椅式(8,8)单壁碳纳米管内偶氮苯的顺反异构化

cis-trans Isomerization of Azobenzene Confined inside an Armchair(8,8) Single-Walled Carbon Nanotube

采用ONIOM(B3LYP/6-31+G*:UFF)方法对偶氮苯受限于椅式单壁碳纳米管CNT(8,8)内的结构、电子光谱和热致顺反异构化势能面进行了计算.结果表明,反式偶氮苯进入CNT(8,8)碳纳米管内是一个放热过程.由于碳纳米管的限制作用,偶氮苯分子的苯环绕CN键有一定的旋转,反式偶氮苯的平面结构发生扭曲,但其它结构参数变化不明显.受限于碳纳米管内的偶氮苯的顺反异构体能量差比非受限状态下增加了8.1 kJ.mol-1,表明碳纳米管的限制作用对偶氮苯顺反异构体的相对热稳定性有一定影响.光谱计算表明,受限于碳纳米管内的偶氮苯最低的三个单线态吸收波长仅蓝移1-5 nm.异构化势能面计算发现,偶氮苯在CNT(8,8)碳纳米管内发生反式到顺式异构化的能垒增加,由顺式回复到反式异构体的能垒无明显变化,表明CNT(8,8)碳纳米管的限制作用可以抑制偶氮苯从反式到顺式异构体的热致异构化过程.此外,受限状态下的偶氮苯主要通过CNN键角反转发生热致异构化.

Molecular structures,electronic spectra,and the thermal trans-cis isomerization of azobezene（AB） confined inside an armchair（8,8） single-walled carbon nanotube（CNT（8,8）） were calculated at the ONIOM（B3LYP/6-31＋G＊：UFF） level.We found that the entrance of azobenzene into CNT（8,8） was exothermic.The geometric parameters of azobenzene were not evidently affected by its confinement in CNT（8,8）.Rotation of the phenyl rings was found to occur around the CN bond to some extent for both trans-and cis-azobenzene,which resulted in a twist configuration for the confined trans-azobenzene.The relative energy between the cis-and trans-azobenzene confined inside the CNT（8,8） increased by ca 8.1 kJ.mol-1 with respect to the case of the isolated azobenzene,suggesting that the relative thermal stability of the isomers of azobenzene is affected by their confinement in CNT（8,8）.Electronic spectrum calculations showed that the lowest three singlet excitation energies of the confined azobenzene were blue-shifted by 1-5 nm in comparison to those of isolated azobenzene.By analyzing the potential energy surfaces,we found that the confinement in CNT（8,8） resulted in the increase in the activation barrier of the trans-to-cis isomerization of azobenzene but had little influence on the barrier of cis-to-trans backward reaction,which means that the confinement in CNT（8,8） restrained the trans-to-cis isomerization of azobenzene.The trans-cis isomerization process of the confined azobenzene mainly involved the bending of the CNN bond angle.