Electronic and assembly properties of a water-soluble blue naphthalene diimide

Article abstract of DOI:10.1039/d1nj02557k

Herein we report on the synthesis and characterisation of a water soluble deep blue naphthalene diimide, (ⁱPrNH)₂NDI-V. The synthesis is performed under mild conditions and careful consideration of the purification of the precuror materials leads to improved results in final yield and purity. The optoelectronic properties are explored through cyclic voltammetry and UV-vis absorption and emission spectroscopy in multiple solvents and the self-assembling properties as part of a multi-component hydrogel are investigated through rheology and small angle neutron scattering.

Full text of DOI:10.1039/d1nj02557k

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Electronic and assembly properties of a
water-soluble blue naphthalene diimide†
Cite this: New J. Chem., 2021,
45, 14005
a
b
b
Thomas A. Welsh,
Emily R. Draper
Olga Matsarskaia,
Ralf Schweins and
a
*
Herein we report on the synthesis and characterisation of a water soluble deep blue naphthalene
i
Received 25th May 2021,
Accepted 14th July 2021
diimide, ( PrNH)
2
NDI–V. The synthesis is performed under mild conditions and careful consideration of
the purification of the precuror materials leads to improved results in final yield and purity. The optoe-
lectronic properties are explored through cyclic voltammetry and UV-vis absorption and emission
spectroscopy in multiple solvents and the self-assembling properties as part of a multi-component
hydrogel are investigated through rheology and small angle neutron scattering.
DOI: 10.1039/d1nj02557k
rsc.li/njc
NDI application in electrochromic devices. We have previously
worked with water soluble amino acid-functionalised NDIs in
Introduction
Research into new organic electronic materials has led to rapid hydrogels and found that they display transparent-to-dark
1
–4
5–7
15
developments in photovoltaics,
light-emitting diodes,
field-effect transistors,
electrochromism, which makes them promising candidates for
8
–10
11,12
13
18
sensors,
and batteries. Organic electrochromic device applications such as smart windows.
materials possess a number of advantages over alternatives
Naphthalene diimides are common moieties found in many
such as their lightweight, flexibility, tuneable optoelectronic electronic materials applications and a wide variety of functio-
properties, solution processability, and derivability from renew- nalisation options are available to them. Aside from the afore-
able feedstocks. Of particular interest to the synthetic chemist mentioned imide functionalisation, functionalisation of the
1
9
are the readily tuneable optoelectronic properties and solution naphthalene core is also very common. While functionalisa-
processability of organic materials. Through simple chemical tion at the imide positions typically only affects the solubility
reactions, a wide variety of modifications can be made to the properties, core functionalisation can also affect the optoelec-
material scaffold that drastically alter optical, electronic, and tronic properties of the species. Many core-functionalised NDIs
1
9–22
solubility properties.
exist with a wide variety of colours and applications.
One
When considering material practicality, solubility is one of such functionalisation is the incorporation of amino groups off
the most important factors. In order to be developed on an the core which has been shown to provide NDIs with excep-
industrial scale, materials should be soluble in environmentally tional photophysical properties including
friendly, nontoxic solvents. Water is the ideal solvent but colour.
a
deep blue
The synthesis can also be done under mild condi-
many organic materials are poorly soluble in water unless tions without the need for co-reagents or transition metal
1
4
22–26
2
6
specially functionalised. In our research, we synthesise water catalysts. Some of these materials have even been made water
soluble naphthalene diimides (NDIs) by functionalising at the soluble. This is typically achieved by using acid to protonate the
imide positions with amino acids, which can be deprotonated amino substituents to make water soluble cationic salts, as was
1
5
23
24
to make fully water-soluble organic molecules (Scheme 1).
reported by Weißenstein et al. and Doria et al., which is in
NDIs are typically used as acceptor moieties in organic electro- contrast to our method of using base to deprotonate amino acid
15
nic devices owing to their ability to form stable radical side chains to make water soluble NDIs. Another strategy
1
6,17
25
anions.
These stable radical anions are often accompanied employed by Higginbotham et al. is to incorporate hydrophilic
by a large change in the absorption properties which allows for
a
School of Chemistry, University of Glasgow, Glasgow G12 8QQ, UK.
E-mail: Emily.Draper@glasgow.ac.uk
b
Institut Laue-Langevin, Large Scale Structures Group, 71 Avenue des Martyrs,
CS 20156, F-38042 Grenoble CEDEX 9, France
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
Scheme 1 The method by which amino-acid functionalised NDIs are
d1nj02557k
made water soluble.
