Layers: Molecular Orientation, Electronic Structure, and Angular

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Well-Ordered 4CzIPN ((4s,6s)-2,4,5,6-tetra(9Hcarbazol-9Yl)isophthalonitrile) Layers: Molecular Orientation, Electronic Structure, and Angular Distribution of Photoluminescence Yuri Hasegawa, Yoichi Yamada, Masahiro Sasaki, Takuya Hosokai, Hajime Nakanotani, and Chihaya Adachi J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b03232 • Publication Date (Web): 25 Jan 2018 Downloaded from http://pubs.acs.org on January 26, 2018

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The Journal of Physical Chemistry Letters

Well-Ordered 4CzIPN ((4s,6s)-2,4,5,6-tetra(9Hcarbazol-9yl)isophthalonitrile) Layers: Molecular Orientation, Electronic Structure, and Angular Distribution of Photoluminescence

Y. Hasegawaa, *Y. Yamadaa, M. Sasakia, T. Hosokaib, H. Nakanotanic and C. Adachic

a

Institute of Pure and Applied Sciences, University of Tsukuba

1-1-1 Tennodai, Tsukuba, Ibaraki, 305-8573, Japan b

National Institute of Advanced Industrial Science and Technology (AIST)

1-1-1 Umezono, Tsukuba, Ibaraki, 305-8568, Japan c

Center for Organic Photonics and Electronics Research (OPERA), Kyushu University,

744 Motooka, Nishi, Fukuoka 819-0395, Japan.

*Corresponding author Yoichi Yamada, [email protected]

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Abstract We fabricated a well-ordered homogeneous monolayer of disk-shaped, carbazolyl dicyanobenzene (CDCB)-based thermally activated delayed fluorescence (TADF) molecule, i.e., 4CzIPN((4s,6s)-2,4,5,6-tetra(9Hcarbazol-9-yl)isophthalonitrile) with the Kagome lattice by slowly depositing 4CzIPN molecules at room temperature on flat Ag(111), Au(111) and Cu(111) surface. The second layer of the 4CzIPN was also found to be well ordered. The electronic states of the well-ordered monolayer and multilayer of 4CzIPN were found to be nearly unchanged from that of the isolated molecule calculated by the density functional theory (DFT), suggesting that the ordered layers retain the TADF properties. Indeed, we demonstrated the delayed fluorescence and the nearly perfect in-plane alignment of the transition dipole moment of a 4CzIPN thin film on glass substrate even in an ambient condition. These results indicate that the well-ordered films of the disc-shaped carbazole-based TADF molecules could potentially be utilized in organic light-emitting diode (OLED) devices with high light outcoupling efficiency.

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In the development of next-generation organic light-emitting diodes (OLEDs), increasing quantum efficiency of exciton-to-photon conversion (internal efficiency; ηint), and light outcoupling efficiency are critical for achieving ultra-high electroluminescence (EL) efficiency (external efficiency; ηext). Recently, extremely high ηint has been achieved in a thermally activated delayed fluorescence (TADF) process, which has been recognized as the 3rd-generation mechanism for OLEDs. In the TADF process, triplet state excitons are up-converted to singlet excitons (a reverse intersystem crossing; RISC) via a thermal excitation, and subsequently the singlet excitons emit delayed fluorescence (DF). Thus, in principle, TADF can realize 100 % of ηint without use of rare metals, and this is well established mechanism1,2. Therefore, achieving the high light outcoupling efficiency in the TADF-based devices is the next challenge. Without outcoupling engineering, the outcoupling efficiency of the conventional emitter layer is often limited in the range of 20 %. Consequently, the ηext of these devices are limited to the same range as the outcoupling efficiency of the emitting layer3. One promising approach to increase the outcoupling efficiency is to control the orientation of the emitter molecules. By controlling the molecular orientation, i.e., alignment of the direction of the transition dipole, the direction of the emitting light can be tuned4,5. Consequently, the horizontal alignment of the transition dipole moment



