White light induced E/Z-photoisomerization of diphenylamine-tethered fluorescent stilbene derivatives: Synthesis, photophysical, and electrochemical investigation

Article abstract of DOI:10.1021/acs.joc.8b00033

A facile synthesis and detailed photophysical investigation of E/Z-isomerization of fluorescent diphenylamine tethered stilbene derivatives (DPASs) under white light exposure have been carried out to understand the effect on fluorescence, electrochemical

Full text of DOI:10.1021/acs.joc.8b00033

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White Light Induced E/Z-Photoisomerization of Diphenylamine-
tethered Fluorescent Stilbene Derivatives: Synthesis,
Photophysical and Electrochemical Investigation
shachi mishra, Pallavi Awasthi, Jagriti Singh, Rahul K Gupta, Vikram
Singh, Ruchir Kant, Ram Jeet, Debabrata Goswami, and Atul Goel
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.8b00033 • Publication Date (Web): 09 Mar 2018
Downloaded from http://pubs.acs.org on March 9, 2018
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White Light Induced E/ZꢀPhotoisomerization of Diphenylamineꢀtethered
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Fluorescent
Stilbene
Derivatives:
Synthesis,
Photophysical
and
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Electrochemical Investigation
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Shachi Mishra†, Pallavi Awasthi†, Jagriti Singh†, Rahul Kumar Gupta§, Vikram Singh†, Ruchir
Kantǁ, Ram Jeet†, Debabrata Goswami§, Atul Goel*,†,
‡
†Medicinal and Process Chemistry Division, CSIRꢀCentral Drug Research Institute,
Lucknow 226031, India; §Department of Chemistry, Indian Institute of Technology Kanpur,
ǁ
Kanpur 208016, Uttar Pradesh, India; Molecular and Structural Biology Division, CSIRꢀ
‡
Central Drug Research Institute, Lucknow 226031, India; Academy of Scientific and
Innovative Research, New Delhi 110001, India
Eꢀmail: atul_goel@cdri.res.in
Table of Contents
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ABSTRACT:
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A facile synthesis and detailed photophysical investigation of E/Zꢀisomerization of fluorescent
diphenylamine tethered stilbene derivatives (DPASs) under white light exposure have been
carried out to understand the effect on fluorescence, electrochemical properties and photoꢀ
stability under various activation/deactivation pathways. In solution state, under dark, Eꢀisomer
of DPASs (6aꢀd) exhibited high fluorescence quantum yields (Φfl ≈ 53% to 60% in DMSO).
However, on white light exposure 1H NMR and HPLC studies revealed that pure Eꢀisomer of the
DPAS 6a (~ 9.5 mM) started converting into its Zꢀform by photoisomerization until it reaches to
nearly equilibrium. At low concentrations (~10 ꢁM), the absorption band of the pure Eꢀisomer in
the range of 350ꢀ450 nm gradually decreased to adopt Zꢀconformation 6a′ until a photoꢀ
stationary state was reached. The structure of the Eꢀisomer 6a was unequivocally confirmed by
Xꢀray diffraction analysis. The synthesized DPAS compounds 6aꢀd possessed positive
solvatochromic properties, two photon absorption properties and good thermal stability. The
electrochemical investigations using DPASs showed reversible oxidation resulting in formation
of a stable radical cation. Owing to useful photophysical, electrochemical and thermal properties,
these DPAS derivatives are suitable for their application in biomedical imaging as well as in
fabrication of electroluminescent materials.
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INTRODUCTION
The photoinduced E/Z isomerization phenomenon has been an interesting topic in organic and
inorganic photochemistry as well as in biology.1 This phenomenon has been exploited for many
applications in photoelectronics, biochemistry and supramolecular chemistry.2 One of the most
fascinating application is the discovery made by the Nobel Laureate G. Wald on the
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photochemical E/Zꢀisomerization of rhodopsin which occurs during the visual cycle.3 Such
molecules have been extensively studied as molecular switches4, data storage materials,5
sensors,6,7 and logic gates.8 Apart from this, another interesting feature of such molecules is their
spectroscopic properties.9 For better exploration of this feature in biological10 and optical
applications,11 design and development of stilbene molecules with understanding of stable E/Z
configurations are required.
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Suitably substituted stilbenes because of their unique chargeꢀtransfer characteristics and
their utility as promising fluorescent materials have drawn much attention.12 However, E/Z
isomerization reaction of such molecules containing a double bond unit are nearly unavoidable
under photoꢀirradiated conditions in most of the organic molecules13 and thus limit their use in
biomedical and material applications where intense emission is required. Therefore, a detailed
study of photoisomerization conditions to get a clear idea about the different energy dissipating
processes is necessary. Several reports on stilbene derivatives have been published with or
without investigating the E/Zꢀphotoisomerization under normal or UVꢀvisible light (See
Supporting Information Table S1).14 For E/Zꢀphotoisomerisation reactions of diꢀ/triphenylamine
tethered stilbene derivatives, most of them have studied the effect on isomerisation upon UV
irradiation. However, white light exposure which is unavoidable under working conditions also
leads to appreciable E/Z isomerisation reaction and thus a detailed study of stability of Eꢀ and Zꢀ
isomers under white light exposure is also a prerequisite for design and development of materials
for organic photoꢀelectronic applications.
Triphenylamine due to its good electronꢀdonating and transporting capabilities as well as
its special propeller starburst molecular structure has been widely used in optoꢀ and electroactive
materials.15 Recently, several diphenylamine based stilbenes have been reported for applications
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in OLEDs16, chemosensors17 and bioꢀimaging18. The synthetic protocol adopted for these
diphenylamine based stilbene derivatives usually involve Wittig reaction, Heck coupling and
condensation reactions.16ꢀ18 In this work, we developed a convenient protocol for the synthesis of
donorꢀacceptor πꢀconjugated diphenylamine based stilbenes through carbanion induced ring
transformation reactions at room temperature and studied their optical and photophysical
properties in solution and solid state under white light exposure. We further carried out the
detailed study of photoisomerization of one of the synthesized dye DPAS 6a in the presence of
white light by using techniques like proton NMR spectroscopy, absorption and emission
spectroscopy, quantum mechanical studies and HPLC technique. Electrochemical, thermal and
two photon absorption studies were also performed for their useful application in biomedical
sciences.
