Self-assembled nanorods of bay functionalized perylenediimide: Cu2+ based 'turn-on' response for INH, complementary NOR/OR and TRANSFER logic functions and fluorosolvatochromism

Article abstract of DOI:10.1039/c6tc00422a

We designed and synthesized the mono bay functionalized perylenediimide (PDI) derivative PDI-DMEA which undergoes self-assembly to form a nanorod morphology. The diameter of these nanorods is in the range of 150-300 nm and the length extended up to the μm range. These nanorods on interaction with Cu²⁺ ions transformed into spherical aggregates having a diameter of 350-600 nm in accordance with DLS studies. In solution, PDI-DMEA shows a blue shift of the internal charge transfer (ICT) absorption band from 605 to 570 nm along with a hyperchromic effect at 570 nm, in the presence of 0.7 equivalents of Cu²⁺ ions. The addition of 2 equivalents of Cu²⁺ ions results in complete disappearance of the ICT band at 570 nm and appearance of a normal absorption band of PDI at 515 nm (a visual colour change from colourless to yellow). On the other hand PDI-DMEA shows a strong fluorescence 'turn-on' response at 537 and 573 nm on addition of higher equivalents of Cu²⁺ ions. Thus, PDI-DMEA represents several examples of molecular switches such as 'OFF-ON-OFF' and 'ON-OFF-ON' at 570/515 nm (Cu²⁺, absorbance), and 'OFF-ON' at 570 nm (Fe²⁺, absorbance) and 537 nm (Cu²⁺, fluorescence) and thus offers the possibility for developing NOR, OR, INH and TRANSFER logic and complimentary logic circuits. Moreover, PDI-DMEA and the PDI-DMEA-Cu²⁺ complex show fluorosolvatochromism which helps to develop a colour array for differentiating organic solvents. The detailed mechanism of interaction of Cu²⁺ and Fe²⁺ with PDI-DMEA, confocal laser scanning microscopic studies, dynamic light scattering, ¹H NMR titration and reversibility are discussed.

Full text of DOI:10.1039/c6tc00422a

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DOI: 10.1039/C6TC00422A
Self-assembled Nano rods of Bay functionalized perylenediimide: Cu2+ based
‘turn on’ response for INH, complementary NOR/OR and TRANSFER logic
functions and fluorosolvatochromism
Prabhpreet Singha,*, Kapil Kumara, Gaurav Bhargavab and Subodh Kumara
aDepartment of Chemistry, UGC Centre for Advanced Studies, Guru Nanak Dev University, Amritsar 143 005, India.
e-mail: prabhpreet.chem@gndu.ac.in; Tel: +91-84271-01534
bDepartment of Applied Sciences, Punjab Technical University, Kapurthala-144601, Punjab, India.
Self-assembled nanorods of bay functionalized perylenediimide with ICT/PET features shows dis-
assembly of nanorods and inhibition of ICT/PET process with Cu2+ for developing INH, TRANSFER and
complimentary OR/NOR logic functions and fluorosolvatochromism for differentiating organic solvents.

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Self-assembled Nano rods of Bay functionalized perylenediimide:
Cu2+ based ‘turn on’ response for INH, complementary NOR/OR
and TRANSFER logic functions and fluorosolvatochromism
Prabhpreet Singha*, Kapil Kumara, Gaurav Bhargavab and Subodh Kumara
Received 00th January 20xx,
Accepted 00th January 20xx
We design and synthesized mono bay functionalized perylenediimide (PDI) derivative PDI-DMEA which undergoes self-
assembly to form Nano rods morphology. The diameter of these Nano rods are in the range of 150-300 nm and length
extended up to µm range. These Nano rods on interaction with Cu2+ ions transformed into spherical aggregates having
diameter of 350-600 nm in consonance with DLS studies. In solution, PDI-DMEA shows blue shift of the internal charge
transfer (ICT) absorption band from 605 to 570 nm along with hyperchromic effect at 570 nm, in the presence of 0.7 equiv.
