Research

Chemistry in Crystals, Cages and Capsules

1. Chemistry in Restricted Spaces: Select Photodimerizations in Crystals and Aqueous Solution

Crystals, were long considered a final resting place for molecules and believed to be incapable of undergoing molecular transformations upon activation by either heat or light. A firm groundwork laid in 1960s by G.M.J. Schmidt and his group for solid-state photochemistry established that molecules do react in this state provided they do not have to move much. Examples of photodimerization and geometric isomerization that require large molecular motions in crystals would be discussed in this talk. The concept of preorganization of molecules in crystals through weak interactions has been extended to solution chemistry. The importance of weak interactions in biological systems has been recognized for several decades. However, only during the last three decades has intelligent use of this been made in everyday chemistry. Molecular cages soluble in water offer an ideal environment to orient molecules and transform them to select products. Examples of these would be presented with water-soluble hosts such as cucurbiturils, calixarenes, Pd-nanocage and cyclodextrins. Photodimerizations in crystals and within water-soluble molecular cages illustrate the power of exploitation of weak interactions in controlling products of photoreactions.

2. Chemistry in Cages and Cavities: Solid State Chiral Photochemistry

For centuries, geologists knew of natural zeolites as minerals. For decades, synthetic- and industrial chemists knew them as adsorbents and catalysts. They have been explored in everyday laboratory chemistry only for the last two decades. Commercially available
zeolites contain a large number of cations that could be utilized to orient molecules within the well-defined, confined spaces through cation-􀀀, cation-carbonyl interactions. Use of zeolites as reaction media is ideal to the current environmentally conscious times of green chemistry. In this talk several examples of photochemical reactions in zeolites with chiral final products would be provided. Instances of asymmetric induction within zeolites achieved through supramolecular control would herald the opportunities and uncertainties in chiral photochemistry in zeolites. Elegant examples of asymmetric photochemistry in crystals that would also illustrate the unpredictable nature of photoreactions in crystals and solids would also be included.

3. Chemistry in a Capsule: Controlling excited state processes through confinement and weak interactions

Host-guest chemistry has witnessed a renaissance under the name ‘supramolecular chemistry’ since the award of Nobel Prize to Cram, Lehn and Pederson. During the last five decades ‘supramolecular chemistry’ has enabled the visualization and exploitation of weak interactions toward altering the chemical and physical behavior of a molecule. It has aided in threading several concepts of host-guest chemistry that have existed for over a century. Enclosing a molecule (guest) within another molecule (host) ‘tames’ the 2 former to result in a behavior different from that in solution. In general, the chemical and physical behavior of a molecule within a confined space (cage, cavity, capsule, box, container etc.) is defined by the size and free space within the container, the dynamics of the container, weak interactions between the container and the content, and the content’s restricted freedom. This talk will illustrate with examples the remarkable changes in the photochemical properties of different molecules on placing them within a water-soluble cavitand trivially known as octa acid. Common organic molecules such as olefins, carbonyls and aromatics are used as the contents/guests. The products of resulting altered photoreactions include oxidation, dimerization, geometric isomerization, Norrish Type I reactions and electrocyclic reactions. The primary goal is to demonstrate the different behavior of molecules when enclosed in a space not much larger than itself.

4. Exploring Motions of and Communications between Molecules in Restricted Spaces

5. Photochemistry in a Capsule: Spin, Energy and Electron Transfer Across a Molecular Wall

6. Supramolecular photochemistry: From molecular crystals to water-soluble capsules

It is well known that chemical reactions, activated by heat or light, occurring in biological assemblies differ from that in conventional media such as organic solvents including water. Uniqueness of these reactions has led to continued search for new supramolecular structures that would mimic biological systems. This type of investigation under the name of ‘supramolecular chemistry’ has witnessed a renaissance since the award of Nobel Prize to Cram, Lehn and Pederson. The early investigation (pre-1900) of supramolecular photochemistry dealt with crystals while the recent ones deal with host-guest assemblies involving synthetic hosts in water. Reactions carried out in crystals as well as in host-guest assemblies in water share some features and they both could be understood based on the topochemical principles established to understand reactions in crystals. This lecture will present results from our laboratory on photoreactions of molecular crystals, and photoreactions of organic molecules included within an organic capsule assembled from a synthetic host octa acid in water.

7. Medium Matters: Dynamics of Molecules in a Confined Space

From time immemorial it is well known that curtailment of freedom often leads to changes in the behaviour of living beings. Similar restriction of freedom leads to selectivity in the chemical behavior molecules embedded in biological systems. Extending these well-known concepts supramolecular chemists have established that even small molecules upon confinement in synthetic hosts exhibit behaviour distinctly different from the ones in an isotropic solution.

