Molecular engineering of two-dimensional π-conjugation: expected and unexpected photophysical consequences of a simple particle-in-a-box approach

A highly modular and convergent synthetic route was devised to construct a series of planar Pi-conjugated molecules with systematically varied structural dimensions and electronic characteristics. High-yielding triple Schiff base condensation reactions between Pi-extended bulky anilines and 1,3,5-triformylphloroglucinol furnished a series of pseudo C3-symmetric tris(N-salicylideneamine)s displaying intense absorptions at lambdamax = 445-475 nm and emissions at lambdamax = 470-504 nm. X-Ray crystallographic studies revealed that intricate hydrogen-bonding networks sustain the planar conjugation of these discotic molecules, the HOMO-LUMO gaps of which decrease with increasing conjugation area. This reduction in excitation energy is accompanied by a nearly 4-fold enhancement in emission quantum yield (PhiF). Past a structural threshold, however, increasing conjugation area leads to either (i) decrease in PhiF or (ii) development of localized electronic transitions. These findings provide a well-defined structural window for future elaboration of this emerging family of dynamic 2-D conjugation, the luminescence properties of which have already been shown to reversibly change in response to external stimuli.