Theoretical Modeling of BODIPY-Helicene Circularly Polarized Luminescence

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Bella and coworkers present a computational work on the modelling of a particular helicene-BODIPY molecule where they examined the performance of different DFT levels in the prediction of an already experimentally reported CPL spectra.

The authors use a simple approach where the rotational strengths are computed for the DFT optimized S1 geometries. However I am not fully  convinced that this approach would be well suited here. I think that the sp2-sp2 bond between the BODIPY and helicene moieties may present a very flat potential curve and therefore large excursions from the potential minimum can be expected. I think that this curve should be computed for S0 and S1 geometries and consider whether a static approach makes sense for this kind of molecule. If not, perhaps a simple boltzmann averaging of rotational strengths over this coordinate could provide a better prediction.

The authors finally chose the TPSSTPSS functional as the best one but taking into account how noisy the experimental spectra is I think that the most one could say is that M11L/M06L/TPSSTPSS/HCTH give results compatible with the experimental observed spectra. I am rather surprised by the, in principle, bad performance of CAM-B3LYP.  Also the reported intensities are much higher than the predicted ones. Could there be any scale factor that the authors did not properly consider ?

In summary, this manuscript could be of interest to organic chemists working on chiral luminescence but I think that the authors need to show that a static approach is, at least moderately suited to this particular problem and therefore I need to request a major revision.

Comments on the Quality of English Language

The manuscript would have benefited from employing a simpler language style which would have made the manuscript more readable and even considerably shorter. I must recommend a style revision of the whole manuscript; 

Author Response

We thank the editor and the reviewers for the comments which hopefully will improve the quality of the paper. We have carefully addressed all comments and thoroughly revised the manuscript accordingly. Changes in the revised manuscript are highlighted in yellow, and are listed below as relative answer referred to the posted observations.

 

Reviewer 1:

The authors use a simple approach where the rotational strengths are computed for the DFT optimized S1 geometries. However I am not fully convinced that this approach would be well suited here. I think that the sp2-sp2 bond between the BODIPY and helicene moieties may present a very flat potential curve and therefore large excursions from the potential minimum can be expected. I think that this curve should be computed for S0 and S1 geometries and consider whether a static approach makes sense for this kind of molecule. If not, perhaps a simple boltzmann averaging of rotational strengths over this coordinate could provide a better prediction. Thanks for the comment, in the last decade the computational approach that we adopted was successfully standardized for small organic molecules^{1, 2}, BODIPY cores³ and helicene scaffold^{4, 5}. However, stimulated by your intriguing question we performed an ab-initio excited molecular dynamics in the first singlet excited state to explore the sp2-sp2 BODIPY/helicene bond. In Figure 1, BODIPY-helicene distance showed regular fluctuations, reporting an average value moderately larger than the S0 as a consequence of the photoexcitation.  

Fig. 1. BODIPY-helicene distance evolution plot of excited state trajectories (S1) computed at M06-2X/def2-TZVP level (Born-Oppenheimer approach) over a picosecond scale at 298K. < > refer to average values.

To verify possible discrepances from the static methodology, CPL spectrum were then recreated as a weighted sum of the spectra calculated at the TD-DFT level (TPSSTPSS/6-311G(d,p)) on temporally equispaced (40 fs) molecular conformation snapshots (26) extrapolated from excited MD trajectories. Figure 2 clearly communicated that marginal differences in the peak position and chiral intensity (see Rlength value) were recorded, confirming that static and dynamic approaches exhibit (approximately) the same CPL results.

Fig. 2. Cumulation of TD-DFT calculated CPL spectra of M enantiomer at the TPSSTPSS/6-311G(d,p) level by means static (yellow profile) and excited MD approach (blue). < > refers to average values of rotational strengths during S1 molecular dynamics.

 

The authors finally chose the TPSSTPSS functional as the best one but taking into account how noisy the experimental spectra is I think that the most one could say is that M11L/M06L/TPSSTPSS/HCTH give results compatible with the experimental observed spectra. I am rather surprised by the, in principle, bad performance of CAM-B3LYP.  Also the reported intensities are much higher than the predicted ones. Could there be any scale factor that the authors did not properly consider? Thanks for the question, considering the fragmented (experimental) spectral profile, all along the manuscript the functionals performances were ranked according to the experimental maximum (608 nm) and as you correctly suggested, M11L (590 nm) and M06L (592 nm) functionals represent valid alternatives. Poor outcomes from CAM-B3LYP are due to its very low rotational strengths (-1.6722 in cgs unit) as an effect of an almost orthogonal disposition of transition electric and magnetic dipole moments (90.27 °). Computationally speaking, ΔI is probably the most complex factor to predict when CPL spectra are simulated. Equation (1) in the manuscript is expressed in molecular quantities, (i.e. they furnish CPL for each emitting molecule) while experimentally, besides the instrumental settings, the CPL magnitude depends on the excited state population, a number extremely difficult to estimate. For these reasons, ΔI comparison between theoretical and experimental CPL spectra can only be semi-quantitative. 

