4-Pyridyl-9,9â²-spirobifluorenes as Host Materials ... - ACS Publications

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4-Pyridyl-9,9'-spirobifluorenes as Host Materials for Green and Sky-Blue Phosphorescent OLEDs Sebastien Thiery, Denis Tondelier, Céline Declairieux, Bernard Geffroy, Olivier Jeannin, Rémi Métivier, Joëlle Rault-Berthelot, and Cyril Poriel J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp511385f • Publication Date (Web): 04 Feb 2015 Downloaded from http://pubs.acs.org on February 18, 2015

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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4-Pyridyl-9,9'-Spirobifluorenes As Host Materials For Green And Sky-Blue Phosphorescent OLEDs Sébastien Thiery,† Denis Tondelier,§ Céline Declairieux,§ Bernard Geffroy,§, £ Olivier Jeannin,† Rémi Métivier,‡ Joëlle Rault-Berthelot,†*a and Cyril Poriel†*a †

. Université de Rennes 1-UMR CNRS 6226, "Institut des Sciences Chimiques de Rennes" MaCSE group. Bat 10C, Campus de Beaulieu, 35042 Rennes, France

‡

. PPSM, Institut d'Alembert, ENS Cachan, UMR CNRS 8531, 61 Av. du Président Wilson, 94235 Cachan, France §

. LPICM, Ecole Polytechnique, UMR CNRS 7647, Route de Saclay, 91128, Palaiseau £

. LICSEN, CEA Saclay, IRAMIS/NIMBE, 91191 Gif sur Yvette

KEYWORDS. Phosphorescent OLED, Host Material, Pure Hydrocarbons, 4-pyridyl-spirobifluorene, Ir(ppy)3, FIrpic.

ABSTRACT. We report herein new pyridine substituted-spirobifluorene (SBF) dyes, i.e. 4-(9,9’spirobi[fluoren]-4-yl)pyridine (4-4Py-SBF), 3-(9,9’-spirobi[fluoren]-4-yl)pyridine (4-3Py-SBF) and 2-(9,9’-spirobi[fluoren]-4-yl)pyridine (4-2Py-SBF), built on the association of the 4-substituted spirobifluorenyl core and various regioisomers of pyridine. These organic semi-conductors possess high triplet energy levels (ET around 2.7 eV) in accordance with their use as hosts for green and skyblue phosphorescent organic light-emitting diodes (PhOLEDs). These dyes have been synthesized from 4-bromo-spirobifluorene (4-Br-SBF) platform, obtained from a new and efficient synthetic 1 ACS Paragon Plus Environment


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approach using the promising building block 4-bromofluorenone as key intermediate. Synthesis, structural, thermal, electrochemical and photophysical properties of the three dyes have been investigated in detail and compared to other model compounds, namely 4-phenyl-SBF (4-Ph-SBF), 2-phenyl-SBF (2-Ph-SBF) and 2,4-pyridyl-SBF (2-4Py-SBF) in order to precisely study the influence of (i) the pyridine unit, (ii) the position of the nitrogen atom within the pyridyl core (either in position 4, 3 or 2) and (iii) the substitution at the C4 position of SBF. This rational structure properties relationship study shed light on the effect of the substitution in position 4 of the SBF core and may pave the way to the development of such materials in electronics. Finally, the high performance of green PhOLEDs, ca. 63 cd/A and of sky-blue PhOLED ca.16 cd/A clearly evidence the potential of these new SBF derivatives as hosts for phosphorescent dopants.

Introduction Organic light-emitting diodes (OLEDs) have attracted considerable attention in recent years because of their potential applications in organic flat-panel displays and solid-state lighting.1-4 The OLED technology has rapidly emerged as a promising technology thanks to (i) the synthesis of a large number of new organic semiconductors (OSCs), (ii) the improvement of the devices structures, (iii) the development of new OLEDs fabrication techniques, (iv) a better understanding of the OSCs properties. Despite OLED devices for display and lighting are on the verge of broad commercialization, this technology is still strongly perfectible and particularly, efficient blue light emission is the bottle-neck for efficient full-color OLED display and white OLEDs. In this context, numerous efficient blue fluorophores have been designed for the last twenty years and used as emitting layer in OLED.5-11 However, the research towards highly efficient OLEDs using fluorescent materials has quickly pointed out the limit of this technology due to its maximum achievable internal quantum yield (IQE) of 25 % (which correspond to 5% of an external quantum efficiency (EQE) without enhancement of light out-coupling).12, 13 Significant progress has been made with the host2 ACS Paragon Plus Environment

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guest phosphorescent OLED technology for which IQE of 100 % (EQE ca 20%) may be theoretically reached.12-17 In this technique, phosphorescent emitters are doped into an appropriate host material as emitting layer (EML) to avoid self-aggregation quenching and triplet-triplet annihilation. A precise roadmap for efficient host materials for blue phosphorescent dyes has then been clearly announced. Thus, an ideal host material for blue PhOLEDs should meet the following intrinsic requirements: (i) a high ET at least higher than 2.75 eV to confine the triplet excitons within the blue phosphorescent guest and prevent reverse energy transfer from the guest to the host, (ii) HOMO/LUMO levels adapted to the Fermi levels of the electrodes allowing an efficient charge injection, (iii) good and well balanced mobility of electron and hole in order to optimize their recombination, (iv) thermal and morphological stability to extend the lifetime of the device. Combining all these properties within a single molecule requires hence very precise molecular designs, which can be only obtained through a perfect knowledge and control of the intra- and intermolecular interactions (physical and electronic) occurring in the material. Recently, our group described the potential of 9,9'-spirobifluorene (SBF) and of 4-substituted phenyl-spirobifluorene (4-Ph-SBF), see structure in chart 2, as hosts for blue and green PhOLEDs with current efficiency reaching 20 cd/A for blue PhOLED (EML: SBF doped with 19 % Firpic) and 49 cd/A for green PhOLEDs (EML: 4-Ph-SBF doped with 10% Ir(ppy)3).18 In 4-Ph-SBF, we notably showed that the presence of a pendant phenyl group in the ortho position leads to a twisted configuration, retaining the electronic properties of its constituted building block SBF, while improving thermal properties. This is an important feature in the design of efficient host material for PhOLED. However and despite very promising for hosting phosphorescent emitters, 4-substituted SBF derivatives are only barely reported to date.18-25 As nitrogen heterocycle-containing materials have greatly contributed to the enhancement in device performance of PhOLEDs,26 a further step on the path to efficient and robust host materials with high ET, was reported by our group with the change of the phenyl group by a pyrimidine group in 4-5-pyrimidine-spirobifluorene (4-5Pm-SBF)

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and its use as host for green and blue PhOLEDs.25 In 4-5Pm-SBF, the presence of a pendant pyrimidine group in the ortho position of SBF leads not only to the improvement of the thermal properties but also to the lowering of the LUMO level (calculated from onset reduction potentials ) from -1.95 eV in 4-Ph-SBF to -2.23 eV in 4-5Pm-SBF therefore to the lowering of the threshold working voltage of the blue devices from 4.0 V for 4-Ph-SBF to 3.3 V for 4-5Pm-SBF.25 In the meantime, the efficiency of the green PhOLED was improved from an EQE of 10.4% with 4-PhSBF as host to an EQE of 13.7% with 4-5Pm-SBF as host, but that of the blue PhOLED decreased from 5.7% with 4-Ph-SBF as host to 4.8% with 4-5Pm-SBF as host. The decrease of blue PhOLED efficiency may translate a less efficient exciton transfer from the host to the guest that may be explained by the triplet energy level (ET) of 4-5Pm-SBF measured at 2.75 eV25 and therefore slightly lower than that of 4-Ph-SBF (2.77 eV).18 Kido and co-workers have recently reported a series of hosts based on meta-terphenyl-like derivatives of carbazole (chart 1) all possessing similar π-conjugation but structurally different from each other in the number (and the position) of nitrogen atoms in the central aromatic unit.27 From 2,5-bis(4-(carbazol-9-yl)phenyl)benzene (molecule K1 with a meta-terphenyl central core) to 4,6bis(3-(carbazol-9-yl)phenyl) pyrimidine (molecule K2 with 2,6-diphenylpyrimidine central core), they reported the lowering of (i) the ET (from 2.72 eV to 2.64 eV) and of (ii) the LUMO energy level (from -2.6 to -3.06 eV). Interestingly, when the central phenyl unit was replaced by a pyridine unit as in K3 and independently of the nitrogen position27, a less pronounced lowering of ET (from 2.72 eV to 2.71 eV) and of the LUMO energy level (from -2.6 to -2.75 eV) were recorded. With this in mind, introduction of a pyridyl unit despite less efficient electron withdrawing unit than the pyrimidine core, appears as an appealing strategy to obtain high ET hosts.


