Influence of Substituted Pyridine Rings on Physical Properties and Electron Mobilities of 2-Methylpyrimidine Skeleton-Based Electron Transporters

A series of 2-methylpyrimidine skeleton-based electron-transporting derivatives (BPyMPM) are designed and synthesized to investigate the influence of substituted pyridine rings on the physical properties and electron mobilities (μe). The only structural difference is the position of substituted pyridine rings. The melting point (Tm) of B4PyMPM is estimated to be ca. 50 °C higher than that of B3PyMPM, and ca. 120 °C higher than that of B2PyMPM. The ionization potential is observed to increase in the order B2PyMPM (6.62 eV) < B3PyMPM (6.97 eV) < B4PyMPM (7.30 eV), measured using ultraviolet photoelectron spectroscopy. Furthermore, time-of-fight measurements of vacuum-deposited films demonstrate that the μe at 298 K of B4PyMPM is 10 times higher than that of B3PyMPM and 100 times higher than that of B2PyMPM. To extract the charge transport parameters, the temperature and field dependencies of μe are investigated. Using Bässler's disorder formalism, the degree of energetic disorder is estimated to decrease in the order B2PyMPM (91 meV) > B3PyMPM (88 meV) > B4PyMPM (76 meV), and the positional disorder is 2.7 for B2PyMPM, and < 1.5 for B3PyMPM and B4PyMPM.