Ultra‐Fast Molecular Rotors within Porous Organic Cages

Abstract Using variable temperature 2H static NMR spectra and 13C spin‐lattice relaxation times (T1), we show that two different porous organic cages with tubular architectures are ultra‐fast molecular rotors. The central para‐phenylene rings that frame the “windows” to the cage voids display very rapid rotational rates of the order of 1.2–8×106 Hz at 230 K with low activation energy barriers in the 12–18 kJ mol−1 range. These cages act as hosts to iodine guest molecules, which dramatically slows down the rotational rates of the phenylene groups (5–10×104 Hz at 230 K), demonstrating potential use in applications that require molecular capture and release.

Abstract: Using variable temperature 2 Hs tatic NMR spectra and 13 Cs pin-lattice relaxation times (T 1 ), we show that two different porouso rganic cages with tubular architectures are ultra-fast molecular rotors.T he central paraphenylene rings that frame the "windows" to the cage voids display very rapid rotational rates of the order of 1.2-8 10 6 Hz at 230 Kw ith low activation energy barriers in the 12-18 kJ mol À1 range. These cages act as hosts to iodine guest molecules, which dramatically slows down the rotational rates of the phenylene groups (5-10 10 4 Hz at 230 K), demonstrating potential use in applications that requiremolecular capturea nd release.
Porous organic frameworks (POFs) are supramolecular assemblies that have ordered architectures containinga ni nherent void. Examples include metal organic frameworks (MOFs), covalent organic frameworks (COFs), zeolitic imidazolate frameworks (ZIFs), and porous organicc ages (POCs). [1][2][3][4] POCs differ from these other framework families and consist of discrete molecules containing both an intrinsic void within the cages in addition to voids within the overall lattice;b ecause of these properties, POCs are also solution-processable and have been explored for ar ange of applicationsi ncluding catalysis, molecular separation and gas storage. [5][6][7][8][9][10] For example, POCs have been shown to captureg uest molecules such as iodine, SF 6 , and hydrocarbons. [11] The sequestration of iodine, whichi sa n unwanted fission product, [12][13][14] is of strong importance for the nuclear industry. [15] The selective loading and retention of guests such as iodineo ften relieso nm olecularf lexibility and dynamics, and the understanding of these processes playsa central role in the design of the next generation of POFs.
Recently,anew family of POCs with achiral, tubular covalent cage (TCC) architecture has been discovered. [10] These TCCs consist of three "walls" boundb ytrans-imine cyclohexanel inkers. Twoo ft heses tructures (TCC2-R and TCC3-R)a re shown in Figure1;they differ only by the additional acetylenem oieties betweent he phenylene rings in TCC3-R. The intrinsic pore withint hese molecules permits guests orptioni nto the tubular cavities. [4,16] The static iminec yclohexane linkersa nd rotating phenylene groups allow these POCs to be classified as molecular rotors. [17][18][19] It is likely that the window dynamicsi nt hese molecules control guest loading into the molecular pore, and we therefore set out to understand the response of these cages to externals timuli, such as guest inclusion. 2 Hs olid echo NMR has become an extremelyp owerful tool for the understanding of dynamics in the kHz timescale, [20][21][22] which is enabled by the large change in the 2 HNMR line shape with temperature. This approach provides aqualitative description of the motion as well as its rate anda ssociated activation energies in both pristine fast molecular rotors and those hampered by guest loading (e.g.,H 2 O, acetone, iodine, CH 4 and hydrocarbons); [23][24][25][26][27][28][29][30][31][32][33] these include mesoporous para-phenylene silica, [23] polyaromatic frameworks (PAFs), [24] and MOFs. [28,29,31,33] Additionally, 13 CT 1 values provide correlation times in the MHz frequencies and complementary details of the molecular reorientation.H ere, we performed variablet emperature 2 H solid echo NMR experiments and room temperature 13 CT 1 Figure 1. Chemical structure and side view of the X-ray crystal structure of (a) TCC2-R and (b) TCC3-R. [10] The cyclohexane groups are showninred; other C, grey;N,b lue;Homittedf or clarity in the crystals tructure representation. The green arrows indicatefast molecular rotationoft he para-phenylene.T he 13 Cs pin-lattice relaxation times (T 1 )o btainedf or selectedc arbons are given in the Figure,  measurements on pristine and iodine-loaded TCC2-R and TCC3-R materials to understand the rotational dynamics of the cages and their host-guest interactions.
