Selectivity in organic photochemical reactions within zeolites
Description
In this work, zeolites have been extensively used as matrices to obtain selectivity in organic photochemical and photophysical transformations. Three different types of selectivities have been probed. They are (a) Switching the nature of the lowest triplet state (3npi* - 3 pipi*); (b) enantio and diastereoselectivity and (c) singlet to triplet switching in the excited states (Heavy Atom Effect). The zeolite polarity is made use of to reverse the ordering of the excited triplet states of arylalkyl ketones, cyclic alpha,beta-enones, cyclic beta,gamma-enones and cyclohexadienones, thereby controlling the selectivity in their phototransformations. Both photophysical and photochemical manifestations have been demonstrated in this work. Asymmetric induction is achieved during the rearrangement of cyclohexadienone and photochemically equivalent naphthalenone systems by three different approaches. In the first approach, the zeolite was chirally modified with chiral inductors and the reactant was kept achiral. In the second approach, the chiral information is introduced into the reactant using a covalent linkage. The third approach involves a combination of the above two approaches. Computational analysis was performed on these systems to gain better understanding of the experimental observations. Heavy atom effect was established on alkanone and azo systems that have very poor intersystem crossing rates. In this study heavier cations were incorporated onto the zeolites by ion-exchange and the phosphorescence spectra of the aforementioned compounds were recorded. Alkanones phosphoresce inherently. Their emission intensity increases manyfold on changing the cations of the zeolite from Na+ to Cs+ and Tl +. Azoalkane phosphorescence is not commonly known. In this study phosphorescence spectra of cyclic azoalkanes were recorded within TlY zeolite