Indole-Fused Acridone: Synthesis, Structures, Proton Transfer, and Hole-Transport Properties

Three pairs of regioisomers of the planar acridone derivatives (9 vs 10, 11 vs 12, and 13 vs 14), classified as the 1-cyclized compounds (9, 11, and 13) and the 3-cyclized (or 1,3'-cyclized) regioisomers (10, 12, and 14), have been synthesized, and their X-ray structures have been determined. The 1-cyclized compounds have higher yields and lower energies compared with their 3-cyclized isomers. The fluorescence spectra of the intramolecular H-bond containing compounds (9, 11, 13, and 14) consist of two bands (shorter wavelength band for the keto form and longer wavelength band for the enol form) and exhibit the feature of the excited-state intramolecular proton transfer (ESIPT). The density functional theory (DFT) theoretical investigation of the reorganization energy (λ) with respect to molecular symmetry revealed that planar rigid- C_{2 v}-symmetric polycyclic heteroaromatic molecules (such as acridone, 1, and 13) can have low charge-transport barrier (small λ value) and keep the invariance of the molecular point group in the charge-transport process, and therefore can have high hole mobility.