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Chart 1. Chemical structure of hosts based on meta-terphenyl-like derivatives of carbazole studied by Kido and co-workers.27

In this context, three new pyridine dyes, i.e. 4-4Py-SBF, 4-3Py-SBF and 4-2Py-SBF, with the nitrogen atom in position 4, 3 or 2 (see chart 2), have been designed, synthesized and studied in detail. A 2-substituted analogue, i.e. 4-(9,9’-spirobi[fluoren]-2-yl)pyridine (2-4Py-SBF) has been also synthesized in order to study the effect of the position of substituents on the SBF scaffold. Thus, in this work we first report our synthetic investigations towards the synthesis of the four dyes and we notably present a new efficient route towards 4-bromo-9,9'-spirobifluorene (4-Br-SBF), key fragment of our synthetic approach. Thus, after synthetic considerations, the electrochemical, thermal and photophysical properties of the four pyridine dyes will be discussed and coupled to a theoretical approach. The influence of the position (2- vs 4-) of the pyridine on the SBF scaffold and the influence of the position of the nitrogen atom within the pyridine unit will be notably investigated. Finally, the incorporation of these four semiconductors as host materials for green and sky-blue PhOLEDs will be presented. 4-substituted pyridine dyes lead to efficient green and sky-blue PhOLED with notably EQE of 15.7 % (63.4 cd/A) for 4-4Py-SBF doped with Ir(ppy)3 and EQE of 5.1 % (16.1 cd/A) for 4-4Py-SBF doped with FIrpic. Chart 2. Molecules studied in this work



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Results and discussion Synthesis 4Br-SBF has been previously reported by Ma and co-workers using as key step a mono lithiumhalogen exchange of 2,2'-dibromobiphenyl followed by the trapping of the corresponding lithiated intermediate with 9-fluorenone.19 This efficient synthetic pathway allows obtaining 4-Br-SBF, after a subsequent intramolecular electrophilic cyclization of the resulting fluorenol, with 83 % yield in two steps.19 We developed, in the present work, a new approach allowing obtaining 4-Br-SBF through the synthesis of a key building block: 4-bromo-9-fluorenone 2 (scheme 1). The synthetic strategy is based on the key Miyaura-Suzuki cross coupling between phenylboronic acid possessing an ethylcarboxylate group in position 2 and 1-bromo-2-iodobenzene. The selective cross-coupling reaction proceeds with 65% yield leading to the biphenyl 1 possessing on one ring the ethylcarboxylate and on the other a bromine atom. The intramolecular aromatic electrophilic substitution in methanesulfonic acid leads to the formation of the 4-bromofluorenone 2 with 95 % yield. The further coupling of 2 with 2-lithiated biphenyl followed by intramolecular ring closure leads to the formation of the spiro derivative 4-Br-SBF with high yield (75 % over the two last steps). Although the overall yield of this approach, ie 46%, the second reported to date for 4-BrSBF, is lower than that reported by Ma,19 this route used as key intermediate the versatile 4-bromofluorenone 2 that may be an interesting building block to design, in the future, other organic semiconductors of interest for electronic applications. Finally, a subsequent Suzuki cross-coupling between 4-Br-SBF and either 3- or 4-pyridine-phenyl boronic acid was performed (Pd(dppf)Cl2/K2CO3/ DMF) providing 4-4Py-SBF and 4-3Py-SBF with excellent yields (89 and 86% respectively). Due to the high instability of 2-pyridine-phenyl boronic acid, the synthesis of 42Py-SBF has been performed via a modified approach involving the corresponding 2-(9,9'spirobi[fluorene]-5-yl)-4,4,5,5-tetramethyl-1,3,2-dioxoborolane 3, synthesized from lithium-halogen


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exchange reaction of 4-Br-SBF in the presence of BuLi followed by the trapping of the corresponding lithiated intermediate with 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (yield: 75%). The cross coupling reaction between 3 and 2-bromopyridine (Pd(Ph3)4/K2CO3/THF) finally provides 4-2Py-SBF with a moderate yield of 56%. Toluene, 100 °C

O

CO2Et

I K2CO3, Pd(dppf)Cl2

CO2Et

MsOH, 100°C

+ B(OH)2

Br

(65 %)

(95 %) Br

Br

1

1) n-BuLi/THF -78 °C 2) Ph2Br/THF -78°C to rt 3) AcOH/HCl 70°C

O

2

DMF, 150°C K2CO3 Pd(dppf)Cl2

(75 %)

N

Br

B(OH)2 or (86 %)

Br

2

4-Br-SBF

N

N B(OH) 2

4-4Py-SBF 4-3Py-SBF 4-2Py-SBF

(89 %) O O B O

n-BuLi/THF -78 °C

THF/H2O, 70°C K2CO3, Pd(Ph3)4 N O B

O

Br

(56 %)

3 (75 %)

Scheme 1. Synthesis of 4-4Py-SBF, 4-3Py-SBF and 4-2Py-SBF The model compound 2-4Py-SBF (see structure in chart 2) was obtained from the coupling of 2Br-SBF,25, 28 with 4-pyridine-phenylboronic acid, using similar conditions than those described above (yield : 82%). Thermal Properties The thermal properties of all pyridyl substituted SBFs were investigated by thermogravimetric analysis (TGA) (figures S25, S27, S29, S31 and S33-left in SI) and differential scanning calorimetry (DSC) (figures S26, S28, S30, S32 and S33-right in SI) and the results are summarized in SI (table S3).



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The decomposition temperature Td1 (defined as the temperature at which the sample has lost the first 5% of mass during heating, figures S25, S27, S29, S31 and S33-left in SI) of 4-substitutedpyridine SBFs, 4-4Py-SBF, 4-3Py-SBF and 4-2Py-SBF, lies in a small range between 217°C and 242°C, the 2-pyridine ring leading to the more stable molecule. The three Td are however smaller than that of 4-Ph-SBF (254 °C). Similarly, Td of 2-4Py-SBF occurs at 181°C lower than that of 2Ph-SBF (238 °C). The substitution of the SBF core by a pyridyl units instead of a phenyl unit leads then to less thermally stable compounds. 4-2Py-SBF and 4-4Py-SBF present melting transitions measured from the peak onset at 201 and 188-189°C respectively, whereas that of 4-3Py-SBF and 2-4Py-SBF is detected at ca 147 and 140°. The pyridyl substituent also has an interesting positive influence on the increase of the glass transition temperatures (Tg), key parameter for device stability. Indeed, Tg of 4-substituted-pyridine SBFs, 4-4Py-SBF, 4-3Py-SBF and 4-2Py-SBF, appear to lye between 81 and 92°C (figures S26, S28, S30, S32 and S33-right in SI). These Tg are higher than that of SBF18 (80°C), 2-Ph-SBF25 (78°C) and 4-Ph-SBF25 (76°C). Thus, we note that, independently of the substitution (C2 or C4), pyridyl-substituted compounds always possess slightly higher Tg than their phenyl analogues, Table 2. This higher Tg may be the consequence of stronger intermolecular interactions due to CH…N hydrogen bonding interactions.29 Interestingly, Tg values of 4-4Py-SBF (84°C) and 2-4Py-SBF (92°C) are slightly higher than that of 4-3Py-SBF and 4-2Py-SBF (both 81°C) since their nitrogen atoms are located at the exterior of the molecule to give stronger molecular interactions. A similar influence of the nitrogen position has been noted by Kido and co-workers for meta-terphenyl-like derivatives of carbazole (chart 1).27 It is important to stress that all compounds present Tg higher than those of classical host materials for PhOLEDs such as CBP (62°C)30 or m-CP (55°C).31 Finally, it should be mentioned that except 4-2Py-SBF, the pyridyl-substituted SBFs do no present any crystallization phenomena being hence highly promising for devices incorporation. A

This temperature can be also seen as the sublimation temperature. Indeed, the sublimation of the material at high temperature may also lead to a total mass loss.

1


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crystallization transition was only recorded for 4-2Py-SBF at high temperature, Tc: 140°C, during the second heating cycle in DSC (figure S26 in SI), this Tc being even higher than that reported for 4-Ph-SBF18 (115 °C). Structural properties The molecular structures of regioisomers 4-4Py-SBF and 2-4Py-SBF were further confirmed by X-ray diffraction on single crystals. Due to their very high solubility in common organic solvents, it was unfortunately not possible to obtain single crystals good enough for X-ray diffraction for the other regioisomers 4-3Py-SBF and 4-2Py-SBF.

