Zn Coordination and the Identity of the Halide Ancillary Ligand Dramatically Influence the Excited-State Dynamics and Bimolecular Reactions of 2,3-Di(pyridin-2-yl)benzo[g]quinoxaline

Abstract

The optical properties of coordination complexes with ligands containing nitrogen heterocycles have been extensively studied for decades. One subclass of these materials, metal complexes utilizing substituted pyrazines and quinoxalines as ligands, has been employed in a variety of photochemical applications ranging from photodynamic therapy to organic light-emitting diodes. A vast majority of this work focuses on characterization of the metal-to-ligand charge-transfer states in these metal complexes; however, literature reports rarely investigate the photophysics of the parent pyrazine or quinoxaline ligand or perform control experiments utilizing metal complexes that lack low-lying charge-transfer (CT) states in order to determine how metal-atom coordination influences the photophysical properties of the ligand. With this in mind, we examined the steady-state and time-resolved photophysics of 2,3-di­(pyridin-2-yl)­benzo­[g]­quinoxaline (dpb) and explored how the coordination of ZnX2 (X = Cl–, Br–, I–) affects the photophysical properties of dpb. In dpb, we find that the dominant mode of deactivation from the singlet excited state is intersystem crossing (ISC). Coordination of ZnX2 perturbs the relative energies of the ππ* and nπ* excited states of dpb, leading to drastically different rates of ISC as well as radiative and nonradiative decay in the [Zn­(dpb)­X2] complexes compared to dpb. These differences in the rates change the dominant singlet-excited-state decay pathway from ISC in dpb to a mixture of ISC and fluorescence in [Zn­(dpb)­Cl2] and [Zn­(dpb)­Br2] and to nonradiative decay in [Zn­(dpb)­I2]. Coordination of ZnX2 and the choice of the halide ligand also have profound effects on the rate constants for excited-state bimolecular reactions, including triplet–triplet annihilation and oxygen quenching. These results demonstrate that metal coordination, even in complexes lacking low-lying CT states, and the choice of the ancillary ligand can dramatically alter the photophysical properties of chromophores containing nitrogen heterocycles.

Heterodinuclear Ru–Pt Complexes Bridged with 2,3-Bis(pyridyl)pyrazinyl Ligands: Studies on Kinetics, Deoxyribonucleic Acid/Bovine Serum Albumin Binding and Cleavage, In Vitro Cytotoxicity, and In Vivo Toxicity on Zebrafish Embryo Activities

“…This leads to a smaller Δ E gap, which makes a metal-to-ligand charge transfer easier and then increases the reactivity from C 1 to C 3 . 32 The π-back bonding further increases the electrophilicity of the Pt, leading to higher rates of substitution. The coordination of Ru at one end of each of the complexes enhances water solubility.…”

“…As a result, the Pt­(II) centers become more positive due to an increase in π-back bonding from the Pt 5d-orbitals into the extended π*-MOs of the chelate bridges. This leads to a smaller Δ E gap, which makes a metal-to-ligand charge transfer easier and then increases the reactivity from C 1 to C 3 . The π-back bonding further increases the electrophilicity of the Pt, leading to higher rates of substitution.…”

Luminescence of ESIPT-capable zinc(ii) complexes with a 1-hydroxy-1H-imidazole-based ligand: exploring the impact of substitution in the proton-donating moiety

“…Recently, we demonstrated that 1-hydroxy-1H-imidazoles decorated with proton-accepting groups at position 2 of the imidazole ring and a pyridin-2-yl group at position 4 to assist the N 3 imidazolic atom in binding metal ions can act as ESIPT-capable ligands and coordinate Zn 2+ ions without deprotonation (HL p and HL q , see Scheme 2). Since Zn 2+ ions are known to be able to enhance the emission originating from the ligand-centered excited states due to so-called chela-tion-enhanced fluorescence (CHEF) effect, [125][126][127][128][129][130] their coordination by ESIPT-capable 1-hydroxy-1H-imidazoles led to a noticeable increase in the photoluminescence quantum yield (PLQY). 131,132 Moreover, the emission of the free ligands and complexes appeared to be tunable through the extension of the π-conjugation in the proton-accepting part of the molecule.…”

Complexes on the Base of a Proton Transfer Capable Pyrimidine Derivative: How Protonation and Deprotonation Switch Emission Mechanisms

“…Azole and benzazole ESIPT-capable derivatives are workhorses in the area of ESIPT studies. − The most common molecular pattern of these compounds which is widely relied upon in the design of ESIPT dyes includes the proton-accepting azole/benzazole moiety combined with the proton-donating 2-hydroxyphenyl group or its analogues introduced in the α position to the azolic N atom. In contrast, pyrimidine derivatives are relatively rarely used for the design of ESIPT-fluorophores in comparison with azole and benzazole ones. ,− However, being combined with suitable proton-donating groups, they can serve as proton-accepting moieties. ,, Moreover, introducing additional donor groups in the pyrimidine core can lead to more complex molecular architectures of pyrimidine-based compounds suitable for binding metal ions and providing numerous sites for protonation, including isomeric ones. ,, In turn, the synthesis of isomeric ESIPT-capable compounds allows researchers to shed more light on the impact of structural factors on ESIPT and on relationships between ESIPT and luminescence. − The coordination of conventional dyes and ESIPT emitters to metal ions with d electronic configuration such as Zn 2+ is known to enhance the quantum efficiency of emission. − Earlier we demonstrated that emissions of 4-(1 H -pyrazol-1-yl)-6-(2-hydroxyphenyl)­pyrimidines and 2-(2-hydroxyphenyl)-4-(1 H -pyrazol-1-yl)­pyrimidines, which belong to two different isomeric families (Chart ), share such a common feature as dual emission associated with singlet-to-singlet and triplet-to-singlet transitions, which is contributed by anti-Kasha fluorescence of the tautomeric form. ,, However, their coordination behavior toward Zn 2+ ions appeared to be quite different, whereas the 4-(1 H -pyrazol-1-yl)-6-(2-hydroxyphenyl)­pyrimidine derivative can bind Zn 2+ ions through the N,N-site of the molecule, which produces multicolor emission of the complex, 2-(2-hydroxyphenyl)-4-(3,5-dimethyl-1 H -pyrazol-1-yl)­pyrimidine derivatives cannot do this. We associated this with steric effects imposed by methyl and phenyl substituents introduced in positions 3 and 5 of the pyrazolyl group.…”