within the emitting layer and the resultant perpendicular emission of light increase the light outcoupling efficiency. Yokoyama et al. have demonstrated by controlling the molecular shape of emitters or intermolecular interaction of these emitters, that the degree of the horizontally aligned emitter molecules in the amorphous host-matrix molecules can be enhanced, and as a result, outcoupling efficiency can be increased6,7. For the alignment of the emitter molecules, single-component films without the host-matrix will be the most beneficial. Recently, Kim et al. demonstrated the advantage of using crystalline emitter layer. They showed ηext of as large as 40 % can be realized when 93 % of the transition dipole are aligned horizontally in “perfectly-ordered” single-component Pt(fppz)2-based phosphorescent emitter film8. However, a mechanism for control of the molecular orientation has not been established for TADF materials. While there have been some studies on un-doped films of TADF molecule with nearly 100 % of ηint, their outcoupling efficiency retain in the range of 20 % 9. In this paper, we report that well-ordered films of the carbazolyl dicyanobenzene (CDCB)-based TADF molecule such as 4CzIPN can be formed on the single crystalline metal substrates, in spite of the relatively complicated molecular structure of 4CzIPN as shown in Fig. 1(a). The monolayer of 4CzIPN showed a characteristic ordering with Kagome-lattice and their arrangement was independent of the type of substrate. Stacking


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of layers was also possible, resulting in a highly ordered flat multilayer film. From the photoemission and photoabsorption spectroscopy measurements, the electronic state of the molecules in the monolayer and multilayer of 4CzIPN was found to be almost identical to that of the monomer, indicating that the intermolecular and molecularsubstrate interactions were both weak, and therefore, the TADF property of the monomer could be preserved in the layers. In addition, angular distribution of the fluorescence revealed a nearly perfect horizontal alignment of the transition dipole moments. Therefore, the well-ordered 4CzIPN molecular layer could potentially be used as a TADF emitting layer with high light outcoupling efficiency. In addition, it can be used as a suitable model to investigate the fundamental properties of TADF materials.



First, we discuss the molecular arrangement of 4CzIPN monolayers on single crystalline metal substrates. Fig. 1(b) shows STM image of 4CzIPN monolayer on Ag(111) substrate. Homogeneous and highly ordered arrangement of 4CzIPN molecule with Kagome lattice was observed to cover the entire substrate surface. In the enlarged image (Fig. 1(c)), the molecule appears as a cluster of three bright protrusions. These protrusions arise because of the carbazole groups that are protruded, and hence, are readily captured by STM. It was found that one carbazole group was not visible, possibly due to a large steric hindrance.

Fig. 1 (a) Molecular structure of 4CzIPN. (b) STM image of 4CzIPN monolayer on Ag(111) and (c) its enlarged scan. STM images of 4CzIPN monolayers on (d) Cu(111) and (e) Au(111). Bias voltage was 2.5 V and tunneling current was 100 pA.


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We observed the same pattern of three protrusions for all the metal substrates examined in this work (see below). We also note that the bias dependence of the STM image was not clear and we basically observed the same appearance of the molecule in both occupied and unoccupied state imaging, at room temperature. Although the LUMO of 4CzIPN is localized at the cyano group as was suggested from theoretical calculation2, we speculate that the LUMO was not accessible in the STM image taken at room temperature due to steric hindrance by the protrusive carbazole groups. From the STM image, we consider that 4CzIPN molecules adsorb in nearly "flat on" configuration with the molecular axis (as defined in Fig. 1(a)) parallel to the surface and we thus marked the molecule with white triangles in the Fig. 1(c). The Kagome lattice is made up of the pair of 4CzIPN molecules with "antiparallel" ordering, as depicted in the figure. This can be interpreted to be due to the dipole interaction between molecules, as 4CzIPN molecule has net permanent dipole moment in the molecular axis. Similar molecular arrangement and pattern of 4CzIPN was also observed on Cu(111) and Au(111) substrate, as shown in Fig.1 (d) and (e), respectively. Kagome lattice is therefore suggested to be an intrinsic ordering of 4CzIPN monolayer on flat substrate, suggesting the substrate effect on the formation of this phase is weak.



With increasing coverage, a second layer with close-packing phase of 4CzIPN was subsequently formed on Ag(111). Fig. 2(a) shows the STM image of the 4CzIPN molecules with slightly larger than 1 ML on Ag(111). The area with different molecular stacking appears at the bottom of the image. In this phase, dense ordering of the molecule was observed as shown in Fig. 2(b) and this phase seemed to be the second layer. The area covered with new phase increased with increasing amount of deposition, till it finally covered the entire surface, suggesting the layer-by-layer growth of the second layer. In the present stage, the precise molecular orientation in the multilayer cannot be discussed. However, it can be considered that the molecular axis in the second layer have a certain angle with respect to the horizontal plane due to enhanced intermolecular interaction and reduced molecule-substrate interaction in the second layer, as is common for other πconjugated molecular systems. We have also shown that the multilayer of 4CzIPN is also well ordered with large and flat terraces, as demonstrated on the HOPG substrate10.

