Molecular Insight into Carbonyl-Induced Room-Temperature Phosphorescence of Organic Aggregates

Room-temperature phosphorescence (RTP) in carbonyl-containing organic luminogens has been widely reported in the aggregated state. However, it remains a challenge to unravel the microscopic origin of carbonyl-activated RTP from organic fluorophores. Herein, by taking a fluorescent prototype, tetraphenylpyrrole (TPP), with its carbonyl-substituted analogue (TPM) as prototypical molecules, we elucidate the excited-state mechanism underlying this fluorescence-to-phosphorescence conversion. Our results demonstrate that carbonyl substitution transforms the lowest singlet excited state (S1) from a locally excited character in TPP to a charge-transfer character in TPM. This change leads to a huge reduction in both oscillator strength and singlet–triplet energy gap (ΔEST), thereby strongly suppressing radiative transition and facilitating intersystem crossing (ISC) for populating triplet excitons. Exciton dynamics analysis further reveals that, upon carbonyl substitution, the radiative decay rate decreases by 3 orders of magnitude, while both internal conversion and ISC rates increase by approximately 3 orders of magnitude, accounting for the fluorescence-to-phosphorescence conversion. In addition, the experimentally observed fluorescence and RTP lifetimes are well reproduced by theoretical calculations, validating the proposed mechanism. This work elucidates the origin of carbonyl-induced RTP in organic aggregates and provides general theoretical insights into the rational design of organic phosphorescent materials through excited-state engineering.