Blue emitting 3 π-2 spiro terfluorene-indenofluorene isomers: A structure-properties relationship study

DOI: 10.1002/chem.201102212

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Chemistry - A European Journal p. 14031 - 14046 (2011)

Update date:2022-08-04

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Authors:

Poriel, Cyril

Rault-Berthelot, Joelle

Thirion, Damien

Barriere, Frederic

Vignau, Laurence

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Article abstract of DOI:10.1002/chem.201102212

Two novel terfluorenyl derivatives, 2,2′′,7,7′′- tetrakis(9,9-dioctyl-9H-fluoren-2-yl)dispiro[fluorene-9,11′-indeno-(2,1-a) -fluorene-12′,9′′-fluorene] ((2,1-a)-DST-IF) and 2,2′′,7,7′′-tetrakis(9,9-dioctyl-9H-fluoren-2-yl) dispiro- [fluorene-9,6′-indeno-(1,2-b)-fluorene-12′,9′′- fluorene] ((1,2-b)-DST-IF) have been synthesized by two different synthetic approaches. These terfluorenyl derivatives possess a different central indenofluorene core, namely (2,1-a)-indenofluorene or (1,2-b)-indenofluorene, which imposes two distinct geometry profiles, and different structural environments for the terfluorenyl fluorophores that translates into drastically different optical and electrochemical properties for (2,1-a)-DST-IF and (1,2-b)-DST-IF. These properties have been carefully studied through a combined experimental and theoretical approach. The (2,1-a)-DST-IF isomer has been successfully used as emitting layer in a blue single-layer small-molecule organic light-emitting diode (SMOLED) and appears as the first example of a blue emission arising from intramolecular terfluorenyl excimers. Regarding the importance of terfluorenyl derivatives in organic electronics, the present structure-properties relationship study, may open new avenues in the design of efficient blue fluorophores.

Full text of DOI:10.1002/chem.201102212

FULL PAPER

DOI: 10.1002/chem.201102212

Blue Emitting 3p–2Spiro Terfluorene–Indenofluorene Isomers: A Structure–

Properties Relationship Study

Cyril Poriel,*^[a]Joꢀlle Rault-Berthelot,*^[a]Damien Thirion,^[a]Frꢁdꢁric Barriꢂre,^[a]and

Laurence Vignau^[b]

Abstract: Two novel terfluorenyl deriv-

atives, 2,2’’,7,7’’-tetrakis(9,9-dioctyl-9H-

fluoren-2-yl)dispiro[fluorene-9,11’-

try profiles, and different structural en-

vironments for the terfluorenyl fluoro-

phores that translates into drastically

different optical and electrochemical

properties for (2,1-a)-DST-IF and (1,2-

b)-DST-IF. These properties have been

carefully studied through a combined

experimental and theoretical approach.

The (2,1-a)-DST-IF isomer has been

successfully used as emitting layer in a

blue single-layer small-molecule organ-

ic light-emitting diode (SMOLED) and

appears as the first example of a blue

emission arising from intramolecular

terfluorenyl excimers. Regarding the

importance of terfluorenyl derivatives

in organic electronics, the present

structure–properties relationship study,

may open new avenues in the design of

efficient blue fluorophores.

indeno-(2,1-a)-fluorene-12’,9’’-fluorene]

((2,1-a)-DST-IF) and 2,2’’,7,7’’-tetra-

kis(9,9-dioctyl-9H-fluoren-2-yl)dispiro-

[fluorene-9,6’-indeno-(1,2-b)-fluorene-

12’,9’’-fluorene] ((1,2-b)-DST-IF) have

been synthesized by two different syn-

thetic approaches. These terfluorenyl

derivatives possess a different central

indenofluorene core, namely (2,1-a)-in-

denofluorene or (1,2-b)-indenofluor-

ene, which imposes two distinct geome-

Keywords: blue-emitting materials ·

indenofluorene · OLEDs · semicon-

ductors · spiro compounds · ter-

fluorene

Introduction

chains,^[19]cyclopropane,^[20]and spiro bridges.^{[10–12,15,16,21,22]}

The introduction of a spiro bridge into small molecules is

indeed an efficient strategy to reduce the crystallization ten-

dency of the materials, enhancing their solubility, thermal

stability, and quantum yield.^[23]For the last few years, our

group has been involved in the synthesis of original blue flu-

orescent emitters based on a unique 3p–2 spiro architecture,

in which a central indenofluorene or ladderpentaphenylene

is connected through two spiro carbons to two other p-con-

jugated systems such as xanthene,^[24,25]fluorene,^[26–30]2,7-di-

Organic p-conjugated materials are of great interest for or-

ganic electronics such as organic light-emitting diodes

(OLEDs), organic field-effect transistors (OFETs), and pho-

tovoltaic cells.^[1,2]Among the organic p-conjugated materi-

als, polyfluorenes and oligofluorenes are very promising

candidates for blue OLED applications due to their high lu-

minescent efficiency and good charge-carrier mobility.^[3–9]In

this context, terfluorene derivatives have emerged as an ap-

pealing class of materials, presenting, for example, intriguing

ambipolar carrier–transport properties.^[10]Various terfluor-

enyl derivatives have hence been investigated over the last

ten years not only for blue OLEDs^[11–14]but also for light-

emitting OFETs (LE-OFETs).^[15]As the molecular structure

of the active layer is of key importance in LE-OFET and

OLED devices, different molecular designs of the terfluor-

enyl backbone have thus been developed, including the in-

corporation of heterocycles,^[16–18]hydroxyaminophenyl side

AHCTUNGTRENNUNG

alkyl fluorene,^[31,32]or 2,7-diaryl fluorene^[33,34]units. These

materials, due to their specific three-dimensional architec-

tures, possess appealing properties for blue OLED applica-

tions. In addition, we recently demonstrated that the fluores-

cent properties of 2,7-diaryl fluorenes with a 3p–2 spiro ar-

chitecture may be easily tuned not only by the nature of the

different p-systems but also by their geometric profiles.^[33,34]

The present study describes the structure–properties rela-

tionships between two novel blue-emitting terfluorenyl ma-

terials with a 3p–2 spiro architecture, namely (2,1-a)-DST-IF

and (1,2-b)-DST-IF, which possess two terfluorenyl units

connected to an indenofluorenyl backbone (see molecular

structures in Scheme 1 and 2). These two regioisomers,

which present distinct profiles due to the different geometry

of their indenofluorenyl central cores, (2,1-a)-indenofluor-

ene for (2,1-a)-DST-IF and (1,2-b)-indenofluorene for (1,2-

b)-DST-IF, respectively, possess cofacial terfluorenyl moiet-

ies or not. To the best of our knowledge, this molecular

design, which involves the connection of terfluorenyl units

to an indenofluorenyl framework, has never been reported

[a] Dr. C. Poriel, Dr. J. Rault-Berthelot, D. Thirion, Dr. F. Barriꢀre

Universitꢁ de Rennes 1-CNRS UMR 6226

“Sciences Chimiques de Rennes”-MaCSE group, Bat. 10C

Campus de Beaulieu - 35042 Rennes Cedex (France)

E-mail: cyril.poriel@univ-rennes1.fr

joelle.rault-berthelot@univ-rennes1.fr

[b] Dr. L. Vignau

Universitꢁ de Bordeaux - IMS/UMR CNRS 5218-site ENSCBP

16 Avenue Pey-Berland - 33607 Pessac cedex (France)

Supporting information for this article is available on the WWW

under http://dx.doi.org/10.1002/chem.201102212.

Chem. Eur. J. 2011, 17, 14031 – 14046

ꢂ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

14031

to date and appears to provide an appealing strategy to tune

terfluorene properties. As the synthesis of new materials for

electronics applications is strongly sought after worldwide, it

remains highly important to develop efficient routes towards

promising fluorophores, such as terfluorenyl derivatives, to

fully explore their potential for organic electronics applica-

tions. Herein, we first report our rational synthetic investiga-

tions towards the synthesis of the two terfluorenyl deriva-

tives (2,1-a)-DST-IF and (1,2-b)-DST-IF. Then, their electro-

chemical and optical properties, which appear to be drasti-

cally different due to their different geometry profiles, will

be carefully studied through a combined experimental and

theoretical approach. Finally, (2,1-a)-DST-IF, which surpris-

ingly presents very different fluorescence properties in solu-

tion and in the solid state, has been successfully used in a

single-layer blue small-molecule organic light-emitting diode

(SMOLED) with an electroluminescence spectrum arising

from terfluorenyl excimer emission. The (2,1-a)-DST-IF and

(1,2-b)-DST-IF derivatives appear to be the first examples

indeno

nistic analyses of the reaction for aryl- and alkyl-substituted

difluorenols, we were able to direct the synthesis preferen-

tially towards one positional isomer over the other by alter-

ing medium and through steric effects.^[31,32]Indeed, these

mechanistic investigations have shown that the bulkiness of

ACHTUNGTRENfNGNU luorenyl core found in (1,2-b)-DST-IF. After mecha-

AHCTUNGTRENNUNG

the substituents borne by the fluorACTHUNTGRNEUNGenyl units of the difluore-

nol dictates the ratio of the regioisomers formed. Thus, the

very bulky 9,9-dioctylfluorene attached to the fluorenyl

units of the difluorenol 6 (Scheme 1) should, in principle,

lead almost exclusively to the isomer (2,1-a)-DST-IF. How-

ever, as the nature of the solvent and the temperature of

this intramolecular cyclization reaction allow also a fine

tuning of the isomer ratio,^[31,32]it should also be possible, in

principle, to drive the reaction towards the formation of the

positional isomer (1,2-b)-DST-IF. Thus, this synthetic strat-

egy (route 1, Scheme 1) would theoretically allow access not

only to the terfluorene derivative (2,1-a)-DST-IF but also to

its positional isomer (1,2-b)-DST-IF. Route 1 starts with the

dialkylation of the commercially available 2-bromofluorene

in the presence of octyl bromide in basic medium, which

leads to 2-bromo-9,9-dioctylfluorene (1) in 99% yield.^[29]

The lithium–bromine exchange of 1 with n-butyllithium at

low temperature, followed by addition of 2-isopropoxy-

4,4,5,5-tetramethyl-1,3,2-dioxaborolane easily afforded the

2-fluorene boronate 2.^[29]Further reaction of 2 with 2,7-di-

bromofluoren-9-one (4) under Suzuki–Miyaura cross-cou-

of dispirofluorene–indenofluorene derivatives with

HOMO–LUMO gap almost exclusively controlled by the

terfluorenyl units and not by their indenofluorenyl unit.

