Molecular complexes/exciplexes

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. 

Exciplexes are defined as molecular complexes which are stable under electronic excitation30. The picosecond (or femtosecond) laser photolysis methods are suitable for investigating the very rapid photo-induced processes related to CT complexes. 

A molecular complex may be dissociated in the ground state, and yet be associated in an excited electronic state. An excited molecular complex of defined stoichiometry, which dissociates in the ground state, is known as an exciplex [110,111], the term being derived from exci (ted comp) plex by analogy to excimer (= exci (ted di) mer ). 

The term exciplex (excited complex) is used to describe an electronically excited molecular complex of definite stoichiometry. Complexes which fall into this broad classification include ... 

In the OLEDs there is a high possibility that exciplex formation occurs at the ETL/EML or HTL/EML interfaces because HTL and electron transport layer (ETL) usually have an electron-donating and an electron-accepting nature, respectively. There have been some researches on the application of exciplexes for the tuning of emission colors (Li et al 2006, Liang and Choy 2006) and white emitting OLEDs (Tong et al 2007). Extensive studies on exited bi-molecular complexes and their application in electrophosphorescent devices have been done by Kalinowski et al (2007) and Cocchi et al (2006). 

Quenching is a radiationless process involving two molecules. There are also radiative processes involving two molecules. These are necessarily cooperative processes involving non-covalent molecular complexes. If two molecules act cooperatively to 6sor6 a photon, an absorption complex is involved. If two molecules act together to emit a photon, an exciplex (electronically excited complex) is involved. The particular case of an exciplex in which the two molecules are the same is termed an excimer (electronically excited dimer). 

As mentioned above, an advantage of primary importance of the liquid crystal method is that polarization measurements may be performed in fluid media. This advantage has been exploited recently for fluorescence polarization studies of metastable molecular complexes formed in the excited state (e.g. excimers [111] and exciplexes [99]). 

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. 

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 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... 

Changes in the electronic and molecular structures after CT excitation can also result in chemical bond formation between the excited complex [AB[ and another molecule (Q) of the system, yielding an encounter complex AB-Q. The complex formed by interaction of an excited molecular entity with a ground state partner of the same structure is called an excimer, whereas an electronically excited dimer of definite stoichiometry, formed by interaction of an excited molecular entity with a ground state partner of a different structure, is named an exciplex [29], Both excimers and exciplexes have their own properties and can thus be regarded as new chemical species. Their generation is well documented by the concentration effect on the fluorescence of some solutes or by flash photolysis measurements [11,24],... 

Aromatic substrates are by far the most commonly used substrates in the rapidly expanding area of photoinduced electron transfer [1,2]. This is obviously due to the favourable location of the frontier molecular orbitals in such compounds. The same factor facilitates the formation of electron transfer donor-acceptor (EDA) complexes both in the ground state (these possibly are intermediates in some thermal reactions, e.g. selected electrophilic substitutions), and in the excited state (exciplexes). 

Fig. 6. A diagram showing the dynamic interconveision of solvent-separated ion pairs (SSIP, exdplexes, contact ion pairs (CIP), and free ions in solution. Electron transfer takes place within a cage of solvent molecules to generate a SSIP or more intimate charge-transfer complex, the latter being an exciplex or CIP. The nature of the charge-transfer intermediate generated may depend on the distance separating the reactants. The distance depends on the molecular structures of the reactants, i.e., their sizes, shapes, and steric features. Free ions are produced by ion dissociation from the solvent cage...

Encounter complex An intermolecular ensemble formed by molecular entities in contact or separated by a distance small compared to the diameter of solvent molecules and surrounded by several shells of solvent molecules the innermost shell is the solvent cage . If one of the species is excited, the excitation usually takes place prior to formation of the encounter complex. During the lifetime of the encounter complex the reactants can collide several times to form colHsion complexes, and then undergo structural and electronic changes. If the interaction between the reactants leads to a minimum in the potential energy and one of the entities is electronically excited, the encounter complex may represent an exciplex or excimer. 

---