Synthesis and fluorescence of dicyanoisophorone derivatives

Article abstract of DOI:10.1016/j.dyepig.2013.05.031

A class of dicyanoisophorone derivatives 1-5 with electron-donating groups has been prepared by a two-step condensation reaction. Fluorescence both in solution and in pure solid state are measured. It is found that dicyanoisophorone derivatives 1-5 exhibi

Full text of DOI:10.1016/j.dyepig.2013.05.031

Dyes and Pigments 99 (2013) 531e536
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Synthesis and fluorescence of dicyanoisophorone derivatives
*
Xu Zhang, Yi Chen
Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, The Chinese Academy of Sciences,
Beijing 100190, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A class of dicyanoisophorone derivatives 1e5 with electron-donating groups has been prepared by a
two-step condensation reaction. Fluorescence both in solution and in pure solid state are measured. It is
found that dicyanoisophorone derivatives 1e5 exhibit fluorescence in both solution and pure solid state.
An enhancement of fluorescence in solid state probably results from the aggregation-induced emission. It
is also found that dicyanoisophorone derivatives 1e5 show large Stoke’s shifts in solution as well as in
pure solid state. The origin of fluorescence of dicyanoisophorone derivatives depends on the nature of
substituents, with a strong pushepull chromophores dicyanoisophorone system, red or orange solid-
state fluorescence is obtained.
Received 19 February 2013
Received in revised form
27 May 2013
Accepted 29 May 2013
Available online 20 June 2013
Keywords:
Dicyanoisophorone derivatives
Fluorescence
Solid state
Ó 2013 Elsevier Ltd. All rights reserved.
Stoke’s shift
Substituent
Synthesis
1. Introduction
fluorescence in pure state with large Stokes shifts, and with a strong
pushepull chromophores dicyanoisophorone system, orange and
Solid-state fluorescence has currently attracted considerable
attention because of wide potentials in material science [1e3] and
biology [4e6]. Recent reports highlighted the use of fluorescent
nanocrystals of small organic molecules as biochips [7,8] or sensors
[9e11] for applications in bioimage or biomolecules detection.
Solid-state fluorescence exhibits much higher photostability than
isolated dissolved molecules [12,13], and the crystal structure also
favors the delocalization of the fluorescence excitation leading to
unique fluorescent transmitters behavior [14]. Such properties
greatly enhance the fluorescence contrast over the biological media
thus lowering the thresholds of detection. However, most organic
fluorophores exhibit strong fluorescence in dilute solutions but no
or weak fluorescence in the solid state as a consequence of the tight
molecular packing in the pure microcrystalline state or amorphous
solid phase (thin film) that usually leads to significant molecular
interactions and self-quenching [15,16]. Although a few of notable
solid-state fluorescent families have been developed [17e21],
finding a new family of solid-state fluorophores continues to be an
active research area [22e26]. In this paper, we report a class of
solid-state fluorophores based on dicyanoisophorone. It is found
that dicyanoisophorone derivatives (Scheme 1) show moderate
red solid-state fluorescence are obtained.
2. Experimental
2.1. General
1H and 13C NMR spectra are recorded at 400 and 100 MHz,
respectively, with TMS as an internal reference. MS spectra are
recorded with TOC-MS spectrometer, respectively. UV absorption
spectra and fluorescence spectra in solution are measured with an
absorption spectrophotometer (Hitachi U-3010) and a fluorescence
spectrophotometer (F-2500), respectively. Fluorescence quantum
yields in solid state are measure with a fluorescence spectropho-
tometer (Edinburgh Instruments FLS-920). All chemicals for syn-
thesis are purchased from commercial suppliers, and solvents are
purified according to standard procedures. Reaction is monitored
by TLC silica gel plates (60F-254). Column chromatography is per-
formed on silica gel (70e230 mesh).
