Peer review of: Kinetic prediction of reverse intersystem crossing in organic donor–acceptor molecules (2020)

REVIEWER COMMENTS 

Reviewer #1 (Remarks to the Author): 

This manuscript presents a detailed study of a library of 22 organic molecules for kinetics of reverse 
intersystem crossing from the triplet to the singlet excited state. Molecules were selected where the 
rate constants for reverse intersystem crossing span a range of 5 orders of magnitude. The study is 
mostly focused on computation, but also involved synthesis of some of the molecules and 
photophysial measurements of these synthesized molecules. The authors compared published rate 
constants for reversed intersystem crossing with their computation using two different models. The 
conventional approach using the Marcus parabolic approximation appears to be less reliable than 
the new approach which includes higher-lying triplet excited states. The work seams to be 
conducted carefully and should be novel enough to justify publication in Nature Communications. I 
recommend to accept this manuscript for publication after the minor corrections and suggestions 
listed below have been addressed. 

Page 10, line8: The authors should include what method was used for collecting the 
photoluminescence decays. Was it done by time-correlated single photon counting or bymulti cannel 
scaling or by another method? 

Page 8, line 14: “Our results suggest that explicitly computing singlet–triplet crossing seams leads to 
more reliable predictions… ” Something is wrong with this sentence. Delete “seams” 

Table S1: The quantum yields “ΦPF” and “ΦDF” should be defined in the footnotes. 



Reviewer #2 (Remarks to the Author): 

As I’m not a theoretician, I can’t judge the accuracy of the calculations and methods, nor the 
significance of the work in relation to the state-of-the-art in the field of computational chemistry. 
However, I can comment on the significance of these findings for TDA/RISC research. On the whole 
the paper is very interesting and shows that the prediction of reverse intersystem crossing rates 
should take into account higher-lying triplet excited states. Whilst the work may predict the kinetics 
of RISC, the paper does not lead to design rules which would have increased the impact of this work. 
As such, I do not believe that the paper has sufficient urgency to be published in Nature 
Communications, but it is a very good quality piece of work. If the advances in the theory are 
highlighted and commented on positively by specialist referees then these opinions should override 
my overall judgement on this paper. 
If published, the authors should take note of the following comments: 
The quality of English is not satisfactory and the paper needs thorough revision in this respect. 
On page 3, I believe that the authors mean triazines instead of cyclic amines. 
On page 6 the authors say that “…brominated molecules exhibit similar blue shifts in their broad, 
unstructured CT emissions compared to the corresponding non-brominated analogues…”. However, 
the data show that brominated analogues are red shifted in figure 3d. When the authors say that the 
bromine atoms have an electron withdrawing effect on the donor units, this will deepen the 
corresponding donor HOMO energy level and it should lead to the destabilisation of the CT state. 
Therefore, is the labelling in figure 3d correct? Also, for the broad readership of this journal, the 
term ‘destabilising’ could do with a better definition. In figure 3a, it is difficult to see which lines are 
which between brominated and non-brominated pairs. It would be better if the compounds show an 
arrow towards their respective PL decay curves. 
The experimental data provided for the new compounds are fine, but the melting points of the two 
new solid materials are missing. These should be added to the SI experimental section. 




Reviewer #3 (Remarks to the Author): 

In the present manuscript NCOMMS-20-17872-T, the authors present a kinetic model to describe the 
reverse intersystem crossing in organic emitter molecules that show thermally activated delayed 
fluorescence. In this model they include an intermediate, higher-lying triplet state, i.e. T2, that 
facilitates the actual spin flip. The model is tested on a variety of experimental results known from 
literature and further applied to newly synthesized molecules. Overall, the manuscript is very 
interesting for the field of organic light-emitting diode (OLED) material research and presents a 
methodology to include advanced computational schemes in material discovery. There are some 
points which I believe are important to be addressed before a recommendation can be given. My 
expertise as reviewer lies in the experimental section of organic electronics, so clearly I cannot judge 
with all the detail regarding the theoretical work. Please accept my apologies, if I ask stupid 
questions in this regard. 

1. The quality of the English needs to be improved in multiple parts of the manuscript. I would advise 
the authors to undertake a careful language revision. Here, I indicate some examples: 
- page 2, line 7: grammar mistake: 'Although new materials ...' 
- page 3, line 17: 'for excited-state density' - I am not sure, what this is supposed to mean 

2. The data from literature seems to agree very well with the simulated values of K_RISC. I would 
appreciate, if the authors can comment also in the text or SI, whether there are also materials that 
do not follow this agreement. If so, what are possible explanations. In addition, it would be good, if 
the authors can disclose their selection criteria for literature data. Are these materials randomly 
chosen with only requirement that all experimental data is available or do they represent some sort 
of subset of materials. 

3. On page 2, lines 4ff: The explanation of the approach used by the authors is not made clear 
enough. In which model (Fig. 1a or 1b) do the authors calculate crossing seams and with this k_RISC? 

4. Why are the parabola in Figure 1a crossing between S1 and T1, while in Fig 1b, the S1 and T1 
parabolae do not cross? 

5. On page 4, line 6, the authors discuss the crossing condition as the square energy difference (E_S-
E_T)^2=0. Doesn't this imply that also the simple difference need to be zero (i.e. E_S-E_T=0)? 
6. Page 5, line 24: the second value stated for k_RISC is missing its unit (s^-1). Please correct. 

7. Figure 3 the difference of dashed and solid lines is only hard to distinguish, in particular for the 
transient decays where the noise compromises the readibility. 

8. On page 8, the authors discuss the limit of RISC rates based on their model. Here the presentation 
seems inconsistent to me. In line 4 ff, the authors give the spin-orbit coupling in inverse cm, the 
activation energy (E_A) in eV. Typical values here are 7.7 cm^-1 and 0.1 eV respectively. In their 
simulation (results shown in Figure 5), the authors present the orders of magnitude of k_RISC as a 
color map on a plot where the E_A is the x-axis and the H_SO the y-axis. Here the y-axis spans from 
0.1 to 10 eV. If I convert the typical wavenumber given for H_SO in the text, this would translate to 
the range of meV. How does this compare in this plot (Summary: The absolute values text vs. figure 
for H_SO seem to be off substantially). Please check carefully. 

9. If I am not mistaken, the newly developed materials are not all included in Figure 2b, where the 
correlation of experimental and simulation results is shown. These results should be included and 
compared. Do they fit the overall trend?