Mono-Aryl/Alkylthio-Substituted (Hetero)acenes of Exceptional Thermal and Photochemical Stability by the Thio-Friedel–Crafts/Bradsher Cyclization Reaction

Article abstract of DOI:10.1002/chem.201903027

The highly substituted mono-aryl/alkylthio-(hetero)acenes prepared in this study have been found to be thermally more stable (T_decomp.=331–354 °C) than the known di-aryl/alkylthio-substituted acenes by an average of 25 °C. They are also much mo

Full text of DOI:10.1002/chem.201903027

A Journal of
Accepted Article
Title: Mono-aryl/alkythio-substituted (Hetero)acenes of Exceptional
Thermal and Photochemical Stability via Thio-Friedel Crafts/
Bradsher Cyclization Reaction
Authors: Piotr Balczewski, Emilia Kowalska, Joanna Skalik, Ewa
Różycka-Sokołowska, Krzysztof Owsianik, Marek Koprowski,
Bernard Marciniak, Dariusz Guziejewski, and Witold Ciesielski
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content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.201903027
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Chemistry - A European Journal
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FULL PAPER
Mono-aryl/alkylthio-substituted (Hetero)acenes of Exceptional
Thermal and Photochemical Stability via Thio-Friedel
Crafts/Bradsher Cyclization Reaction
[
a, b]
[a]
[b]
[a]
Piotr Bałczewski,*
Emilia Kowalska, Ewa Różycka-Sokołowska, Joanna Skalik, Krzysztof
[a]
[a]
[b]
[c]
[c]
Owsianik, Marek Koprowski, Bernard Marciniak, Dariusz Guziejewski, Witold Ciesielski
Abstract:
Significantly
substituted
mono-aryl/alkylthio-(RS)- Introduction
(hetero)acenes are thermally more stable (Tdec.= 331-354 °C) than
known di-RS-substituted acenes by average 25 °C. They are also
much more photostable at 254 and 365 nm (both in argon/air) than
parent anthracene and other reported anthracenes. The most
photostable RS-anthracenes (254 nm) were 60-70 (air) and 130
(Hetero)acenes, despite their excellent electronic performance,
are usually poorly soluble in organic solvents, which limits their
direct use and processing. A further limitation is instability of the
conjugated π-system and its destruction upon contact with
oxygen and light due to photooxidation of double bonds to
(argon) times more stable in solution than the unsubstituted
1
-4
5,6
anthracene and much more stable than known EDG/EWG
-
endoperoxides.
photochemical dimerizations.
Other possible reactions are thermal- and
7
-8
substituted anthracenes with extended aromatic core. The
photostability significantly increased at 365 nm both in air and argon.
The mono-RS-(hetero)acenes are blue light emitting fluorophores.
They have been obtained in the novel thio-Friedel-Crafts/Bradsher
cyclization reaction of unknown ortho-1,3-dithianylaryl(aryl)methyl
thioethers. The developed approach gives a general access to
mono-RS-(hetero)acene frameworks that may contain at least 3 or 4
fused (hetero)aromatic rings. The characteristic feature of this
approach leading to significantly substituted acenes is that
substituents, unlike in other methods, may be introduced at the early
stage of the synthesis. DFT and TD-DFT calculations confirmed a
stabilizing role of RS substituent in mono-RS-substituted
anthracenes which are the most stable anthracenes up to date. Their
high photostability is mainly due to the singlet oxygen quenching by
These degradative processes
are especially intensive for acenes larger than pentacene that
are less stable and decompose already in the presence of
visible light and air. In case of applications of smaller
anthracenes as semiconductors, the higher stability toward the
degradative processes compensates the disadvantage of their
lower charge carrier mobilities in comparison with the larger
9
analogs. High photochemical stability is an important property
of all acene materials, especially those suitable for use in small
1
0
molecule and polymeric solar cells.
In the literature of this field, the photochemical stability
from seconds to hours has been reported for various acenes,
and it depended, primarily, on the acene structure, presence or
absence of oxygen, water and adsorbents as well as on the
volume of photon energy delivered to the sample by various light
1
acene and the acene S state quenching by molecular oxygen.
3
, 11-24
sources.
For the acenes, exposed to the low energy light
sources, an increase of their light resistance to days has been
observed. The increase effect has also been achieved by using
stable radicals and utilization of the radicals characteristics to
25
scavenge the energy of the photoexcited state of acenes.
[
a]
Prof. P. Bałczewski, Dr. E. Kowalska, Dr. J. Skalik, Dr. K. Owsianik,
Dr. M. Koprowski
Division of Organic Chemistry, Group of Functional Materials
Synthesis
Centre of Molecular and Macromolecular Studies, Polish Academy
of Sciences
Sienkiewicza 112, 90-363 Łódź (Poland)
E-mail: pbalczew@cbmm.lodz.pl
Prof. P. Bałczewski, Dr. E. Różycka-Sokołowska, Dr. B. Marciniak
Faculty of Mathematics and Natural Sciences, Institute of Chemistry,
Health and Food Sciences, Department of Material & Structural
Chemistry
Jan Długosz University in Częstochowa
Armii Krajowej 13/15, 42-201 Częstochowa (Poland)
Dr. D. Guziejewski, Prof. W. Ciesielski
Department of Inorganic and Analytical Chemistry
University of Łódź
Finally, photostability of acenes has significantly been enhanced
when the acene was produced in a polymeric matrix.
17
Further important factor determining the photochemical
1
0
stability of the fused aromatic materials is presence of water
13,14,17,18,20,22,26
and oxygen.
either electron transfer (Type-I) or energy transfer mechanisms
Type-II) and both types are believed to operate simultaneously,
whenever the acene is excited by irradiation (cf. our
Acenes can react with oxygen via
(
[
[
b]
c]
4,11
investigations).
The oxidation process, as mentioned,
primarily leads to endoperoxides which may reproduce acenes
11,27
upon thermolysis or photolysis.
For some acenes, like
anthracene, photodimerisation occurs in the presence of air,
especially in dilute solutions, in competition with photooxidation,
leading to the corresponding endoperoxide. However, continued
irradiation of anthracene and the endoperoxide in solution
Pomorska 163, 90-236 Łódź (Poland)
Supporting information for this article is given via a link at the end of
the document.
13
ultimately leads to the corresponding dione.
The photostability has also been tested under anaerobic
1
,28
conditions.
In these studies, photostability of acenes was
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Chemistry - A European Journal
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FULL PAPER
2
1
13,15 1
53
55
monitored by 50% decay of absorbance or fluorescene,
H-
Cl,
NO
2
);
Br/RS
double
exchange
in
9,10-
and
17
15
52,56
57
NMR spectra and use of the HPLC technique.
dibromoanthracene,
bisanthryldisulfides, arylation of anthracenethiols, as well as
conversions of anthraquinones,
dithioanthraquinones leading to mono- or di-RS-substituted
anthracenes.
