Bay-Extended, Distorted Perylene Esters Showing Visible

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Bay-Extended, Distorted Perylene Esters Showing Visible Luminescence after Ultraviolet Excitation: Photophysical and Electrochemical Analysis Joachim Vollbrecht, Christian Wiebeler, Adam Neuba, Harald Bock, Stefan Schumacher, and Heinz-Siegfried Kitzerow J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b00954 • Publication Date (Web): 16 Mar 2016 Downloaded from http://pubs.acs.org on March 17, 2016

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The Journal of Physical Chemistry

Bay-extended, Distorted Perylene Esters Showing Visible Luminescence after Ultraviolet Excitation: Photophysical and Electrochemical Analysis Joachim Vollbrecht,a,b, Christian Wiebeler,b,c, ‡ Adam Neuba,d, ‡ Harald Bock,e Stefan Schumacher,b,c and Heinz Kitzerowa,b,* a.

Department of Chemistry, Physical Chemistry, University of Paderborn, Warburger Str.

100, 33098 Paderborn (Germany). b.

Center for Optoelectronics and Photonics Paderborn, University of Paderborn, Warburger

Str. 100, 33098 Paderborn (Germany). c.

Department of Physics, Theoretical Physics, University of Paderborn, Warburger Str.

100, 33098 Paderborn (Germany). d.

Department of Chemistry, Inorganic and Analytical Chemistry, University of Paderborn,

Warburger Str. 100, 33098 Paderborn (Germany). e.

Centre de Recherche Paul Pascal (CRPP), CNRS, Université Bordeaux, 33600 Pessac

(France).

ACS Paragon Plus Environment

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ABSTRACT Perylene derivatives with a unilaterally and bilaterally extended core show luminescence in the visible wavelength range (500 – 600 nm), which can be excited by absorption in the ultraviolet range (360 – 370 nm). This unusual behavior is investigated by means of excitation spectroscopy, cyclic voltammetry and calculations based on (timedependent) density functional theory. The results indicate that the extended compounds show promising features for optoelectronic applications and might be even used as fluorescent dyes in lasing. This is supported by nonadiabatic ab initio molecular dynamics. With respect to applications in organic optoelectronic nanostructures, nanofilms were prepared via spin coating and thermal vapor deposition demonstrating the formation of excimers. The relationship between the excimer induced bathochromic shift and the interplanar distance of the molecules opens up the possibility to vary the perceived color of a nanofilm via tempering.

Introduction Organic semiconductors (OSC) have shown to be promising materials for the application in electronics since working devices using OSCs were demonstrated.1 Tang and van Slyke2 as well as Burroughs et al.3 were able to show that both, low molar mass compounds and semiconducting polymers, respectively, are eligible for the design of organic light emitting diodes (OLED). While the performance of organic semiconductors does not match their inorganic counterparts,4 they do distinguish themselves due to their low cost in production, applicability on flexible substrates and as areal light sources.5,6 Compounds that additionally exhibit liquid crystalline (LC) characteristics have been of particular interest in recent years owing to their self-organizing properties.7 Calamitic8 and discotic9 mesophases offer enhanced charge carrier mobilities due to an enhancement in π-π stacking.10


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A wide variety of different devices implementing liquid crystalline organic semiconductors (LC-OSC) have been reported.11 For instance the application of white OLEDs as an areally emitting alternative to more common point light sources, such as the classical incandescent lamps or inorganic LEDs, has been a goal for organic electronics in general12 and for liquid crystalline organic semiconductor research in particular.13 Another promising field for LC-OSCs is display technology. While liquid crystal displays are nowadays ubiquitous, LC-OSCs can be applied in very thin, flexible OLED displays. The tuning of the emitted color plays a crucial role and can be tailored by changing the chemical structure of the emitter material,14 by combination of several emitting materials15 or by employing a resonant cavity enhanced (RCE) OLED architecture.16-18 Furthermore, LC-OSCs have potential in a plethora of other fields, including organic photovoltaics, where the creation of flexible solar cells at economical production costs is envisioned,19,20 organic field effect transistors, which would allow all-organic devices,21,22 and chemical or thermal sensors.23 For developing these applications, it is necessary to design new compounds to keep the field of organic electronics advancing. Polyaromatic hydrocarbons in general as well as perylene and its vast amount of derivatives specifically have been in the spotlight of organic chemists and material scientists.24 For instance, a lot of different types of functional groups and side chains have been reported for perylene compounds at the peri-, ortho- and bay-positions (Fig. 1),25 including linearly extended derivatives of the aromatic core of perylene imides at the latter position.26,27


