A comparison of optical, electrochemical and self-assembling properties of two structural isomers based on 1,6- and 1,8-pyrenedione chromophores

Article abstract of DOI:10.1039/c7nj04932c

Two isomeric donor-acceptor-donor (DAD) pyrene chromophores were synthesized and their optical, electrochemical and solid-state properties were investigated. Both chromophores showed similar light absorption profiles that spanned the visible region from 3

Full text of DOI:10.1039/c7nj04932c

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A comparison of optical, electrochemical and
self-assembling properties of two structural
isomers based on 1,6- and 1,8-pyrenedione
chromophores†
Cite this:DOI: 10.1039/c7nj04932c
Samantha N. Keller and Todd C. Sutherland
*
Two isomeric donor–acceptor–donor (DAD) pyrene chromophores were synthesized and their optical,
electrochemical and solid-state properties were investigated. Both chromophores showed similar light
absorption profiles that spanned the visible region from 300 nm to 800 nm, in part due to strong
intramolecular charge transfer bands. Both 1,6- and 1,8-pyrenediketone acceptor cores exhibited similar
reversible electrochemical reductions at potentials of À0.91
V
and À0.86
V versus ferrocene/
ferrocenium, respectively, which yields approximate LUMO energy levels of À3.9 eV versus vacuum.
By design, both DAD chromophores displayed both electro- and halochromism. Despite their similar
structure, only the 1,6-pyrenediketone derivative exhibited self-assembly in the solid-state by forming a
soft crystalline phase. Furthermore, the solid-state film absorption profile of the 1,6-pyrenediketone
Received 12th December 2017,
Accepted 19th January 2018
isomer showed
a significant change in the absorption profile upon annealing above its cold-
DOI: 10.1039/c7nj04932c
crystallisation temperature, providing an absorption band that extends to 900 nm, suggesting strong
intermolecular electronic interactions.
rsc.li/njc
absorption profile: (1) extending pi-conjugation and (2) donor–
acceptor. Several examples of pyrenes containing either
Introduction
2
0–23
23,24
The polycyclic aromatic hydrocarbon pyrene, has been of interest vinylic
or acetylenic
linkages to other pi-systems have
1
–3
4–6
to the synthetic organic and photochemistry communities shown a general red-shift in the absorption spectra, dependent
for a long time, as evidenced by extensive review articles. To the on the attachment location. Likewise, the donor–acceptor
synthetic community, pyrene is a challenging substrate to poly- approach has generated several example pyrene-based chromo-
2
4–29
functionalise in a chemo- and regio-selective manner, whereas, phores with lower energy absorption peaks than pyrene.
the photochemistry community is interested in developing Due to pyrene’s energetically accessible frontier molecular
3
0,31
models that pertain to the excimer formation dynamics and their orbitals, it has been exploited as either an electron donor
3
2–36
potential applications. Because of the fundamental research or an acceptor.
conducted on pyrene, there is an extensive list of applications In addition to improving the light absorption properties,
that have been built on this core in areas such as photo- there is interest in designing materials that self-assemble,
7
8,9
10,11
catalysts, organic semiconductors, bio-sensing/imaging,
which has been proposed as a method to enhancing charge
8
,12–16
17,18
37–40
liquid crystals,
organogelators.
covalent organic frameworks,
and transport in the solid-state.
The notion that self-assembling
1
3,19
pi-systems will enhance the overlap of intermolecular frontier
Despite pyrene’s photophysical prominence, it is a relatively molecular orbitals such that efficient charge migration could
poor chromophore in the visible region of the spectrum. occur. Our goal is twofold. First, to increase the light absorption
Unsubstituted pyrene has electronic absorption peaks between properties of pyrene using both pi-extension and donor–acceptor
2
20 nm and 350 nm and molar absorption coefficients of strategies. Second, to add self-assembling functionality to the
À1
À1
approximately 50 000 M cm in cyclohexane. Two approaches pyrene chromophore by including structural features proposed
are generally taken to enhance the chromophore’s intensity and to impart liquid crystalline properties based on previous
8
,12–16
reports
of pyrene derivatives.
2
7,41
In our previous contributions,
we explored the regio-
isomeric and structure–property relationships of different donor
positions on various pyrene acceptor chromophores, as shown
Department of Chemistry, University of Calgary, 2500 University Drive NW,
Calgary, Alberta, T3G 1M1, Canada. E-mail: todd.sutherland@ucalgary.ca
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/c7nj04932c in Chart 1. Previously, the pyrene scaffold was altered to have
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been crushed to a fine powder prior to addition. The mixture
was stirred and heated at 80 1C for 5 h, then poured onto ice
and a red solid was collected by filtration. The red product was
purified on basic alumina, eluting unreacted pyrene with
chloroform, followed by elution of the red product band with
ethyl acetate. Removal of the solvent yielded a bright red solid
as a mixture of isomers (2.5 g, 11 mmol, 43%). Characterisation
data was in agreement with previously reported data for these
4
6 1
compounds. H NMR (300 MHz, chloroform-d) d 8.63 (s, 1H),
.48 (d, J = 7.5 Hz, 1H), 7.83 (d, J = 7.5 Hz, 1H), 7.70–7.63
m, 3H), 6.68 (dd, J = 9.9, 7.7 Hz, 2H).
