Exciplex formation interactions

Argentophilic attraction has been found in Tl[Ag(CN)2] this compound displays photoluminescence that has been explained in terms of excited-state Ag—Ag interactions leading to exciplex formation, [Ag(CN)2-]3. 51 Several photochemical studies have been carried out with this type of compound.252-255... 

Not all sensitized photochemical reactions occur by electronic energy transfer. Schenck<77,78) has proposed that many sensitized photoreactions involve a sensitizer-substrate complex. The nature of this interaction could vary from case to case. At one extreme this interaction could involve a-bond formation and at the other extreme involve loose charge transfer or exciton interaction (exciplex formation). The Schenck mechanism for a photosensitized reaction is illustrated by the following hypothetical reaction ... 

Besides the excited molecule can interact physically with a second molecule, i.e. undergo bimolecular processes. These are either energy transfer (1.7) or exciplex formation (1.8) depending on the relative excitation energies of the molecule to be studied and its partner. 

Extensive investigation has been performed on the interaction of dendrimers 1 and 2 with metal ions [17a, e-h]. Complexation with Zn2+ engages the nitrogen lone pairs and thereby prevents exciplex formation, with a resulting intense naphthyl fluorescence (dashed lines in Fig. 3). This strong fluorescent signal is... 

This chapter describes the characteristics of the fluorescence emission of an excited molecule in solution. We do not consider here the photophysical processes involving interactions with other molecules (electron transfer, proton transfer, energy transfer, excimer or exciplex formation, etc.). These processes will be examined in Chapter 4. 

Class 3 fluorophores linked, via a spacer or not, to a receptor. The design of such sensors, which are based on molecule or ion recognition by a receptor, requires special care in order to fulfil the criteria of affinity and selectivity. These aspects are relevant to the field of supramolecular chemistry. The changes in photophysical properties of the fluorophore upon interaction with the bound analyte are due to the perturbation by the latter of photoinduced processes such as electron transfer, charge transfer, energy transfer, excimer or exciplex formation or disappearance, etc. These aspects are relevant to the field of photophysics. In the case of ion recognition, the receptor is called an ionophore, and the whole molecular sensor is... 

The basic principle of this method of recognition is a cation-induced conformational change bringing closer together (or moving away) two moieties able to interact and induce photophysical effects excimer or exciplex formation (or disappearance), electronic energy transfer and quenching. 

Solute-solvent interactions are of two types (1) universal interaction, and (2) specific interaction. Universal interaction is due to the collective influence of the solvent as a dielectric medium. It depends on the dielectric constant D and refractive index n of the solvent and the dipole moment g of the solute molecule. Such interactions are van der Waals type. Specific interactions are short range interactions and involve H-bonding, charge-transfer or exciplex formation. H-bonding ability may change on excitation specially for n-yxt transitions. 

If definite stoichiometry is maintained in the exciplex formation, an isoemissive point similar to isosbestic point in absorption miy be observed. An interesting example of intra-molecular exciplex formation has been reported foi 9-methoxy-10-phenanthrenecarboxanil. The aniline group is not necessarily coplanar with the phenanthrene moiety but is oriented perpendicular to it. The u-elcctron located on its N-atom interacts with the excited -electron system and an intramolecular exciplex with T-bone type structure is formed in rigid glassy medium where rotation is restricted. Temperature dependence of fluorescence of this compound in methylcyclohexane-isopentane (3 1) solvent shows a definite isoemissive point (Figure 6.8). As the solvent melts and movement is restored to the molecule, structured fluorescence reappears. 

Figure 4.23 Orbital interactions in exciplex formation between an excited molecule M (electron donor) and a ground state molecule N. hvexp is the exciplex emission...

The photodimerization of cinnamic acid and similar molecules is observed in crystals, but reactions of the same type occur in some polymers as well. Polymers such as polystyrene are made of long, saturated hydrocarbon chains with pendant groups in close contact dangling from the chain these chromophores can then interact in bimolecular photoaddition reactions. Polyvinyl car bazole and its derivatives are important examples of polymers which lead to such bimolecular interactions (e.g. exciplex formation). 

The photochemistry of tethered alkenes is more predictable than the nontethered situation. There is evidence of Ti-stacking for closely held moieties which presumably improves the orbital interactions between the alkenes. Exciplex formation is likely involved when the reacting groups are within 5 angstroms. Cornil et al. have shown that k systems can couple within this distance [11]. This exciplex could lead to a concerted cycloaddition from the excited state which would be consistent with the observed products. Although stepwise addition (see Sch. 3) cannot be ruled out even in these tethered singlet reactions. Ring closure must be very rapid if diradicals are involved, since no radical-trapped species have been found. 

Turro [108] proposed a similar mechanism to explain the photodimerization of 2-butene either concerted addition of Spirit ) to 2-butene, or formation of a singlet exciplex by interaction of SJjiji ) and 2-butene, followed by direct collapse... 

Although the phenomenon is more common in organic photochemistry, a coordination entity can also act in the process of excimer or exciplex formation as an excited molecule AB or quencher (Q).The second-sphere donor-acceptor interaction with an acceptor quencher causes oxidative quenching of AB, whereas interaction with a donor quencher yields reductive quenching. 

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