Study On The Second-order Nonlinear Optical Properties Of Supramolecular Complexes Based On Push-pull Porphyrins

Organic second-order nonlinear optical materials have attracted considerable attention due to their potential applications in optical communicating, optical computing, optical information processing, data storage and THZ imaging. However, it is still a challenge to prepare organic second-order nonlinear optical materials that really reach the request of practicality, how to translate large hyperpolarizbility(β) of the chromophores into macroscopic optical nonlinearity of materials is one of the bottlenecks which hinder the development in this field. To understand the mechanism of multi-chromophores and control their supramolecular structures and performance at the mesocopic level is treated as an effective approach to solve this problem. Push-pull porphyrins are promising building blocks to practice such an idea, as they have excellent second-order nonlinear optical properties and are easy to be self-assembled into supramolecular materials or be processed into oligomers and polymers. Herein, we introduce pyridyl-2,6-dicarboxamide and 1,3-isophtalic acid as isolation spacer to built up two different bichromophores systems based on push-pull arylethynyl porphyrins reported by Therien’s Group, and making use of the metal-ligand interactions, we further design and form ladder- and sandwich-typed assemblies, we have a theoretical study on the conformations of bichromophores(TT,CT,CC), ladder-typed assemblies(1,2,3,4), sandwich-typed assemblies(meta@DABCO, para@DABCO). By using B3 LYP method at 6-31G(d,p) to optimize the geometry, ZINDO method to calculate electronic spectra, then combined with sum-over-states(SOS) formalism to calculate their hyper- polarizablities, we have got several constructive results.First, it is found that it is easy to arise different conformations but with small energy barriers in bichromophores linked by pyridyl-2,6-dicarboxamide due to the presence of rotable amide bonds. Results showed obviously different first hyperpolarizabilities for three conformations(TT(-281.27×10-30 esu) > CT(-126.08×10-30 esu)>CC(45.25×10-30 esu)), while compared with the additive sums of two monomers(-263.30×10-30 esu), only TT has an additivity behavior. The analysis indicated that two-level contribution βμ(0),2 of different conformations follow the same trend as dipole moments μ(TT(29.93 Debye) >CC(27.24 Debye) >CT(24.82 Debye) and orientation factor κ2(TT(1.0)>CC(0.2)>CT(0.0)), but not the same as βμ(0), which suggests the origin of βμ(0) is not sufficiently explained by two-level model. βμ(0),3 values follow the same trend as βμ(0) values, which means βμ(0) values mainly depend on three-level contribution. The analysis of orientation factor κ2, frontier molecular orbitals, two-level model and three-level model based on sum-over-states(SOS) method indicate that the first hyperpolarizabilities of different conformations strongly depend on the orientations of transition dipoles and correlate with π-π interactions between different subchromophores. The selected conformation TT is a reliable building blocks to further prepare supramolecular materials that possess large first hyperpolarizabilities.Second, it is found that the ladder-typed assemblies(1,2,3) constructed by the selected conformation TT and different diamine molecules(ethylene-diamine, propane diamine, butyl diamine) have a better performance than the aggregate(0) without any diamine molecules for their first hyperpolarizabilities βμ(0),increase 5%,21% and 9%, respectively, and 2 has the best effect to achieve excellent second-order nonlinear optical properties. The analysis of the geometrical, electronic structures, two-level model and three-level model based on SOS method indicate that the geometrical parameters, βμ(0) values are not linearly correlated with the length of diamine molecules, there is a concept of “suitable bidentate ligand length”, βμ(0) values mainly depend on three-level contribution. Our results suggest that ladder-typed assemblies would provide an excellent assembled mode to translate large hyperpolarizbility(β)of push-pull porphyrins.Third, it is found that the peripheral units linked by 1,3-isophtalic acid at meta- and paraposition with bidentate ligand(DABCO) to constructure sandwich-typed assemblies(meta@ DABCO,para@DABCO) showed much higher static first hyper- polarizabilities βμ(0) than the situations without DABCO(meta, para), increasing 8 and 4 times, respectively. While compared with the additive sums of two monomers(-263.30×10-30 esu), para@DABCO surpasses the additive sums of two monomers 81.16×10-30 esu, its βμ(0) reaches-344.46×10-30 esu,which is ascribed as the 1+1>2 effects of supramolecular structures.Our results indicate that para@ DABCO is an excellent peripheral units used in dendrimeric structures for which to achieve large macroscopic optical nonlinearity.