Z-shaped polycyclic aromatic hydrocarbons with embedded five-membered rings and their application in organic thin-film transistors

Abstract

Z-shaped polycyclic hydrocarbons with embedded five-membered rings as well as their diaza-analogues have been synthesized. Soluble congeners were obtained by the introduction of mesityl substituents and studied by cyclo- as well as differential pulse voltammetry revealing an amphoteric redox behavior. Unsubstituted congeners showed close π-stacking in single crystal X-ray diffraction analyses and thus promising characteristics for the application as semiconducting materials in p-channel thin-film transistors (TFTs). The hole transport characteristics of the corresponding devices were investigated by GIWAXS.

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Introduction

Polycyclic aromatic hydrocarbons with fused five-membered rings (also named cyclopenta-fused or short cp-PAHs) are a subclass of non-alternant PAHs with unique electronic and photophysical properties.^1,2 cp-PAHs often represent cut-outs of fullerenes such as C₆₀ or C₇₀ and have a high electron affinity.^3–8 Thus, cp-PAHs found application as semiconductors in electronic devices.^9–11 For example, the dibenzo[a,m]rubicene (Fig. 1),¹² a cut-out of C₇₀ with two five-membered rings in the aromatic backbone was used as p-type organic semiconductor with a field effect mobility of 1.0 cm² V⁻¹ s⁻¹ in thin-film transistors (TFTs).^13–15 Another cp-PAH with two five-membered rings, a derivative of acenaphto[1,2-k]fluoranthene¹⁶ (Fig. 1), was used as a green emissive dye in organic light-emitting diodes (OLEDs).¹⁷

Fig. 1 Top: Molecular structures of acenaphtho[1,2-k]fluoranthene¹⁶ (left) and 1,2:9,10-dibenzoheptazethrene²⁰ (right) and the hypothetical structural relationship to the dibenzoacenaphthofluoranthenes (DBAFs; middle). Bottom: Structure of rubicene (left),^22,23 dibenzo[a,m]rubicene¹² and diaza zethrenes.²⁷

Besides cp-PAHs, zethrenes are another class of Z-shaped PAHs with exceptional properties.^18,19 They have biradical character with indices up to γ₀ = 0.58. The γ₀-values were tuned by benzannulation as shown in the cases of the 1,2:9,10-, and the 5,6:13,14-dibenzoheptazethrenes (short: DBHZ; Fig. 1).²⁰ Furthermore, for organic field effect transistors (OFETs) based on single crystals of a DBHZ, a hole mobility up to 0.15 cm² V⁻¹ s⁻¹ was obtained.²¹

Rubicenes^14,22–25 and some of their extended congeners are rare examples combining both, a molecular Z-shape with embedded five-membered rings which is created by a phenyl–anthracenyl–phenyl scaffold (Fig. 1).²⁶ We envisioned to extend the portfolio of PAHs (or cp-PAHs) in this respect by a series of compounds which can be understood either as benzannulated acenaphtofluoranthenes or as 1,2:9,10-dibenzoheptazethrenes with two six-membered rings formally substituted by five-membered ones (shown in blue and red in Fig. 1). Therefore, these compounds are named dibenzoacenaphthofluoranthenes, abbreviated as DBAFs. The DBAFs have an “inverted” anthracenyl-phenyl-anthracenyl scaffold in comparison to rubicenes (Fig. 1).²⁶

By simple modifications of the molecular precursors, nitrogen substituted analogues (the DBAF-N₂s) are accessible to investigate the influence of nitrogen substitution on the compounds' properties which was for example realized for diazahepta and octazethrenes^27–29 (Fig. 1) and is generally rare in the field of cp-PAHs.^30,31

Results and discussion

The Z-shaped five-membered ring containing PAHs were obtained by Suzuki–Miyaura cross-coupling reactions followed by palladium-catalysed arylations under C–H activation starting from dichlorodibromobenzene 1³² or -pyrazine 2³³ (Scheme 1). The cross-coupling of 1 or 2 with 9-anthracenylboronic acid 3 gave the corresponding products 5 and 6 in 80% and 56% yield respectively. The mesityl-substituted congeners 7 and 8 were obtained from 1 and 2 with 10-mesityl-anthracenyl boronic pinacol ester 4 in 34% and 68% yield. Compounds 5–8 were fully characterized by NMR as well as IR spectroscopy, mass spectrometry and their constitution was unambiguously proven by single crystal X-ray diffraction (SCXRD; for details see ESI‡).

