1H NMR Chemical Shift as an Index of UV-Vis Absorption/Emission Maxima in Aromatic Dyes

Abstract

We have demonstrated that experimental ¹H NMR chemical shifts can serve as direct indicators of absorption and emission maxima within families of structurally related molecules. Notably, predictions of optical maxima based on chemical shifts have proven to be more accurate than excitation energies obtained from time-dependent density functional theory employing long-range corrected functionals such as CAM-B3LYP. This proof of principle has first been established using experimental data for a series of pyrrolo[3,2-b]pyrroles, for which a linear correlation between the absorption maximum and the ¹H NMR chemical shift has been identified for the first time. The scope of the study has then been extended to diketopyrrolopyrroles, additional pyrrolo[3,2-b]pyrroles, and thieno[3,2-b]thiophenes using density functional theory calculations. Across all investigated compound families, the correlation has remained close to linear. We have shown that two key criteria must be fulfilled to preserve this linearity: steric similarity among the compared molecules and full π-conjugation of the molecular framework. These conditions arise from the dependence of magnetically induced ring-current strengths, i.e., the most influential factor on the ¹H NMR chemical shifts of conjugated molecules, on frontier-orbital energy gaps, together with symmetry selection rules formulated by Fowler and Steiner. These findings have provided new insight into the electronic structure of quadrupolar dyes and have established a practical framework for the rapid and efficient design of optoelectronic materials.