Date of Award

4-2019

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Chemistry

First Advisor

Dr. Thies Thiemann

Second Advisor

Dr. Panče Naumov

Abstract

This thesis is concerned with the synthesis of novel azo-cinnamates and of steroidal azo ethers and with the analysis of the spectroscopic and thermal properties of these materials. The compounds were identified as possible photoswitchable liquid crystals. The molecules were prepared by multi-step sequences, involving etherification, azo coupling, and Appel-type reactions as the key steps in the case of the azo-cinnamates, and Finkelstejn reactions and Williamson-type etherification reactions as the major transformations in the case of the steroidal azo ethers. The preparation of the azo-cinnamates was a further example of an esterification reaction run under Appel-type conditions using BrCCl₃/PPh₃. The azo-cinnamates were shown to be photoswitchable under photoirradiation at λ = 350 nm, where the thermally stable trans-azo compounds isomerized photochemically to the cis-isomers. Subsequently, the energetically less stable cis-isomers isomerized thermally back to the trans-isomers. The azo-cinnamates were found to exhibit thermotropic liquid crystalline behavior as evidenced by differential scanning calorimetric analysis and by optical texture analysis under the polarization microscope of selected samples. Azocinnamates have been found to exhibit liquid crystalline behavior before, however, it is commonly accepted that in systems containing three (or more) aromatic rings, in the compounds that exhibit liquid crystalline (eg., nematic) behavior, the ring systems normally are positioned in close proximity to each other, preferably with less flexible linkers connecting them. In the presently studied compounds, however, the cinnamate unit is separated from the azobenzene substructure by flexible C9- and C11-alkyl chains, and so it is exciting to see that also these compounds exhibit (albeit in a narrow temperature range) thermotropic behavior.

Comments

في هذا البحث تم تحضير مركبات جديدة تصنف ضمن مجموعة ( azo-cinnamates ) و مجموعة ( steroidal azo ethers ) وتحليلها باستخدام أجهزة تحليل المركبات العضوية. وأيضا تمت دراسة الخصائص الفيزيائية والحرارية لهذه المركبات. فقد تم تحضير هذه المركبات عن طريق العديد من التفاعلات الكيميائية من اهمها: تفاعلات تركيب الازو و تفاعلات تركيب الايثر وتفاعلات الاسترة. تم تحليل جميع المركبات والتأكد من نقاوتها بشكل دقيق. بعد دراسة الخصائص الفيزيائية والضوئية لهذه المواد باستخدام اجهزة تحليل الطيف الضوئي وجهاز الرنين المغناطيسي النووي وجدت ان هذه المركبات لها القدرة على التحول بين النظائر )النظير سيس والنظير ترانس( عند تعريضها للأشعة فوق البنفسجية او حتى تعريضها للحرارة. وجدت أيضا بعد الانتهاء من دراسة الخصائص الحرارية لبعض هذه المواد باستخدام جهاز المجهر الحراري وجهاز المسح الحراري التفاضلي أن لها خصائص مشابهه لخصائص مركبات الكرستالات السائلة.

Download

Included in

Chemistry Commons

COinS