Sensitivity of Redox and Optical Properties of Electroactive Carbazole

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C: Plasmonics, Optical Materials, and Hard Matter

Sensitivity of Redox and Optical Properties of Electroactive Carbazole Derivatives to the Molecular Architecture and Methoxy-Substitutions Xavier Sallenave, Audrius Bucinskas, Seyhan Salman, Dmytro Volyniuk, Oleksandr Bezvikonnyi, Viktorija Mimaite, Juozas Vidas Grazulevicius, and Gjergji Sini J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b02148 • Publication Date (Web): 23 Apr 2018 Downloaded from http://pubs.acs.org on April 24, 2018

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The Journal of Physical Chemistry

Sensitivity of Redox and Optical Properties of Electroactive Carbazole Derivatives to the Molecular Architecture and Methoxy-Substitutions

Xavier Sallenavea, Audrius Bucinskasb, Seyhan Salmanc, Dmytro Volyniukb, Oleksandr Bezvikonnyib, Viktorija Mimaiteb, Juozas Vidas Grazuleviciusb*, and Gjergji Sinia* a

Université de Cergy-Pontoise, Laboratoire de Physicochimie des Polymères et des Interfaces, EA 2528, 5 mail Gay-Lussac, Cergy-Pontoise Cedex, 95031, France.

b

Kaunas University of Technology, Department of Polymer Chemistry and Technology, Radvilenu pl. 19, LT-50254, Kaunas, Lithuania. c

Gwinnett Technical College, Basic Sciences Department, Lawrenceville, GA, USA.

*Corresponding authors: G. Sini e-mail: [email protected] and J.V. Grazulevicius, e-mail: [email protected].

Abstract In the domain of OLED applications, organic electroactive materials play a crucial role. In order to allow for more flexibility in their properties, methoxy- or other substituents are frequently used. However, undesirable modifications in their polarity may be additionally obtained, which is particularly important in the case of TADF-based OLEDs. In order to dissociate as much as possible intramolecular and bulk effects, we synthetized two series of methoxy-substituted carbazole-bridge-carbazole (bridge=carbazolyl, phenyl) compounds and characterized them by means of experimental and theoretical methods. V-shape (3,6) substitutions on carbazole bridge, and linear (para-phenyl) bisubstitutions were used in the new compounds. By varying the number of methoxy groups from 0-4 per carbazole unit, we analyze the effect of the number- and the linking topology of the methoxy-substitutions on the thermal, electronic, and optical properties of the molecules.

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The results indicate that the variations of the redox- and fluorescence properties upon methoxy substitutions depend importantly on the linear- versus V-shape D-A-D molecular architecture, due to the absence- and presence respectively of the dipolar moments and of the bulk polarity. The choice of the molecular architecture and methoxy substitutions can be used consequently to switch on / off the molecular polarity and to tune the sensitivity of redox- and optical properties of electroactive materials with respect to medium electrostatic effects. These compounds were additionally utilized in blue phosphorescence organic lightemitting devices and showed good hole-transporting, exciton-blocking and electronblocking properties.

Introduction Organic light emitting diodes (OLEDs) have been utilized in flat-panel displays and solid-state lighting technologies due to the advantages in color tunability, efficiency, transparency and flexibility. 1 - 5 A typical structure of one color OLED contains fluorescent, thermally activated delayed fluorescent (TADF) or phosphorescent lightemitting layers 6 - 10 inserted between several additional functional layers like charge transporting, blocking and injection layers, or exciton blocking ones. Because of this complex architecture, device complexity and high-production cost are the main disadvantages of multilayer OLEDs.11 The discovery of new multifunctional electroactive materials is consequently required to increase OLED efficiency and/or simplify their architecture. The efficiency and lifetime of multilayer OLEDs can be improved by tuning the properties of the individual layers through judicious choices of materials.12 This can be easily achieved by linking to the electroactive molecules several substituents, such as cyano 13 , tert-butyl 14 or methoxy groups 15 . While tuning the properties of organic electroactive materials by means of chemical modifications is widely used in the design of new electroactive materials,16-19 much less is discussed about the possibility for a given


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substituent to impact the target- but also several additional properties. Tuning, for instance, the thermal or optical properties of hosts or emitters by means of methoxy- or other substituents is frequently coupled with changes in the molecular dipole moments and medium polarity. In the following we focus on the structure-polarity correlation in electroactive materials, given the crucial impact of this property on the OLED efficiency. Several studies have reported on the important sensitivity of the OLED efficiency to the orientation of the transition dipole moments of the emitters,20 molecular packing,21 solid state effects,22 and to the host matrix,23 directly pointing to the important impact of the dipole moments of the host and/or guest (emitter) molecules. The singlet-triplet energy splitting, a crucial parameter for efficient TADF emitters, 24 was shown to be very sensitive to the solid state polarity, due to the electrostatic stabilization of the singlet and triplet CT states25. However, depending on the local- or CT nature of the lowest singlet and triplet states, the polarity effect could play a positive- or negative role on the singlettriplet energy splitting. It could be consequently important to dissociate (associate) as much as possible the impact of the chemical modifications on the optical properties from (with) the impact on the molecular dipole moments. A possible design strategy to tune “exclusively” the optical properties by keeping a non-polar molecular character could be to link the chromophore groups in a linear donor-acceptor-donor (D-A-D) molecular architecture. Interestingly, the D-A-D linking was recently shown to allow for the delocalization- and stabilization of the singlet- and triplet CT states with important impact on TADF efficiency26, but no property-polarity correlation was reported. On the other hand, a Vshape molecular architecture makes possible to tune simultaneously the optical properties and the dipole moments. While both linear- and V-shape molecular architectures correspond to the ensemble of the electroactive compounds used in organic electronics, to the best of our knowledge no systematic studies address the sensitivity of optoelectronic properties to the molecular architecture and to the interplay between the later with the nature of the substituent groups.


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Here we address the sensitivity of the redox and photophysical properties of electroactive carbazole-based derivatives to the molecular architecture and methoxy substitutions. To this aim, we present two series of new molecules, in which carbazolyl- and phenyl central core fragments are bi-substituted by carbazole moieties containing 0-4 methoxy groups. The non-substituted triscarbazole was chosen as the parent compound for a series of Vshape molecules, due to excellent hole-transporting characteristics and high triplet energy of carbazole,27 and the successful application of 3,6-Di(9-carbazolyl)-9-(alkyl)carbazole (TCz) 28 - 30 as hole-transporting or host material for blue electrophosphorescence in OLEDs. The phenyl core with linking in para positions (1,4-bis(9-carbazolyl)benzene, mCP)31-33 was chosen as a linear D-A-D alternative to the meta-linking used already in the case of 1,3-di(9-carbazolyl)benzene (mCP). 34-37 The choice of methoxy groups as substituents was motivated by their well-known impact on thermal and optoelectronic properties of amorphous compounds in general 38 - 40 and of carbazole-based ones in particular41-43. The results indicate that the nature of the central fragments in the two series (carbazole and phenyl) has no effect on the redox and optical properties of the two series of compounds, and only allows switching on- or off the molecular polarity, respectively. The new compounds exhibit increased thermal stability as compared to triscarbazole, and the absorption spectra of all compounds are strikingly similar to the parent free carbazoles containing 0-4 methoxy groups. Importantly, big difference in the variation of the optical properties upon methoxy substitution is observed between the two series of compounds, the strongest variation corresponding to the series of linearly linked D-A-D compounds. A joint experimental and DFT approach is applied allowing understanding the impact of the molecular architecture, methoxy substitutions, and the inter-fragment orbital interactions tune the geometrical, redox, and optical properties of the compounds.

1. Experimental section


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1.1 Materials Synthesis: Copper iodide CuI (>99.5%), 18-crown-6 (99%) and 1,4-diiodobenzene (>99%) were purchased from Aldrich. Potassium carbonate K2CO3 (99%), potassium phosphate K3PO4 97%) and trans-1,2-diaminocyclohexane (99%) were purchased from Alfa Aesar. All products were used as received, unless otherwise mentioned. DMF and dioxane were dried by distillation from calcium hydride CaH2 and sodium metal/benzophenone respectively. Column chromatography and TLC were performed on silica gel 60 (230-400 mesh, Merck) and silica gel (60) F254 plates (Merck) respectively. Devices : Molybdenum trioxide (MoO3), 1,3-bis(9-carbazolyl)benzene (mCP), bis[2-(4,6difluorophenyl)pyridinato-C2,N](picolinato)iridium(III)

(FIrpic),

diphenyl-4-

triphenylsilyl-phenylphosphineoxide (TSPO1) and 2,2’,2’’-(1,3,5-benzinetriyl)-tris(1phenyl-1-H-benzimi-dazole) (TPBi) were purchased from Sigma Aldrich and used as received (Figure S6). 3,6-Di(9-carbazolyl)-9-(2-ethylhexyl)carbazole (TCz1) was synthesized according to the published procedure (Figure S6)44.

