Comment pubs.acs.org/JPCC
Comment on “Calculating Optical Absorption Spectra of Thin Polycrystalline Organic Films: Structural Disorder and SiteDependent van der Waals Interaction” Roman Forker* and Torsten Fritz Friedrich-Schiller-Universität Jena, Institut für Festkörperphysik, Helmholtzweg 5, 07743 Jena, Germany
J. Phys. Chem. C 2015, 119 (10), 5747−5751. DOI: 10.1021/acs.jpcc.5b01587 J. Phys. Chem. C 2015, 119. DOI: 10.1021/acs.jpcc.5b05536
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structure of PTCDI as published, e.g., by Tojo and Mizuguchi3 and by Guillermet et al.4 Further, they fail to relate their structural data to comprehensive reports on the identical system PTCDI/KBr(100) investigated also with atomic force microscopy (AFM), but with much higher resolution.5 There, remarkably, disordered phases were not observed. Besides indepth descriptions of the dewetting and the growth behavior of highly ordered molecular multilayer islands Topple et al. make a suggestion for the microscopic thin film structure of PTCDI/ KBr(100) containing two molecules per unit cell5 as in the bulk crystals,3,4 not just one. Although Megow et al. exclude the detailed structural characterization from the scope of their study, they should have at least mentioned and discussed these literature results in the context of their topic so as to meet the criteria for good scientific practice. The disregard of the second molecule in the unit cell and, consequently, the negligence of a possible Davydov splitting in the optical spectra render their interpretations questionable.
n a recent paper, Megow et al. propose a new approach for calculating the change of the absorption spectrum of an aromatic molecule upon condensation from the gas phase into the solid state. As an example, they describe the optical absorption of 3,4,9,10-perylenetetracarboxylic diimide (PTCDI, Figure 1) on KBr(100).1 They claim that the spectral shape
Figure 1. Chemical formulas (hydrogen atoms omitted) of (a) PTCDI (R = H, CAS registry number: 81-33-4), Me-PTCDI (R = CH3, CAS registry number: 5521-31-3); (b) PTCDA (CAS registry number: 12869-8).
observed for PTCDI aggregates is caused by molecules in significantly different environments, i.e., an ordered and a disordered phase. However, their study suffers from several critical flaws. From our point of view, the main issues are the inadequate references to (I) previous reports on the thin film structure of PTCDI on KBr(100) and on the bulk crystal structure of PTCDI, (II) alternative explanations of the aggregation effects on the absorption spectra, and (III) earlier experimental studies that also employed optical differential reflectance spectroscopy (DRS) to investigate an almost identical system. These aspects are vital for the interpretation and well-balanced discussion of their results and should thus have been addressed with care, as outlined in the following.
(II). ALTERNATIVE THEORETICAL EXPLANATIONS OF SPECTRAL BEHAVIOR Megow et al. neglect the extensive work of Hoffmann et al., who successfully explained the optical absorption of aggregates of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA, Figure 1) and N,N′-dimethyl-3,4:9,10-perylenedicarboximide (Me-PTCDI, Figure 1).6,7 Instead, they write that “It was shown in ref 7 that charge transfer states coupling to Frenkel exciton states [...] change the absorption curve only slightly for PTCDA crystals. [...] For the sake of clarity we will neglect charge transfer states in this work.” However, in their ‘ref 7’, a paper by Gisslén and Scholz,8 it is also stated that “Based on a more sophisticated approach to the deformation of excited or charged molecules, it was found that a theoretical analysis of the optical spectra obtained on Me-PTCDI [...] requires a mixing between Frenkel and CT states.” There, the work of Hoffmann et al.6 was cited which makes clear that charge transfer (CT) excitons cannot be readily ignored in such crystals of perylene diimides. Hence, the theoretical model of Hoffmann et al. is clearly relevant in this context and should be mentioned. A systematic discussion of a new model should justify its novelty and benefits in relation to previous theories which were, however, not considered sufficiently by Megow et al.