 

The manuscript would have benefited from employing a simpler language style which would have made the manuscript more readable and even considerably shorter. I must recommend a style revision of the whole manuscript; We appreciate your advice but, although the authors are not native speakers they tried to use a rigorous and scientifically correct English style with the aim of emphasizing the key points of the current reasearch. From our point of view, every manuscript section is intelligible (even for non-expert in the computational field) without being verbose.

 

 

1. M. Pecul and K. Ruud, Phys. Chem. Chem. Phys., 2011, 13, 643-650.
2. G. Longhi, E. Castiglioni, S. Abbate, F. Lebon and D. A. Lightner, Chirality, 2013, 25, 589-599.
3. F. Zinna, T. Bruhn, C. A. Guido, J. Ahrens, M. Bröring, L. Di Bari and G. Pescitelli, Chem. Eur. J., 2016, 22, 16089-16098.
4. S. Abbate, G. Longhi, F. Lebon, E. Castiglioni, S. Superchi, L. Pisani, F. Fontana, F. Torricelli, T. Caronna, C. Villani, R. Sabia, M. Tommasini, A. Lucotti, D. Mendola, A. Mele and D. A. Lightner, J. Phys. Chem. C, 2014, 118, 1682-1695.
5. G. Longhi, E. Castiglioni, C. Villani, R. Sabia, S. Menichetti, C. Viglianisi, F. Devlin and S. Abbate, J. Photochem. Photobiol. A, 2016, 331, 138-145.

 

 

 

Reviewer 2 Report

Comments and Suggestions for Authors

In this work, Bella et al. performed a detailed DFT and TD-DFT study on a chiral luminophore that integrates a BODIPY core with a [6]helicene moiety to investigate the circularly polarized luminescence (CPL). They conducted a benchmark study to explore different  functionals and basis sets, identifying TPSSTPSS/6-311G(d,p) as the optimal combination for reproducing the experimental CPL spectra in terms of band position, intensity, and chiral sign. This study demonstrates the effectiveness of TD-DFT in describing chiral excited states and provides valuable theoretical guidance for the design of CPL-active molecular emitters. Generally, this work meets the scope and criteria of Organics Journal. Some revisions should be before acceptance.

 

1. The computational studies should be performed in dichloromethane solvation model to enable a more meaningful comparison with the experimental CPL spectra. Performing a brief PCM or SMD benchmark test would significantly strengthen the reliability of the conclusions.
2. The authors are suggested to calculate and report the luminescence dissymmetry factor to quantitatively describe the chiroptical properties.
3. It would be beneficial to calculate the RMSD between the ground and excited-state geometries to quantitatively assess structural relaxation upon excitation. In addition, presenting key excited-state parameters—such as electric transition dipole moments, magnetic transition moments (S0 to S1), oscillator strengths, and rotational strengths—in the manuscript would add depth and clarity to the discussion.
4. The authors claimed that the CPL mechanism involves an intramolecular charge transfer (HOMO → LUMO), which should be combined with additional analyses, such as charge-difference density plots or natural transition orbital (NTO) analysis.  

Author Response

We thank the editor and the reviewers for the comments which hopefully will improve the quality of the paper. We have carefully addressed all comments and thoroughly revised the manuscript accordingly. Changes in the revised manuscript are highlighted in yellow, and are listed below as relative answer referred to the posted observations.

 

Reviewer 2:

(I) The computational studies should be performed in dichloromethane solvation model to enable a more meaningful comparison with the experimental CPL spectra. Performing a brief PCM or SMD benchmark test would significantly strengthen the reliability of the conclusions. Thanks for the comment, as you correctly suggested we integrated a solvent treatment in our leader method (TPSSTPSS/6-311G(d,p)). The solvent conditions (state-specific vertical excitation model, VEM) were defined by using the integral equation formalism for the polarizable continuum model (IEF-PCM); the default parameters of Gaussian16 were utilized for the construction of the cavity, built as the envelope of interlocked spheres centered on each atom of the solute (dichloromethane: ε = 8.93). From Figure 1 it instantly emerges that solvent condition produced a slight blue-shift with a modest intensity enlargment.

 

Fig. 1. Cumulation of TD-DFT calculated CPL spectra of M enantiomer at the TPSSTPSS/6-311G(d,p) level by means vacuo (yellow profile) and solvent environment (blue).

