In situ synthesis of stimulus-responsive luminescent organic materials using a reactive inkjet printing approach

Article abstract of DOI:10.1039/c5tc00334b

We have prepared luminescent organic materials using a reactive inkjet printing system. A stimulus (vapour/heat)-responsive fluorescent dye as well as a luminescent conjugated polymer were readily generated by printing of an aldehyde, a phenylenediacetoni

Full text of DOI:10.1039/c5tc00334b

Journal of
Materials Chemistry C
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In situ synthesis of stimulus-responsive
luminescent organic materials using a reactive
inkjet printing approach†
Seongho Jeon,a Jong Pil Leea and Jong-Man Kim*ab
Cite this: J. Mater. Chem. C, 2015,
3, 2732
Received 3rd February 2015,
Accepted 22nd February 2015
DOI: 10.1039/c5tc00334b
www.rsc.org/MaterialsC
We have prepared luminescent organic materials using a reactive inkjet
printing system. A stimulus (vapour/heat)-responsive fluorescent dye as
well as a luminescent conjugated polymer were readily generated by
printing of an aldehyde, a phenylenediacetonitrile, and potassium
t-butoxide.
Owing to its unique advantages including low reagent consump-
tion and direct patterning ability, inkjet printing technology has
been actively employed for the preparation of organic materials
and devices.1–3 For instance, inkjet printing-based fabrication of
light emitting conjugated polymer films,4,5 metallo-copolymer
films,6 conducting polymer circuits7–9 and OLEDs10,11 has been
already invented. We also reported electrothermochromic displays,12
anti-counterfeiting13,14 and volatile organic vapor sensors15 based on
this technology. Especially interesting is the reactive inkjet printing
Fig. 1 Schematic illustration for the preparation of DBDCS using a reac-
tive inkjet printing (RIP) method.
(RIP) system which utilizes chemically reactive materials as ink feasibility of in situ synthesis of stimulus-responsive lumines-
sources.10,11,16,17 Reactive inkjet printing enables preparation of cent organic materials, a cyanostilbene derivative (2Z,20Z)-2,20-
functional materials at specific locations by in situ chemical (1,4-phenylene)bis(3-(4-butoxyphenyl)acrylonitrile) (DBDCS) has
synthesis from printed reagents. Fabrication of metal and metal been selected in the initial phase of this investigation (Fig. 1).
oxide derived patterns has been achieved using the emerging DBDCS has been already reported in the literature and its vapor-
drop-on-demand inkjet printing method.18–24 In addition, there chromic and thermochromic phenomena in the solid state are also
have been some fascinating examples of the RIP-based synthesis well described.36 We envisioned that inkjet printing of 4-butoxy-
of organic materials including polyacrylate libraries,25–30 poly- benzaldehyde (BA), 1,4-phenylenediacetonitrile (B2CN) and potassium
urethane31,32 as well as poly(9-vinylcarbazole)33 and poly- tert-butoxide (t-BuOK) results in the generation of the desired DBDCS
radicals.34 Especially, the concept of overprinting for the fabrication by a condensation reaction (Fig. 1). The product formation was
of a material library is interesting since it allows investigating the monitored by IR and NMR analyses. The stimulus-responsive nature
features of various components in one set of experiments.25–30,35 of the in situ generated DBDCS was probed by exposing the reaction
Herein, we report synthesis of a variety of fluorescent cyanostilbene product to organic vapor and thermal annealing.
derivatives as well as a solid-state fluorescent conjugated polymer
Three reactive inks are prepared ink A: BA (1.0 M) in N-methyl-2-
using the RIP technology. In addition, fabrication of patterned pyrrolidone (NMP), ink B: B2CN (0.5 M) in 10% ethylene glycol-NMP,
fluorescent molecular arrays that undergo a solvatochromic/ ink C: t-BuOK (1.0 M in water). The polar solvent NMP is chosen to
thermochromic transition was also proved. In order to test the facilitate the condensation reaction. In addition, a small amount of
ethylene glycol was added to NMP to retain a wetted state of the
droplets and to guarantee enough reaction time. The relatively
nonvolatile NMP and ethylene glycol result in no coffee ring
effect and also allowed formation of uniform spots which are
hard to obtain when using volatile organic solvents. To dispense
three prepared inks on a specific area, we used a piezoelectric
a Department of Chemical Engineering, Hanyang University, Seoul 133-791, Korea.
