Achieving Dual Persistent Room‐Temperature Phosphorescence from Polycyclic Luminophores via Inter‐/Intramolecular Charge Transfer

Abstract

bioimaging, [3] and anti-counterfeiting applications. [4] Pure organic OPRTP materials are more attractive compared to the transition metal or rare earth-based phosphorescent materials owing to their low production cost, easy synthesis, low toxicity, and good flexibility. [5] However, OPRTP is rare in pure organic molecules because of the forbidden transition from singlet to triplet excited states. The reported metal-free OPRTP materials are usually substituted with aromatic carbonyl groups [6] or heavy atoms [7] to facilitate the intersystem crossing (ISC) process, while crystallization, [8] host-guest interaction, [9] and polymer doping [10] are utilized to suppress the nonradiative decay and stabilize the triplet states.Solid-state phosphorescence is highly dependent on molecular stacking, such as aggregation-caused quenching. [11] Recently, OPRTP has been observed in H- [8a] or J-aggregates, [12] while the influence of T-shaped (edge-to-face) arrangement on phosphorescence has been rarely discussed. Published work mainly focused on the stabilization of triplet excited states by molecular packing, but packing-induced intermolecular charge transfer received less attention in OPRTP materials although it strongly affected ISC efficiency and the energy gap (ΔE ST ) between singlet and triplet excited states. [3c,13] Polycyclic π-conjugated compounds, such as indolo[3,2-b] carbazoles (ICZs) and their analogues, are important organic semiconductors, which have been widely used in organic optoelectronic fields, such as organic field-effect transistors and photovoltaic cells, owing to their excellent carrier transporting properties. [14] In this work, 6,12-diphenyl-5,6,11,12tetrahydroindolo[3,2-b]carbazole (Ben-H, Figure 1), one of the ICZ analogues, unexpectedly exhibited OPRTP after the cease of UV light irradiation. Unlike most reported OPRTP materials, there were no commonly used groups such as aromatic carbonyl groups or heavy atoms in Ben-H. Based on the energy level diagram and single-crystal structure of Ben-H, we hypothesized that charge transfer (CT) played a dominant role in persistent emission (Figure 2). In the isolated molecular state, intramolecular charge transfer (ICT) from the indole rings to the benzene rings gave rise to the ISC process with narrow ΔE ST energy gap, while in the dimers, T-shaped CH···π interactions induced Organic persistent room-temperature phosphorescence (OPRTP) materials show great prospects in optoelectronic and biomedical applications, such as display, anti-counterfeiting, sensing, and bioimaging. However, the reported OPRTP material systems are relatively rare, and it is a challenge to achieve the tunability of OPRTP. In this work, a series of polycyclic luminophores are developed based on an indole derivative (6,12-diphenyl-5,6,11,12tetrahydroindolo[3,2-b] carbazole, Ben-H) as the structural skeleton. These compounds unexpectedly exhibit dual OPRTP at 442 to 623 nm with lifetimes spanning from 2 to 759 ms. Experimental data and theoretical calculations suggest th...

A Simple Organic Molecule Realizing Simultaneous TADF, RTP, AIE, and Mechanoluminescence: Understanding the Mechanism Behind the Multifunctional Emitter

“…Thelager DE ST and enhanced ISC process in dimer can make it efficient for the accumulation of T 1 . [8] Furthermore,t he SOC element from T 1 to S 0 was large (2.645 cm À1 )e nough, which guaranteed the transition rate quick enough from T 1 to S 0 for phosphorescence emission (Supporting Information, Figure S3). And for the crystal form, the molecular structure is rigid and the nonradiation transition is restrained.…”

“…Thef irst one involves in heavy atoms (such as Br, I) or heteroatoms with ne lectrons to promote the intersystem crossing (ISC) process and help the accumulation for triplet excitons,and the other one is creating arigid environment to suppress the non-radiative decay and promote the emission process. [8,9] Mechanoluminescence (ML), [10] an interesting emission process activated by mechanical stimulation, was first found in 1605, but reports about ML-active pure organic materials have been few until now. [11] Until 2014, an aggregationinduced emission (AIE)-active molecule exhibiting bright ML with nearly 100 %p hotoluminescence quantum yield (PLQY) in solid state was reported by the Chi group.…”

σ‐Conjugation and H‐Bond‐Directed Supramolecular Self‐Assembly: Key Features for Efficient Long‐Lived Room Temperature Phosphorescent Organic Molecular Crystals

“…Among the different pathways explored to develop metal‐free organic materials, the molecular crystal approach is the most studied, [9–20] because addressing simultaneously all the above issues at different length scales. At the molecular level, constitutive individual metal‐free organic phosphors mainly consist of π‐conjugated systems encompassing an emitting subunit.…”

“…Another design strategy consists in coupling the carbazole electron‐donating (D) group to an electron‐accepting (A) group into a twisted (D‐A) system to favor a twisted intramolecular charge‐transfer (TICT), [19, 20] one of the ways to generate triplet states via charge transfer states [21] . Indeed, such a twisted structure is well known in the field of molecules exhibiting thermally activated delayed fluorescence (TADF) for promoting a spatial separation of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), that leads to a small Δ E ST and an increase of the ISC rate [22] .…”

“…On the other hand, it was also demonstrated that the more compact face‐to‐face packing of carbazoles leads to a long lifetime (750 ms) attributed as well to the stabilization of the excited triplet state, with an efficiency of ≈3 % [12] . Nevertheless, the arrangement of carbazoles as a herringbone motif without π‐to‐π coplanar interaction is also emphasized to contribute to the aggregation effect [20] …”