A cationic and a neutral donor-acceptor biphenyl-type system are compared with respect to their photophysical properties. The strong intramolecular fluorescence quenching observed only for the ionic system can be traced back to an excited state twisting relaxation. This process is suppressed either by chemical bridging or by rigidizing the environment at low temperatures. Quantum chemical calculations indicate a strongly different shape of the excited state potential energy surface for the ionic compound, namely a barrierless intramolecular twisting towards perpendicularity, and a significant narrowing of the S-1-S-0 energy gap upon twisting relaxation in the excited state. This suggests a close-lying conical intersection in the ionic case and explains the strongly different fluorescence behaviour of the two compounds.