Molecular Design and Crystal Chemistry of Polyfluorinated Naphthalene‐bis‐phenylhydrazimides with Superior Thermal and Polymorphic Stability and High Solution Processability

Abstract

Naphthalene tetracarboxylic diimides (NDIs) are highly promising air-stable n-type molecular semiconductor candidates for flexible and cost-effective organic solar cells and thermoelectrics. Nonetheless, thermal and polymorphic stabilities of environmentally stable NDIs in the low-tomedium temperature regime (< 300 °C) remain challenging properties. Structural, thermal, spectroscopic, and computational features of polyfluorinated NDI-based molecular solids (with up to 14 F atoms per NDI molecule) are discussed upon increasing the fluorination level. Slip-stacked arrangement of the NDI cores with suitable π-π stacking and systematically short interplanar distances (< 3.2 Å) are found. All these materials exhibit superior thermal stability (up to 260 °C or above) and thermal expansion coefficients indicating a response compatible with flexible polymeric substrates. Optical bandgaps increase from 2.78 to 2.93 eV with fluorination, while LUMO energy levels decrease down to À 4.37 eV, as shown per DFT calculations. The compounds exhibit excellent solubility of 30 mg mL À 1 in 1,4-dioxane and DMF.

Polyfluorinated Naphthalene-bis-hydrazimide for Solution-Grown n-Type Semiconducting Films

“…The NTO analysis shows that the most intense transition (λ 1 in Table ) can be described mainly as a HOMO–4 → LUMO: both these orbitals are placed on the polyaromatic core, corroborating the observation that λ 1 hardly changes in the whole series. Likely, this transition is also responsible for the absorption coefficient rise in the (solid-state measured) Tauc plots, as the associated photon energies reasonably agree with the experimentally determined optical E gap values (2.8–2.9 eV) and with their limited dependence on side-chain substitution …”

“…Table presents the GS energies of the frontier molecular orbitals of F0, F3, F5, and F7 (reported in ref ). As per Koopmans’ theorem (valid for perylene diimide compounds), these GS energies represent the first ionization potential (defined as −HOMO energy) and the electron affinity (defined as −LUMO energy) of the isolated molecules.…”

“…Specifically, using a set of 10 common organic solvents and limiting, as an arbitrary operational threshold of 2 mg mL −1 for considering an NDI derivative sufficiently soluble for efficient spin-coating film deposition, the fluorinated species were found to be soluble in three, six, and five cases, respectively (reaching values as high as 40 mg mL −1 ). 30 At variance, F0 proved to be soluble only in DMF (40 mg mL −1 ) and, to a much more limited extent (4 mg mL −1 ), in THF. Additional tests performed of the nonfluorinated p-aminophenyl-derivative characterized by us in 2021 29 showed a maximum solubility of 1 mg mL −1 , well below any practical use.…”

“…After replacing the alkyl chains with aryl residues, much higher thermal/polymorphic stability was obtained (above 500 °C, up to melting), at the expense of solubility issues. In the present work, we focus on solution-grown thin films of highly fluorinated NDI dihydrazimides, possessing, at the same time, low LUMO levels (compatible with ambient stability), high thermal and polymorphic stability, and good solution processability. The strategy behind the new derivatives was adding numerous EW groups (F or CF 3 residues) on the aromatic rings in the molecular periphery, boosting the approach already outlined with benzylamine-based NDIs with single CF 3 , OCF 3 , and SCF 3 residues in para position, for which significantly improved electron mobility and materials stability are reported. , Indeed, it is well-known that the high electronegativity and low molar polarization of the fluorinated (F and CF 3 -based) groups, often provide, for polyfluorinated species, new and appealing features, such as improved dielectric properties and low moisture uptake .…”

“…The species herein investigated, containing the highly fluorinated 1,4,5,8-naphthalenetetracarboxylic-bis-hydrazimides, are illustrated in Figure , where the labeling scheme (F3, F5, F7) recalls the degree of fluorination versus the nonfluorinated precursor (F0). A comprehensive characterization (structural, thermal, polymorphic, and spectroscopic) of the bulk solid materials is presented elsewhere . The first sections of this contribution are dedicated to the spectroscopic characterization of the precursors, complemented by density functional theory (DFT) computational results addressing the LUMO energy variations versus the degree of fluorination and fostering a correct interpretation of the solution UV–vis spectra.…”