Figure 1. Molecular structure of 4-4Py-SBF and of the two molecules contained in a 2-4Py-SBF single crystal asymmetric unit. Contrary to 2-4Py-SBF, for which X-ray diffraction data reveal an asymmetric unit containing two independent molecules 1 and 2, single crystal of 4-4Py-SBF reveals only one molecule (Figure 1). From each molecule X-ray data, we evaluated different structural parameters (angles and distances) in order to compare the present pyridine-SBF molecular structures to that of their phenyl-analogues 4-Ph-SBF (two molecules by asymmetric unit),18 2-Ph-SBF (two molecules by asymmetric unit, only one is provided in table 1)25 and 9,9'-SBF 32 (see table 1). Different important features concerning (i) the relative position of the pendant pyridine ring and (ii) the deformation of the two fluorenyl units and their consequences on the molecule properties need to be pointed out. 9 ACS Paragon Plus Environment


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N/H Ar Hb H5 Ph3

4

Ph4

Ha H3

H3 Ph3

Ph4

Hb

2

Ar

H/N

H1 Ha Ph1

Ph2

Ph1

Ph2

Scheme 2. Numbering of 4- and 2-substituted SBF In 2-4Py-SBF, the pyridyl ring (Ar) is slightly twisted from the mean plane of Ph4 (see scheme 2 and figure 1) with an angle of 37.5° in molecule 1 and 32.8° in molecule 2. These angles are similar to those found (i) in its disubstituted analogue 2,2'-bis(4-Pyridyl)-9,9'-spirobifluorene (32.25° and 33.01°),33 (ii) in 2-Ph-SBF (37.4° in molecule 1)25 and (iii) in other 2-substituted SBFs.34 In the meantime, the distance between the two closest hydrogen atoms of the pyridyl ring (Ha and Hb) and of those of Ph4 (H1 and H3) is measured between 2.27 and 2.42 Å, that is lower or equal to the sum of van der Walls radii35 indicating some interactions between the hydrogens H1/Ha and H3/Hb. This short distance is similar to the distances measured (i) in 2,2'-bis(4-Pyridyl)-9,9'-spirobifluorene (between 2.3 and 2.38 Å),33 (ii) in 2-Ph-SBF (between 2.36 and 2.39 Å in molecule 1)25 and (iii) in other 2-substituted SBFs.34 We note that in 4-substituted compounds, 4-4Py-SBF and 4-Ph-SBF, the H3-Ha distance is longer (from 2.44 to 2.68 Å) than in 2-substituted compounds indicating no interaction between H3/Ha contrary to what is observed for 2-Ph-SBF. There is indeed another feature to consider in 4-substituted compounds, which is the H5-Hb distance, measured at ca 2.61 Å in the case of 4-4Py-SBF. Thus, and in order to avoid a strong steric hindrance between Hb and H5, the angle between the substituted fluorenyl unit and its pendant pyridine ring is larger in the case of a C4 substitution (4-4Py-SBF: 42.2°) than in the case of a C2 substitution (2-4Py-SBF: 37.5 and 32.8°, table 1), clearly highlighting the influence of the position on the twist angle. This difference appears nevertheless smaller than that observed between 2-Ph-SBF and 4-Ph-SBF (Table 1) translating also the influence of the substituent on this important structural parameter which drives


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the electronic properties (see below). Indeed, the angular variation will lead to a more extended πconjugation between the pyridine and the fluorene in 4-4Py-SBF than between the phenyl and the fluorene in 4-Ph-SBF and hence to a more contracted optical gap for the former (see Table 3). In addition, in the case of 4-2Py-SBF, the dihedral angle between Ph4 and the 2-pyridine unit should be smaller than in 4-4Py-SBF due to the presence of nitrogen atom in position 2, instead of a CH. This feature will lead for 4-2Py-SBF to an extension of the π-conjugation, see UV-vis absorption spectra below. Of particular interest, the deformation of the pyridine-substituted fluorene found in 4-4Py-SBF appears also remarkable (Figure 1). Indeed, a dihedral angle as high as 16.8° between Ph3 and Ph4 mean planes is measured for 4-4Py-SBF, four times higher than the deformation found in the non substituted fluorene unit (with a Ph1/Ph2 dihedral angle close to 4°). If the important deformation of the substituted fluorene appears very unusual and characteristic of 4-substituted SBFs, the small deformation of the non substituted fluorene of 4-4Py-SBF appears nevertheless very similar to that measured in 9,9'-SBF32 (4.1 and 1.3°). In 4-Ph-SBF, the substituted fluorene was also distorted, although less than in 4-4Py-SBF, with a Ph3/Ph4 angle strongly larger than the Ph1/Ph2 angle (12.7 vs 4.2° in molecule 1 and 4.8 vs 2.2° in molecule 2).



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Table 1. Structural parameters obtained from X-ray structures of 4-4Py-SBF and 2-4Py-SBF compared to those of 4-Ph-SBF, 2-Ph-SBF and SBF (see SI for details). Angle (°) between Ph4 and Ar (Pyridine or phenyl) planes

Angle (°) between Ph3 and Ph4 planes


Spiro Angle (°)

4-4Py-SBF

42.2

16.8

4.1

4-Ph-SBF molecule 1 4-Ph-SBF molecule 2

51.2

12.7

56.6

2-4Py-SBF molecule 1 2-4Py-SBF molecule 2 2-Ph-SBF molecule 1 9,9'-SBF

87.8

Distance between H5 and Hb (Å) 2.61

Distance between H3 and Ha (Å) 2.44

ref

4.2

88.3

2.56

2.55

18

4.8

2.2

89.7

2.61

2.68

18

Angle (°) between Ph4 and Ar (Pyridine or phenyl) planes



Spiro Angle (°)

Distance between H1 and Ha (Å)

ref

37.5

3.2

6.1

88.1

2.42

Distance between H3 and Hb (Å) 2.39

32.8

5.0

7.4

89.5

2.27

2.40

37.4

7.1

2.7

87.3

2.39

2.36

25

-

4.0

1.3

89.0

-

-

32

This work

This work This work

The 2-4Py-SBF isomer displays a significant different behaviour, highlighting again the difference between 2 and 4-substituted SBFs. Thus, in the two molecules present in the asymmetric unit of 24Py-SBF unit, both fluorenyl cores are weakly twisted (with dihedral angles Ph1 and Ph2 or Ph3 and Ph4 planes measured between 3.2 and 7.4 °). In 2-4Py-SBF, the more twisted fluorene is the unsubstituted one with Ph1/Ph2 or Ph3/Ph4 angles of 6.1 and 3.2° or 7.4 and 5.0° for molecule 1 and molecule 2 respectively. It is interesting to note that the deformation of the 2-4Py-fluorenyl unit (3.2 and 5.0° in molecule 1 and 2 resp.) is comparable to that reported for disubstituted analogue 2,2'bis(4-Pyridyl)-9,9'-spirobifluorene33 (with dihedral angle in fluorenes of 3.81° and 3.88°), meaning that the number of substituents has no influence on the deformations. All these structural features clearly highlights that the substitution in position 4 of the SBF core leads to a highly curved substituted fluorenyl core possessing a large twist angle with its pendant substituent. This is one of the peculiar particularities of 4-substituted SBF derivatives. Finally, the angle between the mean planes of the two central cyclopentadienyl units of spiroconjugated fluorenyl cores is of 87.8° in 4-4Py-SBF, 88.1 and 89.5° in 2-4Py-SBF (molecule 1 and


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molecule 2 respectively), similar to that recorded for 2,2'-bis(4-Pyridyl)-9,9'-spirobifluorene (87.9°)33 and for SBF (89.0°),32 meaning that no significant deformation in the spiro-configuration is induced by the presence of the pyridine ring neither in C4, nor in C2.

Electrochemical Properties Electrochemical properties have been investigated by cyclic voltammetry (CV) in CH2Cl2 in oxidation and reduction and are summarized in table 2. Before all studies, the electrolytic medium was dehydrated by adding Al2O3 in absence of the SBF derivatives. Then, no Al2O3 was added in the electrolytic cell in presence of Pyridyl-substituted SBFs, as all these compounds quickly and strongly adsorb on Al2O3 leading to an important decrease of the intensity of the oxidation and reduction waves. 4-2Py-SBF

4-2Py-SBF

50

4-3Py-SBF

25

I(µA)

25 0 Cycles 1 à 3 Cycles 4 à 6 Cycles 7 à 9 Cycle 10

30 -25 0.0

0.5

1.5

20

2.0

15

E(V) vs SCE

20

I(µA)

1.0

I(µA)

40

Eonsetox: 1.48 V

10

HOMO : -5.88 eV

10

Eonsetox: 1.5 V 5

HOMO: -5.9 eV

0 0

-10 0.0

0.5

1.0

1.5

2.0

-5 0.0

2.5

0.5

E(V) vs SCE

1.0

1.5

2.0

2.5

E(V) vs SCE

4-4Py-SBF

2-4Py-SBF

25 1.5 20

15

1.0

10

I(µA)

I(µA)

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


Eonsetox: 1.48 V

5

Eonsetox: 1.38 V

0.5

HOMO: -5.78 eV

HOMO: -5.88 eV

0

0.0

-5 0.0

0.5

1.0

1.5

2.0

2.5

0.0

E(V) vs SCE

0.5

1.0

1.5

2.0

2.5

E(V) vs SCE

Figure 2. Cyclic voltammetry of 4-2Py-SBF, 4-3Py-SBF, 4-4Py-SBF and 2-4Py-SBF (5 10-3 M) recorded in CH2Cl2 + Bu4NPF6 0.2 M, sweep-rate 100 mV.s-1. Platinum disk (Ø: 1mm) working electrode.



