Dynamics of Molecular Rotors Confined in Two Dimensions: Transition from a 2D Rotational Glass to a 2D Rotational Fluid in a Periodic Mesoporous Organosilica

Abstract

The motional behavior of p-phenylene-d(4) rotators confined within the 2D layers of a hierarchically ordered periodic mesoporous p-divinylbenzenesilica has been elucidated to evaluate the effects of reduced dimensionality on the engineered dynamics of artificial molecular machines. The hybrid mesoporous material, characterized by a honeycomb lattice structure, has arrays of alternating p-divinylbenzene rotors and siloxane layers forming the molecularly ordered walls of the mesoscopic channels. The p-divinylbenzene rotors are strongly anchored between two adjacent siloxane sheets, so that the p-phenylene rotators are unable to experience translational diffusion and are allowed to rotate about only one fixed axis. Variable-temperature (2)H NMR experiments revealed that the p-phenylene rotators undergo an exchange process between sites related by 180° and a non-Arrhenius temperature dependence of the dynamics, with reorientational rates ranging from 10(3) to 10(8) Hz between 215 to 305 K. The regime of motion changes rapidly at about 280 K indicating the occurrence of a dynamical transition. The transition was also recognized by a steep change in the heat capacity at constant pressure. As a result of the robust lamellar architecture comprising the pore walls, the orientational dynamic disorder related to the phase transition is only realized in two dimensions within the layers, that is in the plane perpendicular to the channel axis. Thus, the aligned rotors that form the organic layers exhibit unique anisotropic dynamical properties as a result of the architecture's reduced dimensionality. The dynamical disorder restricted to two dimensions constitutes a highly mobile fluidlike rotational phase at room temperature, which upon cooling undergoes a transition to a more rigid glasslike phase. Activation energies of 5.9 and 9.5 kcal/mol respectively have been measured for the two dynamical regimes of rotation. Collectively, our investigation has led to the discovery of an orientationally disordered 2D rotational glass and its transition from rigid to soft at increasing temperature. The spectral narrowing observed in the (2)H NMR experiments at higher temperatures (310-420 K) is consistent with fast rotational dynamics, which remain anisotropic in nature within the robust lamellar architecture. This study suggests that exploiting reduced dimensionality in the design of solid-state artificial molecular machines and functional materials may yield access to behavior previously unrealized in 3D materials.

Crystal Fluidity Reflected by Fast Rotational Motion at the Core, Branches, and Peripheral Aromatic Groups of a Dendrimeric Molecular Rotor

“…For example, when exploring the temperature-dependence of the rotational dynamics of a p -phenylene group in a periodic mesoporous organosilicate (PMO), we obtained non-linear data within a temperature region indicating an apparent activation energy of 47 kcal/mol and an apparent pre-exponential value of 4.0x10 41 s −1 , both of which are, of course, nonsensical. 25 Further analysis based on differential scanning calorimetry (DSC) supported a reasonable interpretation for the observed results. It was shown that the steep slope of the rotational frequency vs. inverse temperature in the corresponding Arrhenius plot was the result of the structural softening that occurs when the 2D rigid glass becomes a 2D rotational fluid, during a second order glass transition.…”

“…24 Numerous previous studies with crystalline phenylene rotors have also revealed pre-exponential values of this magnitude, with exception encountered when non-crystalline samples are analyzed. 25 We have previously suggested that abnormally high pre-exponential factors in the solid state can be associated with changes in the fluidity of the structure, rather than with a static potential of the corresponding motions. For example, when exploring the temperature-dependence of the rotational dynamics of a p -phenylene group in a periodic mesoporous organosilicate (PMO), we obtained non-linear data within a temperature region indicating an apparent activation energy of 47 kcal/mol and an apparent pre-exponential value of 4.0x10 41 s −1 , both of which are, of course, nonsensical.…”

Phenyl Ring Dynamics in a Tetraphenylethylene-Bridged Metal–Organic Framework: Implications for the Mechanism of Aggregation-Induced Emission

“…The pre-exponential factor, which can be interpreted as the barrier-less flipping rate of the pure phe-nylene bridge, 28,29 is 2.2×10 11 Hz and is somewhat smaller than those of phenylene bridges in related porous materials such as MOF-5 30,31 and periodically-ordered mesoporous organosili-ca. 22 It is conceivable, however, that intramolecular steric effects converge to decrease the pre-exponential factor in TPE derivatives relative to phenylene itself.…”

Ultra‐Fast Molecular Rotors within Porous Organic Cages

“…Moreover, while 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 larger than in any exclusively organic systems reported previously below ≈200 K (Table S3) 18, 19, 24, 29, 32, 37, 38, 39. In particular, below this temperature, the dynamics of [D 12 ]TCC3‐ R are faster than the ones observed very recently for the para ‐phenylene reorientation in bis(sulfophenylethynyl)‐benzene frameworks based on an overall similar architecture of a phenylene molecular rotor sandwiched between two acetylene moieties (Figure 1 (b)), which previously showed the largest reorientation rate for porous organic materials to date 32…”

“…The 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  Hz22 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, 28, 29, 32, 36, 37, 40, 41. The associated change in entropy (Δ S ) is negative and is tentatively assigned to correlated rotational motion (Table S2) 41, 42…”