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neutral functional groups such as glycols. However, this method to obtain Br
2
NTCDA in a 29% yield. Purifying Br
2
NTCDA in this
was not as effective for making water soluble materials as produ- way leads to improved results in the subsequent reactions.
cing charged species for the neutral materials formed micelles in
aqueous solution.
The Br NTCDA can be reacted with an amino acid, in this
case L-valine, in glacial acetic acid to form the dibrominated
2
Herein, we describe the synthesis of a water-soluble NDI NDI, Br NDI–V. The crude product can be obtained as an
2
core-functionalised with isopropylamino groups using mild orange solid in about a 75% yield by precipitating the reaction
reaction conditions. We investigate the optoelectronic and mixture in cold water and vacuum filtering. The crude product
self-assembly properties via cyclic voltammetry (CV), UV-vis can be used without further purification, but it can be purified
absorption and emission spectroscopy, rheology, and small angle by stirring in acetonitrile at room temperature for a few hours
neutron scattering (SANS) and assess its potential applications.
and filtering.
The final reaction involves suspending the orange Br NDI–V
2
in isopropylamine and stirring at room temperature. According
2
6
to the literature, this reaction leads to a 48% yield of the
disubstituted product after 48 hours. The major impurity of
this reaction is the monosubstituted form which is magenta in
colour. However, we have found that leaving the mixture to stir
at room temperature for 10 to 12 days leads to a much higher
Results and discussion
Synthesis
Obtaining a core-functionalised NDI requires beginning with
the unsubstituted naphthalene tetracarboxylic dianhydride
(
80%) yield of the blue disubstituted product after column
(NTCDA). Through our previous research with synthesising
i
purification. The final product, ( PrNH) NDI–V, is isolated as
a dark blue solid and characterised by H and C NMR.
2
NDIs, we have found that the NTCDA supplied by common
chemical suppliers is only about 20% pure with the majority of
the material composition being the monoanhydride form. This
has led to poor reaction outcomes and so we use a method to
1
13
Electronic and optical characterisation
regenerate and purify the dianhydride by refluxing the crude One of our primary interests in water-soluble NDIs lies in their
material in acetic anhydride and recrystallising from N,N- electrochromic properties. The unfunctionalised analogue of
i
dimethylformamide (DMF). A more detailed account of the ( PrNH) NDI–V, NDI–V, for instance, displays distinct colour-
2
purification method can be found in the ESI.† With pure less to dark purple electrochromism upon reduction (Fig. 1).
i
NTCDA, the core-functionalised NDI could be obtained follow- However, ( PrNH)
2
NDI–V does not display any change from its
ing the procedure outlined in Scheme 2.
deep blue colour upon oxidation or reduction, in stark contrast
The first step of preparing a core-substituted NDI is to to its analogue NDI–V or our previously reported electrochro-
1
5
chlorinate or brominate the NTCDA. Such reactions often mic NDI. Thus, while isopropylamino-substitution on the
require harsh conditions and so should be performed before naphthalene core does imbue the species with a vibrant blue
any other functionalisation is done. There are many methods colour, it also prevents any electrochromism. As a result,
2
7–29
i
by which to introduce halides to an aromatic core
and we ( PrNH) NDI–V would not be suitable for application in smart
2
have chosen the more mild route as outlined by W u¨ rthner window or display technology and would be better suited as a
3
0
et al. ‘‘Mild’’ here is of course a relative term as this method water-soluble dye.
does require the use of fuming sulfuric acid. This reaction The electronic characteristics of the blue ( PrNH)
i
2
NDI–V
results in the bright yellow and highly insoluble dibrominated were characterised via CV. The cyclic voltammogram is shown
NTCDA. Due to its insolubility this material is often used in in Fig. 2a. It was measured in dichloromethane (DCM) with
further reactions without purification or even NMR analysis. 0.1 M tetrabutylammonium hexafluorphosphate (NBu PF )
4
6
However, we have found that it is soluble enough in DMSO-d
electrolyte. There is a reversible oxidation wave with an E1/2
6
1
to obtain an NMR spectrum (ESI,† Fig. S1). The H NMR potential 0.64 V and an onset of 0.55 V which gives an ioniza-
spectrum of the crude product shows that it is a mixture of tion potential of 5.4 eV. There also appears to be two reduction
species of varying levels of bromination (ESI,† Fig. S2) which events: an initial irreversible reduction with an onset of ꢀ1.01 V
can be purified by boiling in acetonitrile and filtering while hot and a partially reversible reduction with an E1/2 of ꢀ1.58 V.
i
Scheme 2 The synthetic method to obtain core-functionalised blue ( PrNH)
2
2
NDI–V. (a) Dibromoisocyanuric acid (2.5 eq.), H
2
SO
4
3
ꢁ20% SO , 25 1C, 5 h,
9% yield. (b) L-Valine (2 eq.), glacial acetic acid, 120 1C, 18 h, 76% yield. (c) Isopropylamine, 25 1C, 10 days, 80% yield.
