Narrowing emission spectra based on indolocarbazole molecular model system: an experimental and theoretical study

To clarify the structure-property relationship of the narrowed emission spectrum, four indolocarbazole (IDCz) model compounds were designed and synthesized. 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. Theoretical calculations reveal that the vibronic coupling gradually weakens with the increase in π-conjugated plane, arising from the significant reduction in the number of involved vibration modes in both high-frequency and low-frequency regions. Additionally, the n → π* transition reduces the charge variation on the benzene rings, which is then diluted as the number of locked benzene rings increases. These two main factors jointly suppress the geometric changes in locked benzene rings, particularly the stretching vibrations of C-C/C[double bond, length as m-dash]C bonds, resulting in a narrowing of the emission spectrum. This work not only helps to better understand the mechanism of narrowing the emission spectrum but also guides the molecular design of high-color-purity luminescent materials.