Scheme 1 Synthesis of DBAF, DBAF-N₂, mes-DBAF and mes-DBAF-N₂ from dibromodichlorobenzene 1 and dibromodichloropyrazine 2. (a) For 5 and 7: Pd(OAc)₂ (2.5–3 mol%), SPhos (5–6 mol%), K₂CO_{3 aq.} (1 M), THF, 85 °C 16 h; for 6 and 8: (b) Pd(dppf)Cl₂ (10 mol%), K₂CO₃ (1 M), THF, 85 °C, 16 h, (c) PdCl₂(PCy₃)₂, DBU, DMAc, 200 °C, 16 h.

Subjecting the four dichlorinated precursors 5–8 to typical conditions for arylations under C–H-activation (PdCl₂(PCy₃)₂, DBU, DMAc, 200 °C, 48 h)^34–41 gave after sublimation unsubstituted DBAF in 96% and DBAF-N₂ in 55% yield as well as the mesityl substituted mes-DBAF in 33% and mes-DBAF-N₂ in 69% after column chromatography (Scheme 1). The parent DBAF and DBAF-N₂ are poorly soluble in common organic solvents at room temperature and NMR spectroscopy had to be performed in ortho-dichlorobenzene-d₄ at elevated temperatures (323 K; see ESI‡). The successful pentannulations have been proven by 1D- and 2D-NMR spectroscopy as well as mass spectrometry (see ESI‡) and by SCXRD analyses (see discussion below).

The optical properties of the DBAF series were investigated by UV/vis- as well as fluorescence spectroscopy in oDCB (DBAF and DBAF-N₂) or dichloromethane (mes-DBAF and mes-DBAF-N₂) (Fig. 2 and Table 1). DBAF and DBAF-N₂ were only soluble in oDCB after heating and ultrasonication. DBAF shows an intensive absorption maximum at λ_abs = 349 nm and additional low energy absorption maxima at λ_abs = 461, 491, 518 and 550 nm. A red emission can be detected by bare eye, which corresponds to a maximum at λ_em = 572 nm and a shoulder at λ_em ∼ 620 nm with a Stokes shift of [small nu, Greek, tilde] = 699 cm⁻¹ and a photoluminescence quantum yield of φ = 21%.

Fig. 2 UV/vis absorption (solid lines) and fluorescence (dashed lines) spectra of DBAF (red) and mes-DBAF (orange) (top) and DBAF-N₂ (blue) and mes-DBAF-N₂ (violet) (bottom), measured in o-DCB (DBAF and DBAF-N₂) or CH₂Cl₂ (mes-DBAF) and mes-DBAF-N₂ at room temperature.

Table 1 Optical and electronic properties of the DBAF-series

Cmpd	λmax^a^,^b [nm]	λonset^b [nm]	Egap,opt^c [eV]	λem^b [nm]	[small nu, Greek, tilde]Stokes^b [cm^−1]	PLQY^b [%]	EIP,CV^d [eV]	EEA,CV^d [eV]	Egap,CV [eV]	EHOMO,DFT^e [eV]	ELUMO,DFT^e [eV]	EDiff,DFT^e [eV]
a Most-red-shifted absorption maximum.b DBAF and DBAF-N2 were measured in o-DCB at room temperature; mes-DBAF and mes-DBAF-N2 were measured in CH2Cl2 at room temperature.c Estimated from the corresponding λonset by Egap,opt = 1240/λonset.d Cyclic voltammograms (CV) of mes-DBAF and mes-DBAF-N2 in dichloromethane (c = 1 mM) using [Bu4N][PF6] (0.1 M) as electrolyte. CV scan speed 100 mV s^−1. EIP,CV = −(E^Ox1/2 + 4.8 eV); EEA,CV = −(E^red1/2 + 4.8 eV).e Calculated at the u-B3LYP/6-31G(d,p) level of theory.f Not determined due to solubility issues.
DBAF	550	581	2.1	572	699	21	—^f	—^f	—^f	−5.0	−2.4	2.6
DBAF-N2	555	582	2.1	577	687	5	—^f	—^f	—^f	−5.2	−2.6	2.6
mes-DBAF	561	589	2.1	588	819	20	−5.3	−3.0	2.3	−4.9	−2.4	2.6
mes-DBAF-N2	561	592	2.1	593	961	29	−5.6	−3.2	2.4	−5.1	−2.5	2.6

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mes-DBAF shows similar spectroscopic properties as its non-mesitylated analogue with a high intensity absorption band at λ_abs = 350 nm (DBAF: 349 nm) and again poorly resolved maxima at λ_abs = 460, 493, 526 and 561 nm and a slightly red-shifted emission with λ_em = 588 nm (DBAF: 572 nm) leading to a larger Stokes shift of [small nu, Greek, tilde] = 819 cm⁻¹ (φ = 20%, Table 1).

