Full-azobenzene Functionalized Dendrimer Synthesis And Nonlinear Optical Properties And Self-assembly Behavior

Azobenzene moieties can readily undergo reversible trans-cis photoisomerization, leading to the large structural change as reflected in the dipole moment, spatial requirement, and absorption spectra, which make azobenzene moieties good candidates as a light induced "molecular switch", and have been attracting increasing interest for many applications. Incorporating of azobenzene moieties into the skeletons of dendrimers has been demonstrated to provide interesting properties and possible applications, as well as the contribution to theoretical study on the conformational information of dendrimers using the drastic changes in photochemical and photophysical properties between two isomers of azobenzene. Therefore, the design and synthesis of new dendrimers constructed with azobenzene moieties are pursued to fully understand the structure-property correlation and to develop new-style nonlinear optical materials.1. Synthesis and characterization of dendrons and dendrimers skeleton-constructed with azobenzene moietyA series of dendrons and dendrimers skeleton-constructed with azobenzene moiety based upon 4-carboxy-4'-(1,2-propanediolether)-azobenzene as an AB₂ monomer, via a convergent approach, proceeding in a repeated stepwise growth manner starting from 4-carboxy-4'-(n-butylether)-azobenzene as a peripheral monomer, were synthesized, and characterized by NMR, FTIR, and MALDI-TOF-MS analysis. Their regular molecular architecture and thus monodispersed molecular weights were confirmed by GPC. The UV-vis absorbance and ¹H NMR spectrum study indicated that the azobenzene moieties in CHCl₃ solution took fully trans-cis isomerization under UV irradiation, and reversely isomerization back to the trans by visible light irradiation or by heat.2. Second-harmonic properties of dendritic polymers skeleton-constructed with azobenzene moiety used for nonlinear optical materialsThe synthesized dendrons and dendrimers were doped in a high T_g polycarbonate (PC) as a host for nonlinear optical (NLO) materials. The optimal loading density and poling conditions were investigated using UV-vis spectral and second-harmonic generation (SHG) measurements. The results showed that the dendrons and dendrimers have good solubility in PC host, which increased with the generation increased. Moreover, the SHG measurements indicated that the dendrons possess higher nonlinearity than the dendrimers. The chromophores of lower generation dendrons were easier to orient along the poling electric field and gave a cone shape with the azobenzene branching units, which coherently contributed to the molecular hyperpolarizability and resulted in the higher SHG intensity. The temporal stability of the dendrons with 15% loading density was also investigated, and showed that the decaying in nonlinearity was slower for higher generation dendrons.3. Aggregation behaviors and photoresponsive properties of azobenzene constructed phosphate dendrimersThe first to third generation of amphiphilic azobenzene constructed phosphate dendrimers (Gl to G3) were synthesized via a convergent strategy and characterized by NMR and MALDI-TOF-MS measurements. The aggregation behaviors and photoresponsive properties of their aggregates in water were investigated. The TEM observation and UV-vis absorption spectrum study showed that the morphologies of the aggregates are greatly affected by the preparation method. The multiple aggregation states, including H-aggregation and J-aggregation, were formed for azobenzene moieties in G2 and G3 aggregates obtained via sonication, resulting from their different distance from the hydrophilic phosphate group. The controlled release property of G2 vesicle was investigated using calcein as a fluorescence indicator, showing the potential use as a photoregulatable carrier.