Ultrafast Electron/Energy Transfer and Intersystem Crossing Mechanisms in BODIPY-Porphyrin Compounds

Round 1

Reviewer 1 Report

Introduction to the scientific interest for BODIPY and porphyrins is given.

The “statical” measurements are presented and the results seems congruent. The “dynamical - time dependent” interpretation seems,  to me, requiring a better explanation, especially concerning the numerical calculation of the energy transfer. Some crucial claims refer to supplementary material, maybe some plots (as S2) can be inserted in the paper.
  R17  Different -> two different 
  R64 The meaning is not clear (maybe “is important for...") R65 advantages -> advantage? R 68-R71 Not clear the number of compounds that are investigated , two. R 136 Some comment/reference about this attribution    R 188 - 190 To me it is not clear how the values of 22 ps and 3 ps are calculated. Moreover, the word “supports” indicate that the results are compatible with this interpretation but that it is a possible and not the only possible interpretation.     

Author Response

I would like to submit the revised manuscript to Processes. I am grateful to the reviewers for their valuable comments. I have modified the manuscript according to the reviewer’s comments. 

Our Response:

The “dynamical-time dependent” interpretation as well as the numerical calculation of the energy transfer is explained in the manuscript as given below:

“The intensity of the bleaching signal of the BODIPY singlet excited state is greatly diminished with time delay as a result of the rapid energy transfer from BODIPY based singlet excited state to singlet state of porphyrin. In an attempt to obtain the energy transfer rates, decay traces of the bleaching signals were fitted by using a multiexponential function and probing 524 nm wavelength. The intramolecular energy transfer rates were obtained as 22 ps for BDP-TTP and 3 ps for 3BDP-TTP at 524 nm probe wavelength, provided in Figure 5b and Figure 6b, respectively.”

The Figure S2 refers to transient absorption spectra of tetratoly porphyrin (4TTP) compound which is the reference compound in this work. The spectral feature of the 4TTP at 400 nm pump wavelength is compared with 3BDP-TTP compound to confirm that there is no reverse energy transfer from porphyrin  to BODIPY unit. Therefore, both Figure S2 and S3 were inserted in Supplementary Information.

 

R17 different-> is corrected as two different.

R64-65 is rewritten as “Increasing or decreasing the energy transfer rate are important for many applications and modifying this properties in organic molecules provide great advantages.”

R68-71 is rewritten as “We herein investigate the optical properties and the energy transfer mechanism of the two porphyrin based compounds with different numbers of BODIPY groups on the electronic energy levels of porphyrin ring.”

R136 The following references have been added, one of which is already in the reference list.

• Loudet, A.; Burgess, K. BODIPY dyes and their derivatives: Syntheses and spectroscopic properties. Chem. Rev. 2007, 107, 4891–4932, doi:10.1021/cr078381n
• Arbeloa, F. L.; Bañuelos, J.; Martínez, V.; Arbeloa, T.; Arbeloa, I. L. Int. Rev. Phys. Chem. 2005, 24(2), 339-374, doi: 10.1080/01442350500270551
• Qin, W.; Baruah, M.; Stefan, A.; Auweraer, M. V. D.; Boens, N. BODIPY-Based Hydroxyaryl Derivatives as Fluorescent pH Probes. J. Org. Chem. 2005, 70, 10, 4152–4157, doi:10.1002/cphc.200500341
• “Scorpion”-shaped mono(carboxy)porphyrin-(BODIPY)2, a novel triazine bridged triad: synthesis, characterization and dye sensiti-zed solar cell (DSSC) applications† DOI: 10.1039/C4TC02902J

R188-190 It is explained how the energy transfer rates are calculated and the results are interpreted by comparing the reference and 3BDP-TPP compounds with 400 nm pump wavelength.

Reviewer 2 Report

This paper describes the synthesis and characterisation (including ultrafast dynamics) of some BODIPY porphyrins.  The work is well done and shows that there is rapid energy transfer from BODIPY to porphyrin despite the lack of pi conjugation between the chromophores.  None of the results are surprised per se but the work is well done.  The paper may be published once the issues around language in it are addressed.

