Theoretical Design Of Molecular Photoswitches And Study On Charge Transport Properties

With the rapid development of miniaturization and integration of electronic devices in recent years,traditional silicon-based semiconductor components are incapable of meeting the increasing needs in the future.There is an urgent need to find new materials to construct devices in order to continue and advance the development process.Thus,a new field named molecular electronics is developed.The ultimate goal of molecular electronics is designing and manufacturing nano-scale functional electronic components to assemble logic circuits.Constructing the functional device at the single-molecule level(molecular switches,molecular rectifiers,molecular motors,for instance)is one of research hotspots.Among them,molecular switches,which are capable of transmitting and storing information accurately,have attracted much attention because of their potential application in future nano-logic circuits.There are two or more interconvertible steady states that exist in molecular switches.The conversion process between steady states can be driven by external stimuli such as light radiation,temperature,electric field,acid-base and so on.The switch controlled by light,namely the photoswitch,has gained widespred attention due to its simple control means,low toxicity,fast response of the device,and precise control.Photochromism is an indispensable property for molecules used for constructing photoswitches.At present,many photochromic molecules(e.g.,diarylethene,dimethyldihydropyrene,azobenzene)have been experimentally and theoretically studied on device level,along with certain accomplishment.However,due to the complex regulation mechanism and variable molecular properties,it is difficult to summarize the laws and general conclusions.It is necessary to further explore the regulation of both molecular photochromic properties and electronic transport properties of devices.In this thesis,the first-principles calculations based on density functional theory combined with the nonequilibrium Green's function are adopted to study the charge transport properties and switching behaviors of photoswitching devices influenced by many factors(molecular properties,types of electrode,molecule-electrode binding sites,etc.).The functional molecules in devices are the photochromic molecules whose switching progress is realized through cyclization reaction and the ring cleavage reaction.The specific research contents and conclusions are as follows:1.Charge transport properties of diarylethene photoswitches based on Au and graphene nanoribbons electrodes.Transport and switching properties of the double-electrode systems consisting of metal[5×5 Au(111)]and non-metal(single-layer zigzag graphene nanoribbon)electrodes are designed and studied,respectively.It is found that two steady states exist in devices,namely the high-conductivity and low-conductivity states,which are corresponding to the ring-closed and ring-open configurations of devices.The obvious switching behavior of each system is observed,and the on-off ratio is expected to be 103 orders of magnitude.Further analysis eonfirms the structure-performance relationship between molecular configurations and switching behaviors of devices.The negative differential resistance effect is only found in graphene-based devices.It indicates that the electrode type has a certain effect on the transport properties,but it has a small effect on the switching behavior of the device.2.Regulation of arene[e]-annelation on electronic transport properties of dimethyldihydropyrene/cyclophanediene based molecular photoswitches.The performance control of single-molecule devices is a hot topic in the research field of molecular electronics.One of the most direct means of regulating device performance is optimizing molecular properties through annelation,substitution,functionalization and so on.Previous work has shown that introducing chemical groups into photochromic molecules using conjugated connections can extend the conjugation system and modulate the spectral properties.If the molecular conjugation system is extended,electronic transport properties and switching behaviors of devices will inevitably change.In order to explore this problem,the effect of arene[e]-annelation on absorption spectrum of dimethyldihydropyrene/cyclophanediene are discussed based on time-dependent density functional theory combined with experimental results.Then,charge transport properties and switching performance of single-molecule devices are studied by using density functional theory and non-equilibrium Green's function methods.The calculation results show the blue shift of absorption produced by arene[e]-annelation,and the ring-closed/open isomer is corresponding to the device with high/low conductance.Device switches constructed by[e]-fused derivatives exhibit great switching performance.The maximum on-off ratio of each system is greater than 102.Further analysis confirms that the annelation mode(single arene[e]-annelation and double arene[e]-annelation)has a significant effect on molecular properties and electronic transport properties of molecular devices.Fusing arene on[e]-positions of outer carbon-skeleton can not only change molecular properties but also be used to enhance the on-off ratio of devices.3.Effects of molecule-electrode linkage sites on transport properties and switching performance of 13,14-dimethy lcethrene based molecular photoswitches.Molecular-electrode linkage configurations,linkage sites,anchor groups,tensile and compression stress,etc.,are factors which directly affect electronic transport properties of single molecular devices,thus it can be used as regulating means.In this work,we studied electronic transport properties,switching behaviors,and the multiple properties effected by different molecular-electrode linkage sites based on a novel chiroptical molecular switch,that is 13,14-dimethylcethrene.Calculation results indicate the excellent switching behavior of 13,14-dimethylcethrene based molecular devices,and the maximum on-off ratio is predicted to be as large as 103.Further analysis shows that the HOMO-LUMO gap of the ring-closed isomer is larger than the ring-open isomer.It is different from most switching molecules which realize the switching process through the cyclization and the ring cleavage reaction.Additionally,it is found that both iosmers of 13,14-dimethylcethrene molecule are sensitive to linkage sites.This study shows that the linkage sites have a significant impact on the transport properties of the devices,and it should be considered in constructions of single-molecule functional electronic devices.This thesis includes the following six chapters:The first chapter is the introduction.It mainly introduces the generation and development of molecular electronics,the experimental and theoretical research progress of molecular devices,especially molecular switches.The second chapter introduces the first principles calculation method used in all these studies and the self-consistent solution process of electronic transport properties of device.From Chapter 3 to Chapter 5,three works and research results based on the above methods are introduced respectively.The sixth chapter is the summary and prospect of the whole text.In summary,the optimization design and performance regulation of molecular photoswitches are carried out both at molecular and device levels.It has referential significance to other functional molecules and devices,such as molecular motors,molecular rectifiers,molecular wires,and others.It also can provide systematic theoretical guidance for the further design of single-molecule functional devices with better performance and stronger stability.