Red-to-NIR luminescent trinuclear gold(I) complexes with exceptionally wide liquid crystalline temperature range

Abstract

Luminescent trinuclear Au(I) complexes integrated into liquid crystalline (LC) frameworks provide a promising platform for developing anisotropic and stimuli-responsive soft materials. However, simultaneously achieving LC phase stability over a wide temperature range and phosphorescence under ambient conditions remains a challenge. Herein, we employed a combined linker and side-chain engineering strategy to tune the LC behaviour and photophysical properties of pyrazolebased cyclic trinuclear Au(I) complexes. Structural modulation was achieved by introducing flexible alkyl side chains of varying length and branching through ester linkages. The structure-property relationships of the Au(I) complexes were systematically investigated in the context of functional soft material design. The alkyl substituents exerted a pronounced influence on thermal behaviour: alkyl chain branching effectively lowered the melting temperature, while incorporation of ester linkages raised the LC-to-isotropic transition temperature. As a result of this synergistic effect, the complex bearing the longest branched alkyl side chain exhibited an exceptionally wide LC temperature window spanning more than 300 °C.In the LC state, all complexes self-assembled into a hexagonal columnar mesophase and displayed aggregation-induced emission behaviour. Alkyl chain substitution did not significantly affect the emission wavelength and excited-state lifetime but markedly enhanced the thermodynamic stability of the complexes. Thus, thermal and LC properties are governed primarily by molecular design, whereas photoluminescence characteristics are dictated by the aggregate structure. These findings highlight linker and side-chain engineering as an effective strategy for independently modulating the liquid crystallinity, thermal stability, and luminescence of Au(I)-based soft luminescent materials.