Vapor-deposited organic glasses exhibit enhanced stability against photodegradation.

Photochemically stable solids are in demand for applications in organic electronics. Previous work has established the importance of the molecular packing environment by demonstrating that different crystal polymorphs of the same compound react at different rates when illuminated. Here we show, for the first time, that different amorphous packing arrangements of the same compound photodegrade at different rates. For these experiments, we utilize the ability of physical vapor deposition to prepare glasses with an unprecedented range of densities and kinetic stabilities. Indomethacin, a pharmaceutical molecule that can undergo photodecarboxylation when irradiated by UV light, is studied as a model system. Photodegradation is assessed through light-induced changes in the mass of glassy thin films due to the loss of CO2, as measured by a quartz crystal microbalance (QCM). Glasses prepared by physical vapor deposition degraded more slowly under UV illumination than did the liquid-cooled glass, with the difference as large as a factor of 2. Resistance to photodegradation correlated with glass density, with the vapor-deposited glasses being up to 1.3% more dense than the liquid-cooled glass. High density glasses apparently limit the local structural changes required for photodegradation.

Tunable molecular orientation and elevated thermal stability of vapor-deposited organic semiconductors.

Physical vapor deposition is commonly used to prepare organic glasses that serve as the active layers in light-emitting diodes, photovoltaics, and other devices. Recent work has shown that orienting the molecules in such organic semiconductors can significantly enhance device performance. We apply a high-throughput characterization scheme to investigate the effect of the substrate temperature (Tsubstrate) on glasses of three organic molecules used as semiconductors. The optical and material properties are evaluated with spectroscopic ellipsometry. We find that molecular orientation in these glasses is continuously tunable and controlled by Tsubstrate/Tg, where Tg is the glass transition temperature. All three molecules can produce highly anisotropic glasses; the dependence of molecular orientation upon substrate temperature is remarkably similar and nearly independent of molecular length. All three compounds form "stable glasses" with high density and thermal stability, and have properties similar to stable glasses prepared from model glass formers. Simulations reproduce the experimental trends and explain molecular orientation in the deposited glasses in terms of the surface properties of the equilibrium liquid. By showing that organic semiconductors form stable glasses, these results provide an avenue for systematic performance optimization of active layers in organic electronics.

Influence of substrate temperature on the transformation front velocities that determine thermal stability of vapor-deposited glasses.

Stable organic glasses prepared by physical vapor deposition transform into the supercooled liquid via propagating fronts of molecular mobility, a mechanism different from that exhibited by glasses prepared by cooling the liquid. Here we show that spectroscopic ellipsometry can directly observe this front-based mechanism in real time and explore how the velocity of the front depends upon the substrate temperature during deposition. For the model glass former indomethacin, we detect surface-initiated mobility fronts in glasses formed at substrate temperatures between 0.68Tg and 0.94Tg. At each of two annealing temperatures, the substrate temperature during deposition can change the transformation front velocity by a factor of 6, and these changes are imperfectly correlated with the density of the glass. We also observe substrate-initiated fronts at some substrate temperatures. By connecting with theoretical work, we are able to infer the relative mobilities of stable glasses prepared at different substrate temperatures. An understanding of the transformation behavior of vapor-deposited glasses may be relevant for extending the lifetime of organic semiconducting devices.

High-throughput ellipsometric characterization of vapor-deposited indomethacin glasses.

A method for the high-throughput preparation and characterization of vapor-deposited organic glasses is presented. Depositing directly onto a substrate with a large temperature gradient allows many different glasses to be prepared simultaneously. Ellipsometry is used to characterize these glasses, allowing the determination of density, birefringence, and kinetic stability as a function of substrate temperature. For indomethacin, a model glass former, materials up to 1.4% more dense than the liquid-cooled glass can be formed with a continuously tunable range of molecular orientations as determined by the birefringence. By comparing measurements of many properties, we observe three phenomenological temperature regimes. For substrate temperatures from Tg + 11 K to Tg - 8 K, equilibrium states are produced. Between Tg - 8 K and Tg - 31 K, the vapor-deposited materials have the macroscopic properties expected for the equilibrium supercooled liquid while showing local structural anisotropy. At lower substrate temperatures, the properties of the vapor-deposited glasses are strongly influenced by kinetic factors. Different macroscopic properties are no longer correlated with each other in this regime, allowing unusual combinations of properties.

Theoretical study of the spectroscopically relevant parameters for the detection of HNP(q) and HPN(q) (q = 0, +1, -1) in the gas phase.

