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

Abstract

Orientation and distance-dependent intervalence charge transfer phenomenon is demonstrated in far-red capturing zinc phthalocyanine dimers connected by a bis-acetylene-phenyl π-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 ~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.