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Two-dimensional (2D) lead halide perovskites with distinct excitonic feature have shown exciting potential for optoelectronic applications. Compared to their three-dimensional counterparts with large polaron character, how the interplay between long- and short- range exciton-phonon interaction due to polar and soft lattice define the excitons in 2D perovskites is yet to be revealed. Here, we seek to understand the nature of excitons in 2D CsPbBr3 perovskites by static and time-resolved spectroscopy which is further rationalized with Urbach-Martienssen rule. We show quantitatively an intermediate exciton-phonon coupling in 2D CsPbBr3 where exciton polarons are momentarily self-trapped by lattice vibrations. The 0.25 ps ultrafast interconversion between free and self-trapped exciton polaron with a barrier of ~ 34 meV gives rise to intrinsic asymmetric photoluminescence with a low energy tail at room temperature. This study reveals a complex and dynamic picture of exciton polarons in 2D perovskites and emphasizes the importance to regulate exciton-phonon coupling.

We demonstrated that a purely organic stack, namely, TPE-3-stack, can be assembled in high yield by using dynamic imine chemistry. TPE-3-stack emits strong fluorescence not only in the solid state (ϕ PL = 83%) but also in dilute solutions (e.g., ϕ PL = 82% in DMSO), which is significantly distinct from the case of the aggregation-caused quenching (ACQ) and aggregation-induced emission (AIE) chromophores. In addition, it shows high spectral, thermal, and chemical stabilities as well as excellent solubility in common solvents. Therefore, this stack is feasible for fabricating a solution-processed electroluminescent device, which displays brightness over 1000 cd m −2 and a current efficiency up to 2.3 cd A −1 . TPE-3-stack is also demonstrated as a promising fluorescent visualizer for lysosome imaging. Such characteristics of the stack compound were attributed to the efficient suppression of intramolecular rotation and vibration by locking its structure into the rigid framework by means of a self-assembly strategy.

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