Constructing a mixed π-conjugated bridge: a simple and effective approach to realize a large first hyperpolarizability in carbon nanotube-based systems

Abstract

By means of ab initio calculations, a new concept of the mixed π-conjugated bridge is identified to significantly improve the first hyperpolarizability of a D–π–A system based on carbon nanotubes (CNTs). This mixed π-conjugated bridge is constructed through applying a –(CHCH)_x–NH₂ chain to modify the end of the CNT system, which can effectively increase the degree of charge transfer and decrease the transition energy. This further results in a considerable first hyperpolarizability, much larger than that of the corresponding directly NH₂-modified CNT system with the same conjugated length. In particular, the first hyperpolarizabilities of these tube–chain (CNT–(CHCH)_x–NH₂) systems can exhibit a monotonous increase with lengthening the –(CHCH)_x–NH₂ chain. This overcomes the bottleneck encountered with pure CNT serving as the π-conjugated bridge: that further extending the CNT will not produce any positive effect on improving their first hyperpolarizabilities in CNT–NH₂ configurations once the CNT reaches a certain length. Additionally, it is revealed that the length ratio of tube to chain can play a crucial role in increasing the first hyperpolarizabilities of these tube–chain motifs CNT_l–(CHCH)_x–NH₂, and employing a –(CHCH)_x–NH₂ chain with comparable conjugated size to substitute the end of the CNT (namely, l ≈ x) can be an effective approach to achieve an excellent NLO response in a CNT-based D–π–A framework. These findings can provide valuable information for the design of new high-performance NLO materials based on CNTs.