Results and Discussion

Synthesis: Recently, our group has developed several origi-

nal synthetic approaches towards various blue fluorophores

with the (2,1-a)-indenofluorenyl backbone.^[28,32,33]One of

these synthetic approaches involved an intramolecular bicyc-

lization of a difluorenol in the last step (see Scheme 1,

bottom).^[32]This intramolecular bicyclization reaction usual-

ly leads to a mixture of regioisomers with the (2,1-a)-

pling conditions with [Pd⁰

ACHTNUTRGNE(UNG PPh₃)₄] as the catalyst and

sodium carbonate as the base in a mixture of toluene and

water (3/1) led to the terfluonenone 5 in 85% yield.^[35]

With 5 in hand, the access to (2,1-a)-DST-IF and (1,2-b)-

DST-IF required its coupling with diiodoterphenyl 2,2’’-

DITP 3 prepared according to a literature procedure.^[27]

Thus, the lithium–iodine exchange of 2,2’’-DITP at ꢀ788C,

indenoACHTUNGTRENNUNGfluorenyl core found in (2,1-a)-DST-IF or the (1,2-b)-

Scheme 1. Synthesis of terfluorenone 5 (top) and of (2,1-a)-DST-IF and (1,2-b)-DST-IF (route 1 bottom).TEBAC=triethyl benzyl ammonium chloride.

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Chem. Eur. J. 2011, 17, 14031 – 14046

3p–2Spiro Terfluorene–Indenofluorene Isomers

FULL PAPER

Scheme 2. Synthesis of (2,1-a)-DST-IF and (1,2-b)-DST-IF (route 2).

followed by the addition of the terfluorenone 5 afforded the

corresponding difluorenol 6 with the 9,9-dioctylfluorenyl

units attached to the fluorenol cores in 25% yield. As men-

tioned above, our group has carried out detailed studies on

the intramolecular aromatic electrophilic cyclization of di-

fluorenol derivatives such as 6.^[31,32]Notably, we have recent-

ly shown that a polar solvent, combined with very bulky

substituents attached to the fluorenyl units (here a 9,9-di<-

<octylfluorene in 6), favors the synthesis of the isomer with

the (2,1-a)-indenofluorenyl core. The opposite leads prefera-

bly to the isomer with the (1,2-b)-indenofluorenyl core.

Thus, the intramolecular cyclization of difluorenol 6 in ace-

tonitrile, a polar and donor solvent,^[36]led to the formation

of the expected two positional isomers (2,1-a)-DST-IF and

(1,2-b)-DST-IF in a 9/1 ratio.¹However, the separation of

the two isomers by column chromatography turned out to

be problematic due to the small difference in the retention

factors between the isomers, which hinders isolation. Thus,

following the route 1, only the isomer (2,1-a)-DST-IF was

obtained as a pure product. Screening for solvents and tem-

peratures in the light of our previous mechanistic studies^[32]

allowed us to slightly increase the amount of (1,2-b)-DST-IF

over (2,1-a)-DST-IF. In dichloromethane, a low polarity,

weak donor solvent,^[36]the amount of (1,2-b)-DST-IF over

(2,1-a)-DST-IF reaches almost 25%, but we still did not

manage to isolate (2,1-a)-DST-IF as a pure product. In the

light of these results, the synthetic approach has been modi-

fied keeping in mind that the size of the substituents borne

by the fluorene units is paramount in controlling the (1,2-b)-

DST-IF and (2,1-a)-DST-IF ratio. The key feature in the

second approach (route 2, Scheme 2) is to favor the forma-

tion of the (1,2-b)-indenofluorenyl core, through the control

of the size of the substituents borne by the fluorenyl units,

prior to the elaboration of the terfluorenyl backbone. We

then decided to focus on the synthesis of the difluorenol 7

with less bulky bromine substituents attached to the fluor-

AHCTUNGTREGeNNUN nol units, which should, in principle, favor the formation of

the (1,2-b)-indenofluorenyl core (Scheme 2).

Thus, the coupling of 2,2’’-DITP 3 with 2,7-dibromofluor-

enone (4) (nBuLi/THF/ꢀ788C) easily leads to the diol 7

with a particularly high yield (60%) for this type of coupling

reaction. This high yield may be rationalized by the elec-

tronic effect of the two electron-withdrawing bromine atoms

on the carbonyl group of the fluorenone 4. The intramolecu-

lar cyclization of the diol 7 under the same conditions as

those mentioned above for the cyclization of 6, that is

CH₃CN/reflux, leads to the formation of the two tetrabromi-

nated isomers 8 and 9 with an isomer distribution of 45/55.

In addition, when the intramolecular cyclization is carried

out in dichloromethane, a low polarity, weak donor sol-

vent,^[36]at room temperature, a significant excess of the

isomer 9 with the (1,2-b)-indenofluorenyl core (8/9 ~ 3/7) is

obtained. Compounds 8 and 9 are very insoluble in common

organic solvents and no separation could be achieved. The

mixture of isomers 8 and 9 nevertheless undergoes a

Suzuki–Miyaura cross-coupling reaction in the presence of

2-fluorACHTNUTRGNEeUNG ne boronate 2 leading to the mixture of (2,1-a)-DST-

IF and (1,2-b)-DST-IF in a 3/7 ratio. The (1,2-b)-DST-IF de-

rivative, now in large excess, was then further purified by

column chromatography. Despite (2,1-a)-DST-IF and (1,2-

b)-DST-IF being closely related positional isomers, their

¹H NMR spectra differ significantly. An important feature is

related to the differences observed between the chemical

shifts of the hydrogen atoms of the indenofluorenyl cores of

(2,1-a)-DST-IF and (1,2-b)-DST-IF. Thus, (2,1-a)-DST-IF

Determined by the analysis of the crude mixture by ¹H NMR spectros-

copy. For additional details see reference [32].

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C. Poriel, J. Rault-Berthelot et al.

effect observed in (2,1-a)-DST-

IF has been assigned to intra-

molecular p–p interactions be-

tween face-to-face terfluorenyl

moieties. Indeed, transannular

p–p interactions are usually ac-

companied by high-field shifts

in ¹H NMR spectra and have

been observed in different sys-

tems such as acridylnaphtha-

lene,^[37]paracyclophane,^[38]poly-

fluorene,^[39,40]or ethynyltriphe-

nylene^[41]or thiophene deriva-

tives.^[42]Thus, p–p interactions

occur between the two cofacial

terfluorenyl moieties of (2,1-a)-

DST-IF and should lead to sig-

nificantly different electro-

chemical and optical properties

compared to its isomer (1,2-b)-

DST-IF, as discussed next.

Thermal properties: The ther-

mal properties of (2,1-a)-DST-

IF and (1,2-b)-DST-IF were in-

vestigated by using thermogra-

vimetric

analysis

(TGA)

Figure 1. Low-field portion of the ¹H NMR spectra (CDCl₃) of (2,1-a)-DST-IF (top) and (1,2-b)-DST-IF

(Figure 2). The (2,1-a)-DST-IF

and (1,2-b)-DST-IF derivatives

exhibit a high decomposition

(bottom).

presents, for example, a low-field singlet at d=8.13 ppm,

which is assigned to the hydrogen atom H₂of the central

phenyl ring of the (2,1-a)-indenofluorenyl core (Figure 1,

top), whereas (1,2-b)-DST-IF presents a singlet at d=

7.43 ppm, which is assigned to the hydrogen atom H₂of the

central phenyl ring of the (1,2-b)-indenofluorenyl core

(Figure 1, bottom).²Similarly, the hydrogen atom H₁of (2,1-

a)-DST-IF and (1,2-b)-DST-IF are found at d=6.23 and

6.81 ppm, respectively. The chemical shifts of the hydrogen

atoms of the indenofluorenyl core in (2,1-a)-DST-IF are

hence significantly different compared to those of (1,2-b)-

DST-IF, whereas their two non-substituted parent indeno-

fluorenes, that is (2,1-a)-indenofluorene and (1,2-b)-indeno-

fluorene, present almost identical chemical shifts for all

their hydrogen atoms.^[34]This has been assigned to the dif-

ferent arrangement of the fluorene units (face-to-face or

not), which differently affects the resonance of the hydrogen

atoms of the indenofluorenyl cores. Indeed, all the signals of

the hydrogen atoms of the terfluorenyl moieties of (2,1-a)-

DST-IF such as H₃(Figure 1) appear to be strongly deshield-

ed compared to those of (1,2-b)-DST-IF. This shielding

temperature T_d, corresponding to 5% weight loss^[23](315

and 3208C, respectively). Moreover, TGA shows the pres-

ence of two steps in the thermal decomposition processes.

The first decomposition step involves around 43% of mass

loss for both isomers (Figure 2). This first decomposition is

ascribed to the loss of the eight octyl chains borne by the

four external fluorene units of (2,1-a)-DST-IF and (1,2-b)-

DST-IF.^[43]After the first decomposition step, a plateau is re-

corded until around 5508C for (2,1-a)-DST-IF and around

6208C for (1,2-b)-DST-IF. The second decomposition step

may be ascribed to the decomposition of the DST-IF back-

bones. Similar terfluorene derivatives bearing alkyl and/or

aryl groups on the fluorene bridges have been previously re-

ported in the literature for which the T_dvalues are strongly

related to the substitution of their fluorene bridges (alkyl vs.

aryl).^[12,21]It is important to stress that a high T_dvalue is an

asset for OLED applications, as Joule heating occurs under

typical operating conditions.^[44]

Optical properties: As (2,1-a)-DST-IF and (1,2-b)-DST-IF

combine both dispirofluorene–indenofluorene (DSF-IF) and

terfluorene frameworks, it was of interest to compare their

optical properties with those of the two constituting building

blocks: (2,1-a)-DSF-IF,^[28](1,2-b)-DSF-IF,^[28]and 9,9-dioctyl-

terfluorene (3-F)^[35](Table 1). The absorption spectra of

(2,1-a)-DST-IF and (1,2-b)-DST-IF are characterized by a

large and poorly structured band between 300 and 400 nm.

