Photoemission Studies on N-Substituted Dithienylated Phenothiazines.

DOI: 10.1002/cphc.201500095

Articles

Photoemission Studies on N-Substituted Dithienylated Phenothiazines Mathias Fingerle,[a] Maximilian Hemgesberg,[b] Yvonne Schmitt,[b] Stefan Lach,[a] Markus Gerhards,[b] Werner R. Thiel,[b] and Christiane Ziegler*[a] Dithienylated phenothiazines (DTPTs) with different functional groups attached to the central nitrogen atom are presented as a class of versatile metal-free chromophores for the design of dye-sensitized solar cells (DSSCs) and organic light-emitting diodes (OLEDs). The electronic characteristics of spin-coated thin films on polycrystalline gold were studied using photoelectron spectroscopy assisted by theoretical calculations, scan-

ning force microscopy, and UV/Vis spectroscopy. Complementary fluorescence spectra show light emission in the blue region (465 nm). The absorption properties and good holetransporting abilities make DTPTs feasible hole-transporting materials (HTM) and metal-free chromophores in UV-sensitive solar cell designs.

1. Introduction Substituted phenothiazines (PTs) have been explored over the previous years as organic photosensitizers and redox-active chromophores for a variety of applications, including hybrid photovoltaic systems.[1–5] Metal-free PTs form stable nitrogencentered radical cations,[6] which has been utilized in the design of hybrid mesoporous silica[7] and for the coating of TiO2 nanoparticles. Most studies are based on asymmetrically donor–acceptor-modified compounds with a completely delocalized p system. Therefore, thienylated phenothiazines are efficient photosensitizers in dye-sensitized solar cells (DSSC), but, to the best of our knowledge, no derivatives with aliphatic moieties on the nitrogen atom have been introduced so far. Due to the small distance between the nitrogen-centered radical cation and the electron-withdrawing substituents, electronrich N-substituted 3,8-dithienylated phenothiazines (DTPTs) could be a promising tool for further development in organic electronics. Here, X-ray- and UV-photoemission (XPS and UPS), as well as inverse photoemission (IPES) experiments on molecular thin films spin-coated on polycrystalline gold are presented. The electronic properties of the organic–metal interface were verified by density functional theory (DFT) and equivalent core calculations. As complementary methods, both UV/Vis and fluorescence spectroscopy were used. The topography of the films was studied by using scanning force microscopy (SFM). The six studied N-substituted phenothiazines, having thien-2-yl units,

are shown in Figure 1 and their synthesis is described in detail in Refs. [8–10] (1–5) and in Section 4.1 (6). Following the concept of a molecular donor–acceptor system, the electron-rich thiophene groups act as donors, whereas the N-substituents have an electron-acceptor character. Among these substituents is a chemically reactive carboxylic group, whose ability to anchor onto TiO2 nanoparticles is of particular importance for hybrid solar cell concepts.[11] The determination of the electronic properties of the molecules is crucial to understand and further improve the efficiency and charge-transfer behavior of the PTs.

[a] M. Fingerle, Dr. S. Lach, Prof. Dr. C. Ziegler University of Kaiserslautern Department of Physics and Research Center OPTIMAS Erwin-Schrçdinger-Str. 56, 67663 Kaiserslautern (Germany) E-mail: [email protected]

2.1.2. XPS

2. Results and Discussion 2.1. Topographic and Stoichiometric Analysis 2.1.1. SFM A representative SFM image of a freshly spin-coated thin film of 4 on polycrystalline gold is shown in Figure 2, together with a height profile. The average height over all height profiles was 37.9 nm. The molecules agglomerate to particles, forming nonordered structures on a nanometer scale. The dark areas inbetween the crystallites can be attributed to the gold-substrate suface, thus indicating the surface is not fully covered. Comparable morphological properties were obtained for all investigated thin films.

The XPS survey scan of the 6–gold interface in Figure 3 shows that the Au core levels of the substrate and the metallic dband structure of gold in the valence band region can still be obtained. As the inelastic mean-free path (IMFP) of the photoelectrons in the organic layer can be approximated[12, 13] to be only about four nanometers, it is likely that the organic layer is

[b] Dr. M. Hemgesberg, Dr. Y. Schmitt, Prof. Dr. M. Gerhards, Prof. Dr. W. R. Thiel University of Kaiserslautern, Department of Chemistry Erwin-Schrçdinger-Str. 52-54, 67663 Kaiserslautern (Germany)

ChemPhysChem 0000, 00, 0 – 0

These are not the final page numbers! ÞÞ

1

0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

&

Articles

Figure 1. Chemical structure of the dithienylated 3,8-phenothiazines with six different N substitutents.

Figure 3. XPS (Mg K) survey of a thin layer of 6 spin-coated on polycrystalline gold. The peak features are assigned to their specific core level or Auger transition, respectively.

three sulfur atoms, two oxygen species, and one nitrogen atom. Similar results were obtained for all the thin films and are listed in Table 1.