The above synthetic review and mentioned benefits from
the presence of RS-groups in acenes, show a need for
elaboration of new synthetic methodologies based on the
controlled introduction of RS-groups in the desired positions of
acenes which would enable: 1) a general access to both mono-
RS-acenes and mono-RS-heteroacenes, 2) an access to
significantly substituted derivatives, and 3) an access to larger
mono-RS-substituted derivatives containing mixed arene and
heteroarene fused rings. In this work, we are proposing such a
general approach to stable mono-RS-substituted (hetero)acenes
6 containing 3 or 4 fused rings with a potential to a further
extension of (hetero)aromatic π-conjugated systems.
alkylation of anthracenethiols
58
59
Another factor determining acenes photostability is their
chemical structure including a kind and a number of fused rings,
a presence of ring heteroatoms, steric hindrance and electronic
character of substituents. Thus, electron-withdrawing and alkyne
substituents, which enhance photostability, usually lower the
LUMO energy in acenes. The lower LUMO orbital makes
photoinduced electron transfer from the excited acene to O
energetically favorable and results in reduced rate of
photooxidation via the electron transfer to oxygen mechanism
44,55
48,56,49,58,60
anthrones
or
6
1
2
less
a
(
Type I) as well as results in the acene low triplet energy, which
prevents singlet oxygen sensitization (Type II).
3
,28
The suitable substituents to increase photostability, such
2
9-31
32
29
as 2-silylethynyl,
2-phenylethynyl,
2-t-butylethynyl,
2-
4
9,33
34
phenylethenyl, heteroaryl (2-thienyl),
trifluoromethyl, (4-
9
35
trifluoromethyl)phenyl,
pentafluorophenyl,
cyano, alkoxy, and o,o-dialkylphenyl, may be
introduced to acene using a variety of synthetic strategies. The
most impressive substituent effect was observed with alkylthio-
and arylthio- substituted pentacenes that were considerably
more resistant to photooxidation than all other pentacene
halo (Cl, Br,
2
3,34,36
34
37
’
38
F),
30
Results and Discussion
2
3,38
derivatives tested.
Similarly, a combination of arylthio- and
Synthesis of (hetero)acenes 6.
bulky o,o’-dialkylphenyl substituents, dramatically enhanced
The new approach involves, as a key step, an electrophilic
3
9
photooxidative resistance of heptacenes.
In fact, the
cyclization
dithioacetalaryl(aryl)methyl thioethers 5 to give (hetero)acenes 6
substituted by various RS-groups (R = Ph, C CH , 4-MeO-
, 2-Naphthyl, n-C12 25). They are precisely located at the
newly formed benzene ring which is fused to two
hetero)aromatic moieties, derived from (hetero)aromatic
aldehydes (Scheme 1). This synthetic concept is, in fact, a new
of
nonconjugated
precursors,
i.e.
ortho-
sulfenylated acenes turned out to be considerably better than
silylethynyl substituents at enhancing photooxidative resistance
6
H
4
2
2
3,38
of larger acenes.
C
6
H
4
H
A
review of known RS-substituted acenes, such as
3
9,40
40
23,40-42
38
43
anthracenes,
tetra-, penta-,
hepta- and nonacenes
(
revealed acenes with two (from anthracene to heptacene, vide
supra), eight and ten (both for nonacene, vide supra) RS
substituents. The RS-substituted acenes showed not only
6
2
thio-variant of the Friedel-Crafts/Bradsher reaction involving
6
3
sulfur rich reagents.
4
0
38
increased chemical and photochemical stability but also
Realization of our strategy started with protection of the
aldehyde 1 using 1,3-propanedithiol in refluxing benzene to give
the 1,3-dithiane 2 in 82% yield following the procedure of
2
3,38,40,41,43-45
useful optoelectronic properties.
The discussed role of RS-substituents in sulfenylated
acenes shows that alkyl-S and aryl-S groups or their
combinations with other substituents may be considered as
6
4
Takano et al. Then, the Br/Li exchange reaction with n-BuLi in
the latter and condensation of the resulting aryllithium with
various (hetero)aromatic aldehydes 3a-g, afforded ortho-1,3-
dithianylaryl(aryl)methanols 4a-g in good 52-72% yields
alternative or complementary substituents to the mentioned
’
silylethynyl,
o,o-dialkylphenyl,
halo,
cyano,
alkoxy,
6
4
(
fluoro)alkyl/aryl or heteroaryl groups in the design of new
(
Scheme 1, Table 1). Only three examples of the latter have
6
4,65
optoeletronic materials. On the other hand, systematic
investigations on stability and properties of the mono-RS-
substituted (hetero)acenes have not been carried out, so far.
The synthesis of mono-RS-substituted acenes may be
realized via their direct sulfenylation only in a limited number of
positions or via step-by-step introduction of RS-groups in the
desired positions and in the controlled manner. The latter
method is helpful, especially in the syntheses of larger, less
stable (hetero)acenes but it has not been applied to the
synthesis of mono-RS-(hetero)acenes. The 1,4-quinone
methodology combining the above two methodologies, turned
been described in literature, based on this sequence, so far.
Next, the diarylmethanols 4a-g were effectively transformed into
unknown thioethers 5 with various aliphatic, benzyl and aryl
thiols in ethyl acetate, in the presence of stoichiometric amount
2
of p-TsOH·H O, at room temperature (Scheme 1, Table 2; Table
S1, see SI). Although the crude thioethers 5 were formed in
good yields, a significant loss of materials was observed during
their purification with column chromatography over silica gel or
preparative TLC. The thioethers 5 were also sensitive to
chlorinated solvents (CHCl , CH Cl ). In order to avoid the loss
3 2 2
of materials during the silica gel purification, realization of the
final step of the thio-Friedel-Crafts/Bradsher cyclization was
38,43
out to be the most versatile, so far.
The direct synthesis of
RS-substituted acenes is limited to smaller anthracenes only
carried out using crude 5 as substrates and the FeCl
equiv.)/KI (1.2 equiv.) reagents system in refluxing EtOH. After
.5-24 h, the mono-RS-substituted (hetero)acenes were
3
(1.2
and may be realized via the following transformations:
4
6
sulfenylation of the middle, the most reactive anthracene ring
0
6
4
7
48,49
44
50-54
the X/RS exchange reactions (X = OH, OMe,
OAc, Br,
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Scheme 1. Synthesis of mono-RS-substituted (hetero)acenes 6 from ortho-bromo substituted aromatic aldehydes 1.
obtained in 38-62% and up to 43% total yields after two reaction
steps (4→5→6), according to the analogous mechanism which
was proposed by us for the oxygen version of the Friedel-
Crafts/Bradsher cyclization reaction (for the mechanism of the
diarylmethanol 10 with PhSH under standard acidic conditions
(PhSH, p-TsOH, RT, AcOEt) gave only trace amounts of the
desired product 11. Introduction of longer alkyl chains in alkoxy
groups should increase solubility of 12 and 13.
