Synthesis and Photophysical Properties of Phenyleneethynylenes Containing a Combination of Two Main Group Element Moieties of B, Si, or P on the Side Chain

X-shaped 1,4-phenyleneethynylenes
containing a combination of two
main group element moieties of B, Si, or P as part of the side chain
have been synthesized. The halogen–lithium exchange of 2-bromo-5-dimethylphenylsilyl-1,4-bis­(phenylethynyl)­benzene
followed by addition of fluorodimesitylborane or chlorodiphenylphosphine
gave the “X-shaped” B/Si and Si/P, respectively. The
P-containing molecule was oxidized to afford a P­(O) derivative.
The B/PO moieties were introduced to the 1,4-phenyleneethynylene unit
by 3 steps, i.e., the halogen–lithium exchange of 2,5-dibromo-1,4-bis­(trimethylsilylethynyl)­benzene
followed by sequential addition of fluorodimesitylborane and chlorodiphenylphosphine
oxide, desilylation, and the Sonogashira coupling with aryl iodides.
Then, the reduction of the B/PO gave “X-shaped” B/P
derivative. The absorption and fluorescence spectra of the “X-shaped”
molecules showed the molecules have two-dimensional conjugation systems,
i.e., the π-conjugation of the original π-conjugation
backbone and p­(B)π–π* conjugation of B, σ*−π*
conjugation of Si, and n−π and σ*−π*
conjugation of P, extended from the π-core. The highest occupied
molecular orbital (HOMO)–lowest unoccupied molecular orbital
(LUMO) energy levels are significantly altered because of a lowering
of the LUMO level due to p−π* conjugation for B and σ*−π*
conjugation for Si and P compared with the parent phenyleneethynylene,
which are also supported by density functional theory calculations.