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All compounds present at least three successive oxidation waves, with maxima recorded between 1.0 and 2.2 V/SCE (see CVs figure 2). Recurrent cyclic voltammetries including the three waves (see inset in figure 2) lead for 4-2Py-SBF to the appearance and the regular growth of a new reversible redox process at less anodic potential values (between 1.0 and 1.3 V). This signs an electropolymerization process. After such exploration, the electrode is covered with an insoluble electroactive polymer. This process is however largely less efficient than that observed for 9,9'SBF36 or 4-Ph-SBF18 due to (i) the low monomer concentration caused by its adsorption on Al2O3 and to (ii) the presence of weak amount of water in the electrolytic medium (due to an insufficient amount of Al2O3) which efficiently traps the radical-cations formed. From the onset of the first oxidation wave,37 we determined the HOMO energy levels of 4-2PySBF (-5.88 eV), 4-3Py-SBF (-5.9 eV) and 4-4Py-SBF (-5.88 eV). These values are almost identical indicating that the nature of the pyridine ring has almost no influence on the HOMO energy levels. In addition, these HOMO energy levels are slightly higher than that of SBF (HOMO: -5.94 eV),38 indicating that the substitution in C4 by a pyridyl unit even renders the molecule slightly more easily oxidizable. Such a feature is surprising since pyridine unit is an electron deficient unit and it should withdraw electrons from the SBF core. Moreover, an increase of the HOMO energy level was neither observed for 4-Ph-SBF (HOMO: -5.95 eV) nor for the 4-5-pyrimidine-SBF (4-5Pm-SBF HOMO: 5.97 eV) previously reported in literature.25 It appears therefore that the pyridyl group induces on the SBF core a particular effect which may be directly correlated to the X-Ray data obtained for 4-4PySBF. Indeed, the dihedral angle measured between the fluorenyl unit and its substituent at the C4position of SBF is above 51° for 4-Ph-SBF and of only 42.2° for 4-4Py-SBF. As stated above, this may sign the existence of a more intense π-conjugation between pyridine and fluorene in 4-4Py-SBF than between phenyl and fluorene in 4-Ph-SBF. This is true for all the pyridine derivatives and will be confirmed with the analysis of the UV-Vis absorption spectra (see below). Thus, as the pyridine seems to be more conjugated with the fluorene, there are two effects to be considered: the electron


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withdrawing effect, which should decrease the HOMO energy level and the extension (despite small) of the π-conjugation which should increase the HOMO energy level. This hence leads to two opposite electronic effects, which in fine leads for 4-4Py-SBF to a higher HOMO energy level compared to those of 4-Ph-SBF and SBF. Electrochemical data obtained from 2-substituted SBFs strengthen this hypothesis. Indeed, HOMO level of 2-4Py-SBF calculated from its onset oxidation potentials is of –5.78 eV ; 0.10 eV higher than that of 2-Ph-SBF (-5.88 eV) and 0.16 eV higher than that of SBF (HOMO: -5.94 eV).38 First, compare to SBF, the presence of the pyridyl or phenyl unit in C2, extends the conjugation length and renders 2-4Py-SBF and 2-Ph-SBF more easily oxidizable than SBF. Then, compared to 2-Ph-SBF, we note that the substitution in C2 by a pyridyl unit renders the 2-4Py-SBF even more easily oxidizable. Such an effect was observed for the 4-substituted series (see above). It is of interest to mention that in 2-substituted SBFs, the dihedral angle between the fluorene and its substituent is almost identical for 2-4Py-SBF (molecule 1, 37.5 °) and 2-Ph-SBF (37.4 °) but smaller 2-4Py-SBF (molecule 2, 32.8 °) (see X-Ray data in Table 1). In this last molecule the conjugation between the fluorene and the pyridyl unit may be more intense and may facilitate the oxidation. Cathodic explorations in dichloromethane reveal that all compounds are reduced at negative potential in a non-reversible process close to the electrolytic medium reduction (see CVs in figures S58 to S61 right in SI). LUMO levels were determined from the onset reduction potential in CH2Cl2 at -2.10; -2.01; -2.11 and -2.26 eV for 4-2Py-SBF, 4-3Py-SBF, 4-4Py-SBF and 2-4Py-SBF respectively. In the 4-pyridinium-substituted series, all LUMO energy levels are close together with however a maximum difference of ca 0.1 eV. This difference signs that the pyridyl ring has a stronger influence on the LUMO than on the HOMO energy levels (difference of 0.02 eV, see above). The LUMO energy levels appear hence 0.12-0.22 eV lower than that of SBF, signing this time and oppositely to what was observed for the HOMO, the electron-withdrawing effect of the pyridyl unit on the



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substituted-fluorene. This is assigned to the character of the LUMOs, which are fluorene/pyridine centred whereas the HOMOs are centred on the two fluorene units with no density on the pyridine core (see below DFT calculations). We note that the decrease of the LUMO energy level induced by the pyridyl ring is slightly less pronounced than that of a pyrimidine ring (4-5Pm-SBF LUMO level: -2.23 eV25) translating the different electron-withdrawing effects of these two heterocycles. This is in accordance with previous findings reported by Kido and co-workers on terphenyl like molecules.27 They have indeed shown (i) that lower-lying LUMO energy levels are obtained by introducing heterocyclic core instead of benzene and (ii) that the more nitrogen atoms are present within the heterocyclic core, the lower lying of the LUMO levels obtained. The lowest LUMO level is recorded for the 2-substituted molecule 2-4Py-SBF (-2.26 eV), 0.37 eV lower than the LUMO of SBF (-1.89 eV),18 signing the double effect of the extension of conjugation due to the C2-substitution of SBF and to the electron-withdrawing effect of the electron-deficient pyridyl group. Table 2. Electronic properties Theoretical calculation LUMOa

HOMOa

∆Etheo (eV)a

4-2Py-SBF

-1.47

-5.93

4.46

4-3Py-SBF

-1.46

-6.08

4-4Py-SBF

-1.57

2-4Py-SBF

Cyclic voltammetry ET a

LUMOb

HOMOb

2.75

-2.12

4.62

2.81

-6.11

4.54

-1.82

-6.09

4-Ph-SBF25

-1.27

2-Ph-SBF25

-1.48

Optical data

∆Eel (eV)b

∆Eopt (eV)c

ET optd

-5.88

3.76

3.80

2.76

-2.01

-5.90

3.89

3.81

2.79

2.79

-2.11

-5.88

3.77

3.75

2.74

4.27

2.62

-2.26

-5.78

3.52

3.70

2.58

-5.97

4.70

2.82

-1.95

-5.95

4.00

3.82

2.77

-5.87

4.39

2.62

-2.10

-5.88

3.78

3.70

2.56

a

: results extracted from calculations ; b : results extracted from electrochemical analysis; c : results extracted from absorption spectrum in

cyclohexane (see spectra Fig. 4) by using ∆Eopt=hc/λ (∆Eopt(eV)=1237.5/λ(nm); d : results extracted from low temperature photoluminescence spectrum at the maximum of the first phosphorescent emission peak.

Finally, HOMO-LUMO gaps (∆Eel) are evaluated from these data at 3.76 eV for 4-2Py-SBF, 3.89 eV for 4-3Py-SBF and 3.77 eV for 4-4Py-SBF. We note that all 4-pyridine-substituted SBFs present


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a more contracted gap than those of SBF (4.05 eV) and 4-Ph-SBF (4.00 eV) due to the lowering of the LUMO level induced by the pyridyl group.

Theoretical calculations Geometry optimization of the four pyridyl dyes in the singlet and triplet states was performed using Density Functional Theory (DFT) at the Gaussian09 B3LYP/6-311+G(d,p) level of theory. All the results are reported in table 2 and figure 3. The character and the energy levels of HOMOs and LUMOs (and the corresponding energy gaps) have been determined on optimized geometries and compared with the previously reported calculations performed for 4-Ph-SBF and 2-Ph-SBF.25 In accordance with electrochemical measurements, the theoretical HOMO energy levels of the four dyes are close together lying between -6.11 and -5.93 eV. All 4-substituted compounds present a SBF character with no delocalization on the pendant pyridine or phenyl core. Theoretical calculation shows for 2-4Py-SBF a SBF character with only a weak delocalization on the pyridine-fluorene backbone. Thus, the character of the HOMO of 2-4Py-SBF is very different to that of 2-Ph-SBF, for which HOMO presents a clear phenyl-fluorene delocalization, with only a weak participation of the non-substituted fluorene but a strong influence of the pendant phenyl core, leading to the highest HOMO level (-5.87 eV) in the series.25 The present calculation are not in accordance with electrochemical data (see above) which conclude to a better delocalization in 2-4Py-SBF than in 2Ph-SBF and therefore to an increase of the HOMO level from 2-Ph-SBF (-5.95 eV) to 2-4Py-SBF (5.78 eV).



