Towards Highly Near-Infrared Chiroptical Switches: Design, Synthesis, And Properties Of Viologen-containing Organic Molecules And Polymers

Chiral materials are inherently circular birefringent and circular dichroic and may find potential uses in polarization-related photonics applications such as data storage, optical switches and light modulators.To explore the possible uses of such materials in optical communication systems for polarization-control,the thesis research is dedicated to the development of structural models directing structural design of materials with large chiroptical properties,of novel chiral materials with significant chiroptical properties at the NIR wavelengths(e.g.,1300 nm and 1550 nm),and of materials with efficient NIR chiroptically switching properties.1.Structural Modeling for Large Chiroptical PropertiesBy taking advantage of exciton coupling theory and modern quantum mechanical computation techniques,a structural model correlating geometric structures of a chiral molecule with the magnitude of chiroptical properties was come up with.Based on the model,a novel type of redox-based molecular chiroptical switches containing electrochromic viologens and axially dissymmetric 1,1'-binaphthyl-2,2'-diyl and with open-ring or cyclic structures was designed.Structural optimization indicates that the cyclic chiral compound[i.e.(R)-CBEBPB]favors more significant chiroptical properties than the open-ring compound[i.e.(R)-BEBPB].2.Electrically-Driven Molecular NIR Chiroptical SwitchesThe open-ring and the cyclic chiral compounds,i.e.,(R)-CBEBPB and (R)-BEBPB,were successfully prepared and carefully characterized. Cyclic-voltammetry(CV)showed three redox peaks for(R)-CBEBPB in DMF solution and two for(R)-BEBPB,indicating there is significant electronic interaction between the two viologen units in(R)-CBEBPB.UV-vis and CD spectroelectrochemistry studies revealed distinctive spectra changes upon redox processes.Before reduction to their radical cation(or violene)states,(R)-CBEBPB and(R)-BEBPB exhibited strong absorptions and CD signals only below 300 nm, whereas after the reduction,both(R)-CBEBPB and(R)-BEBPB exhibited two new strong UV-vis absorptions as well as two new strong negative CD bands at around 400 and 600 nm.Upon oxidation,these spectral changes disappeared and can be reversibly recovered upon the following reduction process,indicating potential applications as molecular chiroptical switches.In comparison,(R)-CBEBPB exhibited much stronger CD signals than(R)-BEBPB in both the tetracationic and the violene states.The chiroptical switching properties of(R)-CBEBPB and(R)-BEBPB were demonstrated by using chemical reducing agent(i.e.metal tin).After five redox cycles, no deterioration of the sample or the CD signals was observed.Based on the CD spectra,optical rotation dispersion(ORD)curves were calculated by using the Rosenfeld equation.The ORD curves of tetracationic(R)-CBEBPB and(R)-BEBPB agree very well with the experimental specific optical rotations,indicating the validity of the calculation.According to the ORD curves of(R)-CBEBPB and(R)-BEBPB in their violene states,both compounds have very large optical rotations at NIR wavelengths(e.g.,for(R)-CBEBPB:[α]_{1000 nm}= -775°and[α]_{1500 nm}= -234°).By translating the switching CD spectra into ORD curves,it was found that,for a solution of(R)-CBEBPB in DMF(c=3.0 mM)in a cell of 1 mm length,an optical rotation on the mdeg scale can be readily switched in the wavelength range of 1000-2000 nm upon redox cycles between its violene and tetracationic states.3.Chiroptically Switching Polymer with Grooved Main-Chain ConformationElectrochromic viologens were successfully introduced onto the side chains of a 1,1'-binaphthyl-based conjugated polymer to yield a novel type of redox-based chiroptically switching polymer.The main-chain chiral conformation(i.e.the grooved conformation)was demonstrated by comparison of optical rotations between model compounds and polymers and by measurement of the CD spectra.The resulting polymer has good solubility in common organic solvents and good thin-film forming ability.Although a relatively poor electrochemical reversibility was observed,the polymer showed good redox reversibility upon using chemical reducing agent(i.e. metal tin).Similar to small molecular compounds[(R)-CBEBPB and(R)-BEBPB], the polymer exhibited significant spectral(UV-vis and CD)changes and good switching properties upon redox cycles.In contrast to that of(R)-BEBPB,the reduced polymer(i.e.,in the Violene state)showed a blue shift of the CD band,opposite sign and much weaker CD signals,which may be attributed to the overlap of multiple electronic interactions between viologen units and between viologen and polymer backbone.4.Chiroptically Switching Polymer with Helical Main-Chain ConformationOptically active polyacetylene bearing viologen groups in the side chains were successfully prepared and characterized.The very large optical rotations(e.g.[α] _D＞300°)and negative CD signals corresponding to the main chain absorption of the polymer indicate a main-chain helical structure.Similar to the above prepared chiral molecules and polymers,the resulting chiral polyacetylene exhibited distinctive UV-vis and CD spectral changes and good chiroptically switching properties upon redox processes.According to the ORD curves derived from the corresponding CD spectra,the polymer shows opposite signs in optical rotation before(negative)and after the reduction(positive)to the violene state.Thus,although the optical rotations of the polymer are not significant at the NIR wavelengths,it is suitable for NIR chiroptically switching applications due to the readiness in detection of the optical rotation changes.