66
sulfur variant, see SI). It is worth mentioning that in some
cases, when cyclization reactions were carried out in smaller
scales, the products 5a, 5b and 5d precipitated after the
standard work up procedure as white solids and they did not
require a further purification. The crude products 6, synthesized
in bigger scales (>1.0 mmol), required purification with column
chromatography over silica gel (Table 2). The thioethers 5a, 5b,
Table 1. Synthesis of a series of ortho-1,3-dithianylaryl(aryl)methanols 4a-g.
Aromatic
aldehyde 3
ortho-1,3-
Dithianylaryl(aryl)methanol 4
Yield
[%]
a
52
5e-g cyclized most effectively to give pure (hetero)acenes in
good 50-62% yields (31-43% in the variant after two steps). The
latter also precipitated from reaction mixtures and did not require
further purification. The remaining thioethers 5c, 5d and 5h
cyclized to 6 less effectively (<46%) due to instability of the
substrates 5 rather than products 6 under the reaction conditions.
Cyclization of 5d to 10-S-benzyl anthracene 6d was
accompanied by formation of the corresponding 10-ethoxy
derivative (MS-EI, m/z = 356) which was formed in 11% yield, as
a result of the S-CH Ph/OEt (from EtOH) group exchange,
2
occurring via the corresponding intermediate dibenzylic
carbocation.
In our further studies, we also considered an interesting
approach to di-RS-substituted pentacenes 14 by a double thio-
Friedel-Crafts/Bradsher cyclization of dithioethers 13 starting
from 2,5-dibromobenzene-1,4-dicarbaldehyde 7 (Scheme 2).
5
5
68
72
6
7
68
Thus, acetalization of the dialdehyde 7 with 1,3-propanediol,
62
66
45
69
afforded the corresponding diacetal 8 in excellent 99% yield.
7
0
Next, bis(trans-thioacetalization)
propanedithiol gave the bis(dithioacetal) 9 also in 99% yield
Scheme 2). The direct bis(dithioacetalization) of the dialdehyde
of the latter with 1,3-
(
7
with 1,3-propanedithiol failed. Surprisingly, lithiation of 9 with
2.2 equiv. of n-BuLi followed by condensation with 3,4,5-
trimethoxybenzaldehyde afforded
bis(diarylmethanols) 10 and 12, respectively in low 13% yields
Scheme 2). Optimization of the reaction conditions did not
a mixture of mono- and
(
improve the yield of the desired diol 12. Due to a low solubility
of 12 in common organic solvents, the OH/SPh exchange could
not be realized. On the other hand, the reaction of the
[
a] Yields of isolated products.
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Table 2. ortho-1,3-Dithianylaryl(aryl)methyl thioethers 5 and (hetero)acenes 6.
Alcohol
4
Yield of 5
[%] /[%]
(Hetero)acene
Yield of 6
[%] /[%]
RSH
Thioether 5
a
b
Reaction time
d
e
6
4a
51/73
30 min
62/43
4
a
a
42/71
45/-
30 min
45 min
51/42
46/8
4
4
a
c
28/51
45 min
38/6
OMe
S
S
O
O
O
O
d
e
4
32/-
90 min
50 /31
OMe
6e
S
S
5e
O
O
O
S
S
O
O
O
O
O
S
4
d
28/-
54/-
6f
60 min
60 min
52/15
57/32
S
5f
OMe
OMe
S
S
O
O
O
O
S
4f
S
S
S
6g
5
g
OMe
OMe
S
O
O
O
S
O
N
[
c]
Me
4g
-
S
24 h
-/7
6h
N
5
h
S
Me
[
a] Yield of 5 after purification by column chromatography. [b] Yield of 5 after purification by precipitation. [c] Not isolated and directly subjected to the cyclization
step. [d] Yield of after purification by column chromatography. [e] Yield of isolated after two steps (4→5→6).
6
6
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Scheme 2. Synthesis of substrates 10-12 for the double thio-Friedel Crafts/Bradsher cyclization.
71
Photophysical, electrochemical, thermal and photochemical, exponential decays in the least squares analysis. The value of
2
properties of mono-RS-(hetero)acenes 6.
χ is the sum of squares deviations between the measured and
expected values, each divided by squared deviations expected
for the number of detected photons. The lifetimes were relatively
Absorption and emission steady state measurements: Optical
properties of mono-RS-substituted (hetero)acenes 6a-g were
measured in six solvents (ethanol, tetrahydrofuran, ethyl acetate, short in nonpolar solvents, such as cyclohexane (Cy) and
cyclohexane, toluene, acetonitrile) (Tables 4-10, see SI).
The absorption spectra of the anthracenes 6a-d, with
identical alkoxy substituents and different SR groups (6a-SPh,
toluene. For 6d and 6f, two lifetimes, one short (τ
(τ ), were measured (6d - τ /τ = 6; 6f - τ /τ = 11, both in Cy). The
radiative k (Fig. 3) and nonradiative knr (SI) rate constants for
the selected mono-RS-substituted anthracenes 6a, 6b, 6c and
1
) and one long
2
2
1
2
1
r
6b-S-2-Naphthyl, 6c- S-Alkyl, 6d-S-Benzyl) had the same
6
9
absorption pattern in the range of 310-425 nm with only two
absorption maxima at λmaxabs= 380/400 nm. In case of the
anthracene 6e, absorption spectra in the above solvents
exhibited four absorption maxima at λmaxabs= 345/365/380/400
nm, while in case of 6g, containing four fused rings, only two,
low lying maxima at λmaxabs= 345/377 nm in the range of 310-450
nm, were observed. The fluorescence spectra of 6a-f showed a
blue emission in all six different solvents (vide supra) with
emission maxima in the ranges of 435-457 nm for 6a-d, 436-440
nm for 6e, 403-424 nm for 6f and 385-401 nm for 6g (Table S4-
6e were also calculated from the measured Φ and τ values.
The resulting rate constants were then plotted as a function of
N
N
the normalized polarity parameter E
T T
(Fig.3, SI).The E values
are dimensionless numbers which correspond to scale ranges
from 0.000 for TMS (tetrametylsilane) as the least polar solvent
7
2
to 1.000 for water, the most polar solvent. As shown in Fig. 3,
values slightly decrease with the increasing solvent polarity for
k
r
all mono-RS-substituted anthracenes 6a-c, 6e, whereas knr are
almost independent of the kind of solvent (SI).
