Absorption maxima Acceptor-donor complex

Evaluation of the Work Term from Charge Transfer Spectral Data. The intermolecular interaction leading to the precursor complex in Scheme IV is reminiscent of the electron donor-acceptor or EDA complexes formed between electron donors and acceptors (21). The latter is characterized by the presence of a new absorption band in the electronic spectrum. According to the Mulliken charge transfer (CT) theory for weak EDA complexes, the absorption maximum hv rp corresponds to the vertical (Franck-Condon) transition from the neutral ground state to the polar excited state (22). 

The position of the absorption maximum hvDA is dependent on the ionization potential IP of the donor, the electron affinity EA of the acceptor, and the dissociation energy W of the complex, which depends on the coulombic attraction and the environment [65,66]. 

Pure charge-transfer exeited states are most rigorously defined in the weakly coupled limit. Simple examples of this limit are found in ion-pair complexes, complexes in whieh there is no covalent linkage between the donor and acceptor. Standard models work very well in this limit, and the IPCT absorption maximum is successfully described by the sum of the reduction potentials and the electron-transfer reorganizational energies of the constituent ionic partners, as in Eqs. 7 and 9 ( DA 0)-... 

In Table II are collected all the literature data on molecular complexes of small biological molecules which we have been able to locate. It would be superfluous to discuss in detail every case, since the table has been made fairly detailed. In particular, the wavelength at which the association constant was determined, or the maximum of new absorption is given, and compared with the absorption maximum of uncomplexed material. In a very large number of the cases listed in Table II a charge-transfer band is not observed and a donor-acceptor interaction is therefore questionable. 

In case of ionic liquids both the anions and the cations influence the absorption spectra of the indicator and the summary effect can be seen. Table 12.1.10 suimnarizes the absorption band maximum of the complexes and the obtained donor and acceptor numbers in neat 1-butyl-3-methylimidazolium (C4C,im ) based ionic liquids. Figure 12.1.12 shows a linear dependence of the donor and acceptor number. In conventional solvents where a solvent molecule can act as donor and/or as acceptor, e g. methanol is a strong acceptor but can also donate electrons from the lone pairs of the oxygen, and no such correlation can be found. In contrast, in ionic liquids without functional groups the cation is only acting as electron pair acceptor and the anion only as electron pair donor (additional functional groups can of course also act as acceptor and/or donor). 

Bulk crystalline radical ion salts and electron donor-electron acceptor charge transfer complexes have been shown to have room temperature d.c. conductivities up to 500 Scm-1 [457, 720, 721]. Tetrathiafiilvalene (TTF), tetraselenoful-valene (TST), and bis-ethyldithiotetrathiafulvalene (BEDT-TTF) have been the most commonly used electron donors, while tetracyano p-quinodimethane (TCNQ) and nickel 4,5-dimercapto-l,3-dithiol-2-thione Ni(dmit)2 have been the most commonly utilized electron acceptors (see Table 8). Metallic behavior in charge transfer complexes is believed to originate in the facile electron movements in the partially filled bands and in the interaction of the electrons with the vibrations of the atomic lattice (phonons). Lowering the temperature causes fewer lattice vibrations and increases the intermolecular orbital overlap and, hence, the conductivity. The good correlation obtained between the position of the maximum of the charge transfer absorption band (proportional to... 

According to the Mulliken theory of charge transfer complexes, the vertical electron affinity (VEa) of an acceptor and the vertical ionization potential (VIP) of a donor are related to the energy of maximum absorption of the complex (Ect) by the following equation ... 

The theory of charge transfer complexes relates the maximum in the absorption spectrum, the charge transfer energies Ect, and energies for complex formation AGct to the vertical ionization potential of the donor and the vertical electron affinities of the acceptor. The relationship uses constants related to the geometry of the complexes. Mulliken described the theory of charge transfer as follows ... 

The most satisfactory account of this phenomenon is due to Mulliken, according to whom a complex (D, A) is formed from a donor (D) species and an acceptor (A) species. This complex can exist in two energy states, the difference in energy between the two being equal to the energy of a quantum at the maximum of the absorption band. In the ground state of the complex the binding between foe components h chiefly due let the Van der Waais... 

The presence of P-CD favors the formation of a charge-transfer (CT) complex between 2-methoxynaphthalene and o-dicyanobenzene, as demonstrated by the variations in the absorption spectrum. The excitation of the charge-transfer complex caused a structureless emission with maximum at 480 nm. Since methoxynaphthalene and dicyanobenzene do not form CT complexes in water, it was concluded that the complexed methoxynaphthalene and the complexed dicyanobenzene aggregate to form a 2 1 1 complex (CD-methoxynaphthalene-dicyanobenzene) [130]. The y-CD promoted formation of charge-transfer complexes between a- and / -naphthylacetic acid (as donors), picric acid, and four isomeric dinitrobenzoic acids (as electron acceptors) was reported in references 147 and 148. 

---