Study On The Molecular Design,Crystal Growth And Optoelectronic Properties Of Cyano-substituted Styrylbenzene Derivatives

Compared to amorphous films,organic single crystals possess excellent optoelectronic properties,attributed to the thermodynamically stable state,highly ordered molecular stacking and low content of impurities and defects.However,the complicated crystalline engineering makes it hard to build the relations between the chemical and crystal structures.Hence,it is of great importance to design and develop different molecules with similar chemical structures and attempt to discover the connections of crystal morphologies,structures and optoelectronic performances.In this dissertation,we designed and synthesized 15 new compounds and successfully obtained 15 single crystal structures based on styrylbenzene molecular skeleton through introduction of functional groups,such as cyano group.We classified all the molecules according to the chemical structures,and researched the influence of crystal growth conditions on the growth rate of different facets,the unusual temperature-dependent photophysical properties and the applied studies of organic field-effect transistors and photocatalytical hydrogen evolution from water splitting based on organic single crystals.Combined to the chemical structures,single crystal structures and optoelectronic properties,we summarized the connections between the molecular structures and crystal performances.Such connections will be beneficial to establish the relationships between the intrinsic structures of materials and optoelectronic properties,and offer effective guidance for expanding the applied range of crystals as well as developing high-performance crystal materials.In the first chapter,we introduced the crystal growth conditions and characterizations based on styrylbenzene crystals,gave a brief analysis about the impact of molecular packing mode on photophysical properties,and the research situations of applied fields including organic light-emitting diodes,organic field-effect transistors(OFETs),organic solid laser and photocatalytic water splitting for hydrogen evolution.We gave a brief analysis on the crystal structure-performance relationships.In the second chapter,we synthesized 15 new molecules via introducing the functional groups(such as the fluorine atom,nitro and cyano groups)at the molecular skeleton,obtained15 single-crystal structures through physical vapor transport(PVT),and explored the factors of molecular chemical structures,growth time,pressure and temperature on the crystal quality and morphologies.The results displayed that:the lower pressure was advantageous to acquire rod-shaped crystals;the higher temperature was,the thinner crystals were,and the vice versa.According to the equations of crystal nucleation density and molecular translational energy,the decreased temperature and increased pressure are beneficial to enhance the nucleation density and downsize the crystals.With the temperature decreased,the molecular translation energy is decreased.Thus,the growth rate of facets with higher growth barrier was suppressed,and the growth rate of facets with lower growth barrier was faster.The facets with lower growth barrier mainly including the facets along hydrogen bond and?···?directions,are the driving forces for the crystal growth,resulting in forming the sheet-shaped crystals.In the third chapter,some crystals were tested for the temperature-dependent fluorescence spectra and fluorescence decay spectra.The spectra showed an unusual trait at low temperature:when the temperature was decreased but still above 200 K,the fluorescence intensity of PCBDT crystals was increased;while the temperature was constantly decreased below 200 K,the fluorescence intensity was decreased.Such unusual phenomena also occurred for other samples both of films and crystals.As the temperature was increased,the calculated radiative decay rate(k_r)was firstly increased and then decreased,whereas the non-radiative decay rate(k_nr)was descended(the k_nrof PCBDT crystals was increased afterwards).It may be the traps in samples that led to this unusual phenomenon.When the temperature was decreased but above the threshold,the process of thermal vibration was restrained,leading to enhanced fluorescence intensity with increased k_rand diminished k_nr.As the temperature was decreased and below the threshold,there were some shallow traps that trapped the excitons,resulting in the weakened fluorescence intensity.In the fourth chapter,the sheet-shaped and strong fluorescence crystals(DPy BTA,Py PBTA,DPBTA and NBTA)were selected and applied to OFETs.The DPy BTA and Py PBTA crystals behaved typical n-type charge transport,whereas the DPBTA and NBTA crystals possessed ambipolar transport.The obvious electroluminescence for NBTA crystals was observed in the device channel.The maximum external quantum efficiency was up to 2.02%,far beyond the benchmark for the single-crystal light-emitting transistors at that time.Compared to other three materials,the NBTA crystal possessed the most adjustable energy level with the work function of Au via Cs F modification,giving rise to higher hole mobility and more balanced charge transport.In the fifth chapter,the selected sheet-shaped crystals(PCBDT and PCPy BDT)were applied to photocatalytic water splitting for hydrogen evolution.The hydrogen evolution rate of micro/nano-sheets for PCBDT and PCPy BDT were 1181 and 8143?mol g^-1 h^-1,higher than that of most catalysts for organic polymers.Compared to the PCBDT,the introduction of pyridine not only broke through the co-planarity of molecule,but also lowered the lowest unoccupied molecular orbital,advantageous to form donor-acceptor structure and enhanced the excitons dissociation.The mobility test results based on the OFETs showed that PCBDT crystal presented hole transport(?_h=0.20 cm² V^-1 s^-1),whereas the PCPy BDT crystal behaved typical electron transport(?_e=0.25 cm² V^-1 s^-1).This signified the low content of defects in crystals,beneficial to reduce the possibility of the charge being trapped by the defects and suppress the exciton recombination.