Synthesis of selectively 4-substituted 9,9′-spirobifluorenes and modulation of their photophysical properties

Article abstract of DOI:10.1039/c7ob01319a

Synthesis of selectively 4-substituted 9,9′-spirobifluorenes was accomplished by using catalytic [2 + 2 + 2]-cyclotrimerization of specifically substituted diynols with alkynes to the corresponding fluorenols. Further synthetic transformations provided the target molecules. The measurement of the photophysical properties of the prepared 4-substituted 9,9′-spirobifluorenes revealed that their emission maxima depended on the electronic properties of the substituents present in the para position: the presence of an electron accepting group strongly favored the maxima red shift toward the blue VIS region (CF₃λ_max = 361 nm vs. MeO λ_max = 330 nm). Adding further substituents (aryl or arylethynyl moieties) on the phenyl ring in position 4 did not lead to a dramatic improvement in the emission maxima (CF₃C₆H₄, λ_max = 369 nm, CF₃C₆H₄CC, λ_max = 370 nm), but increased their quantum yields considerably. In addition, a series of 9,9′-spirobifluorenes possessing various extended π-systems (pyrene, anthracene, etc.) were synthesized. In general, the emission maxima pattern reflected that of the parent π-systems, but they were red shifted by 10-30 nm. Finally, also a 1-[4-(9,9′-spirobifluorene-4-yl)phenyl]-2-aryl-ortho-carborane was prepared. These data thus may provide guidelines for further design of 9,9′-spirobifluorenes with tailored properties.

Full text of DOI:10.1039/c7ob01319a

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ARTICLE
Synthesis of Selectively 4-Substituted 9,9´-Spirobifluorenes and
Modulation of Their Photophysical Properties.
Reinhard P. Kaiser,a JiříMosinger,b Ivana Císařová,b and Martin Kotoraa*
Received 00th January 20xx,
Accepted 00th January 20xx
Synthesis of selectively 4-substituted 9,9’-spirobifluorenes was accomplished by using catalytic [2+2+2]-cyclotrimerization
of specifically substituted diynols with alkynes to the corresponding fluorenols. Further synthetic transformations provided
target molecules. Measurement of photophysical properties of the prepared 4-substituted 9,9’-spirobifluorenes revealed
that emission maxima depended on electronic properties of the substituents present in the para position: the presence of
electron accepting group strongly favored the maxima red shift toward the blue VIS region (CF3λmax = 361 nm vs MeO λmax
= 330 nm). Adding further substituents (aryl or arylethynyl moieties) on the phenyl ring in the position 4 did not exert a
dramatic improvement on the emission maxima (CF3C6H4, λmax = 369 nm, CF3C6H4C≡C, λmax = 370 nm), but increased
considerably quantum yields. In addition, a series of 9,9’-spirobifluorenes possessing various extended π-systems (pyrene,
anthracene, etc.) was synthesized. In general, the emission maxima pattern reflected that of the parent π-systems, but
they were red shifted by 10~30 nm. Finally, also a 1-[4-(9,9'-spirobifluorene-4-yl)phenyl]-2-aryl-ortho-carborane was
prepared. These data thus may provide guidelines for further design of 9,9’-spirobifluorenes with taylored properties. .
