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 (ΔE ST) for efficient TADF as compared to those attached through a phenylene bridge. A modification of the pyrimidine unit with CN, SCH3, and SO2CH3 functional groups at position 2 is shown to enhance the emission yield up to 0.5 with pronounced TADF activity.

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.

Conformational disorder enabled emission phenomena in heavily doped TADF films.

Thermally activated delayed fluorescence (TADF) compounds doped in solid hosts are prone to undergo solvation effects, similar to those in the solution state. Emission peak shifts and changes in emission decay rates usually follow solid-state solvation (SSS). However, here we show that typical SSS behavior in heavily doped TADF films could be of a completely different origin, mistakenly attributed to SSS. Typically, increasing the doping load was found to redshift the emission peak wavelength and enhance the rISC rate. However, more in-depth analysis revealed that SSS actually is negligible and both phenomena are caused by the specific behavior of delayed emission. Increasing the concentration of the TADF compound was shown to enhance the concentration quenching of long-lived delayed fluorescence from conformer states with the largest singlet energy, eventually leading to a gradual redshift of the delayed emission peak wavelength. Concomitantly, the loss of long-lived delayed fluorescence entailed reverse intersystem crossing rate enhancement, though the rate-governing singlet-triplet energy gap was gradually increasing. The observed phenomena are highly unwanted, burdening molecular structure and OLED performance optimization.

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.

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.

Minimization of solid-state conformational disorder in donor-acceptor TADF compounds.

Thermally activated delayed fluorescence (TADF) compounds with a flexible donor-acceptor structure suffer from conformational disorder in solid-state, which deteriorates their emission properties as well as OLED performance. Accordingly, TADF materials with predictable solid-state emission properties are highly desirable. In this work, we analyse the relation between the molecular rigidity and solid-state TADF properties. Two TADF compounds, 4,6-bis(2-methyl-4-(10H-phenothiazin-10-yl)phenyl)pyrimidine (PTZ-mPYR) and 1,2,3,4-tetrakis(carbazol-9-yl)-5,6-dicyanobenzene (4CzPN), with similar emission properties in toluene solutions but different rigidity of the molecular structure were systematically studied. The analysis was supplemented by comparison of solid-state TADF properties of PTZ-mPYR with its analogue 4,6-bis(4-(10H-phenoxazin-10-yl)phenyl)pyrimidine (PXZ-PYR), bearing a more sterically constrained planar electron-donor unit. All compounds showed conformational disorder in diluted polymer films; however its extent directly depended on the molecular structure. Large dispersion of singlet-triplet energy gaps resulted in remarkably prolonged TADF lifetime for PTZ-mPYR with a less sterically constrained donor unit. In contrast, weakened conformational disorder in rigid 4CzPN with sterically crowded donor units was shown to ensure rapid TADF decay with only a threefold lower solid-state rISC rate as compared to toluene. Similarly, selection of a more sterically constrained planar electron-donor unit was also shown to be preferable for lowering the conformational disorder. Our findings are important for the future design of compounds with efficient solid-state TADF as well as for the further application in OLEDs with low external quantum efficiency roll-off.

Emission wavelength dependence on the rISC rate in TADF compounds with large conformational disorder.

Large vibronic coupling between the local and charge-transfer triplet states is required for efficient reverse intersystem crossing in TADF compounds. This is ensured by low steric hindrance between donor and acceptor molecular units. However, flexible molecular cores show large conformational disorder and emission wavelength instability in solid films.

Inactivation of bacterial biofilms using visible-light-activated unmodified ZnO nanorods.

Various zinc oxide (ZnO) nanostructures are widely used for photocatalytic antibacterial applications. Since ZnO possesses a wide bandgap, it is believed that only UV light may efficiently assist bacterial inactivation, and diverse crystal lattice modifications should be applied in order to narrow the bandgap for efficient visible-light absorption. In this work we show that even unmodified ZnO nanorods grown by an aqueous chemical growth technique are found to possess intrinsic defects that can be activated by visible light (λ = 405 nm) and successfully applied for total inactivation of various highly resistant bacterial biofilms rather than more sensitive planktonic bacteria. Time-resolved fluorescence analysis has revealed that visible-light excitation creates long-lived charge carriers (τ > 1 µs), which might be crucial for destructive biochemical reactions achieving significant bacterial biofilm inactivation. ZnO nanorods covered with bacterial biofilms of Enterococcus faecalis MSCL 302 after illumination by visible light (λ = 405 nm) were inactivated by 2 log, and Listeria monocytogenes ATCL3C 7644 and Escherichia coli O157:H7 biofilms by 4 log. Heterogenic waste-water microbial biofilms, consisting of a mixed population of mesophilic bacteria after illumination with visible light were also completely destroyed.

Non-symmetric 9,10-diphenylanthracene-based deep-blue emitters with enhanced charge transport properties.

Realization of efficient deep-blue anthracene-based emitters with superior film-forming and charge transport properties is challenging. A series of non-symmetric 9,10-diphenylanthracenes (DPA) with phenyl and pentyl moieties at the 2nd position and alkyl groups at para positions of the 9,10-phenyls were synthesized and investigated. The non-symmetric substitution at the 2nd position enabled to improve film forming properties as compared to those of the unsubstituted DPA and resulted in glass transition temperatures of up to 92 °C. Small-sized and poorly conjugated substituents allowed to preserve emission in the deep blue range (<450 nm). Substitution at the 2nd position enabled to achieve high fluorescence quantum yields (up to 0.7 in solution, and up to 0.9 in the polymer host), although it caused an up to 10-fold increase in the intersystem crossing rate as compared to that of the unsubstituted DPA. Further optimization of the film forming properties achieved by varying the length of the alkyl groups attached at the 9,10-phenyls enabled to attain very high hole drift mobilities (∼5 × 10(-3)-1 × 10(-2) cm(2) V(-1) s(-1)) in the solution-processed amorphous films of the DPA compounds.

Enhanced electroluminescence based on thermally activated delayed fluorescence from a carbazole-triazine derivative.

Thermally activated delayed fluorescence (TADF) properties of a dicarbazole-triazine compound, 9-(4,6-diphenyl-1,3,5-triazin-2-yl)-9'-phenyl-3,3'-bicarbazole (CzT), and its OLED characteristics were investigated. An estimated small energy gap of about 90 meV between the singlet and triplet energy states of CzT made the up-conversion of triplet excitons back to a singlet state possible. The origin of the observed delayed fluorescence has been shown to be thermally activated delayed fluorescence. An organic light emitting diode (OLED) with CzT as an emitter showed the maximum external quantum efficiency (EQE) of 6%. For comparison, another carbazole-triazine derivative of 3-(2'-(4,6-diphenyl-1,3,5-triazin-2-yl)-[1,1'-biphenyl]-2-yl)-9-phenyl-9H-carbazole (PhCzTAZ) with a similar structure was also studied. PhCzTAZ showed a low fluorescence quantum yield with no TADF.