Cyclometalated Pt(II) complexes with tetradentate Schiff base ligands: Synthesis, photophysics, electrochemical studies and optical power limiting performance
Introduction
In the past decade, cyclometalated Pt(II) complexes have been attracting considerable interest [[1], [2], [3]], owing to their manifold optical properties and related applications in light-emitting devices, light upconversion, nonlinear transmission [[4], [5], [6], [7], [8], [9]]. Especially, for the reasons of efficient intersystem crossing (ISC), high triplet quantum yield (ФT), long triplet excited state lifetime (τT), intense excited state absorption (ESA) and two-photon absorption (TPA), Pt(II) complexes have expressed excellent application prospects in the field of nonlinear absorbing (NLA) materials [10]. Additionally, the optical properties of Pt(II) complexes were demonstrated to be readily tuned by varying their ligands, including Schiff base, N/P-heterocycles or acetylides, as well as conducting structural modification on these ligands [[11], [12], [13], [14]].
Schiff base ligands based on carbon–nitrogen double bond (–N
C–) are easily synthesized in high yields, the conditions for subsequent complexation to Pt(II) are also much milder than those required for cyclometallating ligands. Therefore, Pt(II) complexes with Schiff base are most widely used in the field of catalysis, biological activity, and triplet−triplet annihilation upconversion [[15], [16], [17]]. Previous work concerning Pt(II) Schiff base complexes have revealed that they qualified rich photophysical properties due to the presence of different electronic interactions (e.g., multiple intramolecular charge transfer) [[18], [19], [20]]. The Wong group has synthesized phosphorescent Pt(II) Schiff base chelates with bulky triphenylamino and tert-butyl substituents, which exhibited long triplet excited-state lifetimes (2.60–12.80 μs) and good thermal stability (Td > 279 °C) [21]. It is noted that introducing organic chromophores on the Schiff base ligand of Pt(II) complexes could tune their triplet excited states properties, such as red-shifting absorption bands, prolonging triple excited lifetime, improving quantum yields and enhancing NLA effects [22,23]. All these intriguing factors make Pt(II) Schiff base complexes ideal candidates as multifunctional optical materials, especially optical power limiting (OPL) materials.
So far, there has been little discussion about Pt(II) Schiff base complexes used as OPL materials. Understanding the correlations between structural modification and tunable photophysical properties of Pt(II) complexes bearing Schiff base ligands is still essential and necessary. In this work, we report five novel cyclometalated tetradentate Pt(II) complexes bearing Schiff base ligands (Pt-1–Pt-5), as shown in Chart 1. Different chromophores were introduced at the 5-position of the salicylaldehyde components. Electron-donating tert-butyl carbazole groups, electron-withdrawing naphthalimide groups, as well as π-conjugated fluorene groups were adopted to evaluate the effect of substituents at the Schiff base ligand. Furthermore in Pt-4 and Pt-5, fluorene units were also introduced to extend the π-conjugation, which could enhance their ESA and TPA characters. The photophysical properties and nonlinear transmission performance of Pt-1–Pt-5 were further discussed in order to develop novel NLA materials. In addition, the results also provide a basis for designing the transition-metal complexes rationally with intensive and broadband ESA for OPL applications.
Section snippets
Synthetic procedures and characterization
The synthetic route to the complexes Pt-1–Pt-5 are illustrated in Scheme 1. For tuning the singlet- and triplet- excited properties, varied electron-donating and electron-withdrawing groups were introduced to the phenyl on salicylaldehyde components. Fluorenyl units were used as building blocks to lengthen the π-conjugation of Pt-4 and Pt-5. Furthermore, large alkyl groups were connected to improve solubility and to avoid intermolecular aggregation of target molecules.
Schiff base ligands (L1–L5
General remarks
All solvents and reagents for synthesis were purchased from Aldrich or Alfa Aesar and used as is unless otherwise stated. The intermediates a1, a2, a3, a4, a5 and Pt(DMSO)2Cl2 were prepared according to the literature procedures [31]. The details of synthetic steps for other intermediates and ligands are provided in supporting information (SI). Silica gel (200–300 mesh) used for chromatography was purchased from Sinopharm Chemical Reagent Co. Ltd. All the target compounds were characterized by 1
Conclusions
A series of Pt(II) Schiff base complexes attaching electron-withdrawing or electron-donating substitutions at the 5-position of salicylaldehyde units were developed. These complexes exhibit 1π,π* and 1MLCT/1ILCT transitions in UV–vis region respectively. The assignments of absorption are supported by DFT calculations. The TA spectra of complexes (except Pt-3) also demonstrate that the strong triplet ESA gives rise to the 3MLCT/3ILCT excited states from the visible to the near-infrared region.
CRediT authorship contribution statement
Senqiang Zhu: Conceptualization, Writing - original draft. Shengliang Zhai: Methodology, Writing - original draft. Zhiyuan Chen: Writing - review & editing. Huan Su: Data curation. Caiting Zhang: Formal analysis. Rui Liu: Funding acquisition, Writing - review & editing, Supervision. Hongjun Zhu: Supervision, Funding acquisition.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors greatly acknowledge the Natural Science Foundation of Jiangsu Province-Outstanding Youth Foundation (BK20170104), the National Natural Science Foundation of China (21602106), “Six Talent Peaks Project” of Jiangsu Province (XCL-037), Strategic Pioneer Program on Space Science, Chinese Academy of Sciences (XDA15013100, XDA15013101) for financial support.
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The authors contributed equally.