DOI: 10.1039/x0xx00000x
www.rsc.org/
examples in fluorene-type compounds.7 Despite of this, a
systematic study assessing electronic effects of various
functional groups on photophysical properties fluorenes (and
Introduction
Substituted 9,9’-spirobifluorenes have attracted considerable
attention in material science due to their properties. They
have been employed as interesting components for material
chemistry in various applications, owing to their high thermal
and morphological stability.1 Studies concerning their
electronic properties, especially for the application as
electroluminescent layer in green and blue organic light
emitting diodes (OLEDs), have appeared in the last decade,.1b,2
In combination with other substances they can be used as well
for white organic light emitting diodes (WOLEDs),3 which are
predicted to become the future of illumination systems.4
However, it is still a challenge to develop blue light emitting
materials with high efficiency by using short and high yielding
syntheses. In addition, the blue OLEDs still lack in long
operation times.5 Pure hydrocarbon 9,9’-spirobifluorene
materials were reported as a host for blue phosphorescent
organic light emitting diodes (PhOLEDs).6
spirobifluorenes) has not been, to the best of our knowledge,
pursued. Only recently (in 2017) a pioneering work in this
respect has been reported and the results show that indeed
the presence of electron rich or poor aromatic moieties in the
position 48 or 39 of the fluorene skeleton can significantly
change photophysical properties of the corresponding 9,9´-
spirobifluorenes. These finding have been recently reviewed.10
Since we have recently reported a new and efficient pathway
for the synthesis of substituted 9,9´-spirobifluorenes based on
a rhodium complex-catalyzed [2+2+2] cyclotrimerization as
route for assembly of the substituted fluorene skeleton,11 we
envisioned that this methodology could provide a useful tool
for preparation of series of specifically substituted fluorenes
and 9,9´-spirobifluorenes possessing various substituents in
the position 4. In this respect, we would like to report further
developments in this area encompassing: a) a unified approach
for the synthesis of series specifically substituted fluorenols in
the position 4 of the fluorene framework and their conversion
into the corresponding 9,9´-spirobifluorenes, b) progress in
tuning of their photophysical properties utilizing electron
donating and electron withdrawing substituents, as well as c)
influence of the presence of extended π-aromatic hydrocarbon
substituents and even carborane structures.
One of the possibilities to tune compound’s properties is to
decorate it with either electron-withdrawing or -accepting
functional groups or both in suitable positions on the
molecule’s framework forming substances with push-pull
characteristics. This has been exemplified in numerous
a.Department of Organic Chemistry, Faculty of Science, Charles University, Albertov
6, 128 43 Praha 2, Czech Republic
Phone: +420 221 951 058, FAX: +420 221 951 326, e-mail: kotora@natur.cuni.cz
b.Department of Inorganic Chemistry, Faculty of Science, Charles University,
Albertov 6, 128 43 Praha 2.
Results and discussion
Our choice of the starting material was at this point driven by
Electronic
DOI: 10.1039/x0xx00000x
Supplementary
Information
(ESI)
available:
See
ease of preparation
1
from a simple starting material as well as
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the fact that the presence of aryl or alkyl groups in the position scaffold are almost perpendicular to each other with the
of the final 9,9´-spirobifluorenes does not have any angles of 86°, 94°, and 83°, respectively. These values are
1
significant influence on their photophysical properties, as it considerably more than those reported for the simple 4-
emerged from the previous studies of 1,2,3,4-tetrasubstituted phenyl-9,9´-spirobifluorene (~50°).8b It should be noted that an
9,9’-spirobifluorenes.11 The respective diynols 1a-1g (Scheme effect of the dihedral angle between the fluorene scaffold and
1) were prepared by using standard synthetic methodology the substituent in the position 4 on photophysical properties in
based on Sonogashira coupling of 2-bromobenzaldehyde with the solid state has been recently observed.13 There is no doubt
various para-substituted phenylacetylenes bearing electron- that their almost perpendicular arrangement is the effect of a)
donating (MeO, Me, H) and electron withdrawing (Ph, Cl, CF3, the presence of the Et group in the ortho position of the
and CN) groups followed by the subsequent alkynylation with phenyl linkage of the fluorenyl unit (C4) and b) the substituent
p-tolylacetylide (for details, see SI). The prepared diynols
1
in the meta-position (C3). Interestingly, no considerable
were cyclotrimerized with 3-hexyne in the presence of deformation or twisting between the phenyl units of the
catalytic amount of Wilkinson’s catalyst (RhCl(PPh3)3, 10 fluorene scaffold was observed for 4a (3.7°), 4c (5.4°), and 4e
mol%). In all cases the reaction proceeded successfully and the (2.1°).
corresponding 4-substituted fluorenols 2a-2g were isolated in
reasonable yields (30-64%) (Scheme 1). With the series of
substituted fluorenols
dichloromethane furnished the corresponding substituted
fluorenones . Also the oxidation proceeded uneventfully to
2 on hand, their oxidation with PCC in
p-Tolyl
1. 2-LiC6H4-Ph, THF
-78 to 20 °C, 3h
Et
3
3
2. 12M HCl, AcOH.