 

Please carefully check the language – just two examples of very clumsy sentnces fomr page 2.

which are disadvantages for effective gathering solar light. To enhance light absorp- 54

which are disadvantageous for the effective gathering of solar energy

and

“while porphyrin has 57

poor absorbing properties near the 500 nm wavelength and absorbs strongly in 420-450 58

nm region and moderately in 500-700 nm region.”

 

while most porphyrins do not absorb strongly at 500 nm

 

 

Author Response

The Reviewer is right. We corrected our explanations as given below:

“Since the porphyrins do not absorb strongly in the blue-green region (450-550 nm) of the spectrum, they have limited usage area in the solar cell applications. To enhance the light-harvesting capability and absorption features of porphyrins in that region, BODIPY chromophore with its strong absorbing properties can be attached to the porphyrin unit. BODIPY absorbs light strongly around 500 nm, while porphyrin absorbs strongly in the 420-450 nm region and weakly in the 500-700 nm region. Therefore, BODIPY and porphyrin have complementary light-absorbing features.”

Reviewer 3 Report

The authors have studied different numbers of BODIPY moiety boned to free-base porphyrin by using steady-state and transient absorption spectroscopy techniques. They demonstrated the energy transfer from BODIPY unit to porphyrin and found the rate of the energy transfer of 3BDP-TTP compound (about 3 ps) is faster than BDP-TTP compound (about 22 ps). However, these several questions should be addressed before this work can be accepted.
1. The title of the manuscript is “Ultrafast electron/energy transfer and intersystem crossing mechanisms in BODIPY-Porphyrin compounds”. However, authors mainly discussed the energy transfer from BODIPY unit to porphyrinin in manuscript. Authors should give a more detailed discussion of intersystem crossing mechanisms(ISC). 
2. The ultrafast pump probe experiments were also performed for tetratoly porphyrin compound (4BDP-TTP), and combined with 3BDP-TTP compound to confirm whether there is an energy transfer from porphyrin unit to BODIPY core or not. Why author do not choose 4BDP-TTP to further confirm the influence of the number of BODIPY units on energy transfer rate.
3.  I think, figure 8, the energy level diagrams for BDP-TTP and 3BDP-TTP compounds should be given separately with more dynamical information as much as possible, such as the wavelength and time scale of each process. 
4. The figures in manuscript need to be checked carefully. For example, “Figure 2. Emission spectra of BDP-TTP and 3BDP-TTP solutions in THF”, the labels in Fig. 2 may be wrong.  “Figure 4. The overlap of the linear absorption and emission spectra of (a) BDP-TTP and (b) 3BDP-TTP with 524 nm excitation wavelengths”, however, figure 4 was not found in the manuscript.
5. Page 7 line 197, “the absorption signal appears around 450 nm which is attributed to the triplet-triplet transition of the porphyrin unit”. The signal around 450nm is the triplet-triplet transition in porphyrin unit or triplet energy transfer between BODIPY(T1) and Porphyrin(T1) ? 

Author Response

Dear Editor,

We would like to submit the revised manuscript to Processes. We are grateful to the reviewers for their valuable comments. We have modified the manuscript according to the reviewer’s comments. All our responses are highlighted as blue in this letter.

 

Title: Ultrafast energy transfer and intersystem crossing mechanisms in BODIPY-Porphyrin compounds

Reviewer#3:

Comments and Suggestions for Authors

 

The authors have studied different numbers of BODIPY moiety boned to free-base porphyrin by using steady-state and transient absorption spectroscopy techniques. They demonstrated the energy transfer from BODIPY unit to porphyrin and found the rate of the energy transfer of 3BDP-TTP compound (about 3 ps) is faster than BDP-TTP compound (about 22 ps). However, these several questions should be addressed before this work can be accepted.

 

1. The title of the manuscript is “Ultrafast electron/energy transfer and intersystem crossing mechanisms in BODIPY-Porphyrin compounds”. However, authors mainly discussed the energy transfer from BODIPY unit to porphyrinin in manuscript. Authors should give a more detailed discussion of intersystem crossing mechanisms(ISC).

Our Response:

1. The Reviewer is right. The title of the manuscript revised as “Ultrafast energy transfer and intersystem crossing mechanisms in BODIPY-Porphyrin compounds” and more information is given about ISC in results section.