2.2. Chemical
2.2.1. Synthesis of 2-(3,5,5-trimethylcyclohex-2-enylidene)
malononitrile
To a solution of isophorone (3.8 g, 27.6 mmol) and malononitrile
(1.82 g, 27.6 mmol) in dry ethanol (150 mL) is added piperidine
* Corresponding author. Tel.: þ86 10 8254 3595; fax: þ86 10 6487 9375.
E-mail address: yichen@mail.ipc.ac.cn (Y. Chen).
0143-7208/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2013.05.031

532
X. Zhang, Y. Chen / Dyes and Pigments 99 (2013) 531e536
Found: C, 80.05; H, 6.59. IR (KCl, cmꢁ1): 2212, 1387, 1200, 1157,
954, 873.
3. Yield: 37%. 1H NMR (400 MHz, CDCl3):
d
¼ 7.70 (d, J ¼ 9.1 Hz,
2H), 7.58 (d, J ¼ 9.0 Hz, 2H), 7.05 (d, J ¼ 16 Hz,1H), 6.80 (d, J ¼ 16 Hz,
1H), 6.72 (s, 1H), 2.53 (s, 2H), 2.40 (s, 2H), 1.22 (s, 6H). 13C NMR
(100 MHz, CDCl3):
d
¼ 169.8, 162.9, 160.2, 157.4, 133.2, 129.8, 129.2,
122.9,118.3,114.4,113.9,110.5,102.5, 78.6, 55.3, 43.1, 39.3, 32.1, 28.4.
HRMS (EI) calcd for C19H17ClN2 (Mþ): 308.1080. Found: 308.1085.
Anal. Calcd for C19H17ClN2: C, 73.90; H, 5.55. Found: C, 74.05; H,
5.69. IR (KCl, cmꢁ1): 2212, 1387, 1196, 1153, 960, 873.
Scheme 1. Chemical structure of target compound.
4. Yield: 66%. 1H NMR (400 MHz, CDCl3):
d
¼ 8.04 (d, J ¼ 9.4 Hz,
1H), 7.11 (d, J ¼ 7.9 Hz, 1H), 7.06 (d, J ¼ 16 Hz, 1H), 6.83 (d, J ¼ 16 Hz,
(23 mg, 0.276 mmol). The solution is stirred at 60 ꢀC till starting
material disappeared (detected by TLC plate). After cooling to room
temperature, the solution is slowly poured into water (200 mL) and
the precipitated solid is filtered. Recrystallization from heptane
affords a brown solid. Yield: 4.5 g (90%). M.p. 73e75 ꢀC. 1H NMR
1H), 6.73 (s, 1H), 6.68 (s, 1H), 4.01 (s, 3H), 3.94 (s, 3H), 2.52 (s, 2H),
2.41 (s, 2H), 1.19 (s, 6H). 13C NMR (100 MHz, CDCl3):
d
¼ 169.2, 161.0,
154.3, 136.8, 129.1, 128.3, 126.8, 122.5, 114.4, 113.6, 112.9, 77.5, 55.3,
42.9, 39.1, 31.9, 27.9. HRMS (EI) calcd for C21H22N2O2 (Mþ):
334.1681. Found: 334.1678. Anal. Calcd. For C21H22N2O2: C, 75.42; H,
6.63. Found: C, 75.55; H, 6.53. IR (KCl, cmꢁ1): 2193, 1381, 1196,
967, 876.
(CDCl3):
d (ppm) 6.60 (s, 1H), 2.53 (s, 2H), 2.14 (s, 2H), 2.01 (s, 3H),
1.32 (s, 6H). 13C NMR (CDCl3):
45.6, 42.3, 32.4, 27.5, 25.1.
d (ppm) 170.3, 161, 120.2, 113.1, 76.4,
5. Yield: 50%. 1H NMR (400 MHz, CDCl3):
d
¼ 7.38 (d, J ¼ 8.9 Hz,
2H), 7.01 (d, J ¼ 16 Hz, 1H), 6.76 (d, J ¼ 16 Hz, 1H), 6.73 (s, 1H), 6.64
(d, J ¼ 8.9 Hz, 2H), 3.12 (s, 6H), 2.74 (s, 2H), 2.65 (s, 2H), 1.06 (s, 6H).