Et
provide the substituted fluorenones 3a-3g in very good
isolated yields (61-86%). These results provide evidence for a
high functional group tolerance of the used methodology.
4
R
OH
OH
p-Tolyl
p-Tolyl
p-Tolyl
p-Tolyl
1
9
Et
Et
Et
Et
2
Et
Et
p-Tolyl
4
3
Et
RhCl(PPh3)3
(10 mol%)
toluene, 90 °C
Et
Et
Et
R
R
MeO
Me
1a
1e
2a
2e
, R = OMe
, R = C
, R = OMe, 47% , R = Cl, 44%
1b
2b
, R = Me 1f
, R = Me, 64%
2c
, R = Ph, 48%
2d, R = H, 61%
, R = CF3
1g
2f
, R = CF3, 49%
2g, R = CN, 30%
4a, 83%
4b, 74%
4c, 63%
1c
, R = Ph
, R = CN
1d, R = H
O
p-Tolyl
3a, R = OMe, 80%
p-Tolyl
p-Tolyl
p-Tolyl
3b
, R = Me, 84%
3c, R = Ph, 82%
3d
Et
Et
Et
Et
PCC
, R = H,8 1%
3e, R = Cl, 86%
3f
Et
CH2Cl2
20 °C
Et
Et
Et
, R = CF3, 61%
3g, R = CN, 74%
R
4d, 73%
Cl
4e, 82%
F3C
4f, 90%
Scheme 1. Synthesis of fluorenones3a-3g from diynols1a-1g.
Scheme 2. Synthesis of 9,9´-spirobifluorenes 4a-4f.
In the last step, fluorenones 3 were converted by a two-step
method12 into desired 9,9´-spirobifluorenes
4. Thus addition of
ortho-lithiumbiphenyl to fluorenones
3 gave rise to
intermediate alcohols that were, without isolation, cyclized
under acidic conditions into the corresponding 9,9´-
spirobifluorenes 4a-4f in sensible isolated yields in the range of
63-90% (Scheme 2). In the case of fluorenone 4g the addition
of ortho-lithiumbiphenyl proceeded preferentially to the CN
group giving rise to an imine. Attempts to override this
undesirable reaction by changing the reaction conditions were
not successful.
The structures of selected 9,9´-spirobifluorenes were
Figure 1. PLATON drawing of 4c showing displacement ellipsoids on the 50% probability
unequivocally confirmed by a single crystal X-ray structure level.
analysis of 4a (SI-Figure 1), 4c (Figure 1) and 4e (SI-Figure 1).
