Solid cyclooctatetraene-based triplet quencher demonstrating excellent suppression of singlet–triplet annihilation in optical and electrical excitation

Triplet excitons have been identified as the major obstacle to the realisation of organic laser diodes, as accumulation of triplet excitons leads to significant losses under continuous wave (CW) operation and/or electrical excitation. Here, we report the design and synthesis of a solid-state organic triplet quencher, as well as in-depth studies of its dispersion into a solution processable bis-stilbene-based laser dye. By blending the laser dye with 20 wt% of the quencher, negligible effects on the ASE thresholds, but a complete suppression of singlet–triplet annihilation (STA) and a 20-fold increase in excited-state photostability of the laser dye under CW excitation, were achieved. We used small-area OLEDs (0.2 mm2) to demonstrate efficient STA suppression by the quencher in the nanosecond range, supported by simulations to provide insights into the observed STA quenching under electrical excitation. The results demonstrate excellent triplet quenching ability under both optical and electrical excitations in the nanosecond range, coupled with excellent solution processability.

* Line 346-350: from fits to EL transients in Fig S10, the authors found STA rate is decreased by >60% as a result of blending mCP-COT in BSBCz-EH. This is a surprise because the function of "triplet quencher" should be to reduce the triplet density, and not to reduce the single-triplet interaction rate. The reason is because triplet-single interaction rate is an intrinsic material property of BSBCz-EH. The authors should explain why STA rate in BSBCz-EH is changed as a result of doing 2% mCP-COT; also, a plot of triplet-density in Figure S10 according to the rate equations should be given to verify that the triplet density is significantly decreased as a result of mCP-COT as triplet quencher.
* Line 328-335 and Figure 5 and Figure S10: There is a small discrepency in the measured brightness and modeled singlet density, in that @ ~ 100 ns after electrical excitation, EL intensity ratio of blend (3.5e5 cd/m2) vs. neat (0.4e5 cd/m2) is ~9, while the modeled single density ratio is ~6.5. The authors should explain.

REVIEWER COMMENTS
Reviewer #2 (Remarks to the Author): In this work, the authors report the design, synthesis of a novel solid-state organic triplet quencher based on cyclooctatetraene, and achieve a complete suppression of STA and a 20fold increase in excited-state photostability of BSBCz-EH under CW excitation. The results are interesting. However, there are some issues must be addressed.
1. The transient absorption decay kinetics analysis is not sufficient, why excited-state absorption bands with maxima at 400 nm and 617 nm could be attributed to the tripled and singlet excited-state absorption, respectively?
Response: We thank the Reviewer for the valuable suggestion. To further clarify this, we performed nanosecond transient absorption spectroscopy (TAS) for mCP and mCP-COT in acetonitrile at ambient and deoxygenated (oxygen free) solutions. Furthermore, we added the the Supplementary Figure S7  We have also made the following changes in the TAS section of the main text as highlighted (pages 9 & 10): "In order to gain further evidence of triplet quenching, we performed nanosecond transient absorption spectroscopy (TAS) for mCP and mCP-COT in acetonitrile. In ambient conditions, mCP showed long-lived excited-state absorption band with maximum at 400 nm (decay lifetime of 52 ns) and broad short lived excited-state feature with maximum at 617 nm (biexponential lifetime of 5.8 and 51 ns) ( Fig. 3a, b, S7a). Herein, the decay kinetics of the shortlived absorption band (5.8 ns) was found to match closely with the singlet emission decay obtained from TCSPC (see Table S5) measurements (5.3 ns), suggesting this transient absorption band is a result of the singlet excited-state absorption. In order to get further insights into the long-lived feature (τ ≈ 50 ns), we performed TAS for mCP under deoxygenated conditions. For deoxygenated solution (by using a freeze-pump-thaw method), the lifetime of the long-lived feature increased by more than two orders of magnitude (τ ≈ 32 µs) (Fig. 3c, d, S7b), suggesting this transient absorption band arises from the triplet excited-states that were otherwise quenched by molecular oxygen under ambient conditions. Fig. S7d shows the normalised comparison of triplet excited-state absorption decay under ambient and degassed conditions. In case of a deoxygenated mCP-COT solution, similar singlet and triplet excited state absorption bands were observed. The decay lifetime of singlet excited-state absorption 2 band was found to be 0.9 and 7.3 ns, which is similar to the singlet emission lifetime obtained in the TCSPC measurements (Table S5). Furthermore, triplet excited-state absorption decay of the mCP moiety at 400 nm was found to be significantly quenched (τ ≈ 26 ns) (Fig. 3e, f,   S7c). The extremely shortened decay lifetime of mCP unit's triplet excited-state absorption in mCP-COT suggests ultra-fast transfer of triplet excitons from the mCP unit to the COT moiety, though due to the extremely low triplet energy level of COT, we could not observe the transient absorption band arising from the COT moiety alone."  2. The transient PL spectra analysis should also be given.

