Energy transport and light propagation mechanisms in organic single crystals

Abstract

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All-optical manipulation of singlet exciton transport in individual supramolecular nanostructures by triplet gating

“…We visualise the spatio-temporal dynamics of singlet excitons, created by the 'singlet laser', as well as their manipulation by the triplet population using a detection-beam scanning approach combined with time-correlated single-photon counting of the prompt fluorescence signal. 10,28,31 We measure fluorescence decay curves at fixed positions of the laser foci, while scanning the detection position along the long (x-) axis of the bundle (centred around the 'singlet laser' focus). We thus create a time-dependent distribution of the prompt fluorescence of singlet excitons, I(x,t), as a function of the distance x relative to the centre of the 'singlet laser' and time t after its arrival on the sample (see Materials and methods, ESI †).…”

“…24,25 Second, while singlet exciton transport is mediated predominantly by long-range Coulomb interactions, triplet exciton transport is driven by short-range exchange interactions. 24,26 Therefore, experimentally observed diffusivities of singlet excitons are typically two to three orders of magnitude greater than those of triplet excitons, 10,[27][28][29][30][31][32][33][34][35] i.e., triplet excitons are essentially immobile on time scales of the short (nanosecond) singlet exciton lifetime. Finally, contrary to singlet-singlet or triplet-triplet annihilation, after singlet-triplet annihilation an excited triplet state survives due to spin conservation: 24…”

“…The singlet exciton transport within a single nanostructure is monitored with time-resolved detection-beam scanning. 10,31 The supramolecular nanostructures are based on a carbonyl-bridged triarylamine trisamide, CBT, 10,36,37 as shown in Fig. 1.…”

Tailoring the excited-state energy landscape in supramolecular nanostructures

“…These long transport distances and high diffusion constants are rather surprizing for self-assembled organic structures with unavoidable electronic and structural disorder. It is even more surprizing considering that these transport distances of singlet excitons exceed those commonly observed in highly ordered organic single crystals 10 as well as in (semicrystalline) organic thin films by sometimes more than two orders of magnitude [111,[113][114][115][116][117][118]. The origin of these long distances in supramolecular nanostructures is not entirely clear yet and subject to on-going discussions [10], in particular, since micrometre transport has been demonstrated on both J-and H-type nanostructures.…”

Excitons: Energetics and spatio-temporal dynamics

“…A recent development in the experimental study of excitons has been the combination of high spatial resolution with the time-resolved methods described above. This spatial resolution can be directly implemented using microscopy techniques, as described by Wittmann et al 10 in their studies of the subdiffusive transport of singlet excitons in oligothiophene crystals. Alternatively, Kunsel et al 11 showed how single molecule measurements on Zn-chlorophyll aggregates can be analyzed to provide detailed information on correlated disorder on 10 nm length scales.…”