All mono-RS-substituted anthracenes 6a-g emitted blue
light with the following coordinates (toluene) corresponding to
the CIE 1931 color space chromaticity diagram: 6a (0.153,
0.089), 6b (0,150, 0.086), 6c (0.153, 0.076), 6d (0.168, 0.283),
6e (0.163, 0.107), 6f (0.153, 0.034) and 6g (0.169, 0.004) (Fig.
4).
10, see SI). The Stokes shifts of 59-78 nm for 6a-d revealed
weak interactions between the six solvents and the excited
molecules 6a-d (Tables S4-10, see SI). Absorption and emission
bands values for (hetero)acenes 6a-g in two selected solvents
(
EtOH and toluene) are summarized in Table 3 (for the
remaining four solvents, Tables S4-10, see SI). Emission and
absorption spectra for 6a-g in these solvents are visualized in
Fig. 1 and 2. Analysis of these spectra in the above solvents
showed neither absorption nor emission solvent effects (Tables
S4-10, see SI).
Time-resolved measurements: In addition to the steady state
measurements, the time-resolved experiments also were carried
out. Table 4 presents the fluorescence lifetimes (τ) of acenes 6a-
f and the fluorescence quantum yields Φ.The highest Φ values
of 42.9% (THF) exhibited 6a with the SPh substituent and 6c
Cyclic voltammetry: Cyclic voltammetry measurements were
carried out in Bu N PF (0.1 mol L )/dichloromethane solution
4 6
+
-
-1
to determine the HOMO and the LUMO energy levels of 6a-c, 6e
and 6g (Fig.5). The oxidation potentials of these compounds
were located in a similar potential range except of 6e and 6g
which could be due to a significant change of fused rings
structure and their surroundings. The reversible oxidation of the
compounds (i.e. 6a) was accompanied with a subsequent
chemical reaction (i.e. 6g) while the reduction processes
proceed towards the molecule decomposition and therefore
results in the recorded irreversibile processes. The molecular
frontier orbital levels were estimated from the electrochemical
with the SC12
showed the lowest quantum yields <1% in all solvents. Lifetimes
τ) of 6a-f were also measured in the above six solvents (SI) for
H25 substituent (32.7%,THF). The compound 6f
7
3
(
analysis using a method described by Sun and Dalton. In order
to calculate the absolute energies of LUMO and HOMO, the
redox data were standardized to ferrocene/ferrocenium couple
2
which the chi-square values χ are given in Table 4. These
values judge the quality of the two- or one-component fit
between the measured data and the calculated single or multiple
74
which has
a calculated absolute energy of -4.8 eV.
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Chemistry - A European Journal
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Table 3. UV/Vis absorption and fluorescence properties of 6a-g in EtOH and
toluene solutions.
a
a
EtOH
Toluene
b
b
λ
maxabs
λ
maxem
υ
λ
maxabs
λ
maxem
υ
Comp.
[
nm]
[nm]
[nm]
[nm]
[nm]
[nm]
6
6
6
6
a
b
c
d
379, 399
379, 400
376, 395
378, 397
442
444
436
440
63
65
60
62
380, 401
380, 403
377, 398
379, 401
445
447
65
67
419, 439 62
457
440
78
73
3
3
45, 365,
80, 401
345, 367,
383, 403
6e
437
405
72
23
Figure 1. Normalized absorption (solid line) and emission (dotted line) spectra
of 6a-g in EtOH.
6f
362, 382
345, 377
364, 384
348, 379
424
392
40
44
6g
385, 397 40
[
a] Absorption and fluorescence spectra recorded in EtOH and toluene
-5
solutions (1 x 10 M), [b] Stokes shift
Table 4. Fluorescence quantum yields and lifetimes of 6a-f in different
solvents.
Solvent
EtOH CH
3
CN AcOEt
THF Toluene Cyclohexane
N (a)
E
T
0.65
4
0.460
0.228 0.207
0.099
0.006
(
b)
Φ
33.3
6.08
1.14
16.6
2.24
1.01
17.1
5.27
1.12
39.2
7.20
1.25
15.3
2.69
1.12
22.7
6.06
1.23
2.5
34.1
6.29
1.26
16.3
2.42
1.13
19.7
5.11
1.15
1.9
42.9
6.83
1.11
16.5
2.63
1.07
32.7
5.77
1.08
10.6
5.78
9.34
1.20
15.0
2.24
1.56
0.16
0.53
2.48
1.05
31.5
5.43
1.03
18.3
2.23
1.17
23.5
4.98
1.10
3.1
34.8
5.17
1.10
15.6
2.02
1.00
26.4
4.58
1.09
1.8
6a
6b
6c
τ [ns]
2
(c)
χ
Φ [%]
τ [ns]
Figure 2. Normalized absorption (solid line) and emission (dotted line) spectra
of 6a-g in toluene.
2
χ
Φ [%]
τ [ns]
2
χ
Φ [%]
0.5
τ1 [ns]
1.00
6.16
1.15
8.04
1.36
12.2
1.97
1.10
0.05
0.71
3.14
1.05
1.23
7.30
1.53
13.1
2.04
1.02
0.05
0.44
2.36
1.08
2.33
10.08
1.56
15.0
1.94
1.10
0.63
0.23
2.17
1.12
1.21
7.30
1.09
13.5
1.88
1.12
1.39
0.28
3.19
0.89
6d
6e
6f
τ [ns]
2
2
χ
1.01
12.9
2.10
1.06
0.10
Φ [%]
τ [ns]
2
χ
Φ [%]
τ1 [ns]
0.47
2.81
τ [ns]
2
2
χ
1.24
7
2
[
a] Normalized polarity parameter ; [b[ Quantum yields Φ were measured with
-5
an integrating sphere in the solvent indicated at μM concetration (1 x 10 M);
Figure 3. Radiative rate constants plotted against
(for the non-radiative
2
[
c] χ – CHISQ (chi square).
rate constants see SI).
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Table 5. Cyclic voltammetry data, HOMO-LUMO levels and energy gaps Eg
values of selected acenes 6.
E
ox
Cmpd.
E
HOMO (eV)
E
red (V)
E
LUMO (eV)
g
E (eV)
(V)
6
a
b
0.704
0.697
-5.485±0.054
-5.478±0.042
-2.562
-2.541
-2.219±0.059
-2.240±0.045
3.266
3.238
6
6
c
e
0.653
0.800
-5.434±0.027
-5.581±0.044
-2.657
-2.500
-2.125±0.040
-2.281±0.047
3.309
3.300
6
6g
0.939
-5.720±0.042
-2.754
-2.028±0.060
3.692
Figure 4. The Commission Internationale de L’Eclairage (CIE) 1931
chromaticity coordinates for mono-RS-substituted anthracenes 6a-g in toluene
solution.