1.2 Characterization 1

H and

13

C NMR spectra were recorded on a Bruker DPX-250 NMR spectrometer in 5

mm tubes with CDCl3 as solvent in all cases (safe for the compound 4, see below). The 1

H and 13C chemical shifts were referenced to CDCl3 solvent peaks, 7.26 ppm and 77.16

ppm respectively. High resolution mass spectrometry was performed by the small molecule mass spectrometry plateform of the CNRS IMAGIF in Gif-sur-Yvette. Thermogravimetric analysis (TGA) was carried out on a TA Instrument Q50 TGA under argon flow at a heating rate of 20°C.min-1. Cyclic voltamperometry (CV) was carried out using an Autobab (AU128N, FRA2) electrochemical analyser with Pt disc used as working and counter electrodes and Ag wire as pseudo-reference. These three electrodes were

immersed

in

a

CH2Cl2

solution

hexafluorophosphate (n-Bu4PF6) and 10

-3

with

0.1

M

tetrabutylammonium

M of C1-C4 or P1-P4 derivatives.

Ferrocenium/ferrocene (Fc/Fc+) redox potential was measured in order to calibrate the pseudo-reference electrode. UV-visible measurements were performed at room temperature in spectrometric grade tetrahydrofuran solutions and on glass substrates using a Jasco V570 spectrometer.


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An Edinburgh Instruments FLS980 spectrometer was used for photophysical measurements of compounds of the C and P families. To separate phosphorescence spectra of the films of emission spectra were recorded at 77 K with the delay time exceeding 50 ms. The singlet (S1) and triplet (T1) energy levels were taken from set-on of PL and Ph spectra for C- P-compounds measured at 77K. Ionization potentials (Ip(EP)) of solid-state samples were estimated by photoelectron emission spectrometry in air 45 as described previously 46 using a deep UV deuterium light source ASBN-D130-CM, a CM110 1/8m monochromator, and an electrometer 6517B Keithley in experimental setup. Having Ip(EP) values for the studied compounds, their electron affinities (EAPE) in solid-state were calculated using question EA(EP)= Ip(EP)-Eg. The optical band-gap energies (Eg) were taken from absorbance spectra of solid-state samples. Both Ip(EP) (HOMO) and EA(EP) (LUMO) values for the studied compounds were used for designing of PhOLEDs. 1.3 Synthesis of target molecules

The disubstituted compounds (2, 3) were prepared in two steps, starting from anisole and carbazole derivatives, respectively, using the previously reported methods Tetramethoxycabazole (4)

was

obtained

from

the

47 - 50

2,7-dimethoxycarbazole,

.

via

bromination49 and methoxylation50 reactions. Methoxylation procedure for the synthesis of compound 4 Metallic sodium (1.0 g, 3.1 mmol), absolute MeOH (16 ml), CuI (2.5 g, 4.2 mmol), dry DMF (33 ml) and 3,6-dibromo-2,7-dimethoxy-9Hcarbazole (1.0 g, 3.1 mmol) were stirred for 2 hours under an argon atmosphere at a reflux temperature. After TLC control, the reaction mixture was cooled and EtOAc was added. Then the mixture was filtered through a glass filter and washed with water. The organic layer was dried with Na2SO4, filtered and concentrated under vacuum. The residue was purified chromatographically using hexane and acetone (10:1) as the eluent. The yield of white crystals was 51 % (0.45 g). 1H NMR (400 MHz, DMSO) δ 10.61 (s, 1H), 7.56 (s, 2H), 6.97 (s, 2H), 3.83-3.82 (m, 12H). 13C NMR (101 MHz, DMSO) δ 147.8, 143.4, 134.2, 115.0, 102.8, 94.9, 56.2, 55.7. IR (ῡ, cm-1): 3405 (N ̶ H, 3500-3200 cm-1), 3003 (arene C ̶ H, 3100-3000 cm-1), 1491,


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1473, 1446 (C-C in Ar, 1600-1585, 1500-1400 cm-1), 1363, 1272(C ̶ N in Ar, 1350-1250 cm-1), 1223, 1196, 1159(C ̶ O ̶ C, 1250-1050 cm-1), 837, 774 (C ̶ H in Ar, 900-675 cm-1); m/z: 288 ([M + H]+).

General procedure for the synthesis of C1-C4 To a solution of N-ethyl-3,6-dibromocarbazole (0.4 mmol) in dry DMF (10 mL) under argon were added carbazole derivative 1-4 (0.88 mmol), CuI (0.8 mmol) and 18-crown-6 (0.06 mmol). The solution was degassed with argon for 30 minutes. Then K2CO3 (1.6 mmol) was added and the reaction mixture was refluxed at 165°C for 3 days. After cooling, the solution was poured into 20 mL of water. The aqueous layer was extracted with CH2Cl2 (3×20mL). The combined organic phases were washed with 1N HCl (2×20mL), H2O (2×20mL) and dried with MgSO4. After evaporation of solvent, the residue was purified by column chromatography (SiO2) with eluent indicated in each case (see below) to obtain white solid. C1 : Eluent : CH2Cl2/cyclohexane (3/2). 72% yield. 1H NMR (CDCl3) δ 8.23 (s, 2H), 8.16 (d, J = 7.8 Hz, 4H), 7.68 (m, 4H), 7.45-7.35 (m, 10H), 7.29 (m, 2H), 4.57 (qu, J = 7.3 Hz, 2H), 1.63 (t, J = 7.3 Hz, 3H);

13

C NMR (CDCl3) δ 141.8, 139.9, 129.3, 125.9,

125.8, 123.7, 123.0, 120.3, 119.9, 119.6, 109.7, 38.2, 14.1; HRMS (ESI+): calculated for M+: 525.2238, found: 525.2267. C2 : Eluent : CH2Cl2/cyclohexane (4/1). 69% yield. 1H NMR (CDCl3) δ 8.21 (s, 2H), 7.64 (d, J = 1.0 Hz, 4H), 7.60 (d, J = 2.4 Hz, 4H), 7.29 (d, J = 8.8 Hz, 4H), 7.05 (d, J = 2.5 Hz, 2H), 7.01 (d, J = 2.5 Hz, 2H), 4.55 (qu, J = 7.0 Hz, 2H), 3.96 (s, 12H), 1.61 (t, J = 7.2 Hz, 3H); 13C NMR (CDCl3) δ 153.8, 139.5, 137.4, 129.8, 125.8, 123.4, 123.2, 119.6, 115.2, 110.6, 109.8, 102.9, 56.2, 38.2, 14.1; HRMS (ESI+): calculated for M+: 645.2644, found: 645.2662. C3 : Eluent : CH2Cl2/cyclohexane (4/1). 67% yield. 1H NMR (CDCl3) δ 8.22 (s, 2H), 7.92 (d, J = 8.2 Hz, 4H), 7.75-7.55 (m, 4H), 6.87 (dd, J = 8.3-1.8 Hz, 4H), 6.76 (d, J = 2.3 Hz, 4H), 4.58 (qu, J = 7.3 Hz, 2H), 3.95 (s, 12H), 1.65 (t, J = 7.3 Hz, 3H); 13C NMR (CDCl3) δ 158.2, 143.4, 139.9, 129.2, 126.0, 123.5, 120.1, 117.0, 110.1, 107.9, 94.1, 56.2, 38.2, 14.1; HRMS (ESI+): calculated for M+: 645.2644, found: 645.2671. C4 : Eluent : AcOEt/cyclohexane (6/4). 55% yield. 1H NMR (CDCl3) δ 8.25 (d, J = 1.5


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Hz, 2H), 7.75-7.60 (m, 4H), 7.49 (s, 4H), 6.81 (s, 4H), 4.59 (qu, J = 6.7 Hz, 2H), 4.03 (s, 12H), 3.81 (s, 12H), 1.67 (t, J = 7.0 Hz, 3H);

13

C NMR (CDCl3) δ 148.5, 144.6, 139.7,

136.5, 129.9, 126.2, 123.6, 119.9, 115.4, 110.2, 102.0, 93.6, 56.8, 56.3, 38.3, 14.3; HRMS (ESI+): calculated for M+: 765.3127, found: 765.3147.

