Halochromic phenolate perylene bisimides with unprecedented NIR spectroscopic properties

Article abstract of DOI:10.1002/anie.201105129

C-C coupling of 1,7-dibromoperylene bisimide with sterically hindered 2,6-di-tert-butylphenol by a carbon nucleophilic substitution reaction in the absence of transition-metal catalysts gave novel halochromic perylene bisimides (see picture). The corresponding phenolate ions of these compounds exhibit unprecedented NIR spectroscopic properties including strong absorption with maxima at around 1200 nm.

Full text of DOI:10.1002/anie.201105129

DOI: 10.1002/anie.201105129
NIR Dyes
Halochromic Phenolate Perylene Bisimides with Unprecedented NIR
Spectroscopic Properties
Mei-Jin Lin, Benjamin Fimmel, Krzysztof Radacki, and Frank Wꢀrthner*
Organic p-conjugated dyes with strong absorptions in the
near-infrared (NIR) region have received increasing attention
in the past decades owing to their highly interesting but less
studied electrical/optical properties and foreseeable applica-
tions in NIR light-emitting diodes,[1] biosensing,[2] photo-
voltaic cells,[3] and telecommunication.[4] The most prevalent
organic dyes such as cyanines,[5] squaraines,[6] Bodipys,[7]
porphyrins,[8] phthalocyanines,[9] and rylenes[10] are absorbing
in the visible region of light, and common strategies to
achieve a bathochromic shift of the absorption maximum of a
dye are the incorporation of donor and acceptor groups into
the parent chromophore and the extension of the p system of
the chromophore.[11] The introduction of both donor and
acceptor substituents into the parent chromophore leads to
charge transfer (CT) bands in the long-wavelength region of
the absorption spectrum, which is a simple but promising
strategy to achieve long-wavelength absorbing dyes. On the
other hand, the extension of the p system of the chromophore
is more elaborate and is often accompanied by decreased
chemical stability, for example, for cyanine and squaraine
dyes, or poor solubility, for example, for porphyrin, phthalo-
cyanine, and rylene bisimide dyes, thus limiting their scope for
applications.
localized on the donor substituents, while the lowest unoccu-
pied molecular orbitals (LUMOs) are similar to those of core-
unsubstituted parent PBIs.[14] Thus, an increase of the donor
strength of substituents is expected to result in PBI deriva-
tives with a strong bathochromic shift of their CT bands.
Analysis of Hammett coefficients[15] suggests that a more
pronounced electron-donating effect can be anticipated upon
replacement of the amino groups (sþp ¼ꢀ1.4 for -NPh2),
which were previously applied to PBIs,[13,14] by an oxide anion
(sþp ¼ꢀ4.27 for -Oꢀ) to achieve strongly bathochromically
shifted absorptions of PBIs. Herein, we report that this
concept is indeed viable. For the realization of this concept,
we have synthesized a pair of novel halochromic 4-hydrox-
yphenyl-substituted PBIs 2 and 3 (Scheme 1), the anions of
which exhibit so far unprecedented NIR absorptions for PBI
dyes beyond 1100 nm because of the exceptionally strong
electron-donating oxide anion introduced into the phenyl
substituent at the para position.
Perylene bisimides (PBIs) are an attractive class of
electron-poor dyes owing to their outstanding optical, elec-
tronic (n-type semiconducting), and light fastness proper-
ties.[12] Owing to the electron-deficient character of the PBI
core, the latter can be considered as an electron-accepting
moiety that may interact with strong electron-donating
functional groups attached to it and thus give rise to CT
bands.[13] So far, only few examples of PBIs bearing strong
electron-donor substituents such as 4-(diphenylamino)-
phenyl, [4-(diphenylamino)phenyl]ethynyl, and oligothio-
phene-based moieties at the bay area showed the spectral
features that are characteristic of charge transfer, but their
absorption maxima remained below 800 nm.[13,14] Molecular
orbital calculations indicated that the highest occupied
molecular orbitals (HOMOs) of such PBIs are mainly
Scheme 1. Synthesis of 4-hydroxyphenyl-substituted PBIs 2 and 3.
