A ruthenium(II) polypyridine-type
complex based on the dipyridophenazine ligand with a directly fused
imidazole unit (L1, dipyrido[3,2-a:2′,3′-c]phenazine-10,11-imidazole) has been synthesized, and its
electrochemical and photophysical properties have been studied. The
cyclic voltammogram of [Ru(tbbpy)2(L1)]2+ (C1) (tbbpy is 4,4′-tert-butyl-2,2′-bipyridine) shows a cathodic shift of the phenazine-based
reduction process compared to similar molecules, while the first detected
reduction wave (−1.34 V vs Fc/Fc+) is assigned to
the imidazole unit within the molecule. On the basis of the TD-DFT
calculations, the strong visible absorption band exhibited by C1 is assigned to a metal-to-ligand charge transfer (MLCT)
transition with a concurrent ligand-centered (LC) transition. At room-temperature, C1 features emission (Φ = 0.04) from its lowest excited
states with time constants of 1.2 and 18.3 μs. These lifetimes
are assigned to emission processes from the 3MLCT and 3LC state, respectively. This is the first time that a long-lived
dual emission has been observed for a ruthenium(II) complex bearing
a directly fused extended π-system. Furthermore, the emission
of C1 is quenched upon water addition. In contrast to
related compounds based on a dipyridophenazine ligand, the excited
state energy is not shifted, and the lifetime is drastically decreased
to 169 ns.
Ruthenium(II) complexes, like [(tbbpy)2Ru(dppz)]2+ (Ru-dppz; tbbpy = 4,4′-di-tert-butyl-2,2′-bipyridine, dppz = dipyrido-[3,2-a:2′,3′-c]phenazine),
have emerged as suitable photosensitizers in photoredox catalysis.
Since then, there has been ongoing interest in the design of π-extended Ru-dppz systems with red-shifted visible absorption maxima
and sufficiently long-lived excited states independent of the solvent
or pH value. Herein, we explore the photophysical properties of protonation
isomers of the linearly π-extended [(tbbpy)2Ru(L)]2+-type complexes bearing a dppz ligand with
directly fused imidazole (im) and methyl-imidazole units
(mim) as L. Steady-state UV–vis absorption,
resonance Raman, as well as time-resolved emission and transient absorption
spectroscopy reveal that Ru-im and Ru-mim show desirable properties for the application in photocatalytic
processes, i.e., strong visible absorbance and two
long-lived excited states in the 3ILCT and 3MLCT manifold, at pH values between 3 and 12. However, protonation
of the (methyl-)imidazole unit at pH ≤ 2 unit causes
decreased excited-state lifetimes and an emission switch-off.
Various types of terminal acetylenic iminium triflates (propyne iminium salts) featuring a ketiminium or an aldiminium group have been prepared for the first time by protiodesilylation of the corresponding trimethylsilyl‐substituted acetylenic iminium salts using triflic acid and catalytic silver(I) triflate. The high reactivity of the terminal C,C‐triple bond is documented herein by the facile Michael addition of nucleophiles and the exceptionally high dienophilic reactivity in Diels‐Alder reactions.
A recently reported ruthenium(II) complex bearing an extended dipyridophenazine ligand exhibits unusual long-lived dual emission at room temperature. In this study, the effect of the introduction of a methyl protecting group to the imidazole moiety of this ligand (L1, 11-methyl-11H-imidazo[4,5i]dipyrido[3,2-a:2′,3′-c]phenazine) on the photophysics of the respective ruthenium(II) complex [(tbbpy) 2 Ru(L1)] 2+ (C1) is demonstrated by means of electrochemistry, UV/vis absorption and emission spectroscopy, as well as emission lifetime measurements, and transient absorption spectroscopy on the nanosecond time scale. At room temperature, C1 shows dual emission both in aprotic and in protic solvents with time constants of 1.1/34.2 and 1.2/8.4 μs, respectively. These lifetimes are assigned to the emission from 3 MLCT and 3 LC states. The introduction of the methyl group increases the lifetime of the 3 LC state in C1 almost by a factor of 2 in acetonitrile solution compared to the previously reported compound. Accordingly, the newly introduced methyl group is described as a protecting group for the imidazole moiety of the heterocyclic ligand, which enables prolonged lifetimes of the dual emissive complex in protic solvents. The stabilization of the electronic structure is further underlined by the enhanced stability toward electrochemical reduction as evidenced by cyclic voltammetry.
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