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RESULTS AND DISCUSSION
We have previously shown that controlled tuning of πꢀconjugation and appropriate positioning
of DꢀA moieties modulates dramatically the optical properties of aromatic and heterocyclic
compounds.19,20 Recently, we reported a novel 5,6ꢀdihydroꢀ2Hꢀpyrano[3,2ꢀg]indolizine (DPI)
class of luminogens, which exhibited unique solutionꢀsolid dual emission (SSDE) behaviour.21
The synthetic methodology adopted for the synthesis of designed donorꢀacceptor (DꢀA) DPASs
is outlined in Scheme 1. The key intermediates 2Hꢀpyranꢀ2ꢀones19cꢀe 3aꢀd were prepared from
easily accessible precursor substituted acetophenones (1aꢀd) and αꢀoxoꢀketeneꢀS,Sꢀacetal (2).
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Scheme 1. Synthesis of DPASs 6a−d
Figure 1. ORTEP diagram drawn with 30% ellipsoid probability for nonꢀH atoms of the crystal structure of
compound 6a determined at 293 K (CCDC No. 1565813).
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The methyl sulfanyl group in 3aꢀd was replaced by a piperidine donor group to furnish DꢀA
appended 6ꢀarylꢀ2ꢀoxoꢀ4ꢀpiperidinꢀ1ꢀylꢀ2Hꢀpyranꢀ3ꢀcarbonitriles (4aꢀd) in good yields.19c,d,20d
The other precursor (E)ꢀ4ꢀ(4ꢀ(diphenylamino)phenyl)butꢀ3ꢀenꢀ2ꢀone (5)22 was prepared in two
steps via formylation of triphenylamine23 followed by reaction with acetone in good yield
(Scheme 1). Further, Michael addition of the conjugate base of 5 to the lactones 4aꢀd at position
6 followed by intramolecular cyclization afforded (E)ꢀ3ꢀ(4ꢀ(diphenylamino)styryl)ꢀ5ꢀ(piperidinꢀ
1ꢀyl)biarylꢀ4ꢀcarbonitriles 6aꢀd. All of the intermediates and target compounds were
characterized by IR, 1H and 13C NMR spectroscopy and mass spectrometry, which confirmed
their expected molecular structures. Single crystal Xꢀray analysis of compound 6a
unambiguously confirmed the structure and the ORTEP drawing is shown in Figure 1.
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E/Z Photoisomerization in Solution in the presence and absence (dark) of white light
E/Z isomerization is commonly observed in aromatic and oligomeric olefins, and a detailed study
on these type of systems is important to obtain fundamental information about the stability and
photophysical properties of the stereoisomers.24 NMR spectroscopy was used for investigation
and confirmation of the E/Z photoisomerization. Starting from solution of pure Eꢀisomer of 6a
(9.5 x 10ꢀ3 M in DMSOꢀd6), it was observed that when the sample was exposed to white light for
few hours, new peaks appeared in the 1H NMR spectrum. Hence, timeꢀdependent NMR analysis
was performed to get a clear view. 1H NMR spectra recorded at 0 min was of pure Eꢀisomer with
two doublets of J value 16.1 Hz at δ 7.25 and 7.58 ppm (Figure 2). As time elapsed, new peaks
emerged which were noticeable after 60 minutes of white light exposure as shown in Figure 2. A
new doublet at about δ 6.66 ppm with J value of 12.0 Hz clearly indicated formation of Zꢀ
isomer.25,26 By integrating this signal at 6.66 ppm relative to the constant peak of Eꢀisomer at
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7.78 ppm, it was found that the integral of this new isomer increased gradually and reached
~27% relative to the sum of the two isomers after 24h of white light exposure in DMSOꢀd6.
Further in order to confirm these observations, HPLC analysis of the compound with same
concentration in DMSO (9.5 x 10ꢀ3 M) was carried after 0, 10 and 24 h white light exposure
under identical conditions (Figure S1, Supporting Information). It was observed in HPLC that
only one additional peak for the Zꢀisomer with retention time of ~45 min appeared apart from the
Eꢀisomer peak (~49 min) on white light exposure. We observed that ~27% of Zꢀisomer was
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formed after 24h which is in correlation with the H NMR results shown in Figure 2.
Furthermore, the HPLC analysis revealed that the photostationary state with ~50% of both E/Zꢀ
isomers was achieved after 48 hours of white light exposure (Figure S1, Supporting
Information). Interestingly, no isomerization was observed in solution under dark conditions as
observed upto 24 h using 1H NMR spectroscopy (Figure S2, Supporting Information).
Figure 2. Time dependent 1H NMR of Eꢀisomer on white light exposure
Isolation of Zꢀ6a (6a’) from pure Eꢀ6a through semiꢀpreparative HPLC
To isolate Zꢀisomer of 6a from Eꢀisomer, a solution of 6a in DCM was exposed to white light
and after concentrating, semiꢀpreparative HPLC analysis was carried out as mentioned in
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experimental section and the two compounds were isolated. The new compound isolated was
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characterized by H and 13CꢀNMR spectroscopy and was found to be Zꢀisomer (6a′). The
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comparative 1H NMR spectra of Eꢀisomer (6a) and Zꢀisomer (6a′) are shown in Figure 3.