 

(II) The authors are suggested to calculate and report the luminescence dissymmetry factor to quantitatively describe the chiroptical properties. (III) It would be beneficial to calculate the RMSD between the ground and excited-state geometries to quantitatively assess structural relaxation upon excitation. In addition, presenting key excited-state parameters—such as electric transition dipole moments, magnetic transition moments (S0 to S1), oscillator strengths, and rotational strengths—in the manuscript would add depth and clarity to the discussion. Accordingly with your suggestion, an extra table reporting g_lum, electric and magnetic transition dipole moments, their angle, oscillator strengths, and rotational strengths was inserted in supplementary materials (Tab. S6). In Figure 3 (manuscript) RMSD value between S0 and S1 was reported.

(IV) The authors claimed that the CPL mechanism involves an intramolecular charge transfer (HOMO → LUMO), which should be combined with additional analyses, such as charge-difference density plots or natural transition orbital (NTO) analysis.  Coherently with your comment we corroborated the intramolecular charge transfer mechanism by including natural transition orbital analysis and hole/electron functions distribution which clarified how intramolecular charge transfer process moves from helicene scaffold to the BODIPY core (Figure 2).

Fig. 2 (A) NTO orbitals directly involved in the intramolecular charge transfer. (B) Hole (blue) and electron (green) real space functions computed at M06-2X/def2-TZVP level in gas phase.

Reviewer 3 Report

Comments and Suggestions for Authors

The manuscript by Bella et al. proposes and tests a DFT/TDDFT workflow for predicting the circularly polarized luminescence of a BODIPY–[6]helicene, comparing computed spectra with experiment and correctly recovering the band position and enantiomeric sign. By systematically evaluating modeling choices, the authors identify a practical protocol and show mirror-image CPL for both enantiomers while offering a clear qualitative picture of the emitting state. The topic is well-organized, and could provide a useful template for CPL modeling of related chromophores. However, one technical clarification is needed before the theoretical conclusions can be fully assessed.

Major comments:
1.    All calculations and screening are in vacuo, whereas the experimental CPL was measured in dichloromethane. The good wavelength agreement may reflect fortuitous error cancellation, as charge-transfer states can shift substantially in polarizable media. Please comment on how including solvent effects might influence the optimal-protocol conclusion, and consider adding implicit-solvent TDDFT (e.g., for the final TPSSTPSS spectra) for comparison.

Minor comments:
1.    Typo in Figure 3 caption: “BODIPY-[6]helicence” → “helicene.”
2.    Typo in Supplementary Material: “Figure S1. HOMO […]” → ”Figure S2. HOMO […]”

Author Response

We thank the editor and the reviewers for the comments which hopefully will improve the quality of the paper. We have carefully addressed all comments and thoroughly revised the manuscript accordingly. Changes in the revised manuscript are highlighted in yellow, and are listed below as relative answer referred to the posted observations.

 

Reviewer 3:

(I) All calculations and screening are in vacuo, whereas the experimental CPL was measured in dichloromethane. The good wavelength agreement may reflect fortuitous error cancellation, as charge-transfer states can shift substantially in polarizable media. Please comment on how including solvent effects might influence the optimal-protocol conclusion, and consider adding implicit-solvent TDDFT (e.g., for the final TPSSTPSS spectra) for comparison.

Thanks for the advice, as you correctly suggest we integrated a solvent treatment in our leader method (TPSSTPSS/6-311G(d,p)). The solvent conditions (state-specific vertical excitation model, VEM) were defined by using the integral equation formalism for the polarizable continuum model (IEF-PCM); the default parameters of Gaussian16 were utilized for the construction of the cavity, built as the envelope of interlocked spheres centered on each atom of the solute (dichloromethane: ε = 8.93). From Figure 1 it instantly emerges that solvent condition produced a slight blue-shift with a modest intensity enlargment.

 

Fig. 1. Cumulation of TD-DFT calculated CPL spectra of M enantiomer at the TPSSTPSS/6-311G(d,p) level by means vacuo (yellow profile) and solvent environment (blue).

 

(II) Typo in Figure 3 caption: “BODIPY-[6]helicence” → “helicene.” Thanks for your observation, the typo was fixed.

(III) Typo in Supplementary Material: “Figure S1. HOMO […]” → ”Figure S2. HOMO […]” Thanks for your observation, the typo was fixed.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors have carefully addressed my main concerns about the influence of molecular flexibility in the predicted chiroptical responses.  I think that those new results are very interesting in their own and could be worth to include them in the Supporting Information. Otherwise the main manuscript can be published in its present form.

Comments on the Quality of English Language

The manuscript would have benefited from employing a simpler language style which would have made the manuscript more readable and even considerably shorter. I must recommend a style revision of the whole manuscript; 

Author Response

Thanks for the comment, as required the new outcomes were inserted in the supplementary materials (Figures S4,S5)

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have made sufficient modifications. This manuscript can be accepted now.

Author Response

We thank the reviewer for the comment.

Reviewer 3 Report

Comments and Suggestions for Authors

N/A

Author Response

We thank the reviewer