2.2.2. General synthesis of dicyanoisophorone derivatives 1e5
Under argon, 2-(3,5,5-trimethylcyclohex-2-enylidene) malono-
nitrile (1.0 equiv.) and corresponding aromatic aldehydes
(1.0 equiv.) are dissolved in dry acetonitrile (100 mL). Piperidine
(0.01 equiv) is added and the solution is stirred at 40 ꢀC till starting
material disappeared (detected by TLC plate). The solution is
concentrated and the product is purified by flash column chro-
matography (elute: petroleum ether/ethyl acetate ¼ 10/1, v/v).
13C NMR (100 MHz, CDCl3):
d
¼ 169.2, 155.3, 151.2, 138.3, 129.5,
129.4, 124.4, 121.4, 114.1, 113.4, 112.3, 75.7, 42.9, 40.3, 39.2, 31.8, 28.1.
HRMS (EI) calcd for C21H24N3 (Mþþ1): 318.1970. Found: 318.1889.
Anal. Calcd. For C21H23N3: C, 79.46; H, 7.30. Found: C, 79.65; H, 7.18.
IR (KCl, cmꢁ1): 2196, 1389, 1205, 1163, 957, 872.
3. Results and discussion
1. Yield: 26%. 1H NMR (400 MHz, CDCl3):
d
¼ 7.68 (d, J ¼ 8.9 Hz,
2H), 7.47 (m, 2H), 7.33 (m, 1H), 7.01 (d, J ¼ 16 Hz, 1H), 6.84 (d,
3.1. Synthesis of dicyanoisophorone derivatives 1e5
J ¼ 16 Hz, 1H), 6.73 (s, 1H), 2.56 (s, 2H), 2.44 (s, 2H), 1.20 (s, 6H). 13
C
NMR (100 MHz, CDCl3):
d
¼ 169.2, 153.8, 137.0, 135.6, 129.7, 129.1,
Dicyanoisophorone derivatives are obtained starting from iso-
phorone, malononitrile and the corresponding aromatic aldehydes
by a double Knoevenagel reaction sequence [27]. Compounds 1e5
were prepared by a two-step condensation reaction [28] which
illustrated in Scheme 2. Treatment isophorone with malononitrile
in the catalyst of piperidine (1% mol) provided 2-(3,5,5-
trimethylcyclohex-2-en-1-ylidene) malononitrile in excellent
yield (90%, isolated yield), followed by the condensation with the
corresponding aromatic aldehydes to give target compounds 1e5
in 33e66% isolated yield after column chromatography. The details
of procedure for the preparation are described in Experimental
Section.
129.0, 127.5, 123.5, 113.4, 112.6, 111.6, 78.7, 43.0, 39.2, 32.0, 28.1.
HRMS (EI) calcd for C19H18N2 (Mþ): 274.1470. Found: 274.1474. Anal.
Calcd for C19H18N2: C, 83.18; H, 6.61. Found: C, 83.03; H, 6.57. IR (KCl,
cmꢁ1): 2212, 1387, 1202, 1157, 962, 874.
2. Yield: 30%. 1H NMR (400 MHz, CDCl3):
d
¼ 7.78 (d, J ¼ 8.9 Hz,
2H), 7.18 (d, J ¼ 8.9 Hz, 2H), 7.04 (d, J ¼ 16 Hz, 1H), 6.82 (d, J ¼ 16 Hz,
1H), 6.74 (s, 1H), 3.94 (s, 3H), 2.54 (s, 2H), 2.42 (s, 2H), 1.21 (s, 6H).
13C NMR (100 MHz, CDCl3):
128.8, 127.2, 122.3, 117.9, 113.9, 113.2, 105.6, 98.5, 76.9, 55.6, 55.5,
43.1, 39.2, 32, 28. HRMS (EI) calcd for C20H20N2O (Mþ): 304.1576.