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Figure 3. 1H NMR spectra of DPASB (a) 6a (Eꢀisomer) and (b) 6a′ (Zꢀisomer) in DMSOꢀd6
These experiments (1H NMR and HPLC) clearly implicated that white light plays an important
role for driving this isomerisation process in solution state. However, these DPAS compounds
are stable in solid state even when exposed to white light for few hours. No isomerisation was
observed in solid state upon white light exposure.
Photophysical properties of DPASs 6aꢀd
The optical properties of the dyes 6aꢀd were investigated by measuring absorption and emission
spectra in ~10ꢀ5 M DMSO solutions. The absorption spectra of the dyes are displayed in Figure 4
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and the relevant data are listed in Table 1. All the dyes displayed two distinguishable absorption
bands. The higher energy absorption band appearing below ~310 nm is assigned to the localized
π–π* electronic transitions originating from the triphenylamine chromophores. The lower energy
absorption peak at 398ꢀ404 nm corresponded to the intramolecular chargeꢀtransfer (ICT)
electronic transitions from the conjugated segments comprising triphenylamineꢀstyrylꢀcyano
units. All the dyes displayed intense green emission in DMSO (Figure 4 and Table 1) and in the
range 480ꢀ513 nm in solid state (Table 1 and Figure S3, Supporting Information). E/Zꢀ
Isomerization was observed on white light irradiation for all the dyes 6aꢀd. Exemplary, detailed
investigation of this phenomenon was done for DPAS 6a.
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Figure 4. Normalized absorbance and fluorescence spectra of 6aꢀd in DMSO (~10ꢀ5 M).
Table 1. One photon and Two Photon Optical Properties of 6aꢀd
e
entry
λmax, abs
λmax, em
λmax, em Stokes Shift
TPEACS (σ/GM)
740 nmf 780 nmf
Фf (%)
a
b
c
ꢀ1 d
(nm)
(nm)
(nm)
(cm )
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298, 398
520
480
5890
57
20.79
13.59
6a
5
6
7
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9
284, 401
299, 398
303, 404
523
520
533
500
481
513
5820
5890
5990
58
60
53
nd
10.22
17.35
15.12
6b
6c
6d
11.56
16.46
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b
c
aLongestꢀwavelength absorption maximum in DMSO. Fluorescence emission maximum in DMSO. Fluorescence
emission maximum in the solid state. Stokes shift (cmꢀ1) = (1/λabs–1/λem). eFluorescence quantum yield relative to
d
f
harmine in 0.1 M H2SO4 as a standard (Φ = 45%). σ:Two photon excitation action cross section in GM. Twoꢀ
photon excitation wavelengths.
TwoꢀPhoton Absorption properties of DPASs 6aꢀd
The two photon excitation action crossꢀsections of chromophores 6aꢀd were measured in the
wide wavelength range from 740–785 nm pumped by femtosecond laser pulse at power of 335
mW and keeping the integration time at 300 ms at different excitation wavelengths, exemplary
shown for 6a in Figure 5a and for others in Figure S4, Supporting Information. The two photon
excitation action cross section, TPEACS (σ) values are described in Table 1. Figure 5b and
Figure S4, Supporting Information shows logarithmic plots of the fluorescence integral versus
pumped power with a slope of 1.88, 1.89, 1.95 and 1.73 when the input laser power is increasing,
suggesting a twoꢀphoton excitation mechanism.
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Figure 5. (a) Two photon excitation action cross section (σ, 1 GM = 10ꢀ50 cm4 s per photon per molecule) of 6a in
the wavelength region of 740–785 nm at 335 mW. (b) Twoꢀphoton absorption verification: Plot of log of Two
photon induced fluorescence intensity vs log of laser power of 6a.
Photophysical studies of EꢀZ and ZꢀE isomerisation of 6a :
The Eꢀisomer of DPAS 6a in DMSO solution (10ꢀ5 M) exhibited absorption maximum at 299 nm
and 398 nm with molar absorption coefficients (εmax) of 34600 M−1 cm−1 and 26110 M−1 cm−1
respectively while Zꢀisomer exhibited an absorption maximum at 299 nm and a shoulder at 356
nm with molar absorption coefficient of 44250 M−1 cm−1 and 17000 M−1 cm−1 respectively
(Figure 6a, Table 2). The molar absorptivity of the Eꢀisomer at 398 nm was found to be larger
than that of the Zꢀisomer at 356 nm. The peak in Eꢀisomer at 299 nm could be assigned to the
local electron transition of triphenylamine group and the peak at 398 nm could be ascribed to the
delocalized π to π* transition on the Eꢀstilbene backbone. This second absorption peak was blue
shifted (λmax 356 nm) in the case of Zꢀisomer. These results indicate that there is better ICT
character in Eꢀisomer as compared to Zꢀform, probably due to the twisted conformation of the
stilbene moiety in Zꢀisomer leading to hindrance of the ICT process. Surprisingly, the Eꢀisomer
of 6a showed bright green fluorescence (emission maximum at 522 nm) with good quantum
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yield of 57 % in DMSO, while Zꢀisomer displayed weak fluorescence (emission maximum at
519 nm) with low quantum yield of 4 % (Figure 6b and Table 2).
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(b)
(a)
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Figure 6. (a) UVꢀVis absorption and (b) FL spectra of pure Eꢀ(6a) and Zꢀ(6a′) isomers in DMSO (~10ꢀ5 M) at room
temperature
Table 2: Absorption Properties of 6a and 6a′.
c
entry
λ
(nm)
ꢀ1
λ
max, em
Ф (%)
max, abs
ꢀ1
3 a
b
f
(ε , M cm X 10 )
(nm)
max
6aꢀE isomer
6a′ꢀZ isomer
298 (34.6), 398 (26.1)
299 (44.2), 356 (17.0)
520
523
57
4
b
aLongestꢀwavelength absorption maximum (molar absorptivity) in DMSO. Fluorescence emission maximum in
DMSO. cFluorescence quantum yield relative to harmine in 0.1 M H2SO4 as a standard (Φ = 45%).
