Vapochromism and vapoluminescence of platinum(II) 3,8-bis-(3-hydroxy-3-methylbut-1-yn-1-yl)-phenanthroline organometallic complexes with bis-arylethynyl derivatives

Highlights

• New vapoluminescent platinum(II) phenanthroline organometallics were synthesized.

• Solid state emission clearly showed that the luminescence in solid depended on the molecular structure.

• The vapochromism and vapoluminescence are induced by sterically hindered substituents.

• Vapoluminescence with the 13 VOCs showed multicolor changes from yellow to dark brown in 9TMS.

Abstract

A new series of platinum(II) organometallic complexes with 3,8-bis-(3-hydroxy-3-methylbut-1-yn-1-yl)-1,10-phenanthroline (3,8-Phen-C(CH₃)₂OH) and nine respective arylethynyl ligands with different substituents (Pt(3,8-Phen-C(CH₃)₂OH)(Ph-R₂)₂: R₂ = H (1OH), 2-F (2OH), 3-F (3OH), 4-F (4OH), 4-Me (5OH), 4-CF₃ (6OH), 4-t-Bu (7OH), 3,5-di-CF₃ (8OH), 3,5-di-t-Bu (9OH)) were synthesized and their luminescences in solution and solid state were characterized by photoluminescence spectroscopy. The luminescence of each of the new complexes, 1OH – 9OH, in solution was assigned to the phosphorescence from the mixed transition of ³MLCT/³LLCT (LLCT = ligand-to-ligand charge transfer), revealing that two series had very similar emission spectra: Pt(3,8-PhenTMS)(-Ph-R)₂ 1TMS – 9TMS and Pt(3,8-PhenH)(Ph-R)₂ 1H – 9H in solution. In solid state, the emission spectra of three series of platinum organometallic complexes, 1OH – 9OH, 1H – 9H, and 1TMS – 9TMS, were observed over a wide range (500–1100 nm) because the phosphorescence of each of these complexes was assigned to the emission from the ³MLCT/³LLCT mixed transition and/or the transition of a metal−metal-to-ligand charge transfer, called ³MMLCT.

Results of vapochromism and vapoluminescence experiments with 13 species of volatile organic compounds (VOCs) for three series of the 27 above-mentioned platinum complexes suggested that the vapochromism of the present complex systems was induced by sterically hindered substituents of the arylethynyl ligands as tert-butyl substituents, and that the expansion of the space among the molecules in solid state is important for selective and multicolor detection with vapochromism and vapoluminescence for VOCs. Especially, the vapoluminescence experiment with VOCs for 9TMS, which was composed of a combination of the bulkiest ligands, 3,8-bis-(trimethylsilyl)ethynyl-1,10-phenanthroline and 3,5-di-t-butyl-phenylehtynyl ligands, among the present 27 complexes showed multicolor changes from yellow to dark brown, in contrast with respective VOCs.

Introduction

Research into vapochromic and vapoluminescent materials continues to inspire clear naked-eye and/or luminescent color changes for chemical sensing, flexible response systems, and memory applications [1], [2], [3], [4], [5], [6], [7], [8] because their compounds undergo a rapid and reversible change in color and/or emission in the presence of some volatile organic compounds (VOCs). In particular, square-planar platinum(II) complexes have been explored to construct chemical sensors for VOC detection due to their tendency to form Pt−Pt interaction and π− π stacking interaction in solid state. [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13] Platinum(II) organometallic complexes with bipyridine or terpyridine derivatives and various arylethynyl ligands, Pt(L)(-aryl)₂ or [Pt(L)(-aryl)]⁺ (L = bipyridine or terpyridine derivatives), have recently focused attention on selective sensing for VOCs such as through vapochromism and vapoluminescence in solid state. [14], [15], [16], [17], [18] Photophysical studies of these platinum(II) organometallic complexes indicated that the emission of many complexes in solid state shows a phosphorescence from the transition of a metal−metal-to-ligand charge transfer (³MMLCT) caused by the Pt−Pt interaction and induced a unique luminescence property such as vapoluminescence. [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24] In particular, various vapochromism and vapoluminescence experiments on platinum(II) complexes with 5,5-bis-(trimethylsilyl)ethynyl-2,2′-bipyridine (TMS-bpy) and various arylethynyl ligands, Pt(TMS-bpy)(-aryl)₂, for VOC detection have been reported by Ni's group. [19], [20], [21], [22], [23], [24]