Fig. 2

(a) STM image of 4CzIPN/Ag(111) partially covered with the second layer and (b) enlarged

image of the second layer. Bias voltage was -2.5 V and tunneling current was 100 pA.


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Next, we discuss the occupied and unoccupied states of the molecular orbitals measured using UPS and NEXAFS, respectively for the 4CzIPN films on Ag(111). UPS spectra of 4CzIPN monolayer (orange) and multilayer (20ML, blue) on Ag(111) are shown in Fig. 3(a). HOMO level was observed at a binding energy of around 3 eV below the Fermi level of Ag(111), and no apparent in-gap state between HOMO level and the Fermi level was observed, indicating a negligible adsorption-induced charge transfer between molecule and substrate. In Fig. 3(b), we show the simulated density of states of an isolated 4CzIPN molecule by means of DFT calculations. The orbital energies of the simulated spectra are shifted so that the HOMO position coincides with the experimental spectrum. It is then seen that the experimental UPS spectrum of 4CzIPN monolayer

Fig. 3

(a) UPS spectrum taken from monolayer (orange, bottom line) and multilayer (blue, upper line)

of 4CzIPN/Ag(111). (b) DOS calculated for single 4CzIPN molecule.



matches well to the calculated results of the monomer, suggesting that the molecular orbitals in the monolayer are not different from those of the isolated molecule. It is also seen that the spectral shape of the multilayer is similar to that of the monolayer. This fact also suggests that the electronic structure of the monolayer is not special and the substrate effect on the molecular orbital is negligible. However, the relative intensities of each peaks in the multilayer were different from those in the monolayer. This difference is likely to be due to the difference in the molecular orientation. Fig. 4(a) shows C-K edge NEXAFS spectra taken from the monolayer (orange) and the multilayer (blue) of 4CzIPN on Ag(111) substrate. Incident angle of the X-ray is 30 degrees with respect to the surface normal. In both spectrum, distinct and broad peaks can be seen around 285 eV and above 290 eV attributable to the electronic transition from C 1s to π* and σ* orbitals, respectively. The shapes of the spectra for the multilayer and monolayer were almost identical, indicating again that the molecular orbitals in the monolayer were not modified by the substrate. From the calculated DOS shown in Fig. 4(b), we found that the DOS of π * orbitals corresponds well to the experimental NEXAFS spectrum. Note that the DOS of LUMO, localized at the cyano-group, is small compared to that of π* orbitals of the carbazole units. The small DOS of LUMO compared to that of the π* orbitals may be the reason why the carbazole units dominated


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the patterns in STM image for both the occupied and unoccupied states. We also note that the peak intensity of π* orbitals significantly changed with the incident angle, for both monolayer and the multilayer (not shown). The distinct polarization dependence of the π* intensity indicates that the carbazole groups have certain fixed orientations in the film, reflecting the molecular ordering. However, since one 4CzIPN molecule has four carbazole units with different orientations, it is difficult to analyze the experimental angular dependence of the π* peak intensity at the present stage.

Fig. 4 (a) C-K edge NEXAFS spectrum taken from monolayer (orange, bottom line) and multilayer (blue, upper line). 4CzIPN/Ag(111) (b) Empty-state DOS calculated for single 4CzIPN molecule.

In order to examine the orientation of the transition dipole moment of the ordered 4CzIPN film, we fabricated the multilayer of 4CzIPN (approximately 5 ML) on the flat



glass substrate using the slow deposition method as demonstrated above, and examined the angular distribution of the PL intensity. The AFM image of the 4CzIPN layer on the glass substrate (Fig. 5(a)) shows a flat and continuous surface with few voids. The RMS surface roughness was found to be as low as 0.2 nm. At the present stage, we have not determined the detailed molecular arrangement in the multilayer system. Instead, we evaluated the molecular alignment in the multilayer film by means of the angular dependence of the PL. The PL spectrum of 4CzIPN/glass is shown in Fig. 5(b), exhibiting the characteristic PL band centered at 540 nm, as expected from the results of previous studies reported for the neat films of 4CzIPN11. The occurrence of the DF was confirmed by the time resolved PL decay profile (at 540 nm) as shown in Fig. 5(c), even in the presence of atmospheric oxygen. The time decay of DF can be fitted well by the multiple exponential function shown as a red line in Fig. 5(c), as similar to the previous reports of 4CzIPN-doped films2. The dominant decay constants of prompt fluorescence (PF) and DF were deduced to be approximately 22 ns and 2.5 s, respectively. The time constant of DF of 4CzIPN/glass was closer to that of a doped film (4s) and monomer dispersed in toluene with N2 bubbling (5 s), rather than that of a monomer dispersed in toluene with O2 bubbling (9 ns)2,11. These observations indicated that the observed PL was TADF. Fig. 5(d) shows the angle-dependence of the PL intensity of the p-polarized fluorescence


