The Effect of Symmetry Breaking on the Nonlinear and Quantum Optical Properties of Novel Zinc Phthalocyanine Dendrimer Systems

Phthalocyanines and oligothiophenes have been extensively studied due to their exceptional electronic and optical properties, which stem from their highly delocalized π-conjugated systems. In this study, novel phthalocyanine-dendritic oligothiophene materials are synthesized, and their electronic and optical properties are investigated. The phthalocyanine systems with tert-butyl peripheral substituents showed enhanced classical and entangled TPA cross-sections with the addition of the thiophene dendrons. However, the octyl-sulfonyl phthalocyanine systems showed very little increase in the classical and entangled TPA cross sections with the addition of thiophene dendrons. For the tert-butyl system, the effects of symmetry breaking and a lifting of the degeneracy of the LUMO orbitals are found to enhance the TPA cross sections. Interestingly, the entangled two-photon absorption (ETPA) measurements reveal that the octyl-sulfonyl phthalocyanine systems have greater cross sections than the tert-butyl phthalocyanine systems, suggesting an excitation mechanism that involves a resonant intermediate state. The results of both the classical and entangled two-photon measurements on the phthalocyanine systems are combined with electronic structure calculations to provide a more detailed analysis of the excitation pathways. Femtosecond transient absorption (fsTA) measurements were employed to investigate the excited state dynamics of interesting tert-butyl systems. These findings suggest that the small alteration to the peripheral substituent of the phthalocyanine (tert-butyl vs octyl-sulfonyl) causes the effects of symmetry breaking and a lifting of the LUMO orbital degeneracy. This, as well as strong electronic coupling, plays a major role in enhancing their nonlinear optical properties.