The timeꢀdependent UVꢀFL studies revealed that pure Eꢀform of 6a (fluorescence intensity
~200 au) started converting into Zꢀform (6a’) on white light exposure and reached to a stationary
state within an hour with fluorescence intensity of ~100 au (Figure 7). Figure 6 and 7a showed
the existence of an isobestic point at 358 nm, which indicated that only two species that vary in
concentration contribute to the absorbance.27 Furthermore, a solution of pure Zꢀisomer was
photoisomerized on white light irradiation and in this case, photostationary state was obtained in
30 minutes only, implying thereby that Zꢀform of 6a is less stable than Eꢀform (Figure S5 in the
Supporting Information). Similar results were obtained for other Eꢀisomer derivatives 6bꢀd
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(Figure S6 in the Supporting Information). Additionally, we observed that the absorbance of the
solution of 6a (10ꢀ5 M in DMSO) under continuous monochromatic light exposure at 398 nm for
60 min remained almost constant with respect to time, while the solution of same concentration
under white light exposure resulted in significant exponential decrease in absorbance (see
Supporting Information Figure S7).
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(b)
(a)
Figure 7. (a) Absorption spectra and (b) Fluorescence spectra of DPASB 6a (Eꢀisomer) in DMSO (~10ꢀ5 M)
recorded at various times under white light irradiation.
Solvatochromic properties of 6aꢀd
Molecules with donorꢀacceptor functionalities are characterized by a prominent solvatochromic
effect. Hence, the absorption and emission spectra of EꢀDPASs in solvents with different
polarities were investigated. The photophysical data of all the dyes in different solvents are
summarized in Table S2, Supporting Information. The absorption spectra of the dyes 6aꢀd are
not affected by the solvents of different polarities indicating a nonꢀpolar ground state. The oneꢀ
photon absorption and emission spectra of 6a in six different solvents are shown in Figure 8
(others in Figure S8, Supporting Information). In the emission spectra, dyes 6aꢀd showed a
remarkable red shift with solvents of increasing polarity suggesting solvatochromic behaviour of
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these DPASs. In order to determine the effect of solvents on EꢀZ photoisomerisation upon white
light exposure, timeꢀdependent UVꢀVis spectra of 6a in different solvents (~10ꢀ5 M) were
investigated. It was observed that EꢀZ photoisomerisation took place in both nonꢀpolar and polar
solvents with photostationary state reached within 60 minutes (Figure S9, Supporting
Information).
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To further evaluate the effect of the solvents on the fluorescence of DPASs, the change in
the emission maximum with the Reichardt polarity parameter ET(30)28 is plotted in Figure 9(a).
A linear line with good correlation coefficients (r2 = 0.99) was obtained, confirming the
remarkable solvatochromism of DPASs. Another confirmation regarding the solvent sensitivity
of the emission spectra of 6aꢀd was obtained by evaluation of its Stokes shift in terms of
Lippert–Mataga plot (Figure 9b).29 For all the dyes, a nice linear correlation (r2 = 0.98) with
respect to the orientation polarizability (∆f) was observed and exhibited a much steeper slope
indicating a larger fluorescence solvatochromism. The positive solvatochromism indicated that
the donor–acceptor DPASs exhibited a strongly stabilized excited state with high intramolecular
charge transfer (ICT) character and larger dipole moments compared to the ground state by the
surrounding solvent molecules.
(b)
(a)
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Figure 8. (a) Absorption and (b) emission spectra of dye 6a in different solvents.
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(b)
(a)
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Figure 9. (a) Stokes shift versus ET(30) and (b) Lippert–Mataga plots of the dyes.
Computational study
To unravel the electronic behaviour of DPASs and to get an insight into the isomerization
behaviour, the geometry of 6aꢀd (Eꢀisomers) was optimized at DFT/B3LYP level using a 6ꢀ
31G(d,p) basis set and TDDFT calculations was performed using a B3LYP/6ꢀ311++G(d,p)
method. For Eꢀisomer of 6a, timeꢀdependent density functional theory (TDꢀDFT) calculations
were carried out taking the coordinates from Xꢀray data using a B3LYP/6ꢀ311++G(d,p) method
with a Gaussian 09 package. For all Eꢀisomer derivatives 6aꢀd, the HOMO and LUMO values
were found to be (ꢀ5.19 to ꢀ5.25 eV) and (ꢀ1.92 to ꢀ2.15 eV) respectively (Figure S10 and Table
S3, Supporting Information). In all the dyes the longerꢀwavelength absorption originates from
the HOMO to LUMO electronic excitation (Table S3, Supporting Information), which involve a
charge migration from the π–conjugated N, Nꢀdiphenylaminophenyl (donor) unit to the CNꢀ
containing phenyl core segment (acceptor) (Figure S10, Supporting Information).
Comparative analysis of molecular orbitals of 6a (Eꢀisomer) and 6a’ (Zꢀisomer) in the ground
state are represented in Figure 10. The electronic transition for Eꢀisomer at 427 nm wavelength
(HOMO to LUMO) with an oscillator strength (f) of 0.7263 corresponded to effective intraꢀ
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molecular chargeꢀtransfer (ICT) band while for Zꢀisomer electronic transition at 361 nm
wavelength (HOMO to LUMO+1) with an oscillator strength (f) of 0.4262 corresponded to less
effective intraꢀmolecular chargeꢀtransfer (ICT) band than former (Table 3).
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Figure 10. Energy levels and corresponding frontier molecular orbitals of both isomers of 6a calculated at
B3LYP/6ꢀ31G* level.