1
4006
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i
Table 1 The optoelectronic properties of ( PrNH)
2
NDI–V. Electronic
properties were measured in DCM with 0.1 M NBu PF electrolyte. Optical
4
6
properties were measured in water with a 40% v/v 0.1 M NaCl pH-adjusted
to 10 with 1 M NaOH
Property
Value
a
Ionization potential, IP (eV)
5.4
3.8
a
Electron affinity, EA (eV)
Electronic band gap, Eelec (eV)
Absorption maximum, labs (nm)
Emission maximum, lem (nm)
1.6
634
681
13 700
0.197
7.5
0.31
1.8
ꢀ
1
ꢀ1
Molar absorptivity coefficient, e, 634 nm (M cm
)
ꢀ1
Integrated molar absorptivity, 300–800 nm, (M
Quantum yield (%)
)
b
Stokes shift (eV)
Optical band gap, Eopt (eV)
c
i
a
33
Fig. 1 The chemical structures of NDI–V and ( PrNH)
2
NDI–V along with a
Energies were calculated by (onset V + 4.8) where Fc HOMO = 4.8 eV.
b
display of the electrochromic properties of NDI–V and the lack of
Stokes shift was calculated by (Eabs ꢀ Eems) where (Emax = h ꢂ c/lmax
;
c
i
h = Planck’s constant, c = speed of light). Optical band gap was
calculated from the onset absorption (700 nm) where (Eopt = h ꢂ c/lonset).
2
electrochromic properties of ( PrNH) NDI–V. Both species were dissolved
in water with 40% v/v 0.1 M NaCl electrolyte at pH 8.
Naphthalene diimides are known to be readily reduced to a
3
1,32
radical anion and dianion
and so the two reduction events
observed in the CV likely correspond to these species. A fully
reversible oxidation peak is usually not observed for NDIs but
i
for ( PrNH)
2
NDI–V the incorporation of the amino substituents
on the NDI core has raised the HOMO energy level and lowered
the ionization potential thereby enabling reversible oxidation.
Density functional theory calculations on similarly functiona-
lised NDIs have shown that the core-amino group contributes
2
3
significantly to the HOMO energy level.
i
2
Due to ( PrNH) NDI–V is water soluble at high pH the CV
was also measured in water with 40% 0.1 M NaCl as an
electrolyte and pH adjusted to 8 with 1 M NaOH (Fig. 2b).
The CV in aqueous solution is less defined than that in DCM,
likely because of the material’s aggregation in aqueous media.
There appears to be a partially reversible oxidation event with
an onset at 0.5 V along with two irreversible reduction events at
ꢀ
0.6 V and ꢀ1.5 V which likely correspond to the two reduction
events observed in the DCM CV. The CV profile remained
consistent for multiple scans and at different scan rates (ESI,†
Fig. S8–S10). Unfunctionalised NDI–V, in contrast, displays a
clearly reversible reduction wave in aqueous media with an E1/2
potential of ꢀ0.54 V (Fig. 2b), which is at similar potential to
i
the first reduction peak observed for ( PrNH) NDI–V. This
2
indicates that, while the addition of amino substituents to
the NDI core has had a significant impact on the HOMO energy
level, the LUMO is relatively unaffected. The differences in
reversibility for the reduction peaks between NDI–V and
i
(
2
PrNH) NDI–V are most likely due to differences in the materi-
i
Fig. 2 (a) The cyclic voltammogram for (PrNH)
0
2
NDI–V measured in DCM with als’ self-assembly in aqueous media caused by the addi-
.1 M NBu
shown. (b) The cyclic voltammograms for (PrNH)
black trace) measured in water with a 40% v/v 0.1 M NaCl electrolyte at pH 8.
4 6
PF electrolyte. Oxidation and reduction onsets and E1/2 potentials are
tional steric and hydrogen-bonding influences of the amino
i
2
NDI–V (blue trace) and NDI–V
substituents.
(
i
The optical properties of ( PrNH) NDI–V were characterised
2
via UV-vis absorption and emission spectroscopy. The UV-vis
From the onset of the first reduction, the electron affinity is absorption profile, measured in water with 40% v/v 0.1 M NaCl
calculated to be 3.8 eV which gives the material an electronic and pH-adjusted to 10 with 1 M NaOH, is shown in Fig. 3
band gap of 1.6 eV. These values are listed in Table 1. along with the emission profile with excitation at 380 nm.