Response:
We thank the Reviewer for the worthwhile suggestion. We have now added the following TCSPC PL decay curves and tables summarising the fitting parameters in the Supplementary Fig. S6 and Table S5, as well as Fig. S11 and Table S6.    i) solid lines were used instead of dotted lines for the absorption, ii) annotation arrows were added to clearly distinguish absorption and photoluminescence, and iii) the colour scheme of green-blue-red was changed into black-blue-red for COT, mCP and mCP-COT, respectively, iv) the weak absorption of COT has been further magnified as an inset in the figure (and as noted there is no COT emission). Revised Fig. 2a is shown below and has now been updated on page 7. 4. These is something wrong with the sentence "both molecules can effectively quench triplet excitons with the same mechanism due to their low triplet energies, in which adi-is adiabatic excitation" in Page 11.
Response: We thank the Review's comment. We have now revised the description of our computational studies so to improve its comprehension. Specifically, the following changes have now been made to the main text:  "where the S2-ver and S1-ver transitions are assigned to HOMO → LUMO+1 (57%) and HOMO−2 → LUMO (100%)" has now been changed to "where the S2-ver and S1-ver transitions of mCP-COT are mainly populated over mCP moiety [HOMO → LUMO+1 (57%)] and COT moiety [HOMO−2 → LUMO (100%)]" (page 12). a 6  Additional explanation of "…, which might be related to the decrease in PLQY for the doped films (vide infra)" has now been added (page 12).
 Additional explanation of "(see S1-adi and T1-adi for the adiabatic excitation), and the triplet quenching by COT is known to include non-vertical triplet energy transfer with conformational changes." has now been added on page 12.
 The explanation of "both molecules can effectively quench triplet excitons with the same mechanism due to their low triplet energies, in which adi is adiabatic excitation" has now been re-written as "it is considered that mCP-COT can also effectively quench triplet excitons with the same mechanism because of its low T1-adi energy." (page 12).
5. The authors employ a non-conjugated n-hexyl linker, why? What about influence of the nonconjugated linker length?
Response: We thank the Reviewer for the remark. As highlighted in the main text (page 4): a non-conjugated linker is crucial "to maintain the individual electronic properties of mCP and COT" in distinction to the use of a conjugated linker, which would result in issues of potential undesirable changes in the electronic properties of each or both moieties due to conjugation effects.
For this work, in addition to commercially available starting materials for the ease in chemical synthesis as outlined in Fig. 1, the length of the non-conjugated linker was chosen so that the target molecule would achieve a fine balance between (i) solution-processability, and (ii) thermal stability. Specifically, we sought a linker length that is long enough for good solubility of the target chromophore but not too long that may adversely affect the thermal property of the target material such as reduction in its glass transition temperature (for good film stability).
Hence, an n-hexyl linker was chosen for the study.
