Interlayer Triplet Energy Transfer in Dion–Jacobson Two-Dimensional Lead Halide Perovskites Containing Naphthalene Diammonium Cations

Abstract

Recently, hybrid perovskites have gained attention as sensitizers for molecular triplet generation. Layered, two-dimensional (2D) perovskites are especially well-suited for this purpose because the triplet donor (inorganic framework) and triplet acceptor (organic layer) are self-assembled into adjacent sheets, so that with the appropriate energetics, triplets can be driven across the interface. Here we examine interlayer energy transfer in a series of mixed-halide Dion–Jacobson 2D perovskites containing divalent naphthalene cations. We find that the sensitized phosphorescence in these compounds is dominated by naphthalene triplet excimer emission, but when the inorganic exciton is tuned near resonance with the naphthalene triplet, naphthalene monomer phosphorescence competes with triplet excimer formation. The interlayer energy-transfer process is further revealed by ultrafast transient absorption spectroscopy through kinetic variations in triplet excimer formation times. Ultimately, gaining control over interlayer interactions in 2D perovskites through cation design will help uncover new functions and applications for these materials.

Interlayer Triplet-Sensitized Luminescence in Layered Two-Dimensional Hybrid Metal-Halide Perovskites

“…Interestingly, when the authors targeted compositions in which the inorganic exciton level was nearly resonant with the triplet level of naphthalene, they observed an enhancement in the oscillator strength of certain transitions between the lowest triplet level of the naphthalene moiety and its ground-state manifold. This resonance concept was also invoked in the recent works of Tian, who used a naphthyl­methyl­ammonium cation substituted in a different position of the molecule, and Lin, who used a diammonium naphthalene cation in a Dion–Jacobson-type perovskite …”

“…The numerous structural possibilities that are available within the family of 2D MHPs can be exploited as structural templates for investigating electronic or optical phenomena within the organic spacer layer. For example, for a given organic chromophore, switching the valency from a monoammonium to diammonium linked cation could dramatically alter the intermolecular orientation and electronic interactions within the layer . The ability to achieve targeted intermolecular orientations is potentially useful for engineering certain polarization-dependent phenomena or achieving multiple exciton effects such as singlet fission and triplet–triplet annihilation upconversion, which depend sensitively on the electronic coupling between the organic chromophores.…”

“…Specifically, the energy transfer is initiated by a hole transfer from the valence band of lead bromide to the HOMO level of naphthalene on a time scale of 0.6 ps, followed by a slower electron transfer to form the triplet on naphthalene . Lin and co-workers also recently used ultrafast spectroscopy techniques to gain further insight into the kinetics of naphthalene triplet formation in a series of Dion–Jacobson 2D perovskites containing naphthalene diammonium cations . By tuning the lead-halide composition from pure bromide to pure iodide, the energetic driving force for interlayer triplet energy transfer to naphthalene was modulated.…”

“…For example, for a given organic chromophore, switching the valency from a monoammonium to diammonium linked cation could dramatically alter the intermolecular orientation and electronic interactions within the layer. 40 The ability to achieve targeted intermolecular orientations is potentially useful for engineering certain polarization-dependent phenomena or achieving multiple exciton effects such as singlet fission 41 and triplet−triplet annihilation upconversion, 42 which depend sensitively on the electronic coupling between the organic chromophores. Moreover, using alkyl-ammonium tethers of varying length gives control over interlayer separation and can be used to manipulate organic layer morphology.…”

“…This resonance concept was also invoked in the recent works of Tian, who used a naphthylmethylammonium cation substituted in a different position of the molecule, 66 and Lin, who used a diammonium naphthalene cation in a Dion−Jacobson-type perovskite. 40 In addition to varying the halide composition, another strategy for tuning the inorganic exciton level is to alter the metal site from lead to tin or by lead−tin alloying. Direct studies of interlayer triplet sensitization in tin-based 2D MHP systems are relatively scarce, and so the impact of tin substitution on the degree of spin−orbit coupling, sensitization efficiency, and exciton relaxation mechanisms are not well understood.…”