The assignments of (2,1-a)-DST-IF and (1,2-b)-DST-IF have been per-

formed by 2D NMR spectroscopy experiments: HMBC (heteronuclear

multiple bond correlation), HMQC (heteronuclear multiple quantum

coherence), and ¹H/¹H COSY (correlation spectroscopy) (see portion

of HMBC spectrum in Figure S1 and S2 in the Supporting Informa-

tion).

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3p–2Spiro Terfluorene–Indenofluorene Isomers

FULL PAPER

Figure 2. Thermogravimetric analysis of (2,1-a)-DST-IF (top) and (1,2-b)-

DST-IF (bottom) (scan rate: 58Cmin^ꢀ1under a nitrogen atmosphere).

Figure 3. Normalized absorption spectra (top) and fluorescence spectra

(bottom) of (2,1-a)-DST-IF (dashed line) and (1,2-b)-DST-IF (solid line)

in THF (c=10^ꢀ6m).

F, might be consequently attrib-

uted to the indenofluorenyl

Table 1. Optical properties.

[a]

[b]

l_abs

l_em

F_sol

Ref

units. Indeed, the p–p* indeno-

fluorene transition (334 nm in

CH₂Cl₂)^[27]is observed at

339 nm for (2,1-a)-DSF-IF and

at 345 nm for (1,2-b)-DSF-IF

(Table 1).^[28]

liq. [nm]

[%]

[c,d]

[35]

[28]

326 (sh), 343, 354

314 (sh), 330 (sh), 347, 355 (sh)

351

402, 422, 450 (sh)

395, 417, 441 (sh), 475 (sh)

395, 416, 444 (sh), 479 (sh)

345, 363, 380 (sh), 400 (sh)

348, 355, 366, 388 (sh), 405 (sh)

295, 311, 323, 339

298, 310, 328, 336, 345

ACHTUNGTRENNUNG

[a] In THF. [b] The relative quantum yield was measured with reference to quinine sulfate in 1n H₂SO₄(F=

0.546). The values are estimated ꢁ10%. [c] In CHCl₃. [d] This work.

The (2,1-a)-DST-IF and (1,2-

b)-DST-IF derivatives present

very similar optical gaps³of

about 3.15 eV. This gap is close

to the band gap recorded for

The absorption spectrum of (1,2-b)-DST-IF displays one

maximum at 347 nm and shoulders at 314, 330, and 355 nm,

whereas the absorption spectrum of (2,1-a)-DST-IF displays

two maxima at 343 and 354 nm and a shoulder at 326 nm

(Figure 3, top). The low-energy bands at 354–355 nm are

similar to that of 3-F (l_max351 nm)^[35]and other terfluoren-

yl-based oligomers^{[12,21,45–47]}and are hence attributed to p–

p* electronic transitions involving the terfluorenyl units.

The high-energy bands and shoulders at 343 nm for (2,1-a)-

DST-IF and 347 nm for (1,2-b)-DST-IF, not observed with 3-

other terfluorenyl derivatives such as 9,9-dihexylterfluor-

ene^[47]or terfluorenyl spiro-bridged with either cyclopropane

(3.25 eV)^[20]or indenothiophene (3.14 eV) units.^[17]On the

other hand, the (2,1-a)-DST-IF and (1,2-b)-DST-IF gaps are

smaller than those recorded for (2,1-a)-DSF-IF (3.57 eV)

and (1,2-b)-DSF-IF (3.51 eV) for which the gap is controlled

by the central indenofluorenyl units.^[28]Consequently, the

Optical gap calculated from DE^opt[eV]=hc/l [nm] (DE^opt[eV]=

1237.5/l [nm]).

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C. Poriel, J. Rault-Berthelot et al.

gap contraction observed for (2,1-a)-DST-IF and (1,2-b)-

DST-IF demonstrates that in the present case, the gaps are

controlled by the terfluorenyl units and not by the indeno-

fluorenyl cores. It should be stressed that DSF-IF molecules

with two aryl groups on the fluorene units previously report-

ed by our group present mixed properties with the gap con-

trolled by both indenofluorenyl and diarylfluorenyl

cores.^[33,34]Thus, (2,1-a)-DST-IF and (1,2-b)-DST-IF appear

to be the first examples of DSF-IFs derivatives with a

HOMO–LUMO gap almost exclusively controlled by the

external p-conjugated systems, namely the terfluorenyl

units.

b)-DST-IF. This observation leads us to assign the emission

shoulder at 450 nm in (2,1-a)-DST-IF to frustrated intramo-

lecular interactions between two face-to-face terfluorene

units. The emission of (2,1-a)-DST-IF in solution thus origi-

nates in part from the terfluorene units as for (1,2-b)-DST-

IF and from frustrated intramolecular terfluorene excimers

due to the specific face-to-face organization of the terfluor-

ene units in this molecule. By using quinine sulfate in sulfu-

ric acid as a standard,^[52]the quantum yields in solution for

(2,1-a)-DST-IF and (1,2-b)-DST-IF are estimated to be

around 80 and 83%, respectively. These quantum yields are

between those of (2,1-a)-DSF-IF (60%) and (1,2-b)-DSF-IF

(62%), and those of 3-F (90%)^[35]and ter-9,9-spirobifluor-

ene (99%).^[43]Thus, both (2,1-a)-DST-IF and (1,2-b)-DST-IF

present very high quantum yields in the blue region and are

promising for further OLED applications. The solid-state

absorption spectra of (2,1-a)-DST-IF and (1,2-b)-DST-IF

(Figure 4, top) appear less defined and slightly red-shifted

compared to those in solution (Figure 3, bottom). Regarding

the solid-state fluorescent properties (Figure 4, bottom),

(1,2-b)-DST-IF presents two main maxima recorded at 399

The (2,1-a)-DST-IF and (1,2-b)-DST-IF derivatives both

exhibit deep blue fluorescence in solution (Figure 3,

bottom) with however significant differences between the

two fluorescent spectra. The fluorescence spectrum of (1,2-

b)-DST-IF is well defined with maxima recorded at 395 and

417 nm fitting well with those of 3-F (395, 416 nm,

Table 1).^[35]It is hence reasonable to infer that the main

emission in (1,2-b)-DST-IF arises from the terfluorenyl

arms. The important Stokes shift (46 nm) and the poor

mirror images between absorption and fluorescence spectra

indicate conformational changes between the ground state

and the first excited state. They suggest that conformations

are more rigid in the first excited state than in the ground

state. Indeed, in the excited state, the bonds joining the fluo-

rene units are shortened and the dihedral angles between

the fluorene units are reduced leading hence to a more

planar conformation.^[48,49]Surprisingly, the (2,1-a)-DST-IF

fluorescence spectrum is significantly different to that of its

isomer as it appears red-shifted (l_em=402, 422 nm), larger,

and less structured with a more intense shoulder around

450 nm and a long tail up to 600 nm (Figure 3, bottom). The

emission bands of (2,1-a)-DST-IF found at 402 and 422 nm

may be ascribed to the terfluorenyl units, nevertheless they

are red-shifted by about 10 nm compared to those of 3-F

(395, 416 nm) and those of (1,2-b)-DST-IF (395, 417 nm).

The low-energy shoulder observed at 450 nm appears diffi-

cult to assign. Nevertheless, reports in the literature indicate

that fluorene-excited dimer (fluorene excimer) displays an

emission in solution centered at 400 nm,^[40,50]whereas elec-

trogenerated terfluorene excimers of the terspirobifluorene

lead to an intense emission band in the 522–583 nm range.^[51]

The shoulder observed around 450 nm in the (2,1-a)-DST-IF

fluorescence spectrum appears between the emission of the

fluorene excimer and the emission of the terfluorene exci-

mer. Assigning the emission at 450 nm is hence not trivial

but should result either from 1) frustrated intramolecular

terfluorene excimers, or 2) intermolecular indenofluorene or

terfluorene excimers. Varying the DST-IF concentrations be-

tween 10^ꢀ7and 10^ꢀ4m should hence provide interesting in-

formation regarding this assignment (see Figures S3–S6 in

the Supporting Information). For both compounds, no modi-

fication of the shape of the absorption nor of the emission

spectra as a function of the concentration was observed, in-

dicating that intermolecular interactions do not occur even

at high concentration either for (2,1-a)-DST-IF or for (1,2-

Figure 4. Normalized thin-film absorption spectra (top) and fluorescence

spectra (bottom) of (2,1-a)-DST-IF (dashed line) and (1,2-b)-DST-IF

(solid line).

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3p–2Spiro Terfluorene–Indenofluorene Isomers

FULL PAPER

and 420 nm and shoulders at 444 and 480 nm, in accordance

with those of 9,9-dihexylterfluorene,^[47]highlighting hence

that the terfluorenyl units are the main emitter in thin films.

In addition, (1,2-b)-DST-IF presents a very well-defined

spectrum almost identical to that in solution. Only a very

small red shift of about 4 nm and a different relative intensi-

ty of the main emissive bands are observed. The fact that

the fluorescent spectrum does not show significant change

when going from solution to solid-state suggests that no spe-

cific intermolecular interactions involving the p-systems

occur when the film is formed. The red shift between solu-

tion and thin-film fluorescence spectra of (1,2-b)-DST-IF is

smaller than those reported in the literature for 9,9-dialkyl-

terfluorene derivatives for which an important self-absorp-

tion of the highest energy emitted band is observed.^[21]Simi-

larly, the marked difference observed in the relative intensi-

ty of the emission bands between solution and thin-film

fluorescence spectra of (1,2-b)-DST-IF (that at 399 nm is

weakened, whereas that at 420 nm is strengthened) may be

also explained by self-absorption phenomena due to the

overlap of the 0–0 transition band and the absorption

band.^[21,53]In contrast, the solid-state fluorescent spectrum

of (2,1-a)-DST-IF (Figure 4, bottom) appears to be signifi-

cantly different both from that in solution and from that of

its isomer (1,2-b)-DST-IF. Thus, the thin-film fluorescence

spectrum of (2,1-a)-DST-IF is poorly resolved with a main

emission band centered at 464 nm and with weak shoulders

at 405 and 437 nm. The emission at 464 nm is close to the

shoulder observed in solution (450 nm) and also to the thin-

film main emission of DSF-IF molecules with two aryl

groups on the fluorene units. Indeed, these present in solu-

tion a poorly resolved emission band, which is assigned to

intramolecular excimer emission arising from face-to-face

aryl–fluorene–aryl units, ranging from 434–463 nm depend-

ing on the steric hindrance induced by the substituents

borne by the aryl units.^[33,34]Thus, with small substituents

such as a methoxy group or a fluorine atom, the solid-state

and solution spectra are almost identical (see Figures S7–S9

in the Supporting Information). In contrast, with bulky sub-

stituents such as tert-butyl groups, solid-state and solution

spectra are drastically different due to the emission of non-

stacked and stacked aryl–fluorene–aryl units in solution and

almost exclusively of stacked aryl–fluorene–aryl units in the

solid state (see Figures S10 and S11 in the Supporting Infor-

mation). In the present case, the emission of (2,1-a)-DST-IF

in solution is very different from that in a thin film. It is

hence reasonable to contend that the emission principally

arises in solution from non-stacked terfluorenyl units and to

a lesser extent from stacked terfluorenyl units. However in

the solid state, the main emission at 464 nm is attributed to

intramolecular excimer formation arising from stacked face-

to-face terfluorenyl units and the shoulders observed at

lower wavelength may reflect the weak emission of non-

stacked terfluorenyl units. Before any further SMOLED ap-

plication, it is of great interest to study the behavior of the

two terfluorenyl derivatives in the thin films and to study

their stability upon heating. Indeed, it is known that an

OLED device can reach a temperature of approximately

868C and the stability of the emitted color upon heating is

of key importance.^[44]Evolution of the fluorescent spectra of

both (2,1-a)-DST-IF and (1,2-b)-DST-IF under thermal

stress conditions was then investigated (Figure 5).

