Unusual mechanohypsochromic luminescence and unique bidirectional thermofluorochromism of long-alkylated simple DPP dyes

Article abstract of DOI:10.1039/c7tc01453h

There have been hundreds of organic and metallorganic compounds found to exhibit mechanobathochromic luminescence within tens of nanometers. This paper presents the first example of unusual mechanohypsochromic luminescence and unique bidirectional thermof

Full text of DOI:10.1039/c7tc01453h

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ARTICLE
Unusual Mechanohypsochromic Luminescence and Unique
Bidirectional Thermofluorochromism of Long-Alkylated Simple
DPP Dyes
8
94Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Shian Ying, Mingshuai Chen, Zhongwei Liu, Meng Zheng, Haichang Zhang, Shanfeng Xue,* and
Wenjun Yang*
There have been hundreds of organic and metallorganic compounds found to exhibit mechanobathochromic luminescence
within tens of nanometers. This paper presents the first example of unusual mechanohypsochromic luminescence and
unique bidirectional thermofluorochromism based on 1,4-diketo-2,5-di(tetradecyl)-3,6-bis-(4-chlorophenyl)pyrrolo[3,4-
c]pyrrole (TDPP). Pristine TDPP powder is a red emitting crystal (645 nm, R state) and can be changed into a yellow
amorphous state (544 nm, Y state) by mechanical grinding or solution spin-coating. The Y state could revert to R state
o
o
upon annealing below 60 C or solvent-fuming, however, annealing the Y state above 70 C produces a new green-emitting
crystalline state (523 nm, G state). Moreover, the G state can transform to Y and R states upon grinding and solvent-
fuming, respectively. In contrast, those novel phenomena are not observed in the butylated DPP, demonstrating that
peripheral alkyl chain lengths of organic fluorophores can play the diverse functional role in determining the solid-state
aggregation behavior and fluorescence properties. This work has enriched kinds of stimuli responsive materials and
developed new optical properties of alkylated DPPs.
context, the design and discovery of MFC materials with
simple and comparable chemical structures are interesting but
still scarce at present. Moreover, although there have been
Introduction
Stimuli-response fluorescent solids exhibiting the reversible
change in emission colour or intensity upon external stimuli
1
such as light irradiation, chemical vapor, heat treatment,
hundreds of organic and metallorganic compounds found to
exhibit MFC behaviour, almost the present MFC dyes are
characterized by mechanobathochromic shift within tens of
2
3
4
and mechanical force, etc. have attracted much attention due
to their potential applications in various sensors and optical
recording and academic importance in understanding photo-
physical properties. Among them, mechanofluorochromic
4
,5
nanometers, and those with high contrast, multicolour and
6
multichannel switching are limited. Further, to the best of our
knowledge, organic fluorescent dyes exhibiting remarkable
7
mechanohypsochromic luminescence are rarely realized.
(MFC) materials involving the physical change of aggregate
8
,9
In recent years, we and others have found that the solid-
state optical and electronic properties and stimuli-response
behaviours of some organic chromophores are significantly
related to the positions and lengths of peripheral alkyl chains,
which has demonstrated the important role of flexible side
chains in tuning the molecular aggregation behaviours and
triggered the research of ‘‘side-chain engineering’’ or
morphology instead of the chemical structure alteration have
drawn much interest only within the past decade because of
4
,5
the easy realization and materials diversity. However, there
are still no rational design strategies for MFC dyes since almost
the identified MFC dyes are isolated events, and it is hard to
predict in advance whether a newly synthesized fluorescent
molecule can exhibit MFC behaviour. Therefore, there is still a
great demand on experimental exploitation of new MFC dyes
and accumulation of structure–property relationships. In this
“
homologous effect”. We are now interested in 3,6-bis(aryl)-
,4(2H,5H)-diketo-pyrrolo[3,4-c]pyrroles (DPPs) because DPPs
1
represent a class of commercial high-performance pigments,
and insoluble DPP pigments can be changed into highly soluble
and fluorescent dyes by introducing alkyl chains on the lactam
moiety to eliminate strong intermolecular hydrogen bonds.
