Charge transfer hypsochromic shift

In the UV spectral range complexation with 18-crown-6 causes a hypsochromic shift of the band with the longest wavelength in various solvents (Bartsch et al., 1976 Hashida and Matsui 1980). Gokel and Cram (1973) reported that complexation with binaphtho-20-crown-6 (11.2) produces a yellow to red color. This phenomenon is very likely to be due to a charge-transfer band between a naphthalene ring as donor (7i-base) and the arenediazonium ion as acceptor (7i-acid). 

Figure 16.4 Principle of the PCT (photoinduced charge transfer), chemically driven, luminescent molecular sensor based on the donor-spacer-acceptor architecture, (a) Binding of analyte trigger to the donor (green) moiety results in hypsochromic shift of absorption (emission) band (b) binding of the same analyte to the acceptor moiety (red) results in bathochro-mic shift of corresponding transition...

As expected, electron-withdrawing substituents cause a bathochromic shift of this absorption band, which exhibits a strong intramolecular charge-transfer character [cf. Section 6.2.1), whereas electron-releasing substituents give rise to a corresponding hypsochromic shift. 

Molecular EDA complexes as well as charge-transfer ion pairs show (negative) solvatochromism [128], i.e. the charge-transfer absorption maxima (2cx) undergo hypsochromic shifts with increasing solvent polarity. The solvatochromic effect is readily explained on the basis of the Marcus correlation for charge-transfer energies in solution [129], (Eq. 9) ... 

Hypso-type spectra look similar to normal spectra but the B and Q bands are blue shifted (i.e. hypsochromically). This type of spectrum is shown by d-block elements with unfilled d-orbitals of the type d -d , which includes all the metals from groups VIII to IB. Here, d-electrons may be donated into the porphyrin s empty iT -orbitals (i.e. metal-to-ring charge transfer), thus raising their energy. This increases the energy of the porphyrin ir-ir transition, with respect to the metal-free porphyrin, leading to a blue... 

Compound 2 (/ i = i 2= H) shows an opposite behavior. Its zwitterionic ground state has a calculated dipole moment of 22.6 D, which is reduced in its quinoid, excited state to a value of 13.7 D. An increase in solvent polarity destabilizes the charge-transfer transition of this compound. shifting the value of its longest wavelength band hypsochromically, from 620 nm in nonpolar CHCI3 to 442 nni in water (negative solvatochromism) (Fig. 3). 

VEH calculations predict that the HOMO LUMO transition corresponds to an electronic transfer from the phenoxazine moiety, acting as a donor, to the acceptor naphthoquinone moiety. This result supports the intramolecular charge-transfer nature of the lowest energy absorption band observed experimentally for these quinones. The second absorption band of this compound corresponds to the lowest energy absorption band of 1,4-naphthoquinone which is hypsochromically shifted due to the destabilization of the LUMO in 83 and 84. 

When a Lewis base is added to a diiodine solution, two pronounced changes in the spectrum occur. They can be sehematically described by the approximate MO diagram of Figure 5.15. The visible transition (arrow 1 in free diiodine, arrow 2 in the complex) undergoes a blue (hypsochromic) shift and a new band arises in the ultraviolet region that is due to a charge-transfer transition (arrow 3). 

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