Figure 5. Evolution of normalized thin-film fluorescence spectra as a

function of the temperature (1 h at each stage) of top: (1,2-b)-DST-IF

(normalized at 422 nm) and bottom: (2,1-a)-DST-IF (normalized at

464 nm for T <1008C and at 424 nm for T ꢂ 1008C).

This study provides two main pieces of information. First,

the fluorescence spectrum of (1,2-b)-DST-IF remains stable

from room temperature to 1508C (Figure 5, top). However,

at 2008C, a green emission band (GEB, l =518 nm) starts

to grow and becomes very intense after one night at 2008C.

The origin of such a contribution at low energy has been ex-

tensively studied for the last decade as it remains an impor-

tant problem in blue OLED technology.^[6,54,55]While the

GEB was initially explained by excimer emission (the for-

mation of dimerized units in the excited state that emit at

lower energies) due to p–p interchromophore interactions

of the p-conjugated backbone,^[56]it seems now that the

GEB is linked to the presence of keto-defects appearing in

the bridged p-conjugated backbone. Thus, Mꢃllen and co-

workers have notably demonstrated that alkylated poly(in-

denofluorene) and ladder-type poly(pentaphenylene) are

less stable toward oxidation degradation than their aryl ana-

Chem. Eur. J. 2011, 17, 14031 – 14046

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14037

C. Poriel, J. Rault-Berthelot et al.

logues due to the dialkyl bridgeheads, which are sensitive to

oxidation.^[6,57,58]Similarly, Bredas and co-workers have also

demonstrated the oxidative degradation process occurring in

ladder-type poly-p-phenylene bearing either a hydrogen

atom or a methyl group at the bridge.^[59]However, the pre-

cise relaxation processes that follow excitation of the keto-

defect sites are nevertheless still being debated.^[54,60,61]To go

deeper in the intimate understanding of this GEB, we decid-

ed to investigate the optical properties of the ketone 5 (see

structure in Scheme 1) as a relevant oxidized model com-

pound of (1,2-b)-DST-IF.^[54]In diluted solution, 5 shows ab-

sorption bands with maxima at 308 and 349 nm that can be

assigned to the p–p* transition of the terfluorene unit, and a

less intense band around 455 nm believed to be due to the

symmetry-forbidden carbonyl group n–p* transition (see

Figure S12 in the Supporting Information).^[62,63]Increasing

the concentration of 5 does not lead to any modification of

its absorption spectrum. In addition, its thin-film absorption

spectrum was also almost identical to its solution spectrum

(see Figure S12 and S13 in the Supporting Information). In

contrast, the fluorescence properties of terfluorenone 5

appear to be strongly dependent on the concentration and

the environment (solution vs. thin film). Indeed, the fluores-

cence spectra of 5 only display one band (l=380 nm) for di-

luted solutions but display two sets of emission bands at

(l=380 nm and 540/570 nm) when increasing the concentra-

tion (see Figure S14 in the Supporting Information). On the

other hand, the thin-film emission spectrum of 5 only dis-

plays one large band at 580 nm (see Figure S15 in the Sup-

porting Information) assigned, as recently proposed by

Holmes and co-workers,^[54]to intermolecular interactions in

the solid state between the ketones units.⁴It is hence rea-

sonable to similarly assign the large band (540/570 nm) ob-

served in concentrated solutions of 5 (vide supra) to inter-

molecular interactions between the ketone units. In light of

these results and since the fluorescence spectrum of a film

of (1,2-b)-DST-IF, redissolved after thermal annealing at

2008C, is identical (with no trace of the GEB) to that of

pristine compound in solution,^[61]it is reasonable to contend

that the GEB arises from intermolecular interactions be-

tween the keto-defects formed after thermal annealing

(Figure 5, top). However, a red shift is observed between

the GEB of 5 (l=580 nm) and the GEB of (1,2-b)-DST-IF

(l=518 nm). This can be explained by the position of the

keto-defect in the terfluorene unit, since the appearance of

keto-defects in (1,2-b)-DST-IF should lead to fluorenone-

terminated terfluorene, whereas the model compound 5 is a

fluorenone-centered terfluorene. Indeed, it has been report-

ed that the main emission band of fluorenone-terminated

terfluorene is recorded at about 528 nm, whereas that of flu-

orenone-centered terfluorene is red shifted to about 550 nm.

This clearly indicates that the position of the ketone within

the p-conjugated backbone significantly influences the

wavelength of the emission band.^[54]

Surprisingly, (2,1-a)-DST-IF displays drastically different

behavior since the gradual heating of a spin-coated thin film

in air from room temperature to 2008C did not lead to the

growth of a GEB. Consistently with the above discussion of

(1,2-b)-DST-IF, we propose that some alkyl bridgeheads of

(2,1-a)-DST-IF are also oxidized in ketones but it seems that

these ketones do not interact, presumably due to the differ-

ent arrangement in the thin film. Consequently, no trace of

GEB in the thin-film fluorescence spectra of (2,1-a)-DST-IF

was detected.⁵

The second key piece of information arises nevertheless

from the modification of the fluorescence spectrum of (2,1-

a)-DST-IF as a function of temperature. The (2,1-a)-DST-IF

fluorescence spectrum remains stable with a main emission

centered at 464 nm, when the sample is heated from room

temperature to 608C. However, heating the sample at

1008C is accompanied by a modification of the fluorescence

spectrum of (2,1-a)-DST-IF, which now displays three

maxima (402, 424, and 441 nm) and a shoulder (464 nm),

surprisingly in accordance with those of its solution fluores-

cence spectra. Interestingly, a similar blue shift of the fluo-

rescence maxima was also observed when heating a thin

film of (2,1-a)-dispiro(3,4,5-trimethoxyphenylfluorene)–in-

denofluorene (see Figure S18 in the Supporting Informa-

tion), which became nearly identical to its emission spec-

trum in solution (see Figure S9 in the Supporting Informa-

tion). Similar changes in the solid-state fluorescence spec-

trum with temperature have been reported for butadiene

derivatives^[64–66]but in these examples, the monomer-like or

excimer-like fluorescence come from specific “intermolecu-

lar packing”, which may be modified with temperature. In a

“herringbone” arrangement, minimal electronic coupling be-

tween molecules led to a monomer-like emission, whereas

in a “brickstone” arrangement, molecular packing led to ex-

cimer-like fluorescence. In the case of (2,1-a)-DST-IF, heat-

ing of the films leads to a reorganization of the face-to-face

terfluorene moieties in the solid, leading to a decrease in

the amount of intramolecular emitting excimers. To our

knowledge, variation of fluorescence with temperature due

to “intramolecular packing reorganization” is almost absent

from the literature^[67]and appears to play a major role in de-

ciding the solid-state fluorescence properties of (2,1-a)-DST-

IF.

Electrochemical properties: The electrochemical behavior

of (2,1-a)-DST-IF and (1,2-b)-DST-IF has been investigated

by using cyclic and differential pulse voltammetry (CV and

DPV, respectively). In the cathodic range (see Figure S18 in

the Supporting Information), (1,2-b)-DST-IF presents an ir-

reversible reduction wave with a poorly resolved maximum

at ꢀ2.6 V and an onset reduction potential at ꢀ2.27 V. In

contrast, (2,1-a)-DST-IF does not show any maximum of its

reduction wave and presents an onset reduction potential at

ꢀ2.35 V. From these onset potentials, the LUMO energy

No evolution of the thin-film absorption and emission spectra was ob-

It should be noted that when a film of (2,1-a)-DST-IF was redissolved

served when increasing the temperature from room temperature to

after thermal annealing at 2008C, its fluorescence spectrum is identical

2008C (see Figure S16 and S17 in the Supporting Information).

to that of pristine compound in solution.

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3p–2Spiro Terfluorene–Indenofluorene Isomers

FULL PAPER

Table 2. Electrochemical data of (2,1-a)-DST-IF, (1,2-b)-DST-IF, (2,1-a)-DSF-IF, (1,2-b)-DSF-IF recorded in CH₂Cl₂–[NBu₄]ACHTNUGRTENUNG[PF₆] 0.2m.

red

E_ox

[V vs. SCE]

E_onset

HOMO

[eV]^[a]

LUMO

[eV]^[b]

LUMO

[eV]^[d]

DE_el

DE_opt

Ref.

