Molecular-Level Understanding of Dual-RTP via Host-Sensitized Multiple Triplet-to-Triplet Energy Transfers and Data Security Application

Carbazole-Benzonitrile-Norbornadiene Conjugates for Photothermally Reversible Ambient Phosphorescence

Organic photoswitches have attracted significant attention across various fields, such as sensing, bioimaging, photopharmacology, molecular machines, and solar energy storage. However, as a result of design complexities, achieving photothermally reversible ambient phosphorescence switching in the condensed state remains elusive. Herein, we explore the impact of norbornadiene (NBD)/quadricyclane (QC) substitution at position 5 of the benzonitrile acceptor covalently attached to the carbazole donor on photothermally reversible luminescence switching. Experimental investigations demonstrated that the CzN and TBCzN switches exhibited photothermally reversible fluorescence switching in solution. Moreover, in the condensed state, fluorescence and ambient phosphorescence switching were observed as a result of a low singlet-triplet (ΔEST) gap (CzN ⇆ CzQ, ΔESTCzN/CzQ = 0.05/0.28 eV; TBCzN ⇆ TBCzQ, ΔESTTBCzN/TBCzQ = 0.06/0.09 eV). Reversible ambient phosphorescence switching is primarily influenced by modulation of acceptor conjugation resulting from NBD ⇆ QC switching. This approach may provide important clues for the design of visible-light-absorbing photothermally reversible phosphorescent materials.

Conformational Effect of Catechol-Terephthalonitrile Emitters Leading to Ambient Violet Phosphorescence

Organic ambient violet phosphorescent (AVP) materials are of great interest due to their involvement of high energy and longer-lived triplet excitons. Here, we show three fused ring functionalized donor-acceptor-donor (D-A-D/D-A-D') emitters (BPT1-BPT3), in which two catechol-based donors (3,4-dihydroxybenzophenone, catechol, or 3,5-ditert-butylcatechol) are covalently fused to the terephthalonitrile acceptor via four O-C single bonds. Spectroscopic analysis revealed that all the molecules show AVP (∼390-394 nm, τAVP = 73-101 μs) with phosphorescence quantum yields (ϕP) of 1.8-27.4% due to low singlet-triplet gaps (0.036-0.046 eV) and conformational effects. BPT3 with bulky tert-butyl groups increases AVP (ϕP = 27.4%). Quantum chemistry calculations reveal flat (F1) and twisted (F2) conformers (ground state) with a low energy difference (∼4-5 kcal/mol) for all molecules; the F1 conformer is responsible for efficient AVP, while weak blue thermally activated delayed fluorescence with longer-lived delayed components is realized from the F2 conformer. This approach may provide important clues for the design of high-energy organic phosphorescent materials.

Ln-HOF Nanofiber Organogels with Time-Resolved Luminescence for Programmable and Reliable Encryption

Developing tunable luminescent materials for high throughput information storage is highly desired following the explosive growth of global data. Although considerable success has been achieved, achieving programmable information encryption remains challenging due to current signal crosstalk problems. Here, we developed long-lived room-temperature phosphorescent organogels enabled by lanthanum-coordinated hydrogen-bonded organic framework nanofibers for time-resolved information programming. Via modulating coassembled lanthanum concentration and Förster resonance energy transfer efficiency, the lifetimes are prolonged and facilely manipulated (20-644 ms), realizing encoding space enlargement and multichannel data outputs. The aggregated strong interfacial supramolecular bonding endows organogels with excellent mechanical toughness (36.16 MJ m-2) and self-healing properties (95.7%), synergistically achieving photostability (97.6% lifetime retention in 10000 fatigue cycles) via suppressing nonradiative decays. This work presents a lifetime-gated information programmable strategy via lanthanum-coordination regulation that promisingly breaks through limitations of current responsive luminescent materials, opening unprecedented avenues for high-level information encryption and protection.

