Theoretical and Experimental Studies of N,N-Dimethyl-N′-Picryl-4,4′-Stilbenediamine

Article abstract of DOI:10.1007/s10895-014-1425-9

N,N-dimethyl-N′-picryl-4,4′-stilbenediamine (DMPSDA) was prepared, purified and crystallised in a form of black lustrous crystals, and its absorption and fluorescence spectra were recorded in cyclohexane, acetonitrile and dimethyl sulfoxide. Non-emissive

Full text of DOI:10.1007/s10895-014-1425-9

J Fluoresc
DOI 10.1007/s10895-014-1425-9
ORIGINAL PAPER
Theoretical and Experimental Studies
of N,N-Dimethyl-N′-Picryl-4,4′-Stilbenediamine
Vladislav Papper & Yuanyuan Wu & Vladimir Kharlanov &
Ayrine Sukharaharja & Terry W. J. Steele &
Robert S. Marks
Received: 23 June 2014 /Accepted: 30 June 2014
#
The Author(s) 2014. This article is published with open access at Springerlink.com
Abstract N,N-dimethyl-N′-picryl-4,4′-stilbenediamine
(DMPSDA) was prepared, purified and crystallised in a form
of black lustrous crystals, and its absorption and fluorescence
spectra were recorded in cyclohexane, acetonitrile and di-
methyl sulfoxide. Non-emissive intramolecular charge trans-
fer state (ICT) was clearly observed in this molecule in all
three solvents. Theoretical calculations demonstrating a beta-
ine electronic structure of the trinitrophenyl group in the
ground state of the molecule and a charge transfer nature of
the long wavelength transition S0→S1 supported the experi-
mental observations of the ICT formation in the molecule.
DMA
TNP
EX
EM
CH
dimethylamino
trinitrophenyl
fluorescence excitation
fluorescence emission
cyclohexane
ACN
DMSO
acetonitrile
dimethyl sulfoxide
Introduction
We have previously discussed various time-scale processes
occurring with 4,4′-disubstituted stilbenes after their irradiation
[1–3]. We found that different 4,4′-disubstituted stilbenes ex-
hibit different sensitivity to intramolecular donor-acceptor ef-
fects of substituents and medium polarity. All the investigated
4,4′-disubstituted stilbenes were divided into three groups ac-
.
Keywords Intramolecular charge transfer Fluorescence
Abbreviations
DMPSDA N,N-dimethyl-N′-picryl-4,4′-stilbenediamine
ICT
intramolecular charge transfer
1
DFT
LYP
density functional theory
cording to their intramolecular stabilization of the excited t*
correlation functional of Lee, Yang, and Parr
Becke’s three-parameter hybrid method with
correlation functional of Lee, Yang, and Parr
state. Those having strong donor dimethylamino (DMA) group
at one phenyl ring and strong acceptor substituent at another
phenyl ring constituted the third group called “push-pull stil-
benes”. They were characterised by large charge delocalisation
between donor and acceptor aromatic moieties, large dipole
B3LYP
TD-B3LYP time-dependent B3LYP
:
:
:
1
V. Papper Y. Wu A. Sukharaharja T. W. J. Steele (*)
School of Materials Science and Engineering, Nanyang
Technological University, 50 Nanyang Avenue, Singapore 639798,
Singapore
moment, low polarity p* state with preferential stabilisation
1
1
of t* compared to less polar p* state, very high activation
barrier for the 1t*→1p* twisted transition, and net increase of
the fluorescence quantum yield and fluorescence life-time with
increase of solvent polarity. All these observations indicated
completely different non-radiative relaxation pathway than the
1t*→1p* twisted transition observed in most stilbene com-
pounds, which are not classified as “push-pull stilbenes” [1–3].
Actually, the third group has been formed by three inves-
tigated “push-pull stilbenes”, namely 4-dimethylamino-4′-
cyanostilbene (DACS), 4-dimethylamino-4′-
carbomethoxystilbene (DACMS) and 4-dimethylamino-4′-
e-mail: wjsteele@ntu.edu.sg
V. Kharlanov
Institute of Chemistry, Humboldt University of Berlin, Berlin,
Germany
R. S. Marks
Department of Biotechnology Engineering, National Institute for
Biotechnology in the Negev, Ilse Kats Institute for Nanoscale Science
and Technology, Ben Gurion University of the Negev, Beer Sheva,
Israel

