| Organic π-conjugated molecules and polymers have attracted considerable attention in academia and industry due to their potential applications in the field of optoelectronic devices.The charge mobility in organic solids plays a key role in controlling the performance of optoelectronic devices.Because of the weak intermolecular interaction,the charge carriers usually are localized on a single molecule or repeat unit of polymer.Under the high-temperature approximation,the charge-carrier transport can be well described using the thermally activated hopping model.Besides the hopping rate,the conductivity of the material is also strongly related to the charge injection from metal electrodes into bulk solid.The ability of charge injection of the electrode is closely related to its Fermi level and the molecular ionization potential(IP)/electron affinity(EA)energy at the interface.The ohmic contact will greatly improve the conductivity of the material in the deviceIt is a critical chemical problem to theoretically evaluate the apparent polarization energy of charged molecule in condensed phase.AMOEBA polarizable force field method is one of the most successful methods for the explicit calculation of the polarization energy of charged molecule in the condensed phase.However,the default AMOEBA force field parameters face two main disadvantages in the conjugated molecule systems:(1)the default atomic polarizabilities adopts "average" way to achieve the transferability,and consequently,loses the inherent characters for the specified system and electronic state(the effect of conjugate length and electronic state on atomic polarizability is averaged),and the parameters of aromatic heteroatoms are still not available currently due to those aromatic molecule are not including in fitting database;(2)The default intramolecular force field parameters adopts the unmodified from the MM3 force field,and the excess electron in the ionic molecule,which is not considered in the default force field.Thus,it is important to develop a new force field beyond the fitting the experimental data from a certain database.In this thesis,the state-specified polarizable force field(SS-PFF)is proposed and developed for each molecule uniquely using state-of-art QM theories,and also,applied to the investigation of organic photoelectric materials.The polarization energy of charge-carrier and the external recombination energy accompanying charge transfer reaction in the molecular crystals of linear oligoacenes and their derivatives were studied using the SS-PFF.The main contents includes the following three aspects:1.State-specific force field:The atomic multipoles,the isotropic atomic polarizabilities and the intramolecular force field parameters of a specified state of a given molecule were individually optimized using the electron density obtained by performing QM calculations.The state-specified atomic multipoles are obtained by fitting the electron density using the GDMA program.Good atomic multipoles can be obtained using the ESP fitting method based on Gauss-Hermite quadrature,when the hydrogen atomic radius is set to be 0.325 A or 0.65 A when the diffuse functions are absent.For the diffuse functions including electron density,the grid-based quadrature plus 0.325 A hydrogen radius is only way to fitting the atomic multipole moments.The state-specified atomic polarizability(SSAP)concept was also proposed,in order to consider the effect of quantum state on the atomic polarizability.The SSAP was parameterized using the quantum theory of atom in molecule(QTAIM)theory.The intramolecular force field parameters are also obtained from fitting the Hessian matrix of state-specific molecule by QM calculations.2.Insight from polarization energy:The SS-PFF method was used to evaluate the polarization energy of charge carriers in the molecular crystals of linear oligoacenes and its derivatives,and results were compared with the outcomes obtained from default AMOEBA force field and QM calculations using implicit PCM model.It turned out that the SS-PFF is more reliable to evaluate the electrostatic polarization.Interestingly,the electrostatic polarization of the hole-carrier is less than that of the electron carrier for the p-type material,while the sequence is reversed for the n-type material.In another word,the polarization energy possible is an indicator to judge transport-type for organic semiconductors.This observation,distinctly,provides a theoretical way to design or modify the transport-type of material with a cheap computational cost.In addition,the different polarization energy for electron and hole carrier can explain using the electrostatic potential distribution on the molecular vdW surface.3.A novel "two-point" model to directly estimate the external reorganization energy for hole-carrier transfer reactions:For the first time,a two-point model was proposed to explicitly calculate the external reorganization energy accompanying the charge transfer(CT)reaction in linear oligoacenes molecular crystals,using the potential energy surface of all-atom SS-PFF.The reliability of SS-PFF method was confirmed compared with Marcus two-sphere model.The distance-dependent external reorganization energy is consistent with the physical picture provided by Marcus two-sphere model,and orientation-dependent reorganization energy is explained by anisotropic dielectric tensors in organic crystals.QM-based SS-PFF can well describe the potential energy surface(PES)of state-specific molecules,which gives both reasonable external and internal reorganization energy in contrast to the QM calculations.In addition,increasing the temperature will slightly enlarge the external reorganization energy,which can explain that why high-temperature readily leads to hopping transfer.This novel theoretical model provides us with a reliable and cheap way to describe the electron-phonon interaction between the system and environment. |
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