Charge-transfer absorption band ground state

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-... 

Charge-transfer (CT) complex A ground-state complex which exhibits an observable charge transfer absorption band. 

Tetracyanoethylene forms a charge-transfer complex with 1,1 -binaphthyl which exhibits two charge-transfer absorption bands (Yorozu et al., 1978). Excitation via the band of higher energy leads to the formation of the triplet state of binaphthyl which, unlike the ground state, is planar. When optically active 1,1 -binaphthyl is used, this change in geometry can be measured by the fact that the production of the triplet state is attended by racemisation. 

The charge-transfer absorption band results from the promotion of an electron from the EDA ground state to the excited state. For weakly interacting donors and acceptors (in which a b), populating this excited state by irradiation of the CT band essentially promotes an electron from the donor orbital to one located on the acceptor — usually represented by the HOMO and LUMO, respectively. This transition effectively corresponds to a direct electron transfer without the intermediacy of a (local) excited state of either D or A. The energy of this transition, and thus the frequency (wavelength) of the absorption is given by... 

As it concerns the band in the UV region (at 315 nm in the present case), Benesi and Hildebrand [5] assigned this absorption to a charge-transfer transition, where the phenyl ring acts as an electron donor (D) and the iodine as an electron acceptor. The interaction can be described in resonance terms as D-I2 <-> D+I2", the band being assigned to the transition from the ground non polar state to the excited polar state. 

Thermal or photochemical activation of the [D, A] pair leads to the contact-ion pair D+, A-, the fate of which is critical to the overall efficiency of donor/acceptor reactivity as described by the electron-transfer paradigm in Scheme 1 (equation 8). In photochemical reactions, the contact ion pair D+, A- is generated either via direct excitation of the ground-state [D, A] complex (i.e., CT path via irradiation of the charge-transfer (CT) absorption band in Scheme 13) or by diffusional collision of either the locally excited acceptor with the donor (A path) or the locally excited donor with the acceptor (D path). 

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). 

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