Chart 1 Summary of our previous DAD chromophores based on pyrene
ketones.
8
(
both donors (D) and acceptors (A) arranged as DAD chromophores, 3: 1,6-bis(triisopropylsiloxy)pyrene
whereby the acceptor functionality was based on ortho-diketone
To a mixture of pyrene 1,6- and 1,8-diketones 1 and 2 (1.3 g,
derivatives of pyrene and the donors were based on 4-alkynyl-
aniline derivatives. Here we present two structural isomers of
pyrene diketones, 1,6- and 1,8-pyrene diketones, which are
produced as a mixture upon oxidation of pyrene with chromic
5.6 mmol) in dry dichloromethane (100 mL) were added zinc
powder (5.5 g, 84 mmol), TMEDA (4.2 mL, 28 mmol) and
TIPS-Cl (4.8 mL, 22 mmol). The mixture was stirred at room
temperature under a nitrogen atmosphere for 4 h. The zinc was
removed by filtration through celite, followed by a DCM silica
plug, and the solvent was removed to yield an off-white solid.
Crystallization in a mixture of DCM and MeOH yielded the
4
2
acid, as shown in Scheme 1. There are few reports of pyrene-
based organic materials with this substitution pattern. None of
our previous pyrene ortho-diketones exhibited self-assembling
solid-state properties, however, there is literature precedence for
liquid crystalline behaviour of compounds based on pyrene-1,6
1
2
5
,6-bis(triisopropylsiloxy)pyrene 3 as a white powder (1.29 g,
+
.36 mmol, 42%). HRMS-MALDI (M ) calcd for C34
H
50
O
2
Si
2
:
8
8
and 1,8-diketones. Hirose and co-workers reported a series of
,6- and 1,8-diketones that featured alkoxy substituents with
varying alkyl chain lengths that displayed discotic lamellar meso-
phases with absorption properties spanning to approximately 13
25 nm. Our motivation is threefold to understand the regioiso-
meric effects of D–A positions on a pyrene scaffold. (1) To generate
strong chromophores; (2) access low energy reduction potentials; 4: 1,8-bis(triisopropylsiloxy)pyrene
1
46.3344, found: 546.3362. H NMR (400 MHz, chloroform-d)
1
d 8.27 (d, J = 9.1 Hz, 2H), 7.95 (d, J = 8.4 Hz, 2H), 7.92 (d, J = 9.2 Hz,
2H), 7.49 (d, J = 8.2 Hz, 2H), 1.55–1.42 (m, 6H), 1.25–1.14 (m, 36H).
C NMR (101 MHz, chloroform-d) d 150.1, 126.7, 126.3, 126.3,
5
124.6, 122.8, 119.7, 116.5, 18.3, 13.4.
(
3) probe solid-state organization of soft materials. Using ketones
In order to isolate the 1,8-bis(triisopropylsiloxy)pyrene, 4, the
solvent was removed from the mother liquor to yield a brown
solid, which was further purified on a silica gel plug eluting
with DCM. Removal of the solvent afforded a yellow oil that
43–45
as acceptor motifs, others
have demonstrated assembly of
fluorenone derivatives into microfibers with enhanced nonlinear
optical properties. The pyrene DAD architectures may lead to large
two-photon absorption cross sections, which are of interest to the
photonics community.
+
slowly solidified. (1.3 g, 2.3 mmol, 41%) HRMS-MALDI (M )
1
calcd for C H O Si : 546.3344, found: 546.3319. H NMR
3
4
50
2
2
Here we present the synthesis, optical, electrochemical and
self-assembling properties of 1,6- and 1,8-pyrenediketones that are
appended to pi-conjugated 4-ethynylanilines donors, yielding DAD
chromophores. Moreover, the chromophores have broad absorption
profiles to 800 nm and show soft crystalline features in the solid-state.
(
400 MHz, chloroform-d) d 8.41 (s, 2H), 7.94 (d, J = 8.3 Hz,
2
1
1
H), 7.80 (s, 2H), 7.50 (d, J = 8.3 Hz, 2H), 1.56–1.45 (m, 6H),
13
.25–1.20 (m, 36H). C NMR (101 MHz, chloroform-d) d 150.0,
26.7, 126.1, 125.0, 124.7, 123.0, 121.0, 116.2, 18.3, 13.4.
