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 ${\text{cm}}^{-1}$. 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 $<70{\text{cm}}^{-1}$ 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.

Figure 1

Fluorescence from a single MeLPPP molecule at $5\mathrm{K}$ detected with a spectral resolution of $\sim 2.5\text{meV}$ under nonresonant, broadband excitation in the vibrational manifold of the molecule at $2.91\text{eV}$ (laser line width $50\text{meV}$). The inset shows the polarization of the two emission lines (labeled with a square and a circle) which is determined by rotating a polarizer in the emission pathway of the microscope. The chemical structure of MeLPPP is also given (X: methyl, $\mathrm{R}:n-\text{hexyl},{\mathrm{R}}^{\prime }:$ 1,4-decylphenyl).

Figure 2

(a) Ensemble fluorescence spectrum of MeLPPP dispersed in polystyrene at ${10}^{-4}$ molar concentration measured at $5\mathrm{K}$. Three vibronic bands are identified and labeled. The absorption spectrum of a typical bulk film is also shown for reference (dotted line). (b) Single molecule fluorescence spectrum under nonresonant broadband excitation at $5\mathrm{K}$ measured with a spectral resolution of $10\text{meV}$. Inset: Vibronic progression of the single molecule fluorescence measured with a spectral resolution of $\sim 2.5\text{meV}$. The solid lines correspond to Voigt fits.

Figure 3

Fluorescence from the first and second vibronic bands of a single MeLPPP molecule under narrow band resonant excitation of the 0-0 transition. The solid line corresponds to a Lorentzian fit and demonstrates the presence of the ${\nu }_2+{\nu }_3$ sum-frequency transition in the 0-2 vibronic sideband.

Figure 4

High resolution fluorescence spectrum of a single MeLPPP molecule shifted by its center frequency and plotted on a logarithmic scale. The solid line shows a Gaussian fit to the high energy tail, which reveals a small degree of broadening to the red of the main transition. Besides a very weak feature at ${190\text{cm}}^{-1}$, no low frequency vibrational modes are apparent. The inset depicts the same spectrum on a linear scale.

Figure 5

Comparison of the linewidths of the 0-0 and vibronic emission bands in a single molecule. The lines show the 0-0 transition at $2.7\text{eV}$, which is shifted in energy and rescaled to overlay the ${\nu }_2$ and ${\nu }_3$ vibronic bands (symbols).

Figure 6

(a) Four different single molecule fluorescence spectra normalized to the 0-0 transition line, which is limited in width by the instrument response. The strength of the vibronic side bands differs from molecule to molecule. The inset shows the distribution of measured intensities of the ${\nu }_3$ vibronic sideband. (b) Magnification of the spectra shown in (a).

Figure 7

(a) Single molecule PL spectrum under narrow band resonant excitation at $2.708\text{eV}$. The vibronic doublet of the 0-1 transition is visible around $2.5\text{eV}$ as well as the laser line at $2.708\text{eV}$. The energetic difference between the ${\nu }_2$ vibronic peak and the laser peak, which is resonant with the 0-0 transition, is extracted from the figure and is defined as $E_{\text{Exc}}-E_{\text{PL},{\nu }_2}$. (b) Energetic difference between the laser line and the vibronic fluorescence band for the ${\nu }_2$ $(▴)$ and ${\nu }_3(•)$ peaks. The energy of the ${\nu }_2$ and ${\nu }_3$ transitions are marked by dashed lines, which should correspond to$E_{\text{Exc}}-E_{\text{PL}}$ in the absence of structural relaxation and energy transfer. The lower panel shows the difference $E_{\text{PL},{\nu }_3}-E_{\text{PL},{\nu }_2}(◻)$, demonstrating that the peak positions are determined within a precision of $15{\text{cm}}^{-1}$.