Temperature Dependence of the Photoisomerization of cis-1-(2-Anthryl)-2-phenylethene. Conformer-Specificity, Torsional Energetics and Mechanism
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- 作者：Jack Saltiel ; Yuxin Zhang ; and Donald F. Sears ; Jr.
- 刊名：Journal of the American Chemical Society
- 出版年：1997
- 出版时间：November 19, 1997
- 年：1997
- 卷：119
- 期：46
- 页码：11202 - 11210
- 全文大小：336K
- 年卷期：v.119,no.46(November 19, 1997)
- ISSN：1520-5126
文摘
Emission from cis-1-(2-anthryl)-2-phenylethene,c-APE*, in toluene is resolved into1t-APEB* and1c-APE* components at temperatures ranging between 4.3 and 59.3 
f">C.Decomposition of effective fluorescence quantumyields, 
fchars/phi.gif" BORDER=0 >fc, into pure componentfluorescence quantum yields,fchars/phi.gif" BORDER=0 >ft-B andfchars/phi.gif" BORDER=0 >fc, shows thatfchars/phi.gif" BORDER=0 >ft-B increases 24%withincreasing temperature while fchars/phi.gif" BORDER=0 >fc decreasesmore than 3-fold over this temperature range. On the basis of thefractionof molecules that escape the 1c-APE* potentialenergy minimum, 1 - fchars/phi.gif" BORDER=0 >fc, the efficiency ofadiabatic formation of1t-APEB* remains remarkablytemperature independent at 50.5 ± 0.7%. These results, togetherwith photoisomerizationquantum yields as a function of [c-APE] in degassed andair-saturated toluene, reveal a detailedphotoisomerizationmechanism. At infinite dilution and in the absence of molecularoxygen, photoisomerization of c-APE occurspredominantly via the adiabatic, conformer-specific1c-APEB* 
f">1t-APEB* pathway. Thistorsional motion experiencesa 4.44 ± 0.14 kcal/mol barrier probablylocated at the perpendicular, 3p*, geometry. Since12% of 1t-APEB*intersystemcross to 3t-APEB*, the known tripletstate quantum chain process enhances photoisomerization quantum yieldsathigher [c-APE]. Triplets formed directly from1c-APE* also contribute to this pathway. Inair-saturated solutions,oxygen eliminates the quantum chain process by reducing the lifetime of3t-APE*. However, the quenchingof1c-APE* by O2 gives3c-APE*, thus enhancing photoisomerizationquantum yields via rapid 3c-APE* f">3t-APE*adiabatic torsional displacement. No photoisomerization of1c-APEA* need be postulated toaccount for ourobservations. The enthalpy difference between ground stateconformers, fchars/Delta.gif" BORDER=0 >HAB, favorsc-APEB by 0.92 ± 0.02kcal/mol.
f">C.Decomposition of effective fluorescence quantumyields, fchars/phi.gif" BORDER=0 >fc, into pure componentfluorescence quantum yields,fchars/phi.gif" BORDER=0 >ft-B andfchars/phi.gif" BORDER=0 >fc, shows thatfchars/phi.gif" BORDER=0 >ft-B increases 24%withincreasing temperature while fchars/phi.gif" BORDER=0 >fc decreasesmore than 3-fold over this temperature range. On the basis of thefractionof molecules that escape the 1c-APE* potentialenergy minimum, 1 - fchars/phi.gif" BORDER=0 >fc, the efficiency ofadiabatic formation of1t-APEB* remains remarkablytemperature independent at 50.5 ± 0.7%. These results, togetherwith photoisomerizationquantum yields as a function of [c-APE] in degassed andair-saturated toluene, reveal a detailedphotoisomerizationmechanism. At infinite dilution and in the absence of molecularoxygen, photoisomerization of c-APE occurspredominantly via the adiabatic, conformer-specific1c-APEB* f">1t-APEB* pathway. Thistorsional motion experiencesa 4.44 ± 0.14 kcal/mol barrier probablylocated at the perpendicular, 3p*, geometry. Since12% of 1t-APEB*intersystemcross to 3t-APEB*, the known tripletstate quantum chain process enhances photoisomerization quantum yieldsathigher [c-APE]. Triplets formed directly from1c-APE* also contribute to this pathway. Inair-saturated solutions,oxygen eliminates the quantum chain process by reducing the lifetime of3t-APE*. However, the quenchingof1c-APE* by O2 gives3c-APE*, thus enhancing photoisomerizationquantum yields via rapid 3c-APE* f">3t-APE*adiabatic torsional displacement. No photoisomerization of1c-APEA* need be postulated toaccount for ourobservations. The enthalpy difference between ground stateconformers, fchars/Delta.gif" BORDER=0 >HAB, favorsc-APEB by 0.92 ± 0.02kcal/mol.