Study On The Excited-state Properties Of Conjugated Molecular Systems Based On Porphyrin Group

Photo-induced electron transfer (PET) and energy transfer processes of conjugated molecular systems, has been proved to play a key role in determining theefficiency of these photoelectric materials, has become the focus of research problemsof the people. Consequently, research substituents and conjugated structural changesof the system, which effect excited states properties of conjugated organic molecules,and explore physical mechanism of the process of excited state dynamics will haveimportant significance in designing organic optoelectronic materials.In this thesis, firstly, the ground and excited state properties of nT-C60dyadsand Por-nT-C60triads (n=4,8and12) have been theoretically investigated by usingthe time-dependent density functional theory together with a set of extensivemultidimensional visualization techniques. The results reveal that the excited-statecharge transfer characters strongly depend on the conjugate structure of molecule.The charge transfer ability is largely strengthened by introducing the porphyrin groupand decreases with the length of the nT moiety. Also, the adjustment of thedeactivation pathways of Por-nT-C60triads by the length of the nT moiety werevisualized. It is found that Por*-4T-C60shows predominantly the energy-transferprocess generating Por-4T-1C60*, but the charge-separation becomes predominant forother triads, such as the direct formation of Por+-12T-C60-via Por*-12T-C60.Furthermore, the processes of Por--8T+-C60â†'Por-8T+-C60-via Por*-8T-C60hasbeen proved to be possible. Finally, the most energetically stable finalcharge-separation state is confirmed to be Por-nT+-C60-. Secondly, we use densityfunctional theory and time-dependent together with a set of extensivemultidimensional visualization techniques to characterize the influence of keto effecton charge distribution at ground state and electronic transitions for neutral andcharged hexaphyrin aromaticity with and without keto-defect. It is found that thearomaticity is the key factor to influence the ground state Mülliken chargesdistribution properties, other than the meso-aryl-substituted effect. But with theenhancement of the keto-defect, the distribution changes of Mülliken charges on the hexaphyrin groups are larger than those on the pentafluorophenyl substituted groups,following with the aromaticity changes from nonaromatic to aromatic. Furthermore,through characterizing by transition density and charge difference density, directvisual evidence for neutral and charged aromaticity with and without keto-defect canbe clearly derived, and the ability of charge transfer between units of monoradical(nonaromaticity) and singlet biradical (aromaticity) forms is much stronger than thatof neutral forms.