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Figure 1. Peri- (red), ortho- (green) and bay-positions (blue) of the perylene molecule. In contrast, perylene esters have not been as widely studied, although it is possible for instance to obtain liquid crystalline mesophases without having to resort to long side chains.28,29 The extension of the perylene core with sterically demanding groups can form [4]-helicene fragments, which can induce LC mesophases due to the steric stress in the core of the molecule. Furthermore, these extensions can cause changes in the absorption spectrum, which are not as easy to predict as the alterations that their linearly extended counter parts undergo.30 Thus, a more in-depth examination of the absorption, fluorescence and electrochemical behavior of this group of compounds is required. In this paper we report the further investigation of these characteristics focusing on perylene3,4,9,10-tetracarboxylic tetraethylester (1), the unilaterally extended perylene derivative phenanthro[1,2,3,4,ghi]perylene-1,6,7,12,13,16-hexacarboxylic hexaester (2) and the bilaterally extended

perylene

derivative

dinaphtho[1,2-a:1’,2’-j]coronene-8,9,18,19-tetracarboxylic

butylester (3) (Fig. 2).


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Figure 2. Perylene-3,4,9,10-tetracarboxylic tetraethylester 1, phenanthro[1,2,3,4,ghi]perylene1,6,7,12,13,16-hexacarboxylic

hexaester

2,

dinaphtho[1,2-a:1’,2’-j]coronene-8,9,18,19-

tetracarboxylic butylester 3 and the corresponding 2-ethylhexylester 4. Since the comparison is focused on the changes due to the extension of the core, the side chains of the compounds were chosen to be as short as feasible (methyl-, ethyl- or butyl-groups). A detailed synthetic route to obtain these compounds can be found in the literature.29,30

Results and Discussion Absorption Spectroscopy Previously we reported the unexpected lack of a red-shift after extending the aromatic core of perylene compounds with naphthalene groups at the bay-positions.30 This behavior was explained with the non-planarity of the extended cores owing to steric inhibition (Fig. 3). While the molecular size of the cores increases, the size of the conjugated system remains the same. Thus a red-shift due to an expansion of the aromatic system, which has been demonstrated for linearly extended cores,25 was not observed. These results were in agreement with calculations based on density functional theory (DFT).


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Figure 3. Three dimensional model of compounds 1, 2 and 3 showing the distortions in the extended cores. The biggest difference in the absorption between compounds 1, 2 und 3 turned out to be a significant increase of the absorption in the regions of λ = 300 – 375 nm. While the unaltered perylene ester 1 shows no absorption in that wavelength region at all, the unilaterally extended derivative 2 already has its strongest absorption in that region. The bilaterally extended derivative 3 shows an even bigger absolute absorption in that range with a molar attenuation coefficient ε of up to 117000 L•mol-1•cm-1 (Fig. 4 (a)). These successive changes can be attributed to strong charge-transfer excitations from the outer parts of the molecule to its inner core. Additionally, all compounds show a minor absorption band in the region of λ = 250 – 290 nm. The dependence of the absorption on the polarity of the solvent was also investigated by measuring spectra not only in CHCl3, but also in toluene. It was found that the polarity of the solvent has only a minor influence on the absorption (see supplement).


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Figure 4. (a) Experimental, absolute (thick graphs) and calculated, vertical (thin graphs) absorption of compounds 1 (black lines), 2 (red dashes) and 3 (blue dots). (b) Calculated, vibrational (thin graphs) and experimental, absolute (thick graphs) absorption relative to the lowest energy peak of the respective series of 1 (black lines), 2 (red dashes) and 3 (blue dots). All quantum chemical calculations presented in this article are based on (time-dependent) density functional theory using the Gaussian 09 suite of programs.31 The vertical absorption was calculated modelling the complete molecules. In the subsequent and computationally more demanding calculations, the side chains were replaced by hydrogen atoms, because they have a negligible influence on the optical properties. The calculation of vertical absorption spectra, i.e. excitation energies obtained for an optimized geometry, was done using PBE032,33/6-311G(d,p) for structure optimizations and PBE0/6311+G(d,p) for excited state calculations, analogous to reference 30. The energies of the corresponding Kohn-Sham orbitals were also obtained using the latter basis set. Absolute absorption is determined analogously to reference 34 using a Gaussian broadening of 0.19 eV. The results shown in Fig. 4 (a) verify that the methodology used yields in general good agreement for the positions of the absorption maxima compared to the first maximum of the corresponding vibronic series, which is found at 429 nm for compound 3. Also, the strength of absorption is similar for the calculated and measured spectra with an exception for the absorption of compound 2 at 360 nm, which is underestimated in the calculations. In order to understand the structure of the low energy absorption found in experiment, i.e. the first excited state of compound 1 and the first two excited states of compounds 2 and 3, vibronic spectra were also calculated similar to references 30 and 35 using the method described in reference 36. The same functional as for the calculation of vertical excitations, i.e. PBE0, was


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chosen and the 6-31G* basis set was used as a compromise between accuracy and computational effort. As shown in Fig. 4 (b), the vibronic progression with regard to the energies relative to the lowest energy peak of the series and the absolute absorption is well reproduced in the quantum chemical calculations for compounds 1 and 2. The calculated absolute absorption of compound 3 is however higher than in experiment, which might be attributed to the presence of another molecular species: If there was a mixture of two molecules, the absolute absorption determined based on the assumption that only one isomer is present would be too low. Following this train of thought and assuming that the absolute absorption of the calculations is in good agreement with the experiment, it can be concluded that about two thirds of the molecules in solution have the structure of compound 3.