While alkylated DPPs have been widely used in building
promising field-effect transistor and photovoltaic materials
Key Laboratory of Rubber-Plastics of Ministry of Education/Shandong Province
(
QUST), School of Polymer Science & Engineering, Qingdao University of Science &
Technology, 53-Zhengzhou Road, Qingdao 266042, P. R. China, E-mail:
sfxue@qust.edu.cn, ywjph2004@qust.edu.cn
†
Electronic Supplementary Informaꢀon (ESI) available: The detail of experimental
section; detailed absorption and emission spectra of compounds in different
solvents; fluorescence images and spectra of TDPP and BDPP solids under
various external stimuli; molecular packing structures and π-backbone
conformations of BDPP and TDPP. See DOI: 10.1039/x0xx00000x
1
0
and fluorescent probes, their solid fluorescence and stimuli-
response properties are rarely investigated. We consider that
changing peripheral alkyl lengths of DPPs should tune the
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intermolecular π–π and the aliphatic–aliphatic interactions, simultaneously and unevenly on the sample surface even if
which probably varies molecular conformations and packing annealing time is extended to 30 hours (Figure S3b). It would
modes to allow the design of functional dyes with polymorphs seem that the Y state could form the two different thermo-
and unique stimuli-response behaviours. Thus here we have chromic states. Thus, we increase the annealing temperature
o
introduced a short (butyl) and a long (tetradecyl) alkyl chains to 50–68 C, and find that the Y state is eventually turned into
at the 2,5-positions of commercial Pigment Red 254 to obtain the R state within a few hours (Figure 1, Figure S3b and S3c).
o
two soluble DPP derivatives (BDPP and TDPP, Scheme 1). It Interestingly, if the annealing temperature rises to 70–90 C, a
was surprisingly found that both the TDPP solid powder and strong green-emitting state (523 nm, G state) without any red
the spin-coated film exhibited unusual mechanohypsochromic spot is obtained within minutes (Figure 1 and Figure S3) .
o
luminescence with large spectral shift up to 100 nm and Therefore, we think 70 C is the favorable temperature that Y
unique bidirectional thermochromism depending on annealing state could be turned into G state. The G state has a higher
temperature. This is the first example of such MFC materials, fluorescence efficiency of 29.5% and shows a narrower
and stimuli responsive materials are enriched and optical emission spectrum. The R and G states form once, they are all
properties of alkylated DPPs are developed.
thermodynamically stable and can not transform each other by
thermal annealing before isotropic melt transition, but G state
can convert to R state by solvent-fuming or change into Y state
by mechanical grinding.
H
N
Cl
R
N
O
O
Cl K2CO /DMF/RBr
3
Cl
N
O
N
R
O
H
BDPP
Cl
Pigment red 254
R =
TDPP
Scheme 1. Structure and synthetic route of DPP dyes studied here.
Results and discussion
Both BDPP and TDPP solutions emit yellow fluorescence (~542
nm), regardless of solvent polarity and alkyl length, except for
slightly changed fluorescence quantum yield (36~47%, Figure
S1 and Table S1). BDPP solid is an orange emitting (600 nm)
and non-MFC active crystal with the fluorescence efficiency of
25.8% (Figure S2). However, TDPP can form two fluorescence
colour solids from the same solution, depending on solvent-
evaporation way or rate (Figure 1). When solvent is removed
using routine rotary evaporator, a red-emitting crystalline
powder (645 nm, R state) with the moderate fluorescence
efficiency of 14.8% is obtained. In contrast, spin coating (3000
Figure 1. Fluorescence images (top, illuminated by 365 nm UV lamp) and spectra
rpm) the solution on a quartz plate affords a yellow film with (down) of TDPP solids under various external stimuli. The spectra are excited at
4
80 nm.
the fluorescence efficiency of 5.6% (Y state, top in Figure 1 and
Figure S3a). This Y state exhibits a broad emission with two
peak-wavelengths of 530 and 560 nm. R and Y states are all
Thermal analysis indicates that TDPP exhibits the complex
stable under common ambient conditions, but R state can but interesting thermal properties (Figure 2). DSC shows that R
o
convert to Y state by mechanical grinding using a metal spatula state performs a dominant isotropic melting transition at 98 C
o
on a glass plate at room temperature (RT). Moreover, the Y upon heating process, and the melt can crystallize at ~50 C
state can easily change back to R state upon exposing to upon cooling process. Heating the cooled melt (second heating)
o
solvent vapour at RT within a few minutes (Figure 1). The produces a broad exothermic peak at 30–40 C and a higher
o
grinding- and spin coating-induced Y states show the similar isotropic melting transition at 106 C. The appearance of the
fluorescence colours and emission spectra with the central exothermic peak implies incompleteness of melt crystallization
wavelength of 544 nm. Compared to absorption spectra of the and relaxation of the molecular motion, and the incomplete
pristine spin-coating film, the G state film has a slight blue-shift. crystals can take place cold crystallization upon heating to
And R state film have a strong red-shift from 458 nm to 578 afford an exothermic peak. It is found that the isotropic melt
nm (Figure S3a).The above fluorescence change process is hardly fluoresces. We could obtain a yellow-emitting solid by
repeatable and indicates a reversible MFC behaviour. Notably, cooling the melt in outage oven, and its emission spectrum
the grinding-induced spectral shift (~100 nm) is not only and stimuli response behaviours are same as above Y state
among the largest values reported to date, but also unique
rare mechanohypsochromic shift.
