meso-Indolo[3,2-b]carbazolyl-Substituted Porphyrinoids: Synthesis, Characterization and Effect of the Number of Indolocarbazole Moieties on the Photophysical Properties

Abstract

meso-Indolocarbazolylporphyrins endowed with a different number of indolocarbazole units have been synthesized via condensation of an appropriately substituted monoformylated 5,11-dihydroindolo[3,2-b]carbazole precursor and mesityldipyrromethane. Under specific conditions, analogous meso-indolocarbazolylcorroles could also be prepared. The photophysical features of the novel luminiscent freebase and Zn-porphyrin derivatives were investigated. The introduction of indolocarbazole substituents results in progressive bathochromic shifts of the porphyrin absorbance and fluorescence bands due to the rising energy of the a2u orbital. The excitation energy is efficiently transferred from the meso-indolocarbazole units to the porphyrin macrocycle. An increased number of indolocarbazole moieties does not lead to porphyrin fluorescence quenching; on the contrary, a small increase of the fluorescence quantum yield is observed. The main route for excitation energy deactivation of all the studied porphyrins is intersystem S1→T1 crossing, with the intersystem crossing quantum yield, as determined by the photosensitized formation of singlet molecular oxygen, being as high as about 70% for the free-bases and more than 80% for the Zn complexes. The intersystem crossing quantum yield seems to be barely affected by meso-indolocarbazole substitution. A noticeable part of the excitation energy was found to deactivate through radiationless internal S1→S0 conversion.