J Fluoresc
nitrostilbene (DANS). These three stilbenes exhibited very
large Stock shifts, compared to the first and second groups
[1–3]. In that case, the highly polarized excited state, which
creates a huge dipole moment, is stabilised extensively by
polar interactions (Stock shift value ΔE was found to be
approximately 22–23 kcal/mol). Experimental observations
of high fluorescence quantum yields and long fluorescence
lifetimes in strongly polar solvents indicated a strongly
preferred pathway toward the intramolecular charge trans-
fer (ICT) formation in the competing photochemical pro-
grade, ≥99 %), used for spectroscopy did not require any
additional purification.
Apparatus and Methods
Semi-preparative flash chromatography used for purification
of the compounds was performed with the Büchi Sepacore®
flash chromatography system using 25 g silica gel cartridges
(particle size 40–63 μm) and dichloromethane as an eluting
1
solvent. H NMR spectra were run on 10 % (w/v) sample
1
cesses starting from t*. Assuming that this point holds in
solutions in CDCl3 with (CH3) Si as an internal standard at
4
less polar solvents as well, the reduced fluorescence quan-
tum yield and increased non-radiative decay rate is proba-
bly due to the insufficient stabilization of the ICT in these
solvents. This can easily be understood by assuming that
1p* is less polar than ICT and 1t* states, and thus increasing
solvent polarity preferentially lowers a highly polar ICT
state with respect to 1p*.
Although three aforementioned “push-pull” stilbenes were
categorised into one group, they do not necessarily conform to
the same relaxation mechanism. In fact, intramolecular charge
transfer in these molecules may be observed as an emissive or
non-emissive process dependent on the 4,4′-substitution pat-
tern. In case of DANS, we assumed a non-emissive ICT state,
which was attributed to a specific interaction of the nitro-
group, which quenches the charge-transfer state emission [4,
5]. Actually, non-emissive ICT states were observed in polar
solvents when the acceptor group is very powerful and has a
low-lying anti-bonding orbital [4]. To generalise this observa-
tion, we prepared and studied a new “push-pull” stilbene
having a relatively strong acceptor substituent, namely
trinitrophenyl (TNP) group. This molecule has been disclosed
only once in literature and only its UV absorption spectrum
has been previously reported [6].
room temperature using a 400 MHz Bruker Fourier transform
spectrometer, equipped with a DMX AVANCE I system. UV
absorption spectra were measured using an Agilent Cary 300
spectrophotometer, and the steady-state fluorescence spectra
were recorded with Horiba Jobin Yvon FluoroLog®-3 modu-
lar spectrofluorometer and Agilent Cary Eclipse fluorescence
spectrophotometer. The fluorescence excitation and emission
spectra were corrected for instrumental sensitivity at different
excitation and emission slits using the instrument internal
excitation-emission matrix (EEM) correction [7]. A solution
of quinine bisulphate in 0.1 N H2SO4 (Фf=0.52) was taken as
fluorescence standard for the determination of fluorescence
quantum yields [8]. Constant-illumination fluorescence inten-
sity decay curves at the photostationary steady-state equilib-
rium between the trans- and cis-isomers of DMPSDA were
recorded with Shimadzu RF-5,301 spectrofluorometer
equipped with the 150 W Xenon lamp as a radiation light
source. The fluorescence decay at the photostationary steady-
state equilibrium was monitored at the emission maximum of
DMPSDA after excitation at the excitation maximum using
typically 5-nm slit width for excitation and 5-nm slit width for
emission. Analysis of the experimental data was performed
using Origin® Pro 9.0 for Windows.
Herein, we report the synthetic preparation and experimen-
tal studies of this molecule, as well as theoretical calculations
aimed at elucidating the absolute geometry of the most stable
conformer, which might be responsible for the experimentally
observed charge-transfer interactions in the ground state of the
molecule.
Fluorescence intensity decay curves were analysed with a
polynomial fit for calculation of the trans-cis isomerization
rate constants using a self-written routine within the Origin®
Pro 9.0 for analysis of the first-order photochemical reaction
rates.
Synthesis of N,N-Dimethyl-N′-Picryl-4,4′-Stilbenediamine
The following reactants were prepared by slight modification
of our procedure that we published in [9]: trans-4-
dimethylamino-4′-nitrostilbene and trans-4-dimethylamino-
4′-aminostilbene.
Experimental
Reactants and Solvents
Reactants were either commercially available or freshly pre-
pared as detailed in the Synthesis section. Commercially
available organic solvents, namely cyclohexane (Sigma-Al-
drich ACS spectrophotometric grade, ≥99 %), dimethyl sulf-
oxide (Sigma-Aldrich ACS spectrophotometric grade, ≥99 %)
and acetonitrile (Sigma-Aldrich ACS spectrophotometric
(1) Trans-4-Dimethylamino-4′-nitrostilbene
An equimolar mixture of 4-nitrophenylacetic acid (Sigma-
Aldrich N20204) (9.1 g) and 4-(dimethylamino) benzalde-
hyde (Sigma-Aldrich 156,477) (7.5 g) in 3 ml of piperidine
was heated for 24 h under reflux in an oil bath maintained at