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electron pairs of the amino groups to the NDI core thereby
2
3
quenching emission. Protonating the amino side groups has
been shown to increase quantum yield of similarly functiona-
2
3,24
i
lised NDIs
but for ( PrNH) NDI–V protonation of the amino
2
groups is not possible without also protonating the carboxylate
groups and rendering the material insoluble in aqueous media.
i
Although ( PrNH)
2
NDI–V is not electrochromic, its optical
properties do present alternative applications. The vivid and
electronically stable colour combined with its water solubility
i
makes ( PrNH)
2
NDI–V well suited as a water-soluble dye, while
the red to NIR emission, despite the low quantum yield in
aqueous media, could be useful in sensory applications. The
i
optical properties of ( PrNH) NDI–V are summarised in Table 1.
2
i
Due to ( PrNH)
2
NDI–V’s water solubility is dependent on pH
similar to other amino-functionalised NDIs,
pH on the optical properties of ( PrNH)
2
3,24
the effects of
NDI–V in aqueous
Fig. 3 The absorption (solid blue) and emission (dot blue) profiles for
i
i
2
(
2
PrNH) NDI–V and absorption (solid black) profile for NDI–V. All spectra
solution were also analysed. As shown in Fig. 4, the optical
profiles at high pH values remain unchanged. At lower pH
values, starting at pH 5, a drop in absorbance across the
spectrum is observed which is an indication of the material
were measured in water with a 40% v/v 0.1 M NaCl electrolyte at pH 10.
Emission profile was obtained with excitation at 360 nm.
The absorption spectrum displays absorption bands at 350 nm precipitating out of solution. There is also a noticeable blue
and 367 nm which are typical for naphthalene diimides and are shift in the absorption maxima as well as an increase in the
2
5
the result of p–p* transition in the NDI core. These same 590 nm shoulder of the broad absorption band relative to the
bands appear in the UV-vis absorption profile for the unfunc- maximum. These changes are most pronounced at pH 3 and 4
tionalised NDI–V (Fig. 3) along with an additional band at and are indicative of a change in aggregation in solution, which
3
84 nm which is not present in the absorption profile of is to be expected for this material. As discussed above, these
i
(
PrNH)
PrNH)
2
2
NDI–V. The main feature of the absorption profile of types of NDIs are water-soluble through deprotonation of the
NDI–V is a large absorption band from 500 nm to amino acid side chains and lowering the pH would lead to
i
(
7
5
00 nm with a maximum at 634 nm and a slight shoulder at protonation of these groups, thereby changing the material’s
90 nm due to vibronic coupling. This band is the result of aggregation in aqueous solution and eventually leading to
2
5
the isopropyl amino substituents raising the HOMO energy precipitation. Cyclic voltammetry was also performed at differ-
level of the material and is commonly observed in other NDIs ent pH values and changing the pH had little to no effect on the
2
3,25,34
with similar core substituents.
As such, it is not present electronic properties (ESI,† Fig. S11–S15). This is in contrast to
in the unfunctionalised analogue which displays little to no our other systems that have different electronic properties
3
5–37
absorption above 400 nm. The absorption profile also indicates depending on their aggregated state.
i
i
that ( PrNH)
2
NDI–V is fully dissolved in water for if it were
The blue ( PrNH) NDI–V also displays solvatochromic prop-
2
forming aggregates such as micelles we would expect to see erties. The absorption profile was measured in acetone, DCM,
2
5
significant broadening of the absorption bands. The material
ꢀ1
ꢀ1
has a high molar absorptivity coefficient of 13 700 M cm
ESI,† Fig. S22) at 634 nm and gives rise to the distinct deep
blue colour of the species. The onset of the absorption at
00 nm gives an optical band gap (i.e. band gap determined
(
7
from the optical absorbance spectrum) of 1.8 eV, which corre-
sponds well to the electrical band gap of 1.6 eV calculated from
i
the CV. The absorption of ( PrNH) NDI–V across the visible
2
spectrum is exceptionally high for an NDI species with
i
(
2
PrNH) NDI–V having an integrated molar absorptivity of
ꢀ1
0.197 M from 300 nm to 800 nm.
i
Upon excitation at 360 nm, ( PrNH)
2
NDI–V emits in the red
to NIR portion of the spectrum from 620 nm to 820 nm with an
emission maximum at 681 nm. An identical emission profile is
also obtained upon excitation at 600 nm (ESI,† Fig. S25). The
i
quantum yield of ( PrNH) NDI–V is 7.5%, which is relatively low
2
but not out of the range of other similarly functionalised NDIs
i
Fig. 4 The UV-vis absorption profile for ( PrNH)
water with a 40% v/v 0.1 M NaCl at various pH values from pH 10 to
to photo-induced electron transfer occurring from the lone pH 3 with absorption maxima shown at pH 10 (blue) and pH 3 (red).