TCC2-R and TCC3-R cages weres ynthesized using literature procedures. [10] Cages deuteratedo nt he para-phenylene rings (i.e., [ D 12 ]TCC2-R and [D 12 ]TCC3-R, Figure 1) were prepared via as imilar approach [10] using [D 4 ]1,4-dibromobenzene-2,3,5,6 and these were used to record the 2 HNMR data (see the Supporting Information for details of all samples preparation and characterisation). In these experiments,t he natural abundance 2 H signals (0.015 %) from the other hydrogens in the cyclohexane linkers and trisubstituted benzene rings are not detected and the 2 HNMR spectra only probe the dynamics of the para-phenylene moieties of the cages. 2 Hs tatic echo NMR spectra in the 105-298 Kt emperature range were recorded on desolvated and iodine-loaded [D 12 ]TCC2-R and [D 12 ]TCC3-R cages ( Figure 2). With decreasing temperature ag radualc hange in the 2 HNMR line shape is observed for these materials, however, the motion induced T 2 anisotropy is dependento nt he particular cage. Line shape simulations of the 2 HNMR spectra support am otion consisting of a rapid two-site 1808 flip reorientation of the para-phenylene ring along its para axis, and this providest he rate of molecular reorientation (k)a te ach temperature on the kHz timescale. [19,34,35] Figure 2(a) and (c) show the variable temperature evolution of the 2 Hs tatic echo NMR spectra of [D 12 ]TCC2-R and [D 12 ]TCC3-R cages. As the temperature decreases from 298 to 105 K, the appearance of the outside horns around AE 60 kHz agreesw ith slower rotation rates at low temperature anda static motional regime at 105 Kw ith the anticipated Pake doublet pattern. The line shape evolution of both cages is analogous, asa nticipatedf or materials with similart ubular covalent architectures. However,t he 2 HNMR spectra of [D 12 ]TCC3-R suggest as ignificantly larger jump rate, as evidenced by the weakening of the spectrals houlderso ft his cage at al ower temperature (230 K) compared to [D 12 ]TCC2-R (261 K), showing slower motion forthe latter.
Rotational rates of around 1.2 10 6 and 8 10 6 Hz were extracted for [D 12 ]TCC2-R and [D 12 ]TCC3-R cages, respectively,a t 230 K. These data suggestt hat the presence of an acetylene group enables easier rotation of the para-phenylene ring in [D 12 ]TCC3-R by reducing the strong steric interactions of the ortho-hydrogensand openingupt he void space (Figure 1). Additionally, it is also possible that small electronic factors play a role. For example, the smaller degree of conjugation between the adjacent phenylene rings in [D 12 ]TCC3-R and [D 12 ]TCC2-R likely contributes to the smaller activation barrier seen for para-phenylene rotation in the former.
Moreover,w hile the [D 12 ]TCC2-R rate is comparable to other organic frameworks, [27][28][29]36] the very fast reorientation rate value obtained for the [D 12 ]TCC3-R is largert han in any exclusively organic systemsr eported previously below % 200 K (Table S3). [18,19,24,29,32,[37][38][39] In particular, below this temperature, ]TCC3-R are faster than the ones observed very recently for the para-phenylener eorientation in bis(sulfophenylethynyl)-benzene frameworks based on an overall similar architecture of ap henylene molecular rotor sandwiched between two acetylene moieties ( Figure 1(b)), which previously showedt he largest reorientation rate for porous organic materials to date. [32] At temperatures highert han 298 K, the 2 HNMR lineshape of the [D 12 ]TCC2-R and [D 12 ]TCC3-R pristinec ages is characteristic of that of the fast motional regime with rates exceeding 10 8 Hz (Figure S5 (a) and (c)). No additional change in lineshape occurs at higher temperature probablyi ndicating an absence of CÀDl ibrational motion and is in sharp contrast to what is observedi nP AFs. [24] This differencel ikely arises from the more flexible nature of the PAFs architecture compared to the relative rigidity of these TCC2-R and TCC3-R cage structures (Figure 1).