Figure 6. Evolution of the lower energy band absorptions of 6a-c, e-g in EtOH
-
5
(
10 M), under the UVP-Pen-ray lamp exposure (254 nm) at room temperature
under ambient atmosphere
.
Figure 5. Cyclic voltammograms of selected compounds (concentration of 1
-
1
mmol L ) recorded at a glassy carbon electrode in dichloromethane solution
+
-
containing 0.1 M Bu
blank.
4 6
N PF as supporting electrolyte after subtraction of a
All the measured redox potentials, calculated HOMO/LUMO
energy levels and energy gaps Eg values are shown in Table 5.
Thermal properties: Thermal properties of the mono-RS-
substituted
anthracenes
6
were
determined
by
thermogravimetric analysis (TGA) and differential scanning
calorimetry (DSC). Generally, these compounds decomposed in
the range of T = 331-354 C, i.e. by average 25 C (297-331 C)
d
o
o
o
Figure 7. Evolution of the lower energy band absorptions of 6a-c, e-g in EtOH
-
5
(
10 M), under the 6 W Tube lamp exposure (365 nm) at room temperature
higher than the reported in literature di-RS-substituted acenes.
o
under ambient atmosphere.
The highest decomposition temperature (354 C) was observed
o
for 6g and the lowest one (331 C) for 6b. Solid materials should
o
o
m m
C and 6g - T = 186 C, while for others T were differentiated:
also exhibit high melting points which depend on chemical
structure and a crystal packing pattern. For some mono-RS-6,
DSC curves showed advantageous melting points: 6f - T = 243
m
o
o
68
o
6
6
b - T
m
=171 C, 6 I - T
m m
= 153 C , 6c - T = 51 C. In case of
o
o
o
a, three peaks (T
m
= 108 C, 129 C, 137 C); 6e, two peaks
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o
o
o
o
(
1
T
m
= 164 C, 180 C) and 6d, three peaks (T
m
= 49 C, 90 C,
17 C) were observed which might indicate formation of
different polymorphic structures. For the unfunctionalized di-RS-
substituted anthracenes, reported in literature (R = n-C12
sulfoxide and/or oxidation of the anthracene 10-C carbon atom
to ketone. In case of 6a and 6d, the SBn/OEt and OMe/OEt
replacements products due to the solvent used (EtOH), were
also detected (see SI).
Stability to photooxidation of compounds 6a-6g upon
exposure to the UV light at 365 nm under ambient atmosphere
was obviously much higher than the stability upon exposure to
the UV light at 254 nm. For instance, the 50% absorbance decay
for 6f was at least 7 times higher at 365 nm than at 254 nm (812
min. at 365 nm versus 115 min. at 254 nm). Further
photostability testing under inert atmosphere at 365 nm were not
continued due to very long half-lives, significantly exceeding the
highest values obtained under ambient conditions.
o
H
25, n-
4
4
C
3
H
7
, C
6
o
H
5
, 2-Naphthyl), melting points gradually increased
o
o
o
from 69 C, 109 C, 213 C to 239 C, respectively.
Interestingly, that the RO-disubstituted anthracene (R= n-
25) also melted low, i.e. at 50 C, revealing a trend to the
lowering of the anthracenes melting points by long alkyl chains
o
42
C
12
H
5
0
o
44
(
R= n-C12H25: di-RO, 50 °C ; 6c-RS, 51 C; di-RS, 69 °C).
Photochemical stability: The photostability of mono-RS-
substituted anthracenes 6a-6g was investigated by monitoring
the 50 % absorbance decay of the lower energy bands.
Ethanolic solutions (10 M) were stored in dark at room
temperature under inert and ambient atmosphere and then
exposed to the UV light sources of 254 nm (UVP-Hg-Pen-ray
lamp, 16.33 mW/cm at distance 1 cm) (Fig. 6) and 365 nm
fluorescent lamp VL-6.LC (6W), 27.4 mW/cm at distance 1 cm)
Fig. 7).The actual irradiation dose (J/cm ) may be calculated by
-5
The various reaction conditions used in literature in the
photostability tests and a large number of the variables, make it
difficult to compare photostabilities of acenes originating from
different literature sources. Thus, our results may be compared
with the results obtained under similar photostability testing
2
2
(
(
2
conditions (244/366 nm, CHCl
3
) for anthracenedithiophenes
2
multiplication of the latter (W/cm ) by exposure time (sec.). The
observed photostability was as follows: for 254 nm under
ambient atmosphere: 6b (249 min.) > 6e (210 min.) > 6a (166
min.) > 6g (153 min.) > 6f (115 min.) > 6c (109 min.) and for 254
nm under inert atmosphere: 6b (519 min.) > 6e (276 min.) > 6a
having extended anthracene core for which half-lives (t1/2) of
11,27
only 25–58 s have been reported.
Photostability of 9-AcO-
1
0-(4-AcO-C )-substituted anthracene has also been reported
6 4
H
14
on a seconds level at 254 nm in chloroform. The half-life of the
significantly substituted 10-benzyloxy-6-bromo-7-formyl-1,2,3-
trimethoxy-anthracene, obtained by us earlier, was 40 - 90 times
(
175 min.) > 6c (115 min.). Generally, at 254 nm under both
conditions, the most stable were anthracenes with bulky S-2-
Naphthyl groups. The remaining anthracenes 6a, 6c, 6f and 6g
higher than the reported t1/2 values for the above anthracenes
59
(t1/2= 40 min., 370 nm, chlorobenzene). For a comparison, half-
6 4
with SPh, S-4-MeO-C H and SC12H25 groups were stable but
lives (t1/2) for the unsubstituted anthracene in aqueous solution,
measured as 50% fluorescence abatement, were less than 4
significantly less stable than 6b and 6e with the 50
%
absorbance decay in the range of 109 - 166 min. A comparison
of 6b and 6e showed another trend to a bit greater stability of
sterically crowded, more substituted anthracenes with three
methoxy groups versus anthracenes with only one methoxy
group (air, 254 nm: 240 min. versus 210 min.), despite the
destabilizing electronic effect of methoxy groups (vide infra).
Significantly bigger differences in the photostability were found
for 6b and 6e under inert atmosphere (Ar, 254 nm: 519 min.
versus 276 min.) Interestingly that stability under air and inert
atmosphere at 254 nm for 6a and 6c was comparable (166, 109
and 175, 115 nm, respectively). Several isosbestic points,
observed in the UV/Vis spectra of 6a-6g (see SI), indicated the
coexistence in solution of the starting materials and their
oxidized or degraded products.
