Aggregation-Induced Intermolecular Charge Transfer Emission for Solution-Processable Bipolar Host Material via Adjusting the Length of Alkyl Chain
Round 1
Reviewer 1 Report
The authors investigated three novel compounds with donor-spacer-acceptor structure, as host materials for phosphorescent dopant for the solution-processable OLEDs, which are quite interesting topic. From reading the article, I got a good impression: the need for research in the introduction is well and thoroughly justified, the materials are well equipped with experimental and theoretical studies, and the presented results are well described and illustrated. I suggest some comments in order to improve the manuscript before the publication.
Line 10: configuration have instead of “has”
Line 37: occurs instead of “occur”
Line 90: Please add dot “.”
Line 97: “N2 atmosphere. 1H NMR and 13C NMR.” The numbers should be as a subscript.
Line 110: “N2 atmosphere.” The number should be as a subscript.
Line 113: “CH2Cl2 solution (10-3 M).” The number should be as a subscript.
Line 114: “Bu4NPF6 as electrolyte”. The number should be as a subscript.
Line 173: “cm-2 and an active pattern size of 2 Χ 2 mm2.” The number should be as a subscript in cm and mm.
Line 177: “Cs2CO3”. The number should be as a subscript.
Lines 204-208: How realistic is the presentation of the RMS values derived by AFM measurements using three decimal digits? What is the error in these calculated values? Generally, the RMS values have low variations between the three materials. I would suggest that the authors should rely on the AFM results to conclude that all films possess good film-forming properties and not for a comparative evaluation between them.
Line 223: Please delete the space.
Line 241: solid-state film instead of “solid film”.
Line 240: No information is given about the excitation wavelength for the PL measurements, please specify.
Lines 246-248: Please specify the wavelengths of the “former” and “latter” peaks
Line 249: It's been reported in the literature that mCP and TRZ molecules could form intermolecular CT-type exciplex. Please cite the appropriate references.
Line 281: “Figure 5a-5c demonstrate that, at a low dopant concentration of 1 wt%, three PL spectra show similar LE-type emission from mCP.” Figures 5a-5c demonstrate PL spectra of solid-state films with different concentrations of PMMA. According to the Figure 3(b), the PL characteristics both of mCP and TRZ located in the region between 320-500nm. So, it is suggested that three PL spectra show similar LE-type emission from mCP or TRZ. Furthermore, the remarkable differentiations in the relative intensities of the characteristic peaks in the wavelength range of 330-375nm, obtained for the first three concentrations (1, 10 and 20 wt%) between the mCP-3C-TRZ, mCP-6C-TRZ and mCP-9C-TRZ doped in PMMA films, should be discussed, and justified.
Lines 303-315: Can the authors explain the origins of the significant and no monotonic variations in the intensities and wavelengths of the peaks 420 – 450nm presented in Figure 5d-5f either for the different mixed solvent THF/H2O ratios as well as between the mCP-3C-TRZ, mCP-6C-TRZ and mCP-9C-TRZ materials?
Line 365: Please delete the space.
Line 375: For the PL measurements, it is also crucial factor in this case to refer the excitation wavelength, as it is evidence for the efficient energy transfer from the host to the guest.
Line 375: “The PL emission of doped film in Figure 6a only exhibits a characteristic peak of Ir(mppy)3, showing an efficient energy transfer process from bipolar host to the phosphorescent guest.” You can add the “as the PL emission profile of host materials is quenched.” to be clearer that the efficient energy transfer takes place in PL emission.
Line 388: Please clarify how the thickness of spin-coated thin films of PEDOT: PSS and emissive layer are calculated in OLED devices?
Line 396: In Figure 7c, one can see that when the length of alkyl chain is increased the FHWM is broader and redshifts, and it is suggested that this fact is very interesting to mention in the manuscript.
Line 405: As I mention above that the FHWM is broader and redshifts, when the length of alkyl chain is increased. Maybe, someone expect the EL CIE Coordinates could also exhibit differences.
Author Response
Reviewer 1
The authors investigated three novel compounds with donor-spacer-acceptor structure, as host materials for phosphorescent dopant for the solution-processable OLEDs, which are quite interesting topic. From reading the article, I got a good impression: the need for research in the introduction is well and thoroughly justified, the materials are well equipped with experimental and theoretical studies, and the presented results are well described and illustrated. I suggest some comments in order to improve the manuscript before the publication.
