Effect of Alkyl Groups in Pyrene Chromophore on the Mechanical Response of Pyrene‐Octafluoronaphthalene Co‐Crystals

Abstract

Changes in the photophysical properties of pyrene (Py)-octafluoronaphthalene (OFN) co-crystals (Py·OFN) upon mechanical stimuli are described herein. The Py·OFN cocrystal showed a mechano-induced bathochromic shift in emission, and a similar tendency was observed for the 1,3,6,8-tetramethylpyrene-OFN co-crystal. These shifts are due to disruption of the microscopic molecular orientation in the co-crystal, which allows for excimer formation. In sharp contrast to the parent Py·OFN and methyl-substituted Py-OFN co-crystals, no mechano-induced bathochromic shift was observed when longer alkyl chains were introduced to the 1-, 3-, 6-, and 8-positions of the Py chromophore. This photophysical opposability against mechanical stimuli could be explained by the orthogonally oriented alkyl groups on the Py ring, which existed between two Py cores like pillars. This fixed OFN to maintain the face-to-face alternatively stacked structure of the co-crystal and thus prevented the formation of the Py excimer. The pillar effect demonstrated herein provides a rational design for co-crystalline systems that are photophysically stable against mechanical stresses.[a] Prof.

Thermally Activated Delayed Fluorescence of Aggregates Induced by Strong π–π Interactions and Reversible Dual-Responsive Luminescence Switching

“…67 In only two cases has grinding-induced red-shifted emission been attributed to the formation of excimer [68][69][70][71] and the change in molecular conformation. 3 First, Iwasaki and co-workers 72 reported that the fluorescence of Pyoctafluoronaphthalene cocrystals showed a red-shift from single-molecular Py emission to Py excimer emission by grinding. Nevertheless, this situation can be easily excluded in terms of the lifetime of amorphous powder because the lifetime of excimer fluorescence often lasts as long as tens or hundreds of nanoseconds (<300 ns) in an organic aromatic system.…”

Improving the Overall Properties of Circularly Polarized Luminescent Materials Through Arene–Perfluoroarene Interactions

“…The distinct AP interaction could further be evidenced by other spectral measurements. As shown in Figure 3 a, compared with the absorption spectrum of individual R ‐Py colloidal suspension, the absorption spectrum of AP complex of R ‐Py/OFN shows no obvious change, suggesting that the addition of OFN does not affect the ground state properties of R ‐Py [13a, 15a, 18] . However, the fluorescence spectrum of R ‐Py/OFN shows significant blue‐shift from 508 nm ( R ‐Py colloidal suspension) to 480 nm, indicating the existence of strong AP interaction (Figure 3 b).…”