We recently reported the photophysical properties of two series of platinum(II) organometallics with two phenanthroline derivatives and nine arylethynyl ligands with different substituents, Pt(3,8-Phen-R₁)(Ph-R₂)₂ (R₁=Si(CH₃)₃: 3,8-bis-(trimethylsilyl)ethynyl-1,10-phenanthroline (1TMS – 9TMS) and R₁=H: 3,8-diethynyl-1,10-phenanthroline (1H – 9H), R₂=H, 2-F, 3-F, 4-F, 4-Me, 4-CF₃, 4-t-Bu, 3,5-di-CF₃, 3,5-di-t-Bu: 9 arylethynyl ligands) as shown in Scheme 1 and revealed an interesting luminescence of these 18 complexes in solution and solid state. [25, 26] It was estimated that the differences in color and luminescence between 1TMS – 9TMS and 1H – 9H with the same arylethynyl ligands were caused mainly by the difference in bulkiness between the trimethylsilyl substituent and no substituent (hydrogen only) at the ethynyl termination in phenanthroline ligands. Furthermore, the vapochromism and vapoluminescence experiments of 1TMS – 9TMS revealed the different responsibility between Pt(TMS-bpy)(-aryl)₂ reported by Ni's group and our corresponding complexes with the same arylethynyl ligands in 1TMS – 9TMS to detect some VOCs. For example, vapochromism experiments on 1TMS with CHCl₃ and CH₂Cl₂ vapors (Fig. S1 in supporting information) showed the opposite color change compared to the similar molecular structure of Pt(TMS-bpy)(-Ph)₂, although the structural difference between the two complexes is mainly the structural difference between bipyridine and the phenanthroline skeleton. The structural difference between their compounds has a very small effect on their photophysical properties in solution state. So, a small structural difference between the bidentate ligands may be the reason for big differences in the vapochromism and vapoluminescence of their platinum(II) complexes. On the other hand, hydrogen bonding is a very influential factor in the intermolecular structure of square-planar platinum(II) complexes, the same as metal−metal and π−π stacking interactions in the aggregation and solid state. [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13] We therefore became interested in the solid state luminescence and vapochromism of platinum(II) organometallic complexes with a new type of phenanthroline that includes a hydroxyl substituent.

We report herein the synthesis and photophysical characterization of novel platinum(II) organometallic complexes with 3,8-bis-(3-hydroxy-3-methylbut-1-yn-1-yl)-1,10-phenanthroline (3,8-Phen-C(CH₃)₂OH) and nine respective arylethynyl ligands with different substituents, as shown in Scheme 1: (Pt(3,8-Phen-C(CH₃)₂OH)(Ph-R)₂: R = H (1OH), 4-F (2OH), 3-F (3OH), 2-F (4OH), 4-Me (5OH), 4-CF₃ (6OH), 4-NO₂ (7OH), 3,5-di-CF₃ (8OH), 3,5-di-t-Bu (9OH)). The ligand 3,8-Phen-C(CH₃)₂OH is slightly less bulky than 3,8-Phen-Si(CH₃)₃ in 1TMS – 9TMS, and the 3,8-Phen-C(CH₃)₂OH has one hydroxyl group. Therefore, some types of hydrogen bonding interaction in solid state might be possible because the hydrogen bonding might interact between the hydroxy substituent and the fluoride in some arylethynyl ligands of platinum complexes in solid state. Furthermore, vapochromic and vapoluminescence experiments for VOC detection with 13 species of major volatile organic solvents were performed with three series of 27 platinum complexes (Scheme 1) in solid state, and the phosphorescence from ³MMLCT caused by the Pt−Pt interaction was discussed. We also show the phosphorescence from the mixed transition of ³MLCT/³LLCT (LLCT = ligand-to-ligand charge transfer) for the present new compounds 1OH – 9OH in solution state.