Delocalized Excitons in Mechanically – Exfoliated Ultrathin Crystalline Octabutoxy-Phthalocyanine (H₂OBPc) Films.

Dual linear dichroism and time -resolved photoluminescence microscopy (LD/TRPL) was employed to study the formation of delocalized, coherent excitons in ultrathin crystalline phthalocyanine thin films. Oriented crystalline films with mm-sized grains were obtained using capillary -assisted solution-based deposition (Rawat et al, JPCL 2016). The resulting grains exhibit strong mechanical anisotropy, with molecules arranged in sheets parallel to the substrate with weaker inter-sheet van der Waals interactions. These films were mechanically exfoliated to produce large flakes (10-100 µm) and subsequently transferred to ITO substrates. High resolution atomic force microscopy (AFM) confirmed the flakes are composed of molecularly - flat crystalline sheets. Direct correlations between the film morphology and the coherent exciton emission were possible thanks to an addressable metallic grid deposited on the substrate and the real-time monitoring of the excitation beam location using a telescope. Transient absorption measurements conducted on the same films show a photoinduced absorption contribution associated with the formation of delocalized states within a picosecond after the excitation of Frenkel excitons in these systems.