In bulk crystals, however, the two former parameters are thermodynamically fixed and can hardly be modified except for polymorphic transitions. In principle, the above interactions can be tuned by varying intermolecular arrangements, dielectric environments, and the extent of coupled units. By exploiting the capability of polarized spectroscopy that directly maps the transition dipoles, more direct proof for the mixing and its spatial progression should be within experimental reach.įor such an exploration, two-dimensional molecular crystals (2DMCs) can be an ideal system 23, 24. Despite many experimental breakthroughs and progress 19, however, it remains unclear how the evolution of Frenkel-CT mixing can be revealed by other than differing energetics 20, 21, 22. Notably, the CT coupling is ubiquitous in π-stacked systems, differentiates the J/H-aggregate behaviors 15, limits the coherence of Frenkel excitons (FE) 16, 17, and facilitates the singlet fission 18. Geometric arrangements are crucial in both interactions in that the former is governed by the long-range coupling among transition dipoles 13, whereas the latter is by the short-range orbital overlap 14. Despite their small footprints spanning a few unit cells, molecular excitons are greatly affected by intermolecular interactions through Kasha-type Coulombic 11 and intermolecular CT 12 couplings. They also present a myriad of intriguing photophysical phenomena such as Davydov splitting (DS) 4, 5 associated with J/H aggregates 6, coherence-induced superradiance 7, mixing of Frenkel and charge-transfer 8 excitons 9, and singlet fissions 10. The current spatial anatomy of 2D molecular excitons will inspire a deeper understanding and groundbreaking applications of low-dimensional molecular systems.Įxcitons in molecular solids mediate efficient light harvesting in photosynthesis complexes 1 and important organo-electronic applications such as photovoltaics 2 and light-emitting diodes 3. As the thickness increases, the transition dipole moments of newly emerging charge transfer excitons are reoriented because of mixing with the Frenkel states. In the truly 2D limit of single layers, two Frenkel emissions Davydov-split by Kasha-type intralayer coupling exhibit energy inversion with decreasing temperature, which enhances excitonic coherence. Complete lattice constants with orientations of two herringbone-configured basis molecules are determined with polarization-resolved spectroscopy and electron diffraction methods. Here we show in-plane and out-of-plane excitonic evolution in quasilayered two-dimensional (2D) perylene-3, 4, 9, 10-tetracarboxylic dianhydride (PTCDA) crystals assembly-grown on hexagonal boron nitride (hBN) crystals. Despite this, the spatial evolution of molecular excitons and their transition dipoles have not been captured in the precision of molecular length scales. Understanding the nature of molecular excitons in low-dimensional molecular solids is of paramount importance in fundamental photophysics and various applications such as energy harvesting, switching electronics and display devices.
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