Photophysical properties of 1,3,6,8-tetraarylpyrenes and their cation radicals

Article abstract of DOI:10.1016/j.jphotochem.2019.01.014

The synthesis of 1,3,6,8-tetraaryl substituted pyrenes is described. The substituents on the aryl groups influence the optoelectronic properties of the pyrene core indicating a strong coupling between the aryl groups and the pyrene core. Electronic absroption spectra of the cation radicals generated in solution by the single electron oxidation of pyrenes reaffirmed the coupling of the aryl rings with the pyrene. The aryl groups at 1,3,6,8-positions completely inhibit the π-stacking of pyrene core in solution. Crystal structure of the cation radical salt of tetraphenyl shows charge delocalization between neutral pyrene unit and two cationic pyrene cores. Complete inhibition of the pyrene core stacking is observed in the solid state of cation radical when the dendritic pentaphenyl groups surround the pyrene core.

Full text of DOI:10.1016/j.jphotochem.2019.01.014

Accepted Manuscript
Title: Photophysical properties of 1,3,6,8-tetraarylpyrenes and
their cation radicals
Authors: Vijay S. Vyas, Sergey V. Lindeman, Rajendra
Rathore
PII:
S1010-6030(18)31583-1
DOI:
Reference:
https://doi.org/10.1016/j.jphotochem.2019.01.014
JPC 11673
To appear in:
Journal of Photochemistry and Photobiology A: Chemistry
Received date:
Revised date:
Accepted date:
31 October 2018
31 December 2018
14 January 2019
Please cite this article as: Vyas VS, Lindeman SV, Rathore R, Photophysical
properties of 1,3,6,8-tetraarylpyrenes and their cation radicals, Journal
of Photochemistry and amp; Photobiology, A: Chemistry (2019),
https://doi.org/10.1016/j.jphotochem.2019.01.014
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Photophysical properties of 1,3,6,8-tetraarylpyrenes and their
cation radicals
Vijay S. Vyas, Sergey V. Lindeman, Rajendra Rathore
Department of Chemistry, Marquette U
vijay.vyas@marquette.edu
Highlights.
 The UV–vis absorption and emission spectra of a series of 1,3,6,8-tetraaryl pyrenes were
obtained in solutions.
 Cyclic voltammograms were recorded.
 Cation radicals were generated in solution and their absorption spectra were recorded.
 Single crystal X-ray structure of neutral compounds and cation radicals were studied.
Graphical abstract
1

Highlights:

The UV–vis absorption and emission spectra of a series of 1,3,6,8-tetraaryl pyrenes were
obtained in solutions.



Cyclic voltammograms were recorded.
Cation radicals were generated in solution and their absorption spectra were recorded.
Single crystal X-ray structure of neutral compounds and cation radicals were studied.
A series of 1,3,6,8-tetraaryl pyrenes was synthesized. Photophysical and electronic properties of the
neutral compounds and the optical spectra of cation radicals were studied in solution
Abstract: The synthesis of 1,3,6,8-tetraaryl substituted pyrenes is described. The substituents on the
aryl groups influence the optoelectronic properties of the pyrene core indicating a strong coupling
between the aryl groups and the pyrene core. Electronic absroption spectra of the cation radicals
generated in solution by the single electron oxidation of pyrenes reaffirmed the coupling of the aryl
rings with the pyrene. The aryl groups at 1,3,6,8-positions completely inhibit the -stacking of pyrene
core in solution. Crystal structure of the cation radical salt of tetraphenyl shows charge delocalization
between neutral pyrene unit and two cationic pyrene cores. Complete inhibition of the pyrene core
stacking is observed in the solid state of cation radical when the dendritic pentaphenyl groups surround
the pyrene core.
1. Introduction
Monodisperse, small, -conjugated organic molecules with high purity and solution
processabily are extensively investigated for their use as opto-electronic materials in modern
photovoltaic devices.1 In this regard, 1,3,6,8-tetraaryl substituted pyrenes are promising
candidates due to their easy accessability from readily available starting materials, discotic
shape and the easy extension of -electrons of the pyrene core to the conjugated aryl groups
that can allow modulation of the photoluminiscence properties of the parent pyrene.2 The
conjugated aryl groups around the planar aromatic core of pyrene provide the viability for the
attachment of substitents for modulating the electronic influence on the core or improve the
solubility and stacking behavior by the introduction of flexible alkyl chains. Thus, tetraaryl
pyrenes have been explored as liquid crystalline materials,3 piezochromic photoluminiscence
materials4, hole transport materials for organic light emitting diode (OLED) and other organic
photovoltaic based applications.5 In addition, 1,3,6,8-tetraaryl substituted dendrimeric
structures of pyrene has also been extensively exploited for the study of core encapsulation by
2

