Tailoring Excited-State Properties and Electroluminescence Performance of Donor–Acceptor Molecules through Tuning the Energy Level of the Charge-Transfer State

Abstract

Three donor−acceptor (D−A) compounds (PTZ-10-AnP, PTZ-10P-AnP, and PTZ-3-AnP) were designed and synthesized through linking the same D and A units with different architectures. The structural change, mainly referring to the torsion angle and distance between D and A, gives rise to the significantly different CT level positions for these compounds. The CT state of PTZ-10-AnP locates at the lowest excited state, while that of PTZ-10P-AnP stays at the higher excited state. For PTZ-3-AnP, the CT energy is close to that of the locally π−π* excited state, and a hybridized local and charge-transfer (HLCT) state dominates the S 1 state. Photophysical experiment data of these compounds demonstrate that the energy level of the charge-transfer (CT) state plays a decisive role in the emissive state properties of donor−acceptor (D−A) compounds. In addition, the exciton utilization efficiency of the PTZ-3-AnP device exceeds the limit of the spin statistics, which enlightens the molecular design toward harvesting triplet excitons in fluorescent organic light-emitting diodes (OLEDs). ■ INTRODUCTIONRecently, substantial progress has been made in the field of organic light-emitting diodes (OLEDs), that is, employing triplet excitons through a reverse intersystem crossing (RISC) process in fluorescent OLEDs (FOLEDs), which will certainly bring about exceeding the spin statistical limit of singlet excitons. 1−10 This phenomenon is mainly concentrated in donor−acceptor (D−A) molecules with unique excited-state properties. The combination of D and A moieties often results in the formation of a charge-transfer (CT) excited state. 11−14 The CT state along with the π−π* state constitute the energy level system in the D−A compound, which determines the optical and electronic properties of the compound. Generally, CT states possess weak binding energy due to their spacially separated transition orbitals. 15 This intrinsic merit facilitates the RISC process of triplet excitons. 16 On the other hand, the separated transition orbitals of CT states can cause undesirably low radiative transition rates and photoluminescence efficiency. 17,18 Consequently, for D−A molecules, their CT energy levels have crucial effects on the final emissive state properties. To deeply understand the relationship between CT energy level and excited-state properties, a systematical modulation for CT energy level in D−A molecules is required.As for molecular design, phenothiazine has been widely used as a donor moiety because of its low ionization potential (around 5.0 eV) and strong electron-donating ability. 19−26 Particularly, phenothiazine contains two sp 3 -hybridized heteroatoms (S, N) in the heterocyclic structure, which leads to a nonplanar butterfly-shaped geometry. 27 This molecular configuration endows the 3,7-position and 10-position of phenothiazine with significantly different steric hindrances. As a result, the D−A torsion angle should be of difference when introducing the same acceptor to the 3,7-position and 10-position of phenothiazine. Through c...

“Rate-limited effect” of reverse intersystem crossing process: the key for tuning thermally activated delayed fluorescence lifetime and efficiency roll-off of organic light emitting diodes

“…However, ambient temperature (300 K) with 25 meV energy endows possible molecular rotation around Δ α 1 = ±25°. 45 , 46 The transition is more permissible because the more planar D–A conformation contributing to larger orbital overlap is found around the equilibrium conformation after activation by thermal energy. Theoretical absorption spectra of DC–ACR and PyCN–ACR were simulated based on OHF% functionals according to different twisting angles (Δ α 1 ) using Multiwfn (Fig.…”

Organocatalyzed Atom Transfer Radical Polymerization: Perspectives on Catalyst Design and Performance

“…For example, methylene blue is a popular histologic stain used in biological research that changes color due to electron transfer reactions involving the phenothiazine core structure . Due to their redox‐active nature, phenothiazines have been applied in dye‐sensitized solar cells, organic light‐emitting diodes (OLEDs), and battery electrode applications . Recently, phenothiazine's ability to act as a photoreductant has been exploited in catalysis for applications such as dehalogenation and CC bond formation on aryl halide substrates .…”

Regio- and conformational isomerization critical to design of efficient thermally-activated delayed fluorescence emitters

“…One way of achieving efficient TADF is to use donor–acceptor–donor (D–A–D) molecules possessing strong intramolecular charge-transfer (CT) 10 11 12 . However, contrary to initial thoughts 4 , rISC is not driven by direct spin–orbit coupling (SOC), which within the one electron limit is forbidden between the 1 CT and 3 CT states.…”