Reaction control photochemical reactions mediated

Photoresponsive systems are seen ubiquitously in nature, and light is intimately associated with the subsequent life processes. In these systems, a photoantenna to capture a photon is neatly combined with a functional group to mediate some subsequent events. Important is the fact that these events are frequently linked with photoinduced structural changes in the photoantennae. This suggests that chemical substances that exhibit photoinduced structural changes may serve as potential candidates for the photoantennae. To date, such photochemical reactions as E/Z isomerism of azobenzenes, dimerization of anthracenes, spiropyran-merocyanine interconversion, and others have been exploited in practical photoantennae. It may be expected that if one of these photoantennae were adroitly combined with a crown ether, it would then be possible to control many crown ether family physical and chemical functions by means of an ON/OFF photoswitch. This is the basic concept underlying the designing of photoresponsive crown ethers. We believe that this is one of the earliest examples of molecular machines . 

As seen in Sec. 2.3, conical intersections that mediate unsuccessful chemical reactions have been shown to provide the decay channel associated with processes that are usually thought to occur through a photophysical mechanism (e.g. controlled by the Fermi Golden Rule ) such as the radiationless deactivation and/or quenching processes. Furthermore, important organic chromophores have been demonstrated to undergo either photochemical reactions or internal conversion processes on an extremely fast (usually sub-picosecond) time scale (i.e. emission is not observed from the excited state, since the time scale of the reaction is faster than the radiative lifetime). 

The capability to understand chemical reactions on the atomic scale and subsequently to design control strategies will be illustrated for pho-toinduced reactions via conical intersections. The ultrafast photochemical ring opening of cyclohexadiene, which occurs within 200 fs in gas as well as in condensed phase, is taken as an example. This reaction has been studied quantum chemically, by resonance-Raman spectroscopy and by femtosecond spectroscopy offering already a wealth of information. After photoexcitation to the S2 state, the molecule decays within a few femtoseconds to the S state, from where the relaxation to the ground state is mediated by at least two conical intersections. 

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