Composites of an Excited State Proton Transfer Dye with Chitosan and ZIF-4 Metal Organic Frameworks: Keto Emission from Aggregates and Anion-Type Optical Behavior

Tuning the emission of synthesized organic dyes in their solid-state composites is an area of research of paramount interest due to their varied uses in optoelectronic applications. In the current work, we present a comprehensive study of the spectrodynamic properties of a reported excited-state intramolecular proton transfer (ESIPT) active compound, 2,6-diformyl 4-methyl phenol (DFP), in two different solid matrices, Chitosan (DFP@Chitosan) and ZIF-4 (DFP@ZIF-4) metal organic frameworks, by means of steady-state and time-resolved emission spectroscopic experiments. Our experiments established the differential behavior of DFP in the two matrices. Although present in the solid state, DFP shows anion-like optical behavior in both cases owing to extensive hydrogen bonding interactions with the two matrices. In addition, DFP exists primarily as aggregates in both DFP@Chitosan and DFP@ZIF-4. In the former case, aggregates having differing emission lifetimes are present, each furnishing their corresponding keto emission, the lifetime values of which range from ∼200 ps to 7.0 ns. On the other hand, the aggregates in DFP@ZIF-4 furnish keto emission lifetime values of only ∼200 ps to 1.2 ns. Such a differential spectrodynamic behavior hinted at preferential stabilization of aggregates having shorter excited-state lifetime values in the ZIF-4 matrix, as opposed to that of DFP@Chitosan, where all types of aggregates were stabilized. The differential behavior arises principally due to extra metal-cluster-fluorophore interactions in DFP@ZIF-4 in addition to the existing hydrogen bonding interactions. Hence, the current report clearly delineates how the excited-state emission behavior of an ESIPT active dye could be tuned at will by simply changing the solid-state matrix. The current results warrant their importance in setting guidelines for the future design of solid-state emissive materials for applications in optics and photonics.