The Design of Refined Triphenylamine-Based Dyes for Dye-Sensitized Solar Cells: Theoretical Perspective

• Main Authors: Chun-ShengWu, 吳俊昇
• Other Authors: Shao-Pin Wang
• Format: Others
• Language: zh-TW
• Published: 2013
• Online Access: http://ndltd.ncl.edu.tw/handle/62124661102123143706

碩士 === 國立成功大學 === 化學系碩博士班 === 101 === The DFT/B3LYP calculations have first been confirmed to provide reliable physical quantities published recently in the area of DSSC studies. The triphenylamine derivatives are promising and popular dyes except their high-lying HOMOs. Through the studies of adjusting the energies of pi-type molecular orbitals using chemical substitution, the conventional electron-withdrawing nature of fluoro suffers poor to failure results in view of lowering pi-type MOs. Conversely, the pi-accepting nature of a trifluoromethyl group gives results complying with expected effects based on its EN values despite of its pi-accepting nature well established by the negative hyperconjugation argument. The two substituents have been adapted for modifying the levels of HOMO/LUMO of the Triphenylamine-based dyes. The MO-destabilization effects exerted by a fluoro have been verified through the site-dependent observation of MO-energy variations. As it is introduced at a carbon having more contribution to the MO wavefunction, the lowering extent gets smaller or would even lead to a higher MO energy. This is more pronounced for HOMO than for LUMO since the former is closer in energy to the valence p-orbital of fluorine, 2p(F). The pushing-up effect of p-orbital on HOMO/LUMO can be explained by analysis of LCBO-MO, in which the bond orbitals or localized MOs are extracted from the DFT wavefunctions by the Weinhold’s NBO method. Fluorination performed at a carbon having higher value of the one-center BO, 2p(F), results in a higher MO-energy of HOMO or LUMO. This destabilization described by the orbital interaction between 2p(F) and HOMO/LUMO can be viewed as the MO-variations accompanied by the well known mesomeric (also termed as resonance or conjugation) effect. In addition to the CF anti-bonding, *CF, the 2p(F) BO of a trifluoromethyl group also contributes to HOMO/LUMO of the derivatives. The pi-accepting role played by *CF, which would press down HOMO/LUMO according to NHC argument, is offset by the 2p(F) destabilization effect. This dual nature, which affects MO-energies via pi-framework, explains the reported “purely inductive effect” found for color-tuning using trifluoromethyl group. As well as we have found for tuning phosphorescent energies of Ir(III) complexes, the steric effect would operate as the exceptional destabilization of MOs is found based on the above-stated electronic effects.