Tailoring Amine Substitution in Fluorene and Dibenzothiophene: A Molecular Strategy for Ultralong Organic Phosphorescence

ABSTRACT

Tailoring molecular structure is a powerful strategy to tune excited electronic states and control photophysical pathways, including triplet harvesting and phosphorescence emission. Purely organic room‐temperature phosphorescence (RTP) materials have attracted significant attention due to their potential applications in optoelectronics, sensing, and security technologies. However, achieving persistent RTP remains challenging because of the spin‐forbidden nature of intersystem crossing (ISC) and the susceptibility of triplet excitons to non‐radiative deactivation. To investigate the role of molecular and conformational factors governing phosphorescence, two series of amine derivatives based on fluorene ( FL ) and dibenzothiophene ( DBT ) cores were designed, incorporating peripheral amino substituents such as diphenylamino, bis(3‐methylbut‐2‐en‐1‐yl)amino, and methyljulolidino groups. These systems provide structural features affecting phosphorescence efficiency, including the presence of sulfur heteroatoms, partial charge‐transfer character in diphenylamine derivatives, and increased structural rigidity induced by methyljulolidino units. Using steady‐state and low‐temperature fluorescence spectroscopy, time‐resolved EPR (TREPR), and computational studies, we investigated how sulfur incorporation and amino substitution influence RTP in fluorene and dibenzothiophene derivatives. Consistent with the well‐established heavy‐atom effect, dibenzothiophene derivatives exhibit enhanced phosphorescence. Embedding the emitters in a rigid poly(vinyl alcohol) (PVA) matrix suppresses non‐radiative decay pathways, enabling ultralong RTP with phosphorescence quantum yields up to 10% and lifetimes of 1 s.