Single molecule probing of the local segmental relaxation dynamics in polymer above the glass transition temperature

We investigate the temporal dynamics of terrylene diimide molecule with four phenoxy rings (TDI) in a poly(styrene) (PS) matrix in the supercooled regime by use of single molecule spectroscopy. By recording both fluorescence lifetime and linear dichroism observables simultaneously, we show that the TDI dye molecule is a versatile probe of the local dynamics in the polymer. The molecule is able to undergo conformational changes, as indicated by lifetime fluctuations and/or reorientation jumps, as indicated by both observables on different time scales. Owing to molecular mechanics and quantum calculations, we could assign the conformational changes to folding/unfolding event(s) of one or more arms with respect to the conjugated core. We tentatively attribute the different spatial extents of the locally probed motions to the alpha and beta relaxation processes occurring in the PS matrix.