Found: 304.1571. Anal. Calcd for C20H20N2O: C, 78.92; H, 6.62.
d
¼ 169.4, 162.5, 159.2, 155.4, 132.2,
Scheme 2. General synthetic route for dicyanoisophorone derivatives 1e5. Reagents and conditions: (a) piperidine cat., dry ethanol, 60 ꢀC, 8 h, 90%; (b) piperidine cat., dry
acetonitrile, 40 ꢀC, 8 h, 33e66%.

X. Zhang, Y. Chen / Dyes and Pigments 99 (2013) 531e536
533
3.2. Optical properties of dicyanoisophorone derivatives 1e5 in
solution
1
2
3
4
5
1.0
0.8
0.6
0.4
0.2
0.0
The optical properties of dicyanoisophorone derivatives 1e5 are
measured in CH3CN solution (20 M) and photophysical data are
m
reported in Table 1. Absorbance and fluorescence spectra of the
representative compounds 1e5 measured at room temperature are
shown in Fig. 1 and Fig. 2, respectively. Absorption data are char-
acteristic of induced charge transfer transitions in pushepull
dipolar molecules. It is found that the maximum absorption shifts
from 386 nm to 492 nm when electron-donating substituents
change from Cl (weak) to NMe2 (strong), which is in agreement
with the assumption of intramolecular charge transfer (ICT), and an
ICT enhancement results in the bathochromic shift of the absorp-
tion bands.
300
350
400
450
500
550
600
650
Dicyanoisophorone derivatives 1e5 show weak fluorescence in
solution. Fluorescence quantum yields (ff) are measured in CH3CN
solution using rubrene (ff ¼ 0.27, in MeOH) as reference [29]. As
presented in Table 1, A significant dependence of the fluorescence
quantum yield with substituents is observed. It is found that the
fluorescence of dicyanoisophorone derivatives is increased with
increase of the power of donor, the largest fluorescence quantum
yield (ff ¼ 0.11) is obtained when substituent is N,N-dimethyla-
mino donor group, which indicates that the fluorescence of
dicyanoisophorone derivatives is increased with the increase of ICT
in pushepull dicyanoisophorone molecules. Besides, the emission
bands of 1e5 are also significant bathochromic shift with increase
of ICT, the maximum emission shifts from 507 nm to 643 nm when
substituent changed from Cl to NMe2.
wavelength (nm)
Fig. 1. The absorption of 1e5 in CH3CN solution.
3.3. Fluorescence of dicyanoisophorone derivatives 1e5 in pure
microcrystalline state
Both absorption and fluorescence of dicyanoisophorone de-
rivatives 1e5 in pure solid state are measured. Relevant photo-
physical data are summarized in Table
2 and normalized
fluorescence spectra of dicyanoisophorone derivatives 1e5 is
shown in Fig. 5. It is found that the maximum absorption shifted
from 418 nm to 528 nm when substituent changed from Cl group to
NMe2 group in a pushepull dicyanoisophorone structure, which is
in agreement with the result in solution. Solid-state fluorescence of
dicyanoisophorone derivatives 1e5 have showed that all com-
pounds exhibited fluorescence emission in pure solid state. The
quantum yields of derivatives 1e5 in solid state are measured by an
Large Stoke’s shifts are observed with dicyanoisophorone de-
rivatives 1e5 in solution. As presented in Table 1, more than
4000 cmꢁ1
(
Dl ꢂ 119 nm) of Stokes shift is obtained with electron-
donating group (most current fluorophores only have 50e90 nm of
Stoke’s shift [22,30]). Such a large Stoke’s shift is beneficial to such a
large Stoke’s shift is beneficial to application since it reduces self-
absorption of fluorescent dye and prevents the decrease of
fluorescence.
integrating sphere and the largest quantum yield of
f
¼ 0.15 are
obtained (Table 2). As compared to solution, the fluorescence of
dicyanoisophorone derivatives 1e4 in solid state is stronger than
that in solution but 5 (whose fluorescence is small as compared to
in the solution). An enhancement of fluorescence of derivatives 1e4
in solid state probably resulted from the aggregation-induced
emission or the aggregation-induced emission enhancement due
to the effects of intramolecular planarization or restricted intra-
molecular vibrational and rotational motions in the solid state or of
specific aggregation or certainly a combination of all those effects
[31,32].