of Cu2+ ions. The addition of 2 equiv. of Cu2+ ions result in complete disappearance of ICT band at 570 nm and appearance
of normal absorption band of PDI at 515 nm (visual colour change from colourless to yellow). On the other hand PDI-
DMEA shows strong fluorescence ‘turn-on’ response at 537 and 573 nm on addition of higher equiv. of Cu2+ ions. Thus,
DOI: 10.1039/x0xx00000x
www.rsc.org/
PDI-DMEA represent several examples of molecular switches such as ‘OFF-ON-OFF’ and ‘ON-OFF-ON’ at 570/515 nm (Cu2+
,
absorbance); ‘OFF-ON’ at 570 nm (Fe2+ absorbance) and 537 nm (Cu2+, fluorescence) and thus offers the possibility for
developing NOR, OR, INH and TRANSFER logic and complimentary logic circuits. Moreover, PDI-DMEA and PDI-DMEA-Cu2+
complex show fluorosolvatochromism which help to develop colour array for differentiating organic solvents. The detailed
mechanism of interaction of Cu2+ and Fe2+ with PDI-DMEA, confocal laser scanning microscopic studies, dynamic light
scattering, 1H NMR titration and reversibility are discussed.
of bay substituted PDIs are scarcely reported.16-17 It could be
attributed that due to presence of bulky substituents at bay
position of PDIs results in deviation of perylene backbone from
INTRODUCTION
Perylenediimides (PDIs) are well documented class of
intensively coloured organic dyes with interesting
photophysical properties.1 Moreover, their chemical and
thermal stabilities, high molar extinction coefficient and
fluorescence quantum yields, possibilities of functionalization
at imide and bay positions with various substituents has led to
planarity and so the examples of self-assembled PDIs are lesser
known.
From literature point of view, PDIs are considered as
promising materials for various optoelectronic devices,
however, the full exploitation of bay functionalized PDIs
scaffold for molecular recognition of metal ions and anion in
organic and aqueous medium has not been achieved. In this
regard, recently, we reported bay functionalized PDI as de-
wide spread interest of PDI in supramolecular chemistry2-8
,
dyes and pigment industries9 and optoelectronic based
devices.10-13 Due to presence of planar π-conjugated core, PDI
has high propensity for aggregation in aqueous medium via π-
π stacking, hydrogen bonding, electrostatic and solvation
interactions. In literature we have seen various reports where
self-assembled nanostructures can be generated after
modification of N-terminal position14-15, however self-assembly
-
aggregation based intracellular PDI probe for ClO4 ions.2
Moreover, we have also reported new class of PDI as an
integral part of ethylenediaminetetraacetic acid (EDTA) as PET
sensor for trivalent metal cation in water and DMSO3a and PDI
based dual chemosensor which allow ratiometric sensing of
Cu2+ ions and subsequently PDI-Cu2+ complex was utilized for
sensing of CN- ions.3b In the literature PDI functionalized with
dipyridine amine or picolinamide units at bay position were
utilized as chemosensor for detection of the Cu2+ ions.4 Zhu et
al utilized displacement approach using ensemble (perylene
bisimide- AuNPs) for detection of Cu2+ ions5a whereas Li et al,
reported PDI-dipeptide conjugate as chemosensor for Cu2+
ions.5b The PDI functionalized with dithioacetal6b, Salen7a and
oligo thymine sequence7b were utilized as chemosensor for
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Hg2+ ions. PDI derivatives functionalized with O-/N-propargyl radiation used was with a Ni filter and a data collection was
groups were utilized as chemosensor for Pd2+/Pd0 ions.8
carried out using flat holder in Bragg Brentano geometry.
Although several organic fluorophores have potential to Dynamic Light Scattering measurements were performed at
mimic molecular analogs of AND, OR, NOR, XOR, INHIBIT and 25.0 ± 0.1°C using a light-scattering apparatus (Zetasizer Nano
NAND logic gates along with half subtractor, half adder, full ZS Malvern Instrument Ltd., U.K.). The solutions were filtered
adder and full subtractor18-19
, however description of with a Millipore membrane filter (Acrodisc syringe filter, 0.2
perylenediimide based logic gate and molecular switches such μm Supor membrane) before measurements. The samples
as “ON-OFF-ON” and “OFF-ON-OFF” are rare in the literature.20 were thermally equilibrated for 10 min before each
Additionally, most of the reported Cu2+ chemosensors show measurement, and an average of 10 measurement runs were
quenching of the fluorescence emission,21-22 whereas considered to be data. The temperature was controlled to an
examples of fluorescence enhancement as a result of copper accuracy of ±0.1 °C using an inbuilt Peltier device. Data was
ion binding always favoured.23-24 In these and other context, analyzed using the standard algorithms. Confocal laser
designing of perylenediimide based chemosensor which scanning microscopy (CLSM) imaging was performed on NIKON
possess charge-transfer (CT) characteristics and applications in AIR machine using laser with excitation at 488 nm. Imaging
electronics, molecular recognition would be a case of interest.