2
NDI–V measured in
2
3,24
in the literature.
The low quantum yield here is likely due
1
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and methanol, as shown in Fig. 5a. The organic solvents blue- solvents so the material would be expected to have different
shift the absorption profile maxima relative to water with aggregation properties which give rise to the changes in the
acetone and methanol leading to the largest shift of about absorption profile and likely account for why absorption in
20 nm (634 nm maximum in water vs. 612 nm and 615 nm water occurs at longer wavelengths.
maxima in acetone and methanol, respectively). DCM causes a The effects of acid on the absorption profile can also be
smaller blue-shift of about 10 nm. Materials with a larger dipole explored in organic solvents without the material precipitating
moment in the ground state than in the excited state will from solution. This was done by adding 2.5% v/v trifluoroacetic
experience greater stabilization of the ground state than the acid (TFA) to the acetone, DCM, and methanol solutions. The
excited state in more polar organic solvents, leading to a larger addition of TFA in acetone and methanol had no effect on the
3
8
band gap and absorption at shorter wavelengths. This is absorption profile (ESI,† Fig. S23, S24) but a significant change
known as negative solvatochromism. For materials where the was observed for the DCM solution (Fig. 5b). A significant blue
excited state has larger dipole moment than the ground state, shift of the broad absorption peak was observed from 620 nm
the excited state is stabilised to a greater degree than the to 551 nm as well as the appearance of a new band at 643 nm.
ground state by more polar organic solvents and positive The intensity of the high frequency bands at 364 nm and
solvatochromism occurs with absorption at longer wave- 347 nm also increased and redshifted to 372 nm and 353 nm,
lengths. The organic solvents also give rise to a more pro- respectively. All of this had the effect of turning the solution
nounced shoulder in the broad absorption band at 575 nm from blue to a deep purple. Clearly, the addition of acid on the
i
than water. These solvatochromic effects have been observed absorption profile of ( PrNH)
2
NDI–V in DCM, which is the least
2
5
for similarly functionalised NDIs. Unlike in water, deprotona- polar of the solvents explored, has significant effects on the
tion is not necessary to dissolve ( PrNH) NDI–V in these organic material’s solution aggregation properties. It is most likely that
i
2
i
the strong acid TFA is protonating ( PrNH) NDI–V at the amino
2
positions and leading to new types of intermolecular
3
8
arrangements. The splitting of the absorption band observed
in Fig. 5b may indicate that these new intermolecular arrange-
ments are an example of oblique aggregation, or a hybrid of H-
3
9,40
and J-aggregates.
Given that these changes are not observed
in acetone and methanol indicates that the TFA interaction
i
with ( PrNH) NDI–V and formation of new intermolecular
2
arrangements are suppressed in these solvents due to their
higher polarity relative to DCM. It is also likely that the
protonation of the amino groups has affected the energy levels
i
of ( PrNH)
2
NDI–V which is also contributing to the change in
absorption.
The organic solvents also influence the emission profile of
i
(
PrNH) NDI–V. Once again, a blue shift is observed in organic
2
solvents with the emission maximum shifting to 642 nm,
49 nm, and 649 nm in acetone, dichloromethane, and metha-
6
nol, respectively. Emission profiles are identical with excitation
wavelengths of either 360 nm or 600 nm (ESI,† Fig. S25). The
addition of a 2.5% v/v TFA aliquot also drastically changes the
emission profile in DCM with much broader emission from
5
6
50 nm to 800 nm and two emission maxima occurring at
77 nm and 592 nm. Here, changing the excitation wavelength
does change the intensity ratio of the emission maxima (ESI,†
Fig. S26). Excitation at 360 nm leads to a higher emission
intensity at 677 nm than at 592 nm while excitation at 500 nm
and 550 nm leads to higher emission intensity at 592 nm than
677 nm. As mentioned above, protonation of the amino groups
in similar functionalised NDIs has been shown to lead to
2
3,24
an increase in quantum yield.
