Intervalence Charge Transfer and Exothermic and Isoenergetic Symmetry Breaking Charge Separation in Far-Red Capturing Zinc Phthalocyanine Dimers

Orientation and distance-dependent intervalence charge transfer phenomenon is demonstrated in far-red capturing zinc phthalocyanine dimers connected by a bis-acetylene-phenyl pi-spacer (ortho, meta, or para positions) upon oxidizing one of the phthalocyanine rings, resulting in split oxidation waves and a new optical transition in the near-infrared region. Optical studies initially probed the symmetry-breaking charge separation events in these dimers wherein solvent polarity-dependent quenching was witnessed. Interestingly, efficient quenching in nonpolar toluene was also seen for the ortho dimer. Free-energy calculations supported symmetry-breaking charge separation in polar solvents and the ortho dimer even in nonpolar solvents under isoenergetic (endothermic by similar to 40 mV) conditions. Molecular electrostatic potential maps generated on DFT-optimized structures revealed quadrupolar charge transfer states, more so in polar media. The time-dependent DFT calculations revealed the conversion of the quadrupolar charge transfer states to bipolar charge-separated states from different singlet-singlet excitations. Femtosecond transient absorption studies covering broad temporal and spatial scales provided evidence for the charge separation process, including that for the ortho dimer in toluene, where an isoenergetic process was predicted. The charge-separated states lasted for 200-500 ps depending upon dimer linkage. These unprecedented findings reveal the potential applications of the investigated phthalocyanine dimers in energy harvesting, photocatalytic, and pertinent optoelectronic applications.