 

2. The ultrafast pump probe experiments were also performed for tetratoly porphyrin compound (4BDP-TTP), and combined with 3BDP-TTP compound to confirm whether there is an energy transfer from porphyrin unit to BODIPY core or not. Why author do not choose 4BDP-TTP to further confirm the influence of the number of BODIPY units on energy transfer rate.

Our Response:

2. Tetratoly porphyrin compound does not include BODIPY unit.  It is the reference compound and we labelled the compound as 4TTP as shown in Figure below. (see the figure in the attached file)

In the synthesis of bodipy-porphyrin compounds, sensitizers that thought to be two and four bodipy substituted porphyrin derivatives were also obtained in trace amounts (verified by thin layer chromatography). And the fact remains that, these compounds have been observed to degrade in purification step when applying column chromatography in a short time. The identical results were observed in both acidic silica and the column filled with neutral alumina. Since laser-based photophysical studies of these compounds with low stability can cause misinterpretations and scientific evidences can be speculative. Therefore, these compounds have not been studied. However, when tetra tolyl porphyrin, bodipy-porphyrin and 3 bodipy-porphyrin compounds were evaluated comparatively, it was concluded that there were intramolecular energy transfers and intermolecular energy transfer rates were directly proportional to the number of substituents.

 

 

 

3. I think, figure 8, the energy level diagrams for BDP-TTP and 3BDP-TTP compounds should be given separately with more dynamical information as much as possible, such as the wavelength and time scale of each process.

Our Response:

3. The Figure 8 is revised with more dynamical information as seen below.

(see the figure in the attached file)

 

4. The figures in manuscript need to be checked carefully. For example, “Figure 2. Emission spectra of BDP-TTP and 3BDP-TTP solutions in THF”, the labels in Fig. 2 may be wrong. “Figure 4. The overlap of the linear absorption and emission spectra of (a) BDP-TTP and (b) 3BDP-TTP with 524 nm excitation wavelengths”, however, figure 4 was not found in the manuscript.

Our Response:

4. The Reviewer is right. The labels in Figure 2 are corrected. The Figure 4 is also presented in the manuscript.
5. Page 7 line 197, “the absorption signal appears around 450 nm which is attributed to the triplet-triplet transition of the porphyrin unit”. The signal around 450nm is the triplet-triplet transition in porphyrin unit or triplet energy transfer between BODIPY(T1) and Porphyrin(T1) ?

Our Response:

5. In the ultrafast pump-probe experiments, upon photoexcitation of BODIPY-porphyrin compounds the energy transfer from BODIPY to porphyrin is observed via FRET mechanism. Next, the excited state electron transfer to triplet state of porphyrin by intersystem crossing mechanism. The porphyrin-based triplet excited states are lower than the corresponding BODIPY-based triplet excited state, so “back transfer” from the porphyrin to the BODIPY triplet is not possible in these cases. Thus, the porphyrin based triplet excited states can be readily generated by initial excitation of the BODIPY part of the dyads[1]. Therefore, we consider that the triplet-triplet transition is more efficient in porphyrin unit than that of BODIPY and the ESA around 450 nm was ascribed to triplet transitions of Porphyrin unit.

We added this explanation into manuscript as follows:

“In the ultrafast pump-probe experiments, upon photoexcitation of BODIPY-porphyrin compounds the energy transfer from BODIPY to porphyrin is observed via FRET mechanism [21]. Next, the excited state electron transfer to triplet state of porphyrin by intersystem crossing mechanism. The porphyrin-based triplet excited states are lower than the corresponding BODIPY-based triplet excited state, so “back transfer” from the porphyrin to the BODIPY triplet is not possible in these cases. Thus, the porphyrin based triplet excited states can be readily generated by initial excitation of the BODIPY part of the dyads1. Therefore, we consider that the triplet-triplet transition is more efficient in porphyrin unit than that of BODIPY and the ESA around 450 nm was ascribed to triplet transitions of Porphyrin unit.”

 

 

[1] dx.doi.org/10.1021/ic201052k | Inorg. Chem. 2011, 50, 8926–8936

Author Response File: Author Response.docx

Round 2

Reviewer 3 Report

The authors made sufficient modification. I believe this manuscript could be accepted for publications.