[f]

[V vs. SCE]

[eV]^[c]

[eV]^[e]

1.12 (1e^ꢀ), 1.18 (1e^ꢀ),

1.64 (3e^ꢀ), 2.04 (2e^ꢀ)

1.29 (2e^ꢀ), 1. 47 (2e^ꢀ),

0.98

ꢀ2.35

ꢀ5.38

ꢀ5.46

ꢀ5.64

ꢀ5.76

ꢀ2.05

ꢀ2.13

ꢀ2.03

ꢀ2.17

ꢀ2.28

ꢀ2.36

ꢀ2.07

ꢀ2.23

3.33

3.61

3.59

3.13

3.16

3.57

3.51

[f]

1.06

1.20

1.36

ꢀ2.27

ꢀ2.37

ꢀ2.23

1.58ACHTUNGTRENNUNG

(1e^ꢀ), 2.01 (2e^ꢀ+1e^ꢀ)

[28]

1.36 (1e^ꢀ), 1.69 (1e^ꢀ),

1.99 (>1e^ꢀ)

1.47 (1e^ꢀ), 1.95 (>1e^ꢀ)

[a] Calculated from the onset oxidation potential. [b] Calculated from the onset reduction potential. [c] DE_elec= jHOMO–LUMOj (in eV). [d] Calculated

from the HOMO energy level and the edge of optical band gap. [e] DE_optis calculated from the edge of the absorption spectrum using DE^opt=hc/l.

[f] This work.

levels of (2,1-a)-DST-IF and (1,2-b)-DST-IF were deter-

mined at ꢀ2.05 and ꢀ2.13 V respectively (Table 2).⁶In the

anodic range, both compounds present successive oxidation

processes between 0.2 and 2.5 V (Figure 6A). HOMO

energy levels of (2,1-a)-DST-IF and (1,2-b)-DST-IF were de-

termined from their onset oxidation potentials (0.98 V for

(2,1-a)-DST-IF and 1.06 V for (1,2-b)-DST-IF) at ꢀ5.38 and

ꢀ5.46 eV, respectively. The electrochemical gap calculated

from the HOMO and LUMO level is 3.33 eV for both com-

pounds. Interestingly, (2,1-a)-DST-IF and (1,2-b)-DST-IF

also possess almost identical optical gap, DE_opt, calculated

from the edge of the solution absorption spectrum (DE_opt

3.16 eV for (1,2-b)-DST-IF and DE_opt=3.13 eV for (2,1-a)-

DST-IF). As theoretical calculations point to a terfluorenyl-

based primary oxidation and reduction in (2,1-a)-DST-IF

and (1,2-b)-DST-IF, (see the calculated nature of their fron-

tier molecular orbitals HOMO/LUMO in Figure 7 and 8),

the HOMO/LUMO energy levels are mainly determined by

their external terfluorenyl units and not by their central in-

denofluorenyl cores. This feature is considerably different

from those recently reported for alkyl-substituted (1,2-b)-

DSF-IF and (2,1-a)-DSF-IF, which possess HOMO/LUMO

energy levels mainly centered on their indenofluorenyl

cores^[28,31]and for aryl-substituted (1,2-b)-DSF-IF and (2,1-

a)-DSF-IF, which possess HOMO/LUMO energy levels cen-

tered both on their indenofluorenyl cores and on their exter-

nal aryl–fluorene–aryl units.^[33,34]

The (2,1-a)-DST-IF and (1,2-b)-DST-IF derivatives pres-

ent in CV successive mono or multiple electronic processes

(Figure 6A) clearly pointed out by DPV (Figure 6B). The

(1,2-b)-DST-IF derivative is first oxidized along two succes-

sive bielectronic oxidations at 1.29 and 1.47 V, followed by a

reversible monoelectronic oxidation at 1.58 V and a three-

electron oxidation at 2.01 V (in black, Figure 6A and B).

The two first oxidations are reversible and are close to the

two first reversible oxidations of terfluorene deriva-

red

The LUMO level was calculated from: LUMO (eV)=ꢀ[E_onset(vs.

SCE) + 4.4] and the HOMO level from: HOMO [eV]=ꢀ[E_onset

(vs.

Figure 6. CVs (A) and DPVs (B) in CH₂Cl₂–[NBu₄]

presence of (2,1-a)-DST-IF (dashed line) and (1,2-b)-DST-IF (solid line),

platinum working electrode. CVs (C) in CH₂Cl₂–[NBu₄][PF₆] 0.2m using

a modified poly((2,1-a)-DST-IF) (dashed line) and poly((1,2-b)-DST-IF)

ACHTUNGTREN[NUNG PF₆] 0.2m in the

SCE) + 4.4], based on an SCE energy level of 4.4 eV relative to the

vacuum.^[68]The electrochemical gap was calculated from : DE^el= jHO-

MOꢀLUMOj (in eV). The estimated errors in the determination of

AHCTUNGTRENNUNG

ACHTUNGTRENNUNG

the onset potential values are ꢁ20 mV for E_onsetand ꢁ50 mV for

(solid line) electrode previously modified by oxidation of their monomers

at 2.0 V during 2 min.

red

E_onset

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14039

C. Poriel, J. Rault-Berthelot et al.

confirmed by theoretical calcu-

lations since the HOMO and

the SOMO levels of (1,2-b)-

DST-IF presents a terfluorenyl

character (see Figure 7 and 8

for the HOMO and Figure S23

and S24 in the Supporting In-

formation for the SOMO mo-

lecular orbitals). The third re-

versible oxidation recorded at

1.58 V is assigned to the oxida-

tion of the indenofluorenyl core

a potential slightly more

anodic than that recorded for

(1,2-b)-DSF-IF (1.47 V).^[27]This

shift to more anodic values is

probably due to the electron-

withdrawing effect of the ter-

fluorenyl arms that are under

their dicationic state at this po-

tential. The electrochemical be-

havior of (2,1-a)-DST-IF (Fig-

ure 6A and B, dashed line) dif-

fers drastically to that of (1,2-

b)-DST-IF. Indeed, (2,1-a)-

DST-IF is oxidized along two

successive one-electron process-

es recorded at 1.12 and 1.18 V.

These two first oxidation poten-

tials are lower than those re-

ported above for (1,2-b)-DST-

IF and lead to a doubly charged

species, stable up to 1.64 V at

which a third three-electron ox-

idation process occurs.

Figure 7. Sketch of frontier molecular orbitals for a simplified model of (1,2-b)-DST-IF (methyl instead of

octyl groups) from Gaussian 03 B3LYP/6-31G* calculations.^[69–71]

The fourth oxidation of (2,1-

a)-DST-IF occurs at more than

2.0 V and appears as an irrever-

sible two-electron oxidation

process followed by a multielec-

tronic process. The main differ-

ence between the oxidation of

(2,1-a)-DST-IF and (1,2-b)-

DST-IF is related to the differ-

ent potentials of their first re-

versible electronic processes.

Indeed, the first oxidation po-

tential

(2,1-a)-DST-IF

(1.12 V) is lower than the first

oxidation potential of (1,2-b)-

DST-IF (1.29 V). It is known

that the oxidation of p-stacked

Figure 8. Sketch of frontier molecular orbitals for a simplified model of (2,1-a)-DST-IF (methyl instead of

octyl groups) from Gaussian 03 B3LYP/6-31G* calculations.^[69–71]

tives.^[35,47,72]It seems then consistent to assign the two first

oxidation processes of (1,2-b)-DST-IF to the oxidation of

the two terfluorenyl arms successively leading to (1,2-b)-

DST-IF bis-radical cations and (1,2-b)-DST-IF bis-dications.

The assignment of these two first oxidation waves is clearly

systems is more facile than that of their non-stacked ana-

logues,^[39,73]and the cofacial arrangement of the two terfluor-

enyl units in (2,1-a)-DST-IF is hence at the origin of its ob-

served low oxidation potential. Therefore, the first oxidation

process of (2,1-a)-DST-IF may be assigned to the oxidation

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3p–2Spiro Terfluorene–Indenofluorene Isomers

FULL PAPER

of a “terfluorene dimer” that results from the specific face-

to-face organization of the two terfluorenyl units in (2,1-a)-

DST-IF. These p–p interactions have also been evidenced in

¹H NMR spectra (vide supra). The second oxidation of (2,1-

a)-DST-IF occurs at 1.18 V, which is also at a lower potential

value than the oxidation of 3-F and (1,2-b)-DST-IF and may

be assigned to the second oxidation of the intramolecular

terfluorene dimer. This double radical cation is stabilized

between the two terfluorenyl arms and appears thermody-

namically stable. Indeed, the further oxidation of the ter-

fluorenyl arms occurs nearly 0.5 V higher, together with the

oxidation of the indenofluorenyl unit. The different oxida-

tion processes of (2,1-a)-DST-IF and (1,2-b)-DST-IF are

summarized in Scheme 3. After these five-electron oxida-

ure 6A). Similarly, the 0.16 V shift observed between poly-

ACHTUNGTRENNUNG

AHCTUNGTRENNUNG

ACHTUNGTRENNUNG

IF)^[27,28](shift: 0.02 V). In the light of these comparative re-

sults and since DSF-IF and DST-IF derivatives presents at

least six sites of coupling (two on the two fluorenyl units

and two on the indenofluorenyl unit), it is hence very diffi-

cult to precisely describe the polymerization processes.

Small-molecule organic light-emitting diodes (SMOLEDs)

using (2,1-a)-DST-IF as the emitting layer: As discussed

above, the gradual heating of a thin film of (2,1-a)-DST-IF

from room temperature to 2008C did not lead, as observed

for (1,2-b)-DST-IF, to the growth of a parasite GEB. Hence,

the final OLED investigations were only carried out with

(2,1-a)-DST-IF. To explore the potential applications of the

new blue-emitting (2,1-a)-DST-IF, a basic spin-coated single-

layer SMOLED was fabricated and characterized. The struc-

ture of the devices was based on ITO as the anode,

poly(3,4-ethylenedioxythiophene) doped with poly(styrene-

sulfonate) (PEDOT/PSS) as the hole injecting layer (HIL),

(2,1-a)-DST-IF as the emitting layer (EML), and Ca as cath-

ode. We decided to focus on a single-layer device to study

the intrinsic properties of the (2,1-a)-DST-IF emitting layer.

