Responsive guest encapsulation of dynamic conjugated microporous polymers

ABSTRACT 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 host-guest 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. SIMILAR CONTENT BEING VIEWED BY OTHERS THE SIMULTANEOUS ROLE OF PORPHYRINS’ H- AND J- AGGREGATES AND HOST–GUEST CHEMISTRY ON THE FABRICATION OF

REVERSIBLE DEXTRAN-PMMA POLYMERSOME Article Open access 02 February 2021 A MICROPOROUS POLYMER BASED ON NONCONJUGATED HINDERED BIPHENYLS THAT EMITS BLUE LIGHT Article Open access 28 June

2024 DESIGNING POLYMERSOMES WITH SURFACE-INTEGRATED NANOPARTICLES THROUGH HIERARCHICAL PHASE SEPARATION Article Open access 11 March 2025 INTRODUCTION The guest encapsulation behavior of

conjugated microporous organic polymers as a host have attracted a lot of attention by researchers all over the world1,2,3,4,5. 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-methyl-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/6-31G(d)11,12,13 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 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 method17,18,19 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 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 after guest binding. The host guest interaction is mainly noncovalent π-π interaction. Secondly, we studied NR encapsulation in PyPP framework. Figure 3b

shows various orientations of NR residing in the host framework. In Fig. 3b 1 ,b 2 , NR molecule resides in the open pore and is orthogonal to the pyrene plane, with different orientations.

In Fig. 3b 3 , NR molecule is parallel to the pyrene plane. The binding energies for NR with host were computed to be −26.1, −44.8 and −40.5 kcal/mol for b1, b2, and b3 orientations

respectively. The negative value of binding energies indicates that binding process of NR is energetically favorable. The main host-guest interaction is π-π stacking with different

orientations. Lastly, the host-guest interaction between PyPP and DMDP was explored. Figure 3c shows three different configurations for this interaction. Figure 3c 1 indicates that DMDP

molecule is parallel to the pyrene plane. In Fig. 3c 2 , DMDP resides in the cavity formed from packing oligomers. Figure 3c 3 shows that DMDP is inside the open pore, on the edge of the

oligomer. The binding energies for DMDP were calculated to be −47.7 kcal/mol, −39.6 kcal/mol, and −46.1 kcal/mol for c1, c2, and c3 respectively. The negative value also indicates that

binding of DMDP molecule to the host framework is energetically favorable. The main host-guest interaction is dipolar interaction between DMDP and aromatic rings in the host framework3.

CONTROL SIMULATIONS FOR HOST-GUEST INTERACTION OF PDVB Figure 4 shows the host-guest interaction for PDVB oligomers. We packed three PDVB oligomers and one guest molecule C60, NR, and DMDP

respectively. We used the same computational methodologies as used for PyPP oligomers. Interestingly, the binding energy of PDVB is much smaller than that of PyPP, indicating that the host

guest interaction for PDVB is weaker. For C60 molecule, it mainly stays inside the cavity formed by packing multiple oligomers, such as a1 and a3. C60 could also stay on top of the framework

such as a2. Next, host-guest interaction for NR and DMDP were also explored. The simulations also show that NR and DMDP reside in the cavity formed from packing oligomers such as b3 and c3.

They could also be parallel to the surface of the framework, such as b1 and c2. Interestingly, we also found configurations where NR or DMDP is inside the closed pore of the PDVB framework.

However, the binding is not energetically favorable in this case, since the binding energies were calculated to be positive (see green values in b2 and c1). GUEST AGGREGATES ENCAPSULATION

AND RESPONSIVE PORE ACCOMMODATION Rao _et al._ mentioned that when the loading of guest molecules is increased, they detected the existence of aggregate guest molecules3. Here, we

investigated the detailed guest aggregation picture computationally. We used Amorphous Cell module to construct two PyPP oligomers and 15 guest molecules C60, NR, and DMDP. Similarly,

multiple configurations were obtained by Configurational bias Monte Carlo method17,18,19 with periodic boundary conditions. Dreiding15 force field in Forcite module was used to optimize the

representative configuration geometries. Figure 5 shows the representative encapsulation results for C60, NR, and DMDP guest aggregates. The right column of Fig. 5 shows the comparison

between the pore size of PyPP oligomer without guest encapsulation, and PyPP oligomer with guest encapsulation. First of all, fullerene aggregates encapsulation was explored. Figure 5a

indicates that C60 aggregates assembled on top of pyrene arrays. This matches previous experiment well2,21, which showed that the porous PyPP framework provides an assembled array of

fullerenes. After near-surface spots are occupied, more C60 molecules will reside layer-by-layer with respect to the pyrene plane. From the comparison between PyPP oligomer pore and C60