Fluorescence Spectroscopy The differences in fluorescence between compounds 1, 2 and 3 had not been studied previously. Thus, these three compounds were solved in chloroform and emission as well as emission-excitation plots were measured (Fig. 5 and Fig. 6, respectively). The emission of the three compounds is in a similar range (λem = 450 – 550 nm) and no red-shift can be detected with the extension of the core. Furthermore, a considerable emission begins for compounds 2 and 3 at an excitation of λexc = 300 nm, which is in good agreement with the absorption detected at the same range (Fig. 4). This indicates that the absorption peaks in the region of λ = 300 – 375 nm, which increase with the extension of the core, contribute in a major way to the fluorescent emission of compounds 2 and 3. Cross sections of the important regions in the plots of Fig. 6 can be found in the supplementary material.


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Figure 5. (a) Relative fluorescence of compounds 1 (black line; λexc = 470 nm), 2 (red dashes;

λexc = 360 nm) and 3 (blue dots; λexc = 365 nm) in a CHCl3 solution. (b) Normalized fluorescence (thick graphs) and calculated spectra (thin graphs) of compounds 1 (black lines), 2 (red dashes) and 3 (blue dots) relative to the lowest energy peak of the respective series.


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Figure 6. Normalized emission λem at varying excitation λexc of compounds 1, 2 and 3 as 1⋅10-3 M solution in CHCl3. The fluorescence measurements were performed in a 90° orientation if not further specified. While the polarity of the solvent does not seem to influence the absorption of the compounds, fluorescence in general is more dependent on solvent changes. Thus, fluorescence measurements with CHCl3 and toluene were also carried out, showing neither shifts of the peaks nor considerable changes in the intensity of the fluorescence (see supplement). It is known that perylene derivatives such as perylenetetracarboxylic dianhydride (PTCDA) show significant changes in the fluorescence depending on the concentration of the solution and the aggregation state in general.37 Also for perylenetetracarboxylic tetraesters, such as compound 1, a red-shift of the emission with less pronounced vibrational structure has previously been detected with increasing concentration of the solution and especially if the compound is in a solid state.38 This was attributed to the increasing fluorescence resulting from excimers (excited dimers; photodimers). These are formed if an electronically excited monomer molecule A* reacts with an unexcited monomer molecule A (Fig. 7). The excitation usually takes place due to the absorption of a photon, but this effect has also been observed for electrically driven emission in OLEDs. Stevens and Ban described two separate contributions to the red-shift.39 If the interplanar distance between the two monomer molecules decreases, then an increase of their ground state energy by a repulsive energy ∆ER is the consequence. Furthermore, the formation of an excimer AA* reduces the energy of the excited state by an association enthalpy ∆HA. Both, ∆ER and ∆HA cause a red-shift of the fluorescence. At the same time, no change in the absorption should be observable, regardless of whether the compound is in solution or in the solid state.


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Figure 7. Energy levels that are of importance during the formation of excited dimers (excimers) AA* out of unexcited monomers A and excited monomers A*. ∆Emonomer (green) is the energy gap of monomer fluorescence, ∆Eexcimer is the energy gap of excimer fluorescence, ∆HA is the energy released after photoassociation and ∆ER is the increase of the ground state due to a smaller interplanar distance d.33 The fluorescence of compounds 1, 2 and 3 as solutions in CHCl3 with different concentrations (c = 4•10-7 M, c = 1•10-5 M, c = 2•10-5 M, c = 1•10-3 M) and as solid thin films was therefore measured (Fig. 8). The thin films were deposited onto a glass substrate via spin coating or thermal vapor deposition (TVD) ranging in thickness from 30 – 100 nm. Then they were measured in a 180° orientation including filters to protect the detector from the excitation radiation. All three compounds show a red-shift of the emission maximum λem,max in the range of 20 – 30 nm with increasing concentration c. While the formation of excimers seems like a valid explanation for this observation, one has to be careful since the absorption and fluorescence spectra of all compounds overlap in the region of 450 – 500 nm, possibly enabling selfabsorption (see supplement).