Table 3: Computed Values of Vertical Excitations, Oscillator Strength, Assignment, HOMO,
LUMO, and Energy Band Gap of Eꢀ and Zꢀisomer.
entry
6a
Oscillator
Strength (f)
Assignment
HOMO
(eV)
LUMO
(eV)
E0ꢀ0
λmax, abs
(nm)
(theoretical)
427
0.7263
HOMO to LUMO
(97%)
ꢀ5.19
ꢀ1.91
3.27
(Eꢀ
isomer)
375
353
0.0092
0.1097
HOMOꢀ1 to
LUMO (81%)
HOMO to
LUMO+1 (80%)
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429
387
361
0.2395
HOMO to LUMO
(96%)
ꢀ5.36
ꢀ1.98
3.38
6a′
(Zꢀ
isomer)
0.0659
0.4262
HOMOꢀ1 to
LUMO (95%)
HOMO to
LUMO+1 (96%)
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Electrochemical and thermal properties
Electrochemical studies were carried out to determine experimental electronic properties of 6aꢀd.
All measurements were performed in a threeꢀelectrode cell setup using Ag/AgCl as reference
electrode and a Pt disc as the working electrode, using a 2 mM solution of compound, and 0.2 M
of the electrolyte tetrabutylammonium hexafluorophosphate (Bu4NPF6) dissolved in DCM. The
cyclic voltammograms obtained were employed to evaluate the highest occupied molecular
orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels (Figure
11a). The HOMO levels were readily estimated from the onset oxidation (Eox) using the equation
EHOMO = ꢀ(Eox + 4.8) eV, where Eox is the onset oxidation potentials relative to the
ferrocene/ferrocenium couple (EFOC = 0.53 V versus Ag/AgCl electrode). The LUMO levels of
the dyes were calculated by Eox ꢀ E0–0, where E0–0 is the zeroth–zeroth energy of the dyes
estimated from the absorption edge (Table 4). The oxidation processes observed were reversible
for 6aꢀd, indicating a stable radical cation ion species. Moreover, the highest occupied molecular
orbital (HOMO) of DPASs was in the range ꢀ5.17 to ꢀ5.21 eV, which matched with the work
function of indiumꢀtinꢀoxide (ITO), used as anode in OLED devices.16c Hence, DPASs can be
utilized as a hole transporting material.
The thermal properties of 6aꢀd were gauged by thermogravimetric analysis (TGA) under an
argon atmosphere. DPASs 6aꢀd were thermally stable up to 368, 357, 368 and 370 °C,
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respectively (5% weight loss temperature) as shown in Table 4 and Figure 11b. The good
thermal, photophysical and electrochemical properties of these DPASs suggest that these
fluorescent compounds are suitable for their application in both biomedical research as well as in
fabricating electroluminescent materials.
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(b)
(a)
Figure 11. (a) Cyclic voltammograms recorded for compounds 6aꢀd in DCM (scan rate = 100 mV/s). (b)
Thermogravimetric analysis of compound 6aꢀd.
Table 4: Electrochemical and thermogravimetric analysis.
Td (oC)
Tmb (oC)
HOMOc (eV)
LUMOd (eV)
E0ꢀ0e(eV)
a
entry
368
357
368
218
197
208
ꢀ5.17
ꢀ5.19
ꢀ5.20
ꢀ2.48
ꢀ2.52
ꢀ2.53
2.69
2.67
2.67
6a
6b
6c
370
200
ꢀ5.21
ꢀ2.56
2.65
6d
aTemperature corresponding to 5% weight loss. Melting temperature gauged by DSC. HOMO = 4.8 + Eox. dLUMO =
HOMO ꢀ E0–0. eOptical band gap obtained from the absorption edge.
b
c
CONCLUSION
In summary, the Eꢀisomer of diphenylamine–tethered stilbene derivatives (DPASs, 6aꢀd) were
successfully synthesized using easily accessible precursors through simple reaction conditions.
These DPAS derivatives showed good green fluorescence in both solution and solid state with
good fluorescence quantum yields (Φfl ~ 53% to 60% in DMSO). A detailed investigation of the
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fluorescence of DPAS 6a in solution state (~10ꢀ5 M in DMSO) under white light exposure
indicated the formation of E/Z isomerization with a photostationary state being reached after
approx. 1 hour under tested conditions. Interestingly, the similar photostationary state with 50 %
E/Zꢀisomers was achieved after 48 hours when DPAS 6a was analysed by HPLC at higher
concentration of approximately 10ꢀ2 M in DMSO. These data suggested that photoisomerization
of DPAS derivatives was concentration dependent and isomerization was faster in dilute solution
as compared to concentrated solution. The electrochemical properties of these DPASs showed
reversible oxidation process with HOMO values close to work function of indium tin oxide used
as anode in OLED devices. These data implicated that DPASs may be used as hole transporting
materials in the fabrication of electroluminescent devices. These derivatives also possessed
positive solvatochromic properties and two photon absorption properties with good thermal
stability. On the basis of such interesting properties and detailed understanding of the
photoisomerization phenomenon, these compounds can be applied for material and biological
imaging applications.
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Experimental Section
General:
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1H and C NMR spectra were taken at 400 and 500 MHz. CDCl3 and DMSOꢀd6 was taken as
solvents. Chemical shifts are reported in parts per million shift (δꢀvalue) from Me4Si (δ 0 ppm
1
13
for H) or based on the middle peak of the solvent (CDCl3) (δ 77.00 ppm for C NMR) as an
internal standard. Signal pattern are indicated as s, singlet; d, doublet; t, triplet; m, multiplet.