(I). STRUCTURE OF PTCDI AGGREGATES Megow et al. state that “PTCDI films deposited on KBr(100) reveal steps with heights of 7 Å at sub-ML coverage [...], indicating that the molecules do not lie flat on the surface but are tilted like in the bulk crystal.” and further “In a single monolayer there is one molecule per unit cell”.1 It is astonishing how they can draw these conclusions from their data without even considering the possible observation of substrate steps, given that the lattice constant of KBr is 6.6 Å. Moreover, they refer to a paper by Klebe et al.2 that compares the bulk crystal structures of many derivatives of PTCDI with larger substituents, however not that of PTCDI itself. Since Megow et al. stress the importance of the specific packing motif themselves, they should have referred to the actual bulk crystal © 2015 American Chemical Society
Received: May 5, 2015 Published: July 21, 2015 18816
DOI: 10.1021/acs.jpcc.5b04295 J. Phys. Chem. C 2015, 119, 18816−18817
Comment
The Journal of Physical Chemistry C
(III). DRS STUDIES ON ALMOST IDENTICAL SYSTEMS Key results of Megow et al. were obtained by means of DRS measurements of a PTCDI submonolayer on KBr(100) exhibiting monomeric behavior and of nominal PTCDI mono- and multilayers with absorbance spectra that are attributed to the coexistence of an ordered and a disordered phase with comparable content. They fail to mention that the very same DRS technique was used previously to investigate an almost identical system, namely, PTCDA/KCl(100), in the submonolayer and monolayer regime.9 There, we described in detail the aggregation behavior yielding basically the same evolution of the optical spectra as described by Megow et al., cf. Figure 2. Quite similar to the structural data of PTCDI on
experimental techniques, yet with rather different interpretations and/or higher accuracy. This casts severe doubts on the validity of their conclusions.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Megow, J.; Kö rzdö rfer, T.; Renger, T.; Sparenberg, M.; Blumstengel, S.; Henneberger, F.; May, V. Calculating Optical Absorption Spectra of Thin Polycrystalline Organic Films: Structural Disorder and Site-Dependent van der Waals Interaction. J. Phys. Chem. C 2015, 119, 5747−5751. (2) Klebe, G.; Graser, F.; Hädicke, E.; Berndt, J. Crystallochromy as a Solid-State Effect: Correlation of Molecular Conformation, Crystal Packing and Colour in Perylene-3,4:9,10-bis(dicarboximide) Pigments. Acta Crystallogr., Sect. B: Struct. Sci. 1989, 45, 69−77. (3) Tojo, K.; Mizuguchi, J. Refinement of the Crystal Structure of 3,4:9,10-Perylene-bis(dicarboximide), C24H10N2O4, at 263 K. Z. Kristallogr. NCS 2002, 217, 45−46. (4) Guillermet, O.; Mossoyan-Déneux, M.; Giorgi, M.; Glachant, A.; Mossoyan, J. C. Structural Study of Vapour Phase Deposited 3,4,9,10Perylene tetracarboxylicacid diimide: Comparison between Single Crystal and Ultra Thin Films Grown on Pt(100). Thin Solid Films 2006, 514, 25−32. (5) Topple, J. M.; Burke, S. A.; Ji, W.; Fostner, S.; Tekiel, A.; Grütter, P. Tailoring the Morphology and Dewetting of an Organic Thin Film. J. Phys. Chem. C 2011, 115, 217−224. (6) Hoffmann, M.; Soos, Z. G. Optical Absorption Spectra of the Holstein Molecular Crystal for Weak and Intermediate Electronic Coupling. Phys. Rev. B: Condens. Matter Mater. Phys. 2002, 66, 024305. (7) Hoffmann, M. Mixing of Frenkel and Charge-Transfer Excitons and Their Quantum Confinement in Thin Films. In Electronic Excitations in Organic Multilayers and Organic Based Heterostructures; Agranovich, V. M., Bassani, G. F., Eds.; Elsevier: Amsterdam, 2003; Chapter 5. (8) Gisslén, L.; Scholz, R. Crystallochromy of Perylene Pigments: Interference between Frenkel Excitons and Charge-Transfer States. Phys. Rev. B: Condens. Matter Mater. Phys. 2009, 80, 115309. (9) Dienel, T.; Loppacher, C.; Mannsfeld, S. C. B.; Forker, R.; Fritz, T. Growth-Mode-Induced Narrowing of Optical Spectra of an Organic Adlayer. Adv. Mater. 2008, 20, 959−963.
Figure 2. DRS data of PTCDA deposited on KCl(100). For nominal thicknesses > 1 ML the molecular film recrystallizes from a brickwall monolayer structure to a herringbone phase consisting of multilayer islands, accompanied by a characteristic spectral evolution. Both phases are highly ordered as evidenced by means of AFM. After 60 min the DR spectrum is almost identical to that of a 5 ML thick polycrystalline PTCDA film on mica, shown for comparison. Adapted from ref 9 with permission from John Wiley and Sons.
KBr(100)5 we exclusively found crystalline islands consisting of flat-lying PTCDA molecules on KCl(100).9 In the view of the identical experimental methodology and strong overlap of the systems investigated, our work should have been cited by Megow et al. as well. In conclusion, Megow et al. achieve agreement between their new model and their experimental data assuming a PTCDI film structure that consists of an ordered and a disordered phase in equal parts. While this assumption contravenes the aforementioned structural characterizations we emphasize that for comparable molecules, such as PTCDA, the optical absorbance of thin films (≥4 ML, or equivalent multilayer islands) is very similar to that of rather thick (polycrystalline) films with insignificant disordered phase content.6,7,9 Consequently, attributing the observed spectral features merely to molecules in “distinctively different environments”, as postulated by Megow et al., may simply be overinterpreted and conflicts with existing theories on the optical crystal spectra of perylene derivatives. In contrast to scientific standards, it appears that reports of immediate relevance for the topic were not properly referenced by Megow et al. Had they performed a more thorough literature survey, the authors would have recognized that several key issues had been detailed before using the same 18817
DOI: 10.1021/acs.jpcc.5b04295 J. Phys. Chem. C 2015, 119, 18816−18817
