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

Changes in the photophysical properties of pyrene (Py)-octafluoronaphthalene (OFN) co-crystals (Py⋅OFN) upon mechanical stimuli are described herein. The Py⋅OFN co-crystal 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.