The triplet exciton dynamics and diffusion properties of zinc and platinum-octaethylporphyrin nanoaggregates

J-type nanosized-aggregates (NA) of zinc and platinum-octaethylporphyrin (ZnOEP and PtOEP) are prepared and observed to undergo fast triplet formation via inter system crossing (ISC) process. The ultrafast transient absorption measurement of ZnOEP NA indicates fast decay of singlet excitonic state with the lifetime of similar to 9 ps. On other hand, the singlet decay rate is even faster for PtOEP NA, having lifetime value of similar to 0.8 ps. Singlet excitonic decays cause subsequent population of triplet states which possess long annihilation free lifetimes of 1.7 and 0.6 ms for ZnOEP and PtOEP NA, respectively. However, at high exciton densities the triplet excitons are observed to decay via triplet-triplet annihilation (TTA) and their natural radiative, non-radiative decay processes. The exciton density dependent triplet decay dynamics further analyzed to estimate TTA rate constants and exciton diffusion parameters. The triplet exciton diffusion coefficient values thus obtained are 4.06 x 10(-7) and 0.94 x 10(-7) cm(2) s(-1) for ZnOEP and PtOEP NAs, respectively. The diffusion coefficients possess nearly two order smaller values as compared to those of the commonly reported singlet excitons. Involvement of spin exchange based short range transfer mechanism of triplet exciton make this diffusion process slower. However, the possible involvement of singlet-triplet cooperative diffusion mechanism results similar to 4.3 time faster triplet diffusion in ZnOEP NAs as compared to that of the PtOEP NAs. The slow exciton diffusion properties of triplet enhance the importance of requirement of long triplet lifetime to obtain an elongated triplet diffusion length. These two nanoparticles are observed to fulfill the long lifetime criteria and own reasonably long triplet diffusion lengths which may be beneficial for solar cell, photon up-conversion and singlet oxygen generation based applications.