Theoretical Studies On The Optical Absorption Properties Of Metalloporphyrin And Polycyclic Aromatic Hydrocarbon Molecules

Metalloporphyrins and polycyclic aromatic hydrocarbons are two of the most commonly used types of molecules in the research of single-molecule optical imaging.The study of the optical absorption properties of these molecules are fundamentally important to our understanding of the resulting optical images.However,some novel features in the experimentally measured absorption spectra of these molecular systems have not been systematically studied and explained.To this end,we used combined first-principles calculations and Franck-Condon and Herzberg-Teller simulations in this thesis to compute the vibrationallyresolved absorption properties of some representative metalloporphyrin and polycyclic aromatic hydrocarbon molecules.The experimental findings are analyzed and interpreted based on the theoretical results.The contents of the thesis include the following two parts:1.We have carried out theoretical investigations to understand the spectral profile changes in the Q-band absorption of magnesium porphyrin(Mg-P),zinc porphyrin(Zn-P)and palladium porphyrin(Pd-P)molecules as observed in the experiments.The vibrationallyresolved Q-band absorption spectra of the three molecules were calculated by using two widely used hybrid density functionals,namely B3 LYP and CAM-B3 LYP.It was found that the CAM-B3 LYP functional with long-range correction effect can reproduce nicely the experimental results.However,the results of the commonly used B3 LYP functional show relatively large errors when compared to the experimental results.We then analyzed the changes to the absorption profiles of the three molecules as observed in the experiment based on the results obtained with CAM-B3 LYP.It was found that for the porphyrins with heavier metal ions,the oscillator strength for the transition between the ground state and the first excited state is much larger than that of the porphyrins with lighter ion.Such increased oscillator strength for metalloporphyrins with heavier metal ions thus leads to stronger Franck-Condon dominated 0-0 absorptions that is responsible for the enhanced absorption in the low energy region of the absorption spectra of Zn-P and Pd-P molecules.Through the analysis of the geometric and electronic structures of the three molecules,we found that the key factor leading to the greater oscillator strength for porphyrin molecules with heavier metal ions is the decrease of the size of the porphyrin cavity.Calculation results show that molecules with smaller porphyrin cavities have larger transition dipole moments and thus stronger oscillator strength.In addition,we have carefully analyzed and identified the vibrational fine structures in the Q-band absorption spectra of three metalloporphyrin molecules.The most active vibration modes of the three molecules were given.This work not only helped us to understand the previously unexplained experimental results,but also determined the effective theoretical methods for describing such molecular systems,which laid the foundation for further studies on such systems.2.In order to understand the length induced energy shift and spectral profile change to the absorption spectra of linear polycyclic aromatic hydrocarbon molecules,we applied three commonly used hybrid density functionals including B3 LYP,CAM-B3 LYP and B97XD to calculate the vibrationally-resolved optical absorption spectra of naphthalene,anthracene,tetracene and pentacene molecules.It was found that all three functionals can nicely reproduce the vibrational fine structures in the experimental spectrum.Theoretical results indicate that the red shift of the absorption spectrum caused by the change of the length of the four molecules can be attributed to the decrease of the energy difference between the lowest unoccupied molecular orbital and the highest occupied molecular orbital.The decrease in energy difference between the two molecular orbitals leads to a decrease in the energy of the first excited state of the molecular system,thereby reducing the excitation energy between the ground state and the first excited state,and causing a red shift to the absorption spectrum.This work can serve as a reference for further studies on such systems,especially for the identification of the experimentally measured absorption spectra.