Photoluminescence and transient spectroscopy of free base porphyrin aggregates

Solutions of free base meso-tetraphenylporphyrin (H2TPP), meso-tetra(4-carboxyphenyl)porphyrin (H2TPPC), and meso-tetra(4-pyridyl)porphyrin (H2TPyP) under various conditions generate aggregates whose absorption spectra are characterized by invariant Soret bands with bandwidths that are independent of the preparative method. One of the Soret bands is blue-shifted (H-aggregate) relative to the monomeric porphyrin band; other Soret bands are red-shifted (J-aggregates). The aggregates are characterized by different nonradiative rate constants for excited singlet-state decay and by different efficiencies of singlet-singlet annihilation at the high energies of laser excitation. The quantum yields of fluorescence vary between 10(-5) and 10(-2), and the corresponding fluorescence lifetimes vary in the range from 10(-12) to 10(-9) s; they are more than 1 order of magnitude smaller than those of the corresponding monomeric porphyrins. Lifetimes (tau) correlate with the characteristic ground-state absorption recovery times of the aggregates. The sizes of the H-aggregate and aggregates that are characterized by a minimal blue shift of the Soret band range from 15 to 27 Angstrom.