Figure 2. SFM 5 5 mm2 image and height profile recorded in intermittent mode of a thin layer of 4 spin-coated on polycrystalline gold.

thin in parts or does not provide full coverage, as already stated in Section 2.1.1. The primary core levels of 6 are S 2p, C 1s, N 1s, and O 1s. The amount of atomic species in the organic layer can be quantitatively determined through stoichiometric analysis. The values given in Table 1 clearly reveal that the molecular structure is not damaged by the wet processing, as the atomic ratio n of all species reflects the structure of the DTPTs, which, in the case of 6, consists of 25 carbon atoms,

&


www.chemphyschem.org

2.2. Core Levels The XPS spectra of the DTPTs atomic energy levels were compared in detail with simulated spectra. To obtain the complex electronic structure from the C 1s signal, an equivalent core approximation was used (see the Experimental Methods); this approximation explicitly takes into account the relaxation effects that are due to the photoemission process, and therefore, 2


ÝÝ These are not the final page numbers!

Articles transition energies were measured as the distance between the peak for each component and its shake-up structure peak, and were found to be close to the optical bandgap energy of Eopt,(1) = 2.9 eV, which was obtained through UV/Vis spectroscopy (see Table 4). Based on these basic assumptions, the shape of the spectrum in the shake-up region can still not be fully described, but owing to the low signal-to-noise ratio in this region, a more detailed discussion would only be speculative. The C 1s spectrum of a thin layer of 3 on polycrystalline gold is shown in Figure 4 on the right. Here, the peak corresponding to the additional NCH2 bridge is located at 286.4 eV. The carboxylic group has a binding energy of 289.0 eV, which is consistent with the literature.[17] No shake-up peaks could be resolved in this spectrum, although, in this case, it might be possible that the shake-up peaks are overlapped by the electronic structure of the substituents’ carbon bonds. The C 1s energy levels for all six DTPTs (1–6) on polycrystalline gold are given in Table 2. The methyl group (CH3) of 2 is observed at a binding energy of 286.2 eV, whereas the NCH2 R bridge of 3 is located at 286.4 eV, that of 4 at 286.7 eV, and that of 5 at 286.6 eV. For 6, the binding energy of the NCH=C atom is 286.4 eV. The remaining C 1s components of 4 are the CH2CH2CN bond at 286.6 eV and the cyano endgroup CN at 287.4 eV. At 290.3 eV, the COOR binding energies for 5 and 6 are slightly higher than that of the COOH group of 3 (289.0 eV). The methyl group of 5 bound to an oxygen atom of the COO group has a binding energy of 286.7 eV. The remaining C 1s components for 6 are the CH2=CH2COOR bond at 283.9 eV, the OCH2CH3 level at 286.7 eV, and the methyl endgroup (OCH2CH3) at 285.4 eV.

Table 1. Stoichiometric analysis of the DTPT thin films spin-coated on polycrystalline gold. Here, A is the peak area, s is the cross-section,[14] n the theoretical number of atoms for one molecule in the organic layer, and nXPS the measured value, referenced to the number of carbon atoms.

1 2 3 4 5 6

s A n/nXPS A n/nXPS A n/nXPS A n/nXPS A n/nXPS A n/nXPS

C 1s

N 1s

O 1s

S 2p

1 4180 20/20.0 4579 21/21.0 7590 22/22.0 5095 23/23.0 6074 23/23.0 7798 25/25.0

1.77 288 1/0.94 318 1/0.99 550 1/1.09 810 2/2.50 405 1/1.05 489 1/1.07

2.8 – – – – 1275 2/2.02 – – 1143 2/2.37 1205 2/2.11

1.75 1400 3/3.22 1263 3/2.79 2346 3/3.27 1304 3/2.83 1637 3/2.98 2041 3/3.14

allows a more detailed interpretation.[15, 16] For the N 1s, S 2p, and O 1s peaks the faster DFT method was preferred. 2.2.1. C 1s The C 1s spectrum of a thin layer of 1 on polycrystalline gold is shown in Figure 4 on the left. Based on an equivalent core approximation, a peakfit was applied that takes into account the atomic ratio of the different bonding types and places the peak corresponding to the twelve aromatic carbon bonds at 284.4 eV. The six CS bonds are located at higher binding energies around a maximum of 284.8 eV, and the three CN bonds are located at 285.6 eV. To resemble the experimental spectrum better, a shake-up structure was added to the fit for each of these three main C 1s components with an intensity ratio of 6:3:1, which reflects the intensity ratio of the main peaks. The binding energies of 287.1, 288.0, and 288.8 eV result in the transition energies for an excitonic state. These

2.2.2. N 1s As shown in Figure 5 for a thin layer of 1, the N 1s peak does not feature any substructures, as there is just one single nitrogen species present. The second nitrogen species of 4 is found at similar binding energies, still leading to a structure that can be fully described by a single peak with a maximum around 400 eV. The exact values for all six derivatives are given in Table 2. For 5 (400.1 eV) and 6 (400.3 eV) the peaks are situated at slightly higher binding energies; this could be attributed to the complex nature and electron-withdrawing properties of the ester groups. 2.2.3. S 2p

Figure 4. XPS (Mg K) C 1s spectrum of a thin layer of 1 (left) and 3 (right) on polycrystalline gold.