The quantum yield for
i
(
PrNH) NDI–V measured in DCM with 2.5% v/v TFA is 15.5%
2
which has increased compared to 7.5% in water. However, the
quantum yield measured in DCM without the addition of TFA
Fig. 5 (a) UV-vis absorption (solid) and emission (dot) profiles for
i
(
PrNH)
2
NDI–V measured in acetone (red), DCM (green), methanol (pur-
was found to be 36.3%. It is likely that the protonation of the
ple), and water at pH 10 (blue); (b) absorption and emission profiles for
i
i
amino groups of ( PrNH)
2
NDI–V leading to new aggregation is
(
(
PrNH)
2
NDI–V measured in DCM (green) and in DCM with 2.5% v/v TFA
orange). Emission was obtained with excitation at 360 nm.
suppressing more than enhancing the emission, in contrast to
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i
Table 2 Solvatochromic data for ( PrNH)
2
NDI–V
multicomponent gels, the gels break at just under 10% strain
and G and G are independent of frequency.
0
00
Solvent
labs max (nm)
lem max (nm)
QY (%)
Time-sweep rheological data was also obtained for single
and multicomponent gels and combined with pH measure-
ments to show how lowering the pH triggers gelation. The data
(ESI,† Fig. S28, S29) clearly show that a decrease in pH is
accompanied by an increase in G and G , which is consistent
with pH-induced gel formation. The self-assembly of the multi-
component gel displays a slightly different behaviour. The
Water, pH 10
Water, pH 3
Acetone
634, 367, 351
625, 365, 347
612, 361, 344
620, 364, 347
615, 361, 344
643, 551, 372, 353
681
681
642
649
649
677, 592
7.5
NA
NA
36.3
NA
CH
Methanol
CH Cl + TFA
Cl
2 2
0
00
2
2
15.5
23,24
what has previously been reported.
tochromic properties of ( PrNH) NDI–V is shown in Table 2.
A summary of the solva- same stages occur corresponding to the decrease in pH and
i
the assembly of the 1-ThNap-FF, although the data is a little
2
i
noisier. This may indicate that ( PrNH) NDI–V is templating
2
Gel formation and characterisation
onto the gel fibres formed by 1-ThNap-FF rather than remain-
ing freely dispersed throughout the hydrogel or incorporating
itself into the gel fibres. Time-sweep rheological and pH data
Previous research with water soluble NDIs has shown that they
can be used to prepare hydrogels when functionalised with
i
1
5
were obtained for the system containing only ( PrNH) NDI–V
2
certain peptides. We saw that the immobilisation of the NDI
as a gel was advantageous for device fabrication as immobilisa-
tion reduced leakage and enabled material patterning. To that
i
and GdL (ESI,† Fig. S30) to show that ( PrNH)
2
NDI–V does not
gel by itself when the pH is lowered. Therefore, gelation in the
multicomponent system is solely due to 1-ThNap-FF. A direct
comparison of the time-sweep rheological data for both single
and multicomponent gels (ESI,† Fig. S31) shows that while
gelation seems to initiate faster in the multicomponent than in
the single component system, both systems seem to reach a
end, we have fabricated a multicomponent hydrogel using the
i
blue ( PrNH) NDI–V and the dipeptide 1-ThNap-FF (Fig. 6), a
2
material that has been shown to form both single component
4
1–44
and multicomponent hydrogels.
This would enable the dye
to be immobilised in or as part of a gel network.
0
00
plateau in G and G after the same amount of time. However, a
direct comparison cannot be made from gelation kinetics or
final rheological values due to the different amounts of mate-
rial present in the single and multicomponent systems and the
different amount of GdL needed to reach the same final pH in
both systems.
The strategy employed to form hydrogels involved dissolving
PrNH) NDI–V and 1-ThNap-FF in aqueous solutions at pH 11
2
i
(
with 1 M NaOH, combining the solutions in a 1 : 1 volumetric
ratio, and mixing with glucono-d-lactone (GdL). In aqueous
solution, GdL hydrolyses which lowers the pH of the solution
i
thereby triggering gelation. Both ( PrNH) NDI–V and 1-ThNap-
2
ꢀ1
ꢀ1
Cyclic voltammetry and UV-vis absorption measurements
were performed on the multi-component gels prepared from
FF were used at 5 mg mL concentrations and 10 mg mL of
GdL was used to achieve slow, uniform acidification of the
solution to a pH of 3.8. After 18 hours the solutions had become
dark blue gels capable of remaining suspended when inverted
ꢀ
1
i
ꢀ1
5
2
mg mL ( PrNH) NDI–V and 5 mg mL 1-ThNap-FF to see if
gelation had altered the optoelectronic properties of the
i
ꢀ
1
( PrNH) NDI–V in any way (Fig. 7). Additional CV data for gels
2
(Fig. 6). Single-component gels of 1-ThNap-FF at 5 mg mL
ꢀ1
i
ꢀ1
prepared with 0.25 mg mL ( PrNH) NDI–V and 5 mg mL
2
were also prepared following the same method to study the
i
1-ThNap-FF is provided in the ESI,† Fig. S18, to more directly
effects of ( PrNH) NDI–V incorporation in the blend gels.