Both absorption and fluorescence of dicyanoisophorone de-
rivatives 1e5 in different solvents have also been measured. As
presented in Fig. 3 and Fig. 4, a positive solvatochromism of 2 is
observed in different solvent. Both absorption and fluorescence
occurred red-shift 20 nm and 50 nm, respectively, when the solvent
is changed from cyclohexane to DMSO. Similar results were ob-
tained when other dicyanoisophorone derivatives were measured
in different solvents. The solvatochromism is characteristic of
induced charge transfer transitions in pushepull dipolar molecules,
which is in agreement with this attribution and in agreement with
reported in this type of molecules [23].
1.0
1
2
3
0.8
4
Table 1
5
0.6
Spectroscopic data of compounds 1e5 in CH3CN solution.a
Compds
lmax
3
(
lmax
)
lem
ff
Dl
(nm)
Dn
(l cmꢁ1 molꢁ1
)
(nm)
(cmꢁ1
)
0.4
0.2
0.0
(nm)
1
2
3
4
5
386
412
386
426
492
26,900
37,550
44,650
41,500
21,700
505
556
507
571
643
0.01
0.03
0.01
0.04
0.11
119
144
121
145
151
6104
6286
6182
5961
4773
a
labs: maximum absorption wavelength, lem: maximum emission wavelength at
: molar absorption coefficients at maximum absorption
lem
500
550
600
650
700
ambient temperature,
3
wavelength (nm)
wavelength, Dl: Stoke’s shift calculated by (Dl
¼
ꢁ
labs nm), Dn: Stoke’s shift
calculated by [Dn ¼ (1/lmax ꢁ 1/lem) ꢃ 107 cmꢁ1], ff: fluorescence quantum yield
using rubrene (ff ¼ 0.27, in MeOH) as reference.
Fig. 2. The fluorescence of 1e5 in CH3CN solution.

534
X. Zhang, Y. Chen / Dyes and Pigments 99 (2013) 531e536
1
2
3
4
5
1.0
1.0
0.8
0.6
0.4
0.2
0.0
in cyclohexane
in tuloene
in acetonitrile
in methanol
in DMSO
0.8
0.6
0.4
0.2
0.0
350
400
450
500
450
500
550
600
650
700
750
wavelength (nm)
wavelength (nm)
Fig. 3. The absorption of 2 in different solution.
Fig. 5. Fluorescence spectral of 1e5 in pure solid state.
50
1.0
0.8
0.6
0.4
0.2
0.0
0% H2O
in cyclohexane
10% H2O
20% H2O
30% H2O
40% H2O
50% H2O
60% H2O
70% H2O
80% H2O
in tuloene
40
30
20
10
0
in acetonitrile
in methanol
in DMSO
450
500
550
600
650
700
wavelength (nm)
450
500
550
600
650
700
Fig. 4. The fluorescence of 2 in different solution.
wavelength (nm)
Fig. 6. Fluorescence spectral of 2 in mixed solvent (THFewater).
To insight into a possible mechanism for the enhancement of
fluorescence of dicyanoisophorone derivatives in solid state, ag-
gregation behaviors of derivatives 1e5 in tetrahydrofuran (THF)
and water (H2O) mixed solvent are investigated. As presented in
Fig. 6, the fluorescence intensity of 2 was enhanced continually
with a red-shift from 549 nm to 571 nm until the water was added
up to 80% (volume ratio). The observed continuous emission
enhancement and red-shift might arise from the concerted effect of
an increase in the polarity of water and the resultant self-
assembled agglomerates, which is similar with a polymer char-
acter [33,34]. Similar results were obtained when other derivatives
1, 3 and 4 were measured in mixed solvent. It is worth noting that, a
opposite result was obtained when 5 was investigated in mixed
solvent, as shown in Fig. 7, the fluorescence intensity was decreased
continually with increase of water, which indicated that the ag-
gregation of 5 caused its fluorescence quenching (the result is
agreement with the fluorescence quantum yield of 5 in solid state).
