Vibrational fluorescence spectroscopy of single conjugated polymer molecules

Fluorescence spectroscopy of conjugated polymers at the single molecule level provides unique insight into the nature of the emitting state of these organic semiconductors. We are able to verify the picture that molecular excitations form the primary photoexcitations in conjugated polymers by identifying individual chromophore units on rigid rod-like chains of a ladder-type polymer. The observation of a well-defined substructure in the vibronic progression as well as the presence of sum-frequency vibrational modes in the higher order vibrational bands demonstrate the sensitivity of the method. We find that conjugated polymers are excellent materials for single molecule experiments, exhibiting narrow transition lines accompanied only by a limited number of discrete vibrational modes offset by hundreds of cm⁻¹. We conclude that the high level of structural rigidity of the molecule as well as the presence of shielding sidegroups on the polymer chain reduces vibrational coupling both to the amorphous matrix as well as limiting the number of internal vibrational modes, in contrast to the case for small dye molecules. By studying the fluorescence from different single molecules we are able to image intramolecular and intermolecular disorder directly. We observe a distribution in energy of the electronic transitions due to the characteristic energetic disorder. The intensity of the vibronic side bands is also found to vary from molecule to molecule, which we propose to be related to conformational influence on the strength of coupling between the electronic excitation and vibrational modes. Structural relaxation and intramolecular energy transfer are studied by single molecule site-selective fluorescence. Our results suggest that even in rigid polymer molecules structural relaxation leads to a small Stokes shift of <70 cm⁻¹ upon electronic excitation of a single chromophore on a polymer chain at low temperatures. The influence of vibrational and structural relaxation on intramolecular energy transfer in these multichromophoric systems is also discussed.