59
min. at 254 nm and less than 8 min. at 350 nm. In another
2
polar medium H O/ACN= 70/30, t1/2= 10 min. (the 310–390 nm
72
range, with a maximum intensity at 350 nm). These data were
13
confirmed by Lehto et al. who reported for the parent
anthracene a photostability of 3.41-4.50 min. at pH 5.6 and 7.6
(
254 nm) under nitrogen, synthetic air or oxygen as purging
gases.
As a concluding remark of this subsection, one may state
that the obtained half-lives for the most photostable mono-RS-
substituted anthracenes 6b and 6e (R= 2-Naphthyl) at 254 nm
were at least 60-70 times higher under ambient, air atmosphere
in polar media than the half-life for the unsubstituted anthracene
13
(t1/2= 3.41 min. at pH 7.6). The change of air atmosphere for
the inert gas (argon), at the same testing conditions, revealed
Indeed, irradiation of ethanolic solutions of 6a-d,f for 3h in
that 6b (t1/2= 519 min.) was c.a. 130 times more photostable
2
each case (air, 254 nm, irradiation dose 176 J/cm ) and
13
than the unsubstituted anthracene (t1/2= 4.09 min. at pH 7.6 ),
subsequent HRMS-(+)-APCI analysis of the resulting crude
reaction mixtures, confirmed the presence of the starting acenes
accompanying with various photo-degradants. The characteristic
feature of the latter is maintanance of integrity of all anthracene
rings without C-C bonds cleavage and preservation of the
longest C-S bonds in most cases due to the C-S bond
i.e. it increased its photoresistance twofold. Similar differences
were observed for the half-life of the most photostable RS-acene
6 4
6f at 365 nm (R= 4-MeO-C H , t1/2= 812 min., air), which was
about 80-100 times higher than the half-life of the unsubstituted
59,72
anthracene (t1/2= 8-10 min.)
media. A further comparison of the photostability of the electron-
rich anthracene 6f (t1/2 812 min.) with electron-rich
anthracenedithiophenes possessing extended anthracene core
under air atmosphere in polar
1 1 0
strengthening in S and T excited states compared to S , as in
=
case of 6a (vide infra). In the mixtures, two main groups of
photo-products may be distinguished. They originate from: 1)
addition of oxygen to the anthracene rings to initially form
endoperoxides or hydroperoxides (not detected) which
decompose to cyclic epoxides, phenols or are further oxidized to
quinones of 1,2- and/or 1,4-type, 2) oxidation of sulfur atom to
11,27
(
t
1/2= 25-40s),
showed that 6f is at least 1200-1900 times
more stable at 365 nm. Finally, the literature 6-RS-pentacenes
under mild photochemical conditions (air, ambient light) revealed
76
t
1/2= 29 min. for R= SPh and t1/2= 43 min. for R= SAc, which is
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3
Σ
g
O
2.
The resulting superoxide radical anion and the
corresponding acene radical cation are prone to further
degradative reactions. In the alternative, i.e. energy transfer
mechanism, initiated by the intersystem crossing (ISC) from the
1 1
S to T state, the triplet acene reacts with the triplet oxygen to
give singlet oxygen and acene in the ground state (Scheme 3).
Singlet oxygen is not a radical, but the energy of its molecule is
Scheme 3. Two possible mechanistic pathways of the acene photooxidation
much higher than energy of oxygen in the triplet state. There are
1
two forms of the singlet oxygen: one, ( Δg O
2
), has two paired
1
electrons on one orbital π* 2p and the other form, sigma ( Σg
), has one electron on each of the π* 2p orbitals. Energy of
O
2
the singlet oxygen sigma is higher (157 kJ/mol) than energy of
the singlet oxygen delta (94 kJ/mol).
In order to explain the relatively high photostability of mono-RS-
acenes 6, we carried out DFT calculations of electronic
structures of compounds 6a and 6e-6h in the gas phase, using
the gradient corrected three-parameter hybrid functional
(
B3LYP) with the 6-311++G(d,p) basis set. Full geometry
optimizations of these compounds in the ground state (S ) were
0
performed using the GAUSSIAN 09 quantum chemistry package.
Analysis of shape and values of the HOMO/LUMO orbitals was
carried out for five mono-RS-acenes derivatives and 6,7-(1,3-
methylenedioxa)-anthracene (6i), and 6,7-(1,3-methylenedioxa)-
9-phenylthio-anthracene (6j), treated as the reference
compounds (Fig. 8). Moreover, the TD-DFT calculations at the
same level of theory, were carried out for 6a as a representative.
First, the electronic excited states in the gas phase were derived,
then geometries of the first singlet (S
states were optimized and finally their excitation energies were
calculated. Figure 8 illustrates the optimized geometries of S
and T states for 6a. In each of these states, the anthracene
An) skeleton is essentially planar, and all oxygen atoms and
also C-21 carbon atom, are nearly coplanar with the plane
formed by this skeleton. In the S state, the deviations of
1 1
) and triplet (T ) excited
0
,
S
1
1
(
0
aromatic carbon C3-...-C16 atoms from this plane are relatively
insignificant (they lie in the range from -0.028 Å to 0.0123 Å) and
increase when passing to the S1 and T1 geometries (the
deviations of C atoms from the An plane lie in the ranges from -
0 1 1
Figure 8. Views of the optimized geometries of S , S and T states for 6a,
with the calculated bond distances (in Ångstroms) and its molecular structure
with the atomic numbering scheme (H atoms are removed for clarity).
0
.0578 Å to 0.0529 Å and from -0.0330 Å to 0.0305 Å,
respectively). The dihedral angle between the planes of An and
phenyl rings is 88.226° in the S state and it decreases to
4.245° and 79.637° for S and T states, respectively. The
a result of lowering HOMO and LUMO energy levels in the
anthracene substituted with a stronger (SAc versus SPh)
electron withdrawing group. Interestingly that 6,13-bis-PhS-
pentacene lived 40 times longer than 6-PhS-pentacene under
the same conditions, most probably due to the substitution of
both reactive positions in pentacene that hinders formation of
endoperoxide. The described differences in half-lives of acenes
are surprisingly large and, indeed, reinforce the need to further
study and understand substituent effects in acenes.
0
7
1
1
decrease of this angle is connected with a change of the bond
distances between C6 and sulfur atoms, which in the S1 and T1
geometries are shorter by 0.025 Å and 0.013 Å, respectively and
thus stronger. The strengthening of the An-S bond with the
retained length of the Ph-S bond in S and both excited states, is
0
one of the factors responsible for a higher photostability of 6a. It
was also confirmed for other photoproducts derived from 6a-d,f
7
6
(
HRMS-(+)-APCI analysis) which retained the C-S bond
untouched upon irradiation (254 nm) at ambient atmosphere
vide supra and SI).This result is rather surprising since in the
The DFT and TD-DFT calculations of electronic structures of
mono-RS-6 and explanation of their high photostability.
