# Efficient persistent afterglow modulation using extended Indolo[2,3-a]carbazoles with six-membered rings in a polymer matrix

## TL;DR

Researchers developed a new method to control the afterglow of organic materials by modifying their molecular structure, achieving high efficiency and long-lasting light emission.

## Contribution

A structural modulation strategy using flexible and rigid six-membered rings to control afterglow emission in a polymer matrix is introduced.

## Key findings

- Flexible HICz promotes hybrid afterglow via reverse intersystem crossing.
- Rigid BICz achieves pure phosphorescence with a 3.05 s lifetime and 17.59% quantum yield.
- BICz-MF films show afterglow lasting over 30 s in ambient conditions.

## Abstract

Organic persistent afterglow materials exhibit tremendous promise for applications in optoelectronic displays, anti-counterfeiting, and information encryption. Here, we introduce a structural modulation strategy in which a flexible cyclohexyl or rigid phenyl substituent is added to a 11,12-dihydroindolo[2,3-a]carbazole (ICz) scaffold, namely HICz and BICz, respectively, to precisely manipulate the (hybrid) afterglow emission, with a long lifetime and high efficiency in a melamine-formaldehyde (MF) polymer matrix. Flexible HICz promotes reverse intersystem crossing (RISC) for hybrid afterglow, whereas rigid BICz enlarges singlet-triplet energy gaps (ΔEST), consequently driving the ISC process for pure phosphorescence afterglow, featuring an ultralong lifetime of 3.05 s and an impressive phosphorescence quantum yield of 17.59%. The afterglow duration for BICz-MF film exceeds 30 s in an ambient environment, substantially outperforming conventional ICz systems. Furthermore, a triplet-sensitized Förster resonance energy transfer (TS-FRET) strategy enables multicolor afterglow for optical encryption. This study establishes a generalizable molecular design principle for high-efficiency and long-afterglow organic materials.

Structure-modulated ICzs with flexible/rigid six-membered rings embedded in melamine-formaldehyde composites show efficient organic afterglow with a phosphorescence quantum yield (ΦPhos.) of up to 17.59% and a lifetime (τPhos.) of up to 3.05 s.