DBAF-N₂ shows two maxima at λ_abs = 379 and 399 nm and a less resolved pattern of overlapping adsorption maxima with a most red-shifted maximum at λ_abs = 519 nm with a shoulder at λ_abs = 555 nm. The corresponding emission maximum is found at λ_em = 577 nm with a shoulder at λ_em = 621 nm and a Stokes shift of [small nu, Greek, tilde] = 687 cm⁻¹ with φ = 5% (Table 1). Similarly, as for mes-DBAF and DBAF the spectral differences of mes-DBAF-N₂ and DBAF-N₂ are negligible (Fig. 2 and Table 1), indicating that the mesityl groups change the solubility without significantly influencing the optoelectronic properties of the molecular scaffolds.

As already mentioned, DBAF and DBAF-N₂ are poorly soluble in organic solvents so that decent cyclovoltammograms could not be recorded. Therefore, only the mesitylated derivatives mes-DBAF and mes-DBAF-N₂ have been investigated by cyclic as well as differential pulse voltammetry revealing an amphoteric redox behaviour in both cases (Fig. 3). Two reversible oxidations can be found for mes-DBAF at E¹_Ox = 0.52 V and E²_Ox = 0.83 V as determined by DPV (CV: E^Ox,1_1/2 = 0.56 V und E^Ox,2_1/2 = 0.86 V) and one reversible reduction was detected at E_red = −1.76 V (CV: E^red_1/2 = −1.80 V). For mes-DBAF-N₂ the corresponding oxidation waves appear at approx. 0.2–0.3 V higher potentials with E¹_Ox = 0.76 V and E²_Ox = 1.11 V (CV: E^Ox,1_1/2 = 0.76 V und E^Ox,2_1/2 = 1.13 V). The reduction is shifted by 0.14 V to higher potentials with E_red = −1.62 V (CV: E^red_1/2 = 1.63 V) indicating the electrochemical stabilization of PAHs by nitrogen substitution.^42–44 This is further displayed in the differences of the corresponding ionization potentials (mes-DBAF: IP = −5.32 eV; mes-DBAF-N₂: IP = −5.56 eV) as well as electron affinities (mes-DBAF: EA = −3.04 eV; mes-DBAF-N₂: EA = −3.18 eV) derived from this data. By UV/vis spectrsocopy it was demonstrated that the mesityl substituents, which are oriented orthogonal to the π backbone, do not significantly influence these electronically. Therefore, it is assumed that the redox behaviour of DBAF and DBAF-N₂ are comparable to mes-DBAF and mes-DBAF-N₂. By DFT calculations (u-B3LYP/6-31G(d,p); Fig. 4 and Table 1) a 0.19 eV lower HOMO energy of mes-DBAF-N₂ (E_HOMO = −5.12 eV) in comparison to mes-DBAF (E_HOMO = −4.93 eV) is found as well as a 0.14 eV lower LUMO energy (mes-DBAF-N2: E_LUMO = −2.52 eV; mes-DBAF: E_LUMO = −2.38 eV), which corroborates with the trend found by CV.

Fig. 3 Cyclic voltammograms (CV) and differential pulse voltammograms (DPV) of mes-DBAF (top) and mes-DBAF-N₂ (bottom) in dichloromethane (c = 1 mM) using [Bu₄N][PF₆] (0.1 M) as electrolyte. CV scan speed 100 mV s⁻¹; DPV: step size of 0.005 V, a modulation amplitude of 0.025 V a modulation time of 0.05 s and an interval time of 0.5 s.

Fig. 4 DFT-calculated frontier molecular orbitals on the u-B3LYP/6-31G(d,p) level of theory (LUMOs top; HOMOs bottom) of (from left to right) DBAF, DBAF-N₂, mes-DBAF and mes-DBAF-N₂ with the calculated energy levels.

