AbstractOligo(phenylenevinylene)s (OPV) with a 2,5‐diaminobenzene centre were prepared via twofold Horner olefinations of substituted terephthalaldehydes. Whereas variations of the environment only slightly alter the electronic excitation spectra, the fluorescence spectra appear to be highly responsive. Besides a positive solvatochromism, the emission is very sensitive towards protonation. Quenching or appearance of new emitting species depends on the substitution pattern and is controlled by the concentration of the acid. Very large Stokes shifts indicate extensive structural changes in the excited states. Copyright © 2006 John Wiley & Sons, Ltd.