The photophysical properties of multichromophoric nanoparticles (mcNPs), crucial for applications in optoelectronics and other fields, are strongly influenced by the number of chromophores and their interactions. These interactions involve exciton diffusion and annihilation processes, which complicate traditional methods of characterizing mcNPs. Exciton diffusion, the movement of excited states between chromophores, and singlet-singlet annihilation (SSA), the quenching of one exciton by another, significantly affect parameters such as brightness, photoluminescence (PL) lifetime, exciton harvesting efficiency, and photostability. Photon antibunching, a hallmark of single quantum systems, has been used to count chromophores; however, the presence of exciton diffusion and SSA renders such measurements unreliable in mcNPs. Existing techniques either count chromophores while neglecting SSA or measure SSA rates assuming a known number of chromophores. This limitation hinders a complete understanding of mcNP photophysics. The current study aims to overcome this challenge by developing a new technique that separates the number of physical chromophores from exciton diffusion and annihilation processes.

Previous studies have explored photon antibunching in multichromophoric systems, but have often struggled to disentangle the contributions of intrinsic single-photon emission from exciton dynamics. The use of DNA origami structures to create well-defined systems for studying energy transfer and exciton annihilation has been established. Existing methods for measuring exciton diffusion lengths in conjugated polymers, such as those based on analyzing non-exponential PL decay, often require high excitation fluences, and are not suitable for measuring the slower exciton diffusion processes which dominate at low excitation fluences relevant to most devices. The lack of a method capable of accurately determining both the number of chromophores and the rates of exciton diffusion and annihilation in a single experiment has limited progress in optimizing the design of mcNPs for various applications.

The researchers developed picosecond time-resolved antibunching (psTRAB), a technique that leverages time-correlated single-photon counting (TCSPC) to analyze the photon stream from mcNPs under pulsed excitation. Photons are grouped according to their arrival time after the laser pulse, and cross-correlations are performed to determine the probability of consecutive photon emission. By analyzing the time dependence of the photon statistics, psTRAB simultaneously measures the number of independent emitters (and hence the true number of chromophores) and the time evolution of exciton diffusion and annihilation. The method was first validated using DNA origami structures, which provided a controllable system with a known number of chromophores and precisely defined inter-chromophore distances. ATTO647N dyes were attached to the DNA origami structures, and their photoluminescence was recorded using a custom confocal microscope. The number of independent chromophores was extracted from the photon statistics histograms for different microtime bins. The exciton annihilation rate was then determined by fitting the time evolution of the number of independent chromophores to a single-exponential or biexponential model, accounting for nearest-neighbor and next-nearest-neighbor interactions. Subsequently, the psTRAB technique was applied to mesoscopic aggregates of conjugated polymers (PPEB-1 and PPEB-2), which were grown with different electronic and structural properties (H-type and J-type aggregates) allowing exciton diffusion in these samples to be investigated. The time evolution of the number of independent chromophores was analyzed to extract information about exciton diffusion and annihilation rates, the dimensionality of the diffusion, and a lower limit on the three-dimensional exciton diffusion length. The analysis involved plotting ln(n/(n-1))Vagg as a function of time, where n is the number of independent chromophores and Vagg is the aggregate volume, to extract the time-dependent exciton annihilation rate.

Using psTRAB, the researchers successfully measured the true number of chromophores in well-defined DNA origami structures, even when exciton annihilation occurred. They also determined the distance-dependent annihilation rates between excitons. In the DNA origami structures, the observed decay of the number of independent chromophores to 1 at long microtimes, even for arrangements where excitons would not directly annihilate, indicated exciton hopping (diffusion) between chromophores. Applying psTRAB to conjugated polymer aggregates revealed distinct exciton diffusion dynamics in H-type and J-type aggregates. In the first 250 ps after excitation, exciton diffusion was found to be primarily one- or two-dimensional in both aggregate types, reflecting the intrachain and interchain couplings. After 250 ps, three-dimensional diffusion dominated, and the annihilation rate was an order of magnitude higher in the highly ordered H-type aggregates compared to the disordered J-type aggregates. Analysis of the data in the region where exciton density was too low to support annihilation provided a lower limit for the three-dimensional exciton diffusion lengths: 8.97 nm for H-type aggregates and 5.2 nm for J-type aggregates. The comparison of a nine-chain and a six-chain J-type aggregate highlighted the influence of aggregate size and order on three-dimensional exciton diffusion, with smaller aggregates exhibiting faster diffusion. This suggested increased order in smaller aggregates leading to more efficient interchain coupling.

The psTRAB method provides a powerful tool for studying exciton dynamics in mcNPs, overcoming the limitations of traditional techniques. The ability to simultaneously determine the number of chromophores and the exciton annihilation rate offers significant advantages over previous approaches. The results from the DNA origami experiments demonstrate the high accuracy and reliability of the psTRAB method. The application of psTRAB to conjugated polymer aggregates reveals valuable insights into the relationship between aggregate structure, exciton diffusion, and annihilation. The observation of different diffusion dimensionality at early and late times is consistent with models of exciton transport in disordered materials. The significant difference in annihilation rates between the H-type and J-type aggregates underscores the impact of morphological order on exciton dynamics. The measured diffusion lengths are consistent with values reported in the literature for similar systems. The study's findings provide crucial information for optimizing the design of materials for applications in optoelectronics, where efficient exciton transport and harvesting are crucial.

This study introduces psTRAB, a novel technique that accurately measures the number of chromophores and the kinetics of exciton annihilation in mcNPs. The method has been validated using DNA origami and applied to conjugated polymer aggregates, providing valuable insights into exciton dynamics. Future research could explore the use of psTRAB to study other types of mcNPs, such as semiconductor quantum dots and perovskite nanoparticles, and to investigate the influence of various environmental factors on exciton transport and annihilation. Furthermore, extending the technique to higher-order photon correlations could provide additional information about multi-exciton interactions.

While psTRAB offers significant advantages, certain limitations should be considered. The accuracy of the method relies on sufficient photon counts, requiring extended measurement times in some cases. The analysis of exciton diffusion in the conjugated polymer aggregates relies on assumptions about the shape and density of the aggregates. Furthermore, the interpretation of the diffusion lengths obtained is based on a simplified model of diffusion in a spherical volume, and deviations from this idealized scenario might exist in real aggregates.