Computational study of the photophysical properties and electronic structure of iridium(iii) photosensitizer complexes with electron-withdrawing groups

Abstract

A series of novel [Ir(tpy)(btp)Cl]⁺ complexes (Ir1–Ir4) have been reported to show excellent performance as photosensitizers. The introduction of electron-withdrawing groups increases visible light absorption and the lifetime of triplet states. To improve the photophysical properties, we theoretically design Ir5–Ir9 with electron-withdrawing groups (Cl, F, COOH, CN and NO₂). Surprisingly, our findings indicate that the photosensitizer performance does not strictly increase with the electron-withdrawing ability of the substituents. In this work, the geometric and electronic structures, transition features, and photophysical properties of Ir1–Ir9 are investigated. The natural transition orbital (NTO) analysis indicates that the T₁ and T₂ states play a role in the photochemical pathways. Ultraviolet-visible (UV-vis) absorption spectra and charge-transfer spectra (CTS) have been investigated to show that the introduction of electron-withdrawing groups not only improves the visible light absorbing ability, but also changes the nature of electron excitation, providing a future molecular design strategy for similar series of photosensitizers. The rates of (reverse) intersystem crossing and the Huang–Rhys factors are evaluated to interpret the experimental results within the framework of Marcus theory. For complexes Ir1–Ir7, the introduction of electron-withdrawing groups leads to a lower efficiency of reverse intersystem crossing and a strong non-radiative process T₂ → T₁, resulting in a long triplet lifetime and excellent performance as a photosensitizer. Furthermore, some newly designed complexes (Ir7–Ir9) show great potential as thermally activated delayed fluorescence emitters, contrary to our initial expectations.