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Figure 3. Calculated frontier molecular orbitals by DFT of 4-2Py-SBF, 4-3Py-SBF, 4-4Py-SBF and 2-4Py-SBF, after geometry optimization at the B3LYP/6-311G+(d,p) level of theory shown with an isovalue of 0.04. Theoretical calculations of 4-Ph-SBF and 2-Ph-SBF used herein for comparison purpose, comes from reference.25 Concerning the LUMO levels of the 4-pyridine-substituted-SBFs, they are calculated between 1.57 and -1.46 eV and present a fluorene-pyridyl character with no contribution of the non-substituted fluorene. These LUMO energy levels are 0.15 eV higher than that of 4-5Pm-SBF (-1.65 eV)25 showing the less intense withdrawing effect of pyridyl unit compared to pyrimidyl unit. The LUMO level of 2-4Py-SBF is calculated at -1.82 eV, lower than that of the 2-Ph-SBF (-1.48 eV)25 and those of the 4-pyridine-substituted-SBFs (around -1.5 eV). The lowering of the LUMO in 2-4Py-SBF signs the withdrawing effect of the pyridyl unit and its efficient conjugation with the fluorene unit rendering this molecule the most easily reducible. The main tendency of the LUMO levels obtained through theoretical calculations is in good accordance with our electrochemical conclusions, with the lowest LUMO recorded for 2-4Py-SBF. The LUMO calculated for 4-2Py-SBF, 4-3Py-SBF and 44Py-SBF (-1.47, -1.46 and -1.57 eV respectively) are very close together and their LUMO values 18 ACS Paragon Plus Environment

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determined through electrochemical measurements are also very similar (-2.12, -2.01 and -2.11 eV respectively). The theoretical energy gap of the 4-pyridine-substituted-SBFs are close to 4.54 eV (±0.08 eV) and that of 2-4Py-SBF lies at 4.27 eV. The lowest (4.27 eV for 2-4Py-SBF) and the highest (4.70 eV for 4-Ph-SBF) energy gaps are also the extreme values obtained from the electrochemical measurements (3.52 eV for 2-4Py-SBF and 4.00 eV for 4-Ph-SBF) showing a good fit between theoretical calculations and electrochemical experimentations. Absorption spectroscopy The UV-Visible absorption spectra of the four pyridine-substituted SBFs, recorded in cyclohexane, are presented in figure 4a. The four compounds present absorption bands in the range 260-315 nm with the same maxima at 296 nm and 308 nm similar to the main absorption band of SBF attributed to the π-π* transition of fluorene units in SBF.18, 38 At longer wavelengths, each compound possesses its own signature with a clear additional large absorption band at 320 nm for 2-4Py-SBF signing an extension of the conjugation between the 2-substituted fluorene and the pyridine. Such a band at low energy was also observed for 2-Ph-SBF25 (319 nm) and 2-2Py-9,9-dihexylfluorene (330 nm).39 4-substituted SBFs show very similar absorption spectra with however an impressive decrease of the large band at ca 320 nm compared to 2-4Py-SBF. Thus, for 4-2Py-SBF, 4-3Py-SBF and 44Py-SBF, we indeed only detect a tail between 312 and 340 nm (Figure 4a). This tail is due to a more limited π-conjugation between the pyridine and the fluorene. It reflects a minor proportion of conformers of the molecules with a more planar structure, allowing a certain degree of π-conjugation between the pyridine and the 4-substituted fluorene moiety. As these bands at low energy seem to be more intense depending on the position of the nitrogen atom within the pyridine ring, one may conclude that more intense π-conjugation seems to occur between fluorene and pyridine units following the sequence 4-3Py-SBF, 4-4Py-SBF and 4-2Py-SBF. This is an interesting finding to control the intensity of π-conjugation in 4-substituted compounds. 19 ACS Paragon Plus Environment


4-Ph-SBF b 4-2Py-SBF 4-3Py-SBF 4-4Py-SBF 2-4Py-SBF 2-Ph-SBF

Normalized (a.u.)

2.0

1.5

1.0

0.5

4-Ph-SBF 4-2Py-SBF 4-3Py-SBF 4-4Py-SBF 2-4Py-SBF 2-Ph-SBF

1.0

Normalized (a.u.)

a

0.8 0.6 0.4 0.2 0.0

0.0 260

280

300

320

340

325

350

Wavelength (nm)


2.0

1.5

1.0

0.5

0.0 250

375

400

425

450

Wavelength (nm)

d

475

500


1.0

Normalized (a.u)

c

Normalized (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

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0.8 0.6 0.4 0.2 0.0

275

300

325

350

375

350

Wavelenght (nm)

400

450

500

Wavelength (nm)

Figure 4. (a) UV-vis absorption and (b) emission spectra of SBF derivatives recorded in 10-6 M cyclohexane solutions. λexc: 300 nm for 4-Ph-SBF and 2-Ph-SBF ; 310 nm for 4-2Py-SBF, 311 nm for 4-4Py-SBF and 312 nm for 4-3Py-SBF and 2-4Py-SBF (c) UV-vis Absorption and (d) emission spectra (λexc: 310 nm ) of thin-film of SBF derivatives obtained by spin-coating from THF solutions (10 mg/mL). Interestingly, the model analogue 4-Ph-SBF presents an even less intense tail at ca 320 nm (Figure 4a, black line), compared to pyridine-substituted SBFs. This less intense absorption is assigned to more efficient π-conjugation between fluorene and pyridine than between fluorene and phenyl. This feature can be correlated to the larger angle formed between the fluorene and the phenyl in C4 in 4Ph-SBF than between the fluorene and the pyridine in C4 in 4Py-SBF (See X-ray part above). Interestingly, simulated absorption spectra obtained by the TD-DFT clearly confirm the tendency of the experimental absorption spectra presented figure 4a (see the comparison of experimental and calculated absorption spectra in figures S17-S18 in SI). First, 2-4Py-SBF calculated spectrum clearly shows an absorption band centred at 327.5 nm and for the three 4-pyridine substituted SBFs, an


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absorption band is calculated at 307, 311 and 318 nm for 4-3Py-SBF, 4-4Py-SBF and 4-2Py-SBF respectively. This order follows the relative intensity of the low energy tail observed around 325 nm in the experimental spectra. For 4-substituted SBFs, the calculated lowest energy bands obtained by TD-DFT strongly present less intense oscillator strengths (f=0.06, 0.0512, 0.0553 and 0.069 for 4-Ph-SBF, 4-3Py-SBF, 4-4PySBF and 4-2Py-SBF respectively) than for 2-4Py-SBF (f: 0.1793) and 2-Ph-SBF (f: 0.3552), see figures S19-S24 in SI, in accordance with the intensity of the low-energy bands observed in the absorption spectra. For all compounds, the most intense transition involved in the band nevertheless corresponds to an HOMO-LUMO transition. This HOMO-LUMO transition involves, in the case of 4-substituted SBFs, different molecular fragments (HOMO possessing a spirobifluorene character and LUMO possessing a pyridyl- or phenyl-fluorene character). In 2-Ph-SBF HOMO and LUMO both mainly present a phenyl-fluorene character with significant orbital overlaps. In 2-4Py-SBF, the HOMO possesses a spirobifluorene character with only a very weak extension on the 4-pyridyl unit whereas the LUMO clearly possesses a pyridyl-fluorene character. These orbital characters are due to the electron-withdrawing character of the pyridine ring. From the onset absorption wavelength, optical energy gap (∆Eopt) varying from 3.70 eV for 2-4PySBF to 3.81 eV for 4-3Py-SBF were determined (see table 2) with a variation following the same trend than that obtained from electrochemical measurements and theoretical calculations. Slightly narrower optical gap values are obtained by introducing pyridine units instead of phenyl unit at the C4 position of SBF (from 3.82 eV in 4-Ph-SBF to 3.81, 3.80 and 3.75 eV in 4-3Py-SBF, 4-2Py-SBF and 4-4Py-SBF respectively). This is mainly caused by the decrease of the LUMO energy levels. The solid state absorption spectra of all dyes (figure 4c) appear very similar to those in solution with only a small red shift of 3 and 8 nm detected for the two lowest energy bands of the 2-4Py-SBF and a decrease of the weak band observed at low energy for the three 4-substituted pyridine SBFs. The decrease of the tail may be assigned to the decrease of molecular motions in the solid state



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blocking the angle between the fluorene and the pyridine and hence restricting the conjugation. Such similarity between absorption spectra in solution and in solid state clearly signs that there are minimal intermolecular interactions in the ground state in thin films. The absorption spectra recorded in solvents with different polarity remain almost identical (see Figures S38, S40, S42, S44 in SI). This means that the characteristics of the ground and Franck Condon excited states do not vary much with a change in solvent polarity.40 Emission spectroscopy Fluorescence spectra of the four dyes show structureless emission spectra for the three 4subsituted-pyridine-SBF and a well resolved emission spectra with maxima at 337/354 nm for 24Py-SBF. The spectrum recorded for 2-4Py-SBF (fig. 4b, green line) is similar in shape and wavelength to that recorded for 2-Ph-SBF (fig. 4b, violet line)25 nevertheless red shifted compared to SBF (λmax = 311/323 nm)38 due to the π-conjugation extension. 4-substituted SBFs present very different spectra with a structureless and large emission band centred at 370 nm for 4-2Py-SBF, 363 nm for 4-3Py-SBF and 369.5 nm for 4-4Py-SBF, red shifted (up to 12 nm) compared to emission of 4-Ph-SBF which is centered at 358 nm.18 The presence of the pyridyl unit instead of the phenyl ring induces a bathochromic shift of the emission. This red shift effect was also observed by Kido and co-workers in the dicarbazoyl-oligophenyl versus oligoheteroaryl series.27 However, another feature is also very important to consider herein. Indeed, we have shown above in the absorption section that more intense π-conjugation seems to occur between fluorene and the C-4 substituent (pyridine or phenyl unit) following the sequence 4-PhSBF, 4-3Py-SBF, 4-4Py-SBF and 4-2Py-SBF (intensity of the tail at ca 320 nm). In emission spectra, we note that the emission maxima also follow this sequence (with however 4-4Py-SBF and 4-2Py-SBF possessing almost the same maxima), being hence in accordance with our above mentioned analyses.


