A Study Towards the Design of Materials for Nonlinear Optical Applications Using First Principle Calculations
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basic requirements for RSA, ionization potentials of NLA dyes must be sufficiently large in order to provide extra stability against direct photoionizations and other photoinduced Ionization Threshold
"Strong
ISC Weak
Triplet
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So Singlet Figure 1. Five-Level Model for Nonlinear Absorption 325 Mat. Res. Soc. Symp. Proc. Vol. 597 ©2000 Materials Research Society
reactions. f3-halogenated zinc meso-tetraphenylporphyrins (ZnTPPX 8) have been shown to have large oxidation potentials in comparison to those of the unsubstituted zinc meso-tetraphenylporphyrin (ZnTPP).[6, 7] The induced changes in electronic structure and photophysical property upon halogenation are likely responsible for the observed enhancement in NLA response of ZnTPPX8 .[5] In the present work, ionization potential and spectral calculations are carried out to gain insight into the electronic origin of the NLA response of halogenated zinc porphyrins. Early theoretical work on the T-T absorption spectrum of tetraphenylporphyrin predicted a large number of allowed bands. Although quantitative agreements with experiment were not obtained, as expected, these PPP calculations by Weiss et al.[8] revealed the complex nature of the observed T-T spectrum. They concluded that the four-orbital model proposed by Gouterman[9] is not valid for interpreting the T-T spectra of porphyrins. Recent advances in scalable parallel algorithms and computers allow the application of ab initio electronic structure theory to large systems to be carried out in a timely manner. The versatile Kohn-Sham density functional theory (DFT) and the time-dependent DFT (TDDFT), in particular, have been quite successful in predicting the ground state spectrum of porphin[10, 11] and the larger ZnTTP and ZnTTPX8.[12] For the excited state open-shell radical cation and triplet porphyrins, DFT has been shown to provide IPs[ 11, 13-15] and S0 -TI splittings[ 11, 12] that are in quantitative agreements with experiment. The computed T-T spectra for free-base porphin[16] and those reported here are found to be in good agreement with available experimental data. COMPUTATIONAL
METHODS
The computed ionization potentials spectra were obtained with the Kohn-Sham (KS)[ 17] density functional theory and the time-dependent density functional response theory,[ 10, 18] respectively, at the previously computed structures. [ 12] These DFT and TDDFT calculations were carried out with the Becke's three-parameter hybrid functional [ 19-21 ] (B3LYP) in conjunction with the all-electron 6-31G(d)[22, 23] basis set (for carbon, nitrogen, fluorine, and hydrogen atoms) and effective core potentials and basis sets (for heavier elements) of Stevens et al. [24, 25] as implemented in the Gaussian 98[26] programs. RESULTS We first discuss the T-T spectrum of ZnTPP, the effects of halogenation on the spectrum, and finally the ionization potentials. The computed T-T excitation energies and oscillator strengths for ZnTPP and ZnTPPBr 8 are listed Table 1. Available experimental data are also included for comparison. The
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