Having the set of six differently substituted 9,9´-
The X-ray diffraction data revealed that in 4a, 4c, and 4e the
spirobifluorenes 4a-4f, their photo-physical properties were
attached phenyl rings and the phenyl ring of the fluorene
2 | J. Name., 2012, 00, 1-3
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studied. All measured compounds exhibited similar absorption undoubtly, an effect of the additional
ARTICLE
π
-system (Ph). The
spectral characteristics in the range of λabs = 250-330 nm, obtained data allowed to conclude that the presence of
except of 4c (possessing an additional biphenyl group), which electron deficient groups at the 4-position of the fluorene core
shows a significant higher absorption in the area of 250-390 bear a beneficial effect for red shift of emission maxima into
nm (Figure 2).The emission quantum yields (
Φ
derivatives were determined in cyclohexane by the standard 4c indicated that the presence of additional
s) of these the blue region. In a similar manner the results obtained with
-systems may
π
procedures with DPA as reference and were in an average result not only in higher shift to blue region but also increase
range of Φs = 0.50-0.87 (Table 1 and Figure 2). The obtained quantum yield. In view aforementioned, it is reasonable to
emission maxima, ranged from 330 to 367 nm clearly assume that the compounds 4 might undergo a stabilization of
indicating a relation between the emission of 9,9´- the excited state, e.g. a planarization of the structure, with the
spirobifluorenes and the different R substituents. The plot of phenyl substituents conjugated to the spirofluorene core. This
emission maxima versus the Hammett para substituent hypothesis is consistent with previously observed emission
14
constants
σ
para
indicates a reasonably good linear correlation red-shift and for 4-aryl substituted spirobifluorenes.8b,c On the
for 4a, 4b, 4d-4f. It is worth mentioning that introduction of other hand, it should be noted that comparison of the
the trifluoromethylphenyl group exerted red shift effect of 28 emission maxima λem for 4d
nm in comparison with the phenyl group. This value is more phenyl-9,9´-spirobifluorene (
(
λem = 333 nm) and the parent 4-
λ
em = 358 nm)8b indicates the blue
than twice larger than the one obtained for 2-pyridyl or 4- shift for the former. This might be caused by the presence two
pyridyl groups, whose presence exerted the red shift of ~12 ethyl groups in position 2 and 3. The blue shift is probably
nm.8c
caused by a large dihedral angle and hence breaking of the π-
conjugation between the substituent in the position 4 and the
The only exception from linearity was observed for 4c
,
featuring an unexpectedly high red shift of emission properties fluorene scaffold.13
into the blue light region at λem = 367 nm (Figure 3) which is,
Table 1. Photophysical properties for 4a-4f (in cyclohexane).
Compound
λ
abs/nm (ε/104∙mol-1∙dm3∙cm-1)
λ
exc/nm
241
241
243
242
241
240
λ
em/nm Φsa)
4a
4b
4c
4d
4e
4f
262 (sh), 269 (sh), 284 (sh), 300 (sh), 313 (1.6)
262 (sh), 284 (sh), 300 (sh), 313 (1.6)
330
331
367
333
344
361
0.50
0.61
0.87
0.66
0.61
0.64
250 (6.0), 255 (sh), 263 (sh), 284 (sh), 300 (sh), 313 (1.7)
261 (sh), 284 (sh), 300 (sh), 313 (1.9)
262 (sh), 284 (sh), 300 (sh), 313 (1.5)
263 (sh), 274 (sh), 300 (sh), 313 (1.6)
a) Quantum yields were calculated relative to DPA (Φr = 0.97).
1.0
4b
4b
4d
4d
4c
4a
4e
4f
0.8
0.6
0.4
0.2
0.0
1.0
0.5
0.0
4c
4a
4e
4f
300
350
400
450
300
400
Wavelength (nm)
Wavelength (nm)
Figure 2. Absorption (left) and emission (right) spectra of 4a-4f.
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data revealed that while for 6a the angle between the mean
plane of the attached phenyl rings and the phenyl ring of the
fluorene scaffold are almost 90° to each other (83°), in case of
7b the angle is only 75°. This arrangement is the result of the
same effects as those observed for 4. Also for 6a and 7b no
considerable deformation or twisting between the phenyl
units of the fluorene scaffold was observed being 5.7° and
4.8°.
I
Br
O
OH
p-Tolyl
n-BuLi
Pd(PPh3)2Cl2
H
p-Tolyl
THF, -78 °C
25 °C, 3h
CuI, DIPA
50 °C, 16h
Figure 3. Plot of emission maxima versus Hammett substituent constant σpara
.
5a, 96%
In view of the aforementioned and inspired by the unexpected
OH
OH
p-Tolyl
behavior of 4c, we decided to investigate the influence of
Et
Et
Et
Et
PCC
extended π-systems at the 4-position of the fluorene skeleton.
p-Tolyl
Thus we prepared 9,9´-spirobifluorene 4h possessing the
bromophenyl moiety as the convenient starting material for
further attachment of other functional groups by using
different cross coupling reactions. The diynol precursor 1h was
prepared by a modified standard procedure from 5a in order
to circumvent undesired homo coupling of 1-bromo-4-
ethynylbenzene in Sonogashira coupling (Scheme 3).