Müllen and coworkers.6
Facile bromination of Pyrene at 1,3,6,8-positions with bromine at high temperatures
forms tetrabromopyrene7 thus providing an efficient synthetic route to prepare tetraaryl
pyrenes in quantitative amounts. While a number of tetraaryl pyrenes have been prepared
towards their exploration of the properties mentioned above,3-6 a systemetic study of the
influence of electronic and steric effect of the substituents on the aryl groups of the pyrene
core was found missing. Hence, we decided to modulate the electronic influence on the pyrene
core by varying the substituents on the four phenyl groups in 1,3,6,8-tetraphenylpyrene – a
compound which is very well known for its efficient blue emission, high quantum yield and
lack of aggregation in solution and solid state.5e,f
A series of tetraaryl pyrenes with aryl groups ranging from parent phenyl to 4-
methylphenyl, 4-methoxyphenyl and its analogue 4-propoxyphenyl, 4-acetoxyphenyl, 2,5-
dimethoxy-4-methyl and the dendrimeric pentaphenylphenyl were prepared. A survey of the
optoelectronic properties of the neutral tetraaryl pyrenes as well as their cation radicals or hole
carriers that are of fundamental importance to organic materials science9 is presented here
along with the isolation and X-ray crystallographic characterization of the netural compounds
and cation-radical salts of representative examples.
Figure 1. Structure of synthesized tetraaryl pyrenes.
2. Results and discussion
1.1. Synthesis
3

A four fold bromination of Pyrene afforded 1,3,6,8-tetrabromopyrene7 in quantitative
yields. Using readily-available phenyl boronic acid, 4-methylphenyl boronic acid, 4-
methoxyphenyl boronic acid and 2,5-dimethoxy-4-methylphenyl boronic acid, tetraaryl
substituted pyrenes 1, 2, 3 and 6 were prepared in 89 %, 85 %, 77 % and 72 % yield,
respectively via Suzuki coupling8 (scheme 1).
Scheme 1: Synthesis of 1,3,6,8-tetraarylpyrenes (1, 2, 3 and 6)
For the synthesis of 1,3,6,8 tetrakis(4-propoxyphenyl)pyrene (4) and 1,3,6,8 tetrakis(4-
acetoxyphenyl)pyrene (5), demethylation of tetraanisyl pyrene (3) was done using BBr3 in
dichloromethane followed by standard workup. The resulting 1,3,6,8 tetrakis(4-
hydroxyphenyl)pyrene (9) was then treated with potassium hydroxide followed by n-propyl
iodide to give 4 in 42 % yield while the treatment of 9 with triethyl amine followed by acetyl
chloride afforded 5 in 74 % yield (scheme 2).
Scheme 2. Synthesis of 1,3,6,8-tetraarylpyrenes (4 and 5)
4

The synthesis of dendritic 7 was done as per literature published procedure.6 Thus, a
four-fold Hagihara–Sonogashira cross-coupling of 8 with phenylacetylene gave 1,3,6,8-
tetrakis(phenylethynyl)pyrene (10) which upon subsequent Diels–Alder reaction with
tetraphenylcyclopentadienone yielded dendrimeric pyrene 7 (scheme 3).
5

Scheme 3. Synthesis of 7
1
All synthesized compounds were thoroughly characterized using H and 13C NMR and
the structure of the final compound was also verified by MALDI-TOF mass spectroscopy as
well as single crystal X-ray diffraction (Figure 2). The final compounds exhibited good
solubility in common organic solvents that allowed purification by column chromatography
and subsequent characterization facile. The crystal structure of pyrenes 1 – 4 show that the aryl
groups are twisted at by 43º – 56º with respect to the pyrene core. However, in solution the
aryl groups in 1 – 5 undergo free rotation as exemplified in their NMR spectrum. In this regard,
it is notable that 1,3,6,8-tetrakis(2,5-dimethoxy-4-methylphenyl)pyrene (6) forms a number of
rotamers due to restricted rotation of aryl groups that not only results in complex NMR
spectrum (see experimental section), but also poses problem in solving the crystal structure.
However, at a temperature of ~ 353 K the rotational barrier is overcome and the the NMR
spectrum simplifies. In a similar manner, the dendritic pentaphenylphenyl groups restrict the
free rotation of the aryl groups in 7, the NMR of which is resolved at 373K.6
6

1
2
3
4
7
Figure 2. ORTEP diagram of synthesized tetraaryl pyrenes 1, 2, 3, 4 and 7. Solvent
molecules have been omitted for clarity.
2.2 Photophysical properties
The absorption spectrum of tetraaryl pyrenes in dichloromethane recorded at room
temperature exhibits two distinct bands – one in the UV region (292 – 317 nm) and the other
in the visible region (383 – 408 nm). Thus, in comparison to the parent pyrene that shows
abosroption maxima at ~337 nm, aryl pyrenes show a bathochromic shift of 60 – 85 nm which
is in accordance with the extension of the conjugation from the pyrene to the aryl rings
(table 1). For example the twin absorption bands in phenyl substitued 1 at λmax (log ε) = 297 nm
(4.75), 384 nm (4.59) are red shifted in 4-methylphenyl substituted 2 at λmax (log ε) = 301 nm
(4.73), 389 nm (4.60) and further to λmax (log ε) = 305 nm (4.65), 392 nm (4.61) in 4-
methoxyphenyl substituted 3. Substitution of aryl groups at 1,3,6,8-positions also leads to the
disappearence of fine structure (figure 3) that is observed in the absorption spectra of parent
pyrene. However, in case in 7, the restriction in free rotation posed by the steric bulk of
7

dendritc pentaphenylphenyl groups results in featured bands at λmax (log ε) = 317 nm (4.84),
386 nm (4.71) and 408 nm (4.85).
6
1
4
2
0
6
2
4
2
0
6
3
4
2
0
4
6
4
2
0
5
6
6
4
2
0
6
4
2
0
8
7
6
4
2
0
270 300 330 360 390 420
Wavelength (nm)
Figure 3. UV-vis. absorption spectra of tetraaryl pyrenes recorded in
dichloromethane at 22 ºC.
The fluorescence spectrum of dilute aryl pyrenes (2  10 -5 M) in CH2Cl2 at 22C shows
8