3

Owing to the spin-orbit interaction, the S 2p features are split into two components, 2p1/2 and 2p3/2, at a peak-to-peak distance of 1.2 eV[18] and an intensity ratio of 1:2, which is equal to the mul 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

&

Articles Table 2. XPS Binding energies (eV) for all organic components of thin films of 1–6 on polycrystalline gold, calculated with equivalent core approximation (C 1s) and DFT (N 1s, S 2p and O 1s) and fitted to the experimentally obtained spectra.

S 2p5/2 CC CS CN NCH3 NCH2 NCH=C CH2CH2CR CH=CHCR CN COOR OCH3 OCH2CH3 OCH2CH3 N 1s C=O COR

1

2

3

4

5

6

error

163.9 284.4 284.8 285.6

164.0 284.4 284.8 285.5 286.2

163.8 284.3 284.3 285.2

164.1 284.7 285.0 285.7

64.2 284.6 285.0 285.6

164.1 284.6 285.0 285.5

286.4

286.7

286.6

0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2

286.4 286.6 283.9 287.4 289.0

399.7

399.8

399.7 531.8 533.4

289.3 286.7

399.7

400.1 532.4 534.1

289.3 286.7 285.4 400.3 531.2 532.8

Figure 6. XPS (Mg K) S 2p spectrum of a thin layer of 1 on polycrystalline gold.

ited from ethanol solutions have shown, chemisorbed sulfur species have relatively low binding energies of around 162.0 and 163.2 eV for the S 2p doublet,[20, 21] whereas species with higher binding energies can be assigned to physisorbed sulfur atoms. As the binding energies of the 2p doublet for the DTPTs are much higher, at around 164.0 and 165.2 eV, chemisorption can be ruled out.

2.2.4. O 1s Only the carboxylic group of 3, and the carboxylate ester groups of 5 and 6, contain oxygen atoms. In the O 1s spectrum shown in Figure 7, the two components of 3 can be identified through a fit at 531.8 eV for the C=O bond and 533.4 eV for the hydroxylic group (COH); this is consistent with the literature.[17] For 5, the two components are shifted to higher binding energies, as can be seen in Table 2. This indicates that the Figure 5. XPS (Mg K) N 1s spectrum of a thin layer of 1 on polycrystalline gold.

tiplicity; this leads to a similar spectrum for all investigated derivatives, as shown representatively for 1 in Figure 6. Fitting the DFT-calculated set of binding energies to the experimental spectrum reveals good agreement between the two. The maxima of the S 2p peaks for all DTPTs can be found in Table 2. Here, a shake-up peak was added to the fit at 166.9 eV to describe the shape of the experimental spectrum more closely. The distance measured from the 2p3/2 peak to the coresponding shake-up peak is with 3.0 eV, which is again close to the optical bandgap energy of Eopt,(1) = 2.9 eV, as already stated for the C 1s structure. Owing to the affinity of organosulfur compounds for gold surfaces,[19] a possible chemical reaction involving the sulfur-containing thiazine ring and the two thiophene units of the DTPTs during film construction had to be taken into account. As studies of thiophenes on Au(111) depos-

&



Figure 7. XPS (Mg K) O 1s spectrum of a thin layer of 3 on polycrystalline gold.

4



Articles sorption band between 300 and 400 nm leads to a high sensitivity of the DTPTs in the UV and near-UV region. For 6, additional absorption bands can be observed in the ranges of 260– 280 and 300–400 nm, leading to an even higher sensitivity. As the dyes might potentially be relevant for OLED (organic light emitting diode) design, too,[9] the analysis of the light emission by fluorescence is also of interest. The fluoresence spectra of all DTPTs consist of two bands: the main emission band at approximately 465 nm and a relatively intense preband in the range of 380–430 nm (Figure 9). In contrast to the absorption spectra, the emission spectrum of 6 does not differ

oxygen species cannot easily withdraw electrons away from the methyl group of the ester moiety, whereas in 3 the OH group can be easily deprotonated. The lower binding energies for 6 can be explained by the extension of the p system of the chromophore with the C=C group and the introduction of the ethyl linked to the COO group.

2.3. UV/Vis and Fluorescence Spectra To determine the optical absorption band onsets in solution (Table 3) the UV/Vis spectra of the six compounds solvated in dicholoromethane were recorded (Figure 8). The shape of the

Table 3. Hole injection barriers (FBh), work functions (F(i)/Au), ionization energies (IE), electron injection barriers (FBe), and electron affinities (EA), as well as experimentally observed and DFT-calculated transport gaps and optical absorption band onsets (UV/Vis) given in eV for thin films of 1–6 on polycrystalline gold.

FBh F(i)/Au IE FBe EA Et Et,DFT Eopt

1

2

3

4

5

6

error

0.8 4.2 5.0 2.9 1.3 3.7 3.6 2.9

1.0 4.2 5.2 3.0 1.2 3.9 3.7 2.9

0.7 4.6 5.3 2.9 1.7 3.6 4.0 3.0 6

1.3 4.2 5.5 2.3 1.9 3.6 – 3.0

1.2 3.7 4.9 3.0 0.7 4.2 3.7 2.9

1.4 4.2 5.6 2.1 2.1 3.5 3.8 3.3

0.2 0.2 0.2 0.2 0.2 0.9 0.1

UV/Vis spectra of compounds 1–5 are similar and the positions of the main absorption bands do not vary significantly. In typical solar cell architectures, wavelengths smaller than 300 nm are cutoff by the transparent conductive oxide (TCO) anode. Therefore, the prominent absorption maximum of the DTPTs of around 290 nm is not relevant for solar cell design. But, the ab-

Figure 9. Normalized fluorescence spectra of 1–6 solvated in dichloromethane; the chosen excitation wavelength for the spectra was 350 nm. The position of each emission band is noted in the graph.Hatched area: Raman scattering of the solvent.

