Photochromic properties of ruthenium complexes with dithienylethene-ethynylthiophene

Highlights

• A new photochromic dithienylethene-ethynylthiophene L1o with an extended π-system at ethene bridge unit has been successfully synthesized.

• A head–head coupled symmetric ruthenium(II) complex and a head-shoulder coupled asymmetric ruthenium(II) complex based were first prepared.

• Only the singly ring-closed species 1co and the fully ring-closed isomer 2cc can be obtained for 1oo and 2oo, attributing to the quenching effect of the first ring-closed dithienylethene moiety and the absence of electronic communication between two DTEs, respectively.

Abstract

In order to extend stepwise photochromism to more metal-coordination systems for multicolors and multistates, a new photochromic dithienylethene-ethynylthiophene (L1o) was synthesized to design head–head coupled symmetric ruthenium(II) complex trans-Ru(dppe)₂(L1o)₂ (1oo) and head-shoulder coupled asymmetric ruthenium(II) complex trans-Ru(dppe)₂(L1o)(L2o) (2oo). However, upon irradiation under UV light, 1oo only converts to 1co without the observation of dually ring-closed species 1cc, and only the dually ring-closed species 2cc is observed without the detection of the mixed ring-open/closed species 2co and 2oc, implying that it is difficult to achieve substantial stepwise photochromic process of 1oo-2oo → 1co-2oc/2co→1cc-2cc due to the rapid intramolecular energy transfer in 1co and the absence of electronic communication between two dithienylethene units in 2oo. The photochemical processes are well demonstrated and explained by NMR, UV–vis, electrochemical, as well as time-dependent density functional theory (TD-DFT) computational studies.

Introduction

Photochromic dithienylethene (DTE) compounds that can transform between open and closed isomers upon irradiation with appropriate light have been widely investigated due to their excellent performance such as thermally irreversible, fast response, and durable fatigue resistance [[1], [2], [3], [4], [5]]. Therefore, they demonstrate great potential applications in optical memory devices and switches [[6], [7], [8], [9], [10]].

Recently, the research effort has been devoted to incorporate two or more photochromic units into one molecule, which are of particular interest because they can display multiple colors and states upon irradiation with appropriate wavelengths of light [11,12], thus affording potential applications in multifrequency optical memories and data storage [13]. However, when multiple DTE moieties are separated by nonconjugated spacers in a combined molecule, they always undergo the photocyclization reaction simultaneously to afford the fully ring-closed form due to none intramolecular interaction [[14], [15], [16], [17]]. In this case, stepwise photochromic species containing both ring-open and ring-closed DTEs are usually unattained. In contrast, if multiple DTE moieties are linked together through π-conjugated organic spacers, the multi-switch systems normally display partial photochromism due to the facile intramolecular energy transfer from the ring-open isomer to the ring-closed one, preventing further photocyclization [[18], [19], [20]]. As a result, stepwise photochromism could not be operated in both electronically isolated and delocalized systems. Interestingly, when multi-DTE acetylides are incorporated into a metallic center to afford metal bis(DTE−acetylide) complexes, stepwise photochromism is attainable [[21], [22], [23], [24], [25], [26], [27], [28], [29], [30]]. Nevertheless, the systems are mostly limited in the metal bis(DTE−acetylide) complexes in which the ethene bridge part of DTEs is perfluorocyclopentene, and the structures are all shoulder–shoulder coupled metal complexes. In order to evaluate and compare the photochromic behavior and understand the electronic interaction between two DTEs in head–head and head-shoulder coupled metal bis(DTE−acetylide) systems, we designed and synthesized a new DTE moieties dithienylethene-ethynylthiophene L1o with an extended π-system at ethene bridge unit (Scheme 1).

It has been reported that ruthenium(II) is a better electronic mediator than gold(I) or platinum(II) when two ferrocenyl-acetylides (Fc−C≡C) are incorporated to the metal center to afford the Fc−C≡C−M−C≡C−Fc array [[31], [32], [33], [34], [35], [36], [37], [38]], in which electronic communication between two ferrocenyl (Fc) moieties is more remarkable for M = Ru(II) [[31], [32], [33], [34]] than that for M = Au(I) [35,36] or Pt(II) [37,38]. With this in mind, head–head coupled symmetric ruthenium(II) complex 1oo and head-shoulder coupled asymmetric ruthenium(II) complex 2oo that contain dithienylethene-ethynylthiophene L1o were synthesized to investigate the possibilities of their stepwise photochromism (Scheme 1). It is demonstrated that distinct photochromic behavior of two ruthenium complexes is exhibited upon irradiation with appropriate wavelengths of light, in which only the singly ring-closed species 1co and the fully ring-closed isomer 2cc can be obtained for 1oo and 2oo, respectively. Thus, they show great different properties compared to those of the reported metal complexes incorporating two dithienylethene-acetylides in shoulder–shoulder coupled forms [[21], [22], [23], [24], [25], [26], [27], [28], [29], [30]].