Intramolecular charge-transfer interactions in a julolidine–Bodipy molecular assembly as revealed via 13C NMR chemical shifts

13C NMR spectra were recorded in a series of six deuterated solvents of disparate polarity for three difluoroborondipyrromethene (Bodipy) derivatives bearing an aryl group at the meso position; these aryl residues were julolidine (JULBD), 4-nitrobenzene (NITBD) and phenylene (PHBD). Complete 13C chemical shift assignments were made for all three compounds using a combination of COSY, HMBC and HMQC NMR spectroscopic techniques. The extreme assembly, JULBD, displays pronounced charge-transfer character in polar solvents owing to effective electron donation from the nitrogen atom of the julolidine unit to the electron-affinic Bodipy residue. The 13C chemical shift differences (Δδ) between the meso and N-α dipyrrin carbon atoms recorded for JULBD exhibit a linear correlation with certain solvent polarity functions, including Reichardt’s ETN parameter and Catalán’s solvent dipolarity/polarizability (SPP) function. Similar correlations are not found for NITBD, where the charge-transfer vector lies in the opposite direction and the computed dipole moment is close to zero. Quantum chemical calculations performed on JULBD, PHBD and four other Bodipy derivatives at the DFT (B3LYP/6-311++G∗∗) level afforded the Mulliken charges resident on the dipyrrin carbon centers. A reasonable correlation is found between these calculated Mulliken charges and the corresponding 13C chemical shifts for the various Bodipy derivatives.