General procedure for the synthesis of P1-P4 To a solution of 1,4-diiodobenzene (0.5 mmol) in dioxane (15 mL) under argon, were added carbazole derivative 1-4 (2 mmol), CuI (0.3 mmol), K3PO4 (2.5 mmol) and trans1,2-diaminocyclohexane (0.3 mmol). The reaction mixture was refluxed for 24 h. After cooling, the solvent was evaporated and the residue was purified by quickly flash column chromatography (SiO2) with eluent mentioned below to obtain white solid. P1 : Eluent : CH2Cl2/cyclohexane (1/3). 64% yield. 1H NMR (CDCl3) δ 8.20 (d, J = 7.8 Hz, 4H), 7.83 (s, 4H), 7.58 (d, J = 8.2 Hz, 4H), 7.49 (t, J = 8.0 Hz, 4H), 7.36 (t, J = 8.2 Hz, 4H);

13

C NMR (CDCl3) δ 140.7, 136.8, 128.3, 126.1, 123.5, 120.3, 120.2, 109.7;

HRMS (ESI+): calculated for M+: 408.1654, found: 408.1623. P2 : Eluent : CH2Cl2/cyclohexane (1/2). 60% yield. 1H NMR (CDCl3) δ 7.76 (s, 4H), 7.59 (d, J = 2.3 Hz, 4H), 7.47 (d, J = 9.2 Hz, 4H), 7.11 (d, J = 8.0 Hz, 4H), 3.97 (s, 12H); 13C NMR (CDCl3) δ 154.2, 136.8, 136.1, 127.8, 123.8, 115.3, 110.7, 102.8, 56.2; HRMS (ESI+): calculated for M+: 528.2018, found: 528.2031. P3 : Eluent : CH2Cl2/cyclohexane (1/2). 44% yield. 1H NMR (CDCl3) δ 7.92 (d, J = 8.3 Hz, 4H), 7.78 (s, 4H), 6.95 (s, 4H), 6.90 (dd, J = 8.3-2.3 Hz, 4H), 3.87 (s, 12H);

13

C

NMR (CDCl3) δ 158.4, 142.2, 136.8, 128.4, 120.3, 117.6, 108.1, 94.9, 55.9; HRMS (ESI+): calculated for M+: 528.2018, found: 528.2005. P4 : Eluent : CH2Cl2/acetone (30/1). 30% yield. 1H NMR (CDCl3) δ 7.84 (s, 4H), 7.50 (s, 4H), 7.06 (s, 4H), 4.06 (s, 12H), 3.93 (s, 12H); 13C NMR (CDCl3) δ 148.6, 145.2, 136.9, 135.0, 127.9, 116.2, 102.1, 94.1, 56.8, 56.6; MS (ESI+): calculated for M+: 648.2532, found: 648.2512.

1.4 Devices Three types of devices were fabricated for testing of compounds of the C and P families as multifunctional hole-transporting, exciton-blocking and electron-blocking materials.


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Type

I:

ITO/MoO3(2nm)/C-

P(45nm)/TCz1:5wt%FIrpic(30nm)/TSPO1(8nm)/TPBi(35nm)/ Ca:Al (devices I:C1-C4, P2

or

P3

using

C1-C4,

P2

or

P3,

respectively);

type

II:

ITO/MoO3(2nm)/C(45nm)/TCz1:5wt%FIrpic(30nm)/TCz1(8nm)/TPBi(35nm)/Ca:Al (devices II:C1-C4 using C1-C4, respectively) and type III: ITO/MoO3(1nm)/CP(45nm)/mCP:10wt% FIrpic(30nm)/TSPO1(8nm)/TPBi(35nm)/Ca:Al (devices III:C1C4 or P3 using C1-C4 or P3 respectively). ITO-coated and pre-patterned glass substrates with a sheet resistance of 15 Ω/sq were cleaned in acetone and isopropyl alcohol ultrasonic baths during at least ca. 10 min before OLED fabrication. Organic layers as well as calcium and aluminium layers were vacuum-deposited under the vacuum higher than 2×10−6 mBar exploiting equipment from Kurt J. Lesker in-built in an MB EcoVap4G glove box. Keithley 2400C sourcemeter and the certificated photodiode PH100-Si-HA-D0 together with a PC-Based Power and Energy Monitor 11S-LINK were utilized for simultaneous recording of current-voltage and luminance-voltage characteristics 51,52 . Electroluminescence (EL) spectra were recorded using an Aventes AvaSpec-2048XL spectrometer. External quantum efficiency was calculated from the luminance, current density, and EL spectrum.

1.5 Computational Details The optimized geometries and relevant energies of the carbazole derivatives are obtained by means of density functional theory (DFT) calculations at the CPCM/DFT/6-31G** in diethylether with ωB97XD functional in which the value of the ω parameter was tuned by considering the effect of the solvent (diethylether, ε=4.12).53 Similar ω−values around 0.012 Å-1 were found for all compounds, which are much smaller than the default value (0.2 Å-1). The tuned-ω∗B97XD functional with a constant value (ω∗=0.012Å-1) was used in this study for all compounds. Test calculations with the default ωB97XD and B3LYP functionals were also performed for comparison reasons. The singlet and triplet excited state energies were calculated from Time-Dependent (TD) DFT method on the optimized ground state (S0) geometries. To obtain a better insight on the nature of some singlet and triplet excited states of interest, Natural Transition Orbital


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(NTO) analysis was performed following TD-DFT calculations. The theoretical transition energies are blue-shifted by roughly 30-40 nm wrt the experimental results. Test calculations by increasing the quality of the basis set seem to reduce the error by roughly 10 nm. Previous studies have suggested for the above theory-experiment dichotomy possible impact of vibronic coupling phenomena and breakdown of the BornOppenheimer approximation.54,55 While several such effects could explain the blueshift of the theoretical values, the general theoretical description is coherent with the experimental one. All calculations were performed with the Gaussian09 software.56

2. Results and discussion 2.1 Synthesis and thermal stability The synthetic route is outlined in Scheme 1 and detailed in the supporting information. Carbazole derivatives 2 and 3 were obtained in one step from 3,6-dibromocarbazole or 2,7-dibromocarbazole, respectively by a copper catalyzed coupling with sodium methanoate. Tetramethoxy 4 was prepared by the methoxylation procedure using metallic sodium. Compounds C1-C4 (carbazolyl core) and P1-P4 (phenyl core) were obtained by Cucatalyzed amination of N-ethyl-3,6-dibromocarbazole or 1,4-diiodobenzene and carbazole derivatives 1-4, respectively. Structures of carbazole intermediates and final compounds were fully characterized by 1H and

13

C NMR and mass spectrometry. As

expected, C1-C4 and P1-P4 have a good solubility in common solvents such as THF, dichloromethane, chloroform, toluene, etc.


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X Y X Y

N

X

X N Y

C1-C4

b) Y

Y N H X = Y = H (1) X = OMe, Y = H (2) X = H, Y = OMe (3) X = Y = OMe (4)

N

Y N

X

a)

Y

X

X

Y

N Y

Y X

X

P1-P4

X

Scheme 1. Synthesis of C1-C4 and P1-P4. Reagents and conditions: a) N-ethyl-3,6dibromocarbazole, CuI, 18-c-6, K2CO3, DMF, 165°C, 72 h. b) 1,4-diiodobenzene, CuI, trans-1,2-diaminocyclohexane, K3PO4, dioxane, 110°C, 24 h. Thermal properties of C1-C4 and P1-P4 were investigated by thermogravimetric analysis (TGA). The thermal decomposition (Td5%) temperatures are summarized in Table 1 (Thermograms are shown in Supporting Information). All the compounds showed significant thermal stability with Td values up to 290°C. In the series of C1-C4, the substitution with methoxy groups increases the degradation temperature with a larger effect in the case of the bi-substituted compounds C2 and C3 (remarkable Td value up to 400°C) than for the tetra-OMe substituted compound C4. The derivatives with phenyl core (P1-P4) showed similar thermal stability (Td around 360 °C). However, the tetraOMe substituted compound P4 exhibits decreased degradation temperature not only compared to the bi-substituted ones (P2 and P3), but also smaller than that of the nonsubstituted compound P1.

