Excimer Formation in Chain Self-Contact Points

Fluorescence spectra of poly(indene) (PIN) and poly(acenaphthylene) (PAN) samples of molecular weight ranging from 1.8 × 10³ to 2.9 × 10⁵ were measured in dilute solution at different temperatures (2,10, 25, and 40 °C). The fluorescence ratio (I_E/I_M) of PIN decreases upon increasing the chain molecular weight and becomes zero for chains of about 500 monomeric units. By contrast, the fluorescence ratio of PAN increases with molecular weight. The temperature dependence is also different for both polymers: in the range of temperatures studied here, PIN is in the high-temperature limit and PAN is in the low-temperature limit. These results are interpreted in terms of the number of chain self-contact points in the equilibrium conformation and also taking into account segmental diffusion. Monte Carlo simulation results of the number of self-contact points in Gaussian chains of different length are also presented.