Generally, there are two mechanisms by which acenes, such as
anthracenes 6, react with oxygen in the presence of light
(
excited states, the C-S bond as the longest one in 6, is expected
to further lengthen until breaking (true for the formation of 1,4-
quinone).The most noticeable change in the bond distances
(
Scheme 3). In the first, i.e. electron transfer mechanism, the
excited singlet acene at the S state is an electron donor to
molecular oxygen, which in the ground state is a biradical triplet
when passing from the S
0
state to the excited states regards to
1
the C-C bonds of the peripherial aromatic rings of the An
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skeleton of 6a (Fig. 8). In case of the two excited states, the
largest differences (i.e. larger than 0.030 Å) with respect to the
S state geometry have been found for the C8-C9, C9-C10, C10-
0
C11, C3-C4, C13-C16 and C15-C16 bonds. These differences
are c.a. 0.01 Å larger in case of the T1 state and additionally
they concern the C5-C6, C6-C7 and C13-C14 bonds, as well.
Longer C-C bonds in peripherial rings were responsible for their
greater reactivity and formation of ketones (6b, 6d) and 1,2-
quinones (6c) upon UV irradiation (254 nm) at ambient
atmosphere (HRMS-(+)-APCI analysis,SI).
As Figure 9 illustrates, HOMO and LUMO orbitals for each
molecule, are mainly located on the anthracene core with
exception of 6f, for which the HOMO orbital is located also on
the 4-methoxyphenylthio group.
The geometries of LUMO orbitals of two (hetero)tetracenes
6g and 6h are very similar to each other, but the HOMO orbitals
vary from one another. In the molecule 6h, the HOMO, similarly
as LUMO, is located mainly on fused aromatic rings, whereas in
the molecule 6g, the HOMO is situated also on the 4-
methoxyphenylthio substituent, i.e. just as in 6f.
Therefore, the LUMOs for these molecules are independent of
the type of RS group and consequently a comparison of the
LUMO energy values (Fig. 9), calculated for the RS-
unsubstituted anthracene 6i and mono-RS-substituted
derivatives 6a, 6e and 6j, showed that (i) the presence of
phenylthio substituent in 6j resulted in the significant decrease of
the LUMO energy value from -1.845 eV (in 6i) to -2.118 eV
(
stabilizing effect), (ii) introduction of one, electron-donating
methoxy group at the 1-anthracene position in 6e, increased the
LUMO energy level to the value of -1.984 eV (destabilizing
effect), and (iii) introduction of next two methoxy groups at the
positions 2 and 3 in 6a caused a further increase of this energy
up to -1.895 eV (further destabilization). The same trend was
also observed for the HOMO energy levels in 6a, 6e, 6i and 6j.
A comparison of the LUMO energy values obtained for 6i and 6j
Figure 9. Plots of the HOMO and LUMO orbitals for molecules 6a and 6e-j
optimized at the DFT theory B3LYP/6-311G(d,p).
reaction (ETR) and the
crossing (EISC), respectively.
2
O -induced enhanced intersystem
77,78
A necessary condition for the
(
-1.845 eV versus -2.118 eV) resulted in an increase of
ETR to occur is the S -T energy gap (ES-T = ES1-ET1) higher
1 1
photostability of the latter due to the presence of the RS
substituent. Despite the presence of the destabilizing MeO
substituents in 6a and 6e, the LUMO energy values calculated
for these molecules (-1.895 eV and -1.984 eV, respectively) are
still lesser than calculated for 6i (-1.845 eV) due to the presence
of the stabilizing RS group, as well. Indeed, the experimental
values of photostability confirmed this relationship (6a (3xMeO,
PhS): t1/2 = 166 min., 6e (1xMeO, Naphth-S: t1/2 = 210 min.).
The HOMO energy levels calculated for compounds 6g and 6h
are lower, and the values of energy gaps are higher than those
calculated for practically all acene derivatives.
In summary of this part, one can conclude that the
presence of RS-substituents causes a clear stabilization of
mono-RS-acenes due to reduction of their LUMO energies. Thus,
the lower LUMO level results in the higher activation energy
which prevents the singlet oxygen sensitization and accounts for
a reduced rate of photooxidation in these materials due to a
than 94 kJ/mol. In the EISC, the oxygen molecule simply
enhances the rate of ISC of the acene molecule and it remains
unchanged in this process.
77-79
In our case, the excitation energies of S
1 1
and T states (ES1 and
T1
E )
calculated for the selected 6a (due to comparable
photostability under ambient and argon atmosphere), are 2.64
eV (255 kJ/mol) and 1.29 eV (124 kJ/mol), respectively. The ET1
value lies between the singlet-triplet energy gap (ES-T) for two
forms of singlet oxygen, and is higher by only 30 kJ/mol than
1
the energy of the singlet oxygen Δ
g
O
2
, and by 53 kJ/mol lower
1
than the energy of the singlet oxygen Σ
has sufficient energy to generate Δ
g
O
2
. Thus, the acene 6a
1
g
O
2
, and too low energy to
1
form Σ
g 2
O .
Taking into account that the S
1 1
-T energy gap calculated
for 6a (ES-T = 131 kJ/mol) is larger than 94 kJ/mol, quenching of
the S state by ETR and EISC is more likely than photooxidation
1
of 6a via the electron transfer mechanism for which activation
energy is 72 kJ/mol (Scheme 5). The compound 6a is
characterized by a high triplet energy, larger than 94 kJ/mol and
possible quenching of the excited state of acenes. (Scheme 4).
3
The quenching of the acene S
1
state by the Σg O
2
in excited
acenes, mainly leads to triplet acene and singlet oxygen or to
triplet acene and molecular oxygen via an energy transfer
relatively low oxidation potential (Eox = 1.390 V versus SCE). So,
1
6
a can quench Δg O
2
, mainly by the physical process, induced
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OMe
OMe
OMe
O
O
O
O
O
O
(
O
O
Acene:
OMe
(6i)
(6j) SR
6e) SR
(6a) SR
LUMO:
-1.845 eV
-2.118 eV
-1.984 eV
-1.895 eV
1S0
1S1
1S0
1S1
E (Acene I)
a
E (Acene II)
a
LUMO
LUMO
HOMO
hv
O2
O2
hv
Products of
photooxidation
HOMO
Energy
Products of
photooxidation
Energy
a
Scheme 4. Two hypothetical situations in photooxidation of acenes 6: a) increasing the LUMO level of the acene I accounts for a decrease in E (activation
energy), easier photooxidation and consequently the lower acene photostability (left), b) decreasing the LUMO level of the acene II accounts for the increase in E
and the higher acene photostability (right).