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The loss of resolution of the emission spectra has been previously reported by our group18, 25 and by other groups23, 24, 41 for the rare examples of 4-substituted SBFs found in literature. This loss of resolution appears to be a unique peculiarity of 4-substituted SBFs. In fact, 2-substituted SBF analogues (and 9,9'dialkylfluorene as well) always present well resolved emission bands which are assigned to the double bond character of the C-C bond linking the substituent and the fluorene in the excited state.42-45 The comparison of the emission maxima of 4-4Py-SBF (369.5 nm) with that of 24Py-SBF (337 nm) point a red shift of more than 30 nm, even more important than the shift of 23 nm previously reported between 4-Ph-SBF (358 nm) and 2-Ph-SBF (335 nm).18 It is clear that the position of the substitution (C4 vs C2) is at the origin of this spectacular effect. We could assume that the C4-substitution on the fluorene differently affect the ground and excited state. Indeed, the absorption spectra of C4-substituted molecules are only slightly altered compared to C2-substituted molecules (decrease of the intensity of the last band), as described above, but their emission spectra are more significantly modified with an impressive Stokes shift. However, the non-resolution of the large bands observed is highly difficult to rationalize and not in accordance with a very rigid structure in the excited state. Thus, this highly unusual photo-physical feature remains to be elucidated and more detailed theoretical/spectroscopic investigations of the ground and excited states need to be conducted to unravel this critical issue. The present results clearly confirm nevertheless this peculiar behaviour. The quantum yields (Φsol) were determined in cyclohexane by using standard procedures with quinine sulphate as reference. 4-2Py-SBF with a Φsol of ca 0.17 and lifetime of 3.88 ns appears clearly less emissive than 4-3Py-SBF, 4-4Py-SBF and 4-Ph-SBF with Φsol of 0.40, 0.40 and 0.42 and lifetimes of 3.89, 3.80 and 4.20 ns respectively (Figure 5). The two other compounds 2-4Py-SBF and 2-Ph-SBF show relatively higher fluorescence quantum yields (0.55 and 0.87) and much shorter lifetimes (1.03 ns and 1.56 ns). The corresponding radiative (kr) and non-radiative (knr) rate constants were calculated in cyclohexane and compiled in Table 3. Therefore, three series of compounds can



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be distinguished, based on their photophysical characteristics. A first set of compounds, 4-3Py-SBF, 4-4Py-SBF, and 4-Ph-SBF, have radiative rate constants in the range of 1.00-1.05×108 s-1, and nonradiative rate constants comprised between 1.38 and 1.58×108 s-1. Second, the derivative 4-2Py-SBF is a singular case, with a relatively low kr value (0.44×108 s-1) and relatively high knr (2.14×108 s-1). A third series of compounds, 2-4Py-SBF and 2-Ph-SBF, show much higher kr values (5.34-5.58×108 s-1), and rather different knr values (4.37×108 s-1 and 0.83×108 s-1, respectively). ). This comparison highlights clear differences between the 2-substituted and 4-substituted SBF derivatives. The radiative rate constant kr is clearly higher in 2-substituted compounds with respect to the 4substituted ones. This observation is consistent with the higher oscillator strength of the first absorption electronic transition for 2-4Py-SBF and 2-Ph-SBF (located at 321 nm and 319 nm, respectively, see Figure 4 and Table 3). Additionally, it is noteworthy to realize that 4-2Py-SBF and 2-4Py-SBF show much higher non-radiative rate constants compared to their congeners, meaning that in these particular cases, vibrational deactivation pathways play an important role in the dynamics of the excited states, contributing to a lowering of the fluorescence quantum yields (0.17 and 0.55, respectively, see Table 3). The twist dihedral angle between the pyridyl ring and the mean plane of the SBF, as highlighted by structural analyses described in previous parts (see Figure 1), could be tentatively identified as a critical degree of freedom of the molecules to deactivate through vibrational motions in solution. It should be noted that in other solvents, such as toluene, THF, dichloromethane and acetonitrile, the emission maximum of the three 4-substituted pyridine SBF is weakly sensitive to the dielectric constant of the environment with a bathochromic shift of the fluorescent maxima of less than 15 nm (11, 9 and 15 nm for 4-2Py-, 4-3Py- and 4-4Py-SBF resp., see Figures S39-41-43 in SI). This translates weak dipole-dipole interactions between the dyes and polar solvents indicative of a weak photo-induced intramolecular charge-transfer between donor fluorene and acceptor pyridine. The


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quantum yields of the different derivatives remain similar in all solvents tested (ca 15%, 38% and 37% for 4-2Py-, 4-3Py- and 4-4Py-SBF resp., see tables S8 in SI). Regarding the solid-state fluorescent properties (Figure 4d), except for 2-4Py-SBF, they are almost identical to those recorded in solution with only a small red shift ranging from 2 nm for 4-3Py-SBF to 7.5 nm for 4-4Py-SBF. Solid-state fluorescence spectrum of 2-4Py-SBF nevertheless appears poorly resolved with two maxima recorded at 375 and 425 nm, largely red shifted compared to that in solution. The large and intense band centred at 425 nm signs the existence of important intermolecular interactions in the solid state, which can be assigned to CH…N interactions.

Figure 5. Fluorescence decays (coloured dots), instrumental response function (IRF, black dots), monoexponential fitting (black solid lines), and weighted residuals of SBF derivatives recorded at Room Temperature in cyclohexane (λexc=300nm). Finally, to examine the suitability of the four dyes as host materials for phosphorescent green and/or blue dopant, the phosphorescence of the molecules was recorded at 77 K in a methylcyclohexane/2-methylpentane mixture (1/1). From those spectra presented figure 6, ET values were determined from the lowest phosphorescence peak. 4-3Py-SBF (blue line) and 2-4Py-SBF (green line) possess the highest and lowest ET respectively evaluated at 2.79 eV and 2.58 eV. 4-4Py25 ACS Paragon Plus Environment


SBF (pink line) and 4-2Py-SBF (red line) respectively possess ET of at 2.74 and 2.76 eV with more intense phosphorescence contributions than those of 4-3Py-SBF (blue line) and 2-4Py-SBF. We can note that 4-2Py-SBF possesses the most intense phosphorescence contribution whereas 2-4Py-SBF possesses the less intense one. This may translate a more or less efficient intersystem crossing between S1 and T1 and can be tentatively correlated to the fluorescence quantum yields, 2-4Py-SBF possessing the highest quantum yield (55%) and 4-2Py-SBF (17%) the lowest. 4-4Py-SBF, ET = 2.74 eV 4-3Py-SBF, ET = 2.79 eV 4-2Py-SBF, ET = 2.76 eV

1.0

2-4Py-SBF, ET = 2.58 eV

0.8

0.14 0.12

Normalized (a. u.)

Normalized (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

0.6 0.4

451 448

0.10 0.08

444

0.06

480

0.04 0.02 0.00 450

500

550

600

Wavelength (nm)

0.2 0.0 300

350

400

450

500

550

600

Wavelength (nm)

Figure 6. Emission spectra of SBF derivatives recorded in a frozen matrix of 2methyltetrahydrofuran at 77K (λexc: 309 nm for 4-2Py-SBF, 4-3Py-SBF, 4-4Py-SBF and λexc: 300 nm for 2-4Py-SBF). It should be noted that the ET values are in accordance with those obtained from theoretical calculations (see table 2). Compared to the ET of SBF (2.87 eV) and that of 4-Ph-SBF (ET: 2.77 eV),18 we note that the substitution by the 3-pyridyl rings (ET: 2.79 eV) less affects the ET than the substitution by the 5-pyrimidyl (ET: 2.75 eV)25 or by the 4- or 2-pyridyl rings(ET: 2.74 or 2.76 eV, resp.). These results display a similar trend than that reported by Kido27 and co-workers on terphenyl derivatives (see chart 1) who demonstrated that the replacement of the central phenyl group in K1 by a 2-pyridyl group in K3 has a less intense effect on the lowering of ET than the substitution by a pyrimidyl group in K2. This substituent effect seems hence to be similar in bridged and non-bridged phenylene systems.