a mirror image relationship between excitation and emission bands (for example 1; Figure 4),
indicating that there is no significant change in molecular structure upon excitation.
250 300 350 400 450 500 550
Wavelength (nm)
Figure 4. Normalized excitation (red dash) and emission (blue) spectra of 2.0  10
-5 M solution of 1 recorded in CH2Cl2 at 22 C.
As observed in absorption spectrum, the featured emission spectrum of the parent
pyrene and the tetraaryl pyrenes is replaced by broad featureless emission spectrum (Figure 5)
due to pronounced rotational freedom of the aryl rings. Expectedly, the restricted rotation of
pentaphenylphenyl groups in 7 allows the retention of fine structure as noted earlier in its
absorption spectrum. The extension of conjugation caused by the aryl groups is also
responsible for the bathochromic shift of the emission band in tetraaryl pyrenes (422 – 435 nm)
when compared with parent pyrene (395 nm). The electronic effect of the attachment of
electron releasing alkyl and alkoxy group to the phenyl rings is also evident as the emission
band shows marginal red shift from 422 nm in phenyl substituted 1 to 426 nm in tolyl
substituted 2 and ~ 434 nm in alkoxy substituted 3 and 4. In case of 5, 6 and 7 the emission
band is observed between 426 – 428 nm (table 1). Like the parent pyrene, the shape and
position of emission spectrum was found to be invariant of the wavelength of excitation.
9

1
2
3
4
5
6
7
390 420 450 480 510 540
Wavelength (nm)
Figure 5. Emission spectra of tetraaryl pyrenes as 2 × 10-5 M dichloromethane
solutions at 22 ºC.
2.3 Electronic properties
The redox behavior of tetraaryl pyrenes was determined by electrochemical oxidation at a
platinum electrode as 2 × 10-3 M solution in anhydrous dichloromethane containing 0.2 M tetra-n-
butylammonium hexaflurophosphate (TBAH) as the supporting electrolyte (Figure 6). The
10

electrochemical oxidations were completely reversible (1 - 4, 7) or quasi-reversible (5 and 6) for
the removal of the first electron at varying scan rates of 100 – 500 mV s-1. The oxidation potential
corresponding to the removal of one electron and thereby formation of a monocation radical was
calibrated with ferrocene as internal standard (Eox = 0.45 V vs. SCE)10 and was found to correlate
directly with the electronic influence of the substituents on the aryl rings. As expected, an
introduction of electron donating methyl in 2 or methoxy/propoxy substituents (3/4) lowers the
first and the second oxidation potentials of 2 and 3/4 when compared to 1 (see Table 1). Similarly,
introduction of acetyl group in 5 increases both the oxidation potentials, while the electroactive
dimethoxymethyl substituents in 6 and pentaphenylphenyl groups in 7 again lower the Eox1 values
when compared with 1. Furthermore, it was observed that the difference between the first and
second oxidation potentials, i.e. the ∆Eox value decreased from 0.51 V in 1 to 0.45 V in 2, due to
the substitution of hydrogens with electron donating methyl substituents and further to 0.30 V in
3 and 4, due to electron donating alkoxy substituents. On the other hand, the substitution of acetyl
groups in 5 results in ∆Eox value of 0.45 V.
11

1
2
3
4
5
6
7
1.8
1.5
1.2
0.9
0.6
Volts vs. SCE
Figure 6. Cyclic voltammograms of 2 × 10-3 M 1-7 in CH2Cl2 containing 0.2 M n-
Bu4NPF4 at a scan rate of 100 mV s-1 at 22 °C.
The introduction of electro-active 2,5-dimethoxy-4-methyl groups in 6 not only lowers the
∆Eox value between first and second oxidation potential to 0.33 V, a third oxidation wave is also
observed at 1.65 V (Figure 7). Interestingly, the pentaphenylphenyl groups in 7 only lower the first
12

oxidation potential to Eox1 = 0.95 V whereas, the second oxidation potential is observed at Eox2
1.56 V resulting in ∆Eox value to 0.61 V.
=
8.0e-6
0.0
-8.0e-6
-1.6e-5
1.8
1.5
1.2
0.9
0.6
Volts vs. SCE
Figure 7. Cyclic Voltammograms of 2 × 10-3 M of 6 in CH2Cl2 containing 0.2 M
n-Bu4NPF4 at a scan rate of 100 mV s-1 at 22 °C.
2.4 Cation radical in solution and in solid state
The electrochemical reversibility and low first oxidation potentials of 1-7, prompted us to
generate their cation radicals by chemical oxidation using stable aromatic cation radical salts –
¯
¯
MA•+ SbCl6 (Ered = 1.11 V vs. SCE)11 and hindered naphthalene – NAP+ SbCl6 (Ered = 1.34 V
vs. SCE)12 as one-electron oxidants, see structures below.
¯
For example, the cation radical 1+ SbCl6 of tetraphenylpyrene 1 (Eox1 = 1.12 V) was
¯
generated by a careful redox titration using the hindered NAP+ SbCl6 in anhydrous
dichloromethane. Figure 8 shows the spectral changes attendant upon the incremental addition of
-5
+
¯

sub-stoichiometric amounts of 1 to 3.51 × 10 M NAP SbCl6 [λmax (log ε) = 672 nm (3.97)] in
dichloromethane at 22 C. The plot of formation of 1+ (i.e. increase of absorbance at 569 nm)
+

against the increments of added neutral 1 established that NAP was completely consumed after
addition of 1 equivalent of 1 and the resulting absorption spectrum with absorption bands at 569 nm
13

