Hydrosoluble and solvatochromic naphthalene diimides with NIR absorption

Article abstract of DOI:10.1039/c3ob41771a

Mimicking biochromophore anions containing phenolate moieties, eight conjugated naphthalene diimides (NDIs) have been synthesized in order to develop probes, displaying charge-transfer transitions affected by the nearby environment. NIR absorption of the resulting phenolates and their solvatochromic properties in both organic solvents and water are described.

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Hydrosoluble and Solvatochromic Naphthalene
Diimides with NIR Absorption
Cite this: DOI: 10.1039/x0xx00000x
Filippo Doria,* Caroline Marie Gallati, Mauro Freccero*
Received 00th January 2013,
Accepted 00th January 2013
DOI: 10.1039/x0xx00000x
www.rsc.org/
Mimicking biochromophore anions containing phenolate NDI dyes exhibiting both CT transition and binding properties
towards G4 nucleic acids in a donor-acceptor type molecule. The
electron acceptor moiety is the NDI core and the donor is a
phenolate anion. They may act as reporters for micro-polarity and
proticity, potentially useful for G4 conformational studies. Effective
moieties, eight conjugated naphthalene diimides (NDIs) have
been synthesized in order to develop probes, displaying
charge-transfer transitions affected by the nearby
environment. NIR absorption of the resulting phenolates and
electron coupling between the NDI and the phenolate has been
their solvatochromic properties in both organic solvents and achieved by a conjugating ethynyl spacer. The resulting NDIs 1-8
water are described.
(Scheme 1), have been synthesized, characterizing both their
remarkable acidity and the solvatochromic effect associated to the
conjugate bases, in organic solvents and water.
Functional π-conjugated organic systems have been thoroughly
investigated in the last decade, exploiting them as molecular sensors,
semiconductor materials for organic solar cells, photo-molecular
1
-4
5
switches,
and molecular wires. Among them, naphthalene
diimides (NDIs) dyes are particularly interesting, thanks to their
functional and optoelectronic properties, which can be fine-tuned by
6
-9
structural effects. In fact, substituents on the aromatic core shift
the absorption and emission properties from the near-UV to the red,
1
0-12
also tuning their redox potentials.
In contrast, the substitutions at
the imide moieties do not affect such properties, as there is a node on
1
3
both the HOMO and LUMO orbitals on the imide nitrogens.
Therefore, imide substitutions have been exploited to direct the
1
4
organization of these dyes by hydrogen bonding and metal-ligand
1
5
coordination, altering the solubility and the aggregation, with
negligible effects on the properties of the aromatic core. Parallel to
such an interest as new materials, the NDIs also attracted a great deal
of interest for their biological and medical applications. In fact, tri
and tetra-substituted NDIs, have been developed as a new class of
Scheme 1. Ethynyl-NDIs (1-8) synthesized.
13,29-33
9,11,20-22,34
Several groups,
including our own
have developed
1
6-18
19-22
and optimized efficient protocols to functionalize the NDIs core
introducing structural features aimed at the tuning of the electronic
properties of this class of dyes. Synthetically mono- and -
disubstituted NDIs at the aromatic core are accessible from 2,6-
dibromonaphthalene dianhydride by imidization with primary
promising ligands
and alkylating agents
targeting quadruplex
DNA (G-quadruplex, G4) with remarkable selectivity over duplex
DNA. Great effort has also been devoted to the engineering of NDIs
2
3,24
9,25-27
effectively responsive to anions
and pH.
Other
environmental changes, involving solvent polarity and proticity,
received much less attention, and to the best of our knowledge only a
amines, followed by aromatic nucleophilic substitution of the
32,33,35,36
1
3
bromine atoms at the naphthalene core, or Sonogashira
and
very limited number of studies have been published. It is well-
known that the choromophores mostly affected by the environment
are anions displaying charge-transfer (CT) transitions, as even a
single water molecule or an hydrogen bonding interaction causes a
5,37,38
Stille cross-couplings.
The synthesis of the NDIs 1−8 was
carried out using a palladium-catalyzed Sonogashira reaction, as key
synthetic step. The cross-couplings were performed with two
protected 4-ethynylphenols 11 and 15 (Scheme 2), and 2,6-
dibromonaphthalene tetracarboxylic diimide, using Pd(0) catalyst,
2
8
systematic blue shift of the CT electronic transition. Therefore, in
an attempt to develop probes for G4 environment, we design new
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Scheme 2. Synthesis of 11 and 15. (i) (CH
3
CO)
2
O, I
2
, microwave 300W, 10 min, (ii)
dry toluene, CuI, PdCl
min 0°C; 1M TBAF in THF solution, THF, (iv) NaBH
v) dimethoxypropane, acetone, PTSA, 1h r.t., (vi) Triethylamine, CuI,
PdCl (PPh , PPh , TMS-acetylene, 10h 90°C
2 3
(PPh )
2
, TMS-acetylene, t-butylamine, 24h 40°C, (iii) 30
4
, THF:H O (15:0.1), 5 min r.t.,
2
(
2
3
)
2
3
.
