Substituent effect on TADF properties of 2-modified 4,6-bis(3,6-di- tert -butyl-9-carbazolyl)-5-methylpyrimidines

The interest in organic materials exhibiting thermally activated delayed fluorescence (TADF) significantly increased in recent years owing to their potential application as emitters in highly efficient organic light emitting diodes (OLEDs). Simple modification of the molecular structure of TADF compounds through the selection of different electron-donating or accepting fragments opens great possibilities to tune the emission properties and rates. Here we present the synthesis of a series of novel pyrimidine–carbazole emitters and their photophysical characterization in view of effects of substituents in the pyrimidine ring on their TADF properties. We demonstrate that electron-withdrawing substituents directly connected to the pyrimidine unit have greater impact on the lowering of the energy gap between singlet and triplet states (Δ E ST ) for efficient TADF as compared to those attached through a phenylene bridge. A modification of the pyrimidine unit with CN, SCH 3 , and SO 2 CH 3 functional groups at position 2 is shown to enhance the emission yield up to 0.5 with pronounced TADF activity

Origin of dual emission in σ-bridged donor–acceptor TADF compounds

The desire to boost the reverse intersystem crossing rate and obtain thermally activated delayed fluorescence with sub-microsecond lifetime fosters the search for novel concepts of molecular geometry. Recently, TADF compounds made of acridine, tetramethylcarbazole and triphenylamine donor and triphenyltriazine acceptor units bound by hyperconjugated spacer units were introduced as having very rapid double TADF decay. Here we present an in-depth time-resolved fluorescence analysis of these intriguing donor–σ–acceptor TADF compounds in various surroundings. Extremely weak coupling of electron-donating and electron-accepting units was found for the σ-bridged TADF compounds, resulting in the coexistence of intramolecular and exciplex fluorescence, whose interplay allowed one to tune the emission properties. The initial fluorescence decay in toluene solutions, previously attributed to rapid TADF, was shown to be prompt intramolecular fluorescence with prolonged fluorescence lifetime, susceptible to molecular oxygen. Only the later delayed fluorescence at the microsecond time-scale, originating from the exciplex states, was attributed to TADF. On the contrary, dominant intramolecular TADF was observed in dilute PMMA films with weaker non-radiative decay. The smooth transition from intramolecular to exciplex TADF was observed by increasing the doping concentration of the polymer films. The DF/PF ratio was found to increase with increasing doping concentration due to the emergence of additional exciplex TADF until a 20 wt% doping load, where concentration quenching emerged at larger doping ratios. The presented findings showcase the unusual fluorescence properties of TADF compounds with weakly bound donor and acceptor units and are important for the future design of novel TADF compounds

Achieving submicrosecond thermally activated delayed fluorescence lifetime and highly efficient electroluminescence by fine-tuning of the phenoxazine−pyrimidine structure

Thermally activated delayed fluorescence (TADF) materials, combining high fluorescence quantum efficiency and short delayed emission lifetime, are highly desirable for application in organic light-emitting diodes (OLEDs) with negligible external quantum efficiency (EQE) roll-off. Here, we present the pathway for shortening the TADF lifetime of highly emissive 4,6-bis[4-(10-phenoxazinyl)phenyl]pyrimidine derivatives. Tiny manipulation of the molecular structure with methyl groups was applied to tune the singlet–triplet energy-level scheme and the corresponding coupling strengths, enabling the boost of the reverse intersystem crossing (rISC) rate (from 0.7 to 6.5) × 106 s–1 and shorten the TADF lifetime down to only 800 ns in toluene solutions. An almost identical TADF lifetime of roughly 860 ns was attained also in solid films for the compound with the most rapid TADF decay in toluene despite the presence of inevitable conformational disorder. Concomitantly, the boost of fluorescence quantum efficiency to near unity was achieved in solid films due to the weakened nonradiative decay. Exceptional EQE peak values of 26.3–29.1% together with adjustable emission wavelength in the range of 502–536 nm were achieved in TADF OLEDs. Reduction of EQE roll-off was demonstrated by lowering the TADF lifetime

TADF parameters in the solid state: an easy way to draw wrong conclusions

The successful development of thermally activated delayed fluorescence (TADF) OLEDs relies on advances in molecular design. To guide the molecular design toward compounds with preferable properties, special care should be taken while estimating the parameters of prompt and delayed fluorescence. Mistakes made in the initial steps of analysis may lead to completely misleading conclusions. Here we show that inaccuracies usually are introduced in the very first steps while estimating the solid-state prompt and delayed fluorescence quantum yields, resulting in an overestimation of prompt fluorescence (PF) parameters and a subsequent underestimation of the delayed emission (DF) yield and rates. As a solution to the problem, a working example of a more sophisticated analysis is provided, stressing the importance of in-depth research of emission properties in both oxygen-saturated and oxygen-free surroundings

Temporal dynamics of solid-state thermally activated delayed fluorescence: disorder or ultraslow solvation?

Time-resolved emission spectra of thermally activated delayed fluorescence (TADF) compounds in solid hosts demonstrate significant temporal shifts. To explain the shifts, two possible mechanisms were suggested, namely, slow solid-state solvation and conformational disorder. Here we employ solid hosts with controllable polarity for analysis of the temporal dynamics of TADF. We show that temporal fluorescence shifts are independent of the dielectric constant of the solid film; however, these shifts evidently depend on the structural parameters of both the host and the TADF dopant. A ≤50% smaller emission peak shift was observed in more rigid polymer host polystyrene than in poly(methyl methacrylate). The obtained results imply that both the host and the dopant should be as rigid as possible to minimize fluorescence instability