(Figure S5). Thermal analysis indicates that Y state also has a
o
broad exothermic peak at 30–40 C (Figure 2), an amorphous
o
To our surprise once again, when the Y state is gently feature. Isothermally cooling the melt (in 45 C oven) forms a
annealed at 40 C, the red and green regions appear green-emitting solid, and its emission spectrum and stimuli
o
2
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response behaviours are same as above G state (Figure S5).
ARTICLE
To further gain an insight into the different solid-state
The G state shows no cold crystallization peak upon heating fluorescence properties of alkyl DPPs, we have cultured and
o
and only a isotropic melting transition at 106 C, indicative of analyzed single crystal structures of BDPP and TDPP (Figure 4
1
its well-ordered crystalline nature. Overall, G state is more and Figure S6, Table S2 and S3). In the orange BDPP crystal,
0
thermodynamically stable, and R state is more kinetically the molecule has a non-planar DPP core with a dihedral angle
o
preferred.
of 173.6 , and its two chlorophenyl units are strongly twisted
relative to lactam ring with two different dihedral angles of
o
1.8 and 47.4 , respectively. Two butyl chains are
o
4
homotropous in DPP out-of-plane and interact weakly. The
adjacent BDPP molecules pack in anti-parallel mode with the
o
laterally slipped angle of 19.5 and a vertical distance 3.329 Å.
This is a H-aggregation with the strong twisted backbone and
less π-π overlap stabilized by the intermolecular phenyl CH---O
interactions. In sharp contrast, in the TDPP single crystal with
red emission as R state, the molecules have the nearly planar
o
o
DPP core (179.6 ) and less twisted chlorophenyl units (13.5 ).
The adjacent TDPPs take stretching all-trans alkyl chains to
pack in face-to-face parallel with a vertical π–π distance of
3.345 Å and strong aliphatic–aliphatic interactions. One
Figure 2. DSC curves of TDPP solids prepared from the different conditions. The
insets are the fluorescence images illuminated by a 365 nm UV lamp.
molecule slides to its neighbour along π-backbone long axis
o
44.1 ), affording a J-aggregation mode with the less twisted
(
backbone and large π-π overlap stabilized by strong
intermolecular alpha-H---O and π-π inter-actions. Thus, the
redder emission for TDPP than BDPP crystals could be ascribed
to the former molecules with less twisted π-backbone
extending effective conjugation, strong π-π inter-actions
facilitating excimer formation, and J-aggregation red-shifting
but enhancing emission. In this context, it could be considered
that TDPP molecules in Y and G states all have more twisted
and bent π-backbones, regardless of different packing modes.
It is also reasonable that, to form well-ordered crystal, the
long-alkyl molecules should pack in parallel fashion along
stretched all-trans alkyl chains, and the intermolecular π-π
interactions could be weaker for crystalline G state than
crystalline R state. Overall, changing the length of alkyl chains
can greatly affect the solid-state aggregation behaviour and
fluorescence properties, and the long alkyl chain is more
suitable for tuning the intermolecular π–π and the aliphatic–
aliphatic interactions to allow the design of functional dyes
with polymorphs and unique stimuli-response behaviours.
To further understand the different emissive states formed
by TDPP molecules, we have measured their IR spectra (Figure
Cross-polarized optical microscopy displays that both R and
G states are crystalline, and both spin-coated and grinding-
induced Y states show amorphous characteristics (top inset,
Figure 3). Thus, MFC behaviour of TDPP solid could be ascribed
to the phase transition between crystalline and amorphous
states upon external stimuli. Powder X-ray diffraction patterns
further confirm that R and G states are in well-ordered but
very different crystalline states (Figure 3), and the obtained Y
state by cooling the melt is a hybrid state of semi-crystalline
and certain amorphous characteristics. However, cooling the
melt in room air could form a quasi Y state with more obvious
amorphous characteristics. It could be considered that flash
solvent-evaporation (spin coating) and fast melt-cooling (in air)
would make the long alkyl DPP molecules with relaxation
feature fail to packing in a well orderly manner, but the local
ordered structures could still be formed (quasi Y state).
