( E)-1-[2-Hydroxy-4-(phenylethynyl)phenyl]-3-[4-(phenylethynyl)phenyl]prop-2-en-1-one ( 1), ( E)-1-[2-hydroxy-4-(phenylethynyl)phenyl]-3-phenylprop-2-en-1-one ( 2), and ( E)-1-(2-hydroxyphenyl)-3-[4-(phenylethynyl)phenyl]prop-2-en-1-one ( 3), which belong to a new class of 2′-hydroxychalcones with phenylethynyl group(s) at the para position of the phenyl ring, were synthesized, and their photochemical properties were investigated. The lowest energy absorption band of 1 peaks at a longer wavelength (383 nm) with a much larger molar extinction coefficient (5.0 × 10 M cm) than that of the parent 2′-hydroxychalcone ( 2′HC) (2.0 × 10 M cm at 318 nm). Upon photoexcitation, all three compounds underwent excited-state intramolecular hydrogen atom transfer (ESIHT) to produce an excited tautomer that emitted fluorescence with a large Stokes shift in the longer wavelength region at 600-700 nm. The quantum yield of the tautomer fluorescence of 1 was not high at 298 K ( Φ = 9.1 × 10), but was highest among 2′HC and its analogues. The Φ values of 1- 3 increased 10-30 fold upon reducing the temperature from 298 to 77 K. [ABSTRACT FROM AUTHOR]