Investigation of Energy-Transfer Mechanisms in Pyrene Crystals

In this paper we have examined the fluorescence emission spectra, intensity, and decay characteristics and the excitation spectra of pure pyrene crystals and of perylene‐doped pyrene crystals at temperatures down to 1.7°K. From these observations we conclude that energy transfer in pyrene crystals occurs by four different mechanisms with the following characteristics: (i) Reabsorption of emitted photons: important at high guest concentrations or with thick samples. (ii) Monomer exciton migration: Operates at all temperatures with a transfer rate constant on the order of ${\text{∼10}}^{12}\hspace{0.17em}{\sec }^{\text{−1}}$ . (iii) Excimer exciton migration: important only at temperatures above about 130°K, thermally activated with an apparent activation energy of 112 cm⁻¹ and a transfer rate constant of ${\text{4×10}}^6\hspace{0.17em}{\sec }^{\text{−1}}$ at room temperature. (iv) Forster transfer from excimers to perylene guest molecules: important only with perylene concentrations greater than about 10⁻³ mole/mole, or at slightly lower concentrations at reduced temperatures. In addition to obtaining information about the nature of energy‐transfer processes which take place in pyrene crystals, we have also examined the crystal absorption spectrum on the long‐wavelength side of the first major absorption band. The low‐temperature excimer excitation spectra of highly purified pyrene crystals exhibits a tail which extends almost to 425 nm and appears to overlap with the origin of the excimer emission. This tail absorption is attributed to direct excitation to the excimer state.