a
Photodegradation
products
Chemical
quenching
1(
electron transfer
6a)
3O2
(6a)
O2
ES1- ET1 > 94 kJ/mol
ETR
EISC
1
O quenching
1
1
1
1
3(
1O2
6a)
3O
energy transfer
2
[6a O ]
[6a O ]
2 CT
1O
2 EC
2
6a
2
by 6a
encounter
complex
exciplex
CT
e-v deactivation
Non-radiative solvent
1
induced relaxation of O2
ISC
3
3
3
6
a + O2
[6a O ]
2 CT
1
CT induced quenching of O2
(
ET1 = 124 kJ/mol, Eox =1.390 V vs SCE)
Scheme 5. Photooxidation of the acene 6a through the energy and electron transfer mechanisms, possible physical and chemical pathways of the non-radiative
1
3
g 2 1 2
singlet oxygen Δ O -deactivation by the acene-6a and deactivation of the acene S state by O .
by CT, in which the initially formed singlet encounter complex
photodegradation products. The latter pathway is well
documented for the compounds containing electron-rich
aromatic systems, amines and sulfides, and so it is also
possible in the case of 6a and other mono-RS-acenes 6. It is
confirmed by the presence of isosbestic points, observed in the
UV/Vis spectra of 6.
1
1
1
1
(
2 2
6a O )EC is transformed into a singlet (6a O )CT exciplex which
77
in turn can decay by the ISC to a triplet CT ground-state
3
3
3
(
6a O
physical CT process of the Δg O
with chemical quenching which
2
)
CT complex. Finally, it dissociates to 6a and
O
2
. The
deactivation may compete
leads to various
1
2
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Although 6a does not meet the conditions necessary for the
singlet oxygen quenching by the e-v mechanism in its pure form,
7
7
for which a high oxidation potential is required, the singlet
1
oxygen
g 2
Δ O can interact in solution with hydrogen atoms of
1
the solvent used (ethanol). Thus, Δ
g
2
O is effectively relaxed by
energy transfer from electronically excited oxygen to vibrational
levels of the solvent (non-radiative, solvent induced relaxation of
1
77,80
1
Δ
g
2
O ).
g 2
Taking into account that the Δ O lifetime in solution
depends on the type of solvent, and for the ethanol used by us is
80
relatively short (10 μs), in the case of 6a and the other mono-
RS-6, the e-v mechanism is highly probable.
In summary, it is worth emphasizing that the high photostability
of mono-RS-acenes 6 under ambient atmosphere is due to the
fact that the rates of photooxidation of these materials via both
electron and energy transfer mechanisms are reduced to a large
extent. The electron transfer from the excited singlet S
acene to molecular oxygen is hindered mainly by the effective
quenching of the acene S state via the ETR and EISC
processes. In turn, the most probable mechanisms involving the
1
state of
1
1
CT-induced quenching of
g 2
Δ O , chemical reactions and non-
1
radiative solvent induced relaxation of
energy transfer difficult.
g 2
Δ O also make the
Molecular and crystal structure of 2.
The molecular and crystal structure of the dithioacetal 2, as well
as the molecular Hirshfeld surface of this molecule, have been
performed in Fig. 10a-c. For the X-ray crystal structure
discussion and the Hirshfeld surface analysis, see SI. Other
compounds synthesized in the series 4, 5 and 6 revealed a
weak tendency to crystallization.
Conclusions
This study presents first examples in which the intriguing and
yet underexplored group of small, mono-RS-substituted
(
hetero)acenes with significant substitution, revealed the highest
in their class thermal and photochemical stability at 254 and 365
nm under aerobic and anaerobic conditions. The results were
supported by DFT and TD-DFT calculations which revealed the
leading role of the RS substituent in the acene stabilization.
Both these calculations and analysis of oxidation products
Figure 10. (a) A view of the two symmetry-independent molecules 2 (A and B)
with the atom numbering scheme and displacement ellipsoids drawn at the
30% probability level. (b) Part of the crystal structure, showing intermolecular
non-covalent interactions: all H atoms not involved in these interactions are
omitted for clarity. (c) the Hirshfeld surface of the molecules 2 (A and B)
mapped with dnorm distance, showing the most dominant interactions in crystal.
Symmetry codes: (i) x, -1+y, z; (ii) 1/2+x,1-y,z; (iii) -x, 1- y, -z; (iii) x, 1+y, z; (iv)
-1/2+x,1-y, z.
(
HRMS) showed that one of the factors responsible for
enhanced photostability of acenes is strengthening, in the
excited S and T states, of the An-S bond which in majority of
the analyzed compounds was not cleaved upon UV irradiation
254 nm) at ambient atmosphere. Moreover, the calculations
1
1
(
allowed to determine the three possible mechanisms of the
singlet oxygen quenching (CT-induced, CR, solvent induced
constitutes a further contribution to the fundamental knowledge
on intramolecular electrophilic aromatic substitution. The
protocol is open and enables synthesis of four or more fused
(hetero)aromatic rings by utilization of larger ortho-bromo
aromatic aldehydes which have recently been synthesized in our
lab. Moreover, mild reaction conditions give access to larger
and highly functionalized fused systems which could not be
obtained under harsh conditions of the original Friedel-
Crafts/Bradsher reaction. We believe that the findings herein will
relaxation) and two processes of the acene S
1
state quenching
3
by molecular oxygen
O
2
(ETR, EISC), which may be
responsible for the high photostability of acenes 6. This group of
blue emitters was obtained via novel variant of the Friedel-
Crafts/Bradsher (thio-F-C/B) reaction involving electrophilic
cyclization of unknown ortho-1,3-dithioacetalaryl(aryl)methyl
thioethers 5. The new approach complements both original
8
1
6
9
73,81,87
Friedel-Crafts/Bradsher protocol and its oxo-variant,
and
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spur further interest and development of next generation RS-
mono-substituted acenes and heteroacenes with significant
substitution for practical applications as optoelectronic materials,
organic ligands for photocatalysts, stable organocatalysts and
other thermally and photochemically resistant materials having
additional blue light emission characteristics.
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FULL PAPER
The role of RS-group: The leading role of
Piotr Bałczewski,* Emilia Kowalska, Ewa
Różycka-Sokołowska, Joanna Skalik,
Krzysztof Owsianik, Marek Koprowski,
Bernard Marciniak, Dariusz Guziejewski,
Witold Ciesielski
one RS-group in high photostabilization
(
254/365 nm, both in Ar/O
substituted and thermally stable mono-RS-
hetero)acene fluorophores, obtained via novel
2
) of the significantly
(
thio-Friedel-Crafts/Bradsher cyclization, has
been examined and explained by means of
DFT and TD-DFT calculations.
Page No. – Page No.
Mono-aryl/alkylthio-substituted
(
Hetero)acenes of Exceptional
Thermal and Photochemical Stability
via Thio-Friedel Crafts/Bradsher
Cyclization Reaction
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