5: 1,6-dibromo-3,8-bis(triisopropylsiloxy)pyrene
To a solution of 1,6-bis(triisopropylsiloxy)pyrene 3 (0.40 g,
Experimental section
0
1
.73 mmol) in DCM (50 mL) a solution of bromine (0.27 g,
.7 mmol) in DCM (10 mL) was added dropwise. The mixture
1
and 2: pyrene-1,6-diketone and pyrene-1,8-diketone
To a solution of Na Cr O (7.7 g, 26 mmol) in aqueous H SO
(
was allowed to stir at room temperature for 30 minutes, after
100 mL, 3 M) was added pyrene (5.0 g, 25 mmol), which had which time the reaction was quenched with Na S O . The
2
2
7
2
4
2
2 3
4
organic solution was washed with water, dried with MgSO ,
and the solvent was removed to yield a brown solid. The crude
solid was purified by column chromatography (hexanes :
DCM 19 : 1) yielding a yellow solid (0.28 g, 0.38 mmol, 52%).
+
HRMS-MALDI (M ) calcd for C H Br O Si : 702.1554; found:
3
4
48
2
2
2
1
7
7
02.1581. H NMR (400 MHz, chloroform-d) d 8.35–8.27 (m, 4H),
13
.78 (s, 2H), 1.56–1.41 (m, 6H), 1.21 (d, J = 7.5 Hz, 36H). C NMR
101 MHz, chloroform-d) d 150.2, 126.8, 125.2, 125.1, 122.7, 121.3,
121.0, 119.4, 18.3, 13.3.
(
2 2 7 2 4
Scheme 1 Chromic acid oxidation of pyrene. (i) Na Cr O , H SO (3 M),
9
0 1C, 4 h (mixture of 1 and 2, 43%).
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6: 1,8-dibromo-3,6-bis(triisopropylsiloxy)pyrene
16ketPyr
To a solution of 1,8-bis(triisopropylsiloxy)pyrene 4 (0.35 g, Compound 7 (0.04 g, 0.03 mmol) was dissolved in dichloro-
0
1
.64 mmol) in DCM (50 mL) a solution of bromine (0.24 g, methane (30 mL) and stirred in an open flask. A 1 M solution of
.5 mmol) in DCM (10 mL) was added dropwise. The mixture tetrabutylammonium fluoride (0.1 mL, 0.10 mmol) was added,
was allowed to stir at room temperature for 30 minutes, after and the yellow solution immediately turned pink. The solution
which time the reaction was quenched with Na . The was stirred for 10 minutes, after which time DDQ (0.007 g,
2 2 3
S O
organic solution was washed with water, dried with MgSO4, 0.03 mmol) was added to the mixture, which turned the
and the solvent was removed to yield a brown solid. The crude solution dark green. The mixture was then extracted with water,
solid was purified by column chromatography (hexanes : DCM dried with MgSO , and the solvent was evaporated to yield a
4
4
: 1) yielding a yellow oil (0.27 g, 0.38 mmol, 60%). HRMS- dark green solid. The crude solid was purified on an alumina
+
MALDI (M ) calcd for
C
34
H
48Br
2
O
2
Si
2
: 702.1554; found: plug, eluting with toluene to yield an emerald green solid (0.029 g,
1
+
7
8
3
1
02.1526. H NMR (400 MHz, chloroform-d) d 8.38 (s, 2H), 0.023 mmol, 86%). HRMS-MALDI (M ) calcd for C80H N O
114 2 2
1
.22 (s, 2H), 7.77 (s, 2H), 1.56–1.44 (m, 6H), 1.22 (d, J = 7.5 Hz, 1134.8875; found 1134.8823. H NMR (400 MHz, chloroform-d)
6H). C NMR (101 MHz, CDCl ) d 150.1, 126.6, 125.0, 124.5, d 8.54 (d, J = 7.7 Hz, 2H), 8.45 (d, J = 7.7 Hz, 2H), 7.50 (d, J = 8.6 Hz,
3
1
3
23.0, 120.9, 120.8, 119.1, 18.1, 13.1.
4H), 6.90 (s, 2H), 6.63 (d, J = 8.7 Hz, 4H), 3.32 (t, J = 7.8 Hz, 8H),
1
.67–1.55 (m, 8H), 1.39–1.24 (m, 72H), 0.88 (t, J = 6.5 Hz, 12H).
C NMR (101 MHz, CDCl ) d 184.5, 149.3, 136.2, 134.1, 133.1,
3
1
3
7
1
3
30.6, 130.5, 130.1, 129.5, 127.3, 111.4, 106.8, 105.9, 84.7, 51.2,
2.1, 29.9, 29.8, 29.8, 29.8, 29.7, 29.5, 27.4, 27.3, 22.9, 14.3.
