Most measurements of fluorescence lifetimes on the single-molecule level are carried out using avalanche photon diodes (APDs). These single-photon counters are inherently slow, and their response shows a strong dependence on photon energy, which can make reconvolution of the instrument response function (IRF) challenging. An ultrafast time resolution in single-molecule fluorescence is crucial, e.g., in determining donor lifetimes in donor–acceptor couples which undergo energy transfer, or in plasmonic antenna structures, where the radiative rate and non-radiative rates are enhanced. We introduce a femtosecond double-excitation (FeDEx) photon correlation technique, which measures the degree of photon antibunching as a function of time delay between two excitation pulses. In this boxcar integration, the time resolution of fluorescence transients is limited solely by the laser pulse length and is independent of the detector IRF. The versatility of the technique is demonstrated with a custom-made donor–acceptor complex with one donor and two acceptors and with single dye molecules positioned accurately between two gold nanoparticles using DNA origami. The latter structures show ∼75-fold radiative-rate enhancement and fluorescence lifetimes down to 19 ps, which is measured without the need for any reconvolution. With the potential of measuring subpicosecond fluorescence lifetimes, plasmonic antenna structures can now be optimized further.

The breaking of molecular symmetry through photoexcitation is a ubiquitous but rather elusive process, which, for example, controls the microscopic efficiency of light harvesting in molecular aggregates. A molecular excitation within a π-conjugated segment will self-localize due to strong coupling to molecular vibrations, locally changing bond alternation in a process which is fundamentally nondeterministic. Probing such symmetry breaking usually relies on polarization-resolved fluorescence, which is most powerful on the level of single molecules. Here, we explore symmetry breaking by designing a large, asymmetric acceptor-donor-acceptor (A-D-A) complex 10 nm in length, where excitation energy can flow from the donor, a π-conjugated oligomer, to either one of the two boron-dipyrromethene (bodipy) dye acceptors of different color. Fluorescence correlation spectroscopy (FCS) reveals a nondeterministic switching between the energy-transfer pathways from the oligomer to the two acceptor groups on the submillisecond timescale. We conclude that excitation energy transfer, and light harvesting in general, are fundamentally nondeterministic processes, which can be strongly perturbed by external stimuli. A simple demonstration of the relation between exciton localization within the extended π-system and energy transfer to the endcap is given by considering the selectivity of endcap emission through the polarization of the excitation light in triads with bent oligomer backbones. Bending leads to increased localization so that the molecule acquires bichromophoric characteristics in terms of its fluorescence photon statistics.

Alkyl- and alkoxy-substituted p-phenylene–butadiynylene tetramers, obtained via a stepwise synthesis, are polymerized under Glaser coupling conditions. The fractionation of the oligomers by recycling gel permeation chromatography (recGPC) yields three sets of monodisperse oligo­(p-phenylene–butadiynylene)­s (OPBs) with degrees of polymerization n = 4, 8, 12, ..., 60. Subsequent GPC analysis of these oligomers provides three sets of size- and substituent-dependent conversion factors that quantify the overestimation of the oligomer molecular weight when a polystyrene (PS) calibrated GPC system is used for the data analysis. Scanning tunneling microscopy (STM) of representative oligomers at the solid/liquid interface visualizes the monodispersity, the length-dependent rigidity, and their assembling behavior.

The photophysical properties of a series of para-substituted donor-acceptor cruciform fluorophores (p1-4) were investigated and compared with their meta and ortho isomers (m1-4 and o1-4). The structural variations were found to have a significant effect on the solvatochromism, fluorescence quantum yields (Φ(fl)), fluorescence lifetimes (τ(fl)), and response upon addition of trifluoroacetic acid. The observed spectral shifts in absorption and emission caused by protonation of the cruciforms make them promising candidates as chemosensors. Additional computational studies provided more insight into the electronic structure of the systems.

We examine the redistribution of energy between electronic and vibrational degrees of freedom that takes place between a π-conjugated oligomer, a phenylene–butadiynylene, and two identical boron–dipyrromethene (bodipy) end-caps using femtosecond transient absorption spectroscopy, single-molecule spectroscopy, and nonadiabatic excited-state molecular dynamics (NEXMD) modeling techniques. The molecular structure represents an excitonic seesaw in that the excitation energy on the oligomer backbone can migrate to either one end-cap or the other, but not to both. The NEXMD simulations closely reproduce the characteristic time scale for redistribution of electronic and vibrational energy of 2.2 ps and uncover the vibrational modes contributing to the intramolecular relaxation. The calculations indicate that the dihedral angle between the bodipy dye and the oligomer change upon excitation of the oligomer. Single-molecule experiments reveal a difference in photoluminescence lifetime of the bodipy dyes depending on whether they are excited by direct absorption or by redistribution of energy from the backbone. This difference in lifetime may be attributed to the difference in dihedral angle. The simulations also suggest that a strong coupling can occur between the two end-caps, giving rise to a reversible shuttling of excitation energy between them. Strong coupling should lead to a pronounced loss in polarization memory of the fluorescence since the oligomer backbone tends to be slightly distorted and the two bodipy transition dipoles have different orientations. A sensitive single-molecule technique is presented to test for such coupling. However, although redistribution of electronic and vibrational energy between the end-caps can occur, it appears to be unidirectional and irreversible, suggesting that an additional localization mechanism is at play which is, as yet, not fully accounted for in the simulations.