2.2 Structural and molecular orbital characteristics Carbazole- and phenyl fragments. In order to better understand the electronic and structural characteristics of compounds C1-C4 and P1-P4, we firstly focus on the electronic characteristics of the core-fragments (unsubstituted carbazole and phenyl). A


11


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selected set of electronic characteristics of these fragments are shown in Figure 1a. The LUMO energy levels are not shown, as they are similarly situated around zero eV. HOMO and HOMO-1 of the unsubstituted carbazole (reported hereafter as HOMO(Cz) and HOMO-1(Cz)) are respectively symmetric- and antisymmetric with respect to the symmetry plane containing the N atom (Figure 1a). In the frame of the symmetry point group C2v, these orbitals are labelled A1 and B2 respectively. The 2,3,6,7- substitution positions at the carbazole fragment are highlighted with blue tics over HOMO(Cz), HOMO-1(Cz), and LUMO representations. Note that HOMO(Cz) contains no coefficients at the 2,7 positions, whereas LUMO(Cz) contains no coefficients at the 3,6 positions. On the contrary, HOMO-1(Cz) contains important coefficients on the four substitution positions. The π-donor effect of the methoxy groups at the 3,6 positions will consequently shift up both HOMO(Cz) and HOMO-1(Cz), thus preserving their energy splitting and order, whereas the 2,7 substitutions impact only HOMO-1(Cz) because of the missing coefficients for HOMO(Cz) at these positions. Accordingly, the HOMO(Cz) / HOMO-1(Cz) energy order is permuted in the case of 2,7 substitutions. In the case of 2,3,6,7- tetra-OMe substitutions, the HOMO(Cz)-HOMO-1(Cz) energy splitting is strongly reduced, but HOMO-1(Cz) remains higher in energy than HOMO(Cz), suggesting dominant impact of the 2,7 substitutions. We finally note that LUMO(Cz) contains no coefficients at the 3,6 positions, as opposed to the large ones at the 2,7 positions. Consequently, the π-donor effect of the methoxy groups at the 3,6 positions shifts up the HOMO(Cz) level much more than the LUMO(Cz) one, thus reducing their energy gap, as opposed to the similar impact on both HOMO-1(Cz) and LUMO(Cz) in the case of 2,7 substitutions. This difference will be shown to importantly impact the optical properties. Compounds C1-C4 and P1-P4. The geometries of the compounds C1-C4 and P1-P4 obtained by means of DFT calculations are shown in Figure 1b. The dihedral angles between the lateral carbazole units and the central core are very large and range a very small window (86.5-89.4° and 70.0-89.1° respectively). Given the importance of this parameter for the correct description of the optical properties of these compounds, several


12

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DFT methods were used. Figure S1 indicates that the dihedral angles obtained with the PCM-tuned57 ω*B97XD functional (ω∗ = 0.012 Å-1, see computational section) are by 20-30° larger than those obtained with the default ωB97XD (ω = 0.2 Å-1) or B3LYP functionals. However, scan calculations for the inter-fragment dihedral angles of compounds C1-C4 and P1-P4 indicate almost flat energy profiles between 70-90° (Figures 1c), strongly pointing to the presence of geometrical disorder around the equilibrium inter-fragment dihedral angle. In the following, we show that comparison with the experimental optical and ionization energies in solution gives confidence to the results obtained with the tuned-ω*B97XD. As for the solid films of these compounds, large deviations from the quasi-orthogonality of the interring dihedrals could be predicted, given that interring torsion barriers smaller than 1 kcal/mol were found for all compounds between 65-90° (Figure 1c). Representations of the highest occupied molecular orbitals (HOMO), the lowest unoccupied ones (LUMO), along with a limited set of occupied orbitals of compounds C1-C4 and P1-P4 are shown in Figure 1d. As a global observation, we highlight the strong localization of all orbitals either in the core or in the lateral carbazoles, which is due to the quasi orthogonal inter-fragment geometries. The LUMOs of compounds C1-C4 are localized on the central fragment, whereas a competition between central and lateral fragments can be observed in the case of compounds P1-P4, with LUMOs localized in the lateral fragments for P1-P2 but in the central Ph fragment for P3 and P4. As for HOMOs, they are mainly localized on the external Cz, with some weak contributions from the bridge only in the case of P2 due to both the dihedral angle of 70.0° and the presence of non-zero HOMO(Cz) coefficients on the N-atoms).


13


(c)

kcal/mol

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

8 7 6 5 4 3 2 1 0

Page 14 of 44

C1 C2 C3 C4 P1 P2 P3 P4 40

50

60

70

80

90

dihedral angle (°)


14

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Figure 1. (a) Energy diagram corresponding to HOMO, HOMO-1 of carbazole as a function of the number and topology of methoxy substitutions. A color code shown at the right of the diagram is used to indicate the nature (symmetry) of the orbitals. The HOMO and LUMO (doubly degenerate ones) of phenyl are also shown. All LUMOs levels (not shown) are situated around zero eV. (b) Geometries of compounds C1-C4 and P1-P4 obtained at the tuned-ωB97XD/6-31G(d,p) level of theory “in diethylether”. The numbers correspond to the dihedral angles (dih, in degree) between the lateral- and central groups. In the case of P4 two values corresponding to isoenergetic conformers are indicated. (c) Evolution of the molecular energy of compounds C1-C4 and P1-P4 as a function of the inter-ring dihedral angles. Results obtained from scan calculations with full geometry optimization at each point. (d) Representations of HOMO-1, HOMO, and LUMO corresponding to compounds C1-C4 and P1-P4. All results were obtained at CPCM-tuned ωB97XD/6-31G(d,p) level by considering diethylether (ε=4.24) as a polarizable medium (solvent effect).

2.3. Electrochemical Properties Ionization potentials (IP) of compounds C1-C4 and P1-P4 deduced from the cyclic voltamperometry measurements and from photoelectron emission spectra are collected in Table 1 and plotted in Figure 2a,b. Larger IP values were deduced from the photoelectron emission spectra as compared to those deduced from electrochemistry measurements, pointing to different electrostatic effects felt by the molecules in the respective environments. The theoretical IP values deduced from the Koopmans’ theorem are also given in Figure 2c, showing good general agreement with the electrochemistry values.


15


Page 16 of 44

Upon methoxy substitutions, the ionization potentials decrease by roughly 0.4-0.7 eV. As expected, the largest decrease in the ionization potentials (0.69 eV, compound P4) is obtained upon tetramethoxy-substitution of carbazoles. Thus, in comparison to their unsubstituted analogues, methoxy-substitution of carbazoles is expected to reduce the hole injection barrier from adjacent electrodes. Comparison of the theoretical results obtained for compounds 1-4, C1-C4, and P1-P4 between them and with the Ipcv ones shown in Figure 2d suggests that the Ip values in these compounds are dominated by the lateral carbazole groups. This conclusion is also supported by the HOMO distributions on carbazole moieties in all the studied compounds (Figure 1d). Close Ip values for the two series should be consequently expected, which is supported by the results shown in Table 1 and Figure 2d: similar Ipcv values can be observed between the two series, varying by 0.46 eV and 0.41 eV across the C- and Pseries respectively. Interestingly, differences by up to 0.2 eV were found between C- and P- series for the solid-state Ipep values, with larger variations across the P-compounds (0.69 eV) as compared to C-compounds (0.38 eV). These differences seem to be consistent with the differences in the film polarity between the compounds of C- and Pseries: the C- compounds are polar due to their important dipole moments ranging 2.5-11 Debye (Table 1), in turn stemming from their V-shape, whereas the P-compounds are non-polar (zero dipole moments) because of the linear D-A-D linking. These results suggest consequently important impact on the Ip values from the bulk polarity, in turn being correlated with the molecular architecture. Note that the intermolecular interactions in the solid films may also impact the Ip values, which have been shown to be strongly enhanced in the presence of methoxy groups. 40


16

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Figure 2. (a) and (b) Cyclic voltammograms of dilute solutions of compounds C1-C4 and P1-P4 in CH2Cl2 / nBu4PF6. (c) Photoelectron emission spectra of the films for vacuum deposited compounds C1-C4 and P1P4 recorded in air at 25 °C. (d) Comparison of IP trends across the series of compounds C1-C4 and P1-P4 deduced from photoelectron emission spectra and from cyclic voltamperometry measurements. The HOMO energies in absolute values are also given, as obtained from DFT calculations in the frame CPCM-tuned ωB97XD/6-31G** “in diethylether“. The lines have no physical meaning and are only guides for the eye.

