Regioselective Fluorination of Acenes: Tailoring of Molecular Electronic Levels and Solid‐State Properties

Abstract

Optoelectronic properties of molecular solids are important for organic electronic devices and are largely determined by the adopted molecular packing motifs. In this study, we analyzed such structure‐property relationships for the partially regioselective fluorinated tetracenes 1,2,12‐trifluorotetracene, 1,2,10,12‐tetrafluorotetracene and 1,2,9,10,11‐pentafluorotetracene that were further compared with tetracene and perfluoro‐tetracene. Quantum chemical DFT calculations in combination with optical absorption spectroscopy data show that the frontier orbital energies are lowered with the degree of fluorination, while their optical gap is barely affected. However, the crystal structure changes from a herringbone packing motif of tetracene towards a planar stacking motif of the fluorinated tetracene derivatives, which is accompanied by the formation of excimers and leads to strongly red‐shifted photoluminescence with larger lifetimes.

Solvent Polarity Influenced Polymorph Selection of Polar Aromatic Molecules

“…The energy difference Δ E between the lowest energy optical excitation in solution and solid can be interpreted as a measure of the dielectric screening in the solid. A similar value of about 120 meV is found for both F 5 TET polymorphs, which appears reasonable in view of their comparable density, but is smaller than that of tetracene (250 meV) . To characterize the optical anisotropy of the crystals, polarization-resolved micrographs were taken.…”

“…The previous crystal structure analysis has shown that the molecules adopt a dipole-parallel (d-p) orientation in a crisscross packing motif (CCDC code: 1998748, depicted in Figure b,c) . Although such a stacking appears to be energetically unfavorable, powder diffraction confirms that the same packing motif also occurs in powder precipitates from the slightly polar solvent dichloromethane during the last synthetic step . Inspired by previous studies showing polymorph-selective growth by varying the solvent, ,, we extended our crystallization analysis of the polar F 5 TET also to nonpolar solvents by using the vacuum-stable silicone oil (47 V 350 Rhodorsil, VWR GmbH) for the liquid-assisted OMBD.…”

“…42 Although such a stacking appears to be energetically unfavorable, powder diffraction confirms that the same packing motif also occurs in powder precipitates from the slightly polar solvent dichloromethane during the last synthetic step. 50 Inspired by previous studies showing polymorph-selective growth by varying the solvent, 18,22,51 we extended our crystallization analysis of the polar F 5 TET also to nonpolar solvents by using the vacuum-stable silicone oil (47 V 350 Rhodorsil, VWR GmbH) for the liquid-assisted OMBD. Previous work on liquid-assisted growth of perylene crystals has further shown that the polymorph selection can also be influenced by the growth rate and the liquid temperature.…”

“…A similar value of about 120 meV is found for both F 5 TET polymorphs, which appears reasonable in view of their comparable density, but is smaller than that of tetracene (250 meV). 50 To characterize the optical anisotropy of the crystals, polarization-resolved micrographs were taken. As shown in Figure 4b,c, only F 5 TET crystals of the d-ap phase reveal a strong dichroism.…”

Singlet‐exciton optics and phonon‐mediated dynamics in oligoacene semiconductor crystals

“…This would be achieved by suppressing the short‐range interaction in these materials 71 or by increasing the direct Coulomb interaction (which we consider in our work). Chemical 7 and structural changes 72 of the organic crystal should allow the exciton separation and phonon modes to be tuned. Strategies to suppress singlet fission in organic semiconductors are not well explored, primarily because singlet fission is beneficial for photovoltaics and as such requires further study.…”