Unfortunately, cyclotrimerization of 1h with 3-hexyne under
the standard conditions (Condition A) furnished the desired
fluorenol 2h in only 26% isolated yield. Despite of the low
yield, other possible products caused by Rh-catalyzed
activation of the C-Br were not detected. This rather
unsatisfactory result prompted us to try to tune reaction
conditions using the Wilkinson’s catalyst under modified
reactions conditions. After considerable screening, it was
found out that carrying out cyclotrimerization with RhCl(PPh3)3
(3 mol%) in the presence of a Ag-salt under microwave
irradiation could deliver 2h in almost double isolated yield of
57% (Condition B) (for details see SI-Table-1). Then 2h was
oxidized to 3h in 93% isolated yield and the formation of
spirobifluorene 4h proceeded well giving 74% isolated yield
(Scheme 3).
Conditions
DCM
25 °C
Br
1h, 95%
2h, A: 26%
B: 57%
Br
O
p-Tolyl
p-Tolyl
1. 2-LiC6H4-Ph, THF
-78 to 20 °C, 3h
Et
Et
Et
2. AcOH, HCl, 110 °C
Et
3h, 93
4h, 74%
Br
Br
Conditions A: RhCl(PPh3)3 (10 mol%), toluene, 90 °C, 16h.
Conditions B: RhCl(PPh3)3 (3 mol%), Ag2CO3, THF, MW, 180 °C, 1.5h.
Scheme 3. Synthesis of 9,9´-spirobifluorene 4h.
Sonogashira
coupling[b]
Suzuki
coupling[a]
p-Tolyl
p-Tolyl
4h
B(OH)2
Et
Et
Et
Et
R
Having 4h on hand, we started to extend conjugated π-system
R
on the phenyl moiety in the position 4 by the use of
Sonogashira or Suzuki coupling (Scheme 4). By using these
cross coupling reactions we attached substituted
phenylethynyl and phenyl groups to 4h yielding the
corresponding spirobifluorenes possessing substituted
biphenyl 6a and 6b and substituted diphenylethynyl moieties
7a-7c in generally acceptable isolated yields (54-72%).
Structures of 6a and 7b (SI-Figure 2) were confirmed by a
single crystal X-ray structure analysis. The X-ray diffraction
R
7a, R = OMe,
6a, R = OMe,
54%
72%
62%
58%
7b
6b
, R = H,
, R = CF3,
R
7c
, R = CF3, 57%
[a] Conditions: Pd(OAc)2 (5 mol%), K2CO3, EtOH/H2O, 80°C, 3 h.
[b] For reaction conditions, see SI.
Scheme 4. Synthesis of extended 9,9´-spirobifluorenes 6 and 7.
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Despite of the presences of the extended
ARTICLE
π
-system and fluorene scaffold then extension of the p-system. Moreover, it
electron-accepting and, especially, electron-withdrawing should be taken into the account a preferential perpendicular
groups, which had a beneficial effect on the red-shift towards rearrangement of the aromatic rings in the pendant chain that
the blue VIS region, these compounds, exhibited only would contribute to breaking of the
negligible emission red shifts in comparison with 4c (λem =367). pendant chain. The emission quantum yields (
emission maxima λem for 6a 6b 7a 7b, and 7c were observed derivatives were determined in cyclohexane by standard
π
-conjugation within the
Φ
s) of the
,
,
,
at 366, 369, 367, 369, and 370 nm, respectively. The similar procedures with DPA as reference (Table 2 and Scheme 4). All
values indicate of λem that the effect of the pending aromatic samples possess high quantum yield of emission, especially 7c
side chains rather depends on the angle between the mean with
plane of the attached phenyl rings and the phenyl ring of the
Φs = 1.00.