Figure 8. Normalized UV/Vis spectra of 1–6 solvated in dichloromethane.

significantly from the emission spectra of the other compounds. The dual character of the emission can be explained in different ways: simultaneous radiative deactivation of two electronically excited states, intermolecular energy transfer, aggregation processes (formation of aggregates, excimers, or exciplexes), emission of different conformers of the compounds in solution and photochemical reactions. The examination of the origin of the dual fluorescence was not the focus of the investigations and requires a lot of further measurements and analysis, best in combination with calculations. Consequently, the origin cannot be discussed in the framework of this paper. Based on the absorption and emission spectra, the DTPTs show potential for the design of DSSCs specialized for light absorption in the UV region or for blue light (465 nm) emission in OLEDs. The range of the absorption and emission can only be varied if the functional group is placed close to the chromophoric core (such as in 6, here the effect might result from the overlap of the p bond and the p system of the chromophore). If the functional group is separated by a (CH2)1,2 chain, no significant influence on the optic properties can be




5


&

Articles observed. This effect can be of great interest, because it can be used to match the N substituent to any photoreactive partner (e.g. TiO2) without changing the optical properties. For more significant changes to the optic properties, the dye between the two thiophenes would need to be replaced by other aromatic systems.[9]

2.4. Electronic Structure In this section the combined UPS and IPES spectra of all six derivatives are presented together with a comparison of the UPS spectrum of 1 with the DFT calculations. Figure 10 illustrates the determination of the relevant electronic properties from

Figure 11. Combined UPS and IPES spectra of thin films of 1–6 on polycrystalline gold and a representative polycrystalline gold substrate.

comparable to the value of 5.1 eV for the 10H-phenothiazine obtained in Ref. [23]. The work function (F(i)/Au) of the system i/gold lies between 3.7 eV for 5 and 4.6 eV for 3. As the work function of the pure gold substrate was found to be FAu = 5.1 eV, it can be assumed that there is a diminishment of the dipole of the gold surface, as a result of the push-back effect of hydrocarbon contamination and the spin-coated molecules under ambient conditions. Additionally, DTPT thin films on native TiO2, which display no push-back effect, show no or only small changes of up to 0.2 eV at the interface.[25] Furthermore, DFT calculations show that the molecular dipoles of the phenothiazines are mainly in plane. The molecules with three triangularly distributed sulfur atoms have a flat adsorption geometry. Thus no additional interface dipole is expected, due to molecular polarization. Therefore, changes in the work function after the application of the organic molecules can be fully attributed to a push-back effect, which has a value of D = 0.9 eV for 1, 2, 4, and 6; this is a common value for small molecules on gold surfaces.[26] For 3, D is only 0.5 eV, which can be explained by the electronegativity of the carboxylic group, adding a negative potential for photoelectrons at the surface. For 5, the ester might induce the reverse effect, thus adding a positive potential to the surface barrier and increasing D to 1.4 eV. The hole-injection barriers FBh for all DTPTs are given in Table 3. The injection barrier of 3 is 0.7 eV, which is the lowest, thus making it a very good hole-transporting material. The more complex substituents of 4 and 6 lead to higher injection barriers FBh and should, therefore, be avoided for molecular design. Figure 12 shows the experimentally obtained UPS spectrum of 1 on polycrystalline gold in the region between the Fermi levels EF = 0 and 14 eV, together with the theoretical DFT spectrum for comparison. In general, there is good agreement in the shape of the whole spectrum, in particular for the p orbitals up to around 5 eV; this is true for the UPS spectra of all DTPTs (the spectra are shown in Figure 11 without DFT calculations).

Figure 10. Combined UPS (black) and IPES (blue) spectra of a thin film of 3 on polycrystalline gold. The determination of the onset values of HOMO and LUMO, the secondary electron cut-off (SECO), and relevant electronic properties are schematically illustrated. The IPES spectrum has been manipulated in the region between EF and the LUMO, where the intensity of the gold substrate has been substracted for a better illustration.

a combined UPS and IPES spectrum of a thin film of 3 on polycrystalline gold. The onset of the HOMO indicated the height of the hole injection barrier (FBh) as the distance to the Fermi level (EF) and gives the ionization energy (IE) as the distance to the vacuum level (Evac) of the system. Together with the LUMO onset, which indicates the electron injection barrier (FBe) as the distance to the Fermi level and gives the electron affinity EA as the distance to the vacuum level, the transport gap Et can be obtained. 2.4.1. UPS UPS studies of the phenothiazines are rare and only a few spectra of 10H-phenothiazine are reported in the literature.[22–24] The prominent three-peak feature of the p electrons and high intensity three-peak feature of the s bonds that were experimentally resolved in Ref. [22] are also present in the UPS spectra of all 3,8-dithienylated phenothiazines (Figure 11). The resulting ionization energies of 1–6 given in Table 3 vary between 4.9 and 5.6 eV. The value for 1 is IE(1) = 5.0 eV, which is

&



6



Articles

Figure 13. Energy level alignment at the interface i/gold referenced to the vacuum level Evac. Figure 12. UPS He I spectrum (black) of a thin film of 1 on polycrystalline gold and the DFT calculated spectrum (green) of 1.

