Insights into the Photophysics and Supramolecular Organization of Congo Red in Solution and the Solid State

Steady‐state and time‐resolved absorption and fluorescence measurements are reported for Congo Red (CR) in aqueous and dimethylsulfoxide (DMSO) solutions. The very low fluorescence quantum yield (≈10−4) for CR in dilute solutions together with the absence of a triplet state indicates that internal conversion is the dominant deactivation route with more than 99.99 % of the quanta loss (attributed to the energy gap law for radiationless transitions). Although no direct evidence for trans–cis photoisomerization was obtained from absorption or fluorescence data, the global analysis of fs‐transient absorption data indicates the presence of a photoproduct with a lifetime of ≈170 ps that is suggested to be associated with such a process. Spectral data for more concentrated CR solutions indicate the presence of oblique or twisted J‐type aggregates. These results are compared with spectra for CR in the solid state (sodium salt) and intercalated in a layered double hydroxide via a one‐step co‐precipitation route. Powder XRD and electronic spectral data for the nanohybrid indicate that the CR guest molecules are intercalated as a monolayer consisting of slipped cofacial J‐type aggregates. The self‐assembly of the anionic diazo dye Congo Red (CR) in the solid state was investigated by intercalating the dye in a layered double hydroxide and characterizing the resultant hybrid material by various techniques. In combination with a deep analysis of the photophysics of CR in solution, the results point towards the presence of slipped cofacial J‐type aggregates in the hybrid material, giving rise to the exceptionally large basal spacing of 26.2 Å (see picture).