Table 1. Thermal decomposition (Td5%) temperatures, redox characteristics, and dipole moments (µ) of compounds C1-C4 and P1-P4 Compound

C1

Td5%

E1/2 vs

Ipcv ,a

Ipep,b

ε HOMO,c

µ,c

(°C)

Fc, V

eV

eV

eV

Debye

291

0.741

5.54

5.76

-5.57

6.6


17


Page 18 of 44

C2

423

0.311

5.11

5.33

-5.11

2.7

C3

432

0.456

5.26

5.58

-5.22

11

C4

325

0.285

5.08

5.38

-4.94

7.0

P1

366

0.720

5.52

5.98

-5.64

0.0

P2

361

0.378

5.18

5.38

-5.25

0.0

P3

371

0.456

5.26

5.56

-5.13

0.0

P4

290

0.311

5.11

5.29

-5.02

0.0

a

Ionization potentials measured by electrochemical studies Ipcv = 4.8 + E1/2 vs Fc.

b

Ionization potentials measured by electron photoemission in air method.

c

HOMO energies calculated at the CPCM/tuned-ωB97XD / 6-31G(d,p) “in diethylether”.

2.4 Optical properties Absorption spectra of the free lateral fragments (compounds 1-4, Scheme 1) and of compounds C1-C4 and P1-P4 in dilute THF solutions are shown in Figure 3, whereas a selected set of spectral data are collected in Table 2 and Table S1. The absorption spectra of solid films of compounds C1-C4 and P1-P4 are given in Figure S2c,d. 2.4.1 Nature of the lowest absorption energy band (LEB) Comparison between the absorption spectra shown in Figures 3a,b,c indicates almost identical low energy bands (LEB) for compounds C1-C4 and P1-P4 with those of compounds 1-4 (free lateral fragments). The LEBs of the non-substituted (1, C1, P1) or 3,6 substituted compounds (2, C2, P2) exhibit clear vibrational structure, whereas less clear but still distinguishable one can be observed for the 2,7- and tetra-OMe substituted compounds 3, 4, C3, C4, P3, P4. The absorption spectra of the thin films (Figure S2) also indicate similar LEBs as compared to those in THF solutions, still preserving some traces of the vibrational structure. These results indicate that the LEB in all the compounds C1-C4 and P1-P4 is dominated by the local carbazole transitions (LCz). The inter-fragment (lateralcentral) charge


18

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transfer (CT) transitions in the compounds C1-C4 and P1-P4 should consequently be of negligible intensity, strongly suggesting very weak inter-fragment π-conjugation. This conclusion is coherent with the large dihedral angles deduced in the case of both series of compounds and with the MO- and NTO representations (Figure 1d and Figure 3g respectively). In order to obtain more insight on the impact of the topology and number of methoxy substitutions on the nature of the LEB, the symmetry of the experimental bands in the 300-400 nm window are indicated in Figure 3 and in Figure S2e. A select data concerning the theoretical absorption spectra in the region 300-400 nm are provided in Figure 3d and in the Table 2. In view of the similarity of the LEB of C1-C4 and P1-P4 with compounds 1-4, in the following section we focus on the local carbazole transitions. The two lowest energy bands of carbazole (belonging to the C2v symmetry point group) are labelled A1 and B2 (compound 1, Figures 3c and S2f), with the theoretical 0-0 transitions appearing at ~339 nm (small intensity band) and ~295 nm (large intensity band) respectively. 58 Due to symmetry differences, in the case of compounds P1-P4 (belonging to D2 symmetry point group) these transitions are labelled B1 and B2 respectively, and A’ and A” respectively for compounds C1-C4 (belonging to Cs symmetry point group). We note that, the lowest energy bands of compounds 1-4, C1C4, and P1-P4 (A1, A”, and B1 respectively) will be commonly reported hereafter as LEB, whereas the next higher absorption band of larger intensity (B2, A’, and B2 respectively) will be reported as HEB (standing for “higher energy band”). Compared to compounds 1, C1, and P1, both LEB and HEB bands are redshifted by roughly 20 and 30 nm respectively for the compounds C2 and P2, which is due to the much stronger impact of 3,6 substitutions on the HOMO of these compounds than on their LUMO (section 2.2). The experimental difference between the 0-0 transitionwavelengths of LEB and HEB (reported hereafter as ∆λ1-2) increases from ~45 nm (0.56 eV) in the case of compound 1, to 57 nm (0.61 eV) for compound 2. The theoretical results for ∆λ1-2 in compounds 1 and 2 are consistent with the experimental ones, giving 28 nm (0.43 eV) and 41 nm (0.53 eV) respectively.


19


Page 20 of 44

The theoretical ∆λ1-2 values for all compounds are given in Table 2. Intriguingly, much smaller ∆λ1-2 values ranging only between 5-14 nm can be seen for the 2,7-OMe substituted compounds as compared to 30-45 nm for the non-substituted- and the 3,6OMe substituted ones. This effect is also visible in the Figure 3d: the theoretical results of compounds X3-X4 show practically total overlap between the weakly absorbing LEB and the strongly absorbing HEB. Given the globally much larger oscillator strengths of the HEB transitions as compared to LEB (ranging respectively between 0.3-1.0 and 0.001-0.3, Table S1 and Figure 3e), the later band in compounds 3, 4, C3, C4, P3, P4 is totally hidden by HEB. This can explain the important differences observed in character and intensity of the absorption bands of these compounds as compared to the nonsubstituted- or 3,6 di-OMe substituted ones (Figure 3a-c). Based on the above analysis, a good qualitative agreement can be deduced between the evolutions of the experimental and theoretical LEB results (Figure 3d). The LEB is redshifted by the 3,6-OMe substitutions but is blueshifted by the 2,7 ones. Examination of natural transition orbitals (NTOs)

59

shown in Figure 3g (and Figure S3g)

corresponding for instance to compounds P2 and P3 gives some insights on the above effect: due to missing coefficients at the 3,6 positions of the electron NTO (see encircled parts), the OMe π-donor effect in 3,6 positions (compound P2) impacts only the hole NTO, thus redshifting the LEB in P2 as compared to P1. On the contrary, the 2,7-OMe substitutions in P3 impact both hole and electron distributions (see encircled parts), obviously resulting in higher-energy LEB transition in P3 as compared to P1. In the case of tetra-OMe substituted compounds, two effects seem to counteract, resulting in almost similar LEB position of C4 and P4 with respect to C1 and P1. As for the HEB, the electron NTOs of both P2 and P3 compounds show missing coefficients at the linking positions (see encircled parts in Figure 3d), as opposed to the hole NTOs. Therefore, methoxy groups have a greater impact on the hole NTOs than the electron ones in both P2 and P3, resulting in similar HEB redshifts for these compounds as compared to P1. In the case of the tetra-OMe substituted compounds, both substitutions redshifting the HEB, some cumulative effect can be observed.


20

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Finally we comment on the impact of the methoxy substitutions on the absorption intensities. Figure 3e show much larger effect of the methoxy substitutions on the oscillator strengths of the HEB (involving B2 local Cz hole-NTOs) as compared to LEB (involving A1 local Cz hole-NTOs). Both 3,6- and 2,7-OMe substitutions increase the HEB oscillator strengths, with stronger impact by the 2,7 substitutions. A cumulative effect is found in the case of 2,3,6,7 substituted compounds C4 and P4, exhibiting the largest theoretical HEB intensities. 2.4.2 Charge transfer (CT) states The theoretical results indicate that the lowest energy transition (S1 state) of all compounds (except for P2) is an almost dark CT transition from the lateral carbazoles to the central fragment, lower in energy by roughly 0.15-0.65 eV for compounds C1-C4 and 0.07-0.46 eV for P1-P4 ones. In the case of P2, the S1 is a mixed LCz and CT state of non-negligible intensity due to the dominance of the LCz transition (Figure 3g). Several other dark CT states are present in the theoretical spectra of all compounds, appearing at lower energies as compared to the LCz (Table S1). Given the LCz nature of the experimental LEBs (see above), the experimental S1 states cannot be determined from the absorption spectra. However, the comparison between the experimental LEBs of the C1-C4 compounds in THF (Figure 3a) indicates presence of weak-intensity tails (inside the dashed black rectangles), extending beyond the LEB onset (local transition). These tails are absent in the case of compounds P1-P4 in THF solutions, but increase importantly for both series of compounds in solid films (Figure S2). We consequently suggest that the experimental LEB-tails stem from CT transitions, in turn being of non-zero oscillator strengths due to the disorder in the dihedral angles. Indeed, the ground-state energy profiles corresponding to the inter-fragment dihedral angles are almost flat between 55-90° (section 2.2, Figure 1c and Figure S1b), strongly pointing to the presence of geometrical disorder around the equilibrium geometry. Accordingly, the intensity of these LEB tails (CT transitions) should translate the disorder in the inter-fragment dihedral angles, increasing in the order P1-P4 < C1-C4, and solution < solid state.


