Poly Excited State Lifetime Measurements

Lochmuller and coworkers used the formation of excimer species to answer a distance between site question related to the organization and distribution of molecules bound to the surface of silica xerogels such as those used for chromatography bound phases. Pyrene is a flat, poly aromatic molecule whose excited state is more pi-acidic than the ground state. An excited state of pyrene that can approach a ground state pyrene within 7A will form an excimer Pyr +Pyr (Pyr)2. Monomer pyrene emits at a wavelength shorter than the excimer and so isolated versus near-neighbor estimates can be made. In order to do this quantitatively, these researchers turned to measure lifetime because the monomer and excimer are known to have different lifetimes in solution. This is also a way to introduce the concept of excited state lifetime. 

Because of its spin-forbidden nature, the decay rate of the first excited triplet state is extremely low, and very difficult to quantify accurately. What can be measured is the phosphorescence lifetime. Typical long time decays for a poly(bi-spirofluorene) (PSBF) are shown in Fig. 6. 

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