Synthesis, photophysical, electrochemical and electrochemiluminescence properties of A sub(2)B sub(2) zinc porphyrins: the effect of pi -extended conjugation

The synthesis of two A sub(2)B sub(2) porphyrins, {5,15-bis-[4-(octyloxy)phenyl]-porphyrinato}zinc(ii) (4) and {5,15-bis-(carbazol-3-yl-ethynyl)-10,20-bis-[4-(octyloxy)phenyl]-p o rphinato}-zinc(ii) (9), is reported. Their photophysical properties were studied by steady-state absorption and emission. Substituting the carbazolylethynyl moieties at two of the mesopositions results in a large bathochromic shift of all the absorption bands, a notable increase in the absorption coefficient of the Q(0,0) band, and higher fluorescence quantum yield compared to porphyrin 4, with two unsubstituted mesopositions. Cyclic voltammetry and digital simulation show that electrogenerated radical ions of 9 are more stable than those of 4. The lack of substituents at the mesopositions of 4 leads to dimerization reactions of the radical cation. Despite this, the annihilation reaction of 4 and 9 produces very similar electrogenerated chemiluminescence (ECL) intensity. Spectroelectrochemical experiments demonstrate that the electroreduction of 9 leads to a strong absorption band that might quench the ECL.