21


Page 22 of 44

Intriguingly, our scan calculations on the free molecules C1 and P1 indicate that the impact of the dihedral disorder on the oscillator strength of the S1 (CT) state is stronger in the case of compound P1-P4 (Figure 3f), which is at odds with the absence of the LEB tails for this compound (Figure 3b). This theory/experiment dichotomy suggests consequently that the potential for geometrical disorder in the case of P1-P4 compounds might be smaller in the condensed media as compared to “gas phase”. We suspect intermolecular interactions (aggregation) to be in play, which might be more efficient in the case of P- compounds as compared to C- ones, due to the larger structural symmetry of the former compounds.


22

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Figure 3. Optical characteristics of compounds 1-4, C1-C4, and P1-P4: (a-c) normalized experimental absorption spectra in 10-5 M THF solutions. (d) Comparison between the experimental energies (Ε 0-0 , eV) corresponding to the 0-0 transition of the lowest energy band (indicated as λ 0-0 in the Figure 3a) extracted from the absorption spectra of compounds 1-4, C1-C4, and P1-P4 in 10-5 M THF solutions, and the theoretical results corresponding to the A”, B1, and A1 LCz transitions, and to the A”, B2, and B2 transitions. (e) Evolution of the oscillator strengths corresponding to LEB- (A”, B1) and to the HEB (A’, B2) theoretical transitions. (f) Results from scan calculations on compounds P1-P4 and C1-C4 indicating the evolution of the oscillator strength corresponding to the S1 transition as a function of the interring dihedral angles. (g) Singlet Natural Transition Orbitals (NTOs) corresponding to the LEB and HEB of compounds C1-C4 and P1-P4. The red circles highlight the presence- or absence of the pi-donor effect of the methoxy groups. The percentage of transitions stemming from the same type of fragment orbitals are added together. The contributions smaller than 2% are not shown.

Table 2. Experimental wavelengths (λ0-0 , nm) corresponding to the 0-0 transition of the lowest energy band (LEB)a; experimental singlet (S1), triplet (T1), and singlet−triplet energy difference (∆EST) values (eV)b; theoreticalc absorption energies (eV) of the singlet (S1) and triplet (T1) energy; theoreticalc absorption wavelengths (λ, nm) corresponding to the LEB (A1, A”, and B1) and HEB (B2, A’, and B2) of compounds 14, C1-C4, and P1-P4 respectively. The bold numbers in the last column correspond to the more intense transitions. The differences ∆λ1-2 between the two transitions are given in parentheses.


23


Page 24 of 44

experimental absorption

λ0-0

emission

λ0-0

Compound

a

theoretical absorption

λ S1 /T1/ ΔEST

S1 /T1 LEB / HEB

THF

Solid

C1

342

350

3.28 / 2.99 / 0.29

3.88 (320) / 3.22

303 / 272 (31)

C2

375

389

3.21 / 2.86 / 0.35

3.50 (354) / 2.97

335 / 290 (45)

C3

322

328

3.37 / 2.91 / 0.46

3.59 (345) / 3.15

277 / 289 (-11)

C4

337

341

3.23 / 2.85 / 0.38

3.38 (367) / 3.01

308 / 303 (5)

P1

340

338

3.56 / 3.04 / 0.52

4.05 ( 306) / 3.22

301 / 270 (31)

P2

371

375

3.32 / 2.86 / 0.46

3.70 (333) / 2.99

336 / 290 (46)

P3

321

321

3.77 / 2.92 / 0.85

3.86 (321) / 3.15

285 / 289 (-4)

P4

338

339

3.58 / 2.86 / 0.72

3.65 (339) / 3.01

306 / 302 (4)

1

339

-

-

-

299 / 271 (28)

2

370

-

-

-

332 / 291 (41)

3

323

-

-

-

284 / 289 (-5)

4

340

-

-

-

306 / 303 (3)

Extracted from the absorption spectra of compounds 1-4, C1-C4, and P1-P4 in 10-5 M THF solutions and in neat films. b Taken

from set-on of PL and Ph spectra for C- P-compounds measured at 77K (Figure 4a). c The theoretical values were obtained by time dependent (TD) calculations at the CPCM/tuned-w*B97XD/6-31G** level “in diethylether”.


24

0.0 350 1.0

400

450

500

1.0

C1

PL Ph

0.5 T=77K

550

600

C2

0.5 0.0 400

450

500

550

1.0

600

C3

0.5 0.0 350 1.0

400

450

500

550

0.5 0.0 350

600

C4 400

450

500

550

Normalized intensity, a.u.

1.0 λ =300nm, ex.

P1

PL Ph

0.5 0.0 350

1.0

λex.=300nm,

0.5

T=77K

400

450

500

550

600

P2

0.0 350

400

450

500

550

1.0

600

P3

0.5 0.0 350

400

450

500

550

1.0

600

P4

0.5 0.0

600

350

Wavelength, nm

400

450

500

550

600

Wavelength, nm

(a) (b)

4.4

PL P1-P4 Exp.

4.2

PL C1-C4 Exp.

(c)

4

4.4 P1-P4 Theory C1-C4 Theory P1-P4 Exp C1-C4 Exp

4.2

LEB C1-C4 Exp.

4

LEB P1-P4 Exp.

3.8

T1 (eV)

3.8

eV

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Normalized intensity, a.u.

Page 25 of 44

3.6 3.4

3.6 3.4

3.2

3.2

3

3

2.8 X1

X2

X3

X4

2.8 X1

X2

X3

X4

Figure 4. (a) Photoluminescence and phosphorescence spectra of THF solutions of derivatives C1-C4 and P1-P4 recorded at 77 K. Phosphorescence spectra were taken using delay after excitation > 50 ms. (b-c) Comparison of absorption and emission properties between the theoretical and experimental results for compounds C1-C4 and


25


Page 26 of 44

P1-P4. (d) NTOs corresponding to the triplet T1 state of compounds C1-C4 and P1-P4. The percentage of transitions stemming from the same type of fragment orbitals are added together. The contributions smaller than 2% are not shown. 2.4.3 Fluorescence The results concerning the fluorescent (FL) properties of compounds C1-C4 and P1-P4 are shown in Figure 4 and Table 2. Figure 4b shows the evolution of the FL onset energies across the two series of compounds. The evolutions of the theoretical absorption S1 energies and of the experimental absorption LEBs are additionally reported for comparison reasons. We focus firstly on the nature of the emissive states. Except for the C2 and C4, unclearto well-pronounced vibrational pics can be detected for the FL spectra of all other compounds. Accordingly, presence- or dominance of local-excitation contributions for the corresponding transitions can be deduced, thus ruling out any dominant CT origin of the FL emissions in these compounds. However, we remember that the presence of LEBtails in the experimental absorption spectra of these compounds (Figure 3a-c and Figure S2c,d) suggests some contribution from the CT states. It could thus be possible that the LCz-CT energy splitting may be small enough for both of them to be thermally populated, allowing for some minor contribution of CT states in the FL of these compounds. This assumption could explain the absence of a clear- and pronounced vibrational progression in all compounds (Figure 4a), despite the dominant contribution from the local carbazole emissions. Based on the above conclusions, one would expect similar FL energies between the C1C4 and P1-P4 series, and similar evolutions upon methoxy substitutions. Intriguingly, Figure 4b shows that the FL energies of C1-C4 compounds are systematically smaller by 0.2-0.5 eV as compared to P1-P4, additionally exhibiting very different evolutions, thus pointing to the impact of intermolecular interactions 40 and/or the medium polarity. In order to explain these observations, we suggest that the redshift of the C1-C4 PL energies as compared to P1-P4 stems from the larger polarity of the former compounds.


