The particle-like nature of light becomes evident in the photon statistics of fluorescence from single quantum systems as photon antibunching. In multichromophoric systems, exciton diffusion and subsequent annihilation occurs. These processes also yield photon antibunching but cannot be interpreted reliably. Here we develop picosecond time-resolved antibunching to identify and decode such processes. We use this method to measure the true number of chromophores on well-defined multichromophoric DNA-origami structures, and precisely determine the distance-dependent rates of annihilation between excitons. Further, this allows us to measure exciton diffusion in mesoscopic H- and J-type conjugated-polymer aggregates. We distinguish between one-dimensional intra-chain and three-dimensional inter-chain exciton diffusion at different times after excitation and determine the disorder-dependent diffusion lengths. Our method provides a powerful lens through which excitons can be studied at the single-particle level, enabling the rational design of improved excitonic probes such as ultra-bright fluorescent nanoparticles and materials for optoelectronic devices.

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BackgroundMore than 8.5 million Germans suffer from chronic diseases attributable to smoking. Education Against Tobacco (EAT) is a multinational network of medical students who volunteer for school-based prevention in the classroom setting, amongst other activities. EAT has been implemented in 28 medical schools in Germany and is present in 13 additional countries around the globe. A recent quasi-experimental study showed significant short-term smoking cessation effects on 11-to-15-year-old adolescents.ObjectiveThe aim of this study was to provide the first randomized long-term evaluation of the optimized 2014 EAT curriculum involving a photoaging software for its effectiveness in reducing the smoking prevalence among 11-to-15-year-old pupils in German secondary schools.MethodsA randomized controlled trial was undertaken with 1504 adolescents from 9 German secondary schools, aged 11-15 years in grades 6-8, of which 718 (47.74%) were identifiable for the prospective sample at the 12-month follow-up. The experimental study design included measurements at baseline (t1), 6 months (t2), and 12 months postintervention (t3), via questionnaire. The study groups consisted of 40 randomized classes that received the standardized EAT intervention (two medical student-led interactive modules taking 120 minutes total) and 34 control classes within the same schools (no intervention). The primary endpoint was the difference in smoking prevalence from t1 to t3 in the control group versus the difference from t1 to t3 in the intervention group. The differences in smoking behavior (smoking onset, quitting) between the two groups, as well as gender-specific effects, were studied as secondary outcomes.ResultsNone of the effects were significant due to a high loss-to-follow-up effect (52.26%, 786/1504). From baseline to the two follow-up time points, the prevalence of smoking increased from 3.1% to 5.2% to 7.2% in the control group and from 3.0% to 5.4% to 5.8% in the intervention group (number needed to treat [NNT]=68). Notable differences were observed between the groups for the female gender (4.2% to 9.5% for control vs 4.0% to 5.2% for intervention; NNT=24 for females vs NNT=207 for males), low educational background (7.3% to 12% for control vs 6.1% to 8.7% for intervention; NNT=30), and migrational background (students who claimed that at least one parent was not born in Germany) at the 12-month follow-up. The intervention appears to prevent smoking onset (NNT=63) but does not appear to initiate quitting.ConclusionsThe intervention appears to prevent smoking, especially in females and students with a low educational background.

Single formamidinium lead bromide (FAPbBr 3) perovskite nanocubes, approximately 10 nm in size, have extinction cross sections orders of magnitude larger than single dye molecules and can therefore be used to photoexcite one single dye molecule within their immediate vicinity by means of excitation-energy transfer (EET). The rate of photon emission by the single dye molecule is increased by 2 orders of magnitude under excitation by EET compared to direct excitation at the same laser fluence. Because the dye cannot accommodate biexcitons, NC biexcitons are filtered out by EET, giving rise to up to an order-of-magnitude improvement in the fidelity of photon antibunching. We demonstrate here that, contrary to expectation, energy transfer from the nanocrystal to dye molecules does not depend on the spectral line widths of the donor and acceptor and is therefore not governed by Forster's theory of resonance energy transfer (FRET). Two different cyanine dye acceptors with substantially different spectral overlaps with the nanocrystal donor show a similar lightharvesting capability. Cooling the sample from room temperature to 5 K reduces the average transition line widths 25-fold but has no apparent effect on the number of molecules emitting, i.e., on the spatial density of single dye molecules being photoexcited by single nanocrystals. Narrow zero-phonon lines are identified for both donor and acceptor, with an energetic separation of over 40 times the line width, implying a complete absence of spectral overlapeven though EET is evident. Both donor and acceptor exhibit spectral fluctuations, but no correlation is apparent between the jitter, which controls spectral overlap, and the overall light harvesting. We conclude that the energy transfer process is fundamentally nonresonant, implying effective energy dissipation in the perovskite donor because of strong electron−phonon coupling of the carriers comprising the exciton. The work highlights the importance of performing cryogenic spectroscopy to reveal the underlying mechanisms of energy transfer in complex donor−acceptor systems.

The extraordinary absorption cross section and high photoluminescence (PL) quantum yield of perovskite nanocrystals make this type of material attractive to a variety of applications in optoelectronics. For the same reasons, nanocrystals are also ideally suited to function as nanoantennae to excite nearby single dye molecules by fluorescence resonance energy transfer (FRET). Here, we demonstrate that FAPbBr 3 perovskite nanocrystals, of cuboidal shape and approximately 10 nm in size, are capable of selectively exciting single cyanine 3 molecules at a concentration 100-fold higher than standard single-molecule concentrations. This FRET antenna mechanism increases the effective brightness of the single dye molecules 100-fold. Photon statistics and emission polarization measurements provide evidence for the FRET process by revealing photon antibunching with unprecedented fidelity and highly polarized emission stemming from single dye molecules. Remarkably, the quality of single-photon emission improves 1.5-fold compared to emission collected directly from the nanocrystals because the higher excited states of the dye molecule act as effective filters to multiexcitons. The same process gives rise to efficient deshelving of the molecular triplet state by reverse intersystem crossing (RISC), translating into a reduction of the PL saturation of the dye, thereby increasing the maximum achievable PL intensity of the dye by a factor of 3.