Publication Date

2013-07-17

Availability

Embargoed

Embargo Period

2014-07-17

Degree Type

Doctoral Essay

Degree Name

Doctor of Philosophy (PHD)

Department

Chemistry (Arts and Sciences)

Date of Defense

2013-07-08

First Committee Member

Vaidhyanathan Ramamurthy

Second Committee Member

Rajeev Prabhakar

Third Committee Member

Jamie D. Walls

Fourth Committee Member

Anna D. Gudmundsdottir

Abstract

This research work is an attempt towards understanding the role of supramolecular organic hosts in altering the photochemical and photophysical properties of small organic molecules. The two most important aspects of this study are: use of “light” to provide the activation energy for the desired chemical changes and exploitation of water soluble supramolecular organic hosts. An organic molecule undergoes chemical changes from an excited state. This excited state can be reached in two ways: (i) in tiny steps, traversing through closely spaced vibrational and rotational levels (heat energy) or, (ii) hopping directly and discreetly to the excited state by absorbing quanta of light energy. In this thesis we focus on the latter means. Most of the chemical reactions carried out in various organic solvents, lack the feature of product selectivity. In this work we exploited water soluble organic hosts like cavitand Octa Acid (OA), Cucurbiturils (CB) and dendrimers to firstly solubilize the otherwise insoluble organic molecules in aqueous medium and then carry out their photolysis in a confined space to achieve product selectivity. We have tried to control and stabilize the highly reactive intermediate species like carbenes, nitrenes and hydroperoxides generated as a result of “ene” reaction with singlet oxygen by encapsulating their precursors in various host systems. Remarkable product selectivity has been witnessed in most of the cases. For instance, carbene precursors-aziadamantane derivatives gives more than 90% of 1, 3- inner molecular insertion products when encapsulated inside CB [7] and CB[8], which is produced only in traces in organic solvents. Similarly nitrene generated by the photolysis of azidoadamantanes encapsulated inside OA, can be used to achieve modified OA derivative. It has been investigated in detail how the size and the spatial orientation of the guest molecules confined in a host, govern the photochemical outcome. For example, due to the restricted space, formation of cis-stilbene is preferred inside the hydrophobic pockets of the dendrimers, in a photoinduced cis-trans isomerization reaction. Most part of this work focuses on understanding the role of weak, non-covalent interactions like: van der Waals forces, columbic ion-pair interactions, hydrophobic forces, etc. in organizing a host and guest complex. By investigating the change in the aggregation behavior of positively charged organic dye molecules, in presence of different hosts, we observed that columbic ion-pair interactions win over hydrophobic interactions. We utilized this observation in designing a FRET donor-acceptor pair, in which the donor being uncharged occupies the hydrophobic cavity of the cavitand OA and the positively charged acceptor is attached to its anionic exterior. The donor-acceptor distance RDA, thus obtained matches closely with the dimensions of host cavitand OA, confirming our hypothesis. We expect this work to be a small contribution towards enhancing our knowledge on the vast field of organic supramolecular photochemistry.

Keywords

Supramolecular; photochemistry

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