J Fluoresc
140°. The resulting dark-red crude solid was recrystallized
from 100 ml of chlorobenzene. Upon cooling, the product
precipitated as lustrous red flakes, then was collected by
vacuum filtration on a Buchner funnel, thoroughly washed
with petroleum ether (60–80°) to remove the adherent piper-
idine and dried in a vacuum oven at 60° for about 2 h. Yield of
the lustrous red flakes – 7.4 g (53 %). Chemical purity and
structural identity of the product was confirmed by 1H NMR.
exothermic and vigour! Well-protective glasses, gloves and
clothes should be worn during this step, and all precautions
for minimising the reaction mixture splashing should be
taken. The crude yellow product was collected by vacuum
filtration, thoroughly washed with 10 % sodium bicarbonate
solution, ultrapure water and then with petroleum ether (60–
80°). The crude yellow product was dissolved in 10 ml of
dichloromethane, and subjected to the flash chromatography
on the regular silica gel (particle size 40–63 μm). The frac-
tions containing trans-isomer (′abs=360 nm) were collected
and evaporated to yield the yellow amorphous powder
(2.82 g, 77 % reaction yield). Chemical purity and structural
identity of the product was confirmed by 1H NMR.
(2) Trans-4-Dimethylamino-4′-aminostilbene
Solution of 120 g of potassium hydroxide pellets dis-
solved in 200 ml of ultrapure water was prepared in 500 ml
Erlenmeyer flask and placed in a fridge for storing. This
solution is used for decomposition of the prepared stannous
complex salt of the product. 15 g of stannous chloride
dihydrate was dissolved in 150 ml of concentrated hydro-
chloric acid at room temperature with stirring. 4 g of fresh
trans-4-dimethylamino-4′-nitrostilbene prepared in the pre-
vious step was introduced into the stirred solution of SnCl2.
The resulted suspension was refluxed with strong stirring for
about 4–5 h until all red flakes and orange viscous particles
of the intermediate stannous complex salt formed during the
reaction have been completely dissolved, and the yellow
reaction mixture has become transparent. The reaction mix-
ture was stirred for additional half an hour and allowed to
cool to the point of turbidity. Then it was poured carefully
into a cold stirred solution of potassium hydroxide taken
from the fridge and placed into an ice bath, in order to
decompose the complex stannous salt and to release the
product. Caution: the neutralization reaction is highly
(3) N,N-dimethyl-N′-picryl-4,4′-stilbenediamine
An equimolar mixture of 0.48 g (2 mmol) of trans-4-
dimethylamino-4′-aminostilbene, prepared in a previous step,
and 0.50 g (2 mmol) of 2-chloro-1,3,5-trinitrobenzene
(Sigma-Aldrich 79,874) in 5 ml absolute ethanol, which
yielded initially a clear solution, was left overnight with
stirring The resulted black crystalline precipitate was collected
by vacuum filtration on a Buchner funnel, washed with 10 %
sodium bicarbonate solution, ultrapure water and then with
petroleum ether (60–80°). The crude product was recrystal-
lized from chlorobenzene, and then dried in a vacuum oven at
60° for about 2 h. Yield of the black crystalline product –
0.76 g (85 %). 1H NMR: δ 3.00 (Me2N, s, 6H); δ 6.88, δ 6.93
(CH=CH AB, d, vinyl 2H); δ 6.72, δ 7.89 (4-Me2N-Ar AA′
XX′, dd, 4H); δ 7.33, δ 7.83 (4′-picrylamine-Ar AA′XX′, dd,
4H); δ 9.24 (picryl ring protons, s, 2H); δ 3.71 (NH).
Fig. 1 Normalised absorption,
excitation and emission spectra of
trans-DMPSDA in different
solvents: a CH, b ACN and c
DMSO, and d the photostationary
fluorescence decay curves under
constant-illumination conditions
in these three solvents

