| Abstract
| - A covalently tethered dyad containing the azulene (Az) and zinc tetraphenylporphyrin (ZnP) chromophoreshas been synthesized and its excited-state dynamics investigated, using the tether-substituted monochromophoricspecies as reference compounds. One photon excitation of the dyad at 270 nm results in selective populationof the S2 state of the azulene moiety, followed by near-quantitative electronic relaxation in the cycle S2(Az)−S2(ZnP)−S1(ZnP)−S1(Az)−S0. Energy transfer from the S2 state of the azulene moiety to the S2 state of theZnP moiety is ultrafast (keet> 2 × 1012 s-1) and quantitative. The ZnP(S2) moiety subsequently undergoesrapid (kic = 3 × 1011 s-1), quantitative internal conversion to its S1 state. Thereafter, the excitation residingon the S1 state of the ZnP is returned to the Az moiety via an efficient (ca. 90%) back S1−S1 energy transferprocess (keet = 2.8 × 109 s-1). Ultimately the system returns to the electronic ground state via conical intersectionof the azulene S1 and S0 surfaces in ca. 1 ps. The cyclic interchromophoric energy transfer rates are nearlythe same in both acetonitrile and cyclohexane, suggesting that the conformation of the tether is similar inboth solvents. Förster theory is inadequate in explaining the efficient energy transfer dynamics, and otherprocesses such as the Dexter mechanism must be invoked.
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