1
,6-Dibromo-3,8-bis(triisopropylsiloxy)pyrene 5 (0.10 g, 0.14 mmol)
was dissolved in dry triethylamine (10 mL) under a nitrogen
atmosphere. PdCl (dppf) (0.01 g, 0.01 mmol) and CuI (0.005 g,
.002 mmol) were added, and the mixture was stirred and heated
to 80 1C. A 1.0 M solution of the alkyne in triethylamine (0.31 mL,
.31 mmol) was added slowly over five minutes, and the mixture
2
18ketPyr
0
Compound 8 (0.1 g, 0.07 mmol) was dissolved in dichloro-
methane (30 mL) and stirred in an open flask. A 1 M solution of
tetrabutylammonium fluoride (0.2 mL, 0.2 mmol) was added,
and the yellow solution immediately turned pink. The solution
was stirred for 10 minutes, after which time DDQ (0.02 g,
0
was then stirred for an additional four hours. The solvent
was removed, and the mixture was then purified by column
chromatography (9 : 1 hexanes : DCM) yielding an orange oil
+
(0.11 g, 0.073 mmol, 52%). HRMS-MALDI (M ) calcd for
0
.09 mmol) was added to the mixture, which turned the
solution dark green. The mixture was then extracted with water,
dried with MgSO , and the solvent was evaporated to yield a
1
C
H N O
98 156 2 2
Si
chloroform-d) d 8.52 (d, J = 9.3 Hz, 2H), 8.33 (d, J = 9.3 Hz, 2H),
.60 (s, 2H), 7.56 (d, J = 9.0 Hz, 4H), 6.65 (d, J = 9.0 Hz, 4H), 3.31
t, J = 7.7 Hz, 8H), 1.62 (t, J = 7.1 Hz, 8H), 1.57–1.43 (m, 6H),
.37–1.25 (m, 72H), 1.22 (d, J = 7.5 Hz, 36H), 0.89 (t, J = 7.2 Hz,
2
1449.1700; found 1449.1689. H NMR (400 MHz,
4
7
(
1
dark green solid. The crude solid was purified on an alumina
plug, eluting with toluene to yield an emerald green solid (0.054 g,
+
0.048 mmol, 72%). HRMS-MALDI (M ) calcd for C80
H
114
N
2
O
2
13
1
1
1
3
2H). C NMR (101 MHz, chloroform-d) d 149.7, 148.2, 133.2, 130.0,
27.4, 126.5, 125.2, 123.4, 120.2, 119.2, 118.8, 111.5, 96.6, 86.7, 51.2,
2.1, 29.8, 29.8, 29.8, 29.7, 29.5, 27.4, 27.3, 22.9, 18.4, 14.3, 13.4.
1134.8875; found 1134.8845. H NMR (400 MHz, chloroform-d)
d 8.58 (s, 2H), 8.30 (s, 2H), 7.49 (d, J = 8.8 Hz, 4H), 6.85 (s, 2H), 6.62
(d, J = 8.9 Hz, 4H), 3.32 (t, J = 8.2 Hz, 8H), 1.66–1.58 (m, 8H), 1.38–
13
3
1.25 (m, 72H), 0.89 (t, J = 6.8 Hz, 12H). C NMR (101 MHz, CDCl )
8
d 184.2, 149.4, 136.4, 134.1, 133.8, 132.5, 130.9, 130.2, 130.0, 129.1,
1
2
27.6, 111.5, 107.1, 105.5, 84.7, 51.2, 32.1, 29.8, 29.8, 29.7, 29.5,
7.4, 27.3, 22.8, 14.3.
1
,8-Dibromo-3,6-bis(triisopropylsiloxy)pyrene 6 (0.10 g, 0.14 mmol)
was dissolved in dry triethylamine (10 mL) under a nitrogen
atmosphere. PdCl (dppf) (0.01 g, 0.01 mmol) and CuI (0.005 g,
.002 mmol) were added, and the mixture was stirred and
heated to 80 1C. A 1.0 M solution of the alkyne in triethylamine
2
0
Results and discussion
Synthesis
(0.31 mL, 0.31 mmol) was added slowly over five minutes, and
the mixture was then stirred for an additional four hours. The
solvent was removed, and the mixture was then purified by The synthesis of the DAD chromophores began with the oxida-
column chromatography (9 : 1 hexanes : DCM) yielding an orange tion of pyrene in chromic acid according to a modification of
+
42,47–49
oil (0.097 g, 0.067 mmol, 48%). HRMS-MALDI (M ) calcd for previously reported procedures.
Pyrene was added to a
SO , and was heated
1
C H N O Si
98 156 2 2 2
1449.1700; found 1449.1689. H NMR (400 MHz, solution of sodium dichromate in 3 M H
2
4
chloroform-d) d 8.48 (s, 2H), 8.36 (s, 2H), 7.59 (s, 2H), 7.56 (d, J = to reflux for four hours as shown in Scheme 1. The resulting
8
1
1
.9 Hz, 4H), 6.65 (d, J = 9.2 Hz, 4H), 3.32 (t, J = 7.7 Hz, 8H), red-orange precipitate was filtered through basic alumina,
.61 (p, J = 7.6 Hz, 8H), 1.58–1.47 (m, 6H), 1.40–1.26 (m, 72H), yielding a mixture of the pyrene-1,6 and 1,8-diketones, 1 and 2,
13
.26–1.20 (m, 36H), 0.89 (d, J = 7.3 Hz, 12H). C NMR (101 MHz, in 43% total yield.
