Abstract
In this work, the emission properties of fac-[Re(CO)3(NN)(py)]+, NN = 1,10-phenanthroline (phen), dipyrido[3,2-f:2’,3’-h]quinoxaline (dpq) and dipyrido[3,2-a:2’3’-c]phenazine (dppz); py = pyridine were investigated in different temperatures, ranging from 80 to 300 K, and in different solvent mixtures and in polymethyl methacrylate. The changes observed in the emission quantum yields were rationalized based on a two-level excited state model, in which the nonemissive upper state is thermally populated and decays faster than the lowest lying emissive state. fac‑[Re(CO)3(dpq)(py)]+ is a metal-to-ligand charge transfer (MLCT) emitter as the complex with phen but exhibits smaller emission quantum yields, being more sensitive to the solvent. This behavior was rationalized by quantum-mechanical calculations including the spin-orbit coupling matrix elements, revealing that intersystem crossing from the lowest singlet excited state in fac- [Re(CO)3(dpq)(py)]+ likely occurs to triplet states lying at higher energies. Similar behavior were observed for fac-[Re(CO)3(dppz)(py)]+, although the later exhibits intraligand emission that are strongly quenched in fluid solutions by low-lying MLCT states. The fundamental studies carried out here provide new insights on the excited state dynamics of ReI complexes with dipyridoquinoxaline and phenazine ligands and can contribute for further advances on their application as luminescent probes.
Highlights
Complexes 1 and 2 are characterized by low energy metal-to-ligand charge transfer (MLCT) bands centered at 370 nm (Figure 1), so the additional pyrazine ring in dpq does not lead to significant shifts on the MLCT band
For complex 3, the MLCT band is overlapped with ligand centered π→π* transitions, so no shifts are observed in relation to the absorption bands of 1 and 2, only an increase in the molar absorptivity
The emission profile is very similar for both complexes, the substitution of phen by dpq leads to a decrease in the emission quantum yield of ca. 50% (0.020 to 0.012) associated with an expected much shorter emission lifetime (1.6 to 0.45 μs), Table 1
Summary
The emissive properties of ReI polypyridyl complexes have called growing attention due their highly modularity in terms of the ligands employed and the surrounding media.[1,2,3,4] Such properties have been successfully applied in the development of light-emissive devices (LEDs), biomarkers, ion sensors, among other technological applications.[5,6,7,8] In fluid solution and in biological environments, the typical long-lived emitting states of ReI polypyridyl complex have been consistently employed as specific biological probes,[9,10,11,12]The useful photochemical and photophysical properties of these complexes arise from different classes of excited states, including metal-to-ligand charge transfer (MLCT), ligand-to-ligand charge transfer (LLCT), ligand-to-metal charge-transfer (LMCT), intraligand (IL) and metal-centered (MC) states, which the dynamics have been object of detailed experimental and theoretical studies.[1,22,23,24,25] Among the different polypyridylTemperature Dependent Emission Properties of ReI Tricarbonyl ComplexesJ. Despite the quantum yields of all investigated complexes in EtOH/MeOH 4:1 are smaller than those obtained in prop/but 5:4, the behavior as a function of the temperature are very similar in both environments.
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