Donor-acceptor-functionalized tetraethynylethenes with nitrothienyl substituents:: Structure-property relationships

Tetraethynylethenes (TEEs) functionalized with donor (4-(dimethylamino)phenyl) and acceptor (5-nitro-2-thienyl) groups were prepared by Pd-0-catalyzed Sonogashira cross-coupling reactions (Schemes 1-6). The physical properties of these novel chromophores were examined and compared with those of analogous systems containing 4-nitrophenyl instead of 5-nitro-2-thienyl acceptor groups. X-Ray crystal-structure analyses showed the pi-conjugated frameworks of 2, 11, and 13, including the TEE core and all aryl moieties, to be nearly perfectly planar (Figs. 1, 3, and 4). Ln contrast, one 4-(dimethylamino)phenyl group in 10 is rotated almost 90 degrees out of the molecular plane, presumably due to crystal-packing effects (Fig. 2). The analysis of bond lengths and bond angles revealed little, if any, evidence of intramolecular ground-state donor-acceptor interactions. The electrochemical behavior of nitrothienyl-substituted TEEs is similar to that of the corresponding nitrophenyl-functionalized derivatives (Table 3). The nitrothienyl groups were reduced at -1.23 V (vs. the ferrocene/ferricinium couple, Fc/Fc(+)), regardless of the degree or pattern of other substitutions. For nonsymmetrical TEE 13, the reduction of the nitrothienyl group at - 1.23 V is followed by a reduction of the nitrophenyl group at - 1.40 V, a potential typical for the reduction of other nitrophenyl-substituted TEEs, such as 17-20. UV/VIS Spectroscopy showed a consistently lower-energy absorption cutoff for nitrothienyl derivatives compared with the analogous nitrophenyl-substituted TEEs that confirms a lowering of the HOMO-LUMO gap as a result of nitrothiophene substitution (Figs. 5 and 6). A comparison of the tetrakis-arylated TEEs 11, 13, and 20 clearly showed a steady bathochromic shift of the longest-wavelength absorption maximum and the end-absorption upon sequential replacement of nitrophenyl by nitrothienyl groups. Quantum-chemical computations were performed to explain a number of complex features of the electronic absorption spectra. All empirical features of relevance in the experimental UV/VIS spectra for 2, 5, 6, and 17-19 were correctly reproduced by computation (Tables 4 and 5). The combination of theory and experiment was found to be very useful to explain the particular acceptor properties of the 5-nitro-2-thienyl group.