Coupling constant (J) are given in hertz. Infrared (IR) spectra were recorded in KBr disc and
reported in wave number (cmꢀ1). ESIMS spectrometer was used for mass spectra analysis. High
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resolution mass spectra were taken with a QꢀTOF Analyzer. All the reactions were monitored by
TLC and visualization was done by UVꢀlight (254 nm). The purity analysis of the compound was
measured using a RPꢀHPLC and isolation of Zꢀisomer of 6a was done using semiꢀpreparative
HPLC with the following specification. Analytical HPLC: Shimadzu HPLC (System controller –
SCLꢀ10A VP), binary pump (model LCꢀ10AT VP), and detector (SPDꢀ10A VP). Semiꢀ
preparative HPLC: Shimadzu HPLC (System controller – CBMꢀ20A (UFLC)), binary pump
(model LCꢀ8A), and PDA detector (SPBꢀM10A VP). Data collection and analysis were
performed using Lab solution software. Single crystal Xꢀray data for compound 6a was collected
on the Xꢀray diffractometer (Supporting Information Figure S11 and Table S4). The
photoisomerization experiments were conducted under white light exposure at room temperature.
The “white light” refers to a continuum of wavelengths that is a broad, smooth distribution of
photons in the electromagnetic spectrum and is usually emitted by compact fluorescent lamps.
During experiments, we simply exposed samples to fluorescent tubes in our lab (commonly used
fluorescent tubes for lighting purpose, Crompton Energy Lux FTL, 28W, color temperature
6500K, 220V, light output 2760 Lumen).
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Isolation of Zꢀisomer (6a′) by HPLC technique
Pure Eꢀisomer (6a, purity 98.9%, 80mg) was dissolved in DCM to prepare a dilute solution of
conc. 10ꢀ4 M. It was exposed to white light for 48h. Then, DCM was evaporated and dried. Semiꢀ
preparative HPLC was done and the system was analyzed in isocratic mode with a mobile phase
1
consisting of acetonitrile–triple distilled water (90:10, v/v) at a flow rate of 0.25 mL min− and
2.5 mL minꢀ1 respectively. The resolution of both the isomers were achieved using a
Phenomenex C18 (5 ꢁm, 4.6 x 250 mm) column for analytical HPLC and YMCꢀPack ODSꢀA
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C18 (5 ꢁm, 250 x 20 mm) column for semiꢀpreparative HPLC. The pure Zꢀisomer (25mg) was
obtained in 99% purity and was characterized by 1H and 13C NMR spectroscopy.
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Measurement of Two photon absorption properties of DPASs 6aꢀd
The twoꢀphoton excited fluorescence (TPEF) spectra of chromophores 6aꢀd were recorded at
740 and 780 nm using mode locked MIRA 900 F (Ti: Saphire) oscillator pumped by Verdiꢀ5
with a pulse duration of 149 fs (integration time 1000 ms) at 76 MHz repetition rate. The TPEF
spectral data of 6aꢀd was measured in DMSO (c = 1 X 10ꢀ3 mol Lꢀ1) and the reference Rh6G was
prepared in methanol (c = 1 X 10ꢀ3 mol Lꢀ1). The TPEACS (Two photon excitation action cross
section (σ)) was determined by comparing their two photon excited fluorescence (TPEF)
intensity to that of Rh6G, according to the following equation:
= r X Fs X Φr X Cr X nr / Fr
σ σ
s
X
Φ
s X Cs X ns, where the subscripts ‘‘s’’ and ‘‘r’’ represent sample and reference (here, Rh6G in
methanol solution). F is the twoꢀphoton excited fluorescence integral intensity of the solution
emitted at the exciting wavelength. Φ, n and c are the fluorescence quantum yield, the refractive
index of the solvent, and the concentration of the solution, respectively. Values of σr are taken
from the literature.30
Synthesis of (E)ꢀ4ꢀ(4ꢀ(diphenylamino)phenyl)butꢀ3ꢀenꢀ2ꢀone (5). A mixture of 4ꢀ
(diphenylamino)benzaldehyde (273 mg, 1 mmol, 1 equiv.) in 10% aqueous NaOH with acetone
o
in excess was stirred at 25 C for 1 hr. The progress of the reaction was monitored by TLC and
on completion the reaction mixture was poured onto crushed ice with vigorous stirring and
finally neutralized with 10% HCl. The precipitate 250 mg (91%), obtained was filtered and used
for further reactions. Yellow solid, Rf = 0.31(nꢀhexane/ethyl acetate, 9:1, v/v), Mp: 105oꢀ107oC,
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MS (ESI) 314 [M + H+]. H NMR (400 MHz, CDCl3)
δ (ppm): 7.44 (d, J = 16.1 Hz, 1H), 7.38
5
6
(d, J = 8.6 Hz, 2H), 7.31ꢀ7.27 (m, 4H), 7.13ꢀ7.07 (m, 6H), 7.00 (d, J = 8.6 Hz, 2H), 6.58 (d, J =
7
8
9
13
16.1 Hz, 1H), 2.35 (s, 3H). C NMR (100 MHz, CDCl3)
δ (ppm): 198.3, 150.1, 147.8, 146.7,
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143.1, 129.5, 129.4, 129.1, 127.2, 125.4, 124.6, 124.1, 124.0, 122.6, 121.5, 27.3. HRMS (m/z):
calcd for C22H20NO [M+H]+ 314.1545, found 314.1543.