2
compare with the solution CV data, which was measured with
The single and multicomponent gels were characterised
using rheology. Strain and frequency measurements show that
the blend gels behave like typical low molecular weight gels
ꢀ
1 i
0
2
.25 mg mL ( PrNH) NDI–V concentration. Although the gel
CV shows even less resolution than the aqueous solu-
1
5
tion measurements, the data does seem to indicate that
previously studied in our group (ESI,† Fig. S27). The multi-
i
0
( PrNH)
2
NDI–V possesses the same electronics in solution and
component gels have slightly lower storage modulus (G ) values
i
00
2
in the gel state, again suggesting that the ( PrNH) NDI–V has
and loss modulus (G ) values than the single-component gels.
i
not been associated into the 1-ThNap-FF fibres. The lack of
2
This is likely due to the assembly of ( PrNH) NDI–V disrupting
gel formation mediated by 1-ThNap-FF though not to a suffi-
cient degree to completely inhibit gelation. For both single and
resolution in the reduction peaks could be due to the
i
(
PrNH) NDI–V being effectively immobilised on the 1-ThNap-FF
2
network, and so diffusion is limited. The UV-vis profile of the
multicomponent gel is identical to the aqueous solution UV-vis
profile, further indicating that gelation has not affected the
i
optoelectronic properties of ( PrNH)
2
NDI–V. This again suggests
i
that the ( PrNH)
2
NDI–V is not in the gel pores, as the difference in
aggregation would likely lead to differences in the absorption
profile, but a part of the network.
Small angle neutron scattering (SANS) was collected on both
the single component and multicomponent systems to assess the
gel networks and structures formed at low pH. The scattering data
Fig. 6 The chemical structure of 1-ThNap-FF and photographs of the
i
ꢀ1
ꢀ1
blend ( PrNH) NDI–V (5 mg mL ) and 1-ThNap-FF (5 mg mL ) hydrogel. for each system is shown in Fig. 8 and ESI,† Fig. S32–S34.
2
1
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i
(
2
PrNH) NDI–V fitted best to a power law and had the lowest
scattering intensity. This was expected as we know from the
i
rheological data (ESI,† Fig. S30) that ( PrNH) NDI–V does not
2
form an extended network and only aggregates upon lowering the
pH. 1-ThNap-FF fitted to a flexible cylinder with a radius of 33 Å
combined with a power law, which has been observed before and
41,42
agrees with it forming a fibrous gel network.
The blend gel
also fitted to a flexible cylinder combined with a power law, but
with a larger radius of 50 Å and a 0.2 polydispersity. This
i
demonstrates that although the ( PrNH)
2
NDI–V does not affect
the ability of 1-ThNap-FF to form a gel or fibres, it does affect the
fibres formed both in uniformity and diameter. This suggests that
the network is self-sorted rather than co-assembled, but that
i
perhaps the ( PrNH)
2
NDI–V has associated itself with the fibres
of 1-ThNap-FF giving a larger fibre diameter, due to their hydro-
phobic nature. This complexity in assembly is also seen in the
rheology time-sweep data. Further information and table of fits
used for the SANS data can be found in the ESI.†
Conclusions
Herein we described the synthesis and optoelectronic charac-
terisation of a new water-soluble NDI core-functionalised with
i
isopropylamino substituents, ( PrNH)
2
NDI–V, which possesses
a vibrant blue colour. The material was synthesised following
2
6,30
previously established procedures from the literature
and it
was found that purifying the starting materials, something that
had not been attempted or discussed in prior literature, led to
improved results. It was also found that leaving the synthesis to
react for 10 to 12 days led to dramatically improved results in
i
2
Fig. 7 (a) Cyclic voltammograms for ( PrNH) NDI–V measured as a
solution (solid blue line) in water at pH 10 with 40% 0.1 M NaCl electrolyte
and as a hydrogel (red dotted line) with 1-ThNap-FF and 40% 0.1 M NaCl
i
electrolyte; (b) UV-vis absorption profiles for ( PrNH)
2
NDI–V measured as a favouring the disubstituted product over the monosubstituted
solution (solid blue line) in water at pH 10 and as a gel with 1-ThNap-FF
red dotted line).
product.