The mixture of diketones was identified by each components
chloroform-d) d 149.5, 148.2, 133.2, 127.2, 126.5, 124.1, 123.6,
1
1
2
21.2, 119.0, 118.9, 111.5, 109.3, 96.7, 86.9, 51.2, 32.1, 29.8, 29.8, diagnostic H NMR spectra. The pyrene-1,6-diketone 1, posses-
9.8, 29.7, 29.5, 27.5, 27.3, 22.9, 18.4, 14.3, 13.4. sing C2h symmetry, displays four sets of doublets, while the
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dibromides 5 and 6 as shown in Scheme 2. The yields obtained
in bromination were modest (50–60%), as the TIPS group is
more labile than the robust TBDMS silyl ether. Whether Br2
or NBS was used as a brominating reagent, the synthesis of
dibromides 5 and 6 consistently showed a small amount of
de-protected and re-oxidized pyrene-diketone as a side product,
which was removed by a silica plug.
The regiochemistry of the bromine atoms relative to the
triisopropylsiloxy groups was determined by HMBC-NMR spectro-
scopic methods; 1D-NMR spectroscopy was insufficient to assign
the structure, as several substitution possibilities would have
produced the observed patterns. Pyrene is known to undergo
electrophilic aromatic substitution reactions primarily at the
9
1
,3,6, and 8 positions and there is literature precedence showing
that meta substitution proceeds even on pyrenes substituted with
50
classical ortho/para directors at the 1,6- or 1,8-positions.
The HMBC spectrum showed that the proton signal at
.78 ppm has four interactions including two quaternary carbons
7
and strong interactions with the TIPS and bromine substituted
carbons. Therefore, it can be concluded that dibromide 5 contains
bromine atoms substituted meta to the triisopropylsiloxy groups.
Subsequently, dibromides 5 and 6 were subjected to
Sonogashira cross coupling with the donor, N,N-didodecyl-4-
ethynylaniline, to produce the bis-alkynes, 7 and 8 in approxi-
mately 50% yields.
Scheme 2 Synthesis of chromophores 16ketPyr and 18ketPyr. (i) TIPS-Cl,
TMEDA, Zn, DCM (3, 42%; 4, 41%); (ii) Br
iii) N,N-didodecyl-4-ethynylaniline (donor), PdCl
7, 52%; 8, 48%); (iv) TBAF, DDQ, DCM (16ketPyr, 86%; 18ketPyr, 72%).
2
, DCM (5, 52%; 6, 60%);
Finally, bis-alkynes 7 and 8 were desilylated using TBAF to
N, 80 1C form the corresponding di-oxyanions followed by oxidation.
(
2
(dppf), CuI, Et
3
(
However, unlike our previous reports of ortho-diketones where
the desilylation using TBAF and exposure to air resulted in
2
oxidation. Here, neither air nor Ag O proved useful in converting
1,8-diketone 2, which is C2v symmetric, displays two doublets the orange di-oxyanionic intermediates into the corresponding
and two singlets.
1,6- or 1,8-diketones. Several oxidants were screened including
2 2
H O , NaClO, and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone
The mixture of diketones was not purified at this point and
instead was subjected to reduction followed by silylation, as (DDQ). DDQ oxidation produced the best results in the conver-
shown in Scheme 2. The two diketones were reduced with zinc sion of the fluorescent solution of intermediates to a non-
in the presence of TMEDA, and the oxygen atoms were pro- emissive dark green solution, which after silica gel column
tected with triisopropylsilyl (TIPS) groups to produce a mixture chromatography purification yielded dark green DAD chromo-
of 1,6-bis-(triisopropylsiloxy)pyrene 3 and 1,8-bis-(triisopropyl- phores 16ketPyr and 18ketPyr, as shown in Scheme 2.
siloxy)pyrene 4 in 83% total yield. At this point, the resulting
Optical and electrochemical properties
mixture was separated into single regioisomers by fractional
crystallization in a mixture of chloroform and methanol. The Table 1 lists the pertinent optoelectronic values obtained for
ability to obtain the 1,6- and 1,8-isomers as separate species is the 1,6- and 1,8-regioisomeric DAD chromophores and places
significant given the difficulty encountered in separating the them in context with analogous previously reported pyrene-
diketone mixture of 1 and 2 by chromatography. The diketone based DAD chromophores. Chloroform solutions of the two
mixture was also reduced and protected with TBDMS group, DAD chromophores based on 1,6- and 1,8-pyrene diketones are
4
1
analogous to our ortho-diketone previous work, but attempts shown in the inset of Fig. 1; both compounds are a dark green
to separate this mixture by either crystallization or chromato- colour, but the 1,8-diketo isomer, 18ketPyr, has more of a blue
graphy were unsuccessful. Each of the bis-(triisopropylsiloxy)- tint than the 1,6-diketo isomer, 16ketPyr.