Cu(I) co-catalyst, and triethylamine as base, according to the
synthetic strategy depicted in Scheme 3. Typically, the reaction
required anhydrous and anaerobic conditions in THF. The protection
of the phenol moiety as acetyl (11) and 1,3-dioxolan (15) derivatives
were mandatory for achieving good reaction yields (>60%). The
protected ethynylphenol 11 was synthesized by phenol acetylation,
further Sonogashira cross-coupling with trimethylsilylacetylene and
final fluoride induced desilylation. Similarly, the substituted ethynyl
phenol 15 was synthesized by sodium borohydride reduction of the
commercially available 5-bromo-2-hydroxybenzaldehyde, protection
of the resulting 2-hydroxybenzyl alcohol 12 as isopropyliden acetal
(
13), and subsequent Sonogashira reaction. Both the cross-couplings
of the intermediate and 13 were performed with
trimethylsilylacetylene in the presence of Pd(PPh ) Cl PPh3 and
9
3
2
2,
Scheme 3. Synthesis of
derivatives 11 15, THF; (ii) K
CH CN, (iv) HCl 10%, r.t. 30 min, THF.
1
-
8
. (i) CuI, Pd(PPh
3
)
4
, Et
3
N, 90 min 40°C, ethynyl-
O 4:1, (iii) MeI, r.t. 12 h,
CuI in Et N at 90°C for 4 h, quantitatively yielding the products 10
3
,
2
CO3, r.t. 1 h, MeOH/H
2
and 14. They were both efficiently desilylated by TBAF 1 M in THF
solution (Scheme 2). To avoid the competing oxidative Cu(II)
mediated homo-coupling of the resulting 11 and 15, the reaction was
carried out at 0°C in anhydrous conditions and in the absence of
molecular oxygen. Subsequently both 11 and 15 have been tethered
to the functionalized NDI 16, 17 resulting in the new NDI-dyes 1-8.
The synthesis of the new compounds relies on the imidization of 2,6-
dibromonaphthalene tetracarboxylic dianhydride by N’,N’-dimethyl-
3
Table 1. Wavelength of maximum absorbance (max/nm) for the NDIs 1, 2, 5
and 6 and their conjugate bases in organic solvents.
Solvent

max/nm
-
-
2-
2-
1
1
2
2
5
5
6
6
DMSO
ACN
AcOEt
482 799 488 735 526 823 533 840
465 728 489 692 510 718 524 773
473 632 477 627 521 665 526 710
493 629 478 621 541 650 542 710
487 600 489 607 539 645 546 683
474 585 484 588 519 642 535 654
1
,2-ethanediamine, giving a 60:40 mixture of the mono and
dibromo-substituted NDIs 16 and 17 (Scheme 3). The crude
containing both 16 and 17) was used without further purifications
CHCl
3
(
Toluene
MeOH
for the next Sonogashira reaction, which was carried out in the
presence of 11 or 15, with tetrakis(triphenylphosphine)palladium(0)
and CuI, in THF-Et N 1:1, at 40°C (Scheme 3). The reactants were
quantitatively converted into the mono-substituted adducts 1 and 2
3
The second ethynyl substitution on the NDI core (5 and 6) causes a
moderate 45-50 nm red shift, with the maximum absorption centred
between 510 and 540 nm (Table 1 and Figure 2S), as result of the
cross-conjugation. The solvation effect is very modest for the di-
substituted NDIs 5 and 6. We attributed such a band to an
intramolecular charge transfer (ICT) transition, following the
suggestion of a previous study on other NDIs. This assignment is
in agreement with the remarkable red shift (~300 nm) caused by the
deprotonation of the phenol moiety, which was achieved by DBU
and the di-substituted NDIs
5 and 6 after final hydrolytic
deprotection of the phenol moiety. Finally, the quaternary
ammonium salts (3, 4, 7, and 8) were synthesized in quantitative
yields by exhaustive methylation of the NDIs 1, 2, 5 and 6,
respectively, using methyl iodide in ACN (acetonitrile) at r.t. for 6 h.
The resulting products were purified by preparative HPLC and
isolated as hydrochlorides, in order to implement water solubility
and to reduce self-aggregation under physiological conditions. The
ground-state optical absorption spectra of the synthesized NDIs were
measured in a range of organic solvents (Figure 1S), including
DMSO (dimethyl sulfoxide), ACN, chloroform, ethyl acetate
1
3
(1,8-diazabicyclo[5.4.0]undec-7-ene) addition (1 equivalent for the
titration of 1 and 2 and 2 equivalents, for the titration of the NDIs 5
and 6, in DMSO). In fact, the conjugate base of both the NDIs 5 and
6
exhibits a broad ICT band centred in the NIR spectroscopic region,
(AcOEt), toluene and methanol. The monofunctional NDIs 1 and 2
at 823 and 840 nm, respectively (Figure 1).
exhibit a maximum absorption between 465 and 519 nm, with a
limited but measurable (28 nm) solvation effect (Table 1).