(log e = 4.52) and 804 nm (log e = 4.10) remained unchanged upon further addition of neutral 1 as
per equation 1.
1.2
1.2
0.9
0.6
0.9
0.3
0.0
0.0 0.5 1.0 1.5 2.0
Equivalents of 1 added
0.6
0.3
0.0
400 600 800 1000 1200 1400
Wavelength (nm)
Figure 8. Spectral changes observed upon the reduction of 3.51 × 10-5 M NAP+●
(red line) by an incremental addition of 2 x 10-3 M of 1 to its radical cation (green
line) in CH2Cl2 at 22 °C. Inset: A plot of increase of the absorbance of 1+●
monitored at 569 nm against the equivalent of added neutral 1.
¯
Using NAP+ SbCl6 , a similar redox titration was performed to generate the cation
radical of tetraacetoxyphenyl pyrene 5. Figure 9 shows the spectral changes attendant upon an
¯
incremental addition of sub-stoichiometric amounts of 5 to 3.22 × 10-5 M NAP+ SbCl6 in

dichloromethane at 22 C. A plot of formation of 5+ (i.e. increase of absorbance at 582 nm) against
+

the increments of added neutral 5, indicated that NAP was completely consumed after addition
of 1 equiv. of 5 and the resulting absorption spectrum with red shifted absorption bands (with
14

respect to 1+) at 582 (log e = 4.52) and 828 nm (log e = 4.12) remained unchanged upon further
addition of neutral 5.
1.2
1.2
0.9
0.6
0.9
0.3
0.0
0.0 0.5 1.0 1.5 2.0
Equivalents of 5 added
0.6
0.3
0.0
400 600 800 1000 1200 1400
Wavelength (nm)
Figure 9. Spectral changes observed upon the reduction of 3.22 × 10-5 M NAP+●
(red line) by an incremental addition of 2 x 10-3 M of 5 to its radical cation (green
line) in CH2Cl2 at 22 °C. Inset: A plot of increase of the absorbance of 5+●
monitored at 582 nm against the equivalent of added neutral 5.
For 2, 3, 4, 6 and 7, where Eox1 value was well below the redox potential of MA+

¯
(Ered = 1.11 V vs SCE), the cation radical was generated by a redox titration using MA+ SbCl6 in
anhydrous dichloromethane using equation 2 as an example.
Thus, Figure 10 shows the spectral changes attendant upon an incremental addition of sub-
¯
stoichiometric amounts of 2 to 2.22 × 10-5 M MA+ SbCl6 [λmax (log ε) = 518 nm (3.86)] in

dichloromethane at 22 C. A plot of formation of 2+ (i.e. increase of absorbance at 591 nm) against
15

+

the increments of added neutral 2, established that MA was completely consumed after addition
of 1 equivalent of 2 and the resulting absorption spectrum with absorption bands at 591 nm (log e
= 4.47) and 851 nm (log e = 4.12) remained unchanged upon further addition of neutral 2.
0.8
0.9
0.6
0.4
0.2
0.6
0.0
0.0 0.5 1.0 1.5 2.0
Equivalents of 2 added
0.3
0.0
400 600 800 1000 1200 1400
Wavelength (nm)
Figure 10. Spectral changes observed upon the reduction of 2.22 × 10-5 M MA+●
(red line) by an incremental addition of 2 x 10-3 M of 2 to its radical cation (green
line) in CH2Cl2 at 22 °C. Inset: A plot of increase of the absorbance of 2+●
monitored at 591 nm against the equivalent of added neutral 2.
A comparison of the absorption bands of 1+ with that of methyl substituted 2+ shows a red
shift of 22 nm in the higher energy band and that of 47 nm in the lower energy band. The
bathochromic shift caused by the substituents on aryl groups is further amplified in the absorption
spectra of alkoxy substituted 3+ and 4+. Hence, as seen figure 11, the absorption spectrum of 3+
with absorption bands at 632 nm (log e = 4.59) and 944 nm (log e = 4.35); and 4+ with absorption
bands at 636 nm (log e = 4.57) and 958 nm (log e = 4.33) show a considerable shift in the twin
absorption bands when compared with the absorption bands in tetraphenylpyrene cation radical
1+. The lower energy 958 nm absorption band is also characterized by shoulders at ~1100 nm and
~1300 nm possibly arising from forbidden or partially forbidden transitions.
16

