Vibrational fluorescence spectroscopy of single conjugated polymer molecules

Abstract

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 Ͻ70 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.

Unraveling the chromophoric disorder of poly(3-hexylthiophene)

“…1D). Strong variations in the PL intensity of the vibronic progression between single chromophores have been reported previously, even in nominally rigid materials such as LPPP (51). Such variations arise due to the slight distribution in ground-state molecular conformations, leading to differing structural relaxation energies and the resulting changes in the Franck-Condon progression, which become visible precisely because electronic and vibronic transitions are so narrow at low temperature.…”

Counting Chromophores in Conjugated Polymers

“…[4g] This effect, which is in part related to structural relaxation (although this is very weak indeed in the ladder-type polymers [19] ), has also been investigated theoretically, [24,25] as a precise knowledge of the size of the emitting and absorbing unit is imperative for understanding and modeling intramolecular energy transfer. [25] The conclusion of extended conjugations naturally raises the question of the origin of static disorder, which gives rise to a distribution in chromophore energies and the associated inhomogeneous broadening witnessed in Figure 2 d. The common notion is that topological defects lead to a confinement of the excitation and thus to a modification in transition energy.…”

“…[12][13][14][15][16][17] We recently showed that SMS can provide unique information on the presence of chromophores in CPs and may be used to investigate intramolecular interchromophoric couplings. [18][19][20][21] Single chromophores exhibit universal photophysical properties independent of the details of the materials chemistry, thus enabling the derivation of universal structure-property relationships. [21] Herein we address the fundamental issue of how the number of chromophores varies with molecular size (i.e.…”

A universal picture of chromophores in π-conjugated polymers derived from single-molecule spectroscopy

“…1b to emission from a single chromophore unit. The PL is linearly polarized in emission and absorption (22,24,25). We recently reported multiple line emission from single MeLPPP molecules and suggested that this corresponds to emission from a number of chromophore units on the chain (22).…”