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at 550 nm. The angular dependence showed a characteristic curve of the emission from the horizontally aligned transition dipoles. From the analysis of the angle-dependence of the PL by the simulation described in ref. 12, we can deduce the orientational order parameters of the transition dipole S = (pz2−px2)/(pz2+2px2)12 , where px, py and pz are the horizontal and the vertical transition dipole moment, respectively. Using the refractive indexes of 1.77 and 1.50 of 4CzIPN13 and glass, respectively, we deduced S of 4CzIPN/glass to be –0.498. This value of S indicates a horizontal transition dipole ratio,

Fig. 5 (a) AFM image of approximately 5ML of 4CzIPN on glass. (b) PL spectrum, (c) Normalized time-resolved PL decay at wavelength of 540 nm, and (d) angular dependence of p-polarized PL spectra from multilayer of 4CzIPN on glass substrate at a wavelength of 550 nm. The emission angle θ is the angle from the surface normal. The red line imposed on the data in (c) is the result of the fitting with the multiple exponential function. 13 ACS Paragon Plus Environment


(px+py)/(px+py+pz),of 0.97, i.e., an almost perfect in-plane alignment of the transition dipole moment in the film. It is worth noting that the derived value of  for 4CzIPN/glass is much higher than that reported for the doped 4CzIPN film of 0.7314. The enhancement in in the case of 4CzIPN/glass should be as a result of the well-ordered nature of the single-phase 4CzIPN film. The formation of ordered monolayer and multilayer of 4CzIPN molecules and the resulting alignment of the transition dipole moment mean that this is a promising option for enhancement of the outcoupling efficiency. Other carbazole-based disc-shaped TADF molecules such as 2CzPN and 5CzBN can also form ordered films 10, despite their relatively complicated molecular structures. This tendency of carbazole-related discshaped TADF molecules to form a well-ordered film make them suitable candidates for use in emitting layer with controlled molecular orientation, although we have to determine the degree of quenching of TADF in the well ordered phase for the practical use. The outcome of our study of highly ordered molecules calls for further research to develop operational device using the well-ordered TADF films. In summary, we have fabricated well-ordered monolayer and multilayer of 4CzIPN and determined their molecular arrangement, electronic states and PL properties. STM images of the 4CzIPN monolayer on Ag(111) depicted homogeneous and highly 14 ACS Paragon Plus Environment

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ordered molecular packing of "flat on" molecules. Multilayers of 4CzIPN were found to be well-ordered with dense molecular packing. Electronic states of these layers measured by UPS and NEXAFS were almost identical to that calculated for a monomer of 4CzIPN, suggesting that the TADF properties of the monomer can be maintained in the film. Indeed, the flat multilayer of 4CzIPN fabricated on a glass substrate showed the DF even in the ambient condition. From the angular distribution of the p-polarized fluorescence, we deduced that the transition dipoles in the film are almost perfectly aligned in the horizontal plane, reflecting the well-ordered nature of the 4CzIPN film.



Experimental methods 4CzIPN synthesized using procedure detailed in a previous study2 was used in the present study. Clean single crystalline metal substrates such as Ag(111), Au(111) and Cu(111) were used as the substrate. Molecular layers of 4CzIPN were deposited on these substrates using a home-made ultra-high vacuum evaporation system equipped with a small homemade Knudsen-cell and quartz crystal microbalance, which enabled a very slow (1 monolayer (ML)/1000 s) and stable deposition of TADF molecules. Molecular arrangements were investigated using a scanning tunneling microscope (STM) system configured for stable molecular imaging at room temperature15–17. Multilayer of 4CzIPN was fabricated on a glass substrate (synthesized quartz crystal, S-grade(Ra

Layers: Molecular Orientation, Electronic Structure, and Angular

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