Exciplex formation, charge transfer

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. 

No exciplex or charge-transfer (CT) complexes between two adjoining organic layers. As HTMs have an electron-donating characteristic and ETMs have an electron-accepting characteristic by nature, most of the HTM-ETM combinations result in exciplex or CT complex formation.62 Bulky substituents/components and a three-dimensional steric configuration also contribute to the relaxation of the complexes.58... 

The intramolecular excited states are essentially Frenkel excitons. However, the intermolecular excited states consist of exciplexes and charge-transfer complexes. The exciplex is generated when an excited molecule A is coupled with an unexcited molecule B to form an intermolecular excited state an excited "complex molecule (A B)." If A and B are the same type of molecules, this intermolecular excited state is called an excimer. The charge-transfer complex is formed when charge transfer occurs within an exciplex. The formation of a... 

We will briefly describe the kinetics of the two-state system, and then apply the result to some common examples of inter and/or intramolecular reactions excimer formation, charge transfer (leading to an exciplex), electron transfer (leading to radical ions), proton transfer or isomerisation. Scheme 15.6 is a condensed representation of the two-state system. 

Itoh M, Mimura T, Usui H, Okamoto T (1973) Intramolecular exciplex and charge transfer complex formations in (9,10-dicyanoanthracene)-(trimethylene)-(naphthalene) systems. J Am Chem Soc 95 4388-4392... 

Electronic energy transfer process may also occur via formation of exciplexes. An exciplex (excited charge transfer complex) is a well defined complex which exists in electronically excited states. An exciplex is formed between an excited donor (D ) and/or an excited acceptor A ) and donor (D) molecules ... 

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 ... 

The exciplex emission is also affected by solvent polarity, where an increase in the solvent polarity results in a lowering of the energy level of the exciplex, at the same time allowing stabilisation of charged species formed by electron transfer (Figure 6.7). Thus, in polar solvents the exciplex emission is shifted to even higher wavelength and accompanied by a decrease in the intensity of the emission, due to competition between exciplex formation and electron transfer. 

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 SET between amine and acceptor may be enhanced by photoexcitation and may lead to the formation of exciplexes2 or molecular complex with charge transfer character3. The photochemistry between aromatic acceptors and amines via the exciplexes has been discussed earlier (Scheme l)4. 

Fig. 7 (a) Molecular orbital (MO) description for the charge-transfer state formation in organic donor/ acceptor systems, (b) Description for CT state emission energy using exciplex MOs in (a)... 

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. 

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. 

Charge Transfer Mechanism Exciplex Formation and Decay... 

If added substances absoroing at higher frequencies are present the quenching mechanism is visualized to proceed through transient complex formation in the excited state. They are known as exciplexes and have pronounced charge transfer character. Exciplexes may decay radiatively, nonradiatively or lead to reaction products. 

Adams and Cherry (78) have investigated the effects of stilbene substitution on the behavior of their excited complexes with fumaronitrile and find that the rate constants for fluorescence and nonradiative decay are insensitive to substitution, but that the rate constant for intersystem crossing is increased by electron-donating substituents (lower stilbene oxidation potential). This trend is attributed to a decrease in the energy gap between the excited complex and locally excited 3t (Fig. 4). The observed energy gap dependence of the exciplex lifetime could also account for the absence of fluorescence (or cycloadduct formation, see Section IV-B) from the excited charge-transfer complexes of t-1 with stronger electron acceptors such as maleic anhydride (76) or tetracyanoethylene (85). 

Oxetane formation is presumed to occur via the singlet exciplex however, excitation of the ground state charge-transfer complex may be necessary in order for the formation of 39 to compete with the rapid isomerization of c-1. The factors which favor oxetane versus cyclobutane formation in this reaction are not understood. 

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