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

Abstract

Triplet energy harvesting via thermally activated delayed fluorescence (TADF) from pure organic systems has attracted great attention in organic lightemitting diodes, sensing, and photocatalysis. However, the realization of thermally enhanced phosphorescence (TEP)-guided efficient TADF with a high rate of reverse intersystem crossing (k RISC ) 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 (k RISC = 1.44 × 10 8 s −1 ) originated from the quasi-equatorial (QE) conformer caused by a low singlet−triplet gap (ΔE S1−T1 = 0.11 eV) and triplet energy transfer from QA to QE owing to the degenerate ground state of the conformers. In contrast, TADF (k RISC = 0.74 × 10 8 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.

White Light Emission via Dual Thermally Activated Delayed Fluorescence from a Single-Component Phenothiazines–Diphenyl Quinoline Conjugate

“…300–360 (intense) and 365–500 nm (broad); the lower energy broadband (CT) arises as a result of donor–acceptor coupling, as both the D and A parts absorb only below 350 nm. In contrast, the higher energy absorption bands, which remain almost unaffected by changing solvent polarities, can be assigned to the π–π* character, which agrees well with earlier reports (Figure a). , A slight modification of the broad absorption bands is observed with a change in the polarity of the solvents, ensuring a π–π* transition with a CT character (Figure S8a of the Supporting Information). Likewise, steady-state photoluminescence (PL) measurements (λ ex = 360 nm) in MCH revealed two emission bands at 430 nm (vibrational) and 591 nm (broad) (Figure a).…”

“…On the other hand, efficient thermally activated delayed fluorescence (TADF) emitters with donor–acceptor (D–A) architectures shed light on OLEDs, sensing, and photocatalysis as a result of 100% triplet exciton harvesting through singlet (S1) ← triplet (T1) upconversion [i.e., reverse intersystem crossing (RISC)] caused by low Δ E ST (typically <0.5 eV). ,− On the basis of the TADF design principle, a few multicomponent TADF-based WLE (hybrid OLEDs) devices have also been reported. − Nevertheless, even if hybrid OLEDs are considered good candidates for lighting applications, using a single-component system is more advantageous than combining compounds, as required in hybrid OLEDs. Moreover, hybrid OLEDs facilitate triplet–triplet energy transfer, leading to triplet harvesting losses, poor control of the emission color, and poor device efficiency.…”

“…Here, we investigated the design of a phenothiazines–diphenyl quinoline (D2–A) conjugate, PT2QP1 (Figure ), through the covalent attachment of phenothiazines (PTz) at the isomeric positions (C6 and C8) of the 2,4-phenyl-substituted quinoline segment via two C–N single bonds. Inspired by the dual emission via LE and TADF achieved in phenothiazine-based D–A conjugates, ,,− we hypothesized that the covalent attachment of two PTz donors might result in intense singlet blue 1 LE and orange 1 CT (OTADF) emission as a result of a mixture of quasi-axial (QA) and quasi-equatorial (QE) conformers (Figure ). We demonstrate that the conjugate in the solid state exhibits WLE via simultaneous blue TADF (BTADF: 1 LE ← 3 CT upconversion, RISC1) and orange TADF (OTADF: 1 CT ← 3 CT upconversion, RISC2) [photoluminescence quantum yield (PLQY), 45 ± 2%; Commission Internationale de l’Éclairage (CIE) coordinates, 0.30, 0.33].…”

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

“…It should be noted that a slight decrease in the S1–T1 gaps ( BPT1 to BPT3 ) resulted in no significant enhancement in the DF/PF ratio (Table S17). Further, the rate of ISC ( k ISC S1′–T1′ ) between the S1′ and T1′ states was calculated using the lifetime method , and found to be 8.27 × 10 7 s –1 ( BPT1 ), 10.6 × 10 7 s –1 ( BPT2 ), and 16.5 × 10 7 s –1 ( BPT3 ) (Table S18). An increasing k ISC on going from BPT1 to BPT3 suggests enhancement of ϕ P (AVP), which is consistent with the previous line of discussion.…”

“…On the other hand, harvesting triplet energy via TADF from orthogonal donor–acceptor (D–A)-based emitters has boosted enormous interest in chemical sciences due to the involvement of the reverse intersystem crossing process (RISC). − ,,,− However, optimizing the D–A dihedral angle is one major challenge that shifts the emission color undesirably. Recently, boron and nitrogen (B and N)-containing multiresonance-TADF systems have been successfully developed to realize efficient TADF with narrow emission. − Despite the currently known optimization strategies of D–A emitters, significant challenges with realizing blue-violet phosphorescence remain because of their high emission energy.…”