Enhancing the charge transport and luminescence properties of ethyl 4-[(E)-(2-hydroxy-4-methoxyphenyl)methyleneamino]benzoate through complexation: a DFT and TD-DFT study

Abstract

Organic light emitting diode (OLED) and organic solar cell (OSC) properties of ethyl 4-[(E)-(2-hydroxy-4-methoxyphenyl)methyleneamino]benzoate (EMAB) and its Pt²⁺, Pd²⁺, Ni²⁺, Ir³⁺, Rh³⁺, and Zn²⁺ complexes have been theoretically studied herein. Geometry optimizations have been performed via the r²SCAN-3c composite method while single-point calculations have been carried out at the PBE0-D3(BJ)/def2-TZVP level of theory. Results have shown that complexation with selected metal ions improves hole and electron transfer rates in Pt[EMAB]₂ and Rh[EMAB]₂⁺. Specifically, the hole transport rate of Pt[EMAB]₂, (k_ct(h) = 6.15 × 10¹⁴ s⁻¹), is found to be 44 times greater than that of [EMAB], (k_ct(h) = 1.42 × 10¹³ s⁻¹), whereas electron transport rate of Pt[EMAB]₂, (k_ct(e) = 4.6 × 10¹³ s⁻¹) is 4 times that of EMAB (k_ct(e) = 1.1 × 10¹³ s⁻¹). Charge mobility for holes and electrons are equal to 19.182 cm² V⁻¹ s⁻¹ and 1.431 cm² V⁻¹ s⁻¹ respectively for Pt[EMAB]₂, and equal to 4.11 × 10⁻¹ cm² V⁻¹ s⁻¹ and 3.43 × 10⁻¹ cm² V⁻¹ s⁻¹ for EMAB respectively. These results show that, charge transport in EMAB can be tuned for better performance through complexation with transition metals such as Pt²⁺. OSC properties of the complexes were also studied by comparing their HOMO/LUMO energies with those of (6,6)-phenyl-C₆₁-butyric acid methyl ester (PCBM) and poly(3-hexylthiophene) (P3HT). It turned out that the energy gap of EMAB reduced significantly upon complexation from 2.904 eV to 0.56 eV in [Rh(EMAB)₂]⁺ and to a lesser extent in the other complexes. The energy values of the HOMOs remained higher than those of PCBM while those of the LUMOs were found to be greater than that of P3HT with the exception of [Rh(EMAB)₂]⁺. These findings show that the aforementioned species are good electron donors to PCBM. The open circuit voltage, V_OC, of the compounds ranged between 0.705 × 10⁻¹⁹ V and 6.617 × 10⁻¹⁹ V, values that are good enough for practical usage in OSC applications. The UV-visible absorption spectra revealed absorption maxima well below 900 nm in all compounds, vital in the efficient functioning of solar cells. In general, this study has shown that platinoid complexation of EMAB can successfully modify both its OLED and OSC properties, making them better precursors in the electronic industry.