was done with 60X objective lens with oil-emersion. The
In continuation of our interest in design and synthesis of absorption spectra were recorded on Shimadzu-2450
bay functionalized PDIs for generating self-assembled spectrophotometer from Shimadzu equipped with Peltier
nanostructure and their application in molecular recognition, system as temperature controller. Quartz cells of appropriate
here in we report self-assembled Nano rods morphology for length were used for sample measurement. The spectral
mono bay functionalized PDI derivative (PDI-DMEA). The Nano bandwidth and the scan rate were fixed at 2 nm and 140 nm
rods respond to Cu2+ ions to generate spherical structures. In min-1, respectively. The fluorescence titrations were
solution, PDI-DMEA on interaction with Cu2+ results in performed on Varian Carey Eclipse fluorescence
inhibition of ICT band at 605 nm with concomitant increase in spectrophotometer with excitation at 490 nm, unless
the absorption band at 515 nm (absorbance) and significant otherwise stated. Fourier transform infrared (FT-IR) spectra
fluorescence enhancement (‘turn-on’ behaviour). Moreover, were recorded on Perkin Elmer 92035. High resolution mass
the applications of PDI-DMEA and its copper complex for spectra were recorded on a BRUKER DALTONIK micrOTOF-Q11
developing “ON-OFF-ON”, “OFF-ON-OFF” and “OFF-ON” spectrometer. 1H NMR titration of PDI-DMEA against Cu(ClO4)2
molecular switches; INH, OR, NOR, TRANSFER logic gates, was performed in CDCl3 on Bruker 500 MHz spectrometer. All
Complementary logic circuits and fluorosolvatochromism absorption and fluorescence scans were saved as ACS II files
properties for differentiating various organic solvents would and further processed in Excel™ to produce all graphs shown.
be delineated.
The spectral data were analyzed through curve fitting
procedures by using non-linear regression analysis SPECFIT
3.0.36 to determine the stability constants and the distribution
3
of various species. PDI 125, 225 PDI 26 and PDI-NH226 have been
Experimental
synthesized by following the literature methods.
Materials and Methods
Chemicals and solvents were of reagent grade and used
without further purification unless otherwise stated. All
reactions were performed under N2 atmosphere. All solvents
used for photophysical studies were of HPLC grade.
Chromatographic purification was done with silica gel 60-120
mesh. TLC was performed on aluminium sheets coated with
silica gel 60 F254 (Merck, Darmstadt). NMR spectra were
Synthesis of PDI-DMEA
The mixture of PDI
1 (0.2 g, 0.280 mmol) and N,N’-
dimethylethylenediamine (7.42 ml, 6.71 mmol) were heated
under N2 atmosphere at 110oC for 4 hrs. After completion of
the reaction (tlc), the reaction mixture was extracted with
chloroform and organic layer was concentrated under reduced
pressure to obtain crude product which was further purified by
column chromatography (SiO2, chloroform/methanol) to
isolate pure PDI-DMEA, bluish-black colour solid, yield 140 mg
1
recorded on Bruker (operating at 500 and 400 for H; 125 and
100 MHz for 13C, respectively). The peak values were obtained
as ppm (δ), and referenced to the TMS in 1H NMR and
deutrated solvent in 13C NMR spectra. Abbreviations used for
splitting patterns are s = singlet, bs = broad singlet, t = triplet, q
= quartet, m = multiplet. SEM measurements were performed
on a ZEISS SUPRATM55 operating at an acceleration voltage of
10 KV with tungsten filament as electron source. Sample of
PDI-DMEA was dissolved in CH3CN and 6 µL aliquot was
deposited on glass surface using drop cast method. After
drying and gold plating of the sample, the glass surface was
imaged. Wide angle X-ray diffraction (WXRDs) were recorded
using a Rigaku diffractometer with CuKα (λ = 1.54 Å) emission
and spectra were recorded in the (2θ) range of 5-50o. The
1
(0.22 mmol, 78.2%); Rf = 0.5 (MeOH:CHCl3 2:8). H NMR (500
MHz, CDCl3, 25 °C): δ 8.88 (d, 1H, J = 8.0, perylene-ArH), 8.37-
8.63 (m, 5H, perylene-ArH), 8.16 (s, 1H, perylene-ArH), 7.08
(bs, 1H, NH), 5.00-5.10 (m, 2H, cyclohexyl), 3.50 (bq, J = 5.0,
