Time-Resolved and Near-Field Scanning Optical Microscopy Study on Porphyrin J-Aggregate

Water-soluble porphyrin, 5,10,15,20-tetraphenyl- 21H,23H-porphine-tetrasulfonic acid (TPPS), forms J-aggregate in aqueous solution depending on experimental conditions such as pH, dye concentration, and/or ionic strength. The steady-state fluorescence and picosecond single-photon timing spectroscopy were applied for protonated monomer and J-aggregate in aqueous solution and in thin films to reveal the dynamics in the S$\text{}_{2}$ and S$\text{}_{1}$ states. The S$\text{}_{2}$ fluorescence spectra from the protonated monomer and J-aggregate were observed in addition to the normal S$\text{}_{1}$ fluorescence. The lifetime of the S$\text{}_{2}$ state was estimated to be ≈5 ps for J-aggregate, whereas the lifetime is shorter than 1 ps for protonated TPPS monomer. The mesoscopic structures of J-aggregate in thin film with and without polymer on the glass surface were examined by scanning near-field optical microscopy. With the surface topography and scanning near-field optical microscopy transmission images, TPPS J-aggregate was found to form a long and narrow tube-like structure which has a few μm length, 0.2-0.5 μm width, and 5-30 nm height. An unidirectional orientation of the structure was also found, which may be originated from the spin-coating process.