The invention relates to compositions including a charged oligo-phenylene ethynylene singlet-oxygen sensitizer and an oppositely-charged surfactant, which show an enhanced biocidal activity relative to a comparable concentration of the oligo-phenylene ethynylene without the oppositely-charged surfactant. The enhancement of biocidal activity is observed with an anionic oligo-phenylene ethynylene in the presence of a cationic surfactant such as TTAB, and with a cationic oligo-phenylene ethynylene in the presence of an anionic surfactant such as SDS.