General procedure for the synthesis of 4aꢀd
Compounds 4aꢀd were synthesized following previously described synthetic procedures.19cꢀe
Synthesis of (E)ꢀ3ꢀ(4ꢀ(diphenylamino)styryl)ꢀ4'ꢀmethoxyꢀ5ꢀ(piperidinꢀ1ꢀyl)biphenylꢀ4ꢀ
carbonitrile (6a). A mixture of 2ꢀoxoꢀ6ꢀ(4′ꢀmethoxyphenyl)ꢀ4ꢀ(piperidinꢀ1ꢀyl)ꢀ2Hꢀpyranꢀ3ꢀ
carbonitrile 4a (620 mg, 2 mmol, 2 equiv.), compound 5 (313 mg, 1 mmol, 1 equiv.), and KOH
o
(112 mg, 2 mmol, 2 equiv.) in dry DMF (10 mL) was stirred at 25 C for 1 hr. The progress of
the reaction was monitored by TLC and on completion the reaction mixture was poured onto
crushed ice with vigorous stirring and finally neutralized with 10% HCl. The precipitate obtained
was filtered and purified on a neutral alumina flash column with 2% ethyl acetate in hexane as
the eluent to afford 286 mg (51%) as a greenish yellow shiny solid, Rf = 0.41 (nꢀhexane/ethyl
o
1
acetate, 8:2, v/v), Mp: 216ꢀ218 C, MS (ESI) 562 [M + H+], IR (KBr) ν/cm−1: 2215 (CN). H
NMR (500 MHz, DMSOꢀd6) (ppm): 7.78 (d, J = 8.7 Hz, 2H); 7.71(s, 1H), 7.58 (d, J = 16.1
δ
Hz, 1H), 7.55 (d, J = 8.5 Hz, 2H), 7.34 (t, J = 7.8 Hz, 4H), 7.25 (d, J = 16.1 Hz, 1H), 7.17(s, 1H),
7.11ꢀ7.06 (m, 8H), 6.99 (d, J = 8.5 Hz, 2H), 3.83 (s, 3H), 3.18 (t, J = 4.7 Hz, 4H), 1.74ꢀ1.70 (m,
4H), 1.60ꢀ1.56 (m, 2H). 13C NMR (100 MHz, CDCl3)
δ (ppm): 160.0, 158.3, 148.2, 147.3,
145.5, 142.8, 132.7, 132.5, 130.3, 129.3, 128.4, 128.0, 124.8, 123.3, 123.2, 123.0, 117.6, 116.1,
115.5, 114.3, 103.7, 55.4, 53.5, 26.1, 24.1. HRMS (m/z): calcd for C39H36N3O [M+H]+
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562.2858, found 562.2849. The Xꢀray analysis data of 6a is shown in the Supporting Information
5
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Table S4.
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Synthesis of (E)ꢀ3ꢀ(4ꢀ(diphenylamino)styryl)ꢀ5ꢀ(piperidinꢀ1ꢀyl)biphenylꢀ4ꢀcarbonitrile (6b):
A mixture of 2ꢀoxoꢀ6ꢀphenylꢀ4ꢀ(piperidinꢀ1ꢀyl)ꢀ2Hꢀpyranꢀ3ꢀcarbonitrile 4b (560 mg, 2 mmol, 2
equiv.), compound 5 (313 mg, 1 mmol, 1 equiv.), and KOH (112 mg, 2 mmol, 2 equiv.) in dry
DMF (10 mL) was stirred at 25 oC for 1 hr. The progress of the reaction was monitored by TLC
and on completion the reaction mixture was poured onto crushed ice with vigorous stirring and
finally neutralized with 10% HCl. The precipitate obtained was filtered and purified on a neutral
alumina flash column with 2% ethyl acetate in hexane as the eluent to afford 303 mg (57%) as a
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o
greenish yellow shiny solid; Rf = 0.54 (nꢀhexane/ethyl acetate, 8:2, v/v); Mp: 197ꢀ199 C; MS
1
(ESI+) 532 [M + H+]; IR (KBr) ν/cm−1: 2212 (CN). H NMR (400 MHz, CDCl3)
δ (ppm): 7.59ꢀ
7.62 (m, 2H), 7.49ꢀ 7.36 (m, 7H), 7.29ꢀ 7.20 (m, 6H), 7.13ꢀ 7.11 (m, 4H), 7.07ꢀ7.03 (m, 5H),
13
3.20 (t, J = 5.16 Hz, 4H), 1.84ꢀ1.79 (m, 4H), 1.65ꢀ1.59 (m, 2H). C NMR (100 MHz, CDCl3)
δ
(ppm): 158.2, 148.2, 147.3, 145.9, 142.8, 140.3, 132.6, 130.2, 129.3, 128.9, 128.3, 128.0,
127.2, 124.7, 123.3, 123.0, 122.9, 117.5, 116.5, 116.0, 104.2, 53.5, 26.1, 24.1. HRMS (m/z):
calcd for C38H34N3 [M+H]+ 532.2753, found 532.2738.
Synthesis
of
(E)ꢀ3ꢀ(4ꢀ(diphenylamino)styryl)ꢀ4'ꢀmethylꢀ5ꢀ(piperidinꢀ1ꢀyl)biphenylꢀ4ꢀ
carbonitrile (6c): A mixture of 2ꢀoxoꢀ4ꢀ(piperidinꢀ1ꢀyl)ꢀ6ꢀpꢀtolylꢀ2Hꢀpyranꢀ3ꢀcarbonitrile 4c
(598 mg, 2 mmol, 2 equiv.), compound 5 (313 mg, 1 mmol, 1 equiv.), and KOH (112 mg, 2
o
mmol, 2 equiv.) in dry DMF (10 mL) was stirred at 25 C for 1 hr. The progress of the reaction
was monitored by TLC and on completion the reaction mixture was poured onto crushed ice with
vigorous stirring and finally neutralized with 10% HCl. The precipitate obtained was filtered and
purified on a neutral alumina flash column with 2% ethyl acetate in hexane as the eluent to
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afford 273 mg (50%) as a greenish yellow solid; Rf = 0.57 (nꢀhexane/ethyl acetate, 8:2, v/v); Mp:
5
6
197ꢀ199 oC, MS (ESI) 546 [M + H+], IR (KBr) ν/cm−1: 2214 (CN). 1H NMR (400 MHz, CDCl3)
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δ
(ppm): 7.51ꢀ7.36 (m, 6H), 7.29ꢀ7.19 (m, 8H), 7.13ꢀ7.03 (m, 9H), 3.19 (t, J = 5.2 Hz, 4H), 2.41
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(s, 3H), 1.84ꢀ1.78 (m, 4H), 1.64ꢀ1.59 (m, 2H). C NMR (100 MHz, CDCl3)
δ (ppm): 158.2,
148.2, 147.3, 145.8, 142.8, 138.4, 137.4, 132.5, 130.2, 129.6, 129.3, 128.0, 127.1, 124.7, 123.3,
123.1, 122.9, 117.6, 116.3, 115.8, 103.9, 53.5, 26.1, 24.1, 21.1. HRMS (m/z): calcd for C39H36N3
[M+H]+ 546.2900, found 546.2900.
