Narrowing Emission Spectra Based on Indolocarbazole Molecular Model System: An Experimental and Theoretical Study †

Abstract

To clarify the structure-property relationship of the narrowed emission spectrum, four indolocarbazole (IDCz) model compounds were designed and synthesized, and their π-conjugated plane size can be adjusted by gradually chemically locking the benzene ring. With increasing π-conjugated plane, both 0-1 and 0-2 vibronic peaks are suppressed significantly, demonstrating a gradual narrowing of the emission spectrum. The theoretical calculations reveal that the vibronic coupling gradually weakens with the increase of π-conjugated plane, arising from the significant reduction in the number of involved vibration modes in both high-frequency and low-frequency regions. More essentially, the n→π* transition reduces the charge variation on the benzene rings, and this charge variation is further diluted as the locked benzene ring increases. These two main factors jointly suppress the geometric changes of locked benzene rings, especially the stretching vibrations of C-C/C=C bonds, resulting in the narrowing of emission spectrum. This work not only helps to better understand the mechanism of narrowing emission spectrum, but also provides guidance for the molecular design of high-color-purity luminescent materials.