Indeed, through careful band engineering (addition of elec-

tron/hole transporting/blocking layers) it is always possible

to improve the efficiencies of the device, whatever the emit-

ting layer. Thus, the single-layer device using a spin-coated

emissive layer of (2,1-a)-DST-IF gives a turn on voltage (at

1 Cdm^ꢀ2) of 7 V, a maximum luminance of 100 Cdm^ꢀ2, and

a luminous efficiency of around 0.05 CdA^ꢀ1(see Figure S20

and S21 in the Supporting Information). It is evident that

the modest efficiencies of this device are attributed to the

wide HOMO–LUMO gap of the blue-emitting material cou-

pled to its very simple structure. Indeed, the electron-injec-

tion barrier from Ca (ꢀ2.9 eV) to (2,1-a)-DST-IF

(ꢀ2.05 eV) is very large and hence the device efficiency

could be easily improved by choosing suitable electron-

transporting materials. It should be however stressed that

the HOMO level of (2,1-a)-DST-IF fits well with the work

function of the ITO/PEDOT anode, avoiding hence the use

of a hole-transporting unit. The single-layer device using

(2,1-a)-DST-IF as emitting layer presents comparable per-

formances to those of other terfluorene single-layer devices

previously reported in the literature. For example, Qin and

co-workers have recently designed several poly(terfluorene)

derivatives bearing electron-transporting units and have re-

ported maximum luminances ranging from 102 to

235 Cdm^ꢀ2.^[13]The normalized electroluminescence spec-

Scheme 3. Step by step five-electron oxidation of (2,1-a)-DST-IF and

(1,2-b)-DST-IF.

tions, (2,1-a)-DST-IF and (1,2-b)-DST-IF may be oxidized to

higher oxidation states and these oxidations lead to poly-

merization processes. Indeed, recurrent scans for both com-

pounds between 0 and 2.0 V lead to a regular polymeri-

zation process that is evidenced by the appearance and

growth of new redox waves, by the gradual shift of the onset

potential, and by the coverage of the electrode with an in-

soluble thin film (see Figure S19 in the Supporting Informa-

tion). Polymerization is also observed by oxidation at fixed

potential. For example, deposits prepared by oxidation at

2 V for 180 s, removed from the electrolyte, rinsed in di-

chloromethane, and studied in a new ꢄmonomer freeꢅ solu-

tion, present the profile shown in Figure 6C. The oxidation

of polyACHTUNGTRENNUNG((2,1-a)-DST-IF) and of polyACHTUGNTREN(NUGN (1,2-b)-DST-IF) start at

0.8 V and 0.96 V, respectively, and both polymers are elec-

trochemically stable up to 1.8 V, leading to an electroactivity

window close to 1 V. The onset oxidation potential of the

two polymers follows the same trend as that of their corre-

sponding monomers. Thus, the oxidation of poly

DST-IF) (onset oxidation potential: 0.8 V) starts at a 0.16 V

less anodic potential than that of poly((1,2-b)-DST-IF)

ACHTUNGTRENN(UNG (2,1-a)-

trum recorded for the ITO/PEDOT/ACTHNGUTERNNU(G 2,1-a)-DST-IF/Ca

(onset oxidation potential: 0.96 V), Figure 6C. This shift of

0.16 V between the two polymers is nevertheless larger than

that observed between their corresponding monomers (2,1-

a)-DST-IF and (1,2-b)-DST-IF, which possess onset oxida-

tion potentials at 0.98 V and 1.06 V, respectively (Fig-

device (Figure 9A) exhibits one emission peak in the blue

region at 464 nm. The chromatic coordinates (Figure 9B)

calculated from the electroluminescence spectrum in the

CIE 1964 chromaticity diagram are (0.19; 0.23), correspond-

ing to a blue color. It is worth noting that no parasite GEB

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14041

C. Poriel, J. Rault-Berthelot et al.

(CH₂Cl₂), the positional isomer (1,2-b)-DST-IF was obtained

in a two-step sequence. The two blue-emitting molecules

(1,2-b)-DST-IF and (2,1-a)-DST-IF possess a large HOMO–

LUMO gap (about 3.15 eV), very high quantum yields, and

are thermally stable. Despite the fact that the geometry pro-

files of (2,1-a)-DST-IF and (1,2-b)-DST-IF are imposed by

their indenofluorenyl cores, the properties are mainly gov-

erned by the two terfluorenyl frameworks. For (1,2-b)-DST-

IF, in solution and in the solid state, the fluorescence around

400/420 nm is attributed to the terfluorenyl units. However,

the heating of (1,2-b)-DST-IF films up to 2008C, leads to a

green emission band at 520 nm. Despite being a positional

isomer, (2,1-a)-DST-IF with face-to-face terfluorenyl units,

displays drastically different fluorescent behavior. The most

striking feature is related to the different emitters (non-

stacked and stacked terfluorenyl units) involved in solution

and in the solid state, leading to different main emission

wavelengths; that is 420 nm in solution and 464 nm in the

solid state. In addition, the intriguing behavior of the (2,1-

a)-DST-IF fluorescence spectra as a function of the temper-

ature was investigated, which clearly showed a blue shift of

the main emission (from 464 nm to 420/440 nm). This intra-

molecular packing reorganization leads to a heated film

with an emission comparable to the emission to that in solu-

tion for (2,1-a)-DST-IF. Finally, (2,1-a)-DST-IF was success-

fully used in a blue single-layer SMOLED (chromatic coor-

dinates : 0.19; 0.23) presenting an electroluminescence spec-

trum in perfect accordance with its thin-film fluorescence

spectrum. This stable blue emission arises from an electro-

generated intramolecular terfluorenyl excimer, which is to

the best of our knowledge the first example, and may hence

open new avenues in the design of efficient blue fluoro-

phores for OLED applications.

Figure 9. A) Electroluminescence spectrum of ITO/ACTHNUGRTENUNG(2,1-a)-DST-IF/Ca

OLED; inset: picture of the device. B) Corresponding color placed on

the CIE 1964 chromaticity diagram (0.19; 0.23).

emission was observed in the (2,1-a)-DST-IF-based OLEDs,

which appear very stable. Hence, this behavior appears

promising for the optimization of future OLEDs. Interest-

ingly, both the maximum and the shape of the electrolumi-

nescence spectrum are almost identical to the thin-film fluo-

rescence spectrum of (2,1-a)-DST-IF recorded at room tem-

perature (Figure 5). Thus, this blue color clearly arises from

electrogenerated intramolecular terfluorenyl excimer emis-

sion, which has not previously been described in the litera-

ture.

Experimental Section

Synthesis: Commercially available reagents and solvents were used with-

out further purification other than those detailed below. Dichlorome-

thane was distilled from P₂O₅drying agent Sicapent (Merck). THF was

distilled from sodium/benzophenone, and toluene was distilled from

sodium prior to use. Light petroleum refers to the fraction with b.p. 40–

608C. The reaction mixtures were stirred magnetically, unless otherwise

indicated. Analytical thin-layer chromatography was carried out using

aluminum-backed plates coated with Merck Kieselgel 60 GF₂₅₄and vi-

sualized under UV light (at 254 and/or 360 nm). Chromatography was

carried out using silica 60A (0.06–0.2 mm). ¹H and ¹³C NMR spectra

were recorded by using Bruker 300 MHz instruments (¹H frequency, cor-

responding ¹³C frequency is 75 MHz); chemical shifts were recorded in

ppm and J values in Hz. The residual signals for the NMR solvents are:

CDCl₃: d=7.26 ppm for the proton and d=77.00 ppm for the carbon;

CD₂Cl₂: d=5.32 ppm for the proton and d=53.80 ppm for the carbon;

[D₈]THF: d=3.58 ppm for the proton and d=67.57 for the carbon. The

following abbreviations have been used for the NMR assignment: s for

singlet, d for doublet, t for triplet, m for multiplet and br for broad.

High-resolution mass spectra were recorded at the Centre Rꢁgional de

Mesures Physiques de l’Ouest (Rennes) on a Micromass MS/MS ZAB-

spec Tof (EBE Tof geometry) and reported as m/z. Names of chemicals

have been generated with the naming service of ACD-I lab, which deter-

mines the chemical name according to systematic application of the no-

menclature rules agreed upon by the International Union of Pure and

Conclusion

To summarize, two new terfluorene derivatives (2,1-a)-DST-

IF and (1,2-b)-DST-IF with different geometry profiles have

been synthesized by a rational approach based on the inti-

mate understanding of the distribution of positional isomers

arising from the bicyclization of a difluorenol. Thus, with a

very bulky substituent attached to the fluorene units (9,9-di-

octylfluorene) and a polar solvent (CH₃CN), we managed to

drive the cyclization toward the isomer (2,1-a)-DST-IF. On

the other hand, with a less bulky substituent attached to the

fluorene units (bromine atom) and a low polarity solvent

14042

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ꢂ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Chem. Eur. J. 2011, 17, 14031 – 14046

3p–2Spiro Terfluorene–Indenofluorene Isomers

FULL PAPER

Applied Chemistry and International Union of Biochemistry and Molec-

ular Biology. 9,9-Dioctylfluorene-2-boronate ester 2 was prepared accord-

ing to literature procedures starting from commercially available 2-bro-

mofluorene.^[29]The compound 2,2’’-DITP 3 was prepared according to lit-

erature procedures starting from commercially available 2-bromoani-

line.^[27]

a)-DST-IF was purified by successive column chromatography on silica,

eluting for each with dichloromethane–light petroleum (2/8) and by crys-

tallization in a mixture of MeOH/CDCl₃. ¹H NMR (CDCl₃): d=8.13 (s,

2H; ArH), 7.86 (d, J=7.5 Hz, 2H; ArH), 7.52–7.43 (m, 8H; ArH), 7.31–

7.17 (m, 26H; ArH), 6.90 (t, J=7.5 Hz, 2H; ArH), 6.74 (s, J=8.5 Hz,

4H; ArH), 6.57 (s, 4H; ArH), 6.23 (d, J=7.5 Hz, 2H; ArH), 1.95–1.82

(m, 16H; ArH), 1.20–0.90 (m, 80H; CH₂), 0.83–0.77 (m, 24H; Me), 0.70–

0.50 ppm (m, 16H; CH₂); ¹³C NMR (CDCl₃): d=151.1 (C), 151.0 (C),

150.9 (C), 147.4 (C), 143.8 (C), 143.4 (C), 140.7 (C), 140.4 (C), 140.2 (C),

140.0 (C), 139.67 (C), 139.66, (C), 127.8 (CH), 127.2 (CH), 126.7 (CH),

126.58 (CH), 126.52 (CH), 126.4 (CH), 122.9 (CHꢆ2), 121.6 (CH), 121.0

(CH), 120.2 (CHꢆ2), 119.7 (CH), 119.48 (CH), 119.41 (CH), 66.5 (C),

54.9 (C), 40.08 (CH₂), 40.06 (CH₂), 31.73 (CH₂), 31.71 (CH₂), 30.02

(CH₂), 29.92 (CH₂), 29.22 (CH₂), 29.18 (CH₂), 29.14 (CH₂), 29.11 (CH₂),

23.91 (CH₂), 23.77 (CH₂), 22.58 (CH₂), 22.57 (CH₂), 14.07 (Me),

14.05 ppm (Me); IR (KBr): n˜ =3060 (aryl CH), 2923 (CH₂, CH₃), 2852

(CH₂, CH₃), 1440,1450, 1403, 1296, 1261 cm^ꢀ1; UV/Vis (THF): l_max=343,

353 nm; HRMS (LSIMS⁺, o-nitrophenyloctyl ether/acetic acid/MeOH)

calcd for C₁₆₀H₁₈₆: 2107.45546 [M]⁺, found: 2107.4606.

