Strain-Enabled Exciton Delocalization in Crystalline Phthalocyanine—A Pathway Towards Room-Temperature Macroscopic Coherence in Organic Semiconductors

Rapid exciton (energy) transport is critical for the realization of efficient organic semiconductor devices that surpass current efficiency limitations. Solution -based fabrication techniques for crystalline molecular thin films with macroscopic ordering show potential for overcoming the limitations of the molecular Frenkel exciton hopping transport. Once static/ structural disorder is removed from these films, spectroscopy experiments reveal the superluminescent signature of exciton delocalization in an entire class of small molecules where the packing motifs favor stronger intermolecular coupling.¹<br/>In small molecule crystals the formation of delocalized and coherent excitons is the result of a complex balance between the long-range Coulomb interactions, short range π- stacking coupling and electron phonon coupling. The delocalization takes place when the latter is of the same order of magnitude as the nearest neighbor intermolecular coupling. Small energy phonon modes assist in the formation of coherent states while high energy molecular vibrations (dynamic disorder) introduce decoherence and localize the exciton.²The delocalization will favor ballistic energy transport through the system.<br/>Here we report on how uniaxial strain induces a more robust coherent delocalized exciton state. The molecular thin films under study are deposited from solutions of metal-free octabutoxy Phthalocyanine (H₂OBPc) using a capillary pen - writing technique that results in macroscopic crystalline grains sizes, with molecules stacked in a J-like configuration along the [110] crystal axis.³ It was shown theoretically and experimentally that exciton delocalization only occurs along this molecular stack making the films a 1D excitonic system.⁴<br/>Temperature, strain-dependent luminescence, absorption and linear dichroism microscopy indicate applying uniaxial strain along the stacking axis has a similar effect lowering the temperature by 100K, in terms of the prevalence of the coherent exciton state. The coherence length is estimate from the ratio between the delocalized exciton photoluminescence intensity and its first phonon replica. Increasing strain to 4.2 % initially shortens the coherence length at low temperatures, however, further increase in the strain results in a surprising recovering of the initial value of 22 molecular units (approx. 10 nm). This can be explained by the trend observed in the ratio between the intermolecular coupling and the electron-phonon interaction, also estimated from the temperature dependence of photoluminescence and the associated radiative lifetime. The applied strain results in stronger intermolecular interactions that maintain the delocalization up to a temperature of 200K.⁵<br/>Finally, the mechanism of delocalized exciton formation is confirmed through strain -dependent transient absorption measurements and confocal photoluminescence microscopy that reveal the excited Frenkel excitons delocalize in less than 500fs. The dynamic disorder induces the formation of a more localized exciton-polaron state 5ps later. This process suffers significant changes in the presence of strain.<br/>¹Rawat et al, <i>J. Phys. Chem. Lett.</i> <b>2015</b>, 6, 1834–1840<br/>²Fornari et al, J. <i>Phys. Chem. C</i> <b>2016</b>, 120, 7987–7996<br/>³Pan et al, <i>Nat. Commun.</i> <b>2015</b>, 6, 8201<br/>⁴Liang et al, <i>J. Phys. Chem. C</i> <b>2021</b>, 125, 27966–27974<br/>⁵Liang et al, <i>J. Phys. Chem. C</i> <b>2022</b>, 126, 8889–8896