26

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Indeed, zero dipole moments were found for the P1-P4 compounds, as compared to the important ones ranging 2-11 Debye found for the C1-C4 compounds. Note that a dipole moment of 6.6 Debye was found for the unsubstituted compound C1, whereas the number- and the different orientation of methoxy groups in C2 and C3 result in a large disorder peaking at roughly 11 Debye. Our calculations indicate that the dipole moments in the excited state corresponding to the LEB remain strictly zero for P1, but increase from 6.6 to 7.7 Debye in the case of C1. This means that the impact of the substitution topology in the evolution of the FL energies is expected to play a minor role in the case of C1-C4 series, given that the major contribution to energy relaxation in the excitedstate is generally due to the medium polarity effect. Indeed, Figure 4b shows much smaller evolution of the FL energies in case of compounds C1-C4 as compared to P1-P4. The evolution of the FL energies for the P1-P4 compounds in the solid films should consequently be dominated by the intramolecular characteristics with negligible effect from the medium polarity. Figure 4b supports the above assumption: the evolutions of the experimental absorption LEBs (bold red line) and of the experimental FL energies of compounds P1-P4 (dot red line) are almost identical and close in energy. The very small energy difference between these lines (roughly 0.1 eV) indicates small relaxation energies in the excited states of compounds P1-P4 in solid films, consistent with both the zero dipole moments found for these compounds, and the localization of the excitons in the rigid carbazole moieties. These results support consequently both assumptions, on the local-carbazole nature of the FL spectra, and on the impact of the medium polarity on the different FL positions between the C- and P- series of compounds. It can be consequently deduced that the FL spectra of compounds C1-C4 and P1-P4 stem from the local carbazole transitions of A” and B1 symmetry, meaning deexcitation from the same states as those being at the origin of the absorption LEB. As for the impact of the methoxy groups on the FL energies, comments similar to the absorption LEBs can be done, given the identical local carbazole origin: the 3,6 substitutions redshift the emission spectra, whereas the 2,7 substitutions blueshift them as


27


Page 28 of 44

compared to the non-substituted compounds, the two effects being roughly cancelled out in the case of tetra-OMe substituted ones. 2.4.4 Phosphorescence Figure 4a shows phosphorescence spectra of the solutions of compounds C1-C4 and P1P4 in THF recorded at 77K. The lowest triplet (T1) energies deduced from these spectra are given in Table 2, the theoretical results concerning the five lowest triplet states are given in Table S2, whereas the comparison between the theoretical and experimental results is shown in Figure 3c. The NTOs corresponding to the T1 states are shown in Figure 4d, indicating local carbazole origin for the phosphorescence transitions of all compounds. Similar trends as those found in the case of FL properties could be consequently expected for the phosphorescence (Phos) properties across the series. Instead, comparison between Figure 4b and 4c indicates a striking difference between the evolution of the Phos and FL energies: while larger FL energies and stronger variations across the series is found for the P1-P4 compounds as compared to C1-C4, the triplet emissions of both series are of identical energy and less sensitive with respect to the number and topology of the methoxy substitutions. In order to understand these results, we have again focused the attention on the polarity effect by calculating the dipole moments in the T1 state for compounds C1 and P1. Interestingly, the T1 dipole moment of compound C1 is slightly smaller as compared to the ground state (6.2 and 6.6 Debye respectively), whereas zero dipole moment is again found for compound P1. Accordingly, absence of medium effect for both compounds can be deduced, which allows to explain the identical triplet energies between C- and Pseries. The methoxy-substitutions globally decrease the triplet energy in all cases by roughly 0.2 eV as compared to that of unsubstituted compounds, with the stronger impact being induced by the 3,6 substitutions. Obviously, the impact of the methoxy substitution topology and number is less important for the triplet states as compared to the singlet


28

Page 29 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ones. Accordingly, all compounds maintain high triplet energies in the range of 2.85-3.05 eV, which are larger when compared to the experimental triplet energy (2.62 eV) of the benchmark

blue

phosphorescent

C2,N](picolinato)iridium (III) (FIrpic).

emitter

Bis[2-(4,6-difluoropheny)

pyridinato-

60

As a final aspect, the experimental S1-T1 energy differences (∆EST) were deduced for C1C4 and P1-P4 compound in order to evaluate their potential for TADF emission. Large ∆EST values ranging between 0.29-0.46 eV and 0.46-0.85 eV were found, respectively, which is consistent with the small contributions (20-60%) of the triplets on the FL efficiency after solution de-oxygenation. These results indicate consequently that these materials does not satisfy the conditions for their use as TADF emitters, but can be successfully used as exciton-blocking layers. 2.5 Performance in Organic Light-Emitting Diodes Owing to suitable properties of C- and P- families in solid-state (hole mobility reaching 2.4×10-3 cm2/V·s at 6.4×105 V/cm, Ipep ranging from 5.06 to 5.83 eV, LUMO ranging from -2.56 to -1.97 eV and T1 ranging from 2.85 to 3.04 eV), they were tested in blue phsophorescent OLEDs as multifunctional materials providing hole-transporting, excitonblocking and electron-blocking functions. Taking in to account the HOMO/LUMO and triplet levels of blue phosphorescent emitter Bis[2-(4,6- difluorophenyl)pyridinatoC2,N](picolinato)iridium (III) (FIrpic), devices with different structures were fabricated to gain insight for the hole-transporting, exciton-blocking and electron-blocking properties of the compounds. The energy diagrams of devices ITO/HI/CXPX/Em/HB/ET/Ca:Al are presented in Figure 5a displaying good electron-blocking properties of the studied compounds due to the LUMO-LUMO differences between those of the compounds and the emitter FIrpic. Molybdenum trioxide (MoO3) was used as holeinjection (HI) material. The compounds of the C- and P- families were used for the preparation of hole-transporting (HT), exciton-blocking (ExB) and electron-blocking (EB) layers. Doped light-emitting (Em) layer consisted of FIrpic as the blue phosphorescent emitter and 3,6-Di(9-carbazolyl)-9-(2-ethylhexyl)carbazole (TCz1)26 or 1,3-bis(9-carbazolyl)benzene

(mCP)

as

the

host


(Figure

S6).

Diphenyl-4-

29


Page 30 of 44

triphenylsilylphenyl-phosphineoxide (TSPO1) was utilized as hole-blocking (EB), electron-transporting (ET) and exciton-blocking (ExB) material while 2,2’,2’’-(1,3,5benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) was employed for the preparation of electron-transporting (ET) layer (Figure S6).

a)

b) Figure 5. (a) Energy diagrams of the devices studied. Ipep (HOMO) levels of C1-C4 and P1-P4 compounds in solid-state were obtained by photoelectron spectroscopy; their optical energy gaps estimated from the absorption edges of neat films were used to estimate LUMO levels. The hole-blocking and electron-blocking properties of the device structures are marked by the arrows. (b) Jablonski diagram for the tested compounds:FIrpic systems and most of conventional hole-transporting materials used in OLEDs. S0, S1, and T1 are ground, the first excited singlet and the first excited triplet energy levels. Ph is phosphorescence. CBP, TCTA, TAPC, m-MTDATA, and NPB are 4,4’-bis(9-carbazolyl)-2,2’-biphenyl, tris(4-carbazoyl-


30

Page 31 of 44

9-ylphenyl)amine, 4,4′-Cyclohexylidenebis[N,N-bis(4methylphenyl)benzenamine], 4,4′,4′′-Tris[phenyl(m-tolyl)amino]triphenylamine, and N,N′-Di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine, respectively. The exciton-blocking properties of the tested system are marked by the arrow.

Compounds of the C- and P- families were tested in devices of type I with the following structure: ITO/MoO3/CX-PX/TCz1:FIrpic/TSPO1/TPBi/Ca:Al. The differences in devices of a series I (marked as device I:C1-C4, P2 or P4) were only in the tested material (C1-C4, P2 or P4, respectively) of one layer while the other layers of the devices were the same and even deposited in the same process. The functional materials for the devices of a series I were mainly selected according to their HOMO and LUMO levels and charge-transporting properties 61 in order to ensure good charge injection, transport and balance in light-emitting layer (Figure 5a), and taking into account their triplet-energy levels for achieving of good exciton confinement on the emitter. As it shown in Figure 5b, the studied compounds are characterized by good exciton-blocking properties with respect of the emitter used. It should be noted that the triplet-energy levels of compounds C1-C4, P1-P4 are higher than those of most of conventional holetransporting materials used in OLEDs (Figure 5b). The key output parameters of the devices are summarized in Table 3. The output characteristics are given in Figures 6 and S4.

