Fluorescence monitoring of polyacrylamide hydrogel using 4-aminophthalimide

4-Aminophthalimide (4-AP) trapped in water-filled micropores of polyacrylamide (PAA) gel is studied using steady state and time-resolved emission spectroscopy. The position of the emission maxima and the temporal decay of 4-AP in the hydrogel indicate the presence of multiple microenvironments arising from the variation of the pore size. In PAA gel, the emission maximum of 4-AP exhibits a marked dependence on the wavelength of excitation and varies from 470 to 550 nm as lambda(ex) is varied from 300 to 400 nm. Global lifetime analysis at various wavelengths suggests the presence of two species of lifetime 1.34 and 7.21 ns with the relative amplitude of the short component increasing with an increase in the emission wavelength. The 550 nm emission band with a short lifetime (amplitude of the short component being 0.86) corresponds to an environment very similar to that of water. The 470 nm band with a much longer lifetime (amplitude of the long component, 0.85), however, implies a medium far less polar than water that does not allow the solvent-mediated proton transfer to occur. No dynamic Stokes shift, is observed, which implies that even inside the semirigid gel the water molecules relax at a very fast rate (<60 ps). It is also observed that the rotational relaxation time of 4-AP in PAA gel is too fast to be detected in a picosecond setup.