Reaction conditions and yields are given in Table 1.
Table 1: Reaction conditions and yields of the isolated products PBIs 2
and 3.
Entry
Solvent
T [8C]
t [h]
Yield of 2 [%][a]
Yield of 3 [%][a]
1
2
3
4
5
DMF[b]
DMF
80
80
120
120
120
3
48
3
3
3
38
31
41
57
53
0
0
0
8.1
12
DMF
NMP[c]
DMSO[d]
[a] The yield is referred to the starting material 1,7-dibrominated PBI 1;
[b] DMF: dimethylformamide; [c] NMP: N-methylpyrrolidin-2-one;
[d] DMSO: dimethyl sulfoxide.
[*] Dr. M.-J. Lin, B. Fimmel, Prof. Dr. F. Wꢀrthner
Universitꢁt Wꢀrzburg, Institut fꢀr Organische Chemie and Rçntgen
Research Center for Complex Material Systems
Am Hubland, 97074 Wꢀrzburg (Germany)
E-mail: wuerthner@chemie.uni-wuerzburg.de
Homepage: http://www-organik.chemie.uni-wuerzburg.de
Unlike the known PBIs that are aryl-substituted at the bay
area, which were usually prepared by transition-metal cata-
lysis,[12c] the present PBIs 2 and 3 bearing one or two 4-
hydroxyl-3,5-di-tert-butylphenyl units at the bay area, respec-
tively, are synthesized by nucleophilic substitution of 1,7-
dibrominated PBI 1[16] with excess amounts of the nucleophile
Dr. K. Radacki
Universitꢁt Wꢀrzburg, Institut fꢀr Anorganische Chemie
Am Hubland, 97074 Wꢀrzburg (Germany)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201105129.
Angew. Chem. Int. Ed. 2011, 50, 10847 –10850
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
10847

Communications
2,6-di-tert-butylphenolate ion,[17] followed by protonation
enantiomers (P and M atropo-enantiomers)[21] with the twist
angles between the two naphthalene units being 18.98 and
19.28, respectively, are found. The chiral self-recognition of
these enantiomers by p–p stacking in the solid state led to
(P,P) and (M,M) homochiral dimers,[22] which are further self-
discriminated by H-bonding interactions between the OH
with acid (Scheme 1). To our surprise, no matter which
reaction conditions we applied (see Table 1), the hydro-
debrominated mono(4-hydroxyphenyl)-substituted PBI 2 was
obtained as the major product in 31–57% yield. However, a
small amount of di(4-hydroxyphenyl)-substituted PBI 3 (8.1–
12% yield) could be obtained when N-methylpyrrolidin-2-
one (NMP) or dimethyl sulfoxide (DMSO) was used as a
reaction solvent.
=
donor in the phenol and the C O acceptor in the imide units
to form a racemic 1D network with an alternate P and M
chirality (see Figure S1a). From the 1H NMR spectra of PBI 2
in CD2Cl2 at different concentrations (10ꢀ5–10ꢀ2 m), a slight
tendency for aggregation by p–p stacking is noticeable at
higher concentration (10ꢀ2 m; see Figure S9). Thus, in order to
avoid aggregation, the spectroscopic properties of PBIs 2 and
3 have been studied in dilute solutions (10ꢀ5 m).
Despite two bulky 4-hydroxyl-3,5-di-tert-butylphenyl
groups at both bay areas, PBI 3 exhibits a less twisted
perylene core (about 16.78) than that of the monosubstituted
PBI 2. As expected, no p–p stacking of perylene cores is
observed in the crystal of PBI 3 (see Figure S1b). In
accordance with the X-ray analysis, no indication for p–p
aggregation could be found in the 1H NMR spectra of PBI 3,
not even for a highly concentrated solution (10ꢀ2 m; see
Figure S10).
