Positional Variation of Monopyridyl-N in Unsymmetrical Anthracenyl π-Conjugates: Difference between Solution- and Aggregate-State Emission Behavior

Fluorescence enhancement on aggregation for π-conjugates linked with pyridyl ring has been established as a part of widely studied smart organic functional materials. Therefore, the photophysical features in the solution and aggregate states for such compounds remain impressive. In this work, we synthesized three series of photostable unsymmetrical aryl-substituted anthracenyl π-conjugates linked to pyridyl ring with a variation of the position of a pyridyl-N atom and examined the difference in the photophysical properties preferably in the aggregate state. The so-called "aggregation-induced emission (AIE)" behavior was discernible for the 2- and 4-pyridyl- but not 3-pyridyl-10- p -tolyl or mesityl-substituted π-conjugates. Curiously, a variation of the position of a pyridyl-N atom does not solely control the AIE phenomenon for 10-thiophenyl-substituted π-conjugates, where all of the isomers are found to be AIE-active. Hence, the dissimilarity in emission behavior in the aggregate state is governed by the position of N-atom for pyridine and also the substituent at the 10th position of the anthracyl ring. The mechanistic insight behind these observations is demonstrated by concentration-dependent fluorescence studies, time-resolved fluorescence, single-crystal X-ray diffraction studies (largely supportive to understand the molecular structure and packing in the aggregate), and average particle size measurement of the aggregates and partly by the density functional theory studies for a few representative molecules.