Guest recognition enhanced by lateral interactions

Tianyu Jiao; Kang Cai; Zhichang Liu; Guangcheng Wu; Libo Shen; Chuyang Cheng; Yuanning Feng; Charlotte L Stern; J Fraser Stoddart; Hao Li

doi:10.1039/c9sc00591a

Guest recognition enhanced by lateral interactions

Chem Sci. 2019 Apr 23;10(19):5114-5123. doi: 10.1039/c9sc00591a. eCollection 2019 May 21.

Authors

Tianyu Jiao^{1

2}, Kang Cai², Zhichang Liu³, Guangcheng Wu¹, Libo Shen¹, Chuyang Cheng², Yuanning Feng², Charlotte L Stern², J Fraser Stoddart^{2

4

5}, Hao Li¹

Affiliations

¹ Department of Chemistry , Zhejiang University , Hangzhou 310027 , P. R. China . Email: lihao2015@zju.edu.cn.
² Department of Chemistry , Northwestern University , 2145 Sheridan Road , Evanston , Illinois 60208 , USA . Email: stoddart@northwestern.edu.
³ School of Science , Westlake University , 18 Shilongshan Road , Hangzhou 310024 , P. R. China.
⁴ Institute for Molecular Design and Synthesis , Tianjin University , Tianjin 300072 , P. R. China.
⁵ School of Chemistry , University of New South Wales , Sydney , NSW 2052 , Australia.

Abstract

A hexacationic triangular covalent organic cage, AzaEx²Cage ⁶⁺, has been synthesized by means of a tetrabutylammonium iodide-catalyzed S_N2 reaction. The prismatic cage is composed of two triangular 2,4,6-triphenyl-1,3,5-triazine (TPT) platforms bridged face-to-face by three 4,4'-bipyridinium (BIPY ²⁺) spacers. The rigidity of these building blocks leads to a shape-persistent cage cavity with an inter-platform distance of approximately 11.0 Å. This distance allows the cage to accommodate two aromatic guests, each of which is able to undergo π-π interactions with one of the two TPT platform simultaneously, in an A-D-D-A manner. In the previously reported prism-shaped cage, the spacers (pillars) are often considered passive or non-interactive. In the current system, the three BIPY ²⁺ spacers are observed to play an important role in guest recognition. Firstly, the BIPY ²⁺ spacers are able to interact with the carbonyl group in a pyrene-1-carbaldehyde (PCA) guest, by introducing lateral dipole-cation or dipole-dipole interactions. As a consequence, the binding affinity of the cage towards the PCA guest is significantly larger than that of pyrene as the guest, even although the latter is often considered to be a better π-electron donor. Secondly, in the case of the guest 1,5-bis[2-(2-(2-(2-hydroxyethoxy)ethoxy)ethoxy)ethoxy]naphthalene (BH4EN), the pillars can provide higher binding forces compared to the TPT platform. Hence, peripheral complexation occurs when AzaEx²Cage ⁶⁺ accommodates BH4EN in MeCN. Thirdly, when both PCA and BH4EN are added into a solution of AzaEx²Cage ⁶⁺, inclusion and peripheral complexation occur simultaneously to PCA and BH4EN respectively, even though the accommodation of the former guest seems to attenuate the external binding of the latter. This discovery of the importance of lateral interactions highlights the relationship between the electrostatic properties of a highly charged host and its complexation behavior, and as such, provides insight into the design of more complex hosts that bind guests in multiple locations and modes.