DTPTs for the near UV region could enable applications in the design of emerging UV-sensitive hybrid solar cells.[29]

2.4.2. IPES

3. Conclusions

To the best of our knowledge, no IPES studies of phenothiazine compoundshave been reported so far. All spectra of the unoccupied states presented in Figure 11 show a broad density of states and a double-peak feature with an onset at 2.1 eV for 6 to 3.0 eV for 2, as stated in Table 3. The low intensity peaks at the Fermi level can be attributed to an intermixed signal from the gold substrate. Subtracting the electron injection barrier FBe from the work function F(i)/Au the electron affinity EA is obtained; this ranges from 0.7 eV for 5 to 1.9 eV for 4 (Table 3).

Thin films of N-substituted dithienylated phenothiazines were successfully spin-coated on polycrystalline gold substrates and characterized through scanning force microscopy and photoemission spectroscopy (XPS, UPS and IPES), assisted by theoretical DFT and equivalent core calculations. By using UV/Vis spectroscopy, a high sensitivity of the DTPTs for light absorption in the UV region was observed and the fluorescence spectra show light emission in the blue region (465 nm). Their optical properties and good hole transporting abilities, therefore make the DTPTs feasible as hole-transporting materials and metal-free chromophores in UV-sensitive solar cell[29] or blue OLED designs. The most promising chromophore is 3 with its carboxylic group, which can easily be deprotonated to chemically bind to TiO2 nanoparticles.[30] Such anchor groups are often used for thin-film coating of and efficient charge transfer to metal oxides in hybrid photovoltaic concepts.[11, 31] Plus, the hole injection barrier FBh of 3 is, at 0.7 eV, very small. As the successful preparation of thin films under ambient conditions shows, DTPTs are easy to process on an industrial scale for applications in organic electronics.

2.4.3. Transport Gaps The resulting experimentally obtained transport gaps Et of 1–6 on gold are presented in Table 3, together with the DFT calculated values Et,DFT and the optical absorption band onsets Eopt in solution, which were measured by UV/Vis spectroscopy. A comparison with the DFT calculations shows that the experimental values for Et are realistic and in agreement within the margin of error, which mainly stems from the broad bandpass of 0.6 eV in the IPES experiment, although there are discrepancies of up to 0.5 eV for 5. As expected, the optical absorption band onsets Eopt, which range from 0.2 eV for 6 to 1.3 eV for 5, are smaller than the transport gaps,[27] due to electron correlation effects. However, it should be noted that a direct comparison would only be possible by obtaining the optical band gaps of solid films with UV/Vis spectroscopy. As depicted in the energy level alignment scheme for the interface i/gold in Figure 13, the electron injection barriers FBe are relatively high, leading to large transport gaps and an optical absorption, which is limited to the near ultraviolet and visible blue region. On the one hand, this is disadvantageous when competing with numerous low-bandgap materials already known for organic photovoltaics;[28] on the other hand, the sensitivity of the ChemPhysChem 0000, 00, 0 – 0



Experimental Section Synthesis The synthesis of the phenothiazines 1–5 was performed according to previously published procedures[8–10] and is shown in Figure 14 for 1–3, 5, and 6. (E)-Ethyl-3-(3,7-di(thiophen-2-yl)-10H-phenothiazin-10-yl) acrylate (6): 2,7-(dithien-2-yl-)10H-phenothiazine (3; 1.09 g, 3.0 mmol) was suspended in a mixture of dry toluene and dry dimethoxyethane (4:1, 50 mL). A 1.50 m solution of tBuLi in n-hexane (2.0 mL; 3.0 mmol) was added dropwise at 25 8C. After 30 min of stirring, a finely ground mixture of [Pd(dba)2] (5 mol %; 86.2 mg, 0.15 mmol) and JohnPhos (10 mol- %; 89.4 mg, 0.30 mmol) was added. The stir-

7


&

Articles with a resonance frequency of 300 KHz. The images were edited using the software Scanning Probe Image Processor (SPIP; Image Metrology A/S, Hørsholm, Denmark).

