José C. S. Costa

Abstract This work reports a comprehensive experimental evaluation of the solid-liquid-gas phase equilibria for five representative phenylene-thiophene co-oligomers (3-ring aromatic compounds having both phenyl and thienyl units). The melting temperatures and corresponding standard molar enthalpies and entropies of fusion were measured by differential scanning calorimetry. The equilibrium vapor pressures of the crystalline solids as a function of temperature were measured by a combined Knudsen/quartz-crystal effusion method, with the consequent derivation of the standard molar enthalpies, entropies, and Gibbs energies of sublimation. The thermodynamic properties of vaporization were estimated from the fusion and sublimation data. The results were analyzed together with the literature data for the corresponding phenylene and thiophene homo-oligomers. The thermodynamic properties of fusion and sublimation exhibited a dependence on ring identity and position that cannot be adequately described by a simple group additivity reasoning. The plot of the Gibbs energy of sublimation as a function of the number of thienyl rings in the co-oligomer showed the existence of two series. Terminal 3-thienyl rings and a linear molecular shape were found to be consistent factors contributing to the stabilization of the crystal phase. The higher melting temperatures and lower volatilities of crystalline 3-thienyl compounds were tentatively explained by the ability of these rings to maximize intermolecular C-H & BULL;& BULL;& BULL;pi interactions independently of the sulfur position. The optical energy gaps, as measured by UV-vis in solution, were found to lie within the values for typical organic semiconductors (< 4 eV) and to decrease for co-oligomers containing more 2-thienyl units, following the increased ring-ring planarity of the molecules. The surface morphology of vapor-deposited thin films suggests a stronger tendency of the co-oligomers, if compared to their corresponding homo-oligomers p-terphenyl and terthiophene, to form less amorphous films.

Abstract Ionic liquids (ILs) have been widely used for energy storage and conversion devices due to their negligible vapor pressure, high thermal stability, and outstanding interfacial properties. Notably, the interfacial nanostructure and the wettability of thin ionic liquid films on solid surfaces are of utmost relevance in nanosurface science and technology. Herein, a reproducible physical vapor deposition methodology was used to fabricate thin films of four alkylimidazolium bis(trifluoromethylsulfonyl)imide ILs. The effect of the cation alkyl chain length on the wettability of ILs was explored on different surfaces: gold (Au); silver (Ag); indium-tin oxide (ITO). High-resolution scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to evaluate the morphology of the produced micro- and nanodroplets and films. SEM and AFM results revealed an island growth for all the ILs deposited on ITO and Ag surfaces, with a lower minimum free area to promote nucleation (MFAN) in Ag and higher wettability for ILs having larger non-polar domains. The low wettability of ITO by the studied ILs was highlighted. For long-chain ILs, nucleation and growth mechanisms were strongly conditioned by coalescence processes. The results also supported the higher affinity of the ILs to the Au surface. The increase in the length of the cation alkyl chain was found to promote a better film adhesion inducing a 2D growth and higher wetting ability.

Abstract This study investigates the nucleation and growth of micro-/nanodroplets of triflate-based ionic liquids (ILs) fabricated by vapor deposition on different surfaces: indium tin oxide (ITO); silver (Ag); gold (Au). The ILs studied are constituted by the alkylimidazolium cation and the triflate anion-[CnC1 im][OTF] series. One of the key issues that determine the potential applications of ILs is the wettability of surfaces. Herein, the wetting behavior was evaluated by changing the cation alkyl chain length (C-2 to C-10). A reproducible control of the deposition rate was conducted employing Knudsen cells, and the thin-film morphology was evaluated by high-resolution scanning electron microscopy (SEM). The study reported here for the [C(n)C(1)im][OTF] series agrees with recent data for the [C(n)C(1)im][NTf2] congeners, highlighting the higher wettability of the solid substrates to long-chain alkylimidazolium cations. Compared to [NTf2], the [OTF] series evidenced an even more pronounced wetting ability on Au and coalescence processes of droplets highly intense on ITO. Higher homogeneity and film cohesion were found for cationic groups associated with larger alkyl side chains. An island growth was observed on both Ag and ITO substrates independently of the cation alkyl chain length. The Ag surface promoted the formation of smaller-size droplets. A quantitative analysis of the number of microdroplets formed on Ag and ITO revealed a trend shift around [C(6)C(1)im][OTF], emphasizing the effect of the nanostructuration intensification due to the formation of nonpolar continuous domains.

Abstract The thermodynamic properties and band gap energies were evaluated for six ortho- and peri-fused polycyclic aromatic hydrocarbons (PAHs): triphenylene; benzo[a]pyrene; benzo[e]pyrene; perylene; benzo[ghi]perylene; coronene. The standard molar enthalpies of formation in the crystalline state and the standard molar enthalpies of sublimation were measured by high precision combustion calorimetry and Knudsen effusion methodology, respectively. The combination of the molar enthalpies of formation in the crystalline state with the respective enthalpies of sublimation was used to evaluate the energetics of the progressive peri-fusion of the aromatic moieties from triphenylene to coronene aiming to investigate the hypothetical superaromaticity character of coronene. The linear trend of the enthalpy of formation in crystalline and gaseous phases in the series (from benzo[e]pyrene to coronene) is an irrefutable indication of a non-superaromaticity character of coronene. High accurate thermodynamic properties of sublimation (volatility, enthalpy, and entropy of sublimation) were derived by the measurement of vapor pressures as a function of temperature, using a Knudsen/quartz crystal effusion methodology. Furthermore, the p-electronic conjugation of these compounds was explored by evaluation of the optical band gaps along with this series of compounds. The morphology of perylene, benzo[ghi]perylene, and coronene thin films, deposited by physical vapor deposition onto transparent conductive oxide substrates (ITO and FTO), was used to analyze the nucleation and growth mechanisms. The morphologies observed were found to be related to the cohesive energy and entropy of the bulk.

Abstract The impact of structural differentiation between phenylcarbazoles (PhC, mCP, CBP, TCB) and phenylamines (TPA, BDB, TPB, TDAB) on the phase equilibria, optical spectrum, band gap, and thin-film morphology is evaluated and discussed. The carbazolyl units lead to a lower electronic conjugation contributing to a wide band gap when compared with the diphenylamine analogs. The fusion and sublimation equilibria indicate that entropic contribution is the key factor for the distinguished melting behavior and solid-phase volatility between phenylcarbazole derivatives and phenylamine analogs. The molecular differentiation between the two classes of compounds is not reflected in the crystal packing and intermolecular interactions. However, compared with the diphenylamino groups, the incorporation of carbazolyl moieties contributes to a less flexible molecule. Moreover, the results evidence that intermolecular bonding disruption along the fusion transition is more extensive for phenylamine derivatives. Due to the asymmetric nonplanar structure, mCP is characterized by a ratio of {T-g/T-m approximate to 3/4} while the more symmetric CBP and TCB molecules display ratios closer to {T-g/T-m approximate to 2/3}. Vapor-deposited thin films of mCP, CBP, and TCB are amorphous and their morphology is highly dependent on the substrate roughness. The lower flexibility of nonplanar phenylcarbazoles induces the formation of a glassy state due to the harder packing mechanism leading to the lower ability of the crystallization process.