E-mail: jmk@hanyang.ac.kr
b Institute of Nano Science and Technology, Hanyang University, Seoul 133-791, Korea
† Electronic supplementary information (ESI) available: Experimental section,
NMR, IR and PL diagrams. See DOI: 10.1039/c5tc00334b
2732 | J. Mater. Chem. C, 2015, 3, 2732--2736
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Fig. 3 Optical (left) and fluorescence (right) microscopic images of
DBDCS arrays obtained by the in situ inkjet synthesis on glass (a) and
PDMS (b) substrates (lex = 330–385 nm).
B-to-G transformation occurs by organic vapor and a reverse
G-to-B transition occurs by thermal annealing. Interestingly, we
have observed substrate-dependent phase control of the DBDCS
arrays. Thus, the B-form of DBDCS was generated predominantly
when the glass substrate was used while the G-form of the
product was obtained when using the PDMS substrate. It seems
like the property of the substrate plays a significant role. Thus, the
hard surfaced glass substrate enables fast evaporation of the solvent
and results in the formation of B-form DBDCS. In contrast, the soft
and flexible nature of PDMS allows penetration of the DBDCS
molecules inside the polymer matrix and affords slow molecular
packing of DBDCS during evaporation of the solvent, yielding the
Fig. 2 FTIR spectra of BA (a), B2CN (b) and reaction mixture obtained
after 5 h (c) and 24 h (d) at 20 1C. FTIR spectrum of independently prepared
DBDCS (e).
dispensing system (GeSiM Nanoplotter, Germany). Owing to its G-form of the product. This was confirmed by preparing DBDCS
high reproducibility and accuracy, the three prepared reactive under different conditions. Thus, preparation of DBDCS at 50 1C
inks can be dropped (5 nl) at the same location on the substrate. resulted in the formation of B form of the dye on both glass and
In an effort to verify DBDCS generation from the inkjet- PDMS substrates. In contrast, lowering the reaction temperature to
printed chemical reaction, IR spectroscopic monitoring was 1 1C afforded generation of the B/G mixture (glass substrate) and G
performed (Fig. 2). It is clear that the distinct bands corre- form (PDMS substrate) of the product (Fig. S2†).
sponding to the aldehyde carbonyl (CQO) and cyanide (CRN)
In the next phase of this investigation, we focused on the
groups at 1694 and 2248 cmꢀ1, respectively, decrease 5 h after feasibility of fluorescence switching of DBDCS prepared by reactive
inkjet printing. It should be noted that the band at 2169 cmꢀ1 inkjet printing. Exposing tetrahydrofuran (THF) vapor to a glass
observed after 5 h reaction (Fig. 2c) is presumably due to the substrate containing the B-form of the DBDCS spot resulted in the
intermediate formation and disappears after 24 h (Fig. 2d). It is generation of the G-form of DBDCS (Fig. 4a, top). Heat treatment
clear that the starting BA and B2CN are almost consumed and (120 1C, 30 min) of the THF-exposed glass afforded recovery of the
the desired product is formed after 24 h reaction at 20 1C initial B-form. This process can be reversibly controlled by an
(Fig. 2d and e). The reaction progress was also monitored using alternative heating and solvent exposure process. Similar results
1H NMR spectroscopy which confirmed the formation of were obtained for a DBDCS spot on a PDMS substrate (Fig. 4a,
DBDCS as the only noticeable major product (Fig. S1†).
bottom). Unlike the glass substrate, the PDMS matrix allows move-
In order to fabricate DBDCS patterns, three starting ink ment of DBDCS molecules inside the polymer matrix. Accordingly,
solutions were dispensed in the form of aligned dot arrays. The repeated solvent and heat treatment cycles resulted in the partial
microarray synthesis was conducted on glass and polydimethyl- destruction of the fluorescent spot. Monitoring of the B-to-G phase
siloxane (PDMS, Sylgard 184, Dow Corning) substrates. Optical transition by photoluminescence (PL) spectra indicated that the
and fluorescence images were obtained after a 24 h reaction emission maximum shifted from 496 to 505 nm (Fig. 4b).
