Aerobic Palladium(II)-Catalyzed Dehydrogenative Heck Reaction in the Synthesis of Pyrenyl Fluorophores. A Photophysical Study of β‑Pyrenyl Acrylates in Solution and in the Solid State

An
aerobic dehydrogenative Heck reaction of pyrene (1a)
and 2,7-di-tert-butylpyrene (1b)
with ethyl acrylate is reported. The reaction is catalyzed by a Pd­(OAc)2/4,5-diazafluoren-9-one (DAF) system and takes place in acetic
or pivalic acid as solvents at 110–130 °C. The reaction
of 1a afforded a 6:1 mixture of C-1- and C-4-alkenylated
pyrenes (2a and 3a, respectively) in 71%
yield. In the case of 1b, only a C-4-substituted product
(3b) was formed in 46% yield. Compounds 2a and 3a,b exhibited fluorescence in solution
and in the solid state. In chloroform and THF solution the fluorescence
maxima were in the range of 440–465 nm, and quantum yields
decreased in the order 2a > 3a> 3b. In the solid state, 3a,b showed
blue-green
fluorescence (ΦF = 0.26 and 0.14, respectively),
whereas 2a emitted yellow-green fluorescence) (ΦF = 0.35). Besides blue-emitting monomers, the presence of
green-emitting aggregated species (preformed dimers) in the crystals
of 3a,b and red-emitting dynamic excimers
in the crystals of 2a has been demonstrated. Single-crystal
X-ray diffraction analyses of 2a and 3b confirmed
π-stacking of pyrenyl moieties in the crystals of the former
and the absence of stacking in the crystals of the latter compound.