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Figure 8. Normalized fluorescence of 1, 2 and 3 at c = 4·10-7 M (black line), c = 1·10-5 M (red short dots), c = 2·10-5 M (blue short dashes), c = 1·10-3 M (orange dash-dots) and as thin films deposited via spin coating (gray dashes) and thermal vapor deposition (green dots). Instead of compound 3 an ethylhexyl-tetraester derivative was evaporated. All thin film measurements were performed in a 180° orientation, including filters to protect the detector from the excitation. However, a significant red-shift can be observed for compounds 1 and 2 (108 nm and 90 nm, respectively) and to a lesser extent for compound 3 (50 nm) in the emission of thin films. In this case, the formation of excimers with a red-shift as a result is the most likely explanation, since there is no significant overlap between absorption and fluorescence. Furthermore, the absorption spectra of the thin films show no remarkable change, except for spectral broadening and a small bathochromic shift (see Fig. 9 and the literature38), which also excludes a reaction or degradation of the compounds during evaporation.

Figure 9. Normalized absorption of compound 2 solved in CHCl3 (black line), as thin film deposited via spin coating (blue dashes), as thin film deposited via thermal vapor deposition (orange dash-dots) and thermally evaporated compound solved again in CHCl3 (red dots). Normalized absorption of compound 3 solved in CHCl3 (black line) and as thin film deposited via spin coating (red dots).


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For the bilaterally extended compound (3) three fluorescence peaks can be observed. At the highest concentration, the peak at the shortest wavelength stops being the main emission peak, whereas the mid-wavelength peak shows the strongest emission. This is also the case for the spin coated thin film and might be due to the incremental increase of excimer emission. Since it was not possible to thermally evaporate compound 3, its derivative with 2-ethylhexyl chains 4 was used instead. The general trend of a bathochromic shift in comparison to the solution is still demonstrated. The significance of these changes is especially noteworthy in the context of color perception. For this reason the chromaticity coordinates of the emission spectra presented in Fig. 8 were calculated based on the 1931 CIE chromaticity color space (Fig. 10 and supplement).

Figure 10. 1931 CIE chromaticity diagram of compounds 1 (black squares), 2 (red circles), 3 (blue triangles) and with the D65 whitepoint, sRGB triangle and Planckian locus. Open symbols


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depict solutions of different concentration in CHCl3, filled symbols describe thin films and the blue diamond represents the 2-ethylhexyl-tetraester derivative of 3 which was evaporated. The exact color coordinates are listed in the supplement. All the described forms of absorption and emission are summarized in a diagram (Fig. 11) in which the changes owing to the successive extensions of the polyaromatic core are easily recognizable. The main distinction between compounds 1, 2 and 3 are the energy levels that can be explained by charge-transfer effects which are dominant in compounds 2 and 3 (S2), but are not present for compound 1. The absorption between levels S0 and S1 are similar for all compounds, while the energy gap between A+A and AA* (excimers) increases with the core size. The layout of the energy levels resembles a four-level system and indicates a possible application of these compounds in lasing, which will be discussed further in the last section.

Figure 11. Energy states of compounds 1 (black), 2 (red) and 3 (blue). The vibrational levels of the main energy levels are depicted with a dotted line. Absorption and emission of photons are described with an arrow with a solid line. Arrows with dashed lines are non-radiative transitions.


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Using the same methodology as for the determination of vibronic absorption spectra (see above), the fluorescence can be determined, too. In this case, it is assumed that according to Kasha’s rule fluorescence starts from the vibrationally relaxed first excited state.40 This leads to good agreement regarding the energies relative to the highest energy peak and the relative intensities of the vibronic progression (see Fig. 5). We note that errors connected to the calculation of vibronic fluorescence spectra are expected to be larger than in the case of absorption.41 Nonetheless, it can be concluded that Kasha’s rule holds in the case of the molecules studied: upon excitation with higher photon energies, the molecules emit from the vibrationally relaxed first excited state.

Interplanar Distances Noteworthy is also the difference in the scale of the observed red-shift of the three compounds as thin films compared to their respective solutions. Considering the mechanism of excimer formation proposed by Stevens and Ban, the necessity of a closer look at the interplanar distances d of the compounds in solid state seems to be self-evident. Thus X-ray diffraction (XRD) measurements of compounds 1, 2, 3 and 4 were performed, focusing especially on the interplanar distance d (Fig. 12). While the signal-to-noise ratio in the diffractogram for compound 1 is rather low, it is still possible to identify a diffraction spot at q = 18.3 nm-1 which corresponds to an interplanar distance of dexp;1 = 3.43 Å. The interplanar distances for compounds 2, 3 and 4 were determined as dexp;2 = 3.71 Å, dexp;3 = 3.99 Å and dexp;4 = 4.12 Å, respectively.


