Donor–Acceptor–Donor Triads with Flexible Spacers: Deciphering Complex Photophysics for Targeted Materials Design

The complex photokinetics of donor–acceptor–donor triads with varying flexible spacer lengths (n = 4–10 carbon atoms) are investigated in liquid and solid solution, as well as in crystals, by steady‐state and transient fluorescence spectroscopy combined with computational studies. For the short spacer (n = 4) in a liquid solution, dynamic charge‐transfer (CT) state formation with subsequent, efficient exciplex emission is observed, effectively competing with quenching through electron transfer (eT) via a radical ion pair. In a solid solution, a fluorescent CT static complex is formed upon freezing for all spacer lengths. This allows the observations of a former seminal report on stimuli‐responsive high‐contrast fluorescence on/off switching in films of the triads to be reassigned (Adv. Mater. 2012, 24, 5487), now providing a holistic picture on varying spacer length. In fact, external stimuli of the film by modulating the geometry of the CT complex, which results in on/off fluorescence switching (for n > 4) or in a change of the emission color (n = 4). The work thus demonstrates how in‐depth analysis of complex photophysics can be put to practical use in materials science. The complex photokinetics of donor–acceptor–donor triads are deciphered for practical use in materials science. In solution, fluorescent exciplexes are formed only for the short spacer (n = 4). Upon freezing, emissive static charge transfer (CT) complexes are formed for all spacer lengths. The molecular solids are stimuli‐responsive, showing on/off fluorescence switching (n > 4) or change of the emission color (n = 4).