9,9,9’’,9’’-Tetraoctyl-9H,9’H,9’’H-2,2’:7’,2’’-terfluoren-9’-one (5): Sodium

carbonate (1.100 g, 10.30 mmol) dissolved in water (6 mL) was added to

a solution of 2,7-dibromofluorenone (0.088 g, 0.26 mmol), 2 (0.464 g,

0.77 mmol), and tetrakis(triphenylphosphine)palladium(0) (0.015 g,

0.013 mmol) in toluene (30 mL). The schlenk tube was degassed and the

mixture was allowed to stir vigorously at 1008C for 24 h under an argon

atmosphere. The reaction mixture was then quenched with water

(30 mL) and extracted with dichloromethane (5ꢆ20 mL). The combined

organic layers were washed with water (50 mL), dried (MgSO₄), and

evaporated in vacuo. Purification by column chromatography on silica

gel, eluting with light petroleum and diethyl ether–light petroleum (1/15

to 1/9), afforded

5 as an orange solid (210 mg, 85%). M.p. 908C;

¹H NMR (CDCl₃): d=8.04 (s, 2H; ArH), 7.85–7.73 (m, 6H; ArH), 7.66–

7.61 (m, 6H; ArH), 7.39–7.30 (m, 6H; ArH), 2.03 (t, J=7.0 Hz, 8H;

ArH), 1.21–1.00 (m, 40H; CH₂), 0.79 (t, J=7.2 Hz, 12H; Me), 0.72–

0.59 ppm (m, 8H; CH₂); ¹³C NMR (CDCl₃): d = 194.1 (C=O), 151.6 (C),

151.0 (C), 142.9 (C), 142.6 (C), 141.1 (C), 140.5 (C), 138.5 (C), 135.2 (C),

133.3 (CH), 127.2 (CH), 126.8 (CH), 125.6 (CH), 123.0 (CH), 122.9

(CH), 121.0 (CH), 120.7 (CH), 120.1 (CH), 119.8 (CH), 55.2 (C), 40.4

(CH₂), 31.8 (CH₂), 30.0 (CH₂), 29.2 (2ꢆCH₂), 23.8 (CH₂), 22.6 (CH₂),

14.1 ppm (Me); HRMS (ESI⁺, MeOH/CH₂Cl₂90/10): calcd for

C₇₁H₈₈ONa: 979.67329 [M+Na]⁺, found: 979.6739; IR (KBr): n˜ =3064

(aryl CH), 2922 (CH₂, CH₃), 2852 (CH₂, CH₃), 1718 (C=O), 1603,

9,9’-(1,1’:4’,1’’-Terphenyl-2,2’’-diyl)bis(2,7-dibromo-9H-fluoren-9-ol) (7):

In a schlenk tube under an argon atmosphere, 2,2’’-DITP 3 (1.00 g,

2.1 mmol) was dissolved in dry THF (90 mL) and the solution was de-

gassed. The mixture was cooled to ꢀ788C and stirred at this temperature

for 10 min. A solution of nBuLi (1.6m in hexane, 5.20 mL, 8.3 mmol) was

added dropwise in 3.5 min. The resulting yellow solution was stirred for a

further 3.5 min and 2,7-dibromofluorenone (1.54 g, 4.6 mmol), dissolved

in dry and degassed THF, was added dropwise in 4 min by using a cannu-

la. The resulting orange solution was allowed to stir overnight (from

ꢀ708C to room temperature) and the mixture was poured into a saturat-

ed solution of ammonium chloride (100 mL) and extracted with dichloro-

methane. The combined extracts were dried (MgSO₄) and the solvent

was removed in vacuo and purified by successive column chromatogra-

phy on silica, eluting for each with ethyl acetate–light petroleum (0.5/9.5–

3/7), dichloromethane–light petroleum (3/1–4/1), and dichloromethane–

ethyl acetate (2/1), to afford 7 as a colorless solid (1.12 g, 60%). M.p.

1466 cm^ꢀ1

9’,9’’-(1,1’:4’,1’’-Terphenyl-2,2’’-diyl)bis(9,9,9’’,9’’-tetraoctyl-9H,9’H,9’’H-

2,2’:7’,2’’-terfluoren-9’-ol) (6): In a schlenk tube under an argon atmos-

phere, 2,2’’-DITP 3 (0.10 g, 0.21 mmol) was dissolved in dry THF (7 mL)

and the solution was degassed. The mixture was cooled to ꢀ788C and

stirred at this temperature for 10 min. A solution of nBuLi (1.6m in

hexane, 0.52 mL, 0.83 mmol) was added dropwise in 3.5 min. The result-

ing yellow solution was stirred for a further 3.5 min and fluorenone 5

(0.44 g, 0.46 mmol), dissolved in dry and degassed THF, was added drop-

wise in 4 min by using a cannula. The reaction mixture was allowed to

stir overnight (from ꢀ708C to room temperature) and the resulting mix-

ture was poured into a saturated solution of ammonium chloride (10 mL)

and extracted with dichloromethane. The combined extracts were dried

(MgSO₄) and the solvent was removed in vacuo and purified by column

chromatography on silica, eluting with dichloromethane–light petroleum

(2/8 to 3/7), to afford as a colorless solid (0.11 g, 25%). M.p. >3008C;

¹H NMR (CDCl₃): d=8.59 (d, J=5.8 Hz, 2H; ArH), 7.65–7.25 (m, 40H;

ArH), 7.23–7.19 (m, 4H; ArH), 6.99 (d, J=7.2 Hz, 2H; ArH), 5.52 (s,

4H; ArH), 2.43 (s, 2H; exch D₂O, OH), 2.0–1.88 (m, 16H; ArH), 1.3–0.9

(m, 80H; CH₂), 0.88–0.75 (m, 24H; Me), 0.72–0.55 ppm (m, 16H; CH₂);

¹³C NMR (75 MHz; CDCl₃): d = 151.5 (C), 151.3 (C), 151 (C), 142 (C),

141.3 (C), 140.7 (C), 140.4 (C), 140.1 (C), 139.9 (C), 138.8 (C), 137.2 (C),

131.2 (CH), 128.2 (CH), 127.3 (CH), 127.08 (CH), 127.01 (CH), 126.8

(CH), 126.58 (CH), 126.55 (CH), 126 (CH), 123.2 (CH), 123 (CH), 121.4

(CH), 120.1 (CH), 119.86 (CH), 119.83 (CH), 82.6 (C), 55.2 (C), 40.29

(CH₂), 40.25 (CH₂), 31.83 (CH₂), 31.79 (CH₂), 30.07 (CH₂), 30.01 (CH₂),

29.27 (CH₂), 29.24 (CH₂), 29.22 (CH₂), 29.15 (CH₂), 23.89 (CH₂), 23.76

(CH₂), 22.65 (CH₂), 22.62 (CH₂), 14.15 (Me), 14.10 ppm (Me); HRMS

(LSIMS⁺, 3-nitrobenzenemethanol) calcd for C₁₆₀H₁₉₀O₂2143.4765 [M]⁺,

found: 2143.4831.

>3008C; H NMR ([D₈]THF): d = 8.46 (d, J=7.8 Hz, 2H; ArH), 7.50 (t,

J=7.1 Hz, 2H; ArH), 7.41 (t, J=7.1 Hz, 2H; ArH), 7.24 (d, J=8.0 Hz,

4H; ArH), 7.16 (s, 4H; ArH), 7.09 (d, J=8.0 Hz, 4H; ArH), 6.88 (d, J=

6.8 Hz, 2H; ArH), 5.43 (s, exch D₂O, 2H; OH), 5.36 ppm (s, 4H; ArH);

¹³C NMR ([D₈]THF): d =154.7 (2ꢆC), 141.8 (C), 140.8 (C), 139.5 (C),

138.6 (C), 132.2 (CH), 131.9 (CH), 128.8 (CH), 128.03 (CH), 128.0 (CH)

127.9 (CH), 127.7 (CH), 122.3 (CH), 82.3 ppm (COH); HRMS (ESI⁺,

MeOH/CH₂Cl₂90/10): calcd for C₄₄H₂₆O₂⁷⁹Br₄Na: 924.85639 [M+Na]⁺;

found: 924.8544; IR (KBr): n˜ =3574 (OH), 3499 (OH), 3068 (aryl CH),

1574, 1590, 1480, 1448, 1413, 1317, 1241, 1163, 1118, 1059, 1005 cm^ꢀ1

2,2’’,7,7’’-Tetrakis(9,9-dioctyl-9H-fluoren-2-yl)dispiro[fluorene-9,6’-

indeno

(150 mg, 0.16 mmol) was dissolved in dichloromethane and stirred at

room temperature for 10 min. Boron trifluoride etherate (48% BF₃)

(150 mL) was added and the resulting mixture was stirred for one night

(until total conversion of the starting difluorenol 7). The crude mixture

was evaporated to dryness and analyzed by H NMR spectroscopy to de-

termine the ratio of isomers 8 and 9 formed. After washing with a THF/

MeOH mixture, the crude mixture of isomers 8 and 9 was involved in the

next step without any further purification. Sodium carbonate (160 mg,

1.5 mmol) dissolved in water (120 mL) was added to the previous crude

mixture of isomers 8 and 9 (130 mg, 0.15 mmol), 2 (464 mg, 0.90 mmol),

and 1,1’-bis(diphenylphosphino)ferrocenepalladium(II) dichloride di-

chloromethane complex (12 mg, 0.015 mmol). DMF (70 mL) was then

added at room temperature, the schlenk tube degassed, and the mixture

was allowed to stir at 1308C for 12 h under an argon atmosphere. The re-

action mixture was then quenched with a saturated ammonium chloride

solution (100 mL) and extracted with dichloromethane (4ꢆ30 mL). The

combined organic layers were washed with water (50 mL), dried

(MgSO₄), and evaporated in vacuo. Purification by column chromatogra-

phy on silica gel eluting with dichloromethane/light petroleum (2/8) af-

forded (1,2-b)-DST-IF as a slightly yellow solid (55 mg, 17%); ¹H NMR

(CD₂Cl₂): d=8.08 (d, J=7.5 Hz, 4H; ArH), 7.81 (d, J=7.5 Hz, 4H;

ArH), 7.67 (d, J=7.2 Hz, 2H; ArH), 7.58 (s, , 4H; ArH), 7.49 (t, J=

2,2’’,7,7’’-Tetrakis(9,9-dioctyl-9H-fluoren-2-yl)dispiro[fluorene-9,11’-

indenoACHTUNGTRENNUNG[2,1-a]fluorene-12’,9’’-fluorene] (2,1-a)-DST-IF (general proce-

dure): Difluorenol 6 (10 mg, 4.7 mmol) was dissolved in dichloromethane

or acetonitrile and stirred for 15 min at room temperature or at reflux.