The different results suggested that the controlled crystalline
80
0% H O
2
60
30% H O
2
80% H O
2
Table 2
Spectroscopic data of compounds 1e5 in pure microcrystalline state.a
40
20
0
Compds
labs (nm)
lem (nm)
ff
Dl(nm)
Dn (cmꢁ1
)
1
2
3
4
5
435
446
418
459
528
558
576
541
592
670
0.01
0.13
0.07
0.15
0.003
123
130
123
133
142
5067
5060
5439
4894
4014
550
600
650
700
750
a
labs: maximum absorption wavelength, lem: maximum emission wavelength at
ambient temperature, ff: measured by an integrating sphere at ambient tempera-
wavelength (nm)
ture, Dl: Stoke’s shift calculated by (Dl
¼
lem
ꢁ
labs nm), Dn: Stoke’s shift calculated
by [Dn ¼ (1/lmax ꢁ 1/lem) ꢃ 107 cmꢁ1].
Fig. 7. Fluorescence spectral of 5 in mixed solvent (THFewater).

X. Zhang, Y. Chen / Dyes and Pigments 99 (2013) 531e536
535
Fig. 8. Photograph of 1e5 in pure solid state (top) and their solid-state under illumination of handled UV lamp at 365 nm (bottom).
[8] Dubuisson E, Monnier V, Sanz-Menez N, Boury B, Usson Y, Pansu RB, et al.
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engineering through different molecular stacking might play a
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g-globulin in human serum with organic nanoparticle biosensor.
Similar to solution, a large red-shift of emission is observed
when substituents changed from Cl group to NMe2 group and large
Stoke’s shift are obtained with dicyanoisophorone derivatives 1e5
in pure solid state, which suggests that ICT also took place in
dicyanoisophorone molecules in pure solid state. With a strong
electron-donating group, both red and orange solid-state fluores-
cence of dicyanoisophorone derivatives are obtained (Fig. 8).
Spectrochim Acta A Mol Biomol Spectrosc 2005;61:129e33.
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4. Conclusions
In summary, a class of fluorophores based on dicyanoisophor-
one derivatives has been developed. It has demonstrated that
dicyanoisophorone derivatives exhibit fluorescence in both solu-
tion and pure solid state with large Stokes shifts. It has also
demonstrated that electronic property of substituents has greatly
effected on the optical properties of dicyanoisophorone derivatives,
with a strong pushepull chromophores dicyanoisophorone system,
a red or orange solid-state fluorescence is obtained.
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emission of 1-methyl-1,2,3,4,5-pentaphenylsilole. Chem Commun 2001;18:
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Acknowledgments
[19] Sartin MM, Boydston AJ, Pagenkopf BL, Bard AJ. Electrochemistry, spectros-
copy, and electrogenerated chemiluminescence of silole-based chromo-
phores. J Am Chem Soc 2006;128:10163e70.
[20] Shi C, Guo Z, Yan Y, Zhu S, Xie Y, Zhao YS, et al. Self-assembly solid-state
enhanced red emission of quinolinemalononitrile: optical waveguides and
stimuli response. ACS Appl Mater Interfaces 2013;5:192e8.
[21] Guo Z, Zhu W, Tian H. Dicyanomethylene-4H-pyran chromophores for OLED
emitters logic gates and optical chemosensors. Chem Commun 2012;48:
6073e84.
This work was supported by the National Natural Science
Foundation of China (No 91123032) and National Basic Research
Program of China (2010CB934103).
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