Bisporphyrin‐Fused Pentacenes Exhibiting Abnormal High Stability

A series of largely π -extended multichromophoric molecules including cross-conjugated, half cross-conjugated, conjugation-interrupted and linearly conjugated systems were synthesized and characterized. These multichromophoric molecular systems revealed interesting structural-property relationships. While the cross-conjugated bisporphyrin-fused pentacenequinone chromophores displayed broad UV-Vis absorption bands spanning from 350 nm through 650 nm, the linearly-conjugated bisporphyrin-fused pentacene chromophores exhibited much red-shifted absorptions at 800 - 900 nm. Bisporphyrin-fused pentacenes Pen-1b and Pen-2a showed rich redox chemistry with 7 and 8 observable redox states respectively. The linearly-conjugated bisporphyrin-fused pentacenes ( Pen-1b and Pen-2a ) possess much narrower HOMO-LUMO gaps (1.65 and 1.42 eV redox, respectively) and higher HOMO energy levels than those of their pentacene analogues (2.23 and 2.01 eV redox, respectively), similar to those of much unstable hexacenes and heptacenes. Counterintuitively, Pen-1b and Pen-2a demonstrated unusual stability. An estimated half-life of > 945 h was obtained for bisporphyrin-fused pentacene Pen-2a, which is remarkably much longer than that of its pentacene analogue ( BPE-P, half-life, 33 h). Investigations of the abnormal stability of Pen-1b and Pen-2a using femtosecond transient absorption technique suggested nickel porphyrin centered photo-events leading to a short-lived intermediate charge transfer state, consequently blocking the photodegradation pathways. This charge transfer event is believed to play a major role in causing the abnormal stability of Pen-1b and Pen-2a. Steric effect and decreased LUMO energy level also contributed to stabilizing the bisporphyrin-fused pentacenes.