Again, the mesityl substituents seem to only have a small influence to the energy levels with the HOMOs being lowered by only 0.09–0.1 eV and the LUMOs by only 0.04–0.06 eV for DBAF and DBAF-N₂ in comparison to their mesitylated congeners (Fig. 4 and Table 1) and the distribution of frontier molecular orbital coefficients are nearly the same for all four compounds (Fig. 4).

By thermal crystallization from ortho-dichlorobenzene as well as by sublimation isomorphic single crystals of DBAF suitable for SCXRD were obtained. DBAF crystallizes in the monoclinic space group P2₁/n with Z = 2. The crystalline packing is dominated by slipped π-stacked molecular columns (Fig. 5b and c) with a spacing of d_π–π = 3.33 Å propagating along the crystallographic a-axis. Edge-to-face π stacking with d_C–H⋯π = 2.70 Å and an angle of the π-systems of θ = 51.8° (Fig. 5c) leads to a herringbone like motif (Fig. 5d). Suitable crystals of DBAF-N₂ have also been obtained by sublimation. DBAF-N₂ also crystallized in the P2₁/n space group and again slipped π-stacked columns can be found, here with a slightly larger distance of d_π–π = 3.40 Å but with a larger overlap of two adjacent π systems (compare Fig. 5b and f). Similar edge-to-face π stacking is found but with a larger angle of θ = 73.1° and a larger distance between adjacent molecules (d_C–H⋯π = 2.87 Å), leading to a herringbone type packing as well (Fig. 5g and h). Single-crystals of mes-DBAF were obtained by slow evaporation of a chloroform solution and crystallized in the monoclinic space group P2₁/n with Z = 2. This time with one enclathrated disordered molecule of chloroform per mes-DBAF (Fig. 5). Due to the almost perpendicular oriented (87.6°) mesityl groups, no direct π stacking of the DBAF central backbone is found (Fig. 5i). Instead, the crystalline packing is dominated by C–H⋯π-interactions of the para-methyl-group of a mesityl substituent with the central benzene ring of the DBAF backbone (d_C–H⋯π = 2.34 Å) leading to a loose herringbone-like arrangement with chloroform molecules in its voids (Fig. 5j–l). Crystals of mes-DBAF-N₂ were obtained by vapor phase diffusion of n-hexane into a saturated chloroform solution. mes-DBAF-N₂ also crystallized in the monoclinic space group P2₁/n with Z = 2. In contrast to mes-DBAF, mes-DBAF-N₂ shows linear columns of face-to-face π stacks along the crystallographic a-axis (Fig. 5n and p). Within these stacks two symmetrically independent molecules with unparallel π-planes and a shortest distance of d_π–π = 3.22 Å are found in a rare twisted offset^45–47 with an angle of ∼32° between the central molecular axes to maximize orbital overlaps by avoiding the steric repulsion of the mesityl groups. The columns themselves interact with each other by dispersion interactions between the methyl groups of the mesityl substituents.

Fig. 5 Single crystal X-ray structures of DBAF (a–d), DBAF-N₂ (e–h), mes-DBAF (i–l) and mes-DBAF-N₂. a, e, i and m: molecular structures. b, f and p: top-view on face-to-face π-stacked dimers. c, g and j: side-view on edge-to-face interactions. d, h, l and o: molecular packings. k: zoom-in on the C–H⋯π-interaction of mes-DBAF. n: side-view on the face-to-face π-stacked dimer of mes-DBAF-N₂.

Charge transfer integrals (t) between the molecules as well as intrinsic charge carrier mobilities (hole and electron) were calculated via fragment based non-adiabatic molecular dynamic simulations (for a detailed description of the method see ESI‡).^48,49 Only hole transport properties will be discussed (calculated electron transport properties can be found in the ESI‡), due to the fact, that the experimentally determined electron affinities of EA = −3.04 to −3.18 eV exclude a potential use in n-type semiconducting devices due to a mismatch with the work functions of commonly used electrode materials such as gold.⁵⁰ The hole transfer integral of DBAF along the axis of the face-to-face stacking was calculated using the non-self-consistent density functional tight binding (DFTB) method with special parameter set⁵¹ to be t_‖,h = 20 ± 14 meV (Fig. 5d). In the direction of the edge-to-face stacking only small transfer integrals were found (t_⊥,h = 1 ± 3 meV). The corresponding theoretical hole mobility of μ_h = 0.31 cm² V⁻¹ s⁻¹ calculated by the fewest switches surface hopping algorithm with implicit relaxation (FSSH-IR), where the reorganization energy is calculated by DFT with B3LYP functional and 6-31G(d,p) level of theory to be λ_{h, B3LYP} = 153 meV indicates the potential use of DBAF in such devices. In the case of DBAF-N₂ (Fig. 5h), a higher electronic coupling with transfer integrals of t_‖,h = 70 ± 24 meV was found. The corresponding hole mobilities of μ_h = 14.41 cm² V⁻¹ s⁻¹ are based on a reorganization energy of λ_{h, B3LYP} = 167 meV. The mesitylated congeners mes-DBAF and mes-DBAF-N₂ showed either no or only negligible theoretical mobilities and have thus been excluded for device fabrication (see ESI‡ for further details).

