Responsive Guest Encapsulation of Dynamic Conjugated Microporous Polymers

www.nature.com/scientificreports OPEN received: 24 March 2016 accepted: 08 June 2016 Published: 30 June 2016 Responsive Guest Encapsulation of Dynamic Conjugated Microporous Polymers Lai Xu & Youyong Li The host-guest complexes of conjugated microporous polymers encapsulating C60 and dye molecules have been investigated systematically. The orientation of guest molecules inside the cavities, have different terms: inside the open cavities of the polymer, or inside the cavities formed by packing different polymers. The host backbone shows responsive dynamic behavior in order to accommodate the size and shape of incoming guest molecule or guest aggregates. Simulations show that the hostguest binding of conjugated polymers is stronger than that of non-conjugated polymers. This detailed study could provide a clear picture for the host-guest interaction for dynamic conjugated microporous polymers. The mechanism obtained could guide designing new conjugated microporous polymers. The guest encapsulation behavior of conjugated microporous organic polymers as a host have attracted a lot of attention by researchers all over the world1–5. These polymers have wide applications such as luminescence6,7, sensing8,9, and photocatalysis10. Now scientists could make it possible to achieve easy and fast encapsulation of guest molecules under ambient conditions. This is because of their flexible backbones. They have amorphous three-dimensional organic framework, which provides noncovalent confinement for guest molecules. Therefore, it is easy to tune host-guest composition without changing the polymer structure itself1. Rao et al. reported guest-responsive reversible swelling in a dynamic microporous polymer network poly-tetraphenyl pyrene (Py-PP). It encapsulates C60, dye molecules red-emitting 4-(dicyanomethylene)-2-met hyl-6-(4-dimethylaminostyryl)-4H-pyran (DMDP) and Nile red (NR) dyes at room temperature2,3. Although there are extensive experimental studies in host-guest interaction for conjugated organic frameworks, there is no computational investigation on the detailed and dynamic picture of guest encapsulation inside the porous structures. Here, we have investigated the orientation of guest molecules inside host frameworks and responsive dynamic behaviour of host backbone computationally for the first time. Our simulation results match experiments and predict new insights to the detailed mechanism of guest encapsulation. Based on the mechanism, we proposed several ways to design new materials. We also performed control simulations for host-guest interactions of non-conjugated system polydivinylbenzene (PDVB). Results Construction of Building Units. We chose PyPP as host materials to encapsulate C60, dye molecule DMDP, and dye molecule NR. We first obtained stable structure of building units of the host-guest system. Here, we used density functional theory to optimize the geometry of building units. Figure 1 shows monomer of PyPP, as well as guest molecules C60, NR, DMDP, and monomer divinylbenzene (DVB) for control simulations. The right column shows the optimized geometry of PyPP monomer, C60, NR, DMDP and DVB, by using DFT method B3LYP/631G(d)11–13 in Gaussian0914. The detailed Gaussian reference and structure information could be found in the supplementary information. Based on the PyPP monomer, we constructed PyPP oligomers and PDVB oligomer for next-step construction of host-guest complex. Here, we used Dreiding15 force field to optimize the oligomers. Figure 2 shows the optimized structure of PyPP oligomers and PDVB oligomers by Dreiding15 force field in Forcite module of Material Studio 7.016. From the optimized structures, we noticed that the PyPP oligomer is extended structure, with linking phenylene orthogonal to the main extended structure plane. The unique conjugated structure of the backbone will generate interesting phenomena when it encapsulates a guest molecule or guest aggregates. The aromatic interaction could occur near the linker phenylene, or near the pyrene plane. Therefore, this structure provides Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, 199 Ren’ai Road, Suzhou, Jiangsu, 215123, PR China. Correspondence and requests for materials should be addressed to L.X. (email: ) Scientific Reports | 6:28784 | DOI: 10.1038/srep28784 1 www.nature.com/scientificreports/ Figure 1. Host polymer monomer of PyPP, monomer of PDVB and guest molecules C60, NR, DMDP, and DVB. Structures on the right are optimized structure by B3LYP/6-31G(d). various possible interaction sites when guests are encapsulated. For PDVB oligomer, since crosslinking behavior of divinylbenzene due to its double bonds, we constructed the oligomer with several monomers crosslinked together. From the structure after geometry optimization, we could see that it is three dimensional structure without conjugated structure. Thus PDVB oligomer is constructed to provide control simulation results for conjugated polymer system PyPP. Single Guest Molecule Encapsulation. In order to study the molecule orientation of single guest molecule inside host framework, three PyPP oligomers with one guest molecule C60, NR, and DMDP were loaded respectively within Amorphous Cell module in Materials Studio 7.016. Fifteen stable configurations were generated from Configurational Bias Monte Carlo method17–19 with periodic boundary conditions, and three representative configurations were selected for PyPP-C60, PyPP-NR, and PyPP-DMDP complex respectively. Then geometry optimization was performed in Forcite module in Materials Studio 7.016 by Dreiding15 force field. We Scientific Reports | 6:28784 | DOI: 10.1038/srep28784 2 www.nature.com/scientificreports/ Figure 2. Optimized structure of oligomer of PyPP and PDVB. also calculated binding energy of these complex structures. Binding energy was calculated based on the formula ∆E =  E(complex) − E(host) − E(guest). The three representative configurations with calculated binding energies of PyPP-C60 complex, PyPP-NR complex, and PyPP-DMDP complex are shown in Fig. 3. We chose large system (3 packed oligomers) as our host to calculate binding energies here. The reason is that previous study showed that the calculation of CO2 binding energy based on small molecule fragment is not accurate, because it does not include the entire framework20. Firstly, the encapsulation of C60 was investigated. Figure 3a shows C60 encapsulated in the host framework. It indicates that C60 could stay inside the open pore of PyPP (a1), or stays within the cavities formed from packed oligomers (a2 and a3). The binding energies were computed to be −59.7, −40.2 and −57.2 kcal/mol respectively for a1, a2, and a3 orientations. Since the binding energy is calculated from the equation ∆E =  E(complex) − E(host) − E(guest), the negative value of binding energy means that the system is stabilized af (...truncated)