Spectroscopic Signature of the aggregation-induced emission phenomena caused by restricted nonradiative decay: A theoretical proposal

It has been suggested that the exotic aggregation-induced emission (AIE) phenomenon was caused by the restriction on the nonradiative decay through intramolecular vibrational/rotational relaxation. There have been other proposed mechanisms such as J-aggregation or excimer formation, etc. Through computational studies, we propose a direct approach to verify the AIE process, namely, using resonance Raman spectroscopy (RRS) to explore the microscopic mechanism of AIE. Taking examples of AIE-active 1,2-diphenyl-3,4-bis(diphenylmethylene)-1-cyclobutene (HPDMCb) and AIE-inactive 2,3-dicyanopyrazino phenanthrene (DCPP) for comparison, we found that for the AIEgen, after aggregation into cluster, the intensities of low-frequency peaks in RRS are evidently reduced relative to the high-frequency peaks, along with a remarkable blueshift. However, the RRS of non-AIEgen remains almost unaffected upon aggregation. Such distinctive spectroscopic characteristics can be ascribed to the intramolecular vibrational relaxation which is hindered for AIEgen, especially for the low-frequency ring-twisting motions, because the RRS amplitude is proportional to the mode vibrational relaxation energy times frequency λ_jω_j. Thus, RRS is a direct way to clarify the recent dispute on the AIE mechanism. If such predictions are true, it will clearly validate the earlier proposed restriction on the nonradiative decay through an intramolecular vibration/rotation relaxation mechanism.