Ap lot of ln k as af unctiono fr eciprocal temperature T À1 (Figure 3) shows al inear Arrheniusb ehaviour from whichr otational activation energies, E a ,w ere obtained (Table 1), along with extrapolated rotationalr ates at infinite temperature (at-tempt frequencies), k 0 ,o f( 9AE 4) 10 10 and (10 AE 7) 10 9 Hz for [D 12 ]TCC2-R and [D 12 ]TCC3-R,r espectively.T he larger E a value obtained for [D 12 ]TCC2-R versus [D 12 ]TCC3-R is again consistent with stronger steric interactions in the terphenylene cage structure. The k 0 values obtained are on the low side of the % 10 12 Hz [22] value often associated with para-phenylene rotation, although these values vary significantly with the systems studied and k 0 in the 10 8 -10 41 Hz range are known. [19, 23, 24, 27-29, 32, 36, 37, 40, 41] The associatedc hange in entropy (DS)i sn egative and is tentatively assigned to correlated rotational motion (Table S2). [41,42] Iodine was loaded into [D 12 ]TCC2-R and [D 12 ]TCC3-R using a chemicalv apour sublimation procedure (Supporting Information) to determine if guest addition hampers motional dynamics in theses ystems. [23,24] Upon exposure to iodine at room temperature, the colour of the cages changed from yellow to black,a nd the guest uptake was monitored gravimetrically (Supporting Information). This revealed ah ighl oading of 10 and 12 iodine atoms per TCC2-R and TCC3-R cage molecule, respectively,a fter 40 hours of iodine exposure. The variable temperature 2 Hs tatic echo NMR spectra of iodine-loaded [D 12 ]TCC2-R and[ D 12 ]TCC3-R are given in Figures 2(b)

and (d).
There is ac lear differencei nr otationalr ates of the molecular rotor between the empty and guest-loaded materials.T he less rapid two-site ring flip is particularly evident at 230 Kf or the iodine-loaded materials, reducing the rotational rates to only 10 5 and 5 10 4 Hz for [D 12 ]TCC2-R and [D 12 ]TCC3-R,r espectively (Table 1). This is smaller than the change seen when iodine is loadedi nto other porous materials, [24] which probablyr elates to the different host-guest properties of the materials.
When iodine-loaded TCC2-R and TCC3-R are heated above room temperature, almost complete releaseo fi odine is observed as detected by thermogravimetric analysis( Supporting Information) and visual inspection of the sample;a sar esult, large reorientation rates are obtained in the corresponding 2 H static echo NMR spectra ( Figures S6 and S7) andh ighlight again that these materials are responsive with the rotational dynamics being modulated by the capturea nd releaseo fa guest molecule.
Additionally,A rrhenius plots yield an increaseo fE a with respect to the guest-free cages:f rom 18 to 21 kJ mol À1 for [D 12 ]TCC2-R and from 12 to 21 kJ mol À1 for [D 12 ]TCC3-R (Table 1). These data show that the presence of the iodine guest within the void of the cages, and potentially in extrinsic voids between cages, hampers but does not totally suppress molecular reorientation of the para-phenylene rings. It has been reported that when the motion of ag uest molecule is restrictedw ithin ac avity,f ast librational motions are expected; [43] this is not apparent in theseT CC2-R and TCC3-R cages, since even when ag uest is located inside the cages, the lineshapes remain consistentw ith a1 808 site reorientation of the paraphenylene rings.
Finally, 13 CT 1 values were also monitored due to their strong dependence on molecular motion on the MHz timescale. [43] The considerably shorter room temperature 13 CT 1 values of 1.3-1.5 so btained in TCC2-R and TCC3-R (see Figures 1, S1 and Ta ble S1)f or the CH carbons on the para-phenylene ring  versus the other carbonsi nt he cages (appearing in the range of 4-7 s) suggestsa ne fficient relaxation mechanism and rapid molecular reorientation of the cage windows,s upporting the fast molecular rotors of these cages. The 13 CT 1 relaxation times were found to increase significantly upon loading (Figure 1, Ta ble S1)a nd is consistent with the change in 2 Hs olid echo NMR line shape is noted above where guest addition into the central void slows reorientation of the phenylene rings within the cage structures.
To conclude, we employed 2 Hs olid echo NMR and determined 13 CT 1 values to probe the rotational dynamics of the para-phenylene rings that define access to the central void in two porous chiral tubular covalent cages, TCC2-R and TCC3-R. Using 2 HNMR, we show that TCC2-R cages show reorientation rates that are comparable with the fastest molecular rotors reported foro ther porous frameworks;T CC3-R cages display even faster dynamics (below 200 K), with av ery small activation energyb arrier, which is ascribed to the facile rotation aroundt he acetylene bonds, due to ar eduction in the steric hindrancep resent. Iodine loading slows the phenylener otation considerably,a sf urthers upported by the lengthening of the 13 CT 1 values, while high temperature treatment induces iodine release and reacceleration of the phenylene rings reorientation rates. These data show that the effect on cage dynamics is highly guest dependent,w hich might have important implicationsf or processes such as competitive loading, molecular separation, and drugr elease. These findings also emphasize that models of porosity derived from static single crystal structures might be misleading, but that "time-averaged" models of the pore space could be equally inappropriate because guest inclusionc an switch the rotational dynamics off. This suggests that computational models for loading in such systemsn eed to capture the interplay of guest inclusion and rotational dynamics in the poroushost.