Molecular Packing and Solid‐State Photophysical Properties of 1,3,6,8‐Tetraalkylpyrenes

Abstract

The relationship between the photophysical properties and molecular orientation of 1,3,6,8‐tetraalkylpyrenes in the solid state is described herein. The introduction of alkyl groups with different chain structures (in terms of length and branching) did not affect the photophysical properties in solution, but significantly shifted the emission wavelengths and fluorescence quantum yields in the solid state for some samples. Pyrenes bearing ethyl, isobutyl, or neopentyl groups at the 1‐, 3‐, 6‐, and 8‐positions showed similar emission profiles in both the solution and solid states. In contrast, pyrenes bearing other alkyl groups exhibited an excimer emission in the solid state, similar to that of the parent pyrene. On studying the photophysical properties in the solid state with respect to the obtained crystal structures, the observed solid‐state photophysical properties were found to depend on the relative position of the pyrene chromophores. The solid‐state photophysical properties can be controlled by the alkyl groups, which provide changing crystal packing. Among the pyrenes tested, 1,3,6,8‐tetraethylpyrene showed the highest fluorescence quantum yield of 0.88 in the solid state.

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

“…These bathochromic shifts are due to increasing HOMO energy levels from CÀ H/CÀ C σ-π hyper conjugation between the Py chromophore and alkyl groups. [17,18] Additionally, the solid-state fluorescence profiles of the co-crystals were essentially the same as a dilute solution of Py derivatives ( Table 1 and Figures S2-S5). These results, along with the interleaved faceto-face packing mode of these co-crystals (vide infra), indicate that the intercalated OFN efficiently separates the Py cores to avoid excimer formation in the co-crystal.…”

“…When Pen 4 Py was recrystallized from MeOH and CHCl 3 , two polymorphic crystals were obtained, which exhibited emission at 420 nm along with shoulders at 440 and 470 nm. [17] Although the contribution of the excimer was small in these crystalline samples, an excimer emission centered at 470 nm was mainly observed in a less organized drop-cast sample. These emission profiles of pure Pen 4 Py in the solid state suggested that the introduction of pentyl groups at the 1-, 3-, 6-, and 8-positions suppresses the excimer formation in the crystal, but unregulated molecular orientation leads to excimer formation in the solid state as observed in the drop-cast sample ( Figure S9).…”

“…The introduction of alkyl groups at the 1‐, 3‐, 6‐, and 8‐positions of Py resulted in 1429 to 2137 cm −1 bathochromic shifts in the emission profile compared with that of unsubstituted Py ⋅ OFN (Table and Figure b–e, solid line). These bathochromic shifts are due to increasing HOMO energy levels from C−H/C−C σ‐π hyper conjugation between the Py chromophore and alkyl groups . Additionally, the solid‐state fluorescence profiles of the co‐crystals were essentially the same as a dilute solution of Py derivatives (Table and Figures S2–S5).…”

“…1,3,6,8-Tetraalkylpyrenes were prepared by according to the literature. [17] Further purification by GPC (eluent: CHCl 3 ) followed by recrystallization from CH 3 Cl/MeOH (twice) gave a pure sample for the analysis of photophysical properties. Octafluoronaphthalene and pyrene were purchased from TCI and used as received unless otherwise noted.…”

“…Because these functional materials are used in the solid state and are exposed to mechanical stresses, a rational strategy to construct co‐crystal systems that are robust against mechanical stresses is highly desirable. To this end, we herein report the co‐crystals of pyrene ( Py ) and 1,3,6,8‐tetraalkylpyrenes ( Alkyl 4 Py ) with octafluoronaphthalene ( OFN ) and the changes in their photophysical properties induced by mechanical stimuli.…”