Weak charge-transfer complex crystals

Fig. 21 Crystal structure of the charge-transfer complex pyrene pyromellitic dianhydride (PMDA). The alternate stacking of donors and acceptors. .. DADADA. .. is characteristic of weak charge-transfer complexes. (After Boeyens and Herbstein, 1965b)...

Several hundred reagents are described as being useful for substance visualization [6,27,52,53]. A relatively common test is to place the sorbent into a tank of iodine crystals. The iodine vapors form weak-charge transfer complexes with unsaturated bonds of the sample. This is visibly detected as brown spots. Reaction with iodine is generally reversible but has been shown to oxidize compounds such as mercaptans and disulfides. 

A. Iodine The universal detection reagent iodine is used either as a 1% alcoholic spray, or the plate is placed in a closed jar or tank containing iodine crystals. The iodine vapor dissolves in, or forms weak charge-transfer complexes with organic compounds, which show up as brown spots on a pale yellow background within a few minutes. After marking zones for future reference, exposure of the plate to air causes the iodine to sublimate and the spots to fade, after which the plate can be sprayed with another reagent or the solute can be eluted from the plate for further analysis. 

For many crystals with pure van der Waals interactions, there are several phases with different lattice structure, but similar lattice energies. Phase changes driven by temperature variations or relatively low external pressure can thus readily be observed in many crystals. This is also true of the weak D-A-complex crystals, in which in the ground state no charge transfer has yet occurred. 

Weak (molecular) complexes in solution do not show any IR or NMR spectroscopic evidence for charge-transfer, i.e. the spectra of the complexes represent merely the superposition of the spectra of the separate (uncomplexed) donor and acceptor components [133]. Accordingly, X-ray crystal structures show the donors and acceptors with unperturbed bond lengths and angles [134, 135]. However, a... 

In crystals which are composed of two different partner molecules, CT excitations and with them CT excitons are frequently the predominant lowest excitation states and are thus responsible for the lowest-energy transitions in the singlet system. We will illustrate this using the example of the weak donor-acceptor complex anthracene/pyromellitic acid dianhydride, (A/PMDA) (Fig. 6.14). The ground state is neutral and nonpolar, with only a small charge-transfer fraction. The lowest optical excitation starts from the ground state of the donor D, anthracene, (from its highest occupied orbital or HOMO) and leads to the lowest unoccupied orbital (LUMO) of the acceptors A, PMDA, within the mixed stack DADADA. A polar ex-... 

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