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Therefore, the ET levels measured for the three 4-pyridyl-substituted-SBFs are sufficiently high for their use as host materials for both green and blue phosphorescent dyes (see below). Finally, the ET of 2-4Py-SBF (2.58 eV) is lower than that of the blue dopant FIrpic (ET: 2.62 eV46) and this molecule may be therefore only used as host material only for the green dopant Ir(ppy)3 (ET: 2.42 eV47). All the photophysical properties are summarized in Table 3. Table 3. Photophysical Properties 4-Ph-SBF18

4-2Py-SBF

4-3Py-SBF

4-4Py-SBF

2-4Py-SBF

2-Ph-SBF25

λmax,abs (nm)a

297, 308

297, 308

297, 308

297, 308

297, 308, 321

297, 308, 319

ε (104, L.mol-1.m-2) a

0.7, 1.4

1.0, 1.5

0.9, 1.4

0.7, 1.2

2.2, 2.1, 1.6

2.4, 2.2, 1.6

358

370

363

369.5

337,354, 370(sh)

335, 351

λmax,fluo, thin film (nm)b

363

366

365

377

375, 425

343, 359

a

447

448

444

451

480

483

42

17

40

40

55

87

4.2

3.88

3.89

3.80

1.03

1.56

1.00

0.44

1.03

1.05

5.34

5.58

2.14

1.54

1.58

4.37

0.83

λmax,fluo solution (nm) a

λmax,phospho (77K) (nm) c

QY (%) Fluorescence lifetime in cyclohexane (in ns) Radiative rate constant (kr) (108s) Non-radiative rate constant (knr) (108s) a

b

1.38 c

in cyclohexane from a THF solution at 10g/L calculated from a quinine sulfate solution in 1N sulfuric acid solution

PhOLED devices Finally, green (Ir(ppy)3) and sky blue (FIrpic) PhOLEDs using these new host materials have been fabricated and characterized. The device configuration was ITO/CuPc(10 nm)/NPB (40 nm)/TCTA (10 nm)/ EML:dopant (20 nm)/TPBi (40 nm)/LiF (1.2 nm)/Al (100 nm). ITO is used as the anode, CuPc (copper phtalocyanine) is the hole injecting layer, NPB (N,N’-di(1-naphtyl)-N,N’-diphenyl[1,1’-biphenyl]-4,4’-diamine) is the hole-transporting layer, TCTA (4,4',4''-Tris(carbazol-9-yl)triphenylamine) is the electron/exciton blocking layer, TPBI (1,3,5-Tris(1-phenyl-1H-benzimidazol2-yl)benzene) is both the electron transporting layer and the hole blocking layer and a thin film of lithium fluoride covered with aluminum is the cathode. Ir(ppy)3 and FIrpic are used as dopant for 27 ACS Paragon Plus Environment


green and sky blue PhOLEDs respectively. The relative energy levels of the successive layers of the

4-4Py-SBF

4-3Py-SBF

4-2Py-SBF

devices are reported in scheme 3.

2-4Py-SBF

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

Scheme 3. Schematic energy level diagram of the different materials used in the devices. Due to their ET values higher than that of Ir(ppy)3, the four pyridyl-substituted-SBFs were used as host for Ir(ppy)3 green dopant. Performance of the green PhOLEDs using 4-4Py-SBF as host is presented in figure 7 and performance of the PhOLEDs using the four hosts are gathered in table 4. For comparison purpose, green PhOLEDs using SBF, 4-Ph-SBF, 4-5Pm-SBF, 2-5Pm-SBF and 2Ph-SBF previously described in literature, have been also reported.18, 25 For a clear and accurate comparison of the performance of each device, the threshold voltage (VTh) is measured for a luminance of 1 cd/m2, the current and power efficiencies (CE and PE) are reported at 1 and 10 mA/cm2 and the external quantum efficiency (EQE) is calculated at 10 mA/cm2. Therefore, values in table 4 and 5 for SBF, 4-Ph-SBF, 4-5Pm-SBF, 2-5Pm-SBF and 2-Ph-SBF can be sometimes slightly different than those previously reported.18, 25 Green PhOLEDs (ca. 10% of Ir(ppy)3) using pyridyl-substituted SBF as host possess EQE between 12.7 and 15.7 %. The devices present impressive better performances than those reported for SBF (8.4%),18 4-Ph-SBF (10.6%)18 and, except PhOLED using 4-2Py-SBF, for 4-5Pm-SBF (13.8%).25


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Table 4. Performance of green (Ir(ppy)3 )devices.

SBF 4-Ph-SBF 4-5Pm-SBF 4-4Py-SBF 4-3Py-SBF 4-2Py-SBF 2-4Py-SBF 2-Ph-SBF 2-5Pm-SBF

Ir(ppy)3 % in mass

VTh (V) at 1 cd/m2

E.Q.E (%) calculated at 10 mA/cm2

CE (cd/A) at 1 / 10 mA/cm2

PE (lm/W) at 1 / 10 mA/cm2

Lmax (cd/m2) at (x mA/cm2)

CIE (x, y) at (10mA/cm2)

9 10 9 11 10 9 10 10 9

3.4 3.8 3.7 3.4 3.6 3.9 3.3 3.9 3.1

8.4 10.6 13.8 15.7 14.9 12.7 14.3 9.5 14.2

32.7/31.2 48.9/41.1 56.6/50.5 63.4/56.8 57.3/55.5 59.0/47.3 56.3/51.9 39.6/35.2 56.7/51.4

19.9/14.8 26.6/17.6 25.9/17.9 33.9/23.1 32.3/24.5 31.0/20.0 31.8/22.3 21.5/15.3 32.6/22.6

18610(190) 26420(240) 28610(150) 27530(140) 22360(170) 25830(220) 22300(120) 20220(220) 30480(160)

0.31 ; 0.62 0.33 ; 0.61 0.34 ; 0.60 0.33 ; 0.61 0.34 ; 0.60 0.31 ; 0.62 0.34 ; 0.61 0.33 ; 0.61 0.33 ; 0.61

Compared one to the other, the four devices using pyridyl-substituted SBF as host present similar performances with the best ones recorded for 4-4Py-SBF doped with 11% Ir(ppy)3. This device emits light with a low threshold voltage VTh of 3.4 V and possesses a current efficiency (CE) as high as 63.4 cd/A, a power efficiency (PE) of 33.9 lm/W (recorded at 1 A/cm2), an EQE of 15.7 % and the luminance maximum reaches 27530 cd/m2 at 140 mA/cm2. PhOLEDs using 4-3Py-SBF and 4-4Py-SBF as host present better performance than that using 45Pm-SBF, with VTh lowered by 0.1-0.3 V and EQE increased by 1-2 % highlighting the efficiency of the present molecular design (pyridine vs pyrimidine). The EQE of device using 2-4Py-SBF is of 14.3 % just slightly higher than that recorded for the device using 2-5Pm-SBF as host (14.2 %)25 but impressively higher than that recorded for the device using 2-Ph-SBF (9.5 %) with additionally a decrease of the VTh from 3.9 to 3.3 V. This is again due to the incorporation of the pyridine ring within the structure. The device performances obtained with those host molecules are comparable to those obtained with other 4-substituted-SBF host materials recently reported with in addition a beneficial effect on the threshold voltage, which appears at least 1V lower (3.4/4V).19, 20, 23, 24, 48



60

Current Efficiency Power Efficiency 4-4Py-SBF:Ir(ppy)3 10% in mass

60

4-4Py-SBF:Ir(ppy)3 (10 % in mass) at 10 mA/cm²

1.0

40

40

20 20

0.8

EL spectra (a.u.)

Current Efficiency (cd/A)

80

Power Efficiency (lm/W)

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

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600

700

800

wavelength (nm)

Current Density (mA/cm²)

Figure 7. Current (empty symbol) and power efficiencies (filled symbol) versus current density of the green devices using 4-4Py-SBF doped with Ir(ppy)3 (11% in mass) as emitting layer (left). Corresponding EL spectra recorded at 10 mA/cm2 (right). The electroluminescence (EL) spectra of the 4-4Py-SBF green device presented figure 7 right and these of the devices based on the different host materials are identical, exclusively showing the emission of the green dopant at 516/540 nm close to the photoluminescence of pure Ir(ppy)3 film (509/540 nm)49 with no parasite emission in the range of the non-doped device (see EL spectra of the devices using 4-3Py-SBF, 4-2Py-SBF and 2-4Py-SBF in figures S50, S52, S54 and S56). This result demonstrates an efficient energy transfer from the four SBF-derivatives to Ir(ppy)3, due to their high triplet energy levels. More importantly and due to their ET higher than that of sky blue phosphorescent emitter FIrpic (ET: 2.62 eV)46, 4-2Py-SBF, 4-3Py-SBF and 4-4Py-SBF (ET: 2.76 eV, 279 eV and 2.74 eV respectively) were used as host in sky blue PhOLEDs. The performance of the resulting PhOLEDs based on these three hosts are very similar with however, slightly better performances for 4-3PySBF and 4-4Py-SBF, indicating that 3-pyridine and 4-pyridine rings are the most efficient pyridyl regioisomers for such application. Thus, an EQE of 5.1 %, a CE of 16.1 cd/A and a VTh of 4.3 V were recorded for 4-4Py-SBF as host and an EQE of 4.9 %, a CE of 16.2 cd/A and a VTh of 4.7 V were recorded for 4-3Py-SBF. We can note that the VTh of 4-2Py-SBF, 5.0 V, is higher than those described above for the other isomers. The same observation was drawn above for green devices. As the LUMO level of 4-2Py30 ACS Paragon Plus Environment

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SBF (-2.12 eV) is not the highest in the series (-2.11 eV for 4-4Py-SBF and -2.01 eV for 4-3PySBF), this result seems to indicate that the injection of electron is not exclusively correlated to this increase of VTh. The blue devices using 4-4Py-SBF as host present performances similar to that recorded with 45Pm-SBF25 in term of VTh (4.1 V with 4-5Pm-SBF vs 4.3 V for 4-4Py-SBF) and EQE (5 % with 45Pm-SBF vs 5.1 % for 4-4Py-SBF), the maximum luminance reached being 20 % higher with 44Py-SBF (2470 cd/m2) than with 4-5Pm-SBF (2071 cd/m2). As 4-4Py-SBF and 4-5Pm-SBF possess similar ET levels (2.74 and 2.80 eV resp.) and similar electrochemical energy gaps (3.77 and 3.74 eV resp.) which can be seen as the singlet state energy level, the similarity of the performance using the two hosts are consistent. Oppositely to our above mentioned conclusion for green devices, the C4 substitution with pyrimidine or pyridine unit, despite lowering the LUMO level and keeping an ET value higher than 2.74 eV does not improve the performances compared to blue devices using SBF or 4-Ph-SBF as host.18, 25 The interactions between the hosts and the dopant FIrpic may be less favorable than with Ir(ppy)3 leading to a less efficient charge transfer from the host to the guest. Table 5. Performance of blue (FIrpic) devices.

