利用氧化性光環化或氧化性赫克環化反應合成多環芳香族分子

Abstract

多環芳香族化合物擁有良好的共軛性與低能隙之特性，有成為新一代半導體材料之潛力，可應用在有機場效電晶體、有機發光二極體及有機太陽能電池等光電元件的開發。近年來，多環芳香族化合物被廣泛的研究，許多有機合成方法被用來開發新的分子結構或構築具有良好平面性共軛系統的分子材料。其中一個經典的合成方法就是氧化性光環化反應，另以二價金屬鈀進行之氧化性赫克反應也是一個相當有潛力的選擇。 在本論文中，我們設計並合成具有噻吩與乙烯基團之前驅物，以紫外光照並氧化合環或藉由醋酸鈀（Ⅱ）催化進行氧化性赫克環化的方式得到多環芳香族稠環化合物。將不同芳香環引入多環芳香族的共軛中心，可以得到不同結構的雙噻吩曲形稠環化合物，透過光物理與電化學性質的分析，進一步探討分子結構與性質間的關係。我們發現相較於環化前趨物而言，關環產物的吸收及放光光譜有紅移現象，能隙也有變窄的趨勢，並且彎曲方式相異的螺旋形與折彎形產物之光譜與能階特性都有不同程度的變化，取決於芳香環骨架之結構對曲形分子芳香性與平整性的影響，此特性得以應用於微調化合物的HOMO及LUMO能階。不同結構化合物的分子間堆疊形式也大相逕庭，其中螺旋形化合物的晶相展現三維堆疊模式，透露其應用於純π﹣π作用力的自組裝行為之潛力。此外，我們也發現氧化性光合環與醋酸鈀（Ⅱ）催化之氧化性赫克環化反應對於化合物關環的位向有明顯選擇性上的差異，前者主要取決於環化中間物之穩定性，後者則由化學鍵之活性與立體效應因素決定。

π-Conjugated polycyclic aromatic compounds have the great potential to act as superior semiconductors due to their unique structure and property, making them good candidates for the applications in optoelectronic devices such as organic field effect transistors (OFETs), organic light-emitting diodes (OLEDs), and organic photovoltaic cells. Recently, polycyclic aromatic compounds have received great attention, numerous synthetic strategies were used to construct novel structures with planarity and π-conjugation. One of the classic synthetic methodologies for large aromatic systems is the oxidative photocyclization. In addition, the well-explored palladium(Ⅱ)-mediated oxidative Heck reaction is also profitable. In this thesis, we report the synthesis of novel thiophene-fused polycyclic aromatics from tailor-made precursors, utilizing either a double oxidative photocyclization reaction under UV-light irradiation in the presence of iodine; or palladium(Ⅱ) acetate-mediated oxidative Heck cyclization. Various aromatic rings including pyridine, benzene, and naphthalene were incorporated as the central linkage, giving various bent arenedithiophenes. Structural identifications of heteroacenes are based on X-ray crystal diffraction analysis. The photophysical and electrochemical properties are characterized by studying their UV-Vis/ photoluminescence (PL) and cyclic voltammetry (CV), respectively. We found that there are bathochromic effects in absorption as well as emission spectra of cyclized products compared to those of precursors. Simultaneously the band gaps narrows after cyclization, while the different bending mode of angular, spiral, and kinked products leads them to have distinctive spectral and electrochemical properties, as well as crystal packing pattern, depending on the effect of bending mode of aromatic skeleton to aromaticity and coplanarity of bent arenedithiophenes. This gives us opportunity for tailoring the HOMO/LUMO energy levels of the cyclized products via varying the fusing form of polycyclic aromatic compounds. The characteristic heterodimer formation in crystal phase of helical compounds also furnishes the potential for self-assembly application, where aggregation by pure nonplanar π-π stacking is expected. In addition, ring closure products via oxidative Heck cyclization shows significant regioselectivity comparing to that from oxidative photocyclization. Product selectivity of photocyclization mainly depends on the stability of dihydro intermediates which is further oxidized to cyclized products, while that of oxidative Heck cyclization is the combination of C-H bond activity and steric effect.