Photochemical reactivity of several dibenzannelated systems has been investigated
to demonstrate that the driving force inherent in many of their reactions is the
attainment of a 4n π-electron system or intermediate in S1. It has been shown that
the inherent driving force for the benzylic C-H bond ionization in 9H-xanthene
(125) and 9H-thioxanthene (136) is the formation of 8π-electron cyclically
conjugated carbanion intermediates 130 and 137, respectively, in S1. Formation
of deuterium incorporated products when 125 and 136 are photolyzed in NaOD-EtOD
solutions and protium incorporation in products when photolysis of 123
and 134 is carried out in NaOH-EtOH is consistent with the intermediacy of
carbanions in these reactions. Fluorescence quenching of 123 and 125 by
ethanolamine in CH3CN gave linear Stern-Volmer plots, with kq = (2.16 ± 0.05) x
[special characters omitted] for 125 and (1.12 ± 0.05) x [special characters omitted] for 123, which corresponds to an
isotope effect for quenching by ethanolamine, of [special characters omitted] = 1.92 ± 0.04. Enhanced
stability of 8π carbanions in S1 is further evidenced in the photodecarboxylation
of xanthene-9-carboxylic acids (142 and 145) and thioxanthene-9-carboxylic acids
(147 and 148) in aqueous solution. The intermediacy of carbanions has been
demonstrated in the product studies carried out in 80% [special characters omitted] solution. A
mechanism has been proposed which involves heterolytic bond cleavage of the
carboxylate ion in S1 to give intermediate carbanions. Additional support for the
excited state stability of 4n systems comes from the studies of 155.
Photoexcitation of bent 155 results in its planarization to give planar 155 in S1.
The driving force for this change of conformation of 155 is believed to be the
attainment of a conjugated 8π-electron internal cyclic array in S1.
Photolysis of substituted suberenes in aqueous [special characters omitted] results in benzylic
C-H bond cleavage in these systems in S1 the efficiency of which is greatly
affected by the nature of substituent present. Results obtained in this study are
consistent with benzylic C-H bond cleavage, with H2C acting as the base, to give
intermediate carbanions in S1. Quantum yields of exchange in [special characters omitted] (L = H or D) and in a variety of other solvent mixtures have been measured.
The photochemistry of xanthenium (99 and 114) and thioxanthenium (219)
cations has been studied in aqueous [special characters omitted] in the presence of di- and
trimethoxybenzenes. It has been shown that the primary photochemical step is
electron transfer from methoxybenzenes to singlet excited 99 and 114. The radical
intermediates thus generated subsequently react with 02 to give peroxy
compounds as final products.
These and other results of the photochemistry of dibenzannelated systems
show that there is much interesting photochemistry to be discovered in these
molecules. / Graduate
| Identifer | oai:union.ndltd.org:uvic.ca/oai:dspace.library.uvic.ca:1828/9678 |
| Date | 11 July 2018 |
| Creators | Shukla, Deepak |
| Contributors | Wan, Peter |
| Source Sets | University of Victoria |
| Language | English, English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf |
| Rights | Available to the World Wide Web |
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