Line 10: configuration have instead of “has”
Reply: Many thanks for your valuable suggestion. We have corrected the mistake in the revised manuscript.
Line 37: occurs instead of “occur”
Reply: Many thanks for your valuable suggestion. We have corrected the mistake in the revised manuscript.
Line 90: Please add dot “.”
Reply: Many thanks for your valuable suggestion. We have corrected the mistake in the revised manuscript.
Line 97: “N2 atmosphere. 1H NMR and 13C NMR.” The numbers should be as a subscript.
Reply: Many thanks for your valuable suggestion. We have corrected the mistake in the revised manuscript.
Line 110: “N2 atmosphere.” The number should be as a subscript.
Reply: Many thanks for your valuable suggestion. We have corrected the mistake in the revised manuscript.
Line 113: “CH2Cl2 solution (10-3 M).” The number should be as a subscript.
Reply: Many thanks for your valuable suggestion. We have corrected the mistake in the revised manuscript.
Line 114: “Bu4NPF6 as electrolyte”. The number should be as a subscript.
Reply: Many thanks for your valuable suggestion. We have corrected the mistake in the revised manuscript.
Line 173: “cm-2 and an active pattern size of 2 Χ 2 mm2.” The number should be as a subscript in cm and mm.
Reply: Many thanks for your valuable suggestion. We have corrected the mistake in the revised manuscript.
Line 177: “Cs2CO3”. The number should be as a subscript.
Reply: Many thanks for your valuable suggestion. We have corrected the mistake in the revised manuscript.
Lines 204-208: How realistic is the presentation of the RMS values derived by AFM measurements using three decimal digits? What is the error in these calculated values? Generally, the RMS values have low variations between the three materials. I would suggest that the authors should rely on the AFM results to conclude that all films possess good film-forming properties and not for a comparative evaluation between them.
Reply: Many thanks for your valuable suggestion. We obtained the RMS values using three decimal digits, which is directly derived by AFM measurements. The error in these calculated values could be ±0.001 nm. As you pointed out, the three materials have low variations of the RMS values, suggesting that all films possess good film-forming properties. The shorter the flexible chain, the larger the RMS value, which reveals a coarser film. This phenomenon happens that the flexible chain is beneficial to improve the film formation.
Line 223: Please delete the space.
Reply: Many thanks for your valuable suggestion. We have corrected the mistake in the revised manuscript.
Line 241: solid-state film instead of “solid film”.
Reply: Many thanks for your valuable suggestion. We have corrected the mistake in the revised manuscript.
Line 240: No information is given about the excitation wavelength for the PL measurements, please specify.
Reply: Many thanks for your valuable suggestion. We have added the excitation wavelength of 300 nm in Chemicals and Measurements section of the revised manuscript.
Lines 246-248: Please specify the wavelengths of the “former” and “latter” peaks
Reply: Many thanks for your valuable suggestion. We have specified the wavelengths of the “former” and “latter” peaks, which are 360 nm and 410 nm, respectively, in the revised manuscript.
Line 249: It's been reported in the literature that mCP and TRZ molecules could form intermolecular CT-type exciplex. Please cite the appropriate references.
Reply: Many thanks for your valuable suggestion. We have added the references (Dyes Pigments 2020, 175, 108188; Org. Electron. 2021, 99, 106309) that mCP and TRZ molecules could form intermolecular CT-type exciplex in the revised manuscript.
Line 281: “Figure 5a-5c demonstrate that, at a low dopant concentration of 1 wt%, three PL spectra show similar LE-type emission from mCP.” Figures 5a-5c demonstrate PL spectra of solid-state films with different concentrations of PMMA. According to the Figure 3(b), the PL characteristics both of mCP and TRZ located in the region between 320-500 nm. So, it is suggested that three PL spectra show similar LE-type emission from mCP or TRZ. Furthermore, the remarkable differentiations in the relative intensities of the characteristic peaks in the wavelength range of 330-375 nm, obtained for the first three concentrations (1, 10 and 20 wt%) between the mCP-3C-TRZ, mCP-6C-TRZ and mCP-9C-TRZ doped in PMMA films, should be discussed, and justified.