2
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The spectra in polar aprotic solvents (DMSO and ACN, red and blue
line, respectively, in Figure 2) was always red shifted in comparison
to that one in methanol (black line, Figure 2), with a 214 nm
-
maximum shift, for the anion 1 . Such a remarkable hypsochromic
effect passing from aprotic polar solvents to protic ones is the most
striking difference between our anionic NDIs and those previously
1
3
investigated by Würthner. The absorption solvatochromism in
organic solvents was evaluated by means of LFER (linear free
energy relationships) using the empirical E (30) and * solvent
T
3
9
polarity scales. It is known that both scales describe solvent
polarity, but they are also differently affected by specific
interactions, such as hydrogen-bonding, for E (30), polarizability
T
and charge transfer effects for the * scale. The correlation of the
-
1
-
-
2-
2-
absorption maximum wavenumbers (cm ) vs * for 1 , 2 , 5 and 6
2
was fairly good (0.93<R <0.98, Figure 4S) in all the solvents, with
the exception of methanol, but is was less satisfying using E (30).
Larger slopes were observed for the anions 1 and 5 lacking of the
T
-
2-
Figure 1. Titration of
6 by DBU in DMSO. Inset: change in the absorbance at 524,
7
90 and 830 nm as a function of the BDU equivalents.
intramolecular hydrogen-bonding with the CH OH moiety. The
2
positive solvatochromism observed in aprotic solvents and the
intramolecular hydrogen bonding effect may be rationalized by a
ground state dominated by a tight ionic couple, with the charge
mainly localized on the phenol moiety (left limit structure in Scheme
Almost identical spectra have been recorded in DMSO replacing
DBU with Cs CO and Na CO . Interestingly, the absorption of the
conjugate base of 5 and 6 exhibits the most red-shifted CT band than
that of any other core-substituted NDI reported in literature. The
spectrophotometric titration in DMSO suggested a very acidic
phenol for both the mono (1 and 2) and the bi-functional NDI (5 and
2
3
2
3
4
), and an excited state with a large anionic delocalization on both
imide carbonyls and charge separation (right structure).
-
2-
Intramolecular and intermolecular hydrogen bonding, within 2 , 6
and in protic solvents, respectively, should both stabilize selectively
the ground state, justifying the hypsochromic shift.
6
6
) with pKa 1.5±0.06 and 0.8±0.05, respectively. For the NDI 5 and
we were not able to unambiguously measure the pKa , probably
2
because it was very close to the pKa . With such an acidic phenol, it
1
was not surprising that the conjugate base of NDIs 6 was also
generated by weaker bases such as Et N, in this case the absorption
3
was blue shifted at 815 nm, suggesting a counter ion effect. The
conjugate base of the NDIs 1 and 2 both exhibit a similar
spectroscopic behaviour with a blue shifted absorption centred at
7
99 and 735 nm, respectively (Table 1, and Figure 3S, for the
-
-
titration of 2). The 64 nm blue shift in DMSO, passing from 1 to 2 ,
which is caused by the intra-molecular H-bonding of the benzyl
alcohol moiety, suggests that these anions are also very sensitive to a
single H-bonding interaction. The anionic NDIs showed
remarkable hypsochromic effect passing from DMSO to less polar Scheme 4. Resonance structures of the anionic NDI
a
-
1
.
solvents and methanol (Table 1, and Figure 2).
Furthermore, we decided to measure acidity and spectroscopic
absorption of the neutral NDIs and their conjugate bases under
aqueous solution, as the NDIs synthesized may be useful as reporters
for micro-polarity and proticity under physiological conditions.
Titration was possible only for 2 in water (Figure 5S), due to the low
solubility and aggregation of the NDIs 5 and 6 at pH higher than 7.
The potentiometric titrations gave three pKa values: 6.9, 7.7 and
1
0.4, and the spectrophotometric one unambiguously suggested that
the pKa had to be assigned to the phenol. pKa and pKa should be
2
1
3
assigned to the ammonium moieties. In order to implement the
solubility and minimize the aggregation of the NDIs at physiological
pH, we decided to methylate both the amino moieties and measure
the spectroscopic properties of the resulting quaternary ammonium
salts (3, 4, 7 and 8) and their conjugate bases (Table 1S). The
spectrophotometric titration of the mono-functional ethynyl-NDIs 3
(Figure 6S) and 4 (Figure 7S), gave pKa values very close to each
other: 7.7 and 7.4, respectively. The spectrophotometric titration of
-
the bi-functional ethynyl NDI 8 generating the phenolates 8 (max=
2
-
6
29 nm) and 8 (max= 704 nm, Figure 3), gave two pKa values: 7.5
and 8.8. Due to solubility problems, the titration of 7 gave only the
first pKa value (7.7).
Figure 2. Solvent dependent absorption of the conjugate bases of 1, 2, 5 and 6 in
organic solvents.
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