Modulation of Delayed Fluorescence Guided by Conformational Effect-Mediated Thermally Enhanced Phosphorescence in Phenothiazines-Quinoline-Cl Conjugates

Triplet energy harvesting via thermally activated delayed fluorescence (TADF) from pure organic systems has attracted great attention in organic light-emitting diodes, sensing, and photocatalysis. However, the realization of thermally enhanced phosphorescence (TEP)-guided efficient TADF with a high rate of reverse intersystem crossing (kRISC) still needs to be discovered. Herein, we report two phenothiazine-quinoline conjugates (P2QC, P2QMC) comprising two phenothiazine donors covalently attached to the chlorine-substituted quinolinyl acceptor. Spectroscopic analysis in conjunction with quantum chemistry calculations reveals that TEP in P2QC originated due to slow internal conversion from higher-lying triplet to lowest triplet (T2' → T1') of the quasi-axial (QA) conformer and TADF (kRISC = 1.44 × 108 s-1) originated from the quasi-equatorial (QE) conformer caused by a low singlet-triplet gap (ΔES1-T1 = 0.11 eV) and triplet energy transfer from QA to QE owing to the degenerate ground state of the conformers. In contrast, TADF (kRISC = 0.74 × 108 s-1) and dual phosphorescence under ambient conditions are observed in P2QMC. This study provides a sustainable guideline for developing efficient TADF emitters via conformation effects and energy transfer mechanisms.

Phenoxazine-Quinoline Conjugates: Impact of Halogenation on Charge Transfer Triplet Energy Harvesting via Aggregate Induced Phosphorescence

Room-temperature phosphorescence (RTP) from organic compounds has attracted increasing attention in the field of data security, sensing, and bioimaging. However, realization of RTP with an aggregate induced phosphorescence (AIP) feature via harvesting supersensitive excited charge transfer triplet (³CT) energy under visible light excitation (VLE) in single-component organic systems at ambient conditions remains unfulfilled. Organic donor-acceptor (D-A) based orthogonal structures can therefore be used to harvest the energy of the ³CT state at ambient conditions under VLE. Here we report three phenoxazine-quinoline conjugates (PQ, PQCl, PQBr), in which D and A parts are held in orthogonal orientation around the C-N single bond; PQCl and PQBr are substituted with halogens (Cl, Br) while PQ has no halogen atom. Spectroscopic studies and quantum chemistry calculations combining reference compounds (Phx, QPP) reveal that all the compounds in film at ambient conditions show fluorescence and green-RTP due to (i) radiative decay of both singlet charge transfer (¹CT) and triplet CT (³CT) states under VLE, (ii) energetic nondegeneracy of ¹CT and ³CT states (¹CT- ³CT, 0.17-0.21 eV), and (iii) spatial separation of highest and lowest unoccupied molecular orbitals. Further, we found in a tetrahydrofuran-water mixture (f _w = 90%, v/v) that both PQCl (10^-5 M) and PQBr (10^-5 M) show concentration-dependent AIP with phosphorescence quantum yields (ϕ_P) of ∼25% and ∼28%, respectively, whereas aggregate induced quenching (ACQ) was observed in PQ. The phosphorescence lifetimes (τ_P) of the PQCl and PQBr aggregates were shown to be ∼22-62 μs and ∼22-59 μs, respectively. The ϕ_P of the powder samples is found to be 0.03% (PQ), 15.6% (PQCl), and 13.0% (PQBr), which are significantly lower than that of the aggregates (10^-5 M, f _w = 90%, v/v). Film (Zeonex, 0.1 wt %) studies revealed that ϕ_P of PQ (7.1%) is relatively high, while PQCl and PQBr exhibit relatively low ϕ_P values (PQCl, 9.7%; PQBr, 8.8%), as compared with that of powder samples. In addition, we found in single-crystal X-ray analysis that multiple noncovalent interactions along with halogen···halogen (Cl···Cl) interactions between the neighboring molecules play an important role to stabilize the ³CT caused by increased rigidity of the molecular backbone. This design principle reveals a method to understand nondegeneracy of ¹CT and ³CT states, and RTP with a concentration-dependent AIP effect using halogen substituted twisted donor-acceptor conjugates.