本研究合成具有光學異構化特性之偶氮苯單體，再以原子轉移自由基聚合法製備含有偶氮苯之雙親性團聯共聚物poly(ethylene glycol)methylether-block-poly(6-(4-(4-dimethylaminophenylazo)phenoxy-hexyl methacrylate) (mPEG-b-PDMA-Azo)，並研究其在溶液中之結構與光異構化行為。為了了解該共聚物的自組裝行為，利用小角度X光散射觀察其在混合溶液中結構的變化，當mPEG-b-PDMA-Azo溶於中性溶劑時，由於中性溶劑對兩鏈段具有良好的相容性，共聚物以random coil構型存在。然而，隨著混合溶液中選擇性溶劑(H2O)比例的增加，由於mPEG鏈段和PDMA-Azo鏈段與水的相容性不同，會逐漸形成以偶氮苯鏈段為核(core)，mPEG鏈段為殼(shell)之微胞結構。
mPEG-b-PDMA-Azo具有偶氮苯分子之光異構化特性，在可見光照射下會由反式異構物轉變為順式異構物，而在黑暗中靜置可誘發逆反應，利用紫外光-可見光分光光譜儀分析共聚物於混合溶液中之光異構化行為。結果顯示，偶氮苯分子在選擇性溶液中的光異構化速率明顯小於其在中性溶劑中的速率，這是由於聚集體的形成使PDMA-Azo鏈段產生聚集，限制其進行光異構化時結構轉換所需之空間，而導致光異構化速率下降。此外，溶劑的選擇性也會影響偶氮苯分子在溶劑中的聚集型態，於中性溶劑中，偶氮苯分子以自由散佈的nonassociation為主要分布型態；當選擇性溶液加入中性溶劑時，溶液對PDMA-Azo鏈段的溶解度降低，因此形成聚集體。當水的比例增加時，THF/H2O和DMF/H2O混合溶液系統中的偶氮苯鏈段會顯現不同的聚集行為。在THF/H2O中，nonassociation型的比例變化不大，可能是因為水的比例不足以造成大量聚集，且J-aggregation比例增加而H-aggregation比例減少，這是因為J-aggregation在高極性溶劑中更為穩定。在DMF/H2O中，隨著水的比例增加，高極性的環境使靜電排斥減弱，促進分子聚集，造成nonassociation型態顯著下降，而H-aggregation和J-aggregation的比例都增加。
mPEG-b-PDMA-Azo具有偶氮苯分子之螢光特性，隨著混合溶液中水的比例增加，螢光放射強度明顯下降且出現微弱紅位移現象，這是由於極性溶劑與螢光基團之間的相互作用，影響分子偶極矩並使溶劑分子重新排列，進而影響螢光放射特性。含偶氮苯的團聯共聚物在THF/H2O和DMF/H2O兩種混合溶液中，由於溶劑極性差異及偶氮苯分子光異構化反應的影響，照光前後呈現出不同的螢光特性。在可見光照射下，偶氮苯分子從低極性反式結構轉變為高極性順式結構。在低極性THF中，反式結構的偶氮苯螢光強度較高，因為其穩定性較高可促進能量轉移；而順式結構因與溶劑分子間相互作用較弱，導致螢光強度降低。相反的，在高極性DMF中，順式結構穩定性和螢光放射效率較高，導致螢光強度增加。此外，高極性溶劑改變偶氮苯分子的電子態，導致螢光放射峰藍移。

In this study, we synthesized azobenzene monomers with photoisomerization properties and used atom transfer radical polymerization (ATRP) to prepare the amphiphilic block copolymer poly(ethylene glycol)methylether-block-poly(6-(4-(4-dimethylaminophenylazo)phenoxy-hexyl methacrylate) (mPEG-b-PDMA-Azo). We investigated the photoisomerization behavior and morphology of mPEG-b-PDMA-Azo in solutions. The morphology of mPEG-b-PDMA-Azo in mixed solvents was characterized by using small angle X-ray scattering (SAXS). In a neutral solvents (THF and DMF), mPEG-b-PDMA-Azo exhibited as a random coiled conformation. However, with an increasing the proportion of selective solvent (H2O) in the mixed solvent, the different compatibilities of mPEG and PDMA-Azo with water led to the gradual formation of micellar structures with the azobenzene segment forming the core and the mPEG segment forming the shell.
mPEG-b-PDMA-Azo exhibited the photoresponsive properties of azobenzene molecules. Under visible light irradiation, we observed a trans-to-cis isomerization, while the reverse reaction occurred in dark. The photoisomerization rate of azobenzene molecules in selective solvents was significantly slower than in neutral solvents. This was attributed to the formation of aggregates which restricted the space needed for the structural transformation of PDMA-Azo segments during photoisomerization, resulting in decreasing the rate. Furthermore, solvent selectivity affected the aggregation behavior of azobenzene. In neutral solvents, azobenzene molecules mainly existed in a nonassociation state. When selective solvents were introduced, the solubility of the PDMA-Azo segment decreased, inducing aggregation. As the proportion of water increased, the aggregation behavior of azobenzene segments differed between THF/H2O and DMF/H2O mixed solvent systems. In THF/H2O, the proportion of nonassociation remained relatively unchanged, possibly due to insufficient water to induce significant aggregation. The proportion of J-aggregation increased while H-aggregation decreased, because J-aggregation exhibited greater stability in high-polarity solvents compared to H-aggregation. In the DMF/H2O system, increasing water content led to a significant reduction in nonassociation, accompanied by an increase in both H-aggregation and J-aggregation. This was attributed to the high-polarity environment reducing electrostatic repulsion, thereby promoting molecular aggregation.
mPEG-b-PDMA-Azo exhibited the fluorescence properties. As the selectivity of the solvent increased, the fluorescence intensity significantly decreased, accompanied by a slight red shift. This was due to the interaction between the polar solvent and the fluorescent groups, affecting the molecular dipole moment and causing solvent molecules to rearrange, thereby influencing the fluorescence properties. In THF/H2O and DMF/H2O mixed solvent systems, the differences in solvent polarity and the effect of photoisomerization on azobenzene molecules led to distinct fluorescence characteristics before and after irradiation. Under visible light irradiation, azobenzene molecules underwent a transition from a low-polarity trans-isomers to a high-polarity cis-isomers. In low-polarity THF, the fluorescence intensity of the trans-isomers was higher due to its greater stability, which promotes energy transfer. In contrast, the cis-isomers had weaker interactions with solvent molecules, resulting in lower fluorescence intensity. Conversely, in high-polarity DMF, the cis-isomers was more stable and had higher fluorescence emission efficiency, leading to increased fluorescence intensity. Additionally, high-polarity solvents changed the electronic states of azobenzene molecules, causing a blue shift in the fluorescence emission peak.

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