pyrenes 3 and 4 can be easily deprotected and re-oxidized to the
The UV/Vis absorption spectra of 16ketPyr and 18ketPyr in
corresponding parent diketone using the conditions described chloroform solution are shown in Fig. 1. Both of the chromo-
later. The reduction and protection reactions were carried phores display several electronic transitions, enabling their
out to add solubility and ease purification, but also to enable absorbance profiles to span almost the entire range from 250
subsequent cross-coupling reactions.
to 800 nm. The intramolecular charge transfer (ICT) absorbance
To continue the synthesis of the DAD chromophores, each of bands of 16ketPyr and 18ketPyr both have onsets of 800 nm,
the isolated silylated pyrenes 3 and 4 was treated independently corresponding to a HOMO–LUMO energy gap of 1.6 eV for both
and subjected to electrophilic bromination with Br to produce of the isomers. However, the lmax of the ICT band of 16ketPyr
2
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Table
1
Comparison of optical and electronic properties of DAD
chromophores 16ketPyr and 18ketPyr with closely related pyrene
chromophores
a
b
c
d
e
(
l
max(onset)
E
opt
E
ox(DPV)
À1
À1
d
M
cm
)
(nm)
(eV) (V)
E
red(DPV)
4
1
4
4
4
1
1
5ketPyr14
7400
23 000
1700
25 000
52 000
575(780)
570(730)
630(880)
627(800)
617(800)
1.6 0.33
1.7 0.40
1.4 0.43
1.6 0.47
1.6 0.47
À1.04
À1.11
1
5ketPyr2
27
5910ketPyr
6ketPyr
À0.84, À1.18
À0.91, À1.15
À0.86
8ketPyr
a
b
Molar absorptivity at the peak of the ICT band. Measured in CHCl
from lowest energy band. Calculated from lonset
referenced to an internal Fc/Fc redox probe in CH Cl containing
2 2
3
c
d
.
DPV potentials are
+
0
.05 M (nBu)
4
PF
6
with a Pt button working electrode, a Ag wire quasi
À1
reference electrode and a Pt wire counter electrode at 20 mV s
.
Fig. 2 Absorbance spectra of (a) 16ketPyr red line and (b) 18ketPyr black
line in toluene solution. Blue lines show resulting spectra after addition of
excess TFA, and dotted lines show regenerated spectra upon neutralization
with triethylamine.
Effectively, the protonation converts the DAD chromophores
into AAA scaffolds, which eliminates the ICT bands. Note the
slight red-shift of the Et
the original spectra in toluene; even after addition of a small
amount of TFA and Et N, the overall polarity of the solution is
3
N neutralized spectra compared to
3
increased enough to slightly affect the absorbance onset,
further supporting the ICT band assignment.
The cyclic and differential pulse voltammograms (DPV) of
chromophores 16ketPyr and 18ketPyr are shown in the ESI.†
In terms of the values of the first oxidation and reduction
potentials, the two isomers display similar characteristics.
Both chromophores oxidize irreversibly at potentials of 0.47 V
Fig. 1 Absorption of compounds 16ketPyr (red, left vial) and 18ketPyr
black, right vial) in CHCl
(
3
.
+
relative to the Fc/Fc redox couple, and their reductions are also
appears at slightly lower energy compared to 18ketPyr (627 nm close in energy with compound 18ketPyr being slightly easier to
vs. 617 nm). Compound 18ketPyr shows more intense transitions reduce with a reduction potential of À0.86 V compared to À0.91 V
than those of 16ketPyr at the same concentration (e of the ICT for 16ketPyr. The largest difference lies in the fact that each
À1
À1
À1
À1
band of 18ketPyr is 52 000 M cm compared to 25 000 M cm
for 16ketPyr), which can be explained in terms of the fact that chromophore displays two separate oxidation events, as well as two
8ketPyr has a greater changes in dipole moment upon undergoing separate reductions. Similar to our previously reported DAD chro-
electrochemical event is doubled for 16ketPyr; the C2h symmetric
1
electronic transitions. The ground state dipole moments for 16ketPyr mophores, the oxidations were irreversible under cyclic voltammetry
and 18ketPyr were calculated using DFT methods to be 0.3 D and experiments using both platinum and carbon working electrodes.
16.7 D, compared to TD-DFT calculated excited state diploes of The electrochemical reductions of 16ketPyr and 18ketPyr were found
0
.4 D and 22.0 D, respectively.
Further evidence for the ICT character of the low-energy
to be quasi-reversible under the electrochemical conditions used.