Organic p-channel TFTs of DBAF gave hole mobilities of 1 × 10⁻⁴ to 1 × 10⁻³ cm² V⁻¹ s⁻¹ independent of the type of the substrate (silicon or flexible polyethylene naphthalate (PEN)) and the substrate temperature during the vacuum deposition of the semiconductor (60 or 80 °C) when using a pentadecafluoro-octadecylphosphonic acid self-assembled monolayer (F-SAM) as part of the gate dielectric.⁵²

These mobilities significantly increased using an n-tetradecylphosphonic acid SAM (H-SAM) instead. In this case, mobilities of 0.08 up to 0.4 cm² V⁻¹ s⁻¹ have been obtained, again independent of the type of substrate and the substrate temperature during the semiconductor deposition. The best TFT performance was obtained on a PEN substrate using a substrate temperature of 80 °C during the semiconductor deposition (for details see ESI‡), with a mobility of μ_h,exp = 0.4 cm² V⁻¹ s⁻¹ (Fig. 6), which is in good agreement to the theoretical mobility discussed above (μ_h,theo = 0.31 cm² V⁻¹ s⁻¹). Although DBAF-N₂ showed even higher theoretical hole mobilities (μ_h,theo = 14.41 cm² V⁻¹ s⁻¹, see discussion above), the mobilities obtained experimentally are much smaller, only 4 × 10⁻⁵ to 5 × 10⁻⁴ cm² V⁻¹ s⁻¹, independent of the type of substrate, the type of SAM, and the substrate temperature during the semiconductor deposition. The calculated values represent the intrinsic mobility in the perfect crystalline structures of DBAF and DBAF-N₂, where no grain boundaries or other defects are present. However, in thin-films, such defects exist, and the thin-film structures may deviate from the ideal single-crystal form. Consequently, the calculated mobilities provide a useful estimation of how closely the deposited thin-films approximate a perfect single crystal. This can be inferred from the similarity of their charge-transport properties. The significant difference between the experimental and theoretical mobilities suggests that the deposited DBAF-N₂ structure deviates considerably from the perfect crystal structure.

Fig. 6 Electrical characteristics of a DBAF TFT fabricated on a flexible PEN substrate in the inverted staggered (bottom-gate, top-contact) device architecture using an n-tetradecylphosphonic acid SAM as part of the gate dielectric, with the substrate held at a temperature of 80 °C during the semiconductor deposition. The effective charge-carrier mobility is 0.4 cm² V⁻¹ s⁻¹.

