Synthesis And Photophysical Properties Of Cyanated Tetracene Derivatives

The energy crisis and environmental pollutions have been key issues which have to be solved for the current society. As one of the clean energy, solar enegy has been vigorously developed which can be converted into electric energy by solar cells. However, Shockley-Queisser theory limits the highest efficiency of single junction solar cell is only 32%, because parts of the photons are not absorbed and some absorbed photons with higher energy than the bandgap of the semiconductor convert the part of their enery into heat energy. However, if a high-energy photon is cutted into two low-energy photons by’quantum cutting’, photoelectric conversion efficiencies of solar cells can be increased to 45%, which is called singlet fission (SF) process for organic molecule. SF is a spin allowed process in which a high-energy singlet exciton splits into two low-energy triplet excitons. Therefore, SF is a feasible way to improve the photoelectric conversion efficiency.Tetracene, as a common small organic molecule, has a wide range of applications in organic solar cells (OSCs), organic field-effect transistors (OFETs), organic light-emitting diodes (OLEDs). The photophysical properties of tetracene materials concerned with the region of SF are mainly investigated in the solid films and crystals, as well as oligomer (dimer and trimer). Especially, covalent bond connected tetracene dimers is a very meaningful system for SF, since it contains the least number of chromophores in this process. Many works have been carried out in the region of SF based on tetracene. It is noted that tetracene is easy to be degradation when exposed to light and oxygen and has poor solubiltiy. Therefore, many attempts have been made to introduce various substituents on the molecular skeleton of tetracene to improve the stability and solubility. The experimental and theoretical studies show that the introduction of cyano group can improve the stability and solubility of tetracene obviously. Based on this, the research work of this thesis has carried out in follows:1. A new method was adopted to synthesize cyanated tetracenes, which lead to simplifying the synthetic method and improving the prodcutivity. The photophysical properties were investigated by UV-vis absorption spectra, steady-state fluorescence spectra and time-resolved fluorescence spectra in solutions, illustrating the effects of position and number of the substituent on photophysical properties. In addition, the stuctures and photophysical properties of three cyanated tetracene in vacuum vapor deposited films were studied, and compared with those in solutions. The experimental results show that no delay fluorescence was detected, namely no SF process. This can be attributed to the formation of excimer due to strong intermolecular coupling, which competes with SF process.2. A stable tetracene dimer linked directly by a carbon-carbon single bond is synthesized for the first time. The stability of this dimer is proved to be better than tetracene obviously. The ground-state interaction between the two tetracene subunits in this dimer becomes stronger than the tetracene dimers linked by phenyl bridge. The excited state of this tetracene dimer is delocalized on two tetracene subunits with strong mixing with the charge transfer states, which is similar with that observed for orthogonal pentacene dimer, but significantly different from that of related anthracene dimer. But the excited state of this dimer decays mostly through fluorescence emission channel, rather than singlet exciton fission. The results of this research suggested that similar orthogonal configuration can lead to significantly different photophysical properties for the dimers of different acenes. This information should be useful in the design of new acene dimers for various photo-functional applications.3. Alkynyl linked tetracene dimer has a good planar structure, and alkynyl can also provide π electronic interactions. At the same time, alkynyl as strong electron-withdrawing groups can improve the stability of dimer. This chapter mainly introduces the synthesis and photophysical properties of alkynyl linked tetracene dimer. The photophysical properties of the dimer are studied in comparison with the monomer, which aims to reveal the impact of introduction of alkynyl in the dimer, therefore to provide good reference for future study on tetracene materials.In a word, researching the photophysical properties of cyanated tetracenes is helpful to further exploring the SF process and its mechanism.