PES The photoemission studies were performed in situ in an ultrahigh vacuum (UHV) system with a base pressure below 2 1010 mbar. The XPS and UPS spectra were recorded with a commercial Phoibos 150 MCD9 hemispherical energy analyzer (Specs, Berlin, Germany). The employed Mg K radiation (1,253.6 eV) and He I radiation (21.2 eV) was provided by standard laboratory excitation sources (XR50 and UVS-10/35, Leybold). By measuring the 3d5/2 peak of a Ag(111) crystal, the energy resoFigure 14. Synthesis of different N-substituted phenothiazines carrying thien-2-yl groups. dba = dibenzylideneacelution of XPS was estimated to be tone , DME = dimethyl ether. better than 1 eV and for UPS better than 100 meV, as estimated from measurements of the Fermi ring was continued for another 15 min. Finally, (Z)-3-bromoethyl acedge. The energy scale was aligned by initial determination of the rylate (540 mg, 3.00 mmol; 364 mL) was added dropwise. Hereby, 4p3/2, 4d5/2, 4f5/2, and 4f7/2 peaks of an Au(111) crystal. The backthe red-colored solution turned black and the fluorescence turned ground of the acquired spectra was subtracted using the Shirley from orange red to blue green. The mixture was heated to reflux method[32] for XPS and the method of Li[33] for UPS. For all spectra for 24 h. After cooling to room temperature, a little Al2O3 was satellite correction was applied. For UPS, the sample was biased added to bind palladium and the suspension was filtered over with 6 V. For IPES, a home-built detector was used according to Celite. The remaining solid was washed three times with ethyl acethe model of Schedin[34] with a bandpass of 0.6 eV. As the electron tate (10 mL) and the combined filtrates were extracted three times source, a commercial electron gun (ELG-2, Kimball Physics Inc., with NaHCO3 (40 mL), dried over MgSO4, and filtered again. CoolWilton, USA) was applied. The subtracted background of the IPES ing the solution to 18 8C led to precipitation of the unreacted spectra was described with a cubic polynomial function.[35] All starting material, which was filtered off at this temperature. The shown IPES spectra were smoothed with a Savitzky–Golay algovolume of the filtrate was reduced to about 66 %. 50 mL of a 4:1 rithm. mixture of ethyl acetate and n-hexane were added, whereby the rest of the unreacted starting material precipitated. After filtration, the solvent of the filtrate was evaporated slowly to give the prodUV/Vis and Fluorescence uct as yellow crystals. By chromatography of the mother liquor The UV/Vis absorption spectra were recorded on a PerkinElmer over silica 60 (EtOAc/n-hexane 4:1, Rf = 0.33) a further smaller Lambda 900 double beam spectrophotometer with cylindrical amount of the product was isolated. Yield: 257 mg (0.56 mmol, quartz cuvettes of 10 mm pathlength. Fluorescence measurements 23 %). 1H NMR (400 MHz, [D6]DMSO): d = 7.93–7.79 (m, 3 H; Har), were performed on a Horiba Jobin–Yvon Fluorolog 3-22 t with 7.70 (dd, 2 H, Har), 7.66–7.55 (m, 6 H; Har), 7.22–7.10 (m, 2 H; Har and 10 10 mm2 quartz glass cuvette. For preparation of the solutions, Holef), 5.48 (d, 1 H; Holef), 4.10 (q, 2 H; CH2), 1.20 ppm (t, 3 H; CH3). 13 dichloromethane in UVASOL quality (Merck) was used; the concenC NMR (101 MHz, [D6]DMSO): d = 167.5 (C=O), 146.1, 141.4, 138.8, tration of the solutions was about 1 105 mol l1. Determination 132.3, 130.0, 128.7, 126.5, 125.3, 124.7, 124.3, 123.3 (NC=CC(O)), of the band gap was done following the procedure in Refs. [36, 37] 92.1 (NC=CC(O)), 59.1 (CH2), 14.4 ppm (CH3). IR (ZnSe-ATR): n˜ = by the use of the intersection point of the zero baseline and the 2974, 1695, 1627, 1589, 1526, 1474, 1428, 1400, 1344, 1254, 1237, 1 tangent to the absorption curve. 1177, 1155, 1127, 1091, 1039, 973, 872, 852, 801, 724, 702 cm . HRMS: m/z: calcd. for: 461.0563 found: 461.0578.

Sample Preparation Experimental Methods

The polycrystalline gold films with a thickness of 100 nm were sputtered (UNIVEX 450 C, Oerlikon, Germany) under high vacuum conditions (p < 1 105 mbar) on cut out 10 mm 10 mm plates of a natively oxidized silicon wafer with a 5 nm chromium layer as the adhesive agent. The gold-coated plates were then cleaned consecutively with acetone, isopropanol, and ethanol, each followed by distilled water, by using an ultrasonic bath. Afterwards

SFM The topography was measured in intermittent mode under ambient conditions using a Multimode scanning force microscope with a Nanoscope IIIa controller (Digital Instruments, Santa Barbara, USA). A cantilever (AC160, Olympus, Tokyo, Japan) was selected

&



8



Articles they were tempered in situ at around 150 8C and sputtered with argon ions for 15 min. Finally, their cleanliness was checked by means of XPS and UPS. The DTPTs were dissolved in acetone 1 g/ 10 mL and spin-coated onto the gold substrates with 3000 rpm under ambient conditions.

thank Christoph van Wllen and Sebastian Schmitt for providing the first theoretical band gaps of the dithienylated phenothiazines based on DFT calculations. Further, we thank Isabelle Faus and Juliusz A. Wolny from the group of Volker Schnemann for technical support and helpful discussions regarding the DFT simulations.

Simulation Keywords: dye-sensitized solar cells · organic electronics · phenothiazines · photoelectron spectroscopy · UV/Vis spectroscopy

The single molecule DFT (B3LYP) calculations were performed using the standard diffusion and polarized 6-31 + G(d,g) basis set. All calculations were made using Spartan’14 software (Wavefunction Inc., Irvine, USA).

