Li Xiao, Valdiviezo Jesús, Banziger Susannah D, Zhang Peng, Ren Tong, Beratan David N, Rubtsov Igor V
Department of Chemistry, Tulane University, New Orleans, LA 70118, USA.
Department of Chemistry, Duke University, Durham, North Carolina 27708, USA.
Phys Chem Chem Phys. 2020 May 6;22(17):9664-9676. doi: 10.1039/d0cp01235a.
Electron transfer (ET) in donor-bridge-acceptor (DBA) compounds depends strongly on the structural and electronic properties of the bridge. Among the bridges that support donor-acceptor conjugation, alkyne bridges have attractive and unique properties: they are compact, possess linear structure permitting access to high symmetry DBA molecules, and allow torsional motion of D and A, especially for longer bridges. We report conformation dependent electron transfer dynamics in a set of novel DBA compounds featuring butadiyne (C4) bridge, N-isopropyl-1,8-napthalimide (NAP) acceptors, and donors that span a range of reduction potentials (trimethyl silane (Si-C4-NAP), phenyl (Ph-C4-NAP), and dimethyl aniline (D-C4-NAP)). Transient mid-IR absorption spectra of the C[triple bond, length as m-dash]C bridge stretching modes, transient spectra in the visible range, and TD-DFT calculations were used to decipher the ET mechanisms. We found that the electronic excited state energies and, especially, the transition dipoles (S0 → Sn) depend strongly on the dihedral angle (θ) between D and A and the frontier orbital symmetry, offering an opportunity to photo-select particular excited states with specific ranges of dihedral angles by exciting at chosen wavelengths. For example, excitation of D-C4-NAP at 400 nm predominantly prepares an S1 excited state in the planar conformations (θ ∼ 0) but selects an S2 state with θ ∼ 90°, indicating the dominant role of the molecular symmetry in the photophysics. Moreover, the symmetry of the frontier orbitals of such DBA compounds not only defines the photo-selection outcome, but also determines the rate of the S2 → S1 charge separation reaction. Unprecedented variation of the S2-S1 electronic coupling with θ by over four orders of magnitude results in slow ET at θ ca. 0° and 90° but extremely fast ET at θ of 20-60°. The unique features of high-symmetry alkyne bridged DBA structures enable frequency dependent ET rate selection and make this family of compounds promising targets for the vibrational excitation control of ET kinetics.
供体-桥-受体（DBA）化合物中的电子转移（ET）强烈依赖于桥的结构和电子性质。在支持供体-受体共轭的桥中，炔烃桥具有吸引人的独特性质：它们结构紧凑，具有线性结构，可形成高对称性的DBA分子，并且允许供体（D）和受体（A）发生扭转运动，对于较长的桥尤其如此。我们报道了一组新型DBA化合物中依赖构象的电子转移动力学，这些化合物具有丁二炔（C4）桥、N-异丙基-1,8-萘二甲酰亚胺（NAP）受体以及一系列具有不同还原电位的供体（三甲基硅烷（Si-C4-NAP）、苯基（Ph-C4-NAP）和二甲基苯胺（D-C4-NAP））。利用C≡C桥伸缩振动模式的瞬态中红外吸收光谱、可见光范围内的瞬态光谱以及含时密度泛函理论（TD-DFT）计算来解读电子转移机制。我们发现，电子激发态能量，尤其是跃迁偶极矩（S0 → Sn）强烈依赖于供体和受体之间的二面角（θ）以及前沿轨道对称性，这为通过在特定波长激发来光选择具有特定二面角范围的特定激发态提供了机会。例如，在400 nm处激发D-C4-NAP主要在平面构象（θ ∼ 0）中产生S1激发态，但在θ ∼ 90°时选择S2态，这表明分子对称性在光物理过程中起主导作用。此外，此类DBA化合物前沿轨道的对称性不仅决定了光选择结果，还决定了S2 → S1电荷分离反应的速率。S2 - S1电子耦合随θ变化超过四个数量级，这一前所未有的变化导致在θ约为0°和90°时电子转移缓慢，但在θ为20 - 60°时电子转移极快。高对称性炔烃桥连DBA结构的独特特性使得能够进行频率依赖的电子转移速率选择，并使这类化合物成为用于振动激发控制电子转移动力学的有前景的目标。