Synthesis
of
(E)ꢀ2ꢀ(4ꢀ(diphenylamino)styryl)ꢀ6ꢀ(piperidinꢀ1ꢀyl)ꢀ4ꢀ(thiophenꢀ2ꢀ
yl)benzonitrile (6d): A mixture of 2ꢀoxoꢀ4ꢀ(piperidinꢀ1ꢀyl)ꢀ6ꢀ(thiophenꢀ2ꢀyl)ꢀ2Hꢀpyranꢀ3ꢀ
carbonitrile 4d (586 mg, 2 mmol, 2 equiv.), compound 5 (313 mg, 1 mmol, 1 equiv.), and KOH
o
(112 mg, 2 mmol, 2 equiv.) in dry DMF (10 mL) was stirred at 25 C for 1 hr. The progress of
the reaction was monitored by TLC and on completion the reaction mixture was poured onto
crushed ice with vigorous stirring and finally neutralized with 10% HCl. The precipitate obtained
was filtered and purified on a neutral alumina flash column with 2% ethyl acetate in hexane as
the eluent to afford 258 mg (48%) as a yellow shiny solid, Rf = 0.58 (nꢀhexane/ethyl acetate, 9:1,
v/v), Mp: 197ꢀ199 C, MS (ESI+) 539 [M + H+], IR (KBr) ν/cm−1: 2215(CN). H NMR (400
o
1
MHz, CDCl3) (ppm): 7.51 (d, J = 1.2 Hz, 1H), 7.46ꢀ7.41 (m, 3H), 7.39ꢀ7.36 (m, 1H), 7.32ꢀ7.19
δ
(m, 7H), 7.14ꢀ7.11 (m, 5H), 7.07ꢀ7.04 (m, 5H), 3.19 (t, J = 5.2 Hz, 4H), 1.84ꢀ1.78 (m, 4H), 1.65ꢀ
13
1.60 (m, 2H). C NMR (100 MHz, CDCl3)
δ (ppm): 158.3, 148.3, 147.3, 143.2, 143.1, 138.7,
132.8, 130.1, 129.3, 128.2, 128.1, 126.4, 124.8, 123.3, 122.9, 122.8, 117.4, 115.1, 114.3, 104.0,
53.4, 26.1, 24.0. HRMS (m/z): calcd for C36H32N3S [M+H]+ 538.2317, found 538.2309.
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(Z)ꢀ3ꢀ(4ꢀ(Diphenylamino)styryl)ꢀ4'ꢀmethoxyꢀ5ꢀ(piperidinꢀ1ꢀyl)biphenylꢀ4ꢀcarbonitrile (6a′):
5
6
o
1
yellow solid, Mp: 87ꢀ89 C, H NMR (500 MHz, DMSOꢀd6)
δ (ppm): 7.47 (d, J = 8.7 Hz, 2H),
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8
9
7.25 (t, J = 7.8 Hz, 4H), 7.12ꢀ7.08 (m, 4H), 7.05ꢀ6.99 (m, 4H), 6.96 (d, J = 7.8 Hz, 4H), 6.84ꢀ
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6.80 (m, 3H), 6.66 (d, J = 12.0 Hz, 1H), 3.80 (s, 3H), 3.13 (t, J = 4.8 Hz, 4H), 1.70ꢀ1.66 (m, 4H),
13
1.58ꢀ1.53 (m, 2H). C NMR (100 MHz, DMSOꢀd6)
δ (ppm): 160.2, 158.1, 147.3, 147.3, 147.2,
144.6, 143.2, 133.8, 131.4, 130.5, 130.3, 129.9, 128.5, 126.0, 124.5, 123.8, 122.9, 120.2, 117.6,
115.5, 114.8, 103.4, 55.7, 53.1, 26.2, 24.0. HRMS (m/z): calcd for C39H36N3O [M+H]+
562.2858, found 562.2844.
ASSOCIATED CONTENT
Supporting Information
1
HPLC chromatograms, Timeꢀdependent H NMR of 6a in dark, solid emission spectra of 6aꢀd,
Two photon absorption spectra, Timeꢀdependent photophysical studies of Zꢀisomer 6a’ and Eꢀ
isomers of 6bꢀd, Solvatochromic data of 6aꢀd, Computational studies of 6aꢀd and Cartesian
coordinates, Xꢀray data of 6a and NMR spectra of 5, 6a, 6a′, 6bꢀd. This material is available free
of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*Eꢀmail: atul_goel@cdri.res.in
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGEMENTS
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8
AG thanks the Department of Atomic Energy (DAEꢀSRC) for Outstanding Investigator Award
(21/13/2015ꢀBRNS/35029). SM, PA, JS and RKG are thankful to UGC, DST and CSIR, New
Delhi, India for research fellowships. We thank Dr. Tejender S. Thakur for supervising the Xꢀray
Data collection and structure determination of the compound reported in this paper. We are
grateful to Dr. R. Ampapathi for his help in obtaining timeꢀdependent NMR spectra and SAIFꢀ
CDRI, Lucknow, India for providing other spectral and analytical data. CDRI communication
number is 9648.
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