Fast radiationless decay

We consider situations for which the orbital overlap between M and M is negligible, this means situations as depicted in the upper and middle part of Figure 1.8, which makes sense because we focus on strongly luminescent materials. In general, orbital overlap causes fast radiationless decay for organic molecules as, for example, observed in dimers [56, 57]. The exchange part ex... 

Low phosphorescence efficiency, however, leaves fast radiationless decay as the prime course of inefficient photochemistry. 

Figure 3 Opening of a fast radiationless decay channel in all-trans octatetraene in (a) matrix-isolated conditions and (b) expanding cool jet. (From Ref. 14.)...

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.

One can wonder whether, besides the oxidation step, the reduction step can also be induced by light. This cannot be the case since in Cu(II) complexes (d9 electronic configuration) the lowest excited state is a low energy, distorted ligand-field level which undergoes very fast radiationless decay to the ground state [34]. Therefore it is not possible to involve Cu(II)N5 in a bimolecular reaction where an excited state of the complex should play the role of the electron acceptor. 

On the experimental side, dramatic progress has been made during the last few decades with the generation and shaping of ultrafast light pulses, see, e.g. Refs. 8 and 9. With the availabihty of pulses as short as a few femtoseconds, an ultimate goal has been achieved Essentially any chemical process can be resolved in real time by an appropriately designed pump-probe-type measurement. This apphes, in particular, also for ultrafast internal conversion processes. In fact, the detection of exceptionally fast radiationless decay processes appears at present to be the only way to establish by purely experimental means the existence of a conical intersection. 

Modern experimental measurements and the new computational techniques just discussed are now providing results that can rationalize issues such as the efficiency of 1C at a surface crossing, the competition with fluorescence when an excited state barrier is present, and the relationship between the molecular structure at the intersection and the structure of the photoproducts. Experiments on isolated molecules in cold-matrices or expanding-jets have revealed the presence of thermally activated fast radiationless decay channels. For example, Christensen et al. have proposed that (under isolated conditions in a cool-jet) trans — cis motion in all-tra 5-octa-1.3,5,7-tetraene (all-trow -OT) induces the opening of an efficient nonadiabatic radiationless deactivation channel on Si (2Ag). We now discuss this experiment and complementary theoretical results that illustrate the way in which theory and experiment can be used in concert. 

Upon formation of the [2]pseudorotaxane [1 2], the CT interaction leads to (1) the appearance of a weak and broad absorption band in the visible region (Fig. 5.15) and (2) the disappearance of the luminescence characteristic of the two components (Fig. 5.15, inset). The latter result is due to the presence of low-energy eharge-transfer excited states which offer fast radiationless decay to the upper-lying luminescent states of the molecular components and 2 [12]. 

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