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Figure 12. X-ray diffractogram of compounds 1 (black), 2 (red), 3 (blue) and 4 (navy blue). The dominant reflexes at q = 15 – 20 nm-1 were interpreted as the interplanar distance, which is in good agreement with the literature.29,34


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Additionally, the experimental results for compounds 1 and 2 are also in good agreement with the literature (dlit;1 = 3.45 Å; dlit;2;untempered = 3.7 Å; dlit;2;tempered = 3.5 Å; also see Table 1).29,42 The difference in the value of d for compound 2 before and after tempering has been attributed to the formation of a liquid crystalline mesophase above 230 °C that also causes an increase of the density.29 Interestingly, the thermal vapor deposition of a thin film of compound 2 requires temperatures of 230 – 240 °C. At the same time a considerable red-shift in the emission can be observed, compared to the thin film deposited via spin coating, a process performed at room temperature. Thus, it is reasonable to assume that this red-shift can be explained due to a denser film forming during the deposition via thermal vapor deposition, with a smaller interplanar distance d and an even further shifted emission spectrum. To quantify the relationship between red-shift and interplanar distance the differences between the emission maxima of the solutions in CHCl3 (c = 4•10-7 M) and the emission maxima of the two types of thin films were calculated in electron volt (Table 1).

Table 1. Interplanar distances determined via X-ray diffraction dexp, values found in the literature dlit, 29,34 the bathochromic shift of the fluorescence emission between solutions (c = 4·10-7 M) and thin films created via thermal vapor deposition in ∆λshift and calculated energy difference ∆Eshift of compounds 1, 2, 3 and 4. For compound 2, dlit was determined for untempered (3.7 Å) and tempered (3.5 Å) samples.29


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Compound

dexp/ Å

dlit/ Å

∆λshift/nm

∆Eshift/eV

1

3.43

3.45

108

0.46

2tempered

---

3.5

90

0.43

2untempered

3.71

3.7

68

0.34

3

3.99

---

50

0.23

4

4.12

---

39

0.18

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This energy difference ∆EShift can also be put into relation with the mechanism explaining the formation of excimers (Fig. 7 and Eq. 1).

∆EShift = ∆HA + ∆ER = ∆Emonomer - ∆Eexcimer

(1)

A general trend can be observed, indicating that with a smaller value of d a higher value of ∆EShift can be expected. This is even more apparent when ∆EShift is plotted against d (Fig. 13).


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Figure 13. Diagram of energy difference ∆Eshift against interplanar distance d of compounds 1 (black square), 2 after tempering (red, open circle), 2 untempered (red, full circle), 3 (blue triangle), 4 (navy blue diamond) and the calculated results (black, open squares). The linear regression (gray dashes) has an adjusted R² of 0.9989. While the main focus of this study is the spectral change due to the extension of the core and not the variation of the side chains, these seem to have an influence on the spectrum due to their role in changing the interplanar distance d. This is demonstrated with the differences of d and subsequently ∆EShift between compounds 3 and 4, which share the same core structure (see supplement). To model the influence of the interplanar distance between two molecules, first the structure of a π-stacked dimolecular system based on compound 1 with ethyl side chains replaced by hydrogen atoms was optimized. Then the distance between two carbon atoms located at different molecules was fixed and relaxed scans of the ground state energy for larger and smaller distances were calculated. Subsequently, excitation energies were determined. For these calculations, the PBE0/6-31G* approximation was used to reduce the computational effort for the relatively large system. With this simple model, we do not attempt to model a film of organic molecules, for which more elaborate methods are needed.43 However, only taking dimolecular interactions into account, the qualitative linear trend found in experiment can be reproduced (Fig. 13).

Electrochemical Analysis In the context of organic semiconductors it is also important to determine the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO, respectively). For this reason


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electrochemical analyses of compounds 1, 2 and 3 were performed. The measurements were recorded with a Pt working electrode in CH2Cl2 containing 0.1 M [N(n-Bu)4]PF6 as supporting electrolyte. Ferrocene was used as internal standard, and all potentials are referenced versus the Fc/Fc+ couple. The cyclic and square wave voltammogrammes (see Fig. 14 and supplementary material) show the electrochemical behaviour of compounds 1, 2 and 3.

Figure 14. Cyclic voltammograms at v = 50 V/s (a) and Tauc plot (b) of compounds 1 (black line), 2 (red dashes) and 3 (blue dots). The value for the optical band gap ∆Egap can be determined by linearly fitting the energetically lowest flank of the Tauc plot and calculating the point of intersection with the energy axis All three compounds exhibit in the potential range from 0 to -2.5 V two single electron reduction processes, which could be assigned to the corresponding mono- and bivalent anions of compounds 1, 2 and 3 (Table 2). Table 2. Oxidation and reduction potentials (UOx and URed), electrochemically determined energies for the lowest unoccupied molecular orbitals (LUMOec) and highest occupied molecular orbitals (HOMOec), optical band gaps (∆Egap) and highest occupied molecular orbitals determined via optical band gap (HOMOopt) of compounds 1, 2 and 3.