In this lecture the role a “Medium” in bringing about changes in the well-established behavior of excited molecules would be illustrated with select examples. Results of steady state and ultrafast experiments will be presented that highlight how the confinement alters the excited state dynamics of stilbenes and azobenzenes, the molecules that act as triggers in various biological systems and man-made devices. Another reaction to be discussed concerns with electron transfer (eT) that plays a fundamental role in a number of biological events including photosynthesis. Examples and ultrafast dynamics of donors, imprisoned within an organic capsule, transferring an electron to an acceptor across a molecular wall would be presented. The main message of the talk is that molecules like humans behave differently when confined within synthetic cages.

8. Ultrafast Dynamics of Excited Molecules in a Confined Space

For over a decade our group has been interested in examining the excited state behaviour of molecules in a well-defined confined space. Three unanticipated observations made concerning dimerization of anthracene, geometric isomerization of stilbenes and electro transfer between a donor and an acceptor in this space led us to investigate in detail the above phenomena at ultrafast time scales. The studies were carried out in collaboration with Pratik Sen (IIT, Kanpur), Chris Elles (University of Kansas) and Clemens Burda (Case Western University) and their students.6-12 The medium is an organic capsule made up of the cavitand octa acid (OA) has a width of ~10 Å and length ~20 Å (Fig 1a).

Photochemistry and photophysics of anthracene have been extensively investigated over a century. Unlike most other aromatic molecules, parent unsusbstituted anthracene does not show excimer emission. This puzzling behavior occupied the interests of photochemists for a few decades without a clear outcome. We noticed that when two molecules of anthracene were encapsulated within the 10 x 20 Å capsule in water showed intense excimer emission. This totally unexpected but welcome observation led us to a detailed study involving computational modelling, quantum mechanical calculations, NMR spectral characterization and ultrafast experiments. This study revealed the elegant dynamics involved in promoting excimer emission within the confined capsular assembly.

Similar to anthracene, stilbene is another unique molecule that has played a central in the development of the concepts related to dynamics of molecules having C=C bonds. Stilbenes upon excitation undergo unrestricted rotation around the central C=C bond to yield both cis and trans isomers. However, we found that when these molecules were confined with the OA capsule rotation was restricted and isomerization was selective, cis to trans. Although such type of selectivity is wellknown in biological systems (e.g., rhodopsin) it is unknown in solution. Given that multiple mechanisms have been proposed for the isomerization of excited olefins we
believed that probing the excited state dynamics of stilbenes and azobenzenes within the OA capsule in fs time scales would yield valuable information concerning the dynamics of isomerization in the excited surface and the point of entry from excited to ground surface.

The last photophysical phenomenon we continue to work on is the electron transfer, a fundamental process that plays an important role in both natural and unnatural events. The mechanism of electron transfer between a donor and an acceptor in the excited state has been fairly well understood and it is believed that the donor and the acceptor should be close to one another at the time of electron transfer or should be covalently linked through a short chain. To our surprise we noted that an electron transfer between a OA encapsulated donor and a free acceptor occurs at ultrafast time scales in spite of them being separated by a molecular wall.

The primary message of this investigation is that fundamental understanding of the dynamics of excited molecules at ultrafast time scales is essential to develop a comprehensive model for the behavior of molecules in confined spaces.

Current interest

Controlling Photochemistry with Cations and Confinement Photochemistry in Bowls, Baskets, Cages and Cavities

Life sustaining highly specific chemical reactions occur in the confined and organized medium of protein. Our projects aimed at achieving similar selectivity in (photo)chemical reactions explore the use of readily available hosts that bear similarity to biological media. In our laboratory, spatially confined cavities provided by crystals, zeolites, polymers (nafions), and water- soluble cyclodextrins (natural and functionalized), organic hosts, micelles and dendrimers are currently being explored as reaction media. Photophysical and photochemical probes as well as structural tools are generously utilized in our research to define the characteristics of confined reaction cavities prior to employing them as reaction media.
Our research interests involving synthesis of organic molecules, X- ray crystallography, solid state NMR, spectroscopic structural tools such as UV, IR and NMR, 2D- NMR, EPR, steady state and time resolved emission spectroscopy, and computational methods provide well- rounded experimental skills to students. Our projects encompassing concepts from currently popular topics such as nano- chemistry, supramolecular chemistry and green chemistry serve to train students in the broader areas of physical organic chemistry and photochemistry.