Excited state complexes

Tunable visible and ultraviolet lasers were available well before tunable infrared and far-infrared lasers. There are many complexes that contain monomers with visible and near-UV spectra. The earliest experiments to give detailed dynamical infonnation on complexes were in fact those of Smalley et al [22], who observed laser-induced fluorescence (LIF) spectra of He-l2 complexes. They excited the complex in the I2 B <—A band, and were able to produce excited-state complexes containing 5-state I2 in a wide range of vibrational states. From line w idths and dispersed fluorescence spectra, they were able to study the rates and pathways of dissociation. Such work was subsequently extended to many other systems, including the rare gas-Cl2 systems, and has given quite detailed infonnation on potential energy surfaces [231. 

Molecular Interaction. The examples of gas lasers described above involve the formation of chemical compounds in their excited states, produced by reaction between positive and negative ions. However, molecules can also interact in a formally nonbonding sense to give complexes of very short lifetimes, as when atoms or molecules collide with each other. If these sticky collisions take place with one of the molecules in an electronically excited state and the other in its ground state, then an excited-state complex (an exciplex) is formed, in which energy can be transferred from the excited-state molecule to the ground-state molecule. The process is illustrated in Figure 18.12. 

Exciplexes are complexes of the excited fluorophore molecule (which can be electron donor or acceptor) with the solvent molecule. Like many bimolecular processes, the formation of excimers and exciplexes are diffusion controlled processes. The fluorescence of these complexes is detected at relatively high concentrations of excited species, so a sufficient number of contacts should occur during the excited state lifetime and, hence, the characteristics of the dual emission depend strongly on the temperature and viscosity of solvents. A well-known example of exciplex is an excited state complex of anthracene and /V,/V-diethylaniline resulting from the transfer of an electron from an amine molecule to an excited anthracene. Molecules of anthracene in toluene fluoresce at 400 nm with contour having vibronic structure. An addition to the same solution of diethylaniline reveals quenching of anthracene accompanied by appearance of a broad, structureless fluorescence band of the exciplex near 500 nm (Fig. 2 )... 

The large Stokes shifts of the emission bands are seen to be the consequence of the tendency for distortion of the excited state complexes. The shifts arise both from the stabilization of the excited state and accompanying destabilization of the ground state and is expected to follow the order Xe>Kr>Ar as observed(30). 

A short excursion into the physics and spectroscopy of intermolecular interactions is intended to illustrate the effects of fluorescence spectra change on the transition of dye molecules from liquid solvents to solid environments, on the change of polarity and hydration in these environments, and on the formation of excited-state complexes (excimers and exciplexes). 

Exciplexes are the excited-state complexes that can be formed by partners of different origin [33]. Their formation on intermolecular interaction can provide a fluorescence reporting signal [28, 34]. The advantage of their formation in high-concentration matrices is the large Stokes shift that, as we will see below, can prohibit the homo-FRET. 

Exciplexes are excited-state complexes (the term exciplex comes from excited complex ). They are formed by collision of an excited molecule (electron donor or acceptor) with an unlike unexcited molecule (electron acceptor or donor) ... 

A well-known example of an exciplex is the excited-state complex of anthracene and N,N-diethylaniline resulting from the transfer of an electron from an amine molecule to an excited anthracene molecule. In nonpolar solvents such as hexane, the quenching is accompanied by the appearance of a broad structureless emission band of the exciplex at higher wavelengths than anthracene (Figure 4.9). The kinetic scheme is somewhat similar to that of excimer formation. 

Complex formation is important in photophysics. Two terms need to be described here first, an exciplex, which is an excited state complex formed between two different kinds of molecules, one that is excited and the other that is in its grown state second, an excimer, which is similar to exciplex except that the complex is formed between like molecules. Here, we will focus on excimer complexes that form between two like polymer chains or within the same polymer chain. Such complexes are often formed between two aromatic structures. Resonance interactions between aromatic structures, such as two phenyl rings in PS, give a weak intermolecular force formed from attractions between the pi-electrons of the two aromatic entities. Excimers involving such aromatic structures give strong fluorescence. 

An engineering plastic is simply one that can be cut, sawed, drilled, or similarly worked with, excimer Similar to an exciplex except that the complex is formed between like molecules, exciplex An excited state complex formed between two different kinds of molecules, one is excited and the other is in its grown state. 