[1] W. Tang, T. Kietzke, P. Vemulamada, Z.-K. Chen, J. Polym. Sci. Part A 2007, 45, 5266. [2] W. Wu, J. Yang, J. Hua, J. Tang, L. Zhang, Y. Long, H. Tian, J. Mater. Chem. 2010, 20, 1772. [3] R. Argazzi, C. A. Bignozzi, T. A. Heimer, F. N. Castellano, G. J. Meyer, J. Phys. Chem. B 1997, 101, 2591. [4] H. Tian, X. Yang, R. Chen, Y. Pan, L. Li, A. Hagfeldt, L. Sun, Chem. Commun. 2007, 3741. [5] G. Sang, Y. Zou, Y. Li, J. Phys. Chem. C 2008, 112, 12058. [6] R. J. Owellen, D. W. Donigian, J. Med. Chem. 1972, 15, 894. [7] M. Hemgesberg, G. Dçrr, Y. Schmitt, A. Seifert, Z. Zhou, R. Klupp Taylor, S. Bay, S. Ernst, M. Gerhards, T. J. J. Mller, W. R. Thiel, Beilstein J. Nanotechnol. 2011, 2, 284. [8] M. Hemgesberg, B. Bayarmagnai, N. Jacobs, S. Bay, S. Follmann, Ch. Wilhelm, Z. Zhou, M. Hartman, T. J. J. Mller, S. Ernst, G. Wittstock, W. R. Thiel, RSC Adv. 2013, 3, 8242. [9] M. Hemgesberg, D. M. Ohlmann, Y. Schmitt, M. R. Wolfe, M. K. Mller, B. Erb, Y. Sun, L. J. Goosen, M. Gerhards, W. R. Thiel, Eur. J. Org. Chem. 2012, 2012, 2142. [10] M. Hemgesberg, PhD thesis, University of Technology Kaiserslautern (Germany), 2014. [11] A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo, H. Pettersson, Chem. Rev. 2010, 110, 6595. [12] M. P. Seah, W. A. Dench, Surf. Interface Anal. 1979, 1, 1. [13] A. L. Bramblett, M. Boeckl, K. D. Hauch, B. D. Ratner, T. Sasaki, J. W. Rogers, Surf. Interface Anal. 2002, 33, 506. [14] J. H. Scofield, J. Electron Spectrosc. Relat. Phenom. 1976, 8, 129. [15] D. W. Davis, D. A. Shirley, Chem. Phys. Lett. 1972, 15, 185. [16] A. Snis, S. F. Matar, O. Plashkevych, H. Agren, J. Chem. Phys. 1999, 111, 9678. [17] G. P. Lpez, D. G. Castner, B. D. Ratner, Surf. Interface Anal. 1991, 17, 267. [18] D. G. Castner, K. Hinds, D. W. Grainger, Langmuir 1996, 12, 5083. [19] R. G. Nuzzo, B. R. Zegarski, L. H. Dubois, J. Am. Chem. Soc. 1987, 109, 733. [20] J. Noh, E. Ito, K. Nakajima, J. Kim, H. Lee, M. Hara, J. Phys. Chem. B 2002, 106, 7139. [21] E. O. Sako, H. Kondoh, I. Nakai, A. Nambu, T. Nakamura, T. Ohta, Chem. Phys. Lett. 2005, 413, 267. [22] L. N. Domelsmith, L. L. Munchausen, K. N. Houk, J. Am. Chem. Soc. 1977, 99, 6506. [23] N. Karl, N. Sato, K. Seki, H. Inokuchi, J. Chem. Phys. 1982, 77, 4870. [24] N. Sato, H. Inokuchi, B. M. Schmid, N. Karl, J. Chem. Phys. 1985, 83, 5413. [25] M. Fingerle, PhD thesis, University of Technology Kaiserslautern (Germany), 2014. [26] H. Ishii, K. Sugiyama, E. Ito, K. Seki, Adv. Mater. 1999, 11, 605. [27] M. Knupfer, Appl. Phys. A 2003, 77, 623. [28] N. Robertson, Angew. Chem. Int. Ed. 2006, 45, 2338; Angew. Chem. 2006, 118, 2398. [29] J. Wu, G. Yue, Y. Xiao, J. Lin, M. Huang, Z. Lan, Q. Tang, Y. Huang, L. Fan, S. Yin, T. Sato, Sci. Rep. 2013, 3, 2058. [30] M. Fingerle, M. Hemgesberg, S. Lach, W. R. Thiel, Ch. Ziegler, unpublished results. [31] C. S. Barkschat, S. Stoycheva, M. Himmelhaus, T. J. J. Mller, Chem. Mater. 2010, 22, 52. [32] D. A. Shirley, Phys. Rev. B 1972, 5, 4709. [33] X. Li, Z. Zhang, V. E. Henrich, J. Electron Spectrosc. Relat. Phenom. 1993, 63, 253.

