Excitation energy transfer of porphyrin in polymer thin films by time-resolved scanning near-field optical microspectroscopy

Thin films of water-soluble free-base porphyrin, 5,10,15,20-tetraphenyl-21H , 23H-porphinetetrasulphonic acid (TPPS) mixed with poly(diallyldimethyl am monium chloride) (PDDA) have been prepared by a spin-coating method, in whi ch the monomeric species were observed in the spin-coat film, whereas dimer was formed in the cast film prepared from TPPS/PDDA solution. Mesoscopic s tructures and dynamics of excitation energy migration and trapping of TPPS/ PDDA spin-coat film have been analysed by time-resolved scanning near-field optical microspectroscopy (SNOM) and atomic force microscope. The observed film structure can be classified roughly into two parts: one is a large, f locculated polymer part, and the other is a smooth part widely spread aroun d the flocculated polymers. En the smooth part, the observed spindle-like s tructure and circular hills and dips are essentially due to PDDA. The ellip soidal small structures with similar to2 mum length and <1 <mu>m width in t he flocculated polymer part show non-exponential fluorescence decays. The n on-exponential dynamics originates from the excitation energy migration amo ng TPPS monomers and energy trapping to dimers. From the analysis of fluore scence decay curves based on the equation developed by Klafter and Blumen, the spectral dimension has been estimated to be similar to1.46 for ellipsoi dal structures. These results indicate that the distribution of the chromop hore is inhomogeneous and a fractal-like structure exists even in the small domains determined by the resolution of the SNOM tip.