Novel metallated imidazole phthalocyanines: synthesis, ultrafast excited-state carrier dynamics and multiphoton absorption properties

Abstract

Metal-substituted phthalocyanines are an important class of materials garnering attention due to their structural diversity and excellent optoelectronic properties. Herein, we comprehensively investigate the photophysical properties of a newly synthesized metal-free and metallated [Zn(II) and Cu(II)] phthalocyanines (Pcs) to explore their nonlinear optical (NLO) phenomena. Femtosecond transient absorption spectroscopy (fs-TAS) was utilized to study the excited-state relaxation dynamics of these Pcs within the visible spectral range of 430–780 nm. After photoexcitation, fs-TAS data confirmed the formation of long-lived triplet states due to intersystem crossing. The lifetimes of various intermediate excited states were retrieved from the TAS data using a target model analysis, and these lifetime values were used to fit the NLO data, which were carried out by the femtosecond Z-scan measurements. The calculated nonlinear refractive indices were (0.36–0.68) × 10⁻¹⁵ cm² W⁻¹ and the values of second hyperpolarizability were (2.34–3.61) × 10⁻³⁰ esu for these Pcs, which were verified through DFT calculations. Compared with the metal-free Pc, an enhancement of the nonlinear characteristics for the metallated Pcs was observed, and we attributed this increment to the reduction of aggregation for the metallated Pcs replaced with organic scaffolds at their peripheral locations. As a proof-of-concept application, we calculated the optical limiting onset and figure of merit. The crucial photophysical and nonlinear phenomena obtained here are believed to provide the avenues to address the critical variables involved and their correlated optoelectronic properties.