Synthesis and high-throughput characterization of structural analogues of molecular glassformers: 1,3,5-trisarylbenzenes

Abstract

We report the synthesis and characterization of an analogous series of small organic molecules derived from a well-known glass former, 1,3-bis(1-naphthyl)-5-(2-naphthyl)benzene (α,α,β-TNB). Synthesized molecules include α,α,β-TNB, 3,5-di(naphthalen-1-yl)-1-phenylbenzene (α,α-P), 9-(3,5-di(naphthalen-1-yl)phenyl)anthracene (α,α-A), 9,9'-(5-(naphthalen-2-yl)-1,3-phenylene)dianthracene (β-AA) and 3,3',5,5'-tetra(naphthalen-1-yl)-1,1'-biphenyl (α,α,α,α-TNBP). The design of molecules was based on increasing molecular weight with varied π-π interactions in one or more substituents. The synthesis is based on Suzuki cross-coupling of 1-bromo-3-chloro-5-iodobenzene with arylboronic acids, which allows attachment of various substituents to tailor the chemical structure. The bulk compounds were characterized using NMR spectroscopy and differential scanning calorimetry (DSC). Thin films of these compounds were produced using physical vapor deposition and were subsequently annealed above the glass transition temperatures (Tg). For each molecular glass, cooling rate-dependent glass transition temperature measurements (CR-Tg) were performed using ellipsometry as a high-throughput method to characterize thin film properties. CR-Tg allows rapid characterization of glassy properties, such as Tg, apparent thermal expansion coefficients, apparent activation energy at Tg and fragility. DSC measurements confirmed the general trend that increasing molecular weight leads to increasing melting point (Tm) and Tg. Furthermore, CR-Tg provided evidence that the introduction of stronger π-interacting substituents in the chosen set of structural analogues increases fragility and decreases the ability to form glasses, such that β-AA has the largest fragility and highest tendency to crystallize among all the compounds. These strong interactions also significantly elevate Tg and promote more harmonic intermolecular potentials, as observed by decreasing value of the apparent thermal expansion coefficient.

Glass versus Crystal: A Balancing Act between Competing Intermolecular Interactions

“…Irregular molecular shapes, non-planarity, the presence of flexible groups, and conformational ambiguity have emerged as structural features that promote glass formation, as opposed to crystallization. [4][5][6][8][9] The design of molecular glasses with improved resistance to crystallization has led to their use in a wide range of practical applications, ranging from photon-ics, [10][11][12] opto-electronics [4][5][6] and nanolithography, [13][14] to formulations for amorphous drugs. [15][16] Although strong and directional intermolecular interactions have long been associated with crystallization and are a well-known tool for engineering crystals, the presence of hydrogen (H) bonds does not necessarily prevent compounds from forming glasses, as evidenced by mexylaminotriazine derivatives.…”

Birefringent Stable Glass with Predominantly Isotropic Molecular Orientation

“…Recent studies discovered that physical vapor deposition (PVD) at substrate temperatures just below T g can yield glasses with properties similar to well-aged glasses in laboratory time scales. Compared to liquid-quenched glasses, these stable glasses have higher density (by ∼ 1.5%) [11,12] and lower heat capacity (by ∼ 4%) [13][14][15]. It has been suggested that surface-mediated equilibration (SME) during PVD provides a route to overcome kinetic barriers for rearrangement, and achieve these low-energy states within hours [16][17][18][19][20][21][22] as opposed to millions of years.…”

“…In most molecular systems, it is exceedingly challenging to distinguish the role of orientation vs. layering in the observed birefringence. Here, we design a unique molecule, 9-(3,5-di(naphthalen-1-yl)phenyl)anthracene (α, α-A) (inset of Figure 1(b)) [12,35], to decouple these two effects. The anthracyl substituent introduces steric hindrance to resist rotations around the central benzo ring, allowing α, α-A to retain a nearly isotropic shape.…”

Long-range correlated dynamics in ultra-thin molecular glass films

“…4(c) (details in SI). CR-T g measurements were used to extend the dynamical range of the bulk measurements to temperatures below bulk T g [14,29,36]. CR-T g measurements were performed on films with a thickness range of 20 nm< h <100 nm as shown in Fig 4(b) (details in SI S8 and S9).…”