guest-encapsulated pores in red, the width of three open pores changes. One increases from 8.6 Å to 9.6 Å, the second decreases from 7.7 Å to 6.7 Å. The third open pore decreases from 11.2 Å

to 10.8 Å. This is because the aromatic arms of the open pore will adjust its size in order to enhance the host-guest π-π interaction with C60 molecule. Then the host-guest interaction

between PyPP and NR aggregates was investigated. Figure 5b shows NR aggregates inside the cavities. NR molecules reside in the open pore of the oligomer. By comparing PyPP oligomer pore and

three pores in red, the width of open pore on the left becomes larger from 7.7 Å to 7.8 Å. For the open pore encapsulating two NR molecules on the right, the width of the pore increases from

8.6 Å to 9.4 Å. This clearly demonstrates the dynamic behavior of oligomer backbones. The pore expands in order to accommodate two large-sized NR molecules. Lastly the encapsulation of DMDP

aggregates was studied. Figure 5c shows DMDP aggregates inside the cavities. The width of the open pore has increases from 8.6 Å to 10.2 Å. Since the aggregates with multiple DMDP molecules

needs larger space, the open pore will provide larger cavity to encapsulate the aggregates. The open pore will also adjust its shape to accommodate to the shape of aggregates. At the same

time, because of the expansion of the open pore, the connected pore shrinks (width decreases from 7.7 Å to 6.9 Å). This indicates the dynamic behavior of the PyPP backbone. In terms of

aggregate fashion of PyPP-C60, PyPP-NR, and PyPP-DMDP complexes, it follows π-π stacking and C-H-π interaction. DISCUSSION When the guest orientation changes within the host framework, the

host-guest interaction will also change. Figure 3a indicates different binding energies with different orientations for the PyPP-C60 system. When C60 molecule is inside the open pore of the

system, the host-guest interaction is strong, such as 3a1. When C60 molecule is inside the cavity formed from packing oligomers, depending on the packing density, the host-guest interaction

varies. If the oligomers are densely packed, the host-guest binding is strong, such as 3a3. However, if cavity formed from packing is large, the host-guest interaction will be weaker, such

as 3a2. NR molecule interacts with host framework with various orientations. NR could be parallel with the framework, or inside the open pore. When the NR molecule is inside the open pore,

depending on the orientation of NR, the binding energy varies. If the interaction occurs between only a portion of NR molecule and the host framework, binding is weak, such as 3b1. However,

if the contact area between NR and host framework is large, leading to larger host-guest interaction, such as 3b2. When NR molecule is parallel with the host plane, such as Fig. 3b 3 , the

contact area between host and guest is large, therefore the host-guest interaction is strong. In terms of the interaction between DMDP and PyPP framework, it has several different types as

well. In Fig. 3c 1 , DMDP molecule plane is parallel to the host plane. In this case, the interaction between DMDP and the host plane is thorough, thus the binding energy is also large. On

the other hand, in Fig. 3c 2 , DMDP resides in the cavity formed by packing oligomers, and DMDP molecule is not parallel to the host plane. The binding is not that strong in this case. When

DMDP molecule is inside the open pore of the host framework (such as 3c3), the binding energy is also large, indicating that the interaction between DMDP and edge of the polymer is very

strong. From the trend of Fig. 3 discussed above, we get the conclusion that the interaction between guest molecule and host framework could be attributed to several reasons. First is the

compactness of the host framework. Second is the contact area between host framework and guest molecule, depending on the distance between host and guest molecule, and the orientation of

guest molecule. Third is that interaction between guest molecule and open pore of the host framework is very strong. Besides conjugated polymer PyPP, we also performed control simulations of

host guest interaction for PDVB, which is non-conjugated polymer. As shown in Fig. 4, the host-guest binding of non-conjugated polymer is not as strong as conjugated polymer. We proposed

that the difference depends on whether it is conjugated polymer. For non-conjugated polymer PDVB, the oligomer is formed by crosslinking and the structure is three dimensional. The multiple

phenylene rings are not at the same plane and separate from each other (see Fig. 2). When guest interacts with the non-conjugated polymer, the interaction is between aromatic rings of guest

and separate phenylene rings in PDVB. On the other hand, for the conjugated PyPP system, the π system is conjugated and extended structure (see Fig. 2), therefore the host-guest interaction

is between guest aromatic groups with a large conjugated and extended π system. Thus the interaction between the host and guest molecule is greatly enhanced. We also found that the binding

process of guest molecule inside the closed pore of PDVB is not energetically favorable. It takes a high energy barrier (26.8 kcal/mol for NR and 13.0 kcal/mol for DMDP), for the guest

molecule to go inside the closed pore of PDVB. Instead, guest molecule tends to stay in the cavity formed by packing PDVB oligomers. For the pore size change after the guest aggregate as

shown in Fig. 5, we find that the dynamic behavior of flexible host backbone of conjugated polymer is the key to accommodate the inclusion of guest aggregates. Firstly, if the guest is

relatively small, the pore will shrink to catch the guest. For example, in Fig. 5a, the top two C60 molecule was inside the open pore, and the open pore will close the arm in order to catch