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UOx/V URed1/V URed2/V LUMOec/eV HOMOec/eV ∆Egap/eV HOMOopt/eV 1 1.03 -1.53

-1.79

-3.47

-6.03

2.46

-5.93

2

---

-1.58

-1.82

-3.42

---

2.61

-6.03

3

---

-1.73

-1.97

-3.27

---

2.44

-5.71

In the anodic region from 0 to 1.2 V only 1 shows electrochemical activity. Cyclic- and square wave voltammogrammes show at 1.03 V vs. Fc/Fc+ the oxidation of 1 to the monokation 1+. To evaluate the reversible behavior of the observed electrochemical processes we determine the peak potentials and the cathodic and anodic peak currents Ipa and Ipc. These observables provide the basis for the diagnostics developed by Nicholson and Shain for analyzing the cyclic voltammetric response.44,45 The results of our detailed study are presented in tables S2 – S8 and figures F11 – F19 in the supplementary material. Our calculations show that a) the potentials of the forward peaks are nearly independent of the scan rate, b) the peak-to-peak separations, ∆E, are in the range of 70 to 80 mV at low scan rates (a typical value also for the Fc/Fc+ couple in CH2Cl2) and c) the ratios between the reverse and the forward peaks, Ipa/Ipc, are nearly 1. Furthermore the forward peak currents increase linearly as a function of the square root of the scan rates and their ratios, (Ipc/v)1/2, are constant according to the Randles-Sevcik equitation.46,47 To summarize, we suggest that the electrochemical reduction of the compounds 1, 2 and 3 and the oxidation of 1 possess reversible character with diffusion controlled electron transfer reactions at the electrode surface. Since it was not possible to measure both, the reduction and oxidation potentials (URed and UOx) for every compound, as an alternative the optical band gap ∆Egap was calculated via a Tauc


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plot and used to determine the electrochemically unavailable values for the HOMO or LUMO, respectively (see Fig. 14 and Table 2). For compound 1 the purely electrochemically determined value for HOMOec and the value for the optically determined HOMOopt deviate by an amount of 0.1 eV. Thus, it is reasonable to assume that the values of HOMOopt for compounds 2 and 3 are of the same magnitude as their HOMOec, which for the lack of UOx, were not directly obtainable.

Ultrafast Dynamics A further application for core-extended perylenes might be the use as fluorescent dyes in lasers. To pump the laser an optical excitation with a wavelength close to 350 nm may be used. Then, ultrafast relaxation into the first excited state should occur for the fluorescence. Finally to avoid losses, there should be no radiationless decay to the ground state, which would quench the fluorescence. Using nonadiabatic ab initio molecular dynamics for compound 2, the principle feasibility of this concept can be supported. For the nonadiabatic dynamics not only the energies of the excited states with high oscillator strengths are important, but also the energies of the darker ones. The latter might have a strong charge transfer character and therefore, it might be necessary to use a long-range corrected functional, e.g. CAM-B3LYP,48 to correctly determine these energies. To assess the reliability of the functionals, more accurate RI-CC249 calculations using Turbomole50 were done for an optimized geometry. The excitation energies obtained with PBE0, CAM-B3LYP and RI-CC2 are shown in the supplement. For the considered excitation energies, the largest deviation of the PBE0 values from the RI-CC2 ones is 0.16 eV whereas the smallest deviation of the CAM-


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B3LYP values from the RI-CC2 ones is already 0.26 eV. Thus PBE0 was chosen as functional for the dynamics as the excitation energies are reasonably well reproduced. Generation of initial conditions and the actual nonadiabatic dynamics after photoexcitation based on Trajectory Surface Hopping with Tully’s fewest switches algorithm51 were calculated for 2 interfacing G09 with Newton-X52 employing the PBE0/6-31G* level of theory. The spectrum simulation using an ensemble of molecular geometries, i.e. the nuclear ensemble approach,53 and the generation of initial conditions for simulating an excitation at about 366 nm for the nonadiabatic dynamics are based on a Wigner distribution (c.f. supplement). In the simulated spectrum 1000 geometries are taken into account and the parameters used are given in the supplement. Also the details of the initial condition generation and of the nonadiabatic dynamics can be found there. In total, 60 initial conditions were accepted. Nonadiabatic couplings were considered with the method of local diabatization and a time step of 0.5 fs was used.54 First insights were obtained calculating trajectories for 200 fs. Upon initial excitation, mainly the third and fourth excited states are populated (see Fig. 15). Within the simulated 200 fs, there is a pronounced decay of the populations in these states. Furthermore, a build-up of population in the first excited state is found. Thus ultrafast and efficient relaxation to this state can already occur without intermolecular interactions, due to the fact that energies of the excited states approach each other during the dynamics. Furthermore, the excitation energy of the first excited state is in all 60 trajectories always larger than 1.89 eV, preventing ultrafast radiationless decay to the ground state. An exemplary trajectory to illustrate these findings is shown in the supplement. This advanced theoretical analysis emphasizes the potential of the molecules studied in future laser applications.