Mechanistically, a SRN1-type reaction sequence (see
Scheme S1 in the Supporting Information) may account for
the observed products.[18] Owing to the electron-deficient
character of the PBI core, the 1,7-dibrominated PBI 1 may
accept an electron from the phenolate ion, thus forming a PBI
radical anion, which after elimination of a bromide ion either
abstracts a proton from the medium to form monobrominated
PBI, or is substituted by the 2,6-di-tert-butylphenolate ion to
generate a monobromo-mono(4-hydroxy-phenyl)-substituted
PBI radical anion. The latter species may undergo another
debromination step and subsequently either react with the
phenolate ion to generate the radical anion of di(4-hydroxy-
phenyl)-substituted PBI 3, or may abstract a proton to form
mono(4-hydroxyphenyl)-substituted PBI 2 (for details, see
the Supporting Information). Evidence for the formation of
PBI radical anions is given by a color change of the solution
from red to green during the reaction, which is in accordance
with the spectral characteristics of spectroelectrochemically
generated PBI radical anions.[19] A certain amount of the
oxidation product 3,3’,5,5’-tetra-tert-butyl-4,4’-dibenzoqui-
The optical properties of the neutral PBIs 2 and 3 in
dichloromethane (DCM) were explored by UV/Vis absorp-
tion and fluorescence spectroscopy (see Figure 2). At room
1
none (for H and 13C NMR spectra, see Figure S3–S4 in the
Supporting Information) of 2,6-di-tert-butylphenoxide radi-
cals and core-unsubstituted PBI (the latter is formed by
debromination of PBI 1) were detected under the various
reaction conditions applied, which strongly supports the
proposed SRN1 mechanism for the formation of 4-hydroxy-
phenyl-substituted PBIs 2 and 3. Moreover, the fact that the
highest yield (12%) of di(4-hydroxyphenyl)-substituted PBI 3
was obtained in DMSO, which is an ideal solvent for SRN1
reactions owing to its low reactivity as a hydrogen-atom donor
towards aryl radicals,[20] corroborates such a radical mecha-
nism.
Figure 2. UV/Vis absorption (solid lines) and normalized emission
spectra (lex =450 nm, dashed-dotted lines) of PBIs 2 (gray) and 3
(black) in dichloromethane at room temperature.
The molecular structures of PBIs 2 and 3 were assigned by
1D and 2D NMR spectroscopy (see Figure S5–S8) as well as
by single-crystal X-ray analyses (for details and crystal data,
see the Supporting Information), which unequivocally con-
ꢀ
firm their structures with a C C coupling between the
perylene core and the functional 4-hydroxyl-3,5-di-tert-butyl-
phenyl unit (Figure 1). In the crystal of PBI 2, two pairs of
temperature, the UV/Vis spectra of PBIs 2 and 3 showed
absorption maxima at 534 nm (18726 cmꢀ1) and 578 nm
(17301 cmꢀ1), respectively, which are shifted bathochromi-
cally by 249 cmꢀ1 and 1674 cmꢀ1 with respect to that of the
parent PBI without bay substituents (absorption maximum at
ca. 527 nm[12]). The most remarkable feature of the absorption
spectra is, however, the loss of vibronic fine structure for both
PBIs 2 and 3 and the very intensive absorption throughout the
whole visible range for PBI 3. The UV/Vis spectra in solvents
of different polarity show quite similar absorption maxima
(Table S1) and spectral shapes (Figure S11). Accordingly, the
absorption properties are not governed by a significant charge
transfer from the 4-hydroxyphenyl substituents to the PBI
core but are more likely to be caused by the distortion of the
Figure 1. Molecular structures of a) PBI 2 and b) PBI 3 determined by
X-ray analysis: red, blue, and black balls represent O, N, and C atoms,
respectively. For clarity, the hydrogen atoms and cyclohexyl groups
were omitted.
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aromatic system evoked by sterical congestion in the bay area.
Similar conclusions can be drawn from the fluorescence
spectra of PBIs 2 and 3 that exhibit broad emission bands with
large Stokes shifts at 675 and 685 nm, and fluorescence
quantum yields of only 4.2% and 2.4%, respectively.
Phenol is a well-known halochromic unit in common pH
indicators such as phenol red, thymol blue, and Reichardt
betaines.[23] As expected, upon addition of organic base
tetrabutylammonium hydroxide (TBAH) to a dichlorome-
thane solution of PBI 2, the color of the solution changes.
