Solid‐State Fluorescence Emission Enhancement in H‐Type Aggregate of Single Benzene Fluorophore via Grinding With Metal Salts: Importance of Dipole‐Metal Ion Interactions

ABSTRACT

The ability of showing solid‐state fluorescence emission intensities were pre‐requisites for any fluorescent material to be used for various applications. Various synthetic routes have been adopting to synthesize the desired molecules having extended conjugation and various functional groups. However economic point of view and aggregation‐induced quenching phenomena are the limiting points in this route. Realizing the fluorescent emission from molecules of single benzene unit with few functional groups is the interesting aspect. In the present study, we have chosen 2‐hydroxybenzhydrazide (2HBH) that showed enhanced solid‐state fluorescence emission intensity than in solution state. Even though it exists as antiparallel H‐type dimer in the solid state, it showed solid state fluorescence emission. On the other hand, the other polymorph of 2HBH showed J‐type aggregates in the crystal structure obtained from acetonitrile + water solvent mixture. Interestingly 2HBH upon grinding with various alkaline metal salts, the solid matrices showed enhanced emission intensities as well as quantum yields than solid 2HBH alone. With potassium salts the enhancement was in order of 180 times when compared to the weight of the active 2‐HBH compound in the solids. The slight enhancement of cell parameters was observed via powder x‐ray diffraction studies probably because of specific dipole‐ion interactions in the solid state. Low‐temperature solid‐state emission investigations reveal the enhancement of emission intensities at 105 K in pristine as well as 2HBH + KCl mixed solid; while the enhancement was more in the latter solid. Computational modeling studies revealed that this 2HBH molecule can undergo keto‐enol excited‐state intramolecular proton transfer (ESIPT) phenomena in the excited state. Also, important point observed is that 2HBH‐metal ion combined optimized systems showed that the two oxygen atoms on 2HBH molecule and showed optimum lower bond lengths with potassium ion, than other metal ions might contribute to the enhancement of fluorescence emission showed by KCl and KBr salt‐grinded samples. Frontier molecular orbital analysis showed the existence of electron cloud on 2HBH moiety in HOMO while it is predominantly existed on the metal ion in the LUMO.