Synthesis, photophysical, electrochemical and electrochemiluminescence properties of A2B2 zinc porphyrins: the effect of π-extended conjugation.

The synthesis of two A2B2 porphyrins, {5,15-bis-[4-(octyloxy)phenyl]-porphyrinato}zinc(ii) () and {5,15-bis-(carbazol-3-yl-ethynyl)-10,20-bis-[4-(octyloxy)phenyl]-porphinato}-zinc(ii) (), is reported. Their photophysical properties were studied by steady-state absorption and emission. Substituting the carbazolylethynyl moieties at two of the meso positions 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 , with two unsubstituted meso positions. Cyclic voltammetry and digital simulation show that electrogenerated radical ions of are more stable than those of . The lack of substituents at the meso positions of leads to dimerization reactions of the radical cation. Despite this, the annihilation reaction of and produces very similar electrogenerated chemiluminescence (ECL) intensity. Spectroelectrochemical experiments demonstrate that the electroreduction of leads to a strong absorption band that might quench the ECL.