To better understand the measured TFT characteristics (schematic device structure shown in Fig. 7a), grazing-incidence wide-angle X-ray scattering (GIWAXS) was employed. The critical angles (α_c) of DBAF and DBAF-N₂ are estimated at 0.17° and 0.18°, respectively.⁵³ Therefore, the incident angle (α_i) of the incoming X-ray was set at 0.3° (α_i > α_c) in order to probe microstructures of the deeper-lying organic/SAM/inorganic interface, rather than those from the topmost surface. Though the AFM and SEM investigations reveal the polycrystallinity of the thin-films with non-specific orientations, the small grain size (∼0.1–0.5 μm) makes the in-plane information difficult or impossible to be detected by GIWAXS. Instead, we obtain the strong out-of-plane signals and are thus able to analyze the interface configurations. Two sets of out-of-plane signals have been observed. As exemplified in Fig. 7c and d, the lower one at ∼21 Å stems from the self-assembled monolayer that is part of the gate dielectric, while the upper one at ∼10 Å belongs to the organic semiconductor thin-films. Specifically, for DBAF, the reflex at 9.99 Å is assigned to the (001) plane (10.5 Å calculated from the signal crystal data) and for DBAF-N₂, the reflex at 9.75 Å to the (101̄) plane (9.77 Å calculated from the signal crystal data). Both molecules show an edge-on orientation and a layered structure. DBAF stays more tilted (43°–44°) than DBAF-N₂ (77°–78°) to the substrate, which could slightly lower the HOMO level. Within one layer, both DBAF and DBAF-N₂ molecules stack in lines. The small π–π distance (DBAF 3.3 Å and DBAF-N₂ 3.4 Å) of two adjacent molecules and a reasonable overlap of their electron clouds (∼1/3 for DBAF and ∼2/3 for DBAF-N₂) favor the horizontal charge-carrier hopping along the stack-propagating direction (Fig. 7b). This agrees with the values of the calculated transfer integrals (for DBAF t_‖,h = 20 ± 14 meV and for DBAF-N₂ t_‖,h = 70 ± 24 meV). The main difference is found in the vertical perspective (parallel to the substrate surface normal) between the adjacent layers. DBAF shows an edge-to-face π–π-arrangement (t_⊥,h = 1 ± 3 meV), whereas DBAF-N₂ has an edge-to-edge arrangement, giving rise to a negligible overlap of the π-electron cloud of molecules stemming from different layers (Fig. 7i). Thus, the vertical charge-carrier transport in the DBAF-N₂ thin-films is inefficient, leading to a larger contact resistance and thereby to a smaller effective charge-carrier mobility⁵⁴ compared to the DBAF TFTs. Another explanation for the substantial difference in measured charge-carrier mobilities is that unlike DBAF, DBAF-N₂ does not form a completely closed film on the substrate surface (as indicated by the AFM images in the ESI in Fig. S74‡), which is expected to greatly impede the lateral charge transport.

Fig. 7 Molecular orientation of DBAF and DBAF-N₂ at the organic/SAM/inorganic interface of the OTFT devices. (a) The OTFT device used in this work has a bottom-gate-top-contact structure. (b) Illustration of charge transport taking place in a working device of DBAF. (c) and (d) are typical GIWAXS data for the DBAF- and DBAF-N₂-based devices, respectively. (e) Side view and (g) top view of the orientation of DBAF molecules. (f) Side view and (h and i) top views of the orientation of DBAF-N₂ molecules. (h) shows the first layer (L1) at the SAM/inorganic substrate (bright green). (g) and (i) show the first two layers, in which molecules colored in grey come from the first layer (L1) and molecules in various blue colors are from the second layer (L2).

Conclusions

The PAHs presented in this study have a Z-shape conformation and can be tuned in terms of their solubility by peripheral substituents without influencing the electronic properties of the core structure as proven by spectroscopic methods and quantum chemical calculations. This made the investigation in solution as e.g. by electrochemical methods possible and revealed suitable characteristics for the application in p-type thin-film transistors. Corresponding devices of DBAF showed experimental mobilities up to μ_h,exp = 0.4 cm² V⁻¹ s⁻¹ which is in agreement to quantum chemical calculations based on single crystal X-ray structure data. A substitution of two carbon atoms in the aromatic backbone with nitrogen was possible by simple modification of the molecular precursors. While an expected stabilization of the frontier molecular orbitals was achieved and higher theoretical hole mobilities were obtained, proving the beneficial intrinsic properties of DBAF-N₂, the corresponding devices suffered from a disadvantageous orientation of the crystalline films on the devices' surface resulting in three orders of magnitude smaller experimental mobilities. This study shows on the one hand the potential of this class of PAHs for the application as semiconducting materials in organic electronics and on the other hand the importance of the interplay of suitable intrinsic molecular properties and the controllable device morphology upon structural changes on a molecular level.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

All authors are grateful for financial support to this project by Deutsche Forschungsgemeinschaft (DFG) in the framework of the Collaborative Research Center SFB1249 (TP-A04, -B02, and -C01). Support by the state of Baden-Württemberg through bwHPC and the DFG through grant no. INST 40/575-1 FUGG (JUSTUS 2 cluster) is acknowledged. Jan Lennart Peters is acknowledged for assistance with synthesis. We thank the Kivala group (Heidelberg) for access to their fluorimeter for PLQY determination.

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Footnotes

† Dedicated to Prof. Dr. Frank Würthner on the occasion of his 60th birthday.

‡ Electronic supplementary information (ESI) available. CCDC 2353527–2353534. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4qo01252f

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