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.