Peeks Martin D, Neuhaus Patrik, Anderson Harry L
Department of Chemistry, University of Oxford, Chemistry Research Laboratory, Oxford, OX1 3TA, UK.
Phys Chem Chem Phys. 2016 Feb 21;18(7):5264-74. doi: 10.1039/c5cp06167a.
The barrier to torsional rotation in a butadiyne-linked porphyrin dimer has been determined in solution using variable temperature UV-vis-NIR spectroscopy: ΔH = 5.27 ± 0.03 kJ mol(-1), ΔS = 10.69 ± 0.14 J K(-1) mol(-1). The value of ΔH agrees well with theoretical predictions. Quantum chemical calculations (DFT) were used to predict the torsion angle dependence of the absorption spectrum, and to calculate the vibronic fine structure of the S0 → S1 absorption for the planar dimer, showing that the absorption band of the planar conformer has a vibronic component overlapping with the 〈0|0〉 absorption of the perpendicular conformer. The torsion barrier in the porphyrin dimer is higher than that of 1,4-diphenylbutadiyne (calculated ΔH = 1.1 kJ mol(-1)). Crystallographic bond lengths and IR vibrational frequencies confirm that there is a greater contribution of the cumulenic resonance form in butadiyne-linked porphyrin dimers than in 1,4-diphenylbutadiyne. The DFT frontier orbitals of the twisted conformer of the porphyrin dimer are helical, when calculated in the absence of symmetry. The helical character of these orbitals disappears when D2d symmetry is enforced in the 90° twisted conformer. Helical representations of the frontier orbitals can be generated by linear combinations of the more localised orbitals from a symmetry-constrained calculation but they do not indicate π-conjugation. This work provides insights into the relationship between electronic structure and conformation in alkyne-linked conjugated oligomers.
利用变温紫外-可见-近红外光谱法在溶液中测定了丁二炔连接的卟啉二聚体中扭转旋转的势垒：ΔH = 5.27 ± 0.03 kJ mol⁻¹，ΔS = 10.69 ± 0.14 J K⁻¹ mol⁻¹。ΔH值与理论预测结果吻合良好。采用量子化学计算（DFT）预测吸收光谱的扭转角依赖性，并计算平面二聚体S0 → S1吸收的振动电子精细结构，结果表明平面构象体的吸收带具有与垂直构象体〈0|0〉吸收重叠的振动电子成分。卟啉二聚体中的扭转势垒高于1,4 - 二苯基丁二炔的扭转势垒（计算得到的ΔH = 1.1 kJ mol⁻¹）。晶体学键长和红外振动频率证实，与1,4 - 二苯基丁二炔相比，丁二炔连接的卟啉二聚体中累积烯共振形式的贡献更大。在没有对称性的情况下进行计算时，卟啉二聚体扭曲构象体的DFT前沿轨道呈螺旋状。当在90°扭曲构象体中强制实施D2d对称性时，这些轨道的螺旋特征消失。前沿轨道的螺旋表示可以通过对称约束计算中更局域化轨道的线性组合生成，但它们并不表明存在π共轭。这项工作为炔烃连接的共轭低聚物中电子结构与构象之间的关系提供了见解。