A
B
1.2
1.2
1.5
1.2
0.9
0.6
0.3
0.0
1.5
1.2
0.9
0.6
0.3
0.0
0.9
0.6
0.3
0.0
0.9
0.6
0.3
0.0
0.0 0.5 1.0 1.5 2.0
Equivalents of 3 added
0.0 0.5 1.0 1.5 2.0
Equivalents of 4 added
400 600 800 1000 1200 1400
400 600 800 1000 1200 1400
Wavelength (nm)
Wavelength (nm)
Figure 11 A. Spectral changes observed upon the reduction of 2.93 × 10-5 M MA+●
(red line) by an incremental addition of 2 x 10-3 M of 3 to its radical cation (green
line) in CH2Cl2 at 22 °C. Inset: A plot of increase of the absorbance of 3+●
monitored at 632 nm against the equivalent of added neutral 3. B. Spectral changes
observed upon the reduction of 3.12 × 10-5 M MA+● (red line) by an incremental
addition of 2 x 10-3 M of 4 to its radical cation (green line) in CH2Cl2 at 22 °C.
Inset: A plot of increase of the absorbance of 4+● monitored at 636 nm against the
equivalent of added neutral 4.
The electro-active dimethoxymethyl substituted 6 with a restricted free rotation of aryl
groups, shows an absorption spectrum with interesting features. A plot for the formation of purple
¯
colored cation radical 6+ SbCl6 , against the increments of added neutral 6, established that MA+

was completely consumed after addition of 1 equiv. of 6 and the resulting absorption spectrum
remained unchanged upon further addition of neutral 1. As seen in figure 12, the absorption
spectrum contains a band at 572 nm (log e = 4.43) – a region where other tetraaryl pyrenes exhibit
similar feature. In contrast to the observations made in the cation radicals of 1 – 5, a broad
gaussian-shaped band from 800 nm to 2500 nm is seen in the absorption spectrum of 6+. Such
low-energy wide bands are characteristic intervalence bands corresponding to a localized
distribution of charge in the ground state and can be compared with the absorption spectra of the
cation radical of a biaryl containing two of such elecroactive groups D–D+ (4’-dimethyl-2,5,2’,5’-
tetramethoxy-1,1’-biphenyl; λmax = 580 nm, 2150 nm)13. This indicates that the aryl groups are
able to achieve significant intramolecular electronic coupling with the pyrene core. Theoretical
calculations would be needed to understand the nature and kind of coupling.
17

1.2
0.9
0.6
0.3
0.0
1.2
0.9
0.6
0.3
0.0
0.0 0.5 1.0 1.5 2.0
Equivalents of 6 added
400 800 1200 1600 2000 2400
Wavelength (nm)
Figure 12. Spectral changes observed upon the reduction of 4.34 × 10-5 M MA+●
(red line) by an incremental addition of 2 x 10-3 M of 6 to its radical cation (green
line) in CH2Cl2 at 22 °C. Inset: A plot of increase of the absorbance of 6+●
monitored at 572 nm against the equivalent of added neutral 6.
¯
Lastly, the cation radical 7+ SbCl6 of dendrimeric 7 was generated using the
¯
hindered MA+ SbCl6 in an analogous manner. Figure 13 shows the spectral changes attendant
-5
+

¯
upon an incremental addition of sub-stoichiometric amounts of 7 to 5.54 × 10 M MA SbCl6 in
dichloromethane at 22 C. Further, a plot for the formation of 7+ (i.e. increase of absorbance at
+

602 nm) against the increments of added neutral 7, shows that MA was completely consumed
after the addition of 1 equivalent of 7 and the resulting absorption spectrum with the absorption
bands at 602 (log e = 4.30) and a broad band at 954 nm (log e = 3.85) remained unchanged upon
further addition of neutral 7.
18

1.2
0.9
0.6
0.3
0.0
1.2
0.9
0.6
0.3
0.0
0.0 0.5 1.0 1.5 2.0
Equivalents of 7 added
400 600 800 1000 1200 1400
Wavelength (nm)
Figure 13. Spectral changes observed upon the reduction of 5.54 × 10-5 M MA+●
(red line) by an incremental addition of 2 x 10-3 M of 7 to its radical cation (green
line) in CH2Cl2 at 22 °C. Inset: A plot of increase of the absorbance of 7+●
monitored at 602 nm against the equivalent of added neutral 7.
The blue/purple colored solution of the cation radical salts were highly persistent and did
not show any decomposition at room temperature during the course of 12 hours. Moreover, a
reduction of dichloromethane solution of the cation radical salts with zinc dust regenerated the
neutral aryl pyrenes quantitatively. The high stability of the cation radicals in solution therefore
prompted us to isolate their crystalline salt. Thus, a 5 mM solution of 1 in anhydrous
+
¯

dichloromethane was added to an equimolar soution of NAP SbCl6 at room temperature and the
solution was layered with toluene. An excellent crop of black prismatic crystals, suitable for X-
ray crystallographic studies, were obtained by a slow diffusion of toluene into the above solution
of 1•+ during a period of 2 days at -30 °C.
19

Figure 14. ORTEP diagram showing compact centrosymmetric triads of
¯
tertraphenylpyrene cation radical salt 1•+ SbCl6 . Hydrogen atoms and the solvent
molecules are omitted for clarity.
¯
The crystal structure of 1•+ SbCl6 revealed that 1 crystallized as a 3:2 salt of
¯
tetraphenylpyrene 1: SbCl6 . The oxidized pyrene moieties form compact centrosymmetric
triads (figure 14). Among the triads, no overlap is observed between the pyrene chromophores.
While the first and third pyrene cores are parallel. the central pyrene unit lies across and is flat
with its phenyl groups making dihedral angles of 47° and 53°. The phenyl groups of this central
pyrene unit hang over the pyrene units above and below and in turn result in significant bending
(by ~ 12°) of the pyrene units above and below. The phenyl groups of the peripheral pyrenes
are twisted by 48 - 60°.
Analysis of the bond length changes in the peripheral pyrene units and its comparison
with the central unit as well as the neutral 1 suggest that the positive charges are localized over
the peripheral (bent) units. Thus, in the peripheral pyrenes, the bonds labelled B and D (figure
15) where the HOMO resides show perceptible lengthening by ~ 2 pm while the bond C shows
shortening by about the same amount. The central bond F, undergoes a shortening of ~ 2 pm
in order to accommodate the changes in the bond lengths of various annulenic bonds (i.e. B,
20

