PRESENTED ON-DEMAND ONLY: Orientation of Exciton-Polarons and Strain-Enhenced Photoluminescence in Soluble Phthalocyanine Thin Films

Organic molecules, such as pentacene, rubrene and phthalocyanine are at the forefront of materials research, due to their desired electric and mechanical properties. They represent the feasible alternative to traditional silicon-based semiconductor applications when flexible electronics platforms are required, where materials will experience complex strain environment. Octabutoxy phthalocyanine (H₂-OBPc) is a soluble derivative of the Phthalocyanine (Pc). They have π-conjugated systems and large directional intermolecular interactions, which allow the formation of molecular crystal with well-defined structure of J-aggregate, where molecules are in a head-to-tail fashion. In the expanded theory of molecular exciton, the Frenkel exciton can couple between lattice and intramolecular vibration to form exciton-polaron state, and consequently exciton-polarons can delocalize along the coherent J-aggregates. In this case, the short-ranged excitonic exchange depends on the relative orientation of the HOMO (Highest Occupied Molecular Orbital) and LUMO (Lowest Unoccupied Molecular Orbital) from NN molecules in the crystal, while the long-ranged excitonic exchange depends on lattice vibration. The formation of these one-dimensional delocalized exciton states grants H₂-OBPc the potential for efficient excitation energy transport in the coherent exciton regime.<br/><br/>In this work, H₂-OBPc polycrystalline thin films were prepared using a solution-based deposition technique. These ordered thin films with macroscopic grain size (millimeter scale) uniquely offer the opportunity to spatially explore optical and excitonic properties within a single crystalline grain, free from the influence of grain boundaries or static disorder. Temperature-dependent absorption and photoluminescence (PL) experiment show when temperature T &lt; 175 K, additional bulk bandgap transition state presents at 900 nm and 930 nm, respectively. Incident light <b>k</b>-vector dependent absorption experiment indicates this bandgap transition is linearly polarized within substrate plane that obeys the Davydov selection rules. [1] The PL studies further reveal the temperature-dependent polarization of these excitons, implying the formation of exciton-polaron due to the coupling between exciton and lattice vibrations in the J-aggregates. Strain-dependent absorption and PL studies imply that external tensile strain can soften the lattice vibration mode and tune the above exciton-phonon coupling. Remarkably, an 80% enhancement in PL intensity is observed with a tensile strain of 4.9%, which is equivalent to a temperature reduction approximately by 100 K below room temperature. [2] Our study demonstrates strain-engineering of coherent delocalized exciton states in flexible organic electronics.<br/>Reference<br/>[1] <i>J. Phys. Chem. C </i><b>2021,</b> 125, (51), 27966-27974. https://doi.org/10.1021/acs.jpcc.1c08253<br/>[2] <i>J. Phys. Chem. C </i><b>2022</b>. https://doi.org/10.1021/acs.jpcc.2c01382