Spectroelectrochemical analysis was also carried out on
absorption band of 16ketPyr and 18ketPyr was found in solvato- compounds 16ketPyr and 18ketPyr, and the spectra are shown
chromic and protonation studies. The absorption spectra of in the ESI.† In both cases, the ICT band of the chromophores
16ketPyr and 18ketPyr in non-polar solvents such as toluene are collapses as the acceptor portions of the chromophores are
hypsochromically shifted compared to their spectra in more converted into their respective anions, in essence changing the
polar solvents, such as chloroform (ESI†), where the lmax values DAD chromophores into DDD systems. However, unlike the
for the ICT transitions of 16ketPyr and 18ketPyr in toluene are compounds reported in our previous work, the original spectra
580 and 570 nm, respectively. In Fig. 2, the effect of a Brønsted could not be fully regenerated when the applied potential was
acid is apparent; when TFA is added to the same toluene returned to 0 V, attesting to the quasi-reversibility of the
solutions of 16ketPyr and 18ketPyr, the broad ICT bands reductions observed in the cyclic voltammograms, suggesting
are suppressed because the aniline nitrogens are protonated, that the anions of 16ketPyr and 18ketPyr are not as stable as
2
7,41
and the ICT peaks are regenerated upon the addition of base. those of the analogous ortho-quinones.
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Theoretical calculations
Solid state properties
The orbital energy levels of diketones 16ketPyr and 18ketPyr The solid-state behaviour of diketone chromophores 16ketPyr
calculated using DFT at the B3LYP/6-31+g(d) level as well as the and 18ketPyr was investigated, and comparisons are made
orbital shapes from the ground state calculations are shown in between the two isomeric compounds in terms of their optical
Fig. 3. TD-DFT calculations were found to reflect the experi- properties in thin films and their thermal properties. Pertinent
mentally determined absorption spectra for chromophores comparisons are also made between the 1,6- and 1,8-diketones
1
6ketPyr and 18ketPyr when the polarizable continuum model 16ketPyr and 18ketPyr, and their ortho-diketone structural
2
7,41
(PCM) was used to simulate a chloroform solution. Given the isomers from previous work.
C2-symmetric DAD design, it was expected that both the HOMO
Diketones 16ketPyr and 18ketPyr were subjected to differen-
and HOMOÀ1 energies were nearly degenerate for both chromo- tial scanning calorimetry in order to determine their melting
phores 16ketPyr and 18ketPyr, as demonstrated in the ESI.† temperatures and to explore the possibility of self-organization
The degenerate HOMO and HOMOÀ1 orbitals of 16ketPyr into mesophases. Fig. 4 shows the DSC thermograms of 16ketPyr
and 18ketPyr are localized predominantly on the ethynyl aniline and 18ketPyr that were obtained at a heating and cooling rate of
donor portions of the molecules, but also include MO coefficients 10 1C per minute.
on the outer edge of the pyrene ring and the carbonyl oxygen.
The 18ketPyr chromophore shows negligible thermal
Interestingly, both the HOMO and HOMOÀ1 orbitals appear to transitions in the temperature range explored save for a broad
have nodes on the central naphthalene portion of the pyrene transition between 0 1C and À15 1C. It is likely that this
ring. As optical transitions can proceed from either of these compound undergoes a slow solidification at these low tem-
orbitals, there are many possible allowed electronic transitions peratures, and at higher temperatures exists as a viscous liquid.
for 16ketPyr and 18ketPyr, correlating with their observed Heating and cooling a drop-cast sample of 18ketPyr on the
multi-peak absorbance spectra shown in Fig. 1. The LUMO cross-polarized optical microscope (POM) stage revealed no
orbitals of both chromophores are localized throughout the birefringence, hence no long-range ordering for this compound,
central pyrene with minor contributions on the ethynyl D–A even at low temperatures. Chromophore 16ketPyr, on the other
bridge. TD-DFT calculations predict the lowest energy transi- hand, showed several transitions in its DSC thermogram.
tion as the one from HOMO to LUMO with a value of 1.57 eV for Upon heating diketone 16ketPyr from room temperature, the
both isomers, which is in good agreement with the observed compound undergoes an exothermic cold crystallization event,
optical HOMO–LUMO energy gap of 1.6 eV. The next predicted similar to those seen for analogous ortho-diketones on their
À1
transition is from the HOMOÀ1 to the LUMO; this transition is DSC heating curves, with an enthalpy change of 50 J g . A melt
DFT-calculated at 1.7 eV but is only an allowed transition for is then observed at 135 1C, and this melt was also observed for
18ketPyr. A summary table, including oscillator strengths, of a drop-cast film of chromophore 16ketPyr under POM. The
the TD-DFT computations is in the ESI.† As 18ketPyr has a enthalpy change of the melting transition was determined to be
À1
higher number of allowed transitions than 16ketPyr, many of 104 J g . As diketone 16ketPyr is cooled, it undergoes two
which are close in energy, it is possible that the enhanced exothermic phase transitions, which is indicative of the existence
molar absorptivity is an additive effect of nearly degenerate of a mesophase between the isotropic liquid and solid phases.
transitions that result in stronger absorbance of 18ketPyr The first transition observed upon cooling is at 75 1C, and its
À1
compared to 16ketPyr.
associated enthalpy change of 53 J g might be indicative of a
transition between an isotropic liquid and a soft-crystalline phase,
as the enthalpy change between an isotropic liquid and a liquid
Fig. 4 DSC thermograms of chromophores 16ketPyr (red) and 18ketPyr
Fig. 3 FMO surfaces calculated for truncated models of 16ketPyr (left)
and 18ketPyr (right). Surfaces calculated at the B3LYP/6-31+g(d) level of
theory and basis set, including the PCM solvent model in chloroform.
