Anomalous Fluorescence of Linear Poly(methylphenylsiloxane) in Dilute Solution at Temperatures below −50 °C

Fluorescence spectroscopy and 1 H NMR were used to study linear poly(methylphenylsiloxane) (PMPS) in dilute solution (2 × 10 -4 to 4 × 10 -3 M) at temperatures below −50 °C. Several PMPS samples covering a range of number-average molecular weights ((1.9−93) × 10 3 ) and of low polydispersity were employed. The temperature dependence of steady-state fluorescence results show two different trends, depending on chain length: (i) for the high molecular weight samples, the Stevens−Ban plot of excimer−monomer intensities levels off when reaching temperatures below −50 °C and yields an apparent rate constant for excimer formation ( k a ) which is independent of temperature; (ii) for the low molecular weight samples, the expected bell-shaped plot is observed, and k a increases with temperature in the whole range here studied (153−293 K), including also the temperatures below −50 °C. Likewise, light scattering at the incident radiation wavelength increases abruptly upon decreasing temperature below −50 °C for the high molecular weight samples. An anomalous behavior of high molecular weight samples is also observed in 1 H NMR spectra; the signals disappear in the baseline at temperatures below −50 °C, indicating a drastic loss of mobility in the millisecond time range. A conformational change coupled with interchain aggregation is argued to explain these results.