression of aggregation, segregation, and self-fluorescence quenching. We believe that the studies presented here using a novel supramolecular assembly have expanded the types of molecules that could serve as candidates for efficient energy-transfer systems, such as in an artificial light-harvesting system.

Restricted rotation due to the lack of free space within a capsule translates into product selectivity: photochemistry of cyclohexyl phenyl ketones within a water-soluble organic capsule.

The rotational mobility of organic guest molecules when included within a confined capsule is restricted and this feature could be translated into product selectivity as established with the photochemical behavior of cyclohexyl phenyl ketones.

Consequences of controlling free space within a reaction cavity with a remote alkyl group: photochemistry of para-alkyl dibenzyl ketones within an organic capsule in water

With the aim of controlling free space available for the reactants, photochemical reactivity of several 4-alkyl dibenzyl ketones included within a water-soluble self-assembled organic capsule (octa acid) has been investigated. One and two-dimensional NMR spectroscopic techniques were employed to characterize the structure of the guest@host complexes. Free space controlled by the remote alkyl group influences the distribution of photoproducts. Correlation between the structures of the guest@host complexes and products selectivity suggested that the free space that could be controlled by a remote tether has a determining role during a photoreaction within the restricted space of the capsule. The fact that the same alkyl tether has no effect on the photobehavior of 4-alkyl dibenzyl ketones in hexane solution suggests that the role of substituents in free (solution) and restricted space (supramolecular assemblies) is different. The current observation suggests that rules of physical organic chemistry and photochemistry developed based on solution chemistry cannot be simply extended to supramolecular assemblies.

Hydrocarbons Depending on the Chain Length and Head Group Adopt Different Conformations within a Water-Soluble Nanocapsule: 1H NMR and Molecular Dynamics Studies

In this study we have examined the conformational preference of phenyl-substituted hydrocarbons (alkanes, alkenes, and alkynes) of different chain lengths included within a confined space provided by a molecular capsule made of two host cavitands known by the trivial name octa acid (OA). One- and two-dimensional (1)H NMR experiments and molecular dynamics (MD) simulations were employed to probe the location and conformation of hydrocarbons within the OA capsule. In general, small hydrocarbons adopted a linear conformation while longer ones preferred a folded conformation. In addition, the extent of folding and the location of the end groups (methyl and phenyl) were dependent on the group (H(2)C-CH(2), HC═CH, and C≡C) adjacent to the phenyl group. In addition, the rotational mobility of the hydrocarbons within the capsule varied; for example, while phenylated alkanes tumbled freely, phenylated alkenes and alkynes resisted such a motion at room temperature. Combined NMR and MD simulation studies have confirmed that molecules could adopt conformations within confined spaces different from that in solution, opening opportunities to modulate chemical behavior of guest molecules.

Chloride sensing via suppression of excited state intramolecular proton transfer in squaramides.

A new design strategy is described for chloride ion sensors based on suppression of excited state intramolecular proton transfer in squaramides as a fluorescence turn on mechanism.

Role of free space and weak interactions on geometric isomerization of stilbenes held in a molecular container

Photochemical geometric isomerization of olefins is long known to depend on the medium in which it occurs. Highest selectivity occurs in flexible biological systems as well as in inflexible crystals. We present results in this report that suggest the isomerization is selective even in an isotropic flexible aqueous medium provided it occurs within an isolated water-soluble inflexible reaction cavity. By examining the photochemistry of twelve stilbenes (trans and corresponding cis isomers) included in an organic cavitand octa acid we have been able to probe the role of free volume, weak interactions and supramolecular steric effects on the geometric isomerization process. Geometric isomerization becomes selective when the olefins mobility is restricted by the medium through weak interactions, supramolecular steric effects and controlled free space (free volume).