Ultrafast energy migration

Fleming CN, Maxwell KA, DeSimone JM, Meyer TJ, Papanikolas JM. Ultrafast excited-state energy migration dynamics in an efficient light-harvesting antenna polymer based on Ru(II) and Os(II) polypyridyl complexes. J Am Chem Soc 2001 123 10336-47. 

B. J. Schwartz, T.-Q. Nguyen, J. Wu, and S. H. Tolbert, Interchain and intrachain exciton transport in conjugated polymers ultrafast studies of energy migration in aligned MEH-PPV/mesoporous silica composites, Synthetic Metals, vol. 116, no. 1-3, pp. 35-40, 2001. 

C H bond (HOMO) with the empty orbital (LUMO) of the divalent carbon would necessarily reduce orbital overlap in 27ax and concommitantly suppress the formation of alkene 28. Of course, the conformation of 27 CyD must resemble that of 26 CyD, and also the classical energy barrier to a 1,2-hydride shift cannot be undermined by ultrafast quantum mechanical (QM) tunneling. The latter caveat, however, can probably be neglected at room temperature.122 Indeed, the H/D primary kinetic isotope effect (KIE) for 1,2-H(D) shifts in scaffolded tricyclo[5.3.0.04,8]decan-2-ylidene (38) is quite low (Scheme 7),112 indicating the absence of QM tunneling. Hence, the photolytic Hax/Heq migration preference of 1.2 that was determined for carbene 38 is a dependable benchmark. 

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