Concise Synthesis and Two-Photon-Excited Deep-Blue Emission of 1,8-Diazapyrenes

Efficient violetblue-emitting molecules are especially useful for applications in full-color displays, solid-state lighting, as well as in two-photon absorption (TPA) excited frequency-upconverted violetblue lasing. However, the reported violetblue-emitting molecules generally possess small TPA cross sections. In this work, new 1,8-diazapyrenes derivatives 3 with blue two-photon-excited fluorescence emission were concisely synthesized by the coupling reaction of readily available 1,4-naphthoquinone O,O-diacetyl dioxime (1) with internal alkynes 2 under the [{RhCl2Cp*}2]Cu(OAc)2 (Cp*=pentamethylcyclopentadienyl ligand) bimetallic catalytic system. Elongation of the p-conjugated length of 1,8-diazapyrenes 3 led to the increase of TPA cross sections without the expense of a redshift of the emission wavelength, probably due to the rigid planar structure of chromophores. It is especially noteworthy that 2,3,6,7-tetra(4-bromophenyl)-1,8-diazapyrene (3?c) has a larger TPA cross section than those of other molecules reported so far. These experimental results are explained in terms of the effects of extension of the p-conjugated system, intramolecular charge transfer, and reduced detuning energy.