Phenothiazine–Quinoline Conjugates Realizing Intrinsic Thermally Activated Delayed Fluorescence and Room‐Temperature Phosphorescence: Understanding the Mechanism and Electroluminescence Devices

Abstract

Understanding the local triplet (3LE), charge transfer triplet (3CT), and charge transfer singlet (1CT) is of great importance in designing thermally activated delayed fluorescence (TADF) and room‐temperature phosphorescence (RTP) materials for their use in organic light‐emitting devices (OLEDs), sensing, and bioimaging. Herein, two phenothiazine–quinoline conjugates (PTzQ1, PTzQ2) in which the donor (PTz) and acceptor (Q1, Q2) parts are held in near‐orthogonal orientation that gives rise to spatial separation of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are reported. Photophysical studies of both conjugates along with their individual components (PTz, Q1, Q2) show that TADF occurs via reverse intersystem crossing (rISC) from the upper‐level local triplet (3LE) to lower‐level singlet 1CT mediated by vibronic coupling between 3LE and 3CT, whereas RTP is realized from 3LE. It is found that all three excited states are close in energy, 0.16–0.19 eV (3LE−1CT), 0.02–0.03 eV (1CT−3CT), and 0.14–0.16 eV (3LE−3CT) with the order of 3LE > 1CT > 3CT. Both conjugates exhibit a high rate constant of rISC (krISC, 7.9–9.7 × 105 s−1), resulting in external quantum efficiency (EQE) values of ≈4.7% (solution processed) with emission from both RTP and TADF components.

Thermally Activated Delayed Fluorescence and Room-Temperature Phosphorescence in Asymmetric Phenoxazine-Quinoline (D2–A) Conjugates and Dual Electroluminescence

“…This includes chemical modification, mechanical force, − conformational switching, intramolecular CT, regioisomeric effect on the excited state, and change of host matrices . For instance, Ray and co-workers demonstrated simultaneous TADF and RTP under ambient conditions by a close energetic proximity of the excited energy levels in phenothiazine-quinoline conjugates . Despite these achievements, structure–property correlation in TADF materials that show efficient RTP becomes a tremendous challenge.…”

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

“…The dual emission feature suggests that the radiative recombination from LE to the ground state is relatively slow due to D–A torsion, which can significantly compete with the electron transfer that populates the lower-level CT state. 34 Further, a close overlap of the steady state spectra of the D part and PQ in toluene clearly indicates that the contribution of the LE band arises from the D component ( Figure 4 , Figure S15 ). On the other hand, PQCl and PQBr exhibit only lower energy CT bands that show significant positive solvatochromic effects with increasing solvent polarity as compared with that of PQ ( Figure 4 , Figure S15 ).…”

“…All the compounds exhibit higher energy absorption bands at 265 and 311 nm which can be attributed to the π → π* transition. 34 , 35 , 50 , 51 In addition, a broad tail ranging from ∼355 to 500 nm which shows a slight hypsochromic shift upon increasing solvent polarity, was observed. A comparison between the absorption spectra of D (Phx) and A (QPP) fragments with all the conjugates confirms that a higher energy region of the absorption spectra of the D–A conjugates arises due to the arithmetic sum of the D and A absorption spectra, while the appearance of lower energy broad absorption bands (ε = 6100–7100 L·mol –1 ·cm –1 ) is due to the charge transfer (CT) caused by D–A torsion ( Figure 4 ), which is conducive for visible light excitation (VLE).…”

“…We anticipate that the radiative recombination from 1 LE to the ground state is relatively slow due to rigidity of the host, which can significantly compete with the electron transfer that populates the lower-level 1 CT state. 34 On the other hand, phosphorescence measurements (λ ex = 425 nm, 0.1 ms delay) showed green-RTP ( 3 CT, broad) peaks at 514 (PQ: τ 1 = 48.28 μs, τ 2 = 152.45 μs, τ 3 = 525.94 μs), 529 nm (PQCl: τ 1 = 313 μs, τ 2 = 1.6 ms), and 530 nm (PQBr: τ 1 = 148 μs, τ 2 = 569 μs), which are red-shifted (∼10–19 nm) as compared with the steady state emission spectra recorded at ambient conditions ( Figure 7 a,b, Table 1 , Figures S26a, S28 ). Moreover, a significant increase in the lifetimes under deoxygenated conditions at RT confirms the involvement of triplet state ( 3 CT) ( Figure S27 ).…”

“… 1 , 33 On the other hand, thermally activated delayed fluorescent (TADF) material development relies on the orthogonal D–A structure with a close proximity of 3 LE, 1 CT, and 3 CT states which results in an increased reverse intersystem crossing (RISC) rate by reducing the energy gap between the 1 CT, 3 CT, and 3 LE states. 34 − 39 However, most of the reports determine the energy and configuration of T1 states from a phosphorescence spectrum measured at 77 K. 29 , 40 In addition, 1 CT and 3 CT states are often considered degenerate states, assuming the orthogonal D–A geometry of the TADF molecules. 36 , 37 , 41 Furthermore, decades of RTP research have shown RTP via ultraviolet light excitation.…”