Picosecond Time-Resolved Fluorescence Spectra of Individual Graphene Nanoribbons by Single-Molecule Fourier Transform...

We demonstrate the simultaneous acquisition of spectral and temporal information in single-molecule fluorescence by means of interferometric autocorrelation at room temperature. Time-correlated single-photon counting is used to assess the spectral dynamics on the picosecond time scale in the search for signatures of intramolecular energy transfer between different emitting sites within single cove-zigzag-edged graphene nanoribbons (GNRs). These GNRs show near-perfect photon antibunching, i.e., single-photon emission, along with sufficient photon count rates and long-term photostability to perform Fourier transform spectroscopy on the luminescence signal. Temporal gating of the fluorescence photons provides insight into intramolecular excited-state dynamics. In some GNRs, we observe that, as excitons diffuse within the nanoribbon, they annihilate with one another, increasing the degree of antibunching for photons with later arrival times. The existence of dark excited states within the GNR gives rise to the opposite effect, with photon bunching increasing as excitons are annihilated by these dark states, while they diffuse through the molecule, even when no perfect antibunching is observed. We conclude that short-lived bright and long-lived dark states exist in analogy to singlet and triplet excitations in multichromophoric molecular aggregates. The pronounced temporal dynamics in the photon statistics are not matched by the time-resolved fluorescence spectra, demonstrating the importance of studying both first- and second-order optical coherence in fluorescence in uncovering the electronic heterogeneity of GNRs, which remains masked in the ensemble.