Fluorescent H‐Aggregates of Merocyanine Dyes

Unexpected lighting up: A slight rotational twist and rigidification of merocyanine dyes in a face-to-face π-π-stacked dimer aggregate resulted in an unexpected increase in the fluorescence intensity and lifetime (see graph). This result contrasts the common perception that the fluorescence of H-aggregates is strongly quenched, but can be rationalized within the concept of exciton theory.