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Figure 15. Populations of the states S1 (black line), S2 (red line), S3 (blue dots) and S4 (dark cyan dots) obtained from non-adiabatic ab initio molecular dynamics for a simulation time of 200 fs. Conclusions The spectroscopic studies reveal the influence of the core structure on the absolute absorption in the UV region (λ = 300 – 400 nm) and they underline the changes of the fluorescence in solution, which mostly consist of the excitation of the compounds with extended cores at shorter wavelengths. These results are summarized in a diagram (Fig. 11), which shows that the extended compounds might be used in organic lasing devices. The suitability of the core extended perylenes for this application is demonstrated by advanced ab initio simulations using non-adiabatic molecular dynamics.


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The excimeric behavior of nanofilms was also studied in detail, revealing a linear relationship between the interplanar distance d and the bathochromic shift resulting due to excimer formation. This also opens up the possibility to influence the color perception of a nanofilm simply through altering the interplanar distance d by tempering, using different side chains or a more steric structure in general. Furthermore the determination of the HOMOs and LUMOs is important for the application of these compounds in organic electronics devices with more than one layer. The fact that similar values were measured through purely electrochemical methods as well as with a combination of electrochemical and optical means, reduces the probability of inaccurate results. Therefore, future studies will focus on the application of core extended perylenes as fluorescent dyes in lasers and as emitter materials in OLEDs with the results of this study as their backbone.

Supporting Information. Additional material about the spectroscopic and electrochemical measurements as well as more details on the theoretical calculations can be found in the supplement. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Heinz Kitzerow: [email protected] ; Tel.: (+49) 5251 60 2156 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally.


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Funding Sources German Research Foundation (DFG: GRK 1464) ACKNOWLEDGMENT We thank the German Research Foundation (DFG: GRK 1464) for financial support and PC2 for computing time. S. Schumacher acknowledges support through the Heisenberg programme of the DFG. J. Vollbrecht thanks P. Schnippering, M. Schmitz, C. Weinberger and A. Paul for the XRD measurements. Furthermore, C. Wiebeler is grateful for the fruitful discussions with Felix Plasser regarding non-adiabatic ab initio molecular dynamics.. REFERENCES 1 McGinness, J.; Corry, P.; Proctor, P. Amorphous Semiconductor Switching in Melanins. Science, 1974, 183, 853-855. 2 Tang, C. W.; VanSlyke, S. A. Organic Electroluminescent Diodes. Appl. Phys. Lett., 1987, 51, 913-915. 3 Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay, K.; Friend, R. H.; Burns, P. L.; Holmes, A. B. Light-emitting Diodes Based on Conjugated Polymers. Nature, 1990, 347, 539-541. 4 Dimitrakopoulos, C. D.; Malenfant, P. R. L. Organic Thin Film Transistors for Large Area Electronics. Adv. Mater., 2002, 14, 99-117. 5 Garnier, F.; Hajlaoui, R.; Yassar, A.; Srivastava, P. All-Polymer Field-Effect Transistor Realized by Printing Techniques. Science, 1994, 265, 1684-1686.


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30 Vollbrecht, J.; Bock, H.; Wiebeler, C.; Schumacher, S.; Kitzerow, H. Polycyclic Aromatic Hydrocarbons Obtained by Lateral Core Extension of Mesogenic Perylenes: Absorption and Optoelectronic Properties. Chem. Eur. J., 2014, 20, 12026-12031. 31 Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et al. Gaussian 09 Revision D.01. 2009, Gaussian Inc., Wallingford CT. 32 Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett., 1996, 77, 3865-3868. 33 Adamo, C.; Barone, V. Toward Reliable Density Functional Methods without Adjustable Parameters: The PBE0 Model. J. Chem. Phys., 1999, 110, 6158-6170. 34 Stephens, P. J.; Harada, N. ECD Cotton Effect Approximation by the Gaussian Curve and Other Methods. Chirality, 2010, 22, 229-233. 35 Oltean, M.; Calborean, A.; Mile, G.; Vidrighin, M.; Iosin, M.; Leopold, L.; Maniu, D.; Leopold, N.; Chis, V. Absorption Spectra of PTCDI: A Combined UV-vis and TD-DFT Study. Spectrochim. Acta Mol. Biomol. Spectrosc., 2012, 97, 703-710. 36 Barone, V.; Bloino, J.; Biczysko, M.; Santoro, F. Fully Integrated Approach to Compute Vibrationally Resolved Optical Spectra: From Small Molecules to Macrosystems. J. Chem. Theory Comput., 2009, 5, 540-554. 37 Bulovic, V.; Burrows, P. E.; Forrest, S. R.; Cronin, J. A.; Thompson, M. E. Study of Localized and Extended Excitons in 3,4,9,10-Perylenetetracarboxylic Dianhydride (PTCDA) I. Spectroscopic Properties of Thin Films and Solutions. Chem. Phys., 1996, 210, 1-12.