Reply: Many thanks for your valuable suggestion. We have corrected our description that “Figure 5a-5c demonstrate that, at a low dopant concentration of 1 wt%, three PL spectra show similar LE-type emission from mCP or TRZ.” Besides, when the doped concentrations are 1, 10 and 20 wt%, the emission peaks in the wavelength range of 330-375 nm gradually weakens due to the formation of intermolecular charge transfer, but there is no obvious regularity when compared with three molecules in Figure 5a-5c. In addition, we couldn’t get the crystal data of these molecules to further analyze the internal theory. Thank you again for your question. We will focus on the cultivation of single crystals in the further research.
Lines 303-315: Can the authors explain the origins of the significant and no monotonic variations in the intensities and wavelengths of the peaks 420-450 nm presented in Figure 5d-5f either for the different mixed solvent THF/H2O ratios as well as between the mCP-3C-TRZ, mCP-6C-TRZ and mCP-9C-TRZ materials?
Reply: Many thanks for your valuable suggestion. When the water ratio is low, intermolecular charge transfer is not obvious enough, so the intensities and wavelengths of the peaks 420-450 nm presented in Figure 5d-5f is not much changed. With high water ratio of 70-99%, nanoparticles are formed and intermolecular charge transfer is strong and obvious, which reflects enhanced intensities of the peaks 420-450 nm. Besides, in view of the nature of charge transfer, the wavelengths of the peaks 420-450 nm obviously fluctuate because the proportion of water causes the polarity of the mixed solvent to change.
Line 365: Please delete the space.
Reply: Many thanks for your valuable suggestion. We have corrected the mistake in the revised manuscript.
Line 375: For the PL measurements, it is also crucial factor in this case to refer the excitation wavelength, as it is evidence for the efficient energy transfer from the host to the guest.
Reply: Thank you very much for your suggestion. We have added the excitation wavelength of 380 nm in Line 375 in the revised manuscript.
Line 375: “The PL emission of doped film in Figure 6a only exhibits a characteristic peak of Ir(mppy)3, showing an efficient energy transfer process from bipolar host to the phosphorescent guest.” You can add the “as the PL emission profile of host materials is quenched.” to be clearer that the efficient energy transfer takes place in PL emission.
Reply: Thank you very much for your suggestion. We have added the “as the PL emission profile of host materials is quenched.” to confirm that the efficient energy transfer takes place in PL emission in the revised manuscript.
Line 388: Please clarify how the thickness of spin-coated thin films of PEDOT: PSS and emissive layer are calculated in OLED devices?
Reply: Thank you for your suggestion. The thickness of spin-coated thin films of PEDOT: PSS and emissive layer are calculated to be 40 nm and 40 nm, respectively, which have been calibrated by AFM.
Line 396: In Figure 7c, one can see that when the length of alkyl chain is increased the FHWM is broader and redshifts, and it is suggested that this fact is very interesting to mention in the manuscript.
Reply: Thank you for your suggestion. The length of alkyl chain of these host materials affects the carrier mobility and balance, which has confirmed by Figure S12. The carrier transport and balance could influence exciton recombination region, and microcavity effect (J. Mater. Chem. 2010, 20, 6131-6137), therefore, which could affect FHWM and redshift of EL spectra.
Line 405: As I mention above that the FHWM is broader and redshifts, when the length of alkyl chain is increased. Maybe, someone expect the EL CIE Coordinates could also exhibit differences.
Reply: Thank you for your suggestion. According to your question, we have checked the EL data including CIE Coordinates. We found the data are accurate and unmistakable. The slight difference of FHWM hardly influence the CIE Coordinates because CIE Coordinates are calculated using two decimal digits.
Author Response File: 
Author Response.docx
Reviewer 2 Report
This manuscript reports the synthesis, characterization and photophysical properties of donor/acceptor diads with different alkyl linkers. Biscarbazolyl phenyl and triphenyl triazine were used as the donor and acceptor, respectively. The length of alkyl chain hardly affected their photophysical properties in both solutions and solids. On the other hand, grass-transition temperature, and carrier transport properties were dependent on it. In particular, the shorter alkyl length resulted in better-balanced carrier mobility. As a result, the diad with the shortest alkyl chain worked as the best host material for the phosphorescent organic light-emitting diode. I think these observations are important for the audience of Molecules. However, there are some minor problems. Therefore, I recommend this manuscript would be published after addressing the following points:
1. The way of illustration of the HOMO and LUMO distributions in Figure 2 is not appropriate. HOMO and LUMO distributions must be visualized in separate figures rather than the difference in colors. The phase of LUMOs must be illustrated in different colors as well as HOMOs.
