Simultaneous Raman and Fluorescence Spectroscopy of Single Conjugated Polymer Chains

Simultaneous surface enhanced Raman scattering (SERS) and fluorescence is demonstrated from single conjugated polymer chains. As resonance enhancement of SERS depends on the spectral overlap of the polymer’s absorption and the incident laser, resonance Raman and fluorescence effectively probe the absorbing and emitting part of the polymer, respectively. The optical phonon energies change along the polymer chain, providing a window to spatially track excited state relaxation. Whereas a mean spatial redistribution of the excitation is witnessed by a change in vibronic fingerprint following interchromophoric energy transfer, intrachromophoric exciton self-trapping leaves the vibrations unchanged.

Figure 1

PL (dotted line) and SERRS spectra of single poly(phenylene-ethynylene-butadiynylene) chains at 5 K under excitation at $21.84\times {10}^3{\mathrm{cm}}^{-1}$. The energy of the PL maximum is shifted to $0{\mathrm{cm}}^{-1}$.

Figure 2

Influence of spectral diffusion on simultaneous SERRS and PL from a single chromophore on a single polymer chain. (a) Intensity time trace showing PL and RRS [logarithmic color coding of intensity given in panel (d)]. (b) Two example spectra taken at times marked by arrows in (a). (c) Correlation between SERRS intensity and PL photon energy of the $\mathrm{C}\mathrm{C}$ mode. (d) Correlation between SERRS and PL intensity of the $\mathrm{C}\mathrm{C}$ mode.

Figure 3

PL and SERRS occurring from different chromophores of a single polymer chain. (a) PL and SERRS (logarithmic scale) as a function of time. (b) Absence of a correlation between SERRS intensity and PL photon energy of the $\mathrm{C}\mathrm{C}$ mode. (c) A correlation is observed following the spectral jump to the blue at $t=610\mathrm{s}$ (box), suggesting SERRS and PL now arise from the same chromophore (sketch).

Figure 4

Scatter plot of phonon energies ${\omega }^{\mathrm{C}\mathrm{C}}$ about the ideal line, derived from 56 different single chains displaying simultaneous PL and SERRS. We distinguish situations for which energy transfer (ET) is (a) probable and (b) improbable. (a) The absorbing chromophore has a different phonon energy (${\omega }_{\mathrm{RS}}^{\mathrm{C}\mathrm{C}}$) to the emitting one (${\omega }_{PL}^{\mathrm{C}\mathrm{C}}$). (b) Absorbing and emitting chromophores are identical. PL and SERRS display the same phonon energies, indicating that exciton self-trapping occurs randomly throughout the chromophore. This is illustrated in the cartoon by the transition from light to dark gray.