Carbazole-modified gold(I) complexes with different substituents: Aggregate-induced luminescence change, various solid-state phosphorescence, temperature-dependent phosphorescence, and contrasting mechanoluminochromic characteristics

Highlights

• Three carbazole-containing mononuclear gold(I) complexes were synthesized.

• All these complexes exhibited aggregate luminescence change effect.

• Complexes 1–3 exhibited various solid-state phosphorescence at room temperature and at 77 K.

• Phosphors 1 and 2 showed reversible mechanoluminochromism characteristics.

• Phosphor 3 showed irreversible mechanoluminochromic behavior.

Abstract

Three carbazole-containing isocyano-bridged mononuclear gold(I) complexes with different substituents are reported. The complexes display an aggregate luminescence change effect involving a color change from blue to yellow-green. In the solid state, luminogens 1–3 exhibit varying room-temperature phosphorescence involving blue, light yellow, and yellow emissions. In contrast, at 77 K, the gold(I) phosphors 1–3 display stronger (blue-green or yellow-green depending on the phosphor) phosphorescence with longer luminescence lifetimes. In addition, the solid-state phosphorescence of 1–3 is adjusted by moderate mechanical stimulus. More specifically, phosphor 1 shows reversible high-contrast mechanoluminochromic behavior involving interconversion between blue and yellow-green luminescence and phosphor 2 shows reversible mechanoluminochromism phenomenon involving luminescent interconversion between light yellow and light green. In contrast, solid-state phosphor 3 displays an irreversible mechanoluminochromism phenomenon—a strong yellow-green luminescence is observed after grinding. Subsequent treatment with dichloromethane vapor results in a blue-emitting sample.

Introduction

Over the past decade, stimuli-responsive luminescent solids have garnered much interest owing to their potential in multifunctional sensors and luminescent switches [[1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16]]. In particular, mechanoluminochromic materials that exhibit a change in luminescence upon external mechanical stimulus, as an example of a stimuli-responsive smart material, have gained increasing attention [[17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30]]. To date, many different types of organic and organometallic mechanochromic luminous molecules have been developed [31,32]. Bright aggregative-state luminescence is key to the preparation of high-performance mechanoluminochromic materials. However, traditional luminescent dyes typically become weak emitters upon aggregation despite displaying bright luminescence in dilute solutions as a result of aggregation-caused quenching [33,34]. Therefore, the development of mechanical force-responsive luminophores with a strong aggregative-state emission is important.

Compared with fluorescent mechanochromism, mechanoluminochromism, which involves phosphorescent color changes, has many advantages such as longer luminescence lifetimes and distinct color contrasts before and after material grinding [[35], [36], [37]]. Particularly, metal-organic complexes are more likely to achieve phosphorescent mechanochromism than organic compounds [[38], [39], [40], [41]]. Furthermore, metal complexes exhibit fascinating photophysical properties owing to the presence of metallophilic interactions [[42], [43], [44]]. For example, gold displays a maximum relativistic effect with respect to its adjacent elements in the periodic table [45]. During the past two decades, the application of gold(I) chemistry in research has gained widespread momentum in the scientific community [[46], [47], [48], [49], [50], [51], [52]]. Intra- and inter-molecular aurophilic interactions are one of the most significant research topics in the development of gold(I) chemistry [[53], [54], [55], [56]]. To date, some gold(I) complexes with phosphorescent mechanochromic behavior have been reported. However, gold(I)-containing luminophores that can display both phosphorescent mechanochromism and bright aggregative-state phosphorescence characteristics are rare. Further development in the field is needed to broaden the scope of these promising materials diverse fields. As a promising organic component candidate, carbazoles are extensively used in emissive materials and photoelectronic devices [[57], [58], [59]]. Thus, in the present study, we tried to introduce three carbazole groups with different substituents to one pentafluorophenyl gold(I) unit, respectively. As a result, three new carbazole-based gold(I) isocyanide complexes were reported (Chart 1). The as-synthesized gold(I) complexes 1–3 showed distinct aggregate-induced luminescence change behavior. Furthermore, luminogens 1–3 displayed temperature-dependent solid-state phosphorescence and the phosphors 1–3 displayed contrasting phosphorescent mechanochromism characteristics.