Comment pubs.acs.org/JPCC
Reply to “Comment on ‘Calculating Optical Absorption Spectra of Thin Polycrystalline Organic Films: Structural Disorder and SiteDependent van der Waals Interaction’” Jörg Megow,*,† Thomas Körzdörfer,† Thomas Renger,‡ Mino Sparenberg,§ Sylke Blumstengel,§ and Volkhard May§ †
Institut für Chemie, Universität Potsdam, Karl-Liebknecht-Straße 24-25, D-14476 Potsdam, F. R. Germany Institut für Theoretische Physik, Johannes Kepler Universität Linz, Altenberger Straße 69, A-4040 Linz, Austria § Institut für Physik, Humboldt−Universität zu Berlin, Newtonstraße 15, D-12489 Berlin, F. R. Germany
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‡
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.5b04295
B
implying that to some extent also other orientations are present. The PTCDI film structure on sapphire whose absorption spectrum is depicted in Figure 1 (ref 1) was analyzed with grazing incidence X-ray diffraction (data not shown). Only reflexes corresponding to the [020] PTCDI lattice planes are also found in this film. The Cambridge Crystallographic Data Center lists three structures of PTCDI: one by Klebe (CIF_LENPEZ, CSD No 55462) (used by us) and the two structures suggested in the comment.4,5 The three structures are nearly identical. The comment implies that we used a crystal structure of a different molecule. This is not the case. On the basis of AFM measurements with molecular resolution Topple et al. suggested three surface-induced structures specific to PTCDI deposited on KBr, NaCl, and surface-modified NaCl deviating from the bulk crystal structure.6 The comment criticizes that we did not consider these structures. Comparing the thin film morphology at similar coverage of our films with those obtained by Topple et al.6 one notices striking differences: (i) The nucleation density is much higher (ca. by a factor of 25) in our films. (ii) The aggregate size and shape is different (15 nm × 8 nm × 0.7 nm vs 150 nm × 30 nm × 1.8 nm ). (iii) Preferred nucleation at step edges as reported by Topple et al. is not observed. The differences can be ascribed to a different preparation of the KBr surface. While Topple et al. cleave KBr in situ, we cleave KBr prior to loading into the UHV system. Short exposure to air causes adsorption of contaminants altering the nucleation behavior of the molecules significantly. This is also pointed out distinctly by Topple et al.6 Therefore, it appears to us not justified to adopt the structure found by Topple et al. for PTCDI on KBr. We wish to stress that sole AFM measurements can give hints but will certainly not provide a comprehensive understanding of the evolution of a thin film structure. This requires complementary techniques such as for example real-time X-ray diffraction experiments. We clearly stated in our manuscript that this is beyond the scope of the present work. However, on the basis of
efore responding in detail to the comments by Roman Forker and Torsten Fritz on our recent publication (ref 1) we wish to emphasize the following: The aim of our paper is the presentation of a methodology for the calculation of the socalled gas-to-crystal shift Δ,m which is usually introduced as an adjustable parameter. This shift refers to the change the molecular optical transition energies undergo when the molecules (labeled by m) are moved from the gas phase into a crystalline morphology. We have been able to suggest a procedure to compute the dominant dispersive contribution to Δ,m and to match the energetic position of experimental absorption spectra belonging to 3,4,9,10-perylenetetracarboxylic diimide (PTCDI) molecular layers on KBr(100). The particular challenge lies in the fact that the theory, besides the dispersive shift, has to account for elements of disorder, Frenkel-exciton (FE) formation, vibrational contributions, and charge transfer (CT) excitations. We consider the determination of Δ,m as a considerable step forward to approach an ab initio computation of optical spectra of complex systems. Recently, the reliability of the approach was demonstrated for exciton spectra measured for tubular dye aggregates formed by particular cyanine derivatives2 as well as for spectra of pheophorbide a dendrimers.3 Our subsequent comments on the criticism by Roman Forker and Torsten Fritz are oriented at the three items they addressed in their comment.
1. STRUCTURE OF PTCDI AGGREGATES The comment implies that the islands visible in the AFM image of Figure 2 in ref 1 are not molecular aggregates but substrate steps of the KBr(100) surface. The two features are clearly distinguishable (see Figure 1a). KBr(100) forms a terraced surface with steps corresponding to the KBr lattice constant. On top of the terraces, small molecular aggregates are visible which are not observed on the bare substrate. The zoom, see Figure 1b (and Figure 2 in ref 1), shows a surface region on top of a single KBr terrace. A large fraction of the molecular aggregates have heights of ca. 0.7 nm which is similar to the distance between [020] lattice planes (0.724 nm) of the bulk PTCDI crystal. This led us to the assumption that (i) the contact plane of those aggregates is the [020] plane and (ii) the molecules arrange similarly as in the bulk crystal. In Figure 1b, one can recognize also aggregates with deviating heights © 2015 American Chemical Society
Received: June 10, 2015 Revised: July 20, 2015 Published: July 21, 2015 18818
DOI: 10.1021/acs.jpcc.5b05536 J. Phys. Chem. C 2015, 119, 18818−18820
Comment
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The Journal of Physical Chemistry C
Figure 1. (a) 1.0 μm × 0.585 μm AFM image of nominally 0.2 ML PTCDI on KBr showing substrate steps. The graph beneath shows a line scan taken along the white dotted line (b) 300 nm × 300 nm scan of the region marked by the dotted square in (a). Line profiles recorded at various positions are shown in the graph below. (c) 300 nm × 300 nm scan of nominally 4.9 ML of PTCDI corresponding to the optical spectra shown in Figure 3 of ref 1.
