Linewidth-Limited Energy Transfer in Single Conjugated Polymer Molecules

Using low temperature single molecule spectroscopy on rigid-rod conjugated polymers we are able to identify homogeneously broadened, strongly polarized emission from individual chromophore units on a single chain. Gated fluorescence spectroscopy allows real time imaging of intramolecular energy transfer as the chain behaves as a series of weakly interacting chromophores. Energy transfer is controlled by the chromophoric spectral linewidth, which depends on temperature. Linewidths exceeding intramolecular disorder lead to incoherent chromophore coupling and collective fluorescence phenomena.

Figure 1

(a) Single molecule PL spectra recorded at 5 K (black or blue), 80 K (light gray or orange), and 160 K (gray or red) of MeLPPP in a polystyrene matrix. The polymer chain consists of individual chromophore units of slightly different energies. At low temperatures the fine structure arising from emission from single chromophores on the chain becomes visible. (b) Single molecule PL spectra of a second molecule detected through a polarization filter at orthogonal polarizations at 20 and 250 K (c), showing the existence of a number of chromophore units with different orientations. The sketch indicates the spectral overlap between donor (D) and acceptor (A) chromophores required for intramolecular energy transfer. The structure of the polymer is given at the top (R1: $n$-hexyl, R2: $n$-decyl, R3: methyl).

Figure 2

Temporal evolution of single chain PL at 5 K showing intramolecular energy transfer from a higher energy to a lower energy chromophore unit. The dotted lines indicate Lorentzian fits to the spectrum, the full lines give the sum of the two peaks.

Figure 3

Comparison of bulk film (a) and single chromophore (b) spectra at three different temperatures. (c) Average SM polarization ratios in emission as a function of temperature. (d) Spectral linewidth $\Gamma$ of a single emitting chromophore as a function of temperature (black triangles). The dashed line gives the average level of intramolecular disorder $\Delta E$. The right axis shows the spectral position determined for SMs (red squares) and a bulk film (dark gray or red circles).

Figure 4

Single chain fluorescence transients recorded at 5 and 295 K for 500 ms integration windows showing fluorescence intermittency at room temperature. The dotted curve indicates the background noise.