Theoretical Study On The Control Of Photoelectronic Properties Based On Molecular Configuration

In recent years,understanding and manipulating microstructures of matters have experienced a rapid progress.However,with increasing collection of data of structures from instruments and calculations,it becomes urgent and important to develop means of understanding and controlling properties of materials with the help of these data.Structure of molecule determines the nature of the material.Therefore,exploring the structure-function relationship of molecules,and regulating properties of the material at the molecular level is a crucial research direction for physical chemistry.In this paper,starting from the molecular configuration,using density functional theory simulations,we have studied the optical and electronic properties of some molecular systems under different conformations,and demonstrated how to regulate the light emission(Chapter 3),the charge transfer(Chapter 4)and the adsorption(Chapter 5)properties by changing the molecular conformation,and the structure-function relationship and mechanism were explored and elaborated.This paper is divided into five chapters:Chapter 1 contains two sections,the first is the principle of light emission of molecules and their applications,and the second is photo-responsive molecules.Organic light-emitting molecules have important applications in many fields.How to control the luminescence properties of molecules is also the focus and difficulty for current research.In addition to the structural modification of light-emitting molecules,the current works focuse mainly on the regulation of molecular conformations.In the content,we mainly introduce two of them,that is.the aggregation-induced emission and the solvatochromism effect.On the other hand,the application of stimuli-responsive molecules is also an important means to regulate the properties of materials,among which light stimulation is the simplest.The researches in this paper mainly involve azobenzene molecules in organic photochromic systems.In Chapter 2,we briefly introduced the theoretical framework of the first-principles-based density functional theory and the quantum chemistry calculation software used in this paper.Density functional theory is based on quantum mechanics,and describes the physical properties of the microscopic system by electron density.Kohn-Sham equation simplifies an interacting multiparticle system to a non-interacting single-particle system,and all the approximations are concentrated in the exchange correlation energy.After that,a series of approximate methods have been developed to obtain an accurate exchange correlation energy,including local density approximation,generalized gradient approximation,hybrid functionals and so on.In the actual calculation,we select the appropriate exchange-correlation functional and quantum chemistry calculation software according to the research system and purpose to carry out simulations,so as to obtain the microscopic properties and mechanism of the system.In Chapter 3,we studied the relationship between molecular configuration and luminescent properties.In this work,we started with the configuration of the TPPE molecule.By adjusting two dihedral angles in the molecule,we found that the deviations in molecular configuration affect the group interactions,leading to the molecular orbital wavefunctions redistribution,which further changes the emission energy.The fluorescence wavelength of TPPE changes regularly with the configuration variations.This rule can be summarized as a trigonometric-functional expression,which describes the relationship between the emission wavelength and molecular configuration quantitatively.And using this relationship,we can successfully predict the fluorescence wavelength range of TPPE molecules in experiments.In Chapter 4.we studied the molecular configuration and charge transfer properties.Due to the high probability of charge recombination and the inability to automatically switch the conductivity in general photoelectric conversion system.we have designed a self-adaptive photoelectronic conversion molecular system which has a donor-switch-acceptor structure.The donor and acceptor are bridged by a switchable light-responsive molecule.In this work,calculations are presented for phenylacetylene terpyridine-platinum complex and azobenzene bridge,demonstrating a photoelectronic conversion mechanism in which the charge transfer behavior can automatically switched.Light-induced charge transfer triggers the trans?cis isomerization of the photoswitch.The resulting conformation suppresses charge recombination.Energized charges are trapped in the acceptor,ready for charge collection by electrodes.The photoswitch then response to visible light and goes through inverse isomerization to restore the conjugation and conductance,making the molecule ready for the next cycle of photoelectronic conversion.In Chapter 5,we studied the molecular configuration and adsoption properties.In this work,we conducted a combined theoretical and experimental study on the microscopic mechanism of photo-controllable adsorption behavior of an azobenzene-based surfactant on silica surface.The photoinduced trans?cis isomerization causes radical conformational change of the tail part of the surfactant,which results in significant change of molecular dipole moment as well as its cross-sectional area.Albeit the adsorption of single molecule just experiences negligible change,the adsorption of collective molecules undergo a profound alternation due to reduction of coverage on the surface and increasing of hydrophilicity.Consequently,the photo-responsive surfactant can be efficiently detached from the surface it initially attaches.Experimental results well confirmed our theoretical predictions.These results show that it is feasible to realize desirable separation of photo-responsive surfactant and surface using light.