Department of Chemistry and Biochemistry, Utah State University, Logan, Utah 84322, USA.
Department of Chemistry, Columbia University, New York, New York 10025, USA.
J Chem Phys. 2019 Mar 7;150(9):094709. doi: 10.1063/1.5080228.
This paper presents the first study of the rotations of rigid molecules in 3 dimensions at the air/water interface, using the femtosecond time resolved sum frequency generation (SFG) technique. For the purpose of this research, the aromatic dye molecule C153 was chosen as an example of a molecule having two functional groups that are SFG active, one being the hydrophilic -C=O group and the other the hydrophobic -CF group. From polarized SFG measurements, the orientations of the two chromophores with respect to the surface normal were obtained. On combining these results with the known relative orientation of the two chromophores in the molecule yields the absolute orientation of C153 at the air/water interface. It was found that the -CF axis projected towards the bulk air at an angle of 59° with respect to the interface normal and the -C=O group projected towards the bulk water at an angle of 144°. In order to observe the rotational motions of C153 at the air/water interface, the approach was used to perturb the ground electronic state equilibrium orientational distribution using a polarized resonant pump pulse, which preferentially excites ground state molecules that have their electronic S → S transition moment aligned closely to the electric field of the incident pump pulse. As a consequence of the photoselection perturbation, the orientational distribution of the remaining ground state molecules was not the equilibrium distribution. Similarly, the orientational distribution of the excited state molecules that were created by the polarized pump pulse was not in their final equilibrium orientational distribution. The rotational motions of the interfacial molecules towards equilibrium were obtained from time dependent measurements of the intensities of the SFG signal generated by the simultaneous incidence at the air/water interface of a visible probe pulse plus an IR probe pulse. In this way, the recovery times to achieve the orientational equilibrium of the two chromophores including the orientation of the normal of the C153 plane with respect to the interface were obtained. The photo-selection process shifts the average orientation angle of the hydrophilic -C=O group by an increase of 4° ± 0.6° with a rotational recovery time constant of 130 ± 20 ps, which is the time to return to an orientational equilibrium distribution. The hydrophobic -CF group undergoes a shift that increases its angle by 8° ± 1.5° with a rotational recovery time constant of 210 ± 38 ps. We find that the orientational change of the molecular normal is 4° ± 0.5° and has a rotational recovery time constant of 125 ± 26 ps. The interface-specific time-dependent polarized measurements allowed us to monitor the orientational motions of molecules at interfaces, both in 3 dimensions and in real time.
本文首次使用飞秒时间分辨和频发生（SFG）技术研究了在空气/水界面处刚性分子在三维空间中的旋转。为此目的，选择芳香染料分子 C153 作为具有两个 SFG 活性官能团的分子的示例，一个是亲水性 -C=O 基团，另一个是疏水性 -CF 基团。通过偏振 SFG 测量，获得了两个发色团相对于表面法线的取向。将这些结果与分子中两个发色团的已知相对取向相结合，得出了 C153 在空气/水界面处的绝对取向。结果发现，-CF 轴相对于界面法线以 59°的角度投影到大气中，而 -C=O 基团以 144°的角度投影到水中。为了观察 C153 在空气/水界面上的旋转运动，采用了一种方法来通过偏振共振泵浦脉冲来扰动基态电子态平衡取向分布，该方法优先激发具有电子 S → S 跃迁矩与入射泵浦脉冲电场紧密对齐的基态分子。由于光选择的扰动，剩余基态分子的取向分布不再是平衡分布。同样，由偏振泵浦脉冲产生的激发态分子的取向分布也不在它们的最终平衡取向分布中。通过同时在空气/水界面上入射可见光探测脉冲和红外探测脉冲来测量 SFG 信号强度的时间依赖性，可以获得分子向平衡的旋转运动。通过这种方式，获得了两个发色团（包括 C153 平面相对于界面的法线取向）达到取向平衡的恢复时间。光选择过程使亲水性 -C=O 基团的平均取向角度增加了 4°±0.6°，旋转恢复时间常数为 130±20 ps，这是回到取向平衡分布的时间。疏水性 -CF 基团的取向角度增加了 8°±1.5°，旋转恢复时间常数为 210±38 ps。我们发现分子法线的取向变化为 4°±0.5°，旋转恢复时间常数为 125±26 ps。界面特定的时变偏振测量使我们能够在三维空间和实时监测界面处分子的取向运动。
