Strain-Enhanced Formation of 1D Coherent Exciton-Polaron States in Small Molecule Semiconductors

The role played by low energy phonons (25-300 cm^-1) in the formation of delocalized, 1D coherent exciton-polaron states is explored in octabutoxy phthalocyanine crystalline thin films using an externally applied uniaxial mechanical strain. The films with macroscopic grain sizes were deposited through a solution-based capillary -writing technique on flexible Kapton substrates. The temperature evolution of photoluminescence (PL) and the radiative decay dynamics pointed to the formation of an exciton-polaron state with binding energy of the order of 30 meV. Strain-dependent low temperature Photoluminescence (PL) microscopy, absorption, linear dichroism, and confocal Raman measurements reveal the strain-induced softening of the phonon modes favors the survival of the coherent exciton-polaron states up to room temperature. (L. Liang et al, J. Phys Chem C 2022) A uniaxial tensile strain of 3% applied at room temperature is equivalent to lowering the temperature to 200K for the unstrained film. For applied strain larger than 3% the crack density is sufficient such that the film is completely relaxed, and the signatures of delocalized states (i. e. redshift, enhanced PL intensity) disappear. Transient Absorption (TA) spectroscopy confirms the behavior of the exciton-polaron state as a function of strain.