FE/CT coupling.7−9 On the other hand, the spectra shown in ref 9 for PTCDA indicate that for this specific perylene derivative the FE/CT coupling affects the spectra only moderately. As discussed in our paper,1 we additionally evaluated the effect of FE/CT coupling on the optical spectra using PTCDI parameters (Table XII from ref 9). These calculations also justified the neglect of coupling of CT with FE states. The comment animated us to perform computations varying those parameters. When changing the CT state energies such that they come close to the FE energies, then the coupling between the two states may not be readily neglected. But, one should note the following: Both dispersive and inductive polarization effects will modify FE/CT coupling. However, coupling between FE and CT states is screened in a different way than excitonic couplings. It is not yet clarified how strong the effect is. If this FE/CT coupling is affected in a similar manner by the environment as the excitonic coupling, it is again a reasonable approximation to neglect the effect. However, even when neglecting screening effects and adapting the CT energy, so that FE−CT state coupling becomes important, we are unable to explain the optical spectra without assuming at least some disorder. Respective disordered areas need to cover about 20%−30% of the total volume. The reason for that is the much smaller excitonic coupling (factor 2 to 5) compared to previous work.7−9 In our approach the excitonic couplings are computed using a microscopic model, and environmental modification/ screening is accounted for. We thus consider our computations as a crucial test of the parameters utilized in earlier work. We conclude that the smaller excitonic coupling is the main reason why we do not achieve agreement with the experimental spectra when assuming perfectly ordered PTCDI. This statement does not change when introducing CT coupling. We believe that further work needs to be invested to
our experimental data, we can certainly not exclude disorder in the present PTCDI films. We wish to point out that there are two molecules per unit cell in the bulk PTCDI crystal. Therefore, Davydov splitting is naturally accounted for in our computations as we diagonalized the excited state Hamiltonian of the whole set of molecules. The Hamiltonian covers the strong excitonic coupling of πstacked molecules (about 20 meV) and the weak excitonic coupling between the two molecules within the unit cell (between −2 and −3 meV). The statement that Davydov splitting is neglected in our computations is wrong.
2. ALTERNATIVE THEORETICAL EXPLANATIONS OF SPECTRAL BEHAVIOR In this section Roman Forker and Torsten Fritz criticize that we did not refer to the extensive work of Hoffmann et al.7,8 published in 2002/03. The inclusion of these papers in our reference list might have been correct. Instead, we quoted the publication of Gisslén and Scholz9 from 2009, which discusses and cites previous work on perylene derivatives including those of Hoffmann et al. It has to be noted that in contrast to Gisslén and Scholz, Hoffmann et al. did not carry out ab initio computations of excited states but fitted the spectra using a parametrized Hamiltonian. Moreover, the comment criticizes that we did not consider previous theories sufficiently. We do not agree. It was just the challenge of our present study to demonstrate that the calculation of the dispersive shift has to be combined with additional effects such as disorder, FE formation, vibrational contributions (intra- and intermolecular), and CT excitations. Quotations about these approaches are considered in our reference list. The comment criticizes that CT states are neglected. On the one hand, we acknowledge the fact that previous work explained spectra of perylene derivatives assuming a strong 18819
DOI: 10.1021/acs.jpcc.5b05536 J. Phys. Chem. C 2015, 119, 18818−18820
Comment
The Journal of Physical Chemistry C
Excitations in Organic Multilayers and Organic Based Heterostructures; Agranovich, V. M., Bassani, G. F., Eds.; Elsevier: Amsterdam, 2003; Chapter 5. (9) 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. (10) 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.
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unambiguously describe optical spectra of such complex systems.
3. DRS STUDIES ON ALMOST IDENTICAL SYSTEMS As outlined by Roman Forker and Torsten Fritz, they reported previously combined DRS and AFM investigations of PTCDA crystalline films deposited on KCl.10 The comment stresses the similarity of PTCDI and PTCDA molecules. This is correct on the single-molecule level; however, intermolecular interactions and consequently the growth behavior, the interaction with the substrate, and the arrangement in bulk crystal are very different. In PTCDI, hydrogen bonding between N−H end groups and oxygen atoms of adjacent molecules causes a row-like arrangement, while in PTCDA electrostatic interactions lead to a nearly orthogonal orientation of the two molecules in the unit cell. We believe some caution has to be taken transferring knowledge acquired for the much more extensively studied PTCDA to PTCDI; therefore, we do not agree to the statement that our assumption of disorder in PTCDI films contravenes with results obtained in ref 10. Finally, the comment states that our work conflicts with existing theories. We do not agree since there is no theoretical description of the PTCDI thin film optical spectra reported yet relying solely on parameters obtained by ab initio methods and taking into account the effect of the environment. It is just the progress we achieved, namely, the explicit calculation of the dispersive shift which is usually taken as a parameter, that allows to approach such a description.
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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) Megow, J.; Röhr, M. I. S.; Schmidt am Busch, M.; Renger, T.; Mitrić, R.; Kirstein, S.; Rabe, J. P.; May, V. Site-dependence of van der waals interaction explains exciton spectra of double-walled tubular jaggregates. Phys. Chem. Chem. Phys. 2015, 17, 6741−6747. (3) Megow, J. How van der Waals interaction influences the absorption spectra of pheophorbide a complexes: a mixed quantumclassical study. ChemPhysChem 2015, DOI: 10.1002/ cphc.201500326R1. (4) Tojo, K.; Mizuguchi, J. Refinement of the crystal structure of 3,4:9,10-perylenebis(dicarboximide), C24H10N2O4, at 223 K. Z. Kristallogr. - New Cryst. Struct. 2002, 217, 45−46. (5) Guillermet, O.; Mossoyan-Deneux, 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. (6) 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. (7) 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. (8) Hoffmann, M. Mixing of Frenkel and Charge-Transfer Excitons and TheirQuantumConfinement in Thin Films. In Electronic 18820
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