Excimer/exciplex formation

Explain the deactivation of excited states by other molecules in terms of quenching processes, excimer/exciplex formation, energy transfer and electron transfer. 

Several studies on CD complexes with aromatic molecules using steady-state and nanosecond spectroscopy have been reported. These studies aimed to understand the photophysical and photochemical behavior of organic guests such as fluorescence and phosphorescence enhancement, excimer/exciplex formation, photocleavage, charge and proton transfer, energy hopping, and cis-trans photo-... 

Recognise situations in which the formation of excimers/exciplexes may affect the observed properties of an excited system. 

In acetonitrile-dichloromethane 1 1 v/v solution, their absorption spectra are dominated by naphthalene absorption bands and they exhibit three types of emission bands, assigned to naphthyl localized excited states (/Wx = 337 nm), naphthyl excimers (Amax ca. 390 nm), and naphthyl-amine exciplexes (/lmax = 480 nm) (solid lines in Fig. 3). The tetraamine cyclam core undergoes only two protonation reactions, which not only prevent exciplex formation for electronic reasons but also cause strong nuclear rearrangements in the cyclam structure which affect excimer formation between the peripheral naphthyl units of the dendrimers. 

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 effects of photophysical intermolecular processes on fluorescence emission are described in Chapter 4, which starts with an overview of the de-excitation processes leading to fluorescence quenching of excited molecules. The main excited-state processes are then presented electron transfer, excimer formation or exciplex formation, proton transfer and energy transfer. 

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. 

It is emphasized that the terms excimer2 and exciplex3,4 are reserved here for homomolecular and heteromolecular excited double molecules formed after the act of light absorption by one component in a process of photoassociation, in the absence of spectroscopic or cryoscopic evidence for molecular association in the ground state. Recent findings indicate that excimer (or exciplex) formation may also result from triplet-triplet annihilation,5,8 cation-anion combination7 (doublet-doublet-annihilation), and electron capture by the (relatively stable) dimer (or complex) cation8 these processes are discussed in Section VII. 

An example of exciplex formation in the solid state may be afforded by perylene doped crystals of pyrene which emit a green structureless fluorescence in addition to the blue and orange-red excimer bands of pyrene and perylene, respectively. Hochstrasser112 has shown that the energy of the emitting species is consistent with that of a charge transfer complex of pyrene and perylene molecules in a bimolecular unit of the pyrene lattice. 

All of the photochemical cycloaddition reactions of the stilbenes are presumed to occur via excited state ir-ir type complexes (excimers, exciplexes, or excited charge-transfer complexes). Both the ground state and excited state complexes of t-1 are more stable than expected on the basis of redox potentials and singlet energy. Exciplex formation helps overcome the entropic problems associated with a bimolecular cycloaddition process and predetermines the adduct stereochemistry. Formation of an excited state complex is a necessary, but not a sufficient condition for cycloaddition. In fact, increased exciplex stability can result in decreased quantum yields for cycloaddition, due to an increased barrier for covalent bond formation (Fig. 2). The cycloaddition reactions of t-1 proceed with complete retention of stilbene and alkene photochemistry, indicative of either a concerted or short-lived singlet biradical mechanism. The observation of acyclic adduct formation in the reactions of It with nonconjugated dienes supports the biradical mechanism. 

The non-linear spectral changes of the naphthyl-localised exciplex and exci-mer bands occur after addition of the first two equivalents of trifluoroacetic acid. It is thus unnecessary to protonate all of the nitrogen atoms in order to suppress exciplex formation since - expressed figuratively - the nitrogen atoms share the protons. Moreover, protonation not only provides protection against exciplex formation but also leads to conformational changes in the cyclam unit itself, which in turn affects excimer formation between peripheral naphthyl units of the dendrimer. 

Photocycloaddition and photoaddition can be utilized for new carbon-carbon and carbon-heteroatom bond formation under mild conditions from synthetic viewpoints. In last three decades, a large number of these photoreactions between electron-donating and electron-accepting molecules have been appeared and discussed in the literature, reviews, and books [1-10]. In these photoreactions, a variety of reactive intermediates such as excimers, exciplexes, triplexes, radical ion pairs, and free-radical ions have been postulated and some of them have been detected as transient species to understand the reaction mechanism. Most of reactive species in solution have been already characterized by laser flash photolysis techniques, but still the prediction for the photochemical process is hard to visualize. In preparative organic photochemistry, the dilemma that the transient species including emission are hardly observed in the reaction system giving high chemical yields remains in most cases [11,12]. 

We will discuss briefly the reactive species such as an exciplex and radical ion species generated by the excitation of organic molecules in the electron-donor (D)-acceptor (A) system. An exciplex is produced usually in nonpolar solvents by an interaction of an electronically excited molecule D (or A ) with a ground-state molecule A (or D). It is often postulated as an important intermediate in the photocycloaddition between D and A. In the case of D = A, an excimer is formed as an excited reactive species to cause photodimerization. In some cases, a ter-molecular interaction of an exciplex with another D or A generates a triplex, which is also a reactive intermediate for photocycloaddition. The evidence for the formation of excimers, exciplexes, and triplexes are shown in the fluorescence quenching. Excimer and exciplex emission is, in some cases, observed and an emission of triplex rarely appears. 

In addition to unimolecular reactions, the excited state may participate in several bimolecular processes. At high concentrations, dimer formation, excimer formation, exciplex formation, solute-solvent complexation, energy transfer, and collosional deactivation may occur. The high-concentration conditions are often experienced when the guest molecules are loaded onto the layered materials with high coverages and specific examples will be provided shortly. 

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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