C, and D), while there is no noticeable change in the bonds marked A and E. These
observations are consistent with the observation made in the analysis of bond length changes
in the cation radical of tetraisopropylpyrene14 together with the comparison with its neutral
form.
E
F
A
C
B
O
M
N
D
P
Figure 15. Left: Numbering scheme for the pyrene skeleton. Right: Showing the
localization of the HOMO of 1, obtained by DFT calculations at B3LYP-631G**
level.
Our attempts to crystallize the other tetraaryl pyrenes resulted in amorphous powders.
However, we succeeded in crystallizing the crystalline salt of the cation radical of dendritic 7.
A 5 mM solution of 7 in anhydrous 1,2-dichloroethane was added to an equimolar soution of
+
¯

MA SbCl6 at room temperature and the solution was layered with toluene. During a period
of 2 days at -30 °C, black colored needles suitable for X-ray crystallographic studies were
obtained. The crystal structure shows that 7 crystallizes as a 1:1 ion-radical salt with heavily
disordered SbCl6 anion (Figure 16). The cationic 7 moieties form chains along z-axis
alternating with SbCl6 anions positioned in their clefts. The ionic chains are separated by
numerous disordered solvent molecules. Thus, the dendrimeric pentaphenyl groups completely
prevent the pyrene chromophore from having any overlap with the neighboring units as seen
in the crystal structure of the tetraphenyl pyrene cation.
21

Figure 16. ORTEP diagram showing compact centrosymmetric triads of
¯
tertraphenylpyrene cation radical salt 7•+ SbCl6 . Hydrogen atoms and the solvent
molecules are omitted for clarity.
Table 1. Optoelectronic properties of tetraaryl pyrenes 1-7.
1
2
3
4
5
6
7
1.00,
1.31,
1.65
E
1.12,
1.64
1.04,
1.49
0.96,
1.26
0.95,
1.25
1.16,
1.61
0.95,
1.56
ox
V
(V vs SCE)
22

297
(4.75),
301
(4.73),
305
(4.65),
306
(4.62),
299
(4.73),
292
(4.65),
317 (4.84),
386 (4.71),
408 (4.85)

(log e)
max
nm
UV-vis
384 (4.59) 389 (4.60) 392 (4.61) 393 (4.60) 387 (4.59) 383 (4.57)

max
nm
nm
422
426
434
435
426
428
428
(emission)
572
(4.43),
1520
569
(4.52),
591
(4.47),
632
(4.59),
636
(4.52),
582
(4.52),

(log e)
602 (4.30),
954 (3.85)
max
CR+
804 (4.10) 851 (4.12) 944 (4.35) 958 (4.33) 828 (4.12)
(3.86)
Table 1. Optoelectronic properties of tetraaryl pyrenes 1-7.
2. Conclusion
In summary, a series of 1,3,6,8-tetraarylpyrenes have been prepared using simple and
practical synthesis from readily available precursors. The emission and absorption
spectroscopy of the neutral and cationic 1-7 as well as their cyclic voltammograms clearly
demonstrate the interaction of the aryl groups with the pyrene core. The presence of aryl groups
at 1,3,6,8-positions inhibit the -stacking of pyrene core in solution. However, the isolation
and X-ray crystal structure determination of 1•+ SbCl6¯ shows that the phenyl groups of a
neutral pyrene unit are still able to form extended trimeric - stacks with two cationic pyrene
cores. These results demonstrate that the coupling of aryl groups with the Pyrene core in the
neutral state as well as in cation radicals is significant and can be explored further for material
applications.
3. Material and methods
All reactions were performed under argon atmosphere unless otherwise noted. All
commercial reagents were used without further purification unless otherwise noted.
Dichloromethane (Aldrich) was repeatedly stirred with fresh aliquots of concentrated sulfuric acid
(~10 % by volume) until the acid layer remained colorless. After separation, it was washed
23

