光化學是研究物質受光照射後所產生的化學效應。根據 Jablonski Diagram，分子吸光後有許多可能的反應途徑，其中一個是激發後的電荷轉移。染料敏化太陽能電池 (dye-sensitized solar cells, DSCs) 即依賴染料分子吸光，再將受激發的電子轉移至半導體中，進而形成一個完整的電池電路。染料分子必須要能夠有效地吸光並進行電荷轉移，因此，設計一個能符合要求的染料分子是必要的。然而，DSC元件效率牽扯到的變因太多，無法瞭解效率高低是否為染料分子所造成的影響。於是我們利用密度泛函理論 (density functional theory, DFT) 及含時密度泛函理論 (time-dependent DFT)，探討分子的結構、光補捉效率及電荷轉移效率等特性。本研究針對具有推拉電子效應的 D- spacer-A 架構之分子做討論，包括 (1) 一系列具有相同推電子基團 (D) 及拉電子基團 (A) 但不同共軛基團的 D- spacer-A 有機染料分子與 (2) 七個已發表的含方酸 (squaraine) 之染料分子。所有的染料分子都是吸附於二氧化鈦模型進行計算研究。我們統整不同共軛基團對吸收波長、吸光度及電荷轉移機率的影響，以提供合理的染料分子設計之準則。 光化學過程也可能發生鍵結斷裂的反應。分子吸收光後，被激發至激發態，結構從 FranckCondon state 轉變為光化學產物。本研究即利用第一原理探討1,2-二苯基環丙烷衍生物在第一激發態的鍵結斷裂反應。計算結果指出，斷裂的鍵為環丙烷結構中最長的鍵；同時，也得到分子在激發態的鍵結斷裂位能曲面及能量障礙。環丙烷衍生物的3號位置之取代基為甲氧基時，其位能曲面上之能量障礙是取代基為甲基時的一半。另外，甲氧基與苯基之間的立體障礙促使以甲氧基為取代基之分子可形成puckered狀態。此計算結果可說明實驗上只能觀察到含甲氧基之分子具有長波長螢光的可能原因。;Photochemistry is the study of chemistry concerned with the effects of light. According to the Jablonski Diagram, the molecule absorbs the light to enter an excited state and there are many reaction pathways may occur. One of the photochemical processes is electron transfer. The working principle of dye-sensitized solar cells (DSCs) relies on the excited electron transferred from the dye to the absorbed semiconductor (electron injection process) and then complete a full circuit of solar cell. It is desired to design excellent dyes for high-efficiency light absorbance and electron injection. However, the effects of the dye on the efficiency (η) of the DSC device are complicated. We employed density functional theory and time-dependent DFT to investigate the molecular structures, light harvesting efficiency and electron transfer properties of dye molecules based on D- spacer-A framework. The two series of studied dyes are (a) a series of D- spacer-A dyes with the same donor and acceptor but different spacer moieties and (b) seven published squaraine-based dyes. All of them are investigated when they are adsorbed on (TiO2)38 cluster. We provide a detailed benchmark for the effects of different spacer on max, oscillator strength, and probability of electron transfer. Some rules for rational dye design are drawn. Photochemical processes include not only electron transfer process but also reactions. After the molecule absorbs the light, it reaches the excited state and then its geometry may be transformed from the FranckCondon state to the photoproduct. The bond-breaking reactions of 1,2-diphenylcyclopropane derivatives at the first singlet excited state was investigated by first-principle calculations in our study. The results indicate that the broken bond (C1C2) is longer than the other two bonds of cyclopropane. Moreover, the calculations show the molecular properties and activation energy on the potential-energy surface of C1C2 bond breaking. The activation energy required to break the C1C2 bond in the molecule with methoxy substituents (CP1a*) is more than two times lower than that in the molecule with methyl substituents (CP2a*), supporting experimental observations. The steric hindrance between methoxy groups and phenyl rings of CP1a* result in the formation of a puckered state.