Furans are able to undergo photocycloaddition of the [W2S+ 2S] and the [W4S+ 4S] type to suitable substrates. With benzene (80JCS(P1)2174) five 1 1 products are obtained. The relative proportions of these products are highly variable and depend on the relative concentration of the reactants, the irradiation time, the light intensity and the temperature of the solution. For the shortest irradiation time with a low-pressure mercury lamp at 15 °C, the relative proportions are 1 1 10 40 2. The major product is the 2,5 l, 4 -adduct (301) and the next most prolific is the 2,3 l, 2 -adduct (302). Adduct (301) is unreactive to dienophiles but gives adduct (302) by Cope reaction at 60-70 °C. This reaction can also be achieved by irradiation of a cyclohexane solution of (301). Adduct (302) reacts readily with dienophiles in ethereal solution to form Diels-Alder adducts. The minor adducts possess structures (303), (304) and (305). The reaction is thought to involve the first excited triplet of benzene or an excited state complex. A [ .4s+ .4g] photoadduct (306) is formed... 

Fluorescence from the excited state complexes of t-1 and electron poor alkenes has been observed only with dimethylfuma-rate and fumaronitrile, both of which form weak ground state complexes with t-1 (76). Fluorescence of the same wavelength and lifetime is observed upon quenching of t or excitation in the charge-transfer absorption band of the complexes of t-1 with these acceptors. Some properties of these excited complexes and other exciplexes of t-1 are summarized in Table 7. Fluorescence maxima, like the absorption maxima, of related charge-transfer complexes, can be correlated with the donor ionization potentials (eq. 16). As shown in Fig. 3, the point for t-1 falls well below the line obtained by Shirota and co-workers (87) for the com-... 

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. 

As early as 1964, Corey had suggested the intermediacy of an oriented pi complex in the cycloaddition of enones (43) and soon thereafter Hammond and coworkers, on the basis of arene fluorescence quenching by dienes, suggested the possible involvement of a polar excited state complex with substantial charge transfer character (44). Since then the possibility of cycloadditions occurring through the intervention of exciplex or excimer intermediates per se or as precursors for radical ions pairs, eq. 12,... 

The cation radical can undergo deprotonation to yield an allyl radical or nucleophilic attack by the solvent to produce a methoxyalkyl radical. Coupling of these radicals with the aromatic radical anion produces acyclic adducts. As an alternative, the anion radical can be protonated, ultimately giving reduction product. Thus, the degree of charge separation within the excited state complex dramatically influences the observable chemistry. 

Whilst [ Ru(bipy)3]2t itself is incapable of splitting water, its electron-transfer properties have been utilized for hydrogen production in a series of reactions involving cocatalysts (see equations 21 to 26). The first step involves electron transfer from the excited state complex to an electron relay (R), which in its reduced form is capable (in the presence of a suitable heterogeneous redox catalyst) of reducing protons to hydrogen. The [Ru(bipy)3]3+ which is formed is then capable of... 

Excited-state complexes between two dissimilar entities, called exciplexes, are also frequently formed. The most thoroughly studied exciplexes are those between an aromatic compound and either an amine or a conjugated olefin or diene. Compound 12, for example, has an absorption spectrum identical with... 

Steady-state fluorescence spectroscopy has also been used to study solvation processes in supercritical fluids. For example, Okada et al. (29) and Kajimoto and co-workers (30) studied intramolecular excited-state complexation (exciplex) and charge-transfer formation, respectively, in supercritical CHF3. In the latter studies, the observed spectral shift was more than expected based on the McRae theory (56,57), this was attributed to cluster formation. In other studies, Brennecke and Eckert (5,31,44,45) examined the fluorescence of pyrene in supercritical CO2, C2HSteady-state emission spectra were used to show density augmentation near the critical point. Additional studies investigated the formation of the pyrene excimer (i.e., the reaction of excited- and ground-state pyrene monomers to form the excited-state dimer). These authors concluded that the observance of the pyrene excimer in the supercritical fluid medium was a consequence of increased solute-solute interactions. 

An excimer or exciplex, E, is an excited state complex formed from the association of M with a ground state molecule that is the same or different, respectively ([MM] = excimer [MM ] = exciplex) [115]. The simple considerations set forth in Fig. 8 for the highest-occupied (HOMO) and lowest-unoccupied molecular orbitals (LUMO) account for E formation. The filled HOMOs of two ground state molecules provide no driving force for ground state interaction (Fig. 8a). Conversely, a formal net bond arises from the interaction of the partially filled... 

Figure 8 Energy level diagram explaining excimer formation, (a) Interaction between two ground state molecules results in no net stabilization of dimer, (b) Interaction of ground state (M) and electronically excited molecules (M ) results in an energetically stabilized excited state complex or excimer (E ). Emission from M and E is denoted by hv(M ) and hv(E ), respectively.

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