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38 Benning, S.; Kitzerow, H.-S.; Bock, H.; Achard, M.-F. Fluorescent Columnar Liquid Crystalline 3,4,9,10-Tetra-(n-alkoxycarbonyl)-perylenes. Liq. Cryst., 2000, 27, 901-906. 39 Stevens, B.; Ban, M. I. Spectrophotometric Determination of Enthalpies and Entropies of Photoassociation for Dissolved Aromatic Hydrocarbons. Trans. Faraday Soc., 1964, 60, 15151524. 40 Kasha, M. Characterization of Electronic Transitions in Complex Molecules. Discuss. Faraday Soc., 1950, 9, 14-19. 41 Charaf-Eddin, A.; Planchat, A.; Mennucci, B.; Adamo, C.; Jacquemin, D. Choosing a Functional for Computing Absorption and Fluorescence Band Shapes with TD-DFT. J. Chem. Theory Comput., 2013, 9, 2749-2760. 42 Seguy, I.; Jolinat, P.; Destruel, P.; Mamy, R.; Allouchi, H.; Courseille, C.; Cotrait, M.; Bock, H. Crystal and Electronic Structure of a Fluorescent Columnar Liquid Crystalline Electron Transport Material. ChemPhysChem, 2001, 7, 448-452. 43 Megow, J.; Körzdörfer, 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. 44 Nicholson, R. S.; Shain, I. Theory of Stationary Electrode Polarography. Single Scan and Cyclic Methods Applied to Reversible, Irreversible and Kinetic Systems. Anal. Chem., 1964, 36, 706-723.


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45 Heinze, J. Cyclic Voltammetry – Electrochemical Spectroscopy. New Analytical Methods. Angew. Chem. Int. Ed. Engl., 1984, 11, 831-847. 46 Randles, J. F. B. A Cathode Ray Polarograph. Part II. – The Current-Voltage Curves. Trans. Faraday Soc., 1948, 44, 327-338. 47 Scholz, F. Electroanalytical Methods; Springer, Heidelberg, 2009. 48 Yanai, T.; Tew, D. P.; Handy, N. C. A New Hybrid Exchange-correlation Functional Using the Coulomb-attenuating Method (CAM-B3LYP). Chem. Phys. Lett., 2004, 393, 51-57. 49 Hättig, C.; Köhn, A. Transition Moments and Excited-state First-order Properties in the Coupled-cluster Model CC2 Using the Resolution-of-the-Identity Approximation. J. Chem. Phys., 2002, 117, 6939-6951. 50 TURBOMOLE V6.5 2013, a development of University of Karlsruhe and Forschungszentrum Karlsruhe GmbH, 1989-2007, TURBOMOLE GmbH, since 2007, available from http://www.turbomole.com. 51 Tully, J. C. Molecular Dynamics with Electronic Transitions. J. Chem. Phys., 1990, 93, 1061-1071. 52 Barbatti, M.; Ruckenbauer, M.; Plasser, F.; Pittner, J.; Granucci, G.; Persico, M.; Lischka, H. Newton-X: A Surface-hopping Program for Nonadiabatic Molecular Dynamics. WIREs: Comp. Mol. Sci., 2014, 4, 26-33. 53 Crespo-Otero, R.; Barbatti, M. Spectrum Simulation and Decomposition with Nuclear Ensemble: Formal Derivation and Application to Benzene, Furan and 2-Phenylfuran. Theor. Chem. Acc., 2012, 131, 1237.


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54 Plasser, F.; Granucci, G.; Pittner, J.; Barbatti, M.; Persico, M.; Lischka, H. Surface Hopping Dynamics Using a Locally Diabatic Formalism: Charge Transfer in the Ethylene Dimer Cation and Excited State Dynamics in the 2-Pyridone Dimer. J. Chem. Phys., 2012, 137, 22A514.

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Bay-Extended, Distorted Perylene Esters Showing Visible

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