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.

A new conjugated ladder polymer with a polyacene skeleton was synthesized in a Aldol-type condensation protocol between benzylic and aryl-ketone side groups of suitably functionalized single-stranded presursor polymers. The photophysical...

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.

By measuring the fluorescence photon statistics of single chains of a conjugated polymer, we determine the lifetime of the metastable dark state, the triplet exciton. The single molecule emits single photons one at a time, giving rise to photon antibunching. These photons appear bunched in time over longer time scales because of excursions to the triplet dark state. Remarkably, this triplet intermittency in the fluorescence is spontaneously suppressed over time scales of seconds, implying that either triplet formation is inhibited or that triplets are selectively quenched without the singlet fluorescence being affected. Such discrete switching in the strength of photon bunching is only seen in highly ordered and rigid chains of a ladder-type conjugated polymer. It does not occur in single dye molecules. We propose that trapped photogenerated charges on the chain selectively quench triplets but not singlets, presumably because the effective diffusion length of triplets is longer along the highly rigid ladder-type backbone.

emission color of an organic light-emitting diode (OLED), for example, arises primarily from the electronic structure of the individual molecular building block making up the thin film of the device, implying that single-molecule fluorescence techniques are ideal probes of the underlying intrinsic electronic structure. [1] Intermolecular interactions can, conceivably, arise between polymer chains, inducing H-and J-type aggregation effects, [2] but such coupling is usually effectively suppressed by resorting to molecules with bulky sidechains. [3] A π-conjugated polymer material that has proven particularly interesting in this regard is ladder-type poly(para-phenylene) (LPPP). [4] This compound not only displays a remarkable structural rigidity, minimizing excited-state relaxation and therefore making absorption and luminescence spectra near-perfect mirror images of each other. [5] It also shows well-resolved vibronic transitions, attesting to the low degree of intermolecular disorder. Interchain electronic aggregation effects are virtually absent, so that ensemble absorption and emission spectra are almost identical in the dissolved form and in bulk films. [5] In fact, intermolecular interactions are so weak that the sum of single-molecule luminescence spectra almost perfectly replicates the bulk-film ensemble spectrum. [6] At the same time, LPPP-based materials have a high fluorescence quantum yield and exhibit substantial photostability, making them ideal for applications involving large excitation densities. [5] Feldmann and Lemmer pioneered the use of LPPP in low-threshold mechanically flexible laser structures, [7] a feat that received widespread attention culminating in the award of the Philip-Morris Research Prize. [8] To this day, a photograph of the far-field intensity pattern generated by such a plastic laser [9] decorates the cover of the journal "Organic Electronics". [10] More recently, the unique characteristics of LPPP have been exploited in optical microcavities to create exciton-polariton condensates by strong light-matter coupling, [11] which can even enable optical transistor-like action [12] and single-photon optical nonlinearities at room temperature. [13] Inferring polymer aggregation effects from spectral signatures alone can be challenging and requires a detailed understanding A fundamental question relating to the nature of light emission and absorption in organic semiconductors is the dimension of the domain within a bulk material responsible for the interaction of light and matter. How large can a nanoparticle become to retain the quantized nature of light emission? Excitons are only a few nanometers in size, but because they diffuse in space, they probe a much larger volume than the single molecule. When excitons meet, they may decay non-radiatively by singlet-singlet or singlet-triplet annihilation (SSA or STA). Fluorescence photon statistics reveal whether single photons are emitted (photon antibunching) or arrive in randomly spaced packets (photon bunching), offering direct insight ...