Long-Lived Interfacial Charge Separation in Pyrenylimidazolyl Phthalocyanine@Transition Metal Dichalchogenide Heterostructures

Abstract

Pyrenylimidazolyl-functionalized free-base phthalocyanines (H2Pc) and their zinc complexes (ZnPc) constitute multifunctional building blocks for realizing heterostructures with liquid phase exfoliated transition-metal dichalcogenides (LPE-TMDs). Here, we report the synthesis, optical features, and excited-state characterization of three different phthalocyanines, namely H2Pc 4, ZnPc 5, and ZnPc 6, which we designed to test how π-extended anchoring groups and metalation modulate interfacial charge transfer (iCT) with LPE-TMDs. Clean isosbestic behavior and the emergence of an interfacial Q-band at ∼720 nm as seen in steady-state titrations with LPE-TMDs, namely s-WS2 and s-MoS2, underline strong ground-state electronic coupling. In the excited state, femtosecond transient absorption spectroscopy corroborates ultrafast formation of interfacial charge-separated states (iCSS) with characteristic signatures in the short-wave infrared (SWIR) and multiphasic decay kinetics. The direction of iCSS depends on the alignment of the phthalocyanine orbitals with respect to the TMDs band structure. Pyrenylimidazolyl groups enhance immobilization onto s-TMD and foster spin-conversion between iCSS of singlet and triplet multiplicities. Responsible are the rigid phthalocanine-to-pyrenylimidazolyl linkage as well as the charge transfer transition from pyrene to the phthalocyanine core. Overall, these results establish how the molecular design impacts interfacial interactions and processes in phthalocyanine@s-TMD and provide key design rules for tailoring excitonic and electronic processes in mixed-dimensional optoelectronic systems.