2H, CH2), 2.78 (t, J = 5.5 Hz, 2H, CH2), 2.53-2.63 (m, 4H,
cyclohexyl), 2.33 (s, 6H, N-CH3), 1.91-1.93 (m, 4H, cyclohexyl),
1.73-1.79 (m, 4H, cyclohexyl), 1.27-152 (m, 8H, cyclohexyl),
ppm; 13C NMR (125 MHz, CDCl3, 25 °C): δ 25.6, 26.7, 29.3, 29.4,
41.6, 45.0, 53.0, 54.1, 57.2, 119.9, 121.0, 121.5, 121.6, 122.7,
123.1, 123.6, 124.0, 126.7, 127.8, 128.4, 130.0, 131.2, 131.7,
132.4, 135.4, 136.5, 140.0, 147.7, 164.2, 164.4 ppm; IR (ATR): ν
2 | J. Name., 2012, 00, 1-3
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3300, 2926, 2852, 2777, 1691, 1650, 1588, 1481, 1341, 1259, Dynamic light scattering (DLS) studies
1114, 748 cm-1. HR-MS: m/z found 663.4618; calcd. 663.2942
The self-assembly of PDI-DMEA in CH3CN was checked by
performing dynamic light scattering (DLS) experiments. The
stock solutions of PDI-DMEA (1x10-4 M, CH3CN) and Cu(ClO4)2
were filtered through 0.02 micron filter membrane to remove
any interfering impurities. The particle size analysis of the
solution of PDI-DMEA (10 µM) in CH3CN revealed the
formation of aggregates with hydrodynamic diameter in the
range of 150-300 nm (Fig. 1a). Further, we anticipated that due
to presence of DMEA unit, there is a possibility for interaction
of PDI-DMEA with Cu2+ ions. To verify this idea, we have also
performed DLS experiment of PDI-DMEA-Cu2+ (1:5) complex in
CH3CN solution. From particle size analysis we observed that
the size of the aggregates drastically increased with
hydrodynamic diameter in the range of 350-600 nm and thus
this preliminary data supports the complexation of PDI-DMEA
with Cu2+ ions (Fig. 1b).
for C40H40N4O4Na. UV/Vis (CH3CN): λmax = 515, 605 nm;
fluorescence: λmax = 537, 575 nm.
Results and discussion
Synthesis of PDI derivatives
The PDI-DMEA was prepared by reacting pure 1,7-Br-PDI (PDI
) with N,N-dimethylethylenediamine at 110oC for 8h (scheme
1
1). Unfortunately only mono-bay substituted derivative was
isolated due to reductive debromination of other bromine
under this reaction condition.4a The presence of NCH2 signals
at δ 2.08 and 3.52 ppm, signature signals for mono-bay
substituted PDIs and signals due to cyclohexyl groups confirm
the formation of PDI-DMEA (Fig. S1, ESI). The presence of
broad peak in wide angle X-ray diffraction (WXRD) shows
amorphous nature of PDI-DMEA (Fig. S2, ESI).
Self-assembly of PDI-DMEA
The self-assembly of PDI-DMEA and PDI-DMEA+Cu2+ (1:2)
complex has been also investigated using scanning electron
microscopic (SEM) technique. The SEM images of thin films on
TLC plate prepared by drop cast method from solution of PDI-
DMEA in CH3CN shows presence of 3D nano rods morphology
all over the surface. The diameter of these nano rods is in the
range of 200-350 nm in consonance with DLS experiment in
solution form (Fig. 2a-d), whereas the length of the nano rods
extended up to several µm. During scanning of the surface of
deposited sample we observed that nano rods are jumbled in
such a manner that superficially the nanostructure looks like
nest of ‘Baya weaver bird’ (Fig. 2a and Fig. S5-6).
In order to provide insight into the mechanism of PDI-
DMEA towards metal ions, control compound PDI-NH2 was
also synthesized. For this purpose, first nitration of PDI
been performed according to modification of the literature
procedure. Briefly, PDI was treated with ceric ammonium
nitrate (CAN) with immediate addition of H2SO4-HNO3 mixture
at room temperature to afford PDI in 10 min (according to
literature report at least 2h was required for nitration).26
Subsequently, PDI was reduced in the presence of Zn dust
2 has
2
3
3
and acetic acid mixture at room temperature to isolate PDI-
NH2. The presence of NH2 signal at δ 5.17 ppm, signature
signals for mono-bay substituted PDIs and signal due to
cyclohexyl groups confirm the formation of PDI-NH2 (Fig. S3-
S4, ESI).
The SEM Images of PDI-DMEA+Cu2+ (1:5) complex show
that addition of 2 equivalents of Cu2+ ions caused the complete
disintegration of nano rods into spherical aggregates.
Scheme 1 Synthesis of PDI-DMEA; reagents and conditions: (a) Br2, H2SO4, I2; (b