Directly Fused Porphyrin-Tetracyanopentacenequinone Conjugates: Role of the Cross-Conjugated, Powerful Electron Acceptor in Promoting Highly Efficient Charge Separation.

Tetracyanopentacenequinone, a powerful electron acceptor, is fused directly to the porphyrin π-system to create a new class of donor-acceptor conjugates.  Owing to the direct fusion and electron-deficient property of tetracyanopentacenequinone, strong intramolecular charge transfer both in the ground and excited states was witnessed.  As a control, porphyrin fused with pentacenequinone was also investigated.  Upon complete spectral and electrochemical characterization, the excited state properties were initially probed by time-dependent DFT studies, and the occurrence of electron transfer from different excited states was established.  Free-energy calculations revealed higher exothermic electron transfer (> 600 mV) than the control pentacenequinone-porphyrin systems.  Pump-probe studies covering broad spatial and temporal regions revealed efficient excited state charge separation.  This was unlike the control pentacenequinone-porphyrin system, where slow charge separation was witnessed only in the case of the zinc derivatives but not the free-base ones.  The lifetimes of the charge-separated states ranged between 30-500 ps depending on the solvent and metal ion in the porphyrin cavity.  The significance of cross-conjugated tetracyanopentacenequinone fused directly to the porphyrin π system in promoting highly exothermic and efficient charge separation, irrespective of its cross conjugation, is borne out from this study.