Grinding the R and G states could leave the residual crystal
seeds, thus, upon solvent-fuming and heat-annealing, the
molecular secondary motions are effectively stimulated and
the well-ordered crystalline states are formed.
−
1
4
and S6). The alkyl stretching (2917, 2848 cm ) and rocking
−
1
723 cm ) vibrations in R state are respectively blue shifted to
(
−
920, 2851, and 724 cm in G state, which implies stronger
1
2
aliphatic–aliphatic interactions in G state. The C=C stretching
−
1
−1
1587, 1578 cm ) and C–H bending (736, 699 cm ) for phenyl
(
rings in R state are also blue shifted in G state (1614 and 1588,
−
1
7
40 and 700 cm , respectively), indicating more twisted π-
skeleton in G state. On the other hand, it is observed that
intermolecular phenyl CH–keto interactions in BDPP crystal
(distances 2.559 and 2.419 Å) are stronger than that in R state
of TDPP (distances 2.987 and 2.786 Å, Figure S5c), and the
−1
corresponding keto stretching appears at 1680 and 1674 cm ,
respectively (Figure S6). In this context, the intermolecular
interactions caused by keto moiety would be even weaker in G
state since the observed keto stretching vibration for G state
Figure 3. Powder x-ray diffraction patterns of TDPP solids prepared under
different conditions. Top inset is the cross-polarized optical microscopic photos
of TDPP film.
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−
1
appears only at 1663 cm . Different molecular π-backbone
conformations and intermolecular interactions for G and R
states could also be evidenced by other attribute-unknown
Notes and references
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peaks, for example, two peaks at 848 and 833 cm existed in
G state disappear in R state, instead, two new peaks at 831
−
1
and 772 cm appear. As for Y state, the IR spectrum is similar
to that of G state (Figure S6), which could be ascribed to their
resembled emission spectra and x-ray diffraction patterns.
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(
1
5
,
3
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1
Figure 4. Adjacent molecular packing π-backbone angle od alkyl DPPs and IR
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We have demonstrated that alkylating simple commercially
available DPP pigments can obtain multi-functional fluorescent
dye, and alkyl DPPs have become a new class of “side-chain
engineering” systems. While there have been hundreds of
organic and metallorganic compounds exhibiting mechano-
bathochromic luminescence, this work has presented the first
example of large mechanohypsochromic luminescence. Thus,
the kinds of stimuli responsive dyes are enriched and new
optical properties of alkyl DPPs developed. Only long-alkylated
DPPs could exhibit unusual mechanochromic and unique
thermochromic multi-colour luminescence, emphasizing the
important role of long alkyl chain in tuning π–π and aliphatic–
aliphatic interactions and obtaining polymorphs and diverse
optical properties. This finding could arouse one’s further
attention to the alkyl length effect in design, synthesis and
structure–property investigation of organic optoelectronic
materials. It is possible that utilizing this strategy design new
functional dyes based on other small organic chromophores,
and further extensive and systematic research is underway.
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Acknowledgements
This work was supported by the National Natural Science
Foundation of China (51473084, 51573081) and the Natural
Science Foundation of Shandong Province (No. ZR2016JL016,
ZR2014EMQ013), and the Natural Science Foundation of
Qingdao City of China (16-5-1-89-jch). We thank Open Project
of the State Key Laboratory of Supramolecular Structure and
Materials of Jilin University (SKLSSM-201728).
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1 CCDC 1497296 and 1497297 contain the supplementary
crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
4
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Unusual Mechanohypsochromic Luminescence and Unique
Bidirectional Thermofluorochromism of Long-Alkylated
Simple DPP Dyes
Shian Ying, Mingshuai Chen, Zhongwei Liu, Meng Zheng, Haichang Zhang, Shanfeng Xue,* and
Wenjun Yang*
Key Laboratory of Rubber-plastics of Ministry of Education/Shandong Province (QUST), School
of Polymer Science & Engineering, Qingdao University of Science &Technology, 53-Zhengzhou
Road, Qingdao, 266042, P. R. China. E-mail: sfxue@qust.edu.cn, ywjph2004@qust.edu.cn
The first example of organic dyes exhibiting very large mechanohypsochromic shift
(
~100 nm) and bidirectional thermofluorochromism (red, yellow, and green) in both
powder and film states was demonstrated based on a long-alkylated Pigment Red 254
TDPP). Thus, the kinds of stimuli responsive dyes are enriched and new optical
properties of alkyl DPPs developed.
(