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

New synthetic routes for soluble derivatives of small molecule semiconductors and thin film solution-based deposition techniques enable sustained efforts to gain more insight into the structure-properties relationships and the role of low energy phonon modes in the electronic and excitonic properties of these materials. Of particular interest are crystalline thin films that exhibit highly directional intermolecular interactions, which lead to the formation of robust coherent delocalized excitonic states. Many of these molecules such as TIPS-pentacene, rubrene or phthalocyanines are already known as prime candidates for photovoltaic or transistor applications. In these films, the exciton-phonon interactions can be of comparable strength to the long-range Coulomb interaction and arbitrate the formation and survival of coherent excitonic states, band-like transport and possibly spin exchange interactions.<br/>In this work, 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. Prior theoretical and experimental studies on unstrained films revealed the formation of delocalized coherent excitonic states is possible through strong coupling to low energy phonon modes. Moreover, 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. This state does not survive at room temperature because high energy molecular vibrational modes destroy coherence and localize the exciton. 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. 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. Confocal PL mapping of the films under different applied strains reveals that residual strain is in fact present in the as-deposited, nominally unstrained films and this residual strain has significant fluctuations on the sub-micron scale across the sample.