| dc.contributor.author | Roberts, Jeffrey Charles. | en_US |
| dc.date.accessioned | 2014-10-21T12:33:37Z | |
| dc.date.available | 2005 | |
| dc.date.issued | 2005 | en_US |
| dc.identifier.other | AAINR00957 | en_US |
| dc.identifier.uri | http://hdl.handle.net/10222/54686 | |
| dc.description | The photochemistry of 1-(3,5-dimethoxyphenyl)-2-(4-methoxyphenyl)ethyl ethanoate was investigated in methanol and 2,2,2-trifluoroethanol (TFE). Following photolysis of this ester in either solvent, two ether products were obtained: one resulting from trapping of the cation formed directly by photofragmentation, and one that was expected to form via a carbocation rearrangement mechanism. Closer inspection of the data revealed that the second product was actually formed by photoaddition of the solvent to trans-3,4 ',5-trimethoxystilbene, a byproduct of the initial ester photo fragmentation. | en_US |
| dc.description | A set of five methoxy-substituted stilbene derivatives demonstrated the same TFE photoaddition chemistry. An important difference in reactivity was observed for trans-3,5-dimethoxystilbene (which reacted after 10 minutes of irradiation in TFE), and trans-4-methoxystilbene (which required one hour to react under the same conditions). The fluorescence of all five trans isomers was quenched by addition of TFE. Nuclear magnetic resonance spectroscopy of the products formed by irradiation in TFE-OD indicated that the proton and nucleophile are attached to two adjacent atoms of the original alkene double bond. Irradiation of the corresponding methoxy-substituted styrenes and trans-1-arylpropenes in TFE produced the analogous solvent adducts. Transient carbocation intermediates were observed following laser flash photolysis of the stilbenes in 1,1,1,3,3,3-hexafluoroisopropanol. The results are consistent with a mechanism that involves photoprotonation of the methoxy-substituted trans-stilbenes by TFE, followed by nucleophilic trapping of short-lived carbocation intermediates. | en_US |
| dc.description | Compared to the other stilbenes, trans-3,5-dimethoxystilbene displayed a large quantum yield of fluorescence (&phis;f = 0.32) and a low quantum yield of trans-cis isomerization (&phis;tc = 0.29). The singlet state of this substrate was unusually long-lived, and decayed according to biexponential kinetics (fast decay taus = 3.9 ns; slow decay taus = 18.5 ns). Analysis of the solvent-induced Stokes' shifts of the fluorescence maxima allowed calculation of the excited state dipole moment, mue = 13.2 D. Several substituted derivatives of trans-3,5-dimethoxystilbene displayed similar photophysical properties and the same high reactivity towards TFE. The results indicate that the excited state of the trans-3,5-dimethoxystilbene chromophore has substantial charge transfer character. | en_US |
| dc.description | Thesis (Ph.D.)--Dalhousie University (Canada), 2005. | en_US |
| dc.language | eng | en_US |
| dc.publisher | Dalhousie University | en_US |
| dc.publisher | | en_US |
| dc.subject | Chemistry, Physical. | en_US |
| dc.subject | Biophysics, General. | en_US |
| dc.title | The formation of carbocations from excited states: The meta methoxy effect in photofragmentations and photoadditions. | en_US |
| dc.type | text | en_US |
| dc.contributor.degree | Ph.D. | en_US |