successively with water, aqueous sodium bicarbonate, water, and aqueous sodium chloride and
dried over anhydrous calcium chloride. The dichloromethane was distilled twice from P2O5 under
an argon atmosphere and stored in a Schlenk flask equipped with a Teflon valve fitted with Viton
O-rings. The hexanes and toluene were distilled from P2O5 under an argon atmosphere and then
refluxed over calcium hydride (~12 h). After distillation from CaH2, the solvents were stored in
Schlenk flasks under an argon atmosphere. Tetrahydrofuran (THF) was dried initially by distilling
over lithium aluminum hydride under an argon atmosphere. The THF was further refluxed over
metallic sodium in the presence of benzophenone until a persistent blue color was obtained and
then it was distilled under an argon atmosphere and stored in a Schlenk flask equipped with a
Teflon valve fitted with Viton O-rings. NMR spectra were recorded on Varian 300 and 400 MHz
NMR spectrometers. MALDI-TOF MS spectra were obtained on a Bruker Daltonics microflex
using Dithranol matrix.
Cyclic Voltammetry (CV) :
The CV cell was of an air-tight design with high vacuum Teflon valves and Viton O-ring
seals to allow an inert atmosphere to be maintained without contamination by grease. The working
electrode consisted of an adjustable platinum disk embedded in a glass seal to allow periodic
polishing (with a fine emery cloth) without changing the surface area (~1 mm2) significantly. The
reference SCE electrode (saturated calomel electrode) and its salt bridge were separated from the
catholyte by a sintered glass frit. The counter electrode consisted of platinum gauze that was
separated from the working electrode by ~3 mm. The CV measurements were carried out in a
solution of 0.2 M supporting electrolyte (tetra-n-butylammonium hexafluorophosphate, TBAH)
and 2 × 10-3 M substrate in dry dichloromethane under an argon atmosphere. All the cyclic
voltammograms were recorded at a sweep rate of 100 mV sec-1, unless otherwise specified and
were IR compensated. The oxidation potentials (E1/2) were referenced to SCE, which was
calibrated with added (equimolar) ferrocene (E1/2 = 0.450 V vs. SCE). The E1/2 values were
calculated by taking the average of anodic and cathodic peak potentials in the reversible cyclic
voltammograms.
-
General Procedure for the Spectral Titration of [MA·+ SbCl6 ] with aryl Pyrenes
An orange-red solution of MA·+ SbCl6¯ in dichloromethane (3 mL, concentrations as
specified in individual cases) was transferred to a 1-cm quartz cuvette at room temperature. A
dichloromethane solution (2.0 × 10-3 M) of aryl pyrenes in was added to this solution. The UV-vis
24

spectra and NIR spectra of the resulting solutions, after the addition of each increment, were
recorded at 22 C
-
Procedure for the Spectral Titration of [Nap·+ SbCl6 ] with aryl pyrenes
¯
A deep blue solution of NAP+ SbCl6 in dichloromethane (3 mL, concentrations as
specified in individual cases) was transferred in a 1-cm quartz cuvette at room temperature. A
dichloromethane solution (2 × 10-3 M) of aryl pyrene was added to this solution. The UV-vis
spectra and NIR spectra of the resulting solutions, after the addition of each increment, were
recorded at 22 C.

Synthesis of Compounds:
1,3,6,8-Tetrabromopyrene (8): 1,3,6,8-Tetrabromopyrene was synthesized in 95% yield by
reacting pyrene with bromine according to a literature published procedure.7
General Procedure for the Suzuki Coupling: Compound 8 (0.52 g, 1 mmol), boronic acid (5
mmol), K2CO3 (2.8 g, 20 mmol) and Pd(PPh3)4 (185 mg) was added to dimethylformamide (20
mL) taken in a Schlenk flask. The flask was repeatedly degassed by evacuation and purging with
argon. The mixture was heated at 160 C under argon atmosphere under complete exclusion of
light for the time mentioned for individual cases after which it was quenched by adding water. The
organic layer was extracted with dichloromethane, washed consecutively with water and brine
before being dried over MgSO4. After having removed the solvent under reduced pressure, the
crude product was purified by column chromatography (silica gel, hexane/ethyl acetate).
1,3,6,8-tetraphenylpyrene (1): Phenyl boronic acid (0.78 g, 5 mmol) was used and the reaction
was carried out for 2 days. The crude product was then chromatographed over silica gel, using
hexanes as eluent to afford 1 (0.45 g, 89%). mp 294-296 C, 1H NMR (400 MHz, CDCl3, δ) 8.18
(s, 4H), 8.02 (s, 2H), 7.67 (m, 8H), 7.54 (m, 8H), 7.46 (m, 4H); 13C NMR (100 MHz, CDCl3, δ)
141.26, 137.44, 130.87, 129.76, 128.66, 128.31, 127.51, 126.14, 125.53; MS : (MALDI) m/z 506.3
(M+).
1,3,6,8-tetrakis(4-methylphenyl)pyrene (2): 4-methylphenylboronic acid (0.68 g, 5 mmol) was
used and the reaction was carried out for 2 days. The crude product was then chromatographed
over silica gel, using hexanes as eluent to afford 2 (0.48 g, 85%). mp 316-318 C, 1H NMR (400
25