High level ab initio electronic structure calculations at different levels of theory have been performed on HNP and HPN neutrals, anions, and cations. This includes standard coupled cluster CCSD(T) level with augmented correlation-consistent basis sets, internally contacted multi-reference configuration interaction, and the newly developed CCSD(T)-F12 methods in connection with the explicitly correlated basis sets. Core-valence correction and scalar relativistic effects were examined. We present optimized equilibrium geometries, harmonic vibrational frequencies, rotational constants, adiabatic ionization energies, electron affinities, vertical detachment energies, and relative energies. In addition, the three-dimensional potential energy surfaces of HNP(-1,0,+1) and of HPN(-1,0,+1) were generated at the (R)CCSD(T)-F12b∕cc-pVTZ-F12 level. The anharmonic terms and fundamentals were derived using second order perturbation theory. For HNP, our best estimate for the adiabatic ionization energy is 7.31 eV, for the adiabatic electron affinity is 0.47 eV. The higher energy isomer, HPN, is 23.23 kcal∕mol above HNP. HPN possesses a rather large adiabatic electron affinity of 1.62 eV. The intramolecular isomerization pathways were computed. Our calculations show that HNP(-) to HPN(-) reaction is subject to electron detachment.

Density and birefringence of a highly stable α,α,ß-trisnaphthylbenzene glass.

Spectroscopic ellipsometry has been used to understand the properties of α,α,ß-trisnaphthylbenzene (ααß-TNB) glasses vapor-deposited at a substrate temperature of 295 K (0.85 T(g)). In a single temperature ramping experiment, a range of properties of the as-deposited glass can be measured, including density, fictive temperature, onset temperature, thermal expansion coefficient, and birefringence. The vapor-deposited ααß-TNB glass is 1.3% more dense than the ordinary glass prepared by cooling at 1 K/min, is found to be birefringent, has a fictive temperature 35 K below that of the ordinary glass, and an onset temperature 20 K above that of the ordinary glass. The thermal expansion coefficient of the vapor-deposited ααß-TNB glass is 14% lower than that of the ordinary glass, indicating that lower portions of the potential energy landscape have more harmonic potential minima than the parts accessible to the ordinary glass.

Molecular Orientation in Stable Glasses of Indomethacin.

Spectroscopic ellipsometry has been used to measure the properties of indomethacin prepared by physical vapor deposition at Tsubstrate/Tg = 0.78, 0.84, and 0.90. The as-deposited glasses exhibited high kinetic stability and had densities 0.8-1.2% higher than the ordinary glass prepared by cooling the liquid at 1 K/min. Deposition at the higher temperatures yielded glasses with positive birefringence (up to Δn = 0.028), while the lowest-temperature sample was negatively birefringent (Δn = -0.015). These results indicate that substrate temperature can be used to manipulate molecular orientation in high-density and high-stability glasses. The data for the supercooled liquid and the ordinary glass of indomethacin are reasonably consistent with the Lorentz-Lorenz equation, but significant deviations are noted with the as-deposited materials.

Hydroxyl radical substitution in halogenated carbonyls: oxalic acid formation.

An ab initio study of OH radical substitution reactions in halogenated carbonyls is conducted. Hydroxyl radical substitution into oxalyl dichloride [ClC(O)C(O)Cl] and oxalyl dibromide [BrC(O)C(O)Br], resulting in the formation of oxalic acid, is presented. Analogous substitution reactions in formyl chloride [ClCH(O)], acetyl chloride [ClC(O)CH(3)], formyl bromide [BrCH(O)], and acetyl bromide [BrC(O)CH(3)] are considered. Energetics of competing hydrogen abstraction reactions for all applicable species are computed for comparison. Geometry optimizations and frequency computations are performed using the second-order Møller-Plesset perturbation theory (MP2) and the 6-31G(d) basis set for all minimum species and transition states. Single point energy computations are performed using fourth-order Møller-Plesset perturbation theory (MP4) and coupled cluster theory [CCSD(T)]. Potential energy surfaces, including activation energies and enthalpies, are determined from the computations. These potential energy surfaces show that OH substitution into ClC(O)C(O)Cl and BrC(O)C(O)Br, resulting in the formation of oxalic acid and other minor products, is energetically favorable. Energetics of analogous reactions with ClCH(O), BrCH(O), ClC(O)CH(3), and BrC(O)CH(3) are also computed.