Excited states dimers

Let us now turn our attention to the newly emerging field of excited state metal atom chemistry. The discovery of the excited state dimerization reaction (34) ... 

The explanation for this behaviour is that at the higher concentration of pyrene an excited-state dimer, or excimer, is formed through an interaction between the electronically-excited pyrene (M ) and the ground-state pyrene (M) ... 

In some cases, simultaneously with the quenching of the normal fluorescence a new structureless emission band appeals at about 6000 cm-1 to the red side of the monomer fluorescence spectrum (Figure 6.4). This phenomenon was first observed in pyrene solution by Forster and was explained as due to transitory complex formation between the ground and the excited state molecules since the absorption spectrum was not modified by increase in concentration. Furthermore, cryoscopic experiments gave negative results for the presence of ground state dimers. These shortlived excited state dimers are called pxcimers to differentiate them from... 

Distinctive properties of excimer and excited state dimers... 

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. 

Many of the present models used to describe fluid-solid phase equilibria require one to assume that the solute is at infinite dilution. That is, researchers have often assumed that solute-solute interactions are nonexistent. Recently, Brennecke et al. used the fluorescent probe pyrene to investigate the possibility of solute-solute interactions in C02, C2H4, and CF3H (7-9). Pyrene is an interesting probe because it can form a characteristic excited-state dimer (excimer) during its excited-state... 

In some cases excited state chromophores form supramolecular complexes either with ground state chromophores on the same molecule or one nearby resulting in the formation of an excimer (excited state dimer) or, if the two chromophores are different to one another an exciplex. Formally an excimer as defined as a dimer which is associated in an electronic excited state and which is dissociative in its ground state.1 Formation of the pyrene excimer is illustrated in Figure 11.3. 

Figure 11.3 The excited state of a chromophore such as pyrene can form a complex with a ground state molecule. If the result is an excited state dimer the complex is known as an excimer, while if the excited complex is formed between two different molecule it is termed and exciplex. Excimers and exciplexes emit at lower energy than the corresponding monomers.

The interactions between the different units in the excited states are called excimers. These excimers can be excited dimers, trimers, tetramers, etc. These excited oligomers have different wavelengths and emission lifetimes. The extent of the interactions in the excited state (dimers, trimers, tetramers) is hard to predict because it depends on the amplitude of the interactions... 

For instance, out of many studies carried out with crystalline olefins that possess slightly different interbond distances and orientations one would only answer Yes or No to questions of reactivity and selectivity. An oversimplified and pictorial representation of this analysis is illustrated in Fig. 2 with a reaction coordinate for the excited state dimerization of a hypothetical set of crystalline alkenes. The reaction coordinate in this case would be given primarily by the displacement between the two Tr-systems which, for simplicity, may be assumed to have the required parallel arrangement. In the figure, different crystals can be though of as positioning the prospective reactants at different distances. Each crystal represents a point along the reaction coordinate [50]. Some molecules may... 

Excited state complexes are relevant in connection with photo-induced electron transfer, since their formation frequently competes with or precedes electron transfer. The simplest examples, excited state dimers (excimers), were discussed by Kautsky as early as 1939 [77], The first organic excimer, the dimer of pyrene, was identified by its characteristic, red-shifted, structureless emission spectrum by... 

Charge-annihilation processes preceding emission may occur from solvent-separated or contact ion pairs, and there is a possibility that excited-state dimers... 

For some fluorophores (well-known examples are pyrene and its derivatives) the emission spectrum depends dramatically on their concentration. This is due to the formation of excited-state dimers, so-called excimers, consisting of a ground-state and an excited-state monomer. For instance, lipids that are substituted with a pyrene moiety in each of their acyl chains can be used to study intramolecular excimer formation. In this way information on lateral organization and intramolecular dynamics can be obtained. 

Solute/solute Interactions are revealed by the forinatlon of exclmers. Exclmers are excited state dimers that result In a broad structureless band at significantly longer wavelengths than the normal fluorescence. While they are not dimers In the sense of a ground state conqplex, their existence does Indicate that there Is sufficient Interaction In the approximately 10 second lifetime of the excited state (33) to form the excited state complex. We have observed the formation of pyrene exclmers even at extremely low concentrations In supercritical fluids. Figure 7 shows the spectra of pyrene In SCF CO2 at two concentrations. 

Depending on the strength of the electron-phonon coupling, one may observe the formation of a polaron or an exclmer. The formation of a polaron does not lead to the loss of the identity of the monomer in the excited state, but simply the excitation is localized by local lattice-deformation. The excimer formation requires a severe distortion oi the local structure which leads to an excited state dimer. It may also be pointed out that the polaron mechanism is a purely dynamic effect which can occur even in a defect-free lattice. In contrast, the excimer formation can occur either by a dynamic effect due to strong electron-phonon coupling or by a static effect due to sites deformed by the presence of defects. 

Excimer Formation Interaction of a ground-state species (M) with an excited state (M ) results in the formation of an excited state dimer, known as an excimer. The process is outlined below... 

So far, preassociation and the possibility of photorecombinative laser action have been investigated in the formation of NO 85>83>, n2 85) CN 85>, and the halogens 87>. A somewhat different system is the dimol" emission from an excited-state dimer of molecular oxygen in either the or 1Af state 89>. 

Bonds to Oxygen.—Lower Oxidation States. The origin of the cool green phosphorus flame is an excited-state dimer of phosphorus monoxide, and marked... 

Two commonly used terms for intermolecular charge-transfer emission materials are exciplex and excimer. The former refers to an excited state charge-transfer complex and the latter to an excited state dimer. For example, anthracene and diethylaniline form an exciplex that has a broad emission band at A, = 485 nm. The best-known excimer is pyrene, which displays an intense excimer emission band at A = 470 nm in a relatively concentrated solution M). Excimers formed by aromatic... 

Similarly, l-anilinonaphthalene-8-sulfonic acid (ANS) only emits in a hydrophobic environment, being almost completely quenched in aqueous solution. ANS and some other dyes, including 6-(p-toluidinyl)naphthalene-2-sulfo-nate, pyrene, l,6-diphenyl-l,3,5-hexatriene, fluorescein, and rhodamine derivatives attached to long acyl chains or to fatty acids that localize in the cellular membranes were used as probes for hydrophobic sites in proteins, protein folding, imaging of membranes of the cell, and solvent polarity. Pyrene-labeled fatty acids were used to detect the fusion of two membranes. When present in a membrane at sufficiently high concentrations, pyrene excimers (excited-state dimers) are formed that emit at 470 nm. Upon fusion with other membranes, probe concentration decreases, and excimer fluorescence is replaced by monomer fluorescence at 400 nm. This process can be monitored by ratiometric detection of pyrene labels. 

Some fluorophores can also form complexes with themselves. The best-known example is pyrene. At low concentration, pyrene displays a highly structured emission (Figure 1.11). At higher concentrations, the previously invisible UV emission of pyrene becomes visible at 470 nm. This long-wavelength emission is due to excimer formation, the term excimer being an abbreviation for an excited-state dimer. 

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