Effects of microstructure formation on the stability of vapor-deposited glasses.

Glasses formed by physical vapor deposition (PVD) are an interesting new class of materials, exhibiting properties thought to be equivalent to those of glasses aged for thousands of years. Exerting control over the structure and properties of PVD glasses formed with different types of glass-forming molecules is now an emerging challenge. In this work, we study coarse-grained models of organic glass formers containing fluorocarbon tails of increasing length, corresponding to an increased tendency to form microstructures. We use simulated PVD to examine how the presence of the microphase-separated domains in the supercooled liquid influences the ability to form stable glasses. This model suggests that increasing molecule tail length results in decreased thermodynamic stability of the molecules in PVD films. The reduced stability is further linked to the reduced ability of these molecules to equilibrate at the free surface during PVD. We find that, as the tail length is increased, the relaxation times near the surface of the supercooled equilibrium liquid films of these molecules are slowed and become essentially bulk-like, due to the segregation of the fluorocarbon tails to the free surface. Surface diffusion is also markedly reduced due to clustering of the molecules at the surface. Based on these results, we propose a trapping mechanism where tails are unable to move between local phase-separated domains on the relevant deposition time scales.

Design of a homologous series of molecular glassformers.

We design and synthesize a set of homologous organic molecules by taking advantage of facile and tailorable Suzuki cross coupling reactions to produce triarylbenzene derivatives. By adjusting the number and the arrangement of conjugated rings, the identity of heteroatoms, lengths of fluorinated alkyl chains, and other interaction parameters, we create a library of glassformers with a wide range of properties. Measurements of the glass transition temperature (Tg) show a power-law relationship between Tg and molecular weight (MW), with of the molecules, with an exponent of 0.3 ± 0.1, for Tg values spanning a range of 300-450 K. The trends in indices of refraction and expansion coefficients indicate a general increase in the glass density with MW, consistent with the trends observed in Tg variations. A notable exception to these trends was observed with the addition of alkyl and fluorinated alkyl groups, which significantly reduced Tg and increased the dynamical fragility (which is otherwise insensitive to MW). This is an indication of reduced density and increased packing frustrations in these systems, which is also corroborated by the observations of the decreasing index of refraction with increasing length of these groups. These data were used to launch a new database for glassforming materials, glass.apps.sas.upenn.edu.

Exploring the Importance of Surface Diffusion in Stability of Vapor-Deposited Organic Glasses.

Stable glasses are formed during physical vapor deposition (PVD), through the surface-mediated equilibration process. Understanding surface relaxation dynamics is important in understanding the details of this process. Direct measurements of the surface relaxation times in molecular glass systems are challenging. As such, surface diffusion measurements have been used in the past as a proxy for the surface relaxation process. In this study, we show that the absence of enhanced surface diffusion is not a reliable predictor of reduced ability to produce stable glasses. To demonstrate, we have prepared stable glasses (SGs) from two structurally similar organic molecules, 1,3-bis(1-naphthyl)-5-(2-naphthyl)benzene (TNB) and 9-(3,5-di(naphthalen-1-yl)phenyl)anthracene (α,α-A), with similar density increase and improved kinetic stability as compared to their liquid-quenched (LQ) counterparts. The surface diffusion values of these glasses were measured both in the LQ and SG states below their glass transition temperatures ( Tgs) using gold nanorod probes. While TNB shows enhanced surface diffusion in both SG and LQ states, no significant surface Tg diffusion is observed on the surface of α,α-A within our experimental time scales. However, isothermal dewetting experiments on ultrathin films of both molecules below Tg indicate the existence of enhanced dynamics in ultrathin films for both molecules, indirectly showing the existence of an enhanced mobile surface layer. Both films produce stable glasses, which is another indication for the existence of the mobile surface layer. Our results suggest that lateral surface diffusion may not be a good proxy for enhanced surface relaxation dynamics required to produce stable glasses, and thus, other types of measurements to directly probe the surface relaxation times may be necessary.

Transition-metal-free chemo- and regioselective vinylation of azaallyls.

Direct C(sp3)-C(sp2) bond formation under transition-metal-free conditions offers an atom-economical, inexpensive and environmentally benign alternative to traditional transition-metal-catalysed cross-coupling reactions. A new chemo- and regioselective coupling protocol between 3-aryl-substituted-1,1-diphenyl-2-azaallyl derivatives and vinyl bromides has been developed. This is the first transition-metal-free cross-coupling of azaallyls with vinyl bromide electrophiles and delivers allylic amines in excellent yields (up to 99%). This relatively simple and mild protocol offers a direct and practical strategy for the synthesis of high-value allylic amine building blocks that does not require the use of transition metals, special initiators or photoredox catalysts. Radical clock experiments, electron paramagnetic resonance studies and density functional theory calculations point to an unprecedented substrate-dependent coupling mechanism. Furthermore, an electron paramagnetic resonance signal was observed when the N-benzyl benzophenone ketimine was subjected to silylamide base, supporting the formation of radical species upon deprotonation. The unique mechanisms outlined herein could pave the way for new approaches to transition-metal-free C-C bond formations.