The optoelectronic properties of ( PrNH)
(
i
2
NDI–V were char-
acterised using CV and UV-vis absorption and emission
spectroscopy in both aqueous and organic solvents. Unlike
i
the unfunctionalised NDI–V, ( PrNH) NDI–V did not possess
2
electrochromic properties, but was highly absorbing in the
visible spectrum to give it a vibrant blue colour as well as being
i
emissive in the red to NIR. The ( PrNH)
2
NDI–V also displayed
i
2
solvatochromic properties. Dissolving ( PrNH) NDI–V in com-
mon organic solvents led to blue shifts in both the absorption
i
and emission maxima. ( PrNH) NDI–V was also combined with
2
a known gelator and was found to make strongly coloured blue
hydrogels that possessed rheological properties typical for low-
molecular weight hydrogels with little change in its absorption
or electronic properties. This makes it a promising candidate
for use in gel-based electronic devices.
i
With the synthesis of ( PrNH)
2
NDI–V, we have provided an
efficient framework to make water-soluble organic materials
with simple functionalisations that lead to dramatic changes in
the optoelectronic properties of the products obtained. Ease of
functionalisation and tuneable optoelectronic properties is a
well-known advantage of organic materials and synthesising
water-soluble organic materials is very important in developing
cost-effective and environmentally friendly organic electronic
Fig. 8 Small angle neutron scattering (SANS) for the single component
i
(
(
PrNH)
2
NDI–V solution (light blue), single component 1-ThNap-FF gel
i
black), and multicomponent 1-ThNap-FF + ( PrNH)
2
NDI–V gel (dark blue)
with fit lines for each shown in red.
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devices. With its unique blue colour and high visible light 10 R.-P. Xu, Y.-Q. Li and J.-X. Tang, J. Mater. Chem. C, 2016, 4,
i
absorption ( PrNH)
acceptor in various organic electronic devices. Future efforts 11 A. Rostami and M. S. Taylor, Macromol. Rapid Commun.,
will be devoted to developing other types of water-soluble NDIs
2012, 33, 21–34.
with simple core functionalisations to give rise to new and 12 S. Yamaguchi and A. Wakamiya, Pure Appl. Chem., 2006, 78,
2
NDI–V has potential application as a dye or
9116–9142.
unique optoelectronic properties.
1413–1424.
13 T. B. Schon, B. T. McAllister, P.-F. Li and D. S. Seferos,
Chem. Soc. Rev., 2016, 45, 6345–6404.
Author contributions
14 F. P. Byrne, S. Jin, G. Paggiola, T. H. M. Petchey, J. H. Clark,
T. J. Farmer, A. J. Hunt, C. Robert McElroy and J. Sherwood,
Sustainable Chem. Processes, 2016, 4, 7.
Experiments conceived by ERD and TAW. All authors contrib-
uted towards the writing of the paper. Funding was from ERD.
TAW performed the synthesis and purification for ( PrNH)
NDI–V and its precursors. TAW also performed the NMR, cyclic
voltammetry, optical spectroscopy, and rheology characterisa-
tion. SANS data was collected by RS and OM. SANS was
15 L. Gonzalez, C. Liu, B. Dietrich, H. Su, S. Sproules, H. Cui,
i
2
D. Honecker, D. J. Adams and E. R. Draper, Commun. Chem.,
2
018, 1, 1–8.
6 M. Saha and S. Bandyopadhyay, J. Phys. Chem. C, 2021, 125,
427–6432.
7 M. B. Avinash, K. Swathi, K. S. Narayan and T. Govindaraju,
ACS Appl. Mater. Interfaces, 2016, 8, 8678–8685.
1
1
1
6
4
5
processed by ERD using SAS View.
8 D. R. Rosseinsky and R. J. Mortimer, Adv. Mater., 2001, 13,
Conflicts of interest
7
83–793.
19 N. Sakai, J. Mareda, E. Vauthey and S. Matile, Chem. Commun.,
010, 46, 4225–4237.
There are no conflicts to declare.
2
20 S. V. Bhosale, M. B. Kalyankar, S. V. Bhosale, S. J. Langford,
Acknowledgements
E. F. Reid and C. F. Hogan, New J. Chem., 2009, 33,
2409–2413.
ERD would like to thank EPSRC for a NIA award (EP/S032673/1)
which also funds TAW. ERD also thanks the Leverhulme Trust
for an Early Career Fellowship (ECF-2017-223) and the Univer-
sity of Glasgow for a LKAS Leadership Fellowship. We acknow-
ledge beamtime allocation on D11 at Institut Laue-Langevin
2
1 Y. Hu, Y. Qin, X. Gao, F. Zhang, C. Di, Z. Zhao, H. Li and
D. Zhu, Org. Lett., 2012, 14, 292–295.
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