Intermolecular Interactions Determine Exciton Lifetimes in Neat Films and Solid State Solutions of Metal-Free Phthalocyanine

Abstract

Thin films of vapor-deposited metal-free phthalocyanine (H2Pc) were studied using ultrafast transient absorption spectroscopy in the visible region. Following photoexcitation, an excited state absorption feature located near 532 nm was observed which served as a probe of the excited state. For exciton densities larger than 5 × 1018 excitons/cm3 the time-dependent measurements of the excited state absorption included the presence of nonexponential decay kinetics attributed to exciton–exciton annihilation. At exciton densities less than 5 × 1018 excitons/cm3 annihilation was negligible, and the decay kinetics appeared single exponential within the signal-to-noise. The fitted time constant, 239 ± 24 ps, was attributed to the lifetime decay of the singlet excitons. When the H2Pc was diluted into a wide energy gap host via vapor deposition, the observed lifetime was significantly reduced, reaching 87 ± 9 ps for a concentration of 25% H2Pc. The decreased exciton lifetime with dilution was remarkable since it ha...