To simulate the complex electronic structure of the C 1s signal, an equivalent core approximation was used. It explicitly takes into account relaxation effects, due to the photoemission process.[38] Thereby, the photoionization process is simulated by assuming an additional proton in the nucleus. In a Hartree–Fock calculation, the electrostatic potential of the ionized core with an atomic number of Z can then be simulated by using a cation of the next higher atomic number (Z + 1; for carbon with Z = 6, this is nitrogen N + with Z = 7). The chemical shift in the binding energy of an electron in the C 1s state is then given as [Eq. (1)]: DEb ðC1sÞ ¼

1 e eC1s;reference þ eNþ 1s eNþ 1s;reference 2 C1s

ð1Þ

in which e is the Hartree–Fock orbital energy. Here, methane (CH4) was used as the reference molecule for the (Z + 1) approximation. The calculated energy levels were convoluted using a pseudoVoigt function [Eq. (2)]: " f ðE Þ ¼ y0 þ A mu

# pffiffiffiffiffiffiffiffiffiffiffi 4 ln 2 4wln 2ðxxC Þ2 2 wL ffiffiffiffiffiffiffiffiffi p þ ð 1 m Þ e u p 4ðx xc Þ2 þwL2 pwG G

ð2Þ inwhich wl is the FWHM of the Lorentzian function, wg is the FWHM of the Gaussian function, and mu is the ratio between the Lorentzian and Gaussian distributions. For the ratio, a constant factor mu = 0.2 was chosen and the Lorentzian FWHM was varied with the binding energy of the experimentally probed energy levels, as stated in Table 4. For comparability between the theoretical data and the experimental UPS spectra, a scaling factor of 1.2 was applied.[39]

Table 4. Fit parameters for the convolution of the calculated energy levels using a pseudo Voigt function.

valence band S 2p C 1s N 1s O 1s

mu

wl

wg

0.2 0.2 0.2 0.2 0.2

0.7 1.2 1.4 1.6 1.8

0.7 1.4 1.4 1.4 1.4

Acknowledgements We thank Dimitri Imanbaew for performing the UV/Vis measurements under the supervision of Yvonne Schmitt. Additionally, we




9


&

Articles [34] G. Schedin, R. I. G. Uhrberg, Rev. Sci. Instrum. 1997, 68, 41. [35] D. R. T. Zahn, G. N. Gavrila, M. Gorgoi, Chem. Phys. 2006, 325, 99. [36] R. Schlaf, P. G. Schroeder, M. W. Nelson, B. A. Parkinson, C. D. Merritt, L. A. Crisafulli, H. Murata, Z. H. Kafafi, Surf. Sci. 2000, 450, 142. [37] G. A. Martinez-Castanon, M. G. Sanchez-Loredo, J. R. Martinez-Mendoza, F. Ruiz, Adv. Technol. Mater. Mater. Process. J. 2005, 7, 171. [38] S. Hfner, Photoelectron Spectroscopy, Springer, New York, 2003.

&



[39] J. Hwang, E.-G. Kim, J. Liu, J.-L. Bredas, A. Duggal, A. Kahn, J. Phys. Chem. C 2007, 111, 1378.

Received: February 4, 2015 Published online on && &&, 0000

10



ARTICLES UV-sensitive molecules for organic photovoltaics: Six different phenothiazines with acceptor groups attached to the central nitrogen atom were symmetrically substituted with thiophene donor groups (see picture) and studied by using photoelectron spectroscopy and complementary methods. UV sensitivity and light emission in the blue region were observed. With a carboxylic group as the N-substituent, the dithienylated phenothiazine can be utilized as a dye for TiO2-based photovoltaics.




M. Fingerle, M. Hemgesberg, Y. Schmitt, S. Lach, M. Gerhards, W. R. Thiel, C. Ziegler* && – && Photoemission Studies on NSubstituted Dithienylated Phenothiazines

11


&

[Studies on antipsychotic drug of phenothiazines. I. Electric conductivity of their aqueous solutions (author's transl)].

Letter: Sudden death while on phenothiazines.

Letter: long-acting phenothiazines.

Phenothiazines and the electrocardiogram.

Antitumor properties of phenothiazines.

Influence of Al fraction on photoemission performance of AlGaN photocathode.

Long-acting phenothiazines in alcoholism.

Estimate of the Coulomb correlation energy in CeAg2Ge2 from inverse photoemission and high resolution photoemission spectroscopy.

Study of the metabolism of phenothiazines: determination of N-demethylated phenothiazines in urine.

Momentum resolution in inverse photoemission.

Photoemission studies of the near EF spectral weight shifts in FeSe1-xTex superconductor.

Urinary incontinence during treatment with depot phenothiazines.

Urinary incontinence during treatment with depot phenothiazines.

Gate-tunable photoemission from graphene transistors.

Trypanosoma cruzi: effect of phenothiazines on the parasite and its interaction with host cells.

Effect of formulation of intramuscular injections of phenothiazines on duration of activity.

Revisiting photoemission and inverse photoemission spectra of nickel oxide from first principles: implications for solar energy conversion.

The use of phenothiazines during pregnancy and lactation.

A subset of N-substituted phenothiazines inhibits NADPH oxidases.

Novel non-neuroleptic phenothiazines inhibit Mycobacterium tuberculosis replication.

Photoemission performance of thin graded structure AlGaN photocathode.

Rapid fluorimetric determination of phenothiazines employing in situ photochemical oxidation.

Potential CNS antitumor agents-phenothiazines II: fluphenazine analogs.

Chlorpromazine and related phenothiazines inhibit the ATP-sensitive K+ channel.