Exciplex minimum

The yield of 71 increases with increasing polarity of either the solvent or the substituent X in the 9-position of anthracene. This result has been explained by invoking a stabilization of the exciplex, which should reduce the barrier between the exciplex minimum and the pericyclic minimum. The observation that in the presence of methyl iodide 72 becomes the mqjor product for X = H, due to the heavy-atom effect, is compatible with the obvious assumption that the multistep process is a triplet reaction. However, if X = CN, no methyl iodide heavy-atom effect is observed (N. C. Yang et al., 1979, 1981). 

Here, R(Sq) stands for a reactant molecule in the Sq minimum at its equilibrium ground state geometry, the arrow labeled hi/ indicates vertical excitation by photon absorption or by energy transfer to a spectroscopic minimum S in the Si or Ti state, and the other arrows indicate further fate of this initial vertically excited species. Travel on the Si or Ti surface may take the molecule to an excimer or exciplex minimum E and to a biradicaloid minimum B (an Sq - Si conical intersection is viewed as a limiting case of B). Vertical radiative or non-radiative return from S or E restores R in its ground state (a photophysical outcome). If the return to the Sq state occurs from B, it is still possible for R to be restored, but there also is some probability of forming a new product P (a photochemical outcome). 

It is instructive to consider the stability of other excitations in DNA, ex-cimers and exciplexes. An excimer (exciplex) is formed when two identical (nonidentical) molecules that do not interact in their ground states do so when one of the molecules is in an excited state. As a result of charge-transfer and exchange interactions of the overlapping n electrons of the two molecules on the one hand, and their mutual repulsion on the other, the molecules are drawn together in a potential minimum at a separation smaller... 

FIGURE 2. Schematic potential energy surfaces for [2+2] cycloaddition in the ground (G), singlet (S), and doubly excited (D) states. E is the exciplex and P the pericyclic minimum. 

Cycloaddition is a singlet state reaction, triplet quenching yielding only stilbene isomerization. In the limit of high t-1 concentration, the quantum yield for the formation of 89 and 90 is 0.66 and no c-1 is formed. Nonradiative exciplex decay is proposed to occur by partitioning at the pericyclic minimum (Fig. 2) between products and reactants. In the limit of high c-1 concentration, 91 is formed with a quantum yield of 0.05 and the predominant exciplex decay pathway is dissociation to yield f-c, which decays to a mixture of t-1 and c-1. 

Although the relative orientations of the two addends must be different for ortho and meta addition, it is conceivable that both processes should proceed via the same exciplex. One may speculate that the exciplex does not have one favorite rigid geometry, but that it is in a double-minimum energy well on the excited-state potential-energy surface, with the minima separated by a small barrier. 

Encounter complex An intermolecular ensemble formed by molecular entities in contact or separated by a distance small compared to the diameter of solvent molecules and surrounded by several shells of solvent molecules the innermost shell is the solvent cage . If one of the species is excited, the excitation usually takes place prior to formation of the encounter complex. During the lifetime of the encounter complex the reactants can collide several times to form colHsion complexes, and then undergo structural and electronic changes. If the interaction between the reactants leads to a minimum in the potential energy and one of the entities is electronically excited, the encounter complex may represent an exciplex or excimer. 

Their results are shown in Fig. 8-5, from which the following MFEs could be obtained (1) For each of the compounds with 8 < n < 11, the intensity (7(B)//(0T)) decreased with increasing B from 0 T, attained a minimum value at the minimum field (Smin), and started to increase with increasing B from Smin- (2) Because the singlet exciplex is in equilibrium with the singlet biradical as shown by process (8-2f), the observed MFEs should be due to the LCM and the observed Bmin values correspond to the Blc ones. These Bmin values are very similar to the Bmax ones obtained from the intensity (Et(B)) of the transition absorption due to A as listed in Table 8-1. (3) For each of the compounds with n > 12, the intensity... 

Emission from the exciplex will occur according to the Franck-Condon principle i.e., vertically from the excited-state minimum (no change of the nuclear configuration during the emission process). The separation of M and N in the ex-... 

When two molecules in their ground state approach each other, no ground state complex can be obtained if the corresponding potential energy curve does not have any minimum. However, it may happen that, when one of the two molecules is excited, the approach between the excited molecule and the other molecule in its ground state leads to a minimum in the potential energy curve. In such a case, an exciplex is formed (Reactions 7 and 8). The exciplex formation results in the... 

Exciplexes and Second Sphere Interactions The concept of exdplex formation in inorganic systems has received considerable attention in recent years. Exciplexes can be observed when ground state complex formation is forbidden but the excited state complex has a shallow energy minimum that can radiatively decay to the ground state (Equation (6) and (7)). McMillin and co-workers postulated exdplex contributions to nonradiative relaxation of Cu phenanthroline... 

In the gas phase, there usually is a very shallow van der Waals minimum at a somewhat larger separation between ground-state partners, but in solution, this is often insufficient for keeping them in close contact, due to their prevalent interaction with the solvent. In some cases, pairwise complexation of ground-state reactant molecules occurs even in solution, as in so-called charge-transfer complexes, and in that case the designation of the excited species as an excimer or an exciplex is not strictly correct the minimum in Si or Ti then refers to an excited charge-transfer complex. 

S is greatly stabilized compared to the others, through exciton as well as CT interactions—that is, exactly by the factors responsible for the stability of excimers and exciplexes. (Cf. Sections 5.4.2 and 5.4.3.) The S state has a minimum at the biradicaloid square geometry that may be referred to as the excimer minimum. A strong absorption band in the near-IR region corresponding to a transition between the S and S states of the fluorene excimer has been observed by photodissociation spectroscopy (Sun et al., 1993). 

Another important photoreaction is the 1,3 cycloaddition of ethene to benzene. Although a 1,2 and a 1,4 photocycloaddition can also take place, the 1,3 photochemical addition is the most important, since it yields a variety of natural products depending on the substitution pattern of the alkene and arene. The reaction mechanism which was obtained by experimental results indicates an exciplex mechanism, whereas a SINDOl calculation favors the prefulvene mechanism.Since the experiments were done in solution and the calculations were performed in a vaccum, this is not at all surprising. According to the SINDOl calculations, there is a conical intersection near the prefulvene minimum, which is very flat. The semiempirical calculations are supported by MCSCF ab initio calculations, which show a remarkable similiarity in the important structures for both the exciplex and the prefulvene mechanisms.This demonstrates that semiempirical programs on the Cl level are well suited for photochemical problems. 

Figure 21 Potential energy diagram of the ground and the first excited electronic states of [Ag(CN)32 (eclipsed configuration) as plotted from extended Huckel calculations. The excimer [Ag(CN)32 corresponds to the potential minimum of the excited state. The optical transitions shown are (a) excimer emission, (b) solid state excitation and (c) dilute solution absorption. (Reproduced with permission from Omary MA and Patterson HH (1998) Luminescent homoatomic exciplexes in dicyanoargentate 0) ions doped in alkali halide crystals 1. Exciplex tuning by site-selective excitation. Journal of the American Chemical Society 120 7606-7706.

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