Syntheses and electronic properties of the nickel and palladium complexes of the octaethylporphyrin(M1)–(dihexylbithiophene)n–octaethylporphyrin(M2) system [OEP(M1)–(DHBTh)n–OEP(M2)] connected with the diacetylene linkage. A methodology for molecular design of the particular electronic structure