Pyrene-Labeled Poly(aryl ether) Monodendrons:  Synthesis, Characterization, Diffusion Coefficients, and Photophysical Studies

Abstract

Four generations of poly(aryl ether) monodendrons labeled with pyrene at the focus were synthesized and characterized by a combination of NMR, size-exclusion chromatography with light scattering detection, and electronic spectroscopy. The monodendrons were then studied by magnetic resonance and fluorescence techniques. Translational diffusion coefficients in THF-d 8, acetonitrile-d3, and cyclohexane-d12 were measured by pulsed-field-gradient NMR and ranged from 2.2 × 10 -5 cm 2 /s for methoxypyrene in acetonitrile to 3.4 × 10 -6 cm 2 /s for the fourth generation monodendron in THF. Molecular radii were calculated from the diffusion coefficients by the Stokes-Einstein equation. In THF, the radii increased from 2.8 Å for methoxypyrene to 14 Å for the fourth generation monodendron. In acetonitrile the radii were smaller, increasing from 2.7 Å for methoxypyrene to 5.4 Å for the third generation monodendron. In both solvents, a change in solution structure between the second and third generation monodendrons was observed in the diffusion data. The rate of fluorescence quenching by molecular oxygen was measured for the pyrene-labeled monodendrons in THF, acetonitrile, and cyclohexane and was found to decrease for monodendrons of increasing generation. This decrease cannot be fully explained by the slower diffusion of the larger monodendrons. Several simple models for the reduced quenching rate of the pyrene chromophore were developed. These models suggest that the increased density of the larger monodendrons provides a more effective barrier to diffusing dioxygen.

Diffusion NMR Spectroscopy in Supramolecular and Combinatorial Chemistry: An Old Parameter—New Insights

“…The authors pointed out that the different breaking points arise from the different probes used in both processes (THF or dioxygen). [93] The last two examples demonstrate how diffusion NMR spectroscopy gives a better insight into the solution structure of dendritic materials, which, in turn, may affect their properties, and demonstrate the potential of such measurements to establish the structure/functional activity of such complex systems. [93] 4.4.…”

“…[93] The last two examples demonstrate how diffusion NMR spectroscopy gives a better insight into the solution structure of dendritic materials, which, in turn, may affect their properties, and demonstrate the potential of such measurements to establish the structure/functional activity of such complex systems. [93] 4.4. Reactive Intermediates, Ion Pairing, and Organometallic Systems…”

“…The structure is even more collapsed in cyclohexane. [93] The change in the diffusion behavior in THF occurs between G2 and G3, while the quenching data shows the change in behavior occurs between G3 and G4. The authors pointed out that the different breaking points arise from the different probes used in both processes (THF or dioxygen).…”

“…A comparison of the structural features of these two classes of dendrimers in the different solvents with the electron-transfer rates in these systems led the authors to evoke a molecular structure/property relationship for the attenuation of the electron transfer in these systems. [92] Riley et al studied the relationship between the diffusion characteristics of pyrene-labeled poly(aryl ether) monodendrons [93] (Scheme 5) and their photophysical performance. Interestingly, it was found that the increase in the Stokes radii with the increase in dendron generation is much more pronounced in THF than in CH 3 CN (Table 5).…”

“…Table 5: Diffusion coefficients D, radii r, volumes V, and densities 1 from pulsed-field-gradient NMR experiments. [93] Compound [a] Calculated from NMR diffusion data. [b] Calculated from Equation (4) obtained from the diffusion coefficients and the van der Waals radii for the neutral polycyclic systems.…”