The photodebromination of selected bromoarenes has been studied at 300 nm
to determine the the possible mechanistic pathways leading to product.
Irradiation of 4-bromobiphenyl (BpBr) at this wavelength leads to the product
biphenyl. The quantum yield of intersystem crossing (0.98) and quenching
studies with cis-1 ,3-pentadiene suggest that the reaction occurs only via the
triplet state. The observed increase of quantum yield of reaction with
increasing concentration of BpBr suggests formation of a triplet excimer
between the triplet state and ground state of BpBr as the key reactive
intermediate. The log of the rate constant for excimer formation showed a
linear increase with solvent polarity. The correlation of rate constants for
excimer formation with linear solvation energy parameters indicates only a
weak polarization of the excimer species. In order to understand the extent
of radical anion character in the excimer, the regiochemistry of the photo-debromination
of 3,4-dibromobiphenyl (3,4-BpBr) was studied.
3,4-BpBr was irradiated with and without an electron donor
(triethylamine). 3,4-BpBr was also chemically reduced with lithium
biphenylide (LiDBB). The difference in the regiochemistries under these
conditions has been interpreted in terms of reaction via a free radical anion
in the case of reactions with triethylamine and LiDBB and a weakly
polarized excimer in the case of direct irradiation.
In order to understand the extent of polarization in the excimers and
their conformation, it was decided to study systems where the structure of
the molecule would fix the geometry of potential intramolecular charge-transfer
complexes. Towards this goal, brominated [2.2]paracyclophanes,
4-bromo[2.2]para-cylophane (CpBr), pseudo-para-dibromo[2.2]cyclophane
(ps-p-CpBr) and pseudo-ortho-dibromo[2.2]cyclophane (ps-o-CpBr), were
studied. The effect of substitution pattern of the bromines on the efficiency
of excimer formation was also investigated. The brominated [2.2]para-cyclophanes
showed varying efficiencies of formation of intermolecular
excimer and intramolecular charge-transfer. A probable conformation for the
excimer from BpBr has been proposed based on these results. / Graduation date: 1991
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/37091 |
| Date | 21 September 1990 |
| Creators | Jang, Jung-suk |
| Contributors | Freeman, Peter K. |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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