Synthesis of extremely pi-extended porphyrin tapes from hybrid meso-meso linked porphyrin arrays: an approach towards the conjugation length.

Directly meso-meso, beta-beta, beta-beta triply linked porphyrin arrays are exceptional pi-conjugated molecules exhibiting remarkably red-shifted absorption bands extending deeply in the IR region. In order to determine the effective conjugated length (ECL), we embarked on the synthesis of the porphyrin tapes far beyond the 12-mer, which is the longest we have prepared so far. In this study, to find the compromise between the feasibility of the meso-meso coupling reaction up to longer arrays and the sufficient solubility and chemical stability of the resultant porphyrin tapes, we prepared hybrid meso-meso linked porphyrin arrays BOn up to 24-mer, which have two different aryl groups, a 2,4,6-tris(3,5-di-tert-butylphenoxy) phenyl group (Ar(1)) and a 3,5-dioctyloxy phenyl group (Ar(2)). All these arrays were effectively converted into the corresponding triply linked porphyrin tapes TBOn by oxidation with DDQ-Sc(OTf)(3). Importantly, the low energy Q-band-like absorption bands of TBOn are progressively red-shifted with an increase in the number of porphyrins n until 16 but the red-shift is saturated at n=16, indicating the ECL of the porphyrin tape to be around 14-16. The regularly introduced meso-aryl bulky substituents impose facial encumbrance, hence leading to the effective suppression of pi-pi interactions as well as improvement of the chemical stabilities of TBOn.