Proton-Induced Reversible Modulation of the Luminescent Output of Rhenium(I), Iridium(III), and Ruthenium(II) Tetrazolate Complexes

One of the distinct features of metal–tetrazolate
complexes is the possibility of performing electrophilic additions
onto the imine-type nitrogens of the coordinated five-membered ring.
These reactions, in particular, provide a useful tool for varying
the main structural and electronic properties of the starting tetrazolate
complexes. In this paper, we demonstrate how the use of a simple protonation–deprotonation
protocol enables us to reversibly change, to a significant extent,
the light-emission output and performance of a series of Re­(I)-tetrazolate-based
phosphors of the general formulation fac-[Re­(N∧N)­(CO)3L], where N∧N denotes
diimine-type ligands such as 2,2′-bipyridine (bpy) or 1,10-phenanthroline
(phen) and L represents a series of different 5-aryl tetrazolates.
Indeed, upon addition of triflic acid to these neutral Re­(I) complexes,
a consistent blue shift (Δλmax ca. 50 nm) of
the emission maximum is observed and the protonated species also display
increased quantum yield values (4–13 times greater than the
starting compounds) and longer decay lifetimes. This alteration can
be reversed to the initial condition by further treating the protonated
Re­(I) complex with a base such as triethylamine. Interestingly, the
reversible modulation of luminescent features by the same protonation–deprotonation
mechanism appears as a quite general characteristic of photoactive
metal tetrazolate complexes, even for compounds in which the 2-pyridyl
tetrazolate ligands coordinate the metal center with a bidentate mode,
such as the corresponding Ir­(III) cyclometalates [Ir­(C∧N)2L] and the Ru­(II) polypyridyl derivatives [Ru­(bpy)2L]+. In these cases, the protonation of the starting
materials leads to red-shifted and more intense emissions for the
Ir­(III) complexes, while almost complete quenching is observed in
the case of the Ru­(II) analogues.

金属-四唑配合物（metal–tetrazolate complexes）的显著特征之一，是可在配位五元环的亚胺型氮原子上进行亲电加成反应。此类反应尤为实用，可为调控起始四唑配合物的核心结构与电子性质提供有效手段。本文阐明，通过简易的质子化-去质子化方案，可在较大程度上可逆改变一系列通式为fac-[Re(N∧N)(CO)₃L]的Re(I)基四唑磷光体的发光输出与性能；其中N∧N代表二亚胺类配体，如2,2′-联吡啶(2,2′-bipyridine, bpy)或1,10-菲咯啉(1,10-phenanthroline, phen)，L则对应一系列不同的5-芳基四唑盐。事实上，向这些中性Re(I)配合物中加入三氟甲磺酸(triflic acid)后，可观测到发射最大值出现一致的蓝移（Δλmax约50 nm），且质子化物种的量子产率显著提升（为起始化合物的4~13倍），衰变寿命也有所延长。该变化可通过向质子化的Re(I)配合物中进一步加入三乙胺(triethylamine)等碱类试剂，可逆恢复至初始状态。值得注意的是，通过相同的质子化-去质子化机制实现发光特性的可逆调控，似乎是光活性金属四唑配合物的普遍特征，即便对于那些2-吡啶基四唑配体以双齿模式与金属中心配位的化合物亦成立，例如对应的Ir(III)环金属配合物[Ir(C∧N)₂L]以及Ru(II)多吡啶衍生物[Ru(bpy)₂L]⁺。在这些体系中，起始原料的质子化会使Ir(III)配合物的发射发生红移且强度提升，而Ru(II)类似物则几乎完全发生发光猝灭。