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

Tetraethynylethenes (TEEs) functionalized with donor (4-(dimethylamino)phen yl) and acceptor (5-nitro-2-thienyl) groups were prepared by Pd-0-catalyzed Sonogashira cross-coupling reactions (Schemes 1-6). The physical propertie s of these novel chromophores were examined and compared with those of anal ogous systems containing 4-nitrophenyl instead of 5-nitro-2-thienyl accepto r groups. X-Ray crystal-structure analyses showed the pi-conjugated framewo rks of 2, 11, and 13, including the TEE core and all aryl moieties, to be n early perfectly planar (Figs. 1, 3, and 4). Ln contrast, one 4-(dimethylami no)phenyl group in 10 is rotated almost 90 degrees out of the molecular pla ne, presumably due to crystal-packing effects (Fig. 2). The analysis of bon d lengths and bond angles revealed little, if any, evidence of intramolecul ar ground-state donor-acceptor interactions. The electrochemical behavior o f nitrothienyl-substituted TEEs is similar to that of the corresponding nit rophenyl-functionalized derivatives (Table 3). The nitrothienyl groups were reduced at -1.23 V (vs. the ferrocene/ferricinium couple, Fc/Fc(+)), regar dless of the degree or pattern of other substitutions. For nonsymmetrical T EE 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 th e reduction of other nitrophenyl-substituted TEEs, such as 17-20. UV/VIS Sp ectroscopy showed a consistently lower-energy absorption cutoff for nitroth ienyl 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 1 1, 13, and 20 clearly showed a steady bathochromic shift of the longest-wav elength absorption maximum and the end-absorption upon sequential replaceme nt of nitrophenyl by nitrothienyl groups. Quantum-chemical computations wer e performed to explain a number of complex features of the electronic absor ption spectra. All empirical features of relevance in the experimental UV/V IS 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.