C60. The open pore shrinks in order to catch guest molecule and keep it locked within the pore. Secondly, if the size of the guest aggregates is larger than the pore size, the pore will

expand in order to encapsulate the aggregates, such as the right pore of Fig. 5b, and the right pore in Fig. 5c. Lastly, when the pore expands (right pore in Fig. 5c), the neighboring pore

(left pore in Fig. 5c) will shrink accordingly. This is also due to the flexibility of the backbone. From the simulations, we discovered that there are two types of cavities within the PyPP

host framework. Figure 6 shows these two pore types. The first type is the internal open pore formed between pyrene and phenylene groups. The second type is cavity formed from packing

different PyPP oligomers. For the open pore, there are also three types: the pore labeled in red, black and green respectively. The red pore has diameter of 8.6 Å, black pore has diameter of

7.7 Å, and green pore has diameter of 11.2 Å. For the cavity formed from packing oligomers, it could also be divided into two types, one is that guest molecule is parallel to the framework

surface, the other is that guest molecule is deep inside the cavity from packing oligomers. In conclusion, we explored guest encapsulation for conjugated microporous polymers by theoretical

calculations for the first time, discovered different orientations of guest molecules inside the host framework, and confirmed the responsive dynamic behaviour of the polymer backbone by

simulations. We also performed control simulations for non-conjugated polymers. The host guest interaction for conjugated polymers is stronger than that of non-conjugated system. This work

provides detailed picture of guest encapsulation and could be helpful guidance for future design of dynamic porous materials. For example, future design could focus on new conjugated

microporous polymers with pore size appropriate to the size of guest. We found two types of pore and get new insights of open pore, and pore formed from packing oligomers. In order to

increase the host-guest interaction, we could adjust the monomer size, or adjust the compactness of the backbone, and ultimately increase the interaction area between the host and guest

molecule or guest aggregates. METHODS The structure of PyPP monomer, C60, NR, DMDP and PDVB monomer, were optimized by using DFT method B3LYP/6-31G(d)11,12,13 in Gaussian0914. The PyPP

oligomers was geometry optimized by Dreiding15 force field in Forcite module of Material Studio 7.016. The guest encapsulation inside PyPP host framework, was studied by Amorphous Cell

module of Material Studio 7.016. PyPP oligomers with guest molecule or guest aggregates of C60, NR, and DMDP were loaded respectively. Multiple stable configurations were generated from

Configurational Bias Monte Carlo method17,18,19 with periodic boundary conditions, and 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. The host guest interaction between PDVB host framework and guest molecule C60,

NR, and DMDP, was also studied by the same methods mentioned above. ADDITIONAL INFORMATION HOW TO CITE THIS ARTICLE: Xu, L. and Li, Y. Responsive Guest Encapsulation of Dynamic Conjugated

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Startup Funding (Grant No. Q421400215) from Institute of Functional Nano & Soft Materials (FUNSOM) in Soochow University. This project is also funded by the Collaborative Innovation

Center of Suzhou Nano Science & Technology (Grant No. SX21400113, Grant No. 21146104), and by the Priority Academic Program Development of Jiangsu Higher Education Institutions. AUTHOR

INFORMATION AUTHORS AND AFFILIATIONS * Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow

University, 199 Ren’ai Road, Suzhou, 215123, Jiangsu, PR China Lai Xu & Youyong Li Authors * Lai Xu View author publications You can also search for this author inPubMed Google Scholar *

Youyong Li View author publications You can also search for this author inPubMed Google Scholar CONTRIBUTIONS L.X. designed the project, performed the simulations, and analyzed the results.

Y.L. provided the computational facilities. All authors reviewed the manuscript. CORRESPONDING AUTHOR Correspondence to Lai Xu. ETHICS DECLARATIONS COMPETING INTERESTS The authors declare

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Polymers. _Sci Rep_ 6, 28784 (2016). https://doi.org/10.1038/srep28784 Download citation * Received: 24 March 2016 * Accepted: 08 June 2016 * Published: 30 June 2016 * DOI:

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