Table 2.Photophysical properties for 6and 7(in cyclohexane).
Compound
λ
abs/nm (ε/104∙mol-1∙dm3∙cm-1)
λ
exc/nm
254
λ
em/nm Φsa)
6a
6b
7a
7b
7c
216 (1.1), 264 (0.9), 284 (sh), 313 (sh)
218 (1.1), 250 (sh), 261 (sh), 300 (sh)
220 (1.4), 285 (sh), 296 (sh), 313 (sh)
220 (1.6), 287 (sh), 305 (sh), 312 (sh), 328 (sh)
366
369
367
369
370
0.65
0.99
0.82
0.79
1.00
251
253
254
220 (1.0), 252 (sh), 266 (sh), 276 (sh), 284 (sh), 291 (sh) 300 (sh), 312 (sh)
248
a) Quantum yields were calculated relative to DPA (Φr = 0.97).
1,5
1,0
0,5
0,0
1,0
0,8
0,6
0,4
0,2
0,0
7a
7b
7c
6a
6b
7a
7b
7c
6a
6b
200
300
400
350
400
450
Wavelength (nm)
Wavelength (nm)
Figure 4. Absorption (left) and emission (right) spectra of 6and 7.
The above mentioned results indicating that attachment of mean plane of the attached phenyl rings and the phenyl ring of
rather simple aromatic substituents in the position 4 was the fluorene scaffold are almost 91.8° and no considerable
sufficient to induce the shift of emission into the blue VIS deformation or twisting between the phenyl units of the
region, prompted us to prepare derivatives with larger
π
-
fluorene scaffold was observed being 0.1°.
extended systems. In this respect as potential candidates
appeared to be naphthyl, anthracenyl, phenanthrenyl, and
pyrenyl moieties. In this instance synthesis of diynol precursors
8a-8d (45-59% isolated yields) was carried out by using a
slightly changed version of the previously developed general
protocol, starting from 2-ethynylbenzaldehyde with addition of
n-propynylacetylide followed by Sonogashira coupling with the
corresponding aryl halides. The cyclotrimerization with 3-
hexyne gave rise to the corresponding fluorenols 9a-9d in
good isolated yields (45-59%).
The subsequent oxidation with PCC in dichloromethane
proceeded uneventfully as expected, providing 10a-10d in
good isolated yields (84-91%). Finally, the addition of ortho-
lithiumbiphenyl followed by acidic treatment furnished 9,9´-
spirobifluorenes 11a-11d in good to high yields (51-91%,
Scheme 6). The structure of 11b was confirmed by a single
crystal X-ray structure analysis (SI-Figure 2). The X-ray
diffraction data revealed that for 11b the angle between the
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The set of four 9,9´-spirobifluorenes 11a-11d with extended π-
aromatic hydrocarbon systems was also investigated for their
photophysical properties. All measured compounds exhibit
different absorption spectra in the range of λabs = 250-330 nm
(Table 3 and Figure 5). The emission quantum yields (
Φs) of the
derivatives exhibit low Φs = 0.11-0.12 except of 11c, with Φs
=
0.87. The emission bands show a high red shift into the visible
blue region. For 11b: λem = 353, 370, 391, 413 nm.The emission
spectra for 11c and 11d had a similar structure of emission
spectra to condensed aromatic hydrocarbons anthracene and
pyrene but red shifted by ~30 and ~10 nm.15 The sample 11a
,
possessing the naphthyl group did not exhibit any emission at
all.
Scheme 5. Synthesis of 11a-11d with extended aromatic π-systems.
Table 3. Photophysical properties for 11 (in cyclohexane).