While the color change from red to orange appears modest
(Figure 3a, inset), UV/Vis-NIR spectroscopy reveals that the
major effect of TBAH addition is not the change in the visible
TBAH base to give the respective phenolate-substituted
anionic PBIs 2ꢀ and 32ꢀ, the formation of which could indeed
1
be substantiated by H NMR spectroscopy (see Figure S14a
and S15a). Accordingly, the unprecedented absorption bands
in the NIR region at 1179 nm for the PBI 2ꢀ anion
ꢀ1
~
(Dn ¼10494 cm compared to that of the bay-unsubstituted
parent PBI[12]) and 1185 nm for PBI 32ꢀ ion in dichloro-
methane are attributed to a pronounced charge transfer from
the electron-rich phenolate bay substituents to the electron-
poor PBI, which is supported by molecular orbital calcula-
tions (see the Supporting Information). Although CT bands
were previously observed for PBIs bearing 4-(diphenylami-
no)phenyl and [4-(diphenylamino)-phenyl]ethynyl substitu-
ents at the bay positions, the bathochromic shifts in those
cases are only modest and could not attain the highly
desirable NIR spectral range, that is, their absorption
maxima are at 653 nm[13a] and 718 nm,[14b] respectively. The
formation of PBI 2ꢀ and 32ꢀ ions could also be followed by
fluorescence spectroscopy as the fluorescence intensities of
PBI 2 and PBI 3 in DCM decrease gradually with an increase
in concentration of TBAH (Figure 3b and Figure S12b).
The color, UV/Vis-NIR, and fluorescence spectra of
neutral PBIs 2 and 3 could be recovered by addition of
trifluoroacetic acid (TFA) to solutions of PBIs 2 and 3 in
dichloromethane containing TBAH, hence confirming the
reversible nature of the deprotonation/reprotonation process
(see Figure S14 and S15). This reversible behavior has also
1
been confirmed by the H NMR spectra showing that all the
proton signals, except those for the alkyl groups, changed
upon addition of TBAH to solutions of PBIs 2 and 3 in
deuterated dichloromethane, and that their initial spectra are
recovered after the addition of TFA to the TBAH-containing
solutions (see Figure S14 and S15).
In conclusion, we have discovered a facile synthetic
pathway for novel halochromic 4-hydroxyphenyl-substituted
PBIs. To the best of our knowledge, such a synthetic pathway
ꢀ
for C C coupling without transition-metal catalysis in the bay
area of perylene bisimide dyes is unprecedented. More
importantly, the anions of the hitherto unknown halochromic
4-hydroxyphenyl-functionalized PBIs exhibit NIR absorp-
tions close to 1200 nm, which were so far elusive for the
perylene dye family. Thus, further studies on this unique class
of halochromic PBIs in various directions are well motivated.
Figure 3. UV/Vis absorption (a) and emission (b, lex =450 nm) spec-
tral changes of PBI 2 in dichloromethane (10ꢀ5 m) with increasing
amounts of TBAH ((0.0, 0.5, 1.0, 1.5, 2.0, 6.0, 20, 30)ꢂ10ꢀ5 m). The
insets show the color changes in absorption (a) and fluorescence
upon excitation with a UV lamp (b) of PBI 2 in dichloromethane.
Received: July 21, 2011
Published online: September 23, 2011
ꢀ
Keywords: C C coupling · chromophores · dyes/pigments ·
halochromism · near-infrared absorption
spectral range (decrease of the band at 534 nm accompanied
by a slightly hypsochromic shift to 528 nm and an increase of
the band at 457 nm accompanied by a slightly hypsochromic
shift to 439 nm) but the appearance of an entirely new band in
the NIR region with an absorption maximum at 1179 nm
(8481 cmꢀ1; Figure 3a). Almost the same spectral changes are
observed for PBI 3 upon addition of TBAH (Figure S12a).
For both PBIs 2 and 3, spectral changes similar to those in
dichloromethane are observed in other solvents such as
DMSO and nitrobenzene (see Figure S13).
.
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