SBF18 4-Ph-SBF18 4-5Pm-SBF25 4-4Py-SBF 4-3Py-SBF 4-2Py-SBF 2-4Py-SBF

FIrpic % in mass

VTh (V) at 1 cd/m2

E.Q.E (%) calculated at 10 mA/cm2

CE (cd/A) at 1 / 10 mA/cm2

PE (lm/W) at 1 / 10 mA/cm2

Lmax (cd/m2) at (x mA/cm2)

CIE (x, y) at (10mA/cm2)

19 19 19 19 27 19 20

4.0 4.7 4.1 4.3 4.7 5.0 4.8

6.5 6.0 5.0 5.1 4.9 3.9 0.2

20.1/18.7 18.4/16.7 15.2/14.5 16.1/14.6 16.2/14.2 10.9/10.4 0.7/0.6

10.8/7.9 8.5/6.1 7.9/5.7 8.3/5.9 7.5/5.1 4.8/3.6 0.4/0.3

4354(70) 4022(70) 2071(50) 2470(60) 1773(50) 2160(60) 189(130)

0.19 ; 0.45 0.21 ; 0.45 0.21 ; 0.46 0.20 ; 0.45 0.21 ; 0.44 0.18 ; 0.43 0.33 ; 0.44

In order to highlight the importance of the C4 substitution on the device performance, a benchmark device was performed using 2-4Py-SBF as host for FIrpic. As expected and due to its too low ET (2.58 eV), very low EQE (0.2 %) was recorded due to energy back transfers from the guest to the host.



40

10

Power efficiency 4-4Py-SBF:FIrPic 19%

30

1.0

4-4Py-SBF:FIrPic 19% at 10 mA/cm²

8

6 20 4

10 2

0.8

EL spectra (a.u.)

Current Efficiency (cd/A)

Current efficiency 4-4Py-SBF:FIrPic 19%

Power Efficiency (lm/W)

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

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0.6

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0 0

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300

400

500

600

700

800

wavelength (nm)

Current Density (mA/cm²)

Figure 8. Current (empty symbol) and power efficiencies (filled symbol) versus current density of the blue devices 4-4Py-SBF doped with FIrPic (19% in mass) as emitting layer (left). Corresponding EL spectra recorded at a current density of 10 mA/cm2 (right). Finally, EL spectra of the device using 4-4Py-SBF as host for FIrpic (Figure 8 right) points an exclusive emission of the blue dopant with maxima at 473 and 500 nm close to the photoluminescence of pure FIrpic film (475/500 nm)50 with no parasite emission. It is important to note that other devices using SBF,18 4-Ph-SBF,18 4-5Pm-SBF25 and 4-3Py-SBF or 4-2Py-SBF all present an additional tiny emission in the range of the undoped device (360/440 nm) (see in figures S52, S54 and S56 in SI the EL spectra of devices using 4-3Py-SBF, 4-2Py-SBF and 2-4Py-SBF as host). This tiny parasite emission may be due to a very small amount of recombination in the other organic layers. Indeed this tiny emission is more important in the case of 2-4Py-SBF due to its ET (2.58 eV) lower than the ET value of the dopant FIrpic (2.62 eV). Although the ET of 4-4Py-SBF (ET: 2.74 eV) is slightly lower than that of 4-2Py-SBF and 4-3Py-SBF (ET: 2.76 and 2.79 eV), the energy transfers from the host to the guest seems to be more efficient with 4-4Py-SBF indicating that other factors may also have an influence on such a transfer.

Conclusion To conclude, four pyridyl-substituted SBF dyes have been synthesized and their thermal, electrochemical and photophysical properties have been deeply studied and compared to those of a series of model compounds. As 4-substituted SBFs have been rarely studied in literature and in the


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light of their potential as hosts for PhOLEDs, such structure properties relationship study is of great interest to further design highly efficient materials based on this scaffold. Thus, we have notably shown that the position (C2 vs C4) and the nature of the substituent on the SBF core leads to drastic alterations of the resulting properties and we have shed light on their origin. Substitution at the C2 position leads to an increase of the conjugation length and therefore to a decrease of the singlet and triplet state energy level, while the substitution at the C4 only has a weak influence on these two key parameters, keeping them almost identical to those of their constituting building block SBF. This has been assigned to π-conjugation breaking induced by the large torsion angle made by the substituents in C4. By introducing pyridyl units at the C4 position, we have shown that a decrease of the LUMO level can be easily obtained compared to model compound 4-Ph-SBF due to the pyridine electron affinity. This study has clearly highlighted that the twist angle between the substituted fluorene and its C4 substituent has a drastic influence on the electronic properties. More importantly, we have shown that this dihedral angle can be tuned depending on the substituent borne by the fluorene unit (4-pyridine vs phenyl), which allows a very interesting tuning of the electronic properties. We are convinced that this finding may be used in other applications in the future to design 4-substituted SBF based semiconductors with tunable properties. In addition to this fundamental findings, the present chemical design allows obtaining organic semiconductors based on a C4-substituted SBF scaffold with high Tg and Td, HOMO/LUMO levels adapted for hole and electron injection/transport in the devices and ET higher than classical green (Ir(ppy)3 , ET= 2.42 eV)47 and sky blue phosphorescent emitter FIrpic (2.62 eV).46 The best device using 4-4Py-SBF as host for Ir(ppy)3 emits light at 3.4 V, with a current efficiency (CE) as high as 63.4 cd/A and a power efficiency (PE) of 33.9 lm/W (recorded at 1 A/cm2), an EQE of 15.7 % and the maximum of luminance reaching 27530 cd/m2 at 140 mA/cm2. These performances are better than those recorded for SBF,18 4-Ph-SBF18 and 4-5Pm-SBF25 showing the interest of the incorporation of the pyridyl unit.



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

For blue devices, best results are also obtained with 4-4Py-SBF as host for FIrpic (VTh: 4.3 V and EQE: 5.1 %). These performances are similar to the blue devices using 4-5Pm-SBF25 as host but not better than that recorded using 4-Ph-SBF or SBF18 as host for FIrpic, showing that electronic properties (HOMO, LUMO and ET) are not the only parameters that drive the performance of the PhOLED but interactions between host and guest in the solid state may have also an important responsibility in the energy transfer cascades.

ACKNOWLEDGEMENTS S.T. and C.D. thank the Agence Nationale de la Recherche (HOME-OLED Project n°ANR-11BS07-020-01) for a studentship (ST) and for a postdoctoral position (CD). We wish to thank the C.R.M.P.O (Centre Régional de Mesures Physiques de l'Ouest, Rennes) for high resolution mass measurements, Service de Microanalyse-CNRS (Gif sur Yvette) for CHN analyses, CINES (Montpellier) for computing time, the CDIFX (Rennes) for X-Ray data collection, the "Institut des Sciences Analytiques" (UMR CNRS 5280, Villeurbanne) for TGA. Arnaud Brosseau (Cachan) is thanked for his precious assistance in photophysical measurements. The Agence Nationale de la Recherche (HOME-OLED Project n°ANR-11-BS07-020-01 and MEN IN BLUE project n°ANR-14CE05-0024-01) is also warmly thanked for financial supports. ASSOCIATED CONTENT Supporting Information. Experimental section, structural properties, theoretical modeling, thermal properties, electrochemical properties, 2D NMR Studies, Copy of NMR and Mass spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author Joëlle Rault-Berthelot and Cyril Poriel [email protected] ; [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.


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Table of contents graphic

-1.20 eV -1.32 eV

-1.34 eV

-1.47 eV

-1.46 eV

4.46 eV

4.62 eV

-5.93 eV

-6.08 eV

6.11eV

-6.33 eV

-6.37eV

4-3Py-SBF

4-4Py-SBF

N

-1.57 eV

N N

4-2Py-SBF 4-3Py-SBF 4-4Py-SBF ET: 2.75 eV

ET: 2.81 eV

4.54 eV

-6.11eV

ET: 2.79 eV

4-2Py-SBF


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