S-Space College of Engineering/Engineering Practice School (공과대학/대학원) Dept. of Material Science and Engineering (재료공학부) Theses (Master's Degree_재료공학부)
Novel Photoresponsive Soft Materials based on Highly Emissive Cyanostilbene Derivatives: Studies on Photophysical Property, Liquid Crystalline Behavior
고형광성 시아노스틸벤 유도체를 기반으로 하는 새로운 광응답성 연성재료: 광물리적 특성 및 액정 거동에 대한 연구
- 공과대학 재료공학부
- Issue Date
- 서울대학교 대학원
- liquid crystal; photoinduced isomerization; fluorescence pattern; mass migration; surface relief grating
- 학위논문 (석사)-- 서울대학교 대학원 : 재료공학부, 2013. 2. 박수영.
- Photo-responsive materials are promising research areas for a wide range of applications such as optical device, and photo-actuator. Especially, azobenzene, and stilbene based soft materials are commonly known for implementing such applications due to their fast and reversible photo-induced isomerization behavior. Despite those remarkable properties, the molecules show low or non-fluorescence behavior in solid state. In order to solve such problem, fluorescent dyes are attached in azobenzene or stilbene unit via covalent, secondary bonding interactions, or host-guest mixed systems. Nevertheless, at the current stage, there are only a few reports to be successfully demonstrated.
To date, we have reported a novel class of α-cyano-substituted stilbenic derivatives exhibiting a unique and peculiar fluorescence behavior, that is, aggregation-induced enhanced emission (AIEE): it is virtually non-fluorescent in the monomer state in solution but becomes highly fluorescent upon self-assembly into supramolecules. Furthermore, the cyanostilbene unit in typical AIEE molecules has a multiple function of enabling AIEE-type molecule to undergo a trans-cis photoisomerization, as commonly observed in stilbene and azobenzene materials. However, so far, there is no report on fluorescence property in combination with photoisomerization behavior by using such molecules.
Herein, we report on a demonstration of fluorescence patterning technique via photo-induced isomerization behavior in soft state, LC or soft crystal, based on cyanostilbene derivatives for the first time. The molecules consist of rigid cyanostilbene/dicyanodistrylbenzene(DCS) backbone which shows AIEE behaivor and photo-responsive characters with different lengths of flexible alkoxy chains. In this respect, to clearly understand thermotropic LC and optical property in solid state, we study the following issues: i) the role of –CN group, ii) the role of length of alkyl chain, and iii) understanding of the differences in photoisomerization behavior between cyanostilbene and DCS beackbone. The photo-isomerization behavior is monitored by change of UV-vis abosrbance spectra and confirmed the conversition ratio of trans to cis isomer by 1H NMR data. Based on such characters, we demonstrated a fabrication of fluorescence patterning.
Furthermore, we have synthesized a new cyanostilbene based material, GCS, which forms columnar hexagonal LC phase at room temperature. Thin film of GCS was prepared by spin-coated method. Initially, as-prepared film looks like amorphous state with a high transparency. However, a partial crystallization in the film occurs subsequently. During overnight under dark at near RT (25~30 °C), it was transformed into the perfectly uniform crystalline film that had same crystal arrangement to GCS bulk powder. In crystalline state, GCS exhibits intense blue fluorescence. Interestingly, such crystalline film shows a soft character. To understand such property, we have carried out GI-XRD measurement. In soft state, GCS shows phase transition behavior via photoisomerization process. Thus, we successfully demonstrate highly fluorescent micro patterns via unique property, so-called photo-tirrgered mass migration behavior, with phase transition by using soft crystalline material. In this work, we propose a new mechanism of ‘photo-triggered mass migration’ with phase transition, crystal to LC phase, for the first time.