Time sensitive, temperature and pH responsive photoluminescence behaviour of a melamine containing bicomponent hydrogel

Abstract

Melamine (M) produces supramolecular complexes MQ31, MQ11 and MQ13 with 6,7-dimethoxy-2,4[1H,3H]-quinazolinedione (Q) (numbers indicate respective molar ratios of components) and they produce thermoreversible hydrogels [≥0.1% (w/v)] at 30 °C. Optical microscopy shows fibrillar network morphology and on irradiation with 300 nm light, the fibrils emit blue light. DSC results indicate a thermoreversible first order phase transition and the storage modulus (G') is invariant with frequency supporting the formation of thermoreversible hydrogel. The magnitude of G' follows the order MQ11 > MQ13 > MQ31 and a probable cause is attributed to number of H-bonds, which follows the same order in the nanofibers. FTIR-spectra suggest H-bonding interaction between components and WAXS results indicate a different crystalline structure of the complexes. The UV-vis spectra of the MQ systems shows red shift of both π-π and n-π∗ bands and a maximum red shift is observed for the MQ13 system. The PL intensity decreases with increase in M concentration and lifetime data indicates PL quenching due to the formation of less fluorescent complexes. The PL property of the gel depends on aging time. A gradual development of a new emission peak at the expanse of an initial peak suggests the formation of an initial metastable state, which transforms into the stable state after 24 h of aging. The gel is stable in the pH range 6-9, above or below which the gel breaks down showing a significant PL-quenching and a blue shift of the emission peak. With increase of temperature, the emission peak intensity increases at first up to 45 °C, followed by an abrupt decrease at ≥55 °C. Possible reasons are attributed to the gradual disassembly of the complexes converting thinner fibers and finally melting.