Homoconjugation and Tautomeric Isomerism in Triptycene-Fused Pyridylbenzimidazoles

The facile, metal-free synthesis and characterization
of three
new series of triptycene-fused pyridylbenzimidazoles are reported;
such compounds possess an imidazole moiety fused within the benzene
rings of the trypticene and a pyridine ring installed at position
2 of the imidazole rings. The position of the nitrogen atom of the
pyridyl moiety linked to position 2 of the fused benzimidazole scaffold
is systematically changed from the ortho to para position. The number of substituted blades bearing
the pyridyl-substituted fused benzimidazole scaffolds has been increased
from one to three. Such a library of compounds allowed us to evaluate
the enhancement of two main effects: tautomeric isomerism and homoconjugation.
The characteristic dynamic equilibrium between different isomers induced
by prototropic tautomerization was examined by 1H nuclear
magnetic resonance spectroscopy. By comparison of the photophysical
properties of the new compounds with those of classical planar pyridylbenzimidazoles,
the presence of the homoconjugation effect between the different triptycene
blades was demonstrated. Fine details of the electronic structure
of the new derivatives were unraveled by a computational analysis.
The novel compounds can be employed for the construction of intriguing
self-assembled supramolecular architectures.