Dimerized mixed stack

Some semiconducting organic CT complexes of mixed-stack architecture exhibit the rather unusual neutral-to-ionic (N-I) phase transition upon variation of an external variable of parameter hyperspace, such as P or r. The transition manifests itself by a change of q and a dimerization distortion with the formation of donor-acceptor dimers along the stacking axis in the I-phase. 

TTF-CA more ionic, increasing q up to 0.7. The space group of the I-phase is Pn with two equivalent donor-acceptor dimers related by a glide plane with a ferroelectric arrangement (see Fig. 6.33(b)). Further examples of mixed-stack organic CT materials exhibiting N-I transitions are tetramethylbenzidine-TCNQ (Tn-i — 205 K) (Iwasa et al, 1990) and DMTTF-CA (Tn-i 65 K) (Aoki et al, 1993). 

PP-TCNQ could not be obtained as crystals suitable for a full structure but unit cell parameters are listed in Table 4. The structure of BP-TCNQ was determined and related to the TSE spectra With the exception of the folded butyl group and a slight torsion (2.3° and 4.3° respectively) of the C = (CN) groups, all the molecules are planar and there is effective it-it overlap of the TCNQ ions at 3.15(3) A. The complete structure and single crystal epr was also obtained for BP-TCNQF4, whose novel mixed stack in Fig. 9 clearly shows the face-to-face TCNQ"" dimers. 

The 5.10-dihydro-5-methyl-10-ethylphenazine (MEP) complex of TCNQ has been prepared but not characterized in detail. We anticipate the MEP-TCNQ structure and physical properties to resemble MjP-TCNQ and E2P-TCNQ. The 5.10-dihydro-5.10-diethylphenazine (E2P)-TCNQ complex s structure has been determined. The E2P moeities are folded along the N-N axis, as shown in Fig. 11 for MjP-TCNQ the two benzene halves have a dihedral angle of 12.7°. The mixed stack is again dimerized. Both ethyls are bent to one side of the average phenazine plane, a configuration that reinforces the dimerization or is reinforced by the dimerization. 

Fig. 1.9 Mixed columnar stacks of MOPV dimers in dodecane. Energy transfer (ENT) within mixed stacks is studied from MOPV3 to MOPV4. Reproduced from Ref. [101] by permission of John Wiley Sons Ltd...

S = segregated stacks M = mixed stacks L2 =lone dimers... 

Finally, the dimerized mixed [dm) stack does not possess symmetry at all. Each mode is both infrared and Raman-active and perturbed by the vibronic interactions. Coincidence of frequencies between infrared and Raman spectra should therefore be expected for dm stacks. 

Finally, we must emphasize that the selection rules outlined before are only valid for the case of dimerized and/or alternating (mixed) stack systems. More complicated structures such as trimerized, tetramerized or even incommensurate ones... 

In 1996 Stack and co-workers reported an unusual 3 1 (copper 02 stoichiometry) reaction between a mononuclear copper(I) complex of a A-permethylated (lR,2R)-cyclohexanediamine ligand with dioxygen. The end product of this reaction, stable at only low temperatures (X-ray structure at —40 °C) is a discrete, mixed-valence trinuclear copper cluster (1), with two Cu11 and a Cu111 center (Cu-Cu 2.641 and 2.704 A).27 Its spectroscopic and magnetic behavior were also investigated in detail. The relevance of this synthetic complex to the reduction of 02 at the trinuclear active sites of multicopper oxidases4-8 was discussed. Once formed, it exhibits moderate thermal stability, decomposed by a non-first-order process in about 3h at —10 °C. In the presence of trace water, the major isolated product was the bis(/i-hydroxo)dicopper(II) dimer (2). 

A few examples in which aromatic cation radicals have been isolated as crystalline salts, actually consist of mixed valence units. For example, crystal structure analysis showed that the naphthalene radical cation (NAP) + forms a mixed valence dimer (NAPy4 in which the two components are arranged face to face in n-stacks with an interplanar separation significantly closer than van der Waals contacts. Such an intermolecular organization arises from the sponta-... 

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