Compound
11a
λ
abs/nm (ε/104∙mol-1∙dm3∙cm-1)
λ
exc/nm
-
λ
em/nm
-
Φsa)
-
229 (1.2), 270 (sh), 282 (sh), 299 (sh), 312 (0.2)
11b
215 (sh), 228 (sh), 231 (sh), 249 (sh), 255 (sh), 270 (sh), 279 (sh), 289 (sh), 300 (sh), 312 (0.1)
219 (sh), 220 (sh), 260 (1.1), 282 (sh), 299 (sh), 312 (0.1), 361 (sh), 380 (0.1), 401 (0.1)
237 (sh), 246 (1.0), 267 (sh) , 278 (0.7), 300 (0.2), 312 (0.3), 328 (0.3), 345 (0.5)
267
270
267
353,370, 391, 413 0.12
11c
405, 429,454
378, 389, 398
0.80
0.11
11d
a) Quantum yields were calculated relative to DPA (Φr = 0.97).
11c
11d
11b
1,0
0,8
0,6
0,4
0,2
0,0
11a
11b
11c
11d
1,0
0,5
0,0
350
400
450
500
200
300
Wavelength (nm)
400
Wavelength (nm)
Figure 5. Absorption (left) and emission (right) spectra of 11a-11d.
Finally, having the 9,9´-spirobifluorenes with the internal triple photophysical properties due to the a “Face-On” conformation
bonds we decided to investigate also the influence of of the aryl moieties with respect to each other. Hence we were
7
carborane structures with regard to absorption and emission curious whether a synthesis of fluorene derived carborane
properties. It was shown that suitably 1,2-aryl substituted might have similar behavior.16 7a was reacted with 6,9-
carboranes and their related compounds may have interesting (Me2S)2-arachno-B10H12 under standard conditions17-19 to give
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1,2-disubstituted carborane 12 (Scheme 6). The desired 330-460 nm (in cyclohexane) and in many cases leads to high
product was obtained in 60% isolated yield (and its structure quantum yields of emission (Φs = 0.67-1.00). Namely, the
was again unequivocally confirmed by a single crystal X-ray presence of phenyl rings with electron accepting substituents
structure analysis (Figure 6).
in para position or other condensed aromatic systems induces
the red shift of emission bands. It was demonstrated that the
presence of trifluoromethylphenyl group causes a red shift of
28 nm in comparison with phenyl substituted derivative (4d),
the shift is more than twice larger than the shift induced by
pyridyl groups. From a general point of view, it should be
taken in to account that emission spectra of the prepared
derivatives are generally blue shifted in comparison with
derivatives that do not bear substituents in the positions 2 and
3. The blue shift is probably caused by a large dihedral angle
between the substituent in the position 4 and the fluorene
p-Tolyl
p-Tolyl
Et
Et
Et
6,9-(Me2S)2-B10H12
MeO
Et
toluene, reflux
C
C
scaffold leading to disruption of π-conjugation. Nonetheless, it
is reasonable to presume that a similar effect, i.e. red shift of
emission peaks induced by the presence of benzene ring
possessing an electron withdrawing substituents in the
position 4, could be observed also in simple 4-substituted 9,9´-
spirobifluorenes. These results thus might provide useful
information for further design of other substituted 9,9´-
spirobifluorenes making them interesting potential materials
for blue light emitting OLEDs and for further research.
MeO
Scheme 6. Reaction of 6,9-(Me2S)2-arachno-B10H12 with alkyne 7a
7a
12, 60%
.
The X-ray diffraction data revealed that whereas for 7a the
angle between the mean plane of the attached phenyl rings
and the phenyl ring of the fluorene scaffold is only 74.8° and
no considerable deformation or twisting between the phenyl
units of the fluorene scaffold was observed being 6.8°. The
photophysical properties of carborane containing molecule
12was investigated as well. It exhibited similar absorption
Acknowledgements
spectrum to spirobifluorenes4 in the range of λabs = 250-330
nm, but revealed no emission.
This work was supported by Czech Science Foundation (grant
No. 17-07707S) and the Grant Agency of the Charles University
(GAUK 430516). R. P. Kaiser would like gratefully acknowledge
the support from the Cusanuswerk e. V. The authors would
also like to express their thanks to Robert Kuchta (an
ERASMUS intern from University of Cologne) for participation
on the project.
Conflict of interest
The authors declare no competing financial interest.
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