A density functional study of the optical spectra and nonlinear optical properties of heteroleptic tetrapyrrole sandwich complexes: The porphyrinato-porphyrazinato-zirconium(IV) complex as a case study

Time-dependent density functional theory calculations are presented for the excitation energies and oscillator strengths of the title mixed sandwich. They prove to agree very well with the experimental data, providing that an accurate description of the salient features of the UV-vis spectrum is given. The nature of the excitations, intraligand (excitonic) or interligand (charge-resonance), is discussed and compared to previous theoretical deductions for bisporphyrins. An experimental measurement of the first hyperpolarizability beta (-2 omega; omega, omega), related to a second harmonic generation, yielded a large, negative beta at omega = 0.65 eV and, by extrapolation, also a large, negative static value. This suggests that this complex has very interesting nonlinear optical properties. Our calculations, however, show beta to have a pole very close to the laser frequency used in the experiment, with large, negative beta values only in its vicinity. Off-resonance values of beta at omega < 0.6 eV are small and positive.