J Fluoresc
Table 1 Absorption, fluorescence excitation and emission maximum
wavelengths (λabs, λex and λem), fluorescence quantum yields (Φf) and
apparent fluorescence decay rate constants (kapp) for N,N-dimethyl-N′-
picryl-4,4′-stilbenediamine in acetonitrile (ACN), dimethyl sulfoxide
(DMSO) and cyclohexane (CH) at room temperature
Results and Discussion
Experimental Studies
We succeeded to synthesize and recrystallize a stable trans-
isomer of DMPSDA. Figure 1 shows the absorption, excita-
tion and emission spectra of trans-DMPSDA in different
solvents (CH, ACN and DMSO), as well as the
photostationary fluorescence decay curves under constant-
illumination conditions. Figure 2 demonstrates the fluores-
cence excitation and emission spectra of trans-DMPSDA in
Solvent
λ
abs (nm)
λ
ex (nm)
λ
em (nm)
Φf
kapp (s−1
)
ACN
DMSO
CH
360
365
350
360
365
350
420
0.009
0.013
0.001
0.643
0.614
−
425
390/410
three solvents. Table 1 brings together all the spectroscopic
properties of the molecule in these solvents at room tempera-
ture including absorption, fluorescence excitation and emis-
sion maxima (λabs, λex and λem), fluorescence quantum yields
(Φf) and apparent fluorescence decay rate constants (kapp), for
the investigated compound in three solvents at room temper-
ature. There is a clear non-emissive ICT band observed at the
longer wavelength in the absorption spectra of the molecule in
all three solvents. Theoretical calculations, which will be
presented in the next section, show the possible electronic
structure of trans-DMPSDA responsible for this non-
emissive ICT state.
The photostationary fluorescence decay of trans-
DMPSDA is observed only in relatively polar solvents
1
(ACN and DMSO) and originates from the non-radiative
t*→1p* twisted transition, which is responsible for trans-cis
isomerisation of the molecule and acts as a quenching funnel
on fluorescence emission. CH is a non-polar solvent, and
vibrational relaxation of the Franck-Condon state plays the
primary role in its stabilisation of the excited state, thereby
considerably slowing the 1t*→1p* transition [1, 2].
The measured fluorescence quantum yield values in all
three solvents, shown in Table 1, are relatively low compared
to the parent trans-4-dimethylamino-4′-aminostilbene [1–3].
Fig. 2 Fluorescence excitation and fluorescence emission spectra of
trans-DMPSDA in three solvents at the different excitation and emission
wavelengths: a CH, emission range 380–500 nm; excitation 275–350 nm;
solid bold line – λem=390 nm, λex=320 nm; b ACN, emission range
400–520 nm; excitation 300–400 nm; solid bold line – λem=420 nm,
λex=360 nm; and c DMSO, emission range 420–520 nm; excitation 300–
Fig. 3 Stereoisomers 1E, 1E′, 1Z, 1Z′ of DMPSDA studied with the ab
400 nm; solid bold line – λem=420 nm, λex=360 nm
initio calculations

J Fluoresc
Fig. 4 B3LYP optimized 3D
structures of 1E, 1E′ and 1Z with
the bond lengths
We assume that part of the additional non-radiative losses are
caused by the electronic energy transfer onto the TNP frag-
ment, which intrinsically shows negligible fluorescence, due
to the presence of an efficient conical intersection of the S1
state with the ground state S0 [10].
Theoretical Calculations
Theoretical calculations were conducted with the following
software suites: Gaussian 03 [11], Gaussian 09 [12] and
Turbomole 6.1 [13]. The ground state studies were carried
out by the density functional theory (DFT) using the
Becke’s Three Parameter Hybrid Method and the correla-
tion functional of Lee, Yang, and Parr (LYP) with the
correlation potentials VWM (III) (calculated in Gaussian
03 or Gaussian 09) and VWM (V) (calculated in Turbomole
6.1). The optimized structures were controlled by frequen-
cy calculations with the vibration analysis using Hessian
matrix. The excited states were calculated by the Time
Dependent B3LYP hybrid method for the ground state
optimized structures.
Table 2 Absolute energy Eh, relative energy ΔE, dipole moment μg and
number of negative Hessian of the equilibrium structures for isomers 1E,
1Z and 1E′calculated by B3LYP/6–31+G (3df,2pd)
Isomers
Eh, Hartree
ΔE, kcal/mol
μg, D
x
1E
1E′
1Z
−1,574.7860481
−1,574.7860106
−1,574.7780105
0
10.5
10.3
7.52
0
0
0
0.024
5.04

