Conical intersection control

We now proceed to look at three examples from recent work in some depth. In the first example, we wish to illustrate that a knowledge of the VB structure or of the states involved in photophysics and photochemistry rationalize the potential surface topology in an intuitively appealing way. We then proceed to look at an example where the extended hyperline concept has interesting mechanistic implications. Finally, we shall look at an example of how conical intersections can control electron transfer problems. 

In Fig. 1 (top right) we show a sloped conical intersection in the terminology of Ruedenberg et al (29). Here the cone is tilted due to the fact that the force (gradient) vectors on both the upper and lower surfaces point in the same direction. The first-order topology (sloped vs. peaked) controls the nature of the photochemical reaction dynamics, and whether reactants are regenerated or photoproducts are formed (23,24). 

Figure 4 Opening of a fast radiationless decay channel via conical intersection for (a) a barrier controlled reaction, (b) a barrierless path, and (c) an uphill path without transition state (sloped conical intersection). M" is an excited state intermediate and FC is a Franck-Condon point.

The selectivity C/D is controlled by the geometry of the conical intersection for the singlet reaction and by the optimal ISC-geometry of the 2-oxatetramethylene biradical (3X) for the triplet reaction. The situation is described for the propionaldehyde/2,3-dihydrofuran photocycloaddition reaction. At low concentrations (0.005 M, triplet conditions), the diastereo-selectivity (endo-159 / exo-159) approaches a maximum endo/exo value of 85 15 (Sch. 56). At high concentrations (0.5 M, singlet conditions), the diastereoselectivity decreased to 52 48. 

Therefore, the photoisomerization in the NasFa cluster through a conical intersection will be addressed first and then in Section 111 the new strategy for optimal control will be applied in order to suppress the passage through the conical intersection and to selectively populate one of the chosen isomers. 

Using the strategy for optimal pump-dump control based on the intermediate target, we have shown that the isomerization pathway through the conical intersection can be suppressed and that optimized pulses can drive the isomerization process to the desired objective (isomer 11). This means that the complex systems are amenable to control, provided that the intermediate target exists. Furthermore, the analysis of the MD and of the tailored pulses allows for the identification of the mechanism responsible for the selection of appropriate vibronic modes necessary for the optimal control. 

Photochemistry is controlled in two ways. First, there is the avoidance of excited-state energy barriers, which plays a major role. Second, the placement of conical intersections and avoided crossings determine reaction success vs. radiationless decay. A conical intersection positioned close to the Franck-Condon geometry and before to an appreciable barrier favors decay to the reactant ground state and a low or zero quantum yield. A conical intersection positioned after the first energy barrier on the excited state hypersurface facilitates reaction, epecially if the conical intersection appears close to a product geometry. 

Basic questions are analyzed, as is the case for the photochemistry of formaldehyde. Contrary to previous results, direct quantum dynamics simulations showed that the H2 + CO H + HCO branching ratio in the Si/Sq nonadiabatic photodissociation of formaldehyde is controlled by the direction and size of the mean momentum of the wavepacket when it crosses the seam of conical intersection. In practice, if the wavepacket falls down from the barrier to the conical intersection with no initial momentum the system leads to H2 + CO, while an extra momentum toward products favors... 

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). 

Detection and Control of Chemical Dynamics at Conical Intersections... 

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