Device I:C1 Device I:C2 Device I:C3 Device I:C4 Device I:P2 Device I:P4

10 10000 2

1000

100

0

2

4

6

8

10

10 12

EQE, %

900 850 800 750 700 650 600 550 500 450 400 350 300 250 200 150 100 50 0

Brightness (cd/m )

2

Current density (mA/cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



1

0.1 1

10

100

Current density, mA cm

Voltage (V)

a)

2

1000

b)


31


0.8

0.6

0.4

Device III:C1 Device III:C2 Device III:C3 Device III:C4 Device III:P4

1.0

Intensity, a.u.


1.0

Intensity, a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 44

0.8

0.6

0.4

0.2

0.2

0.0 400

500

600

700

0.0 400

Wavelength, nm

500

600

700

Wavelength, nm

c)

d)

Figure 6. (a) Voltage-current density and voltage-brightness characteristics; (b) current density-external quantum efficiency characteristics; (c) normalized electroluminescence spectra of devices of a series I. (d) Normalized electroluminescence spectra of devices of a series III.

The turn on voltages (Von) ranging from 2.3 to 2.5 V observed for most of the devices of the series I confirmed perfect charge-injection and transporting properties in the devices, except device I:C1 which showed Von of 5.0 V. High value of Von observed for this device can apparently be explained by the high energy barrier for holes due to low HOMO (-5.56 eV) of C1 (Table 3). Despite of the similarity in the turn on voltages, the voltage-current density curves showed different trends mainly due to the differences in charge-transporting properties of C1-C4, P2, P4 under different electric fields (Figure 6a). All the devices of the series I exhibited high maximum brightness ranging from 15500 to 33100 cd/m2 due to the charge balance and exciton confinement in the emitter. Due to the relatively low turn-on voltages and high brightness, the maximum current and power efficiencies (CEs and PEs) of the devices reached high values of 47.8 cd/A and 36 lm/W, respectively (Table 3, Figure S3). For the device I:C2, the value of maximum external quantum efficiency (EQE) exceeding 20% was observed. The lowest EQE of 6.94 % was observed for device I:C1 apparently due to the charge balance problems. Electroluminescence (EL) spectra of the devices of a series I were characterized by blue


32

Page 33 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(sky-blue) emission related to the FIrpic emission. In addition, EL spectra contained high-energy low-intensity band showing that the radiative recombination of excitons occurred not only on FIrpic but also on the host and/or charge-transporting materials. Taking into account the shape and position of the high-energy band one can presume, that this emission is related to recombination of singlet excitons on TCz1 host which also is an efficient fluorescent emitter.62 Higher relative intensity of the high-energy band was observed

in

EL spectra

of

the

devices

of

a

series

II having

structure

ITO/MoO3/CX/TCz1:FIrpic/TCz1/TPBi/Ca:Al in which the non-doped TCz1 layer was additionally used as the exciton-blocking layer (Figures 5a and S5a). Due to the exciton recombination on the fluorescence emitter (TCz1), the output characteristics of the devices of a series II were found to be worse than those of the devices of a series I (Table 3, Figure S4b-e). To avoid the high-energy emission, TCz1 host was replaced by mCP host

in

devices

of

a

series

III

having

structure

ITO/MoO3/C-

PX/mCP:FIrpic/TSPO1/TPBi/Ca:Al (Figures 5 and S5, Table 3). EL spectra of devices of a series III contained only emission of FIrpic (Figure 6d). The emission related to other used materials was absent. However, the devises of a series I based on TCz1 showed better characteristics than the devices of a series III based on mCP host because of the bipolar charge transport and appropriate HOMO/LUMO levels of TCz1 (Figure 5a).63 Inferior performances of devises of a series III can also be explained by higher concentration of FIrpic in the emitting layer. Nevertheless, the results observed for the devices of the series I, II, and III confirm that compounds of the C- and P- series can be efficiently utilized as multifunctional hole-transporting, exciton-blocking and electronblocking materials for blue OLEDs. The devices showed relatively high EQEs and exciton recombination was absent in the layers of compounds of the C- and P- families.

Table 3. EL parameters of phosphorescent OLEDs

Devices

Von [V]

Max brightness

Current efficiency

Power efficiency

[cd/m2]

[cd/A]

[lm/W]

External quantum efficiency [%]


33


Page 34 of 44

10/1000 max cd/m2 I:C1

5.0/8.0

15500

15.2

8.1

6.94

I:C2

2.3/4.2

32400

40.3

33.2

16.5

I:C3

2.5/5.2

30100

47.8

34.9

21.1

I:C4

2.5/5.3

33100

46.3

33.2

20.5

I:P2

2.3/3.6

20400

39.4

33.2

16.3

I:P4

2.4/3.9

32800

34.3

36.0

19.4

II:C1

5.3/8.8

9100

6.0

11.0

4.8

II:C2

4.1/7.5

7900

3.0

5.0

3.1

II:C3

4.4/7.2

9500

3.4

8.3

5.3

II:C4

3.8/8.3

10700

5.6

9.5

5.0

III:C1

7.5/10.9

2200

2.4

2.7

0.8

III:C2

5.4/8.5

4200

3.0

6.8

3.2

III:C3

8.5/11.2

2100

6.6

14.1

4.9

III:C4

5.4/9.3

3200

4.2

9.4

3.6

III:P4

7.2/10.7

2500

4.1

9.3

3.5

Conclusions Six carbazole derivatives containing different number of methoxy substituents (2-4 per carbazole unit) in different positions (2,3,6,7) were synthetized and characterized. The compounds were found to adopt quasi-orthogonal inter-ring configurations in solution, which disrupts their through-bridge π-conjugation and makes the redox and optical properties being dominated by the lateral carbazole fragments. Upon methoxy substitutions the ionization potentials and the fluorescence and phosphorescence energies globally decrease, with the largest effect being observed for the tetra-methoxy substituted compounds. Very high triplet energies (2.85-3.05 eV) were found in all cases, exhibiting continuous- but small variations upon methoxy substitutions. On the contrary, large- and opposite effects from the 3,6-methoxy substitutions (redshift) and the 2,7- substitutions


34

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(blueshift) as compared to the non-substituted compounds were found for the FL spectra. Despite of the above mentioned similarities, compounds C1-C4 exhibit larger variation of the ionization potentials (by ~0.3 eV) and systematically redshifted FL spectra (by ~0.2-0.5 eV) as compared to P1-P4. This is explained by the large dipole moments in both the ground- and excited states of the V-shape compounds C1-C4, which are missing in the case of linear D-A-D P1-P4 compounds. While explained on the basis of wellknown effects, this result highlights an important design strategy for new materials in optoelectronic devices: the sensitivity of the intramolecular properties of electroactive materials to the impact of the bulk electrostatic effects can be reduced or enhanced by means of linear D-A-D or V-shape molecular architecture respectively. Our results show that additional fine-tuning of these properties can be achieved by means of methoxy substitutions. Finally, due to their quite high T1 energy levels, the new C- and P- compounds were efficiently utilized as multifunctional hole-transporting, exciton-blocking and electronblocking materials in blue phosphorescent organic light-emitting devices showing high maximum current-, power-, and external quantum efficiencies of 47.6 cd/A, 36 lm/W, and 21.1 %, respectively. These compounds constitute consequently good candidates for host- and for exciton blocking materials in OLED applications.

Supporting Information Details on thermogravimetric analysis (TGA), theoretical data corresponding to optical transitions, comparison of dihedral angles obtained by means of two different theoretical methods, OLED characteristics.

Acknowledgements This research was funded by the European Regional Development Fund according to the supported activity ‘Research Projects Implemented by World-class Researcher Groups’


35


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under Measure No. 01.2.2-LMT-K-718. G.S gratefully acknowledges the calculation center of Cergy-Pontoise university for computing time and support, S.S. gratefully acknowledges the Institute of Advanced Studies of the Cergy-Pontoise University and the LPPI research group for funding several invited professorship stays at LPPI.

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Sensitivity of Redox and Optical Properties of Electroactive Carbazole

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