J Fluoresc
Table 3 The S0→S1 transition characteristics for the optimised isomers
(Franck-Condon energy EhFC, transition energy ΔE, the oscillator strength
f, the excited state dipole moment μe, the state nature) calculated by TD-
B3LYP/6–31+G (3df,2pd)
equilibrium structures, the long wavelength transitions S0→
S1 have a clear charge transfer nature. The dipole moments of
the Franck-Condon states are listed in Table 3, which support
the above conclusion.
Isomers EhFC, Hartree
ΔE01 (f), eV μe, D State Nature
1E
1E′
1Z
−1,574.7317932 1.48 (0.04)
−1,574.7319603 1.47 (0.04)
−1,574.7240979 1.47 (0.02)
57.0
56.1
49.5
Charge Transfer
Charge Transfer
Charge Transfer
Conclusions
N,N-dimethyl-N′-picryl-4,4′-stilbenediamine (DMPSDA) was
prepared in a form of lustrous black crystals, and its absorption
and fluorescence spectra were recorded in cyclohexane, acetoni-
trile and dimethyl sulfoxide. Non-emissive intramolecular charge
transfer (ICT) was clearly observed in this molecule at the longer
wavelength in the absorption spectra in all three solvents. The
measured fluorescence quantum yield values in all three solvents
were found to be relatively low compared to the parent trans-4-
dimethylamino-4′-aminostilbene. We assumed that part of the
additional non-radiative losses are caused by the electronic ener-
gy transfer onto the TNP fragment, which intrinsically shows
negligible fluorescence, due to the presence of an efficient con-
ical intersection of the S1 state with the ground state S0. Theo-
retical calculations demonstrating a betaine electronic structure of
the trinitrophenyl group in the ground state of the molecule and a
charge transfer nature of the long wavelength transition S0→S1
supported the aforementioned experimental observations of the
ICT formation in the molecule.
Figure 3 shows the possible isomers 1E, 1E′, 1Z, 1Z′ of
DMPSDA studied with the ab initio calculations. The B3LYP
optimized structures with the bond lengths are shown in
Fig. 4. The calculated properties of the isomers are given in
Tables 2 and 3. The obtained results clearly demonstrate that
trans-isomer 1E is the most electronically stable isomer. How-
ever, the energy difference (ΔE=0.024 kcal/mol) to another
trans-isomer 1E′ is very small. On the other hand, cis-isomer
1Z having ΔE=5.04 kcal/mol is relatively unstable. The fre-
quency calculations showed that all optimized isomeric struc-
tures are equilibrium ones. The vibration frequencies are
positive and the number of negative Hessian is zero.
Both trans-isomers 1E and 1E′of DMPSDA possess a
planar geometry of the TNP-amino fragment and a weak
hybridized dimethylamino group on the opposite stilbene ring.
TNP rings are non-planar to the stilbene plane of symmetry.
Torsion angles of the TNP rings are 23.2° for 1E and 23.0° for
1E′. The crystal str ucture of 2, 4, 6-trinitro-
N-[4-(phenyldiazenyl) phenyl] aniline shows the similar ge-
ometry of the TNP ring with the torsion angle of 27.7° [14].
According to the calculations, the bond alternation of the
trans-isomers is similar and slightly differs from the bond
alternation of the cis-isomer 1Z, as seen in Fig. 4. The corre-
sponding bond lengths of all calculated isomers are also
similar. Bond distance between the nitrogen atom of the
DMA group and the phenyl ring is 1.3789 Å (1E), 1.3790 Å
(1E′) and 1.3816 Å (1Z). The bond lengths connecting the
phenyl rings are 1.4595 Å, 1.3505 Å, 1.4561 Å (for 1E),
1.4594 Å, 1.3505 Å, 1.4561 Å (for 1E′) and 1.4703 Å,
1.3508 Å, 1.4654 Å (for 1Z). The bond alteration of the
TNP ring in all isomers clearly demonstrates the betaine
electronic structure of this fragment, shown in Fig. 5. Accord-
ing to the TD-B3LYP calculations for the optimized
Acknowledgements This research is funded by the Singapore National
Research Foundation and the publication is supported under the Campus for
Research Excellence and Technological Enterprise (CREATE) programme
(13–04–00364 А). The authors thank Huang Ruo Cheng, Tan Chong Yuan
and Terence Mak of River Valley High School for their active participation in
this project in the frame of the program for Singapore Science & Engineering
Fair. In addition, the authors thank NTU for providing partial financial support
to this program. Funding was also greatly appreciated from the Ministry of
Education Tier 1 Grant: Photochrome aptamer switch assay: A universal
bioassay device - RG54/13.
Open Access This article is distributed under the terms of the Creative
Commons Attribution License which permits any use, distribution, and
reproduction in any medium, provided the original author(s) and the
source are credited.
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