Michael Therien

Printing thin-film transistors (TFTs) using nanomaterials is a promising approach for future electronics. Yet, most inks rely on environmentally harmful solvents for solubilizing and postprint processing the nanomaterials. In this work, we demonstrate water-only TFTs printed from all-carbon inks of semiconducting carbon nanotubes (CNTs), conducting graphene, and insulating crystalline nanocellulose (CNC). While suspending these nanomaterials into aqueous inks is readily achieved, printing the inks into thin films of sufficient surface coverage and in multilayer stacks to form TFTs has proven elusive without high temperatures, hazardous chemicals, and/or lengthy postprocessing. Using aerosol jet printing, our approach involves a maximum temperature of 70 °C and no hazardous chemicals─all inks are aqueous and only water is used for processing. An intermittent rinsing technique was utilized to address the surface adhesion challenges that limit film density of printed aqueous CNTs. These findings provide promising steps toward an environmentally friendly realization of thin-film electronics.

Synthetic strategies for electron-deficient meso-perfluoroalkylporphyrins bearing diverse functional groups are described. Scalable and efficient syntheses for 5-triisopropylsilylethynyl-10,15,20-tris(heptafluoropropyl)porphyrin and 5-triisopropylsilylethynyl-10,20-bis(heptafluoropropyl)porphyrin that equip meso-ethynyl functional groups via the bilane route have been established, along with a refined route to [5,15-bis(heptafluoropropyl)porphinato]zinc(II). meso-Position halogenation of [5,15-bis(heptafluoropropyl)porphinato]zinc(II) was achieved by selective meso-nitration and subsequent reduction, diazonium salt formation, and iodination reactions. Computational data describe the low energy excited states of these chromophores and the electronic structural factors that control reactivity of these meso-perfluoroalkyl substituted porphyrin complexes. meso-Functionalized [5-triisopropylsilylethynyl-10,20-bis(heptafluoropropyl)porphinato]zinc(II) and [5-iodo-10,20-bis(heptafluoropropyl)porphinato]zinc(II) building blocks lay the foundation for the construction of highly conjugated multiporphyrin arrays that feature electronic structural properties important for the development of n-type materials and high potential photooxidants.

A series of new π-stacked compounds, 1,8-bis(2',5'-dimethoxybenzene-1'-yl)naphthalene (1), 1,4-bis(8'-(2″,5″-dimethoxybenzene-1″-yl)naphthalen-1'-yl)benzene (2), and 1,8-bis(4'-(8″-(2‴,5‴-dimethoxybenzene-1‴-yl)naphthalen-1″-yl)benzene-1'-yl)naphthalene (3), have been synthesized and characterized herein as precursor molecules of monocationic mixed-valence systems (MVSs). The three-dimensional geometries of these compounds were determined by X-ray crystallography. A near-orthogonal alignment of the naphthalene pillaring motif to the dimethoxybenzene redox center, or the phenylene spacer, imposes cofacial alignment of these units in a juxtaposed manner with sub-van der Waals interplanar distances. Cyclic and differential pulse voltammograms reveal that the ΔE values between two sequential oxidation potentials are 0.30, 0.11, and 0.10 V for 1, 2, and 3, respectively. MVSs derived from these compounds are recognized as class II according to the Robin and Day classification. The decay parameter β, which describes the distance dependence of the squared electronic coupling in the three mixed-valence systems, was experimentally determined via Mulliken-Hush analysis of the intervalence charge transfer band (β = 0.37 Å-1) and theoretically assessed from charge-resonance contributions derived from DFT computations (β = 0.37 Å-1). These values are extraordinarily mild, indicating that the electronic interaction between redox centers in the longitudinal direction may be comparable to that in the transverse direction, if the MVS system is appropriately designed.

Printed carbon nanotube thin-film transistors (CNT-TFTs) are candidates for flexible electronics with printability on a wide range of substrates. Among the layers comprising a CNT-TFT, the gate dielectric has proven most difficult to additively print owing to challenges in film uniformity, thickness, and post-processing requirements. Printed ionic dielectrics show promise for addressing these issues and yielding devices that operate at low voltages thanks to their high-capacitance electric double layers. However, the printing of ionic dielectrics in their various compositions is not well understood, nor is the impact of certain stresses on these materials. In this work, we studied three compositionally distinct ionic dielectrics in fully printed CNT-TFTs: the polar-fluorinated polymer elastomer PVDF-HFP; an ion gel consisting of triblock polymer PS-PMMA-PS and ionic liquid EMIM-TFSI; and crystalline nanocellulose (CNC) with a salt concentration of 0.05%. Although ion gel has been thoroughly studied, e-PVDF-HFP and CNC printing are relatively new and this study provides insights into their ink formulation, print processing, and performance as gate dielectrics. Using a consistent aerosol jet printing approach, each ionic dielectric was printed into similar CNT-TFTs, allowing for direct comparison through extensive characterization, including mechanical and electrical stress tests. The ionic dielectrics were found to have distinct operational dependencies based on their compositional and ionic attributes. Overall, the results reveal a number of trade-offs that must be managed when selecting a printable ionic dielectric, with CNC showing the strongest performance for low-voltage operation but the ion gel and elastomer exhibiting better stability under bias and mechanical stresses.

Control of the singlet-triplet energy gap (Δ<i>E</i>_ST) is central to realizing productive energy conversion reactions, photochemical reaction trajectories, and emergent applications that exploit molecular spin physics. Despite this, no systematic methods have been defined to tune Δ<i>E</i>_ST in simple molecular frameworks, let alone by an approach that also holds chromophore size and electronic structural parameters (such as the HOMO-LUMO gap) constant. Using a combination of molecular design, photophysical and potentiometric experiments, and quantum chemical analyses, we show that the degree of electron-electron repulsion in excited singlet and triplet states may be finely controlled through the substitution pattern of a simple porphyrin absorber, enabling regulation of relative electronically excited singlet and triplet state energies by the designed restriction of the electron-electron Coulomb (<i>J</i>) and exchange (<i>K</i>) interaction magnitudes. This approach modulates the Δ<i>E</i>_ST magnitude by controlling the densities of state in the occupied and virtual molecular orbital manifolds, natural transition orbital polarization, and the relative contributions of one electron transitions involving select natural transition orbital pairs. This road map, which regulates electron density overlaps in the occupied and virtual states that define the singlet and triplet wave functions of these chromophores, enables new approaches to preserve excitation energy despite intersystem crossing.