Terahertz-Field-Induced Stark Effect in Metal Phthalocyanine Thin Film

Abstract

Molecular-scale electric-field control is an important direction for future nanoscale switching and information-storage materials. Metal phthalocyanines provide a useful platform for studying molecular electric-field responses because of their chemical stability and tunable molecular structure. In molecular thin films, electric-field control is often reflected in changes in electronic transitions and exciton energies. Meanwhile, these field-induced responses are strongly influenced by intermolecular coupling, molecular packing, and exciton character, and therefore cannot be understood simply as isolated-molecule Stark responses.Here, we investigate how strong ultrafast terahertz (THz) electric fields modify molecular excitonic responses in metal phthalocyanine thin films using THz-pump optical-probe transient absorption spectroscopy. We observe an anomalous blue-shifting response of the Q-band absorption, in contrast to the conventional redshift expected from a quadratic Stark effect when the excited state is more polarizable than the ground state. By comparing the transient absorption spectrum with the derivatives of the static absorption spectrum, we find that the signal closely resembles the negative first derivative, indicating a field-induced blueshift of the absorption band. We attribute this blueshift to a reduction of the exciton binding energy caused by THz-field-enhanced intermolecular charge-transfer character in stacked metal phthalocyanine films. This result suggests that field-driven charge separation can overcome the conventional Stark redshift and produce a net transient blueshift in molecular excitonic systems.