环状芴基张力半导体拉曼光谱理论与实验研究

Theoretical and Experimental Studies on Raman Spectroscopy of Cyclic Fluorene-Based Strained Semiconductors

环状芴基张力半导体由于其特殊的分子结构,发射状排布的p轨道和纳米级空腔等诸多特点,引起了科学家广泛的关注.相对于直链型芴基半导体,环状芴基张力半导体展现出独特的光电性质.然而,迄今为止其振动性质没有被报道.对四元环芴的晶体与直链四聚芴粉末样品进行拉曼光谱表征和归一化处理,并结合理论计算.结果表明,与直链寡聚芴相比,环芴类似于碳纳米管的G峰发生了偏移,并且低频区域拉曼频移峰的峰强增加.其原因是由于张力的引入,改变了芴基主链的骨架和电子结构,加强芴骨架上π电子离域,同时使环芴中每个芴单元都参与到振动中.该研究结果为芴基张力半导体材料拉曼光谱的研究提供了一定的工作基础.

Cyclic fluorene-based strained semiconductors which achieve both merits of hoop-shaped cycloparaphenylenes and fluorene-based emitters with high-efficiency feature have attracted increasing attention from synthetic chemists and theoreticians due to their aesthetic molecular structure, radial p orbitals and nanosized cavities. Compared with linear fluorene- based semiconductors, Cyclic fluorene-based strained semiconductors exhibit unique photoelectrical properties. For example, contrary to the deep blue emission of linear fluorene-based molecules, the controlled cyclic fluorene-based strained molecules show stronger green emission. However, the properties of molecular vibrations of cyclic fluorene-based strained materials have not been reported so far. In this article,[4]Cyclo-9,9-dipropyl-2,7-fluorene (CF) and linear quaterfluorenes (LF) were synthesized as modeling compounds to explore the differences of Raman spectra on structures by theoretical and experimental studies. Raman spectroscopy measurements have been presented on polymer poly(9,9-dioctylfluorene)(PFO) and LF, and compare them with CF. In addition, we have calculated the theoretical Raman spectra of CF and LF based on time-dependent density functional theory (TDDFT), which are then compared to the experimental results for the assignment of different modes. All calculations were performed at 6-31G (d) basis set along with the range corrected B3LYP density functional. The results demonstrate that the Raman peak positions of CF which are analogous to those of carbon nanotubes such as G band are shifted. Compared to the Raman spectra of LF, G1 and G2 peaks of CF shifted to lower frequency region, however G3 peaks shifted to higher frequency region. The relative intensity of Raman peaks in CF especially in low frequency region has increased. These properties of Raman in CF can be assigned to the changed structure of conjugated backbone and electrical structure due to strain and every fluorene unit of CF has involved in vibration and the delocalization of π electrons gets higher. These results provide powerful basis for correlating structure and properties on strain organic semiconductors by Raman spectra.