Boron trifluoride etherate (48% BF₃) (5 mL) was added and the resulting

mixture was stirred for one hour (until total conversion of the starting di-

fluorenol) at the desired temperature. The crude mixture was evaporated

to dryness, analyzed by ¹H NMR spectroscopy to determine the ratio of

isomers (1,2-b)-DST-IF and (2,1-a)-DST-IF formed, and the isomer (2,1-

Chem. Eur. J. 2011, 17, 14031 – 14046

ꢂ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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14043

C. Poriel, J. Rault-Berthelot et al.

7.5 Hz, 8H; ArH), 7.43 (s, 2H; ArH), 7.34–7.21 (m, 18H; ArH), 7.15 (s,

4H; ArH), 7.08 (t, J=7.5 Hz, 2H; ArH), 6.81 (d, J=7.5 Hz, 2H; ArH),

2.03–1.89 (m, 16H; ArH), 1.20–0.90 (m, 80H; CH₂), 0.80–0.73 (t, J=

7.5 Hz, 24H; Me), 0.68–0.50 ppm (m, 16H; CH₂); ¹³C NMR (CD₂Cl₂): d

= 151.8 (C), 151.4 (C), 150.5 (C), 149.5 (C), 149.4 (C), 142.6 (C), 142.1

(C), 141.7 (C), 141.1 (C), 141.0 (C), 140.9 (C), 139.9 (C), 128.2 (CH),

127.5 (CH), 127.4 (CH), 127.2 (2ꢆCH), 126.1 (CH), 124.3 (CH), 123.3

(CH), 122.8 (CH), 121.7 (CH), 120.9 (CH), 120.7 (CH), 120.2 (2ꢆCH),

116.4 (CH), 66.5 (C), 55.5 (C), 40.69 (CH₂), 40.64 (CH₂), 32.17 (CH₂),

32.13 (CH₂), 30.38 (CH₂), 30.35 (CH₂), 29.60 (CH₂), 29.58 (CH₂), 29.56

(CH₂), 29.52 (CH₂), 24.25 (CH₂), 24.19 (CH₂), 23.00 (CH₂), 22.96 (CH₂),

14.30 (Me), 14.28 ppm (Me); IR (KBr): n˜ =3060 (aryl CH), 2925 (CH₂,

CH₃), 2852 (CH₂, CH₃), 1440, 1451, 1403, 1260 cm^ꢀ1; UV/Vis (THF):

l_max=347, 350 nm (br); HRMS (ESI⁺, MeOH/CHCl₃90/10): calcd for

functional^[79](B3LYP) implemented in the Gaussian 09 (Revision B.01)

program suite^[69]using the 6–31G* basis set^[80]and the default conver-

gence criterion implemented in the program. The figures were generated

with MOLEKEL 4.3.^[70]

Fabrication and characterization of OLEDs: Organic light-emitting

diodes were prepared according to the following procedure. The indium

tin oxide (ITO) coated glass electrodes were partially etched and cleaned

in successive ultrasonic baths using acetone, ethanol, and 2-propanol.

UV-ozone treatment was applied to the substrates to increase the hydro-

philic nature of the surface and to remove any residual contamination.^[81]

A water dispersion of poly(3,4-ethylenedioxythiophene) doped with pol-

y(styrenesulfonate) (PEDOT:PSS, Clevios) was spin-coated on top of the

ITO sheets at 6000 rpm to form a 40 nm-thick layer. A thermal annealing

under vacuum (10 mbar) at 808C for 30 min was subsequently applied to

the PEDOT:PSS layer to remove any residual moisture. PEDOT:PSS

possesses a HOMO level of ꢀ5.1 eV, which enables a better hole injec-

tion from the anode to the active layer. It also smoothes the ITO surface

and therefore prevents any possible short circuits due to the ITO

spikes.^[82]The further device elaboration steps and characterizations were

carried out under an inert atmosphere (N₂) inside a glove box system.

Then the emissive layer was spin-coated on top of the PEDOT–PSS layer

at 1000 rpm from a xylene solution. Calcium cathodes were finally evapo-

rated through a shadow mask under a secondary vacuum (ca. 10^ꢀ6mbar).

Current–voltage–luminance (I–V–L) curves were recorded by using a

Keithley 4200 SCS instrument. Light emission was collected by using a

calibrated photodiode coupled to a Hamamatsu Photosensor amplifier

C9329. Electroluminescence spectra were measured with a CDD spec-

trometer (Ocean Optics HR 2000).

₁₆₀H₁₈₆: 2107.45546 [M]⁺, found: 2107.4556.

Spectroscopic studies: UV/Vis spectra were recorded in solution using a

UV-Visible-NIR spectrophotometer CARY 5000-Varian or SHIMADZU

UV-1605 spectrophotometer. The optical band gap was calculated from

the absorption edge of the UV/Vis absorption spectrum using the formu-

la DE^opt[eV]=hc/l, l being the absorption edge (in meter). With h=

6.6ꢆ10^ꢀ34Js (1 eV=1.6ꢆ10^ꢀ19J) and c=3.0ꢆ10⁸ms^ꢀ1, this equation may

be simplified as: DE^opt[eV]=1237.5/l (in nm). Photoluminescence spec-

tra were recorded with a PTI spectrofluorimeter (PTI-814 PDS, MD

5020, LPS 220B) using a Xenon lamp either in solution (THF). Quantum

yields in solution (f_sol) were calculated relative to quinine sulfate (f_sol

0.546 in H₂SO₄1n) using standard procedures.^[52]f_solwas determined ac-

cording to Equation (1),

ꢀ

ꢁ

ðT_sꢃ A_rÞ n_s

ð1Þ

ꢀ_sol¼ ꢀ_refꢃ 100 ꢃ

n_r

ðT_rꢃ A_sÞ

where subscripts s and r refer to the sample and reference, respectively.

The integrated area of the emission peak in arbitrary units is given as T,

n is the refracting index of the solvent (n_s=1.407 for THF), and A is the

absorbance (A<0.1). IR spectra were recorded on a VARIAN 640-IR

using a PIKE Technologies MIRacle(TM) ATR (single attenuated total

reflectance) with a diamond crystal or on a BIORAD IRFTS175C (with

KBR pellets).

Acknowledgements

D.T. thanks Rꢁgion Bretagne for a studentship. We thank Dr. Nathalie

Audebrand (Universitꢁ de Rennes 1) for TGA measurements, Dr Rꢁmi

Mꢁtivier (ENS Cachan) for helpful discussions, Vincent Quꢁmꢁner for

his contribution to the synthesis of terfluorenone 5, Stꢁphanie Fryars for

IR data and technical assistance, the C.R.M.P.O (Centre Rꢁgional de

Mesure Physique de l’Ouest, Rennes) for high-resolution mass measure-

ments, and the CINES (Montpellier) for computing time.

Thermal analysis: Thermogravimetric analyses (TGA) were carried out

with a Rigaku Thermoflex instrument under a nitrogen atmosphere be-

tween with a heating rate of 58Cmin^ꢀ1. Melting points were determined

by using an electrothermal melting point apparatus.

Electrochemical studies: All electrochemical experiments were per-

formed under argon, using a Pt disk working electrode (diameter 1 mm),

the counter electrode was a vitreous carbon rod and the reference elec-

trode was a silver wire in a 0.1m AgNO₃solution in CH₃CN. Ferrocene

was added to the electrolyte solution at the end of a series of experi-

ments. The ferrocene/ferrocenium (Fc/Fc⁺) couple served as internal

standard. For a further comparison of the electrochemical and optical

properties, all potentials are referred to the SCE electrode that was cali-

brated at ꢀ0.405 V versus the Fc/Fc⁺system. The three-electrode cell

was connected to a PAR Model 273 potentiostat/galvanostat (PAR,

EG&G, USA) monitored with the ECHEM Software. Dichloromethane

with less than 100 ppm of water (ref. SDS 02910E21) was used without

purification. Activated Al₂O₃was added to the electrolytic solution to

remove excess moisture. Following the work of Jenekhe and co-work-

ers,^[68]we estimated the electron affinity (EA) or lowest unoccupied mo-

lecular orbital (LUMO) and the ionization potential (IP) or highest occu-

pied molecular orbital (HOMO) from the redox data. The LUMO level

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red

was calculated from: LUMO [eV]=ꢀ[E_onset(vs. SCE) + 4.4] and the

HOMO level from: HOMO [eV]=ꢀ[E_onset

ACHTUNGERTN(NUNG vs. SCE) + 4.4], based on

an SCE energy level of 4.4 eV relative to the vacuum. The electrochemi-

cal gap was calculated from: DE^el= jHOMOꢀLUMOj (in eV). The esti-

mated errors in the determination of the onset potential values are

red

ꢁ20 mV for E_onsetand ꢁ50 mV for E_onset

Computational details: Full geometry optimization with density function-

al theory (DFT)^{[75, 76]}were performed with the hybrid Becke-3 parameter

exchange^[71,77,78]functional and the Lee–Yang–Parr non-local correlation

14044

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ꢂ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Chem. Eur. J. 2011, 17, 14031 – 14046

3p–2Spiro Terfluorene–Indenofluorene Isomers

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Received: July 19, 2011

Published online: November 14, 2011

14046

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