2. Why does the shorter spacer lead to the balanced charge transport property? Do you have any information on the packing structures of your molecules?
3. The potential of the redox couple Fc+/Fc relative to the vacuum must be stated.
4. The description for (a–f) of Figure 4 must be in its caption.
Author Response
Reviewer 2
This manuscript reports the synthesis, characterization and photophysical properties of donor/acceptor diads with different alkyl linkers. Biscarbazolyl phenyl and triphenyl triazine were used as the donor and acceptor, respectively. The length of alkyl chain hardly affected their photophysical properties in both solutions and solids. On the other hand, grass-transition temperature, and carrier transport properties were dependent on it. In particular, the shorter alkyl length resulted in better-balanced carrier mobility. As a result, the diad with the shortest alkyl chain worked as the best host material for the phosphorescent organic light-emitting diode. I think these observations are important for the audience of Molecules. However, there are some minor problems. Therefore, I recommend this manuscript would be published after addressing the following points:
- The way of illustration of the HOMO and LUMO distributions in Figure 2 is not appropriate. HOMO and LUMO distributions must be visualized in separate figures rather than the difference in colors. The phase of LUMOs must be illustrated in different colors as well as HOMOs.
Reply: Thank you very much for your suggestion. We have adjusted the illustration of the HOMO and LUMO distributions in Figure 2 in the revised manuscript.
Figure 2. HOMO, LUMO distributions and 3D structure of the optimized ground state calculated by B3LYP/6-31G(d) level for mCP-3C-TRZ, mCP-6C-TRZ and mCP-9C-TRZ.
- Why does the shorter spacer lead to the balanced charge transport property? Do you have any information on the packing structures of your molecules?
Reply: Thank you for your comments. For organic semiconducting materials applied to organic field-effect transistor or organic photovoltaics, the alkyl chain length could influence the hole mobility, the interchain π-π stacking interaction of conjugated molecules (Nature 1999, 401, 685-688; Synth. Met. 2005, 148, 169-173; Macromolecules 2011, 44, 6649-6652; Adv. Mater. 2012, 24, 4618-4622). Besides, the longer alkyl chains could induce increased degree of phase separation due to the morphological changes (Adv. Funct. Mater. 2009, 19, 3300-3306). Meanwhile, given that alkyl chains feature non-conjugated linker, acting as the role of insulators, the charge transport and balance are influenced. Therefore, in this article, the shorter the alkyl chain, the closer the π groups, which is beneficial to carrier transfer and balance. It is good agreement with the carrier-only devices in Figure S12.
- The potential of the redox couple Fc+/Fc relative to the vacuum must be stated.
Reply: Thank you for your valuable suggestion. According to the reference (J. Appl. Phys. 2001, 89, 5442), EHOMO (ferrocene)=4.8 eV. We have added the value in Table 1, and the reference has been cited in the revised manuscript.
- The description for (a–f) of Figure 4 must be in its caption.
Reply: Thank you very much for your suggestion. We have added the description for (a–f) of Figure 4 in its caption in the revised manuscript.
Author Response File: Author Response.docx
Reviewer 3 Report
This work reports solution processable bipolar host materials with adjusting the length of alkyl chain, showing intermolecular exciplex (charge transfer) properties and tunable electro-optical properties. They are also successfully used as host for phosphorescent OLEDs, giving decent EL properties. The work is comprehensive and clearly presented. It can be recommended for publication after further improving English weiting.
Author Response
Reviewer 3
This work reports solution processable bipolar host materials with adjusting the length of alkyl chain, showing intermolecular exciplex (charge transfer) properties and tunable electro-optical properties. They are also successfully used as host for phosphorescent OLEDs, giving decent EL properties. The work is comprehensive and clearly presented. It can be recommended for publication after further improving English writing.
Reply: Thank you very much for your recognition of our work. We have improved English writing, which hopes to meet your requirements.
Author Response File: Author Response.docx
