Лако-красочные материалы — производство

During investigations to synthesize N, N’-substituted DPP derivatives, Langhals et al. have been able to condense the diketofurofurane 8 with anilines in the pres­ence of dicyclohexylcarbodiimide (DCC) and isolate the tetraphenyl DPP 9 in 46% yield (Scheme 11.6, top) [9].

An alternative approach to N, N’-diaryl DPPs 9A is based on the classical Stobbe condensation [10] followed by an oxidation with 2,3-dichloro-5,6-dicyanobenzoqui — none (DDQ) [11] (Scheme 11.6, middle).

Recently, a third approach to DPPs of type 9A has been disclosed wherein a ketoamide and its bromo derivatives are first condensed followed by a ring closure to form the pyrrolopyrrole system [12] (Scheme 11.6, bottom). This reaction sequence also allows to prepare unsymmetrically substituted derivatives.

11.3

Molecular Structure and Properties

11.3.1

Spectral Properties

The diketopyrrolopyrrole is a bicyclic 8p electron system containing two lactam units. The characteristic physical properties of DPP 2 are the high melting point (>350 °C), low solubility (<110 mg/l in DMF at 25 °C [3]) and an absorption in the visible region with a molar extinction coefficient є of 33 000 (dm2 mol-1). In solu­tion, DPP 2 shows a greenish yellow fluorescence, whereas in the solid state it is a vivid red. The absorption spectra of 2 in solution (dimethylsulfoxide) and in the solid state (of an evaporated film) are shown in Figure 11.1 [5,13].

Figure 11.1 Absorption spectra of DPP 2 in solution (DMSO) and in the solid state (evaporated film).

Table 11.1 shows the absorption maxima (kmax) of differently substituted DPPs measured in solution together with the є values, as well as the absorption maxima in the solid state, obtained by reflectance measurement on PVC-white reductions.

R

Table 11.1 Influence of substituents on shade and absorption

(kmax) ofdiaryl-DPPs

As with many other classes of pigments, all DPP pigments show a bathochro — mic shift of the maximum absorption in the solid state with respect to the maxi­mum absorption in solution. The solid-state absorption maxima of DPP depends strongly on the nature and position of the substituent. In comparison with the unsubstituted DPP 2, the kmax of m, m’ substituted DPP often show a hypsochro — mic shift and the kmax of p, p’ substituted DPP a bathochromic shift.

The large bathochromic shift of the kmax in the solid state compared to that in solution is due to the strong intermolecular interactions, i. e., the hydrogen bond­ing, p-p — and van der Waals interactions in the solid state. The extent to which different interactions contribute to the bathochromic shift is not known. The de­tailed investigations of the influence of the intermolecular hydrogen bonding and the p-p-interactions on the spectral properties of DPPs have been published [5, 13, 14]. The spectroscopic (1H-, 13C-NMR, UV-Vis, and IR), thermogravimetric, and conductivity investigations on DPP, N-methyl-DPP and N, N’-dimethyl-DPP lead to the conclusion that intermolecular hydrogen bonding is the predominant interaction, and the reason for the strong bathochromic shift of the kmax in the sol­id state of the DPP molecule compared to the kmax in solution.

Theoretical considerations and calculations have been done as well to explain the strong bathochromic shift of the DPP molecule [15]. In semiempirical MO INDO/S calculations, the authors investigated the influence of the geometry of aggregates on the electronic and spectral properties. The results of these calcula­tions were in accordance with the experimental investigations, which indicated that the source of the above bathochromic shift is the intermolecular hydrogen bonding. In addition, the calculations also found that the p-p-stacking would result in a small hypsochromic shift.

11.3.2