(black). The first cooling and second heating curves are shown. DSC
À1
measurements were taken at 10 1C min . Exothermic transitions are in
the positive direction.
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crystal is usually smaller. As 16ketPyr is cooled from the isotropic
liquid to the mesophase on the POM stage, birefringent patterns
were observed, as shown in the POM images in Fig. 5. A second
exothermic transition takes place at 41 1C, and its associated
À1
enthalpy change of 23 J g corresponds to a transition from the
mesophase to a crystalline solid.
Spin-cast films of 16ketPyr and 18ketPyr were obtained by
depositing a saturated chloroform solution of each compound
on a 2.5 cm  2.5 cm glass slide at a spinning rate of 700 rpm
for 1.5 minutes. Each slide was examined under POM, as well as
UV/Visible absorption spectroscopy. Chromophore 18ketPyr,
which displayed no long-range ordering in the DSC studies,
formed a uniform green film on the glass slide that was
observed to be amorphous under POM, as was its corresponding
drop cast sample. The absorption spectrum of a spin-cast film of
1
8ketPyr is plotted in Fig. 6b overlaid with its solution absorption
spectrum in chloroform.
For the ortho-diketone DAD chromophores in our previous Fig. 6 Normalized absorbance spectra of diketones 16ketPyr and
1
8ketPyr. (a) Compound 16ketPyr in toluene solution (dotted line), and
as a spin-cast film before heating (solid red line) and after heating to 50 1C
solid blue line). (b) Diketone 18ketPyr in chloroform solution (dashed
black line) and spin-cast film (solid black line).
work, there has been a trend in the film absorbances showing
that compounds with larger dipole moments show film absor-
bance spectra that are similar to or even red-shifted compared
to their solution absorbance spectra in more polar solvents
such as chloroform. Compounds with no overall dipole
(
2
7
51–53
moment, such as our previous pyrene-tetraketone, showed absorption spectra can be found elsewhere.
In this limited
film absorption spectra indicative of a more non-polar solid- series of pyrene DAD chromophores, 18ketPyr has a film
state environment, with the lmax of the ICT transition similar to absorbance indicative of a more polar solid-state environment;
the absorbance in non-polar solvents such as toluene. Note, the its lmax in the spin-cast film is 613 nm, which is close to that of
correlation between peak positions in the solid-state absorption 617 nm obtained in chloroform.
spectra and the polarity of solvent represents a simplistic view
However, when chromophore 16ketPyr was spin-cast, as
because solid-state solvatochromic features are a combination opposed to drop-cast, at room temperature, the film initially
of a variety of complex interactions, such as molecular aggregation, formed a uniform but amorphous green film on the glass slide,
orientation, dielectric constants, doping and film forming aside from a few small needle shaped microcrystals, which were
conditions. More detail on the solid-state packing effects on observed under POM, shown in Fig. 5d. The absorption spectrum
of the amorphous film, shown in Fig. 6a, is similar to the solution
absorbance of 16ketPyr in toluene with a lmax of 581 nm. This is
consistent with the overall trend for series of related pyrene DAD
chromophores; the near zero dipole moment of 16ketPyr causes
the amorphous solid to behave as if it were in a non-polar
medium. However, the onset of absorption of 16ketPyr in the
solid-state is still quite far into the low-energy region of the
absorption spectrum at 820 nm, which corresponds to a small
decrease in the HOMO–LUMO energy gap of 16ketPyr compared
to the value calculated from the chloroform absorbance spectrum.
As the spin-cast film of 16ketPyr was heated and maintained
at a temperature of 50 1C on the POM stage, birefringent
patterns emerged, as show in Fig. 5e. This observation is
consistent with the recrystallization event observed in the
DSC trace of 16ketPyr. The birefringent patterns were retained
even after the film of 16ketPyr was re-cooled to room temperature.
The large difference between the POM images of the drop-cast
versus spin-cast films is due to both the kinetics of film formation
and the thicker drop-cast films.
A second absorption spectrum of 16ketPyr film was recorded
after thermal annealing at the cold crystallization temperature
from DSC and the film revealed a dramatic change from the
original amorphous film. As shown by the POM image of Fig. 5e
Fig. 5 Polarized optical micrographs (100Â magnification) of drop-cast
a–c) and spin-cast (d and e) films of diketone 16ketPyr. (a) As-cast room
temperature image of 16ketPyr; (b) melted, then cooled to 75 1C image of
6ketPyr; (c) melted film of 16ketPyr returned to room temperature;
d) spin-cast film of 16ketPyr; (e) spin-cast film of 16ketPyr annealed at
0 1C for 10 minutes.
(
1
(
5
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and the blue line in Fig. 6a, the lmax of the ICT absorbance Notes and references
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