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-C2v-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.