MHz, CDCl3, δ) 8.18 (s, 4H), 7.99 (s, 2H), 7.57 (d, J = 7.6 Hz, 8H), 7.35 (d, J = 7.6 Hz, 8H), 2.49
(s, 12H); 13C NMR (100 MHz, CDCl3, δ) 138.41, 137.28, 137.15, 130.74, 129.78, 129.27, 128.21,
126.25, 125.40, 21.49; MS : (MALDI) m/z 562.3 (M+).
1,3,6,8-tetrakis(4-methoxyphenyl)pyrene (3): 4-methoxyphenyl boronic acid (0.78 g, 5 mmol)
was used and the reaction was carried out for 2 days. The crude product was then chromatographed
over silica gel, using hexanes/ethyl acetate as eluent to afford 3 (0.48 g, 77%). mp 264-266 C, 1H
NMR (400 MHz, CDCl3, δ) 8.17 (s, 4H), 7.97 (s, 2H), 7.60 (d, J = 8.2 Hz, 8H), 7.08 (d, J = 8.2
Hz, 8H), 3.92 (s, 12H); 13C NMR (100 MHz, CDCl3, δ) 159.2, 136.9, 133.7, 131.91, 131.87, 129.8,
128.2, 126.3, 125.3, 114.0, 55.6; MS : (MALDI) m/z 626.3 (M+).
1,3,6,8-tetrakis(4-n-hydroxyphenyl)pyrene (9): Compound 3 (1.125 g, 2 mmol) was dissolved
in dichloromethane (100 mL) taken in a Schlenk flask fitted with a rubber septum. The flask was
cooled to 0 C and BBr3 (1.9 mL, 20 mmol) was added dropwise. The reaction was then warmed
to room temperature (22 C) at which it was allowed to stir for 14 h. The contents were poured in
water (1 L) and filtered. The product was washed repeatedly with water to yield dark green solid
(1 g, 88%) that was used further without any purification. 1H NMR (300 MHz, d6-acetone, δ) 8.20
(s, 4H), 7.93 (s, 2H), 7.53 (d, J = 8.52 Hz, 8H), 7.08 (d, J = 8.52 Hz, 8H), 3.13 (broad s, 4H ).
1,3,6,8-tetrakis(4-n-propoxyphenyl)pyrene (4): Compound 9 (0.57 g, 1 mmol) was dissolved in
1,2-dimethoxyethane (20 mL) taken in a schlenk flask flushed with Argon. To this was added
potassium hydroxide (1.4 g, 25 mmol) dissolved in water (10 mL) and the reaction was stirred.
After 30 min. 1-iodopropane (1.95 mL, 20 mmol) was added and the reaction was allowed to stir
further for 24 hour at 22 C. The reaction was worked up by pouring the contents in water (500
mL). The product was extracted with dichloromethane and washed with water (3  500 mL) before
being dried over MgSO4. The solvent was removed under reduced pressure and the crude product
was then chromatographed over silica gel, using hexanes/ethyl acetate as eluent. The product was
then recrystallized with dichloromethane/methanol to afford 6 as orange-yellow crystals (0.31 g,
42%). mp 256-258 C, 1H NMR (300 MHz, CDCl3, δ) 8.16 (s, 4H), 7.96 (s, 2H), 7.58 (d, J = 8.6
Hz, 8H), 7.07 (d, J = 8.6 Hz, 8H), 4.03 (t, J = 6.5 Hz, 2H), 1.88 (m, 2H), 1.10 (t, J = 7.4 Hz, 3H);
13C NMR (75 MHz, CDCl3, δ) 158.75, 136.98, 133.57, 131.88, 129.83, 128.20, 126.35, 125.31,
114.59, 69.84, 22.87, 10.81; MS : (MALDI) m/z 738.5 (M+).
26

1,3,6,8-tetrakis(4-acetylphenyl)pyrene (5): Compound 9 (0.28 g, 0.5 mmol) was added to a
solution of triethyl amine (0.35 mL, 2.5 mmol) in acetone (10 mL). The mixture was stirred for 2
h. Acetyl chloride (0.71 mL, 10 mmol) was then added and the reaction was stirred further for 20
h at 22 C. The product was extracted with dichloromethane, dried over magnesium sulphate and
then chromatographed over silica gel using hexanes/ethyl acetate as eluent to afford 8 as light
1
yellow solid (0.27 g, 74%). mp 282 d C, H NMR (400 MHz, CDCl3, δ) 8.19 (s, 4H), 8.00 (s,
2H), 7.67 (d, J = 8.6 Hz, 8H), 7.28 (d, J = 8.6 Hz, 8H), 2.37 (s, 12H); 13C NMR (100 MHz, CDCl3,
δ) 169.72, 150.31, 138.65, 136.58, 131.79, 129.88, 128.37, 126.09, 125.56, 121.78, 21.43; MS :
(MALDI) m/z 738.3 (M+).
1,3,6,8-tetrakis(2,5-dimethoxy-4-methylphenyl)pyrene
(6):
2,5-dimethoxy-4-
methylphenylboronic acid (0.98 g, 5 mmol) was used and the reaction was carried out for 2 days.
The crude product was then chromatographed over silica gel, using hexanes/ethyl acetate as eluent
to afford 6 (0.58 g, 72%). mp 300-302 C, 1H NMR (400 MHz, (CD3)2SO, 353 K, δ) 7.82 (s, 2H),
7.78 (s, 4H), 7.06 (s, 4H), 6.98 (s, 4H), 3.79 (s, 12H), 3.61 (s, 12H), 2.31 (s, 12H); 13C NMR (100
MHz, CDCl3, δ) 151.21, 133.11 br, 127.99, 127.67, 126.79 br, 126.29, 124.62 br, 117.06, 55.74
br, 15.48 ; MS : (MALDI) m/z 802.5 (M+).
1,3,6,8-tetrakis(pentaphenylphenyl)pyrene (7): This was prepared in 72% overall yield in two
steps by reacting 1,3,6,8-tetrabromopyrene (8) with phenylethylene followed by cyloaddition of
the resulting 1,3,6,8-tetrakis(phenylethynyl)pyrene with tetraphenylcyclopentadienone according
to a literature published procedure.6
27

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