(Fig. 3). As shown in Fig. 3a, the DBDCS microarrays prepared on
The versatility of our system was further proved by preparing
the glass substrate emitted a blue-green fluorescence (excitation: a variety of luminescent cyanostilbene derivatives (Fig. 5). As
330–385 nm). We also fabricated the DBDCS patterns on a PDMS shown in Fig. 5, the RIP method allowed a facile synthesis of
film which display a yellow-green emission (Fig. 3b). DBDCS cyanostilbene derivatives with unsubstituted (1), electron rich
can exist in two different phases; a blue-emitting B-form and a (2,3) as well as electron poor (4) substituents. In addition,
green-emitting G-form.36 The two phases are switchable and a aldehydes derived from phenyl (5), thienyl (6) and anthracene
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matched with those obtained using independent methods
(Fig. S4†).
The scope of the reactive inkjet printing is not limited to the
preparation of small molecular luminescent materials. We have
successfully applied this method to the in situ synthesis of a
luminescent conjugated polymer. As shown in Scheme 1, con-
densation reaction between terephthaldehyde (TPA) and B2CN
in the presence of base yielded a cyano-substituted conjugated
polymer poly(phenylenevinylene) (CN-PPV).
Inkjet printing of these reactive inks on a glass substrate
afforded patterned luminescent microarrays (Fig. 6a and b).
The versatility of the method was also proved by fabricating
Fig. 4 (a) Fluorescence microscope images of DBDCS spots prepared on
glass (top) and PDMS (bottom) upon solvent/heat treatment. (b) PL spectra
of B- (blue line) and G (green line)-phases of DBDCS (lex = 330 nm). The
emission spectrum of the G-form of DBDCS was obtained by exposing
THF to the B-form DBDCS prepared on a glass substrate.
Scheme 1 Ink components and a conjugated polymer obtained by reactive
inkjet printing.
Fig. 5 Structures of cyanostilbene derivatives prepared by the reactive
inkjet printing method.
(7) groups were also converted to the desired products without
difficulty. It should be noted that we were not able to obtain the
cyanostilbene derivative with 4-nitrobenzaldehyde. 4-Nitrobenzal-
dehyde was found to be ineffective with the plotting solvent NMP
while the desired product was formed in ethanol when the
conventional solution based synthesis was carried out. The
cyanostilbene derivatives displayed in Fig. 5 are also prepared
independently using the conventional solution phase synthesis
to compare emission features. Fluorescence microscopic
images confirm that the RIP-based approach is affordable
for the preparation of diverse luminescent cyanostilbene deri-
vatives (Fig. S3†). In addition, the photoluminescence spectra
of some cyanostilbene derivatives that are prepared by RIP were
Fig. 6 (a, b) Photograph (a) and fluorescence microscopic image (b) of
CN-PPV arrays prepared on a glass substrate at 20 1C. (c, d) Photograph
(c) and fluorescence microscopic image (b, d) of CN-PPV arrays prepared
on a PDMS film (lex = 510–550 nm). (e) Photoluminescence (PL) spectra of
CN-PPV prepared by reactive inkjet printing (red line) and conventional
solution phase synthesis (black line) (lex = 470 nm).
2734 | J. Mater. Chem. C, 2015, 3, 2732--2736
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film (Fig. 6c and d). FTIR spectroscopic monitoring of the
reaction indicates that the starting TPA and B2CN disappeared
24 h after inkjet printing at 20 1C (Fig. S5†). Comparison of the
FTIR spectrum of the conjugated polymer obtained by the
inkjet printing with that of CN-PPV independently prepared
by the conventional solution synthesis demonstrates the effi-
ciency of the reactive inkjet printing method. In addition, the
photoluminescence (PL) spectrum of the product obtained
using the inkjet printing method (Fig. 6e, red line) is almost
identical to the one prepared by the solution based approach
(Fig. 6e, black line). The molecular weight of the polymer was
difficult to obtain owing to the insoluble nature of the polymer.
However, comparison of the emission spectra displayed in
Fig. 6e suggests that at least oligomeric conjugated materials
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We have utilized a reactive ink system to fabricate fluorescent
organic molecular arrays. Inkjet printing of aromatic aldehyde,
aromatic nitrile, and base solutions afforded the efficient
synthesis of a luminescent cyanostilbene derivative DBDCS by
an in situ condensation reaction. The reaction was probed by IR
and NMR analyses, and clean product formation was observed
after a 24 h reaction. The luminescent cyanostilbene derivative
displayed two different phases (B- and G-forms) depending on
the substrate used. The B-form product was obtained when a
glass substrate was employed, while the G-form cyanostilbene
was produced when the reactive inkjet printing was carried out
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