To further clarify our choice of the non-conjugated n-hexyl linker, we have now revised the statement of "To maintain the individual electronic properties of mCP and COT, we employed a non-conjugated n-hexyl linker to give mCP-COT with high solubility in common organic solvents for solution processing." (page 4) to "To maintain the individual electronic properties of mCP and COT, it is essential that a non-conjugated linker is employed. For mCP-COT to achieve a high solubility in common organic solvents for solution processing without adversely affecting its thermal property, an n-hexyl linker was chosen for our initial study".

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Regarding the influence of the non-conjugated linker length, we acknowledge that the effect of varying the linker length is beyond the scope of this work and may be a research topic of future work.
6. How about the negative influence of the non-conjugated n-hexyl linker on the electrical properties?
Response: We thank the Reviewer's remark. n-Hexyl group has been a common solubilising moiety employed in multiple high-performing organic semiconductors devices [e.g., poly (  The manuscript reported a novel triplet quencher mCP-COT with the potential to significantly reduce singlet-triplet annihilation. The authors demonstrated the effectiveness of the triplet quencher by mixing mCP-COT in BSBCZ-EH and showed significantly improved EL and PL characteristics. This is the first report to my knowledge on a successfully designed solid state triplet quencher, which represents a good step towards organic laser diode. I have a few comment on the some wording and technical details of the manuscript as below.
* In the abstract, the wordings "this is the first report of a solid state triplet quencher, exhibiting excellent…" should be revised to "this is the first report of a solid state triplet quencher that exhibits excellent…" or something alike. The reason is because solid state triplet quencher has previously been demonstrated, and the novelty in the current report is in the improved triplet quenching ability.
Response: We thank the Reviewer for the valid remark. The according text has now been corrected in the Abstract (page 2); "…, this is the first report of a solid-state triplet quencher that exhibits excellent triplet quenching ability…" * Line 261-266 and Figure 4: The data presented in this report is not an apple-to-apple comparison for triplet quenching capability of mCP-COT to ADN in Ref 31. The reason is because baseline (i.e. without triplet quencher) STA is ~ 50% in Ref 31 while baseline STA in the current study is < 25%. The authors should clarify that.
Response: We agree with the Reviewer that apple-to-apple comparison with Ref 31 using direct values is not ideal in this case since the two systems have different STA baselines. We have now clarified our comparison between the two triplet quenchers using the relative magnitude of STA losses (i.e. by taking the initial STA baseline into consideration) in these two systems. The following changes have now been made:  In main text, the statement of "were not effective" has now been changed to "were not as effective" (page 14).

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 The statement of "In particular, at a 70vol% blend concentration of ADN, the reported PL transient quenching due to STA was still at 17% 31 . In contrast, mCP-COT showed complete suppression of STA at a blending concentration as low as 5wt%." (page 14) has also been changed to "A comparative analysis of the triplet quenching performance of ADN and mCP-COT was conducted to show the relative drop in the initial STA in the two systems (Fig. S8). The results support the superior triplet management properties of mCP-COT since at the 10wt% concentration mCP-COT removes over 98% STA present originally in the neat system, while the same concentration of ADN results in approximately 25% reduction of STA (Fig. S8)".  for BSBCz-EH and Alq3/DCM2 10 , respectively. In the absence of STA, there would be no singlet-triplet interaction between populations; therefore, the singlet population should shortly (under 1 µs) saturate at a steady value where the positive pumping term is balanced out by negative fluorescent ISC and SSA terms (assuming positive contribution of TTA to be negligible) and there is no impact of growing triplet population. Since the singlet population directly correlates to the light intensity, one can treat the difference between peak and steady state in a neat film as a total (i.e., 100%) loss due to STA in a system without triplet quencher. Then, the relative decrease in STA plotted against the quencher concentration can be a rough measure of how successful the triplet manager is in the system. *Also, by looking at the comparison between Figure 4a and 4b, it seems that the amount of triplet quenching is also dependent on PL pump condition, so  Fig. 4a and Fig. 4b, respectively (where previous Fig. 5 has also been updated as Fig. 6 accordingly).  (now Fig. 5). Furthermore, as noted by the Reviewer that the pump densities are very different in the two measurements. The temporal characteristics in previous Fig. 4a (now Fig. 4) were performed by using 355 nm excitation source at low excitation densities to avoid photodegradation while the long term photostability was conducted by using 405 nm excitation as 405 nm excitation source provided higher power densities which are important for photostability study.
* Line 346-350: from fits to EL transients in Fig S10, the authors found STA rate is decreased by >60% as a result of blending mCP-COT in BSBCz-EH. This is a surprise because the function of "triplet quencher" should be to reduce the triplet density, and not to reduce the single-triplet interaction rate. The reason is because triplet-single interaction rate is an intrinsic material property of BSBCz-EH. The authors should explain why STA rate in BSBCz-EH is changed as a result of doing 2% mCP-COT; also, a plot of triplet-density in Figure S10 according to the rate equations should be given to verify that the triplet density is significantly decreased as a result of mCP-COT as triplet quencher.
Response: We agree with the Reviewer that changing the value of kSTA during the fit of different samples is not the best way of describing physical processes going in the system (as kSTA is an intrinsic material property). In the revised manuscript, we kept the kSTA (4.3 × 10 -8 cm 3 s -1 ) constant between neat and blended case while adding another term, kmCP-COTT1, to the triplet rate equation, where the kmCP-COTT1 term is the rate of triplet quenching (depopulation) in the presence of mCP-COT. In the neat case its value is kept at 0, while in the blend case the obtained quenching rate of 1 ×10 10 s -1 results in significant drop of triplet population, and thus reduction of kSTAS1T1 component in the singlet equation. kmCP-COTT1 has now been added to the triplet rate equation in the Supplementary Information (on page S23) for the blend system.
We have also added simulated triplet populations to Fig. S10 (i.e. now the new Fig. S14c).
We have also modified the discussion on EL simulation in the main text to the following (on page 20); In order to confirm STA quenching by mCP-COT in the blend films, rate equations for polaron, singlet and triplet generation were simulated in MATLAB ® and the STA rate along with other annihilation rates was extracted from the program. Simulation of neat and blend device EL characteristics from rate equations (see Supplementary Eq. S1) suggests an STA rate (kSTA) of 4.3 × 10 −8 cm 3 s −1 for the neat OLEDs. For blend OLEDs, kSTA was kept the same and a new term, kmCP-COT, was introduced in the triplet equation depicting contribution of mCP-COT towards rapid triplet depopulation. kmCP-COT was extracted to be 1 × 10 10 s −1 . Fig. S14a, b shows the result of rate equation fitting for the EL response of the neat and blend devices, respectively.
It must be noted that the plotted singlet density for both neat and blend devices is for the same current (50 A cm −2 ) going through both devices. However, singlet density for the blend can be seen as being around eight times the singlet density in neat device (an indication of more STA quenching in neat device). The results of reduced STA quenching indicate the triplet quencher mCP-COT is efficient for the fast triplet decay. Fig. S14c gives evidence of triplet populations extracted from neat and blend devices. The triplet population obtained for neat devices is almost 30 times more than that of the blend.
14 Fig. S14: Simulated singlet density evolution over time compared with experimental data at 50 A cm -2 . a Evolution of singlets in a neat device, actual signal (black) versus simulated fit (red); STA rate recovered from simulation was 4.3 × 10 −8 cm 3 s −1 . b EL fitting for a 2wt% mCP-COT blend device actual signal (black) vs simulated fit (red) with the same STA rate (as neat is shown). A new parameter kmCP-COT was introduced in the blend case signifying triplet depopulation rate due to COT. kmCP-COT was found to be 1 × 10 10 s -1 . c Evolution of triplet population in neat and blend case is shown depicting efficient triplet recycling in the blend case.
* Line 328-335 and Figure 5 and Figure S10: There is a small discrepancy in the measured brightness and modeled singlet density, in that @ ~ 100 ns after electrical excitation, EL intensity ratio of blend (3.5e5 cd/m2) vs. neat (0.4e5 cd/m2) is ~9, while the modeled single density ratio is ~6.5. The authors should explain.