Electronic structure and spectrum third-order nonlinear optics of the metal phthalocyanines PcM (M = Zn, Ni, TiO) -: art. no. 125109

The electronic structures and spectra have been calculated using the INDO-single- and double-excitation configuration interaction method, and following combination with the sum-over-states method the dynamic third-order polarizabilities in the third-harmonic generation (THG), electric-field-induced second harmonic generation, and degenerate four-wave mixing (DFWM) optical processes have been computed for the metal-phthalocyanines PcM (M = Zn, Ni, TiO). The calculated electronic structures show that the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are exclusively formed from macrocycle pi orbitals and the negative values of the HOMO energies well reproduce the observed values of threshold ionization potentials, and the obtained lowest transition energies are agreement with the observed values of the Q bands in PcM (M = Zn, Ni, TiO). The Q band in the absorption spectrum is the contributions from the pi-->pi* electronic transitions and the B band seems to be more sensitive to the center metal atom. The calculated dynamic third-order polarizabilities show a correlation between the dispersion behavior of [gamma] and the absorption spectrum of Q band region, and the first resonance enhancement in the dispersion curve appears at one third or a half of transition energy for the THG or DFWM optical process at ground state. Additionally, an enhancement of optical nonlinearities in excited state is presented in PcM, and it has a large enhancement at excited state if there is a small third-order polarizability at ground state. The calculated susceptibilities of chi(-3 omega;omega,omega,omega) of PcM films are agreement with the available measuring values in the order of magnitude at an input wavelength of 1907 nm. Both the theoretical and experimental results show the decrease in the order of PcTiO>PcZn>PcNi as the distances between the centered metal atom and N-4 plane increase in the order of PcNi<PcZn<PcTiO. The electronic origin of the next to leading order (NLO) response is analyzed from the state component and the frontier orbital character. The pi-->pi* charge transfers among macrocycle make the dominant contributions to the third-order NLO response for PcM (M = Zn, Ni) and LMCT make a substantial contribution to the third-order NLO response for PcTiO. The role of axial substitute is more important than that of centered metal atom to the third-order NLO response in PcM.