Optical properties of free-base tetraphenylporphyrin embedded in fluorinated polyimides and their ethanol and water vapours sensing capabilities

Thin films of different fluorinated polyimides derived from 4,4’-hexafluoroisopropylidene diphthalic anhydride (6FDA), 2,3,5,6-tetramethyl paraphenylene diamine (DAD) and 3,3’- diaminodiphenylsulfone (DDS) with dispersed 5,10,15,20-tetraphenylporphyrin (H2TPP) were produced. The chemical structure and the surface morphology and topography of the final films were studied by Fourier transform infrared spectroscopy (FTIR) and atomic force microscopy (AFM). The photophysical properties of the films as derived from absorption, excitation and emission spectra were thoroughly investigated and correlated to the distribution and possible aggregation of H2TPP moieties in the polyimide matrix. Optical sensing capabilities of these composite films were tested by exposing them to ethanol and water vapour atmospheres. Unusual emission spectral changes of these materials upon exposure to ethanol and water vapours have been interpreted in terms of porphyrin macrocycle distortions induced by the presence of ethanol molecules which ultimately affects the position and shape of the potential curves representing the S0 and S1 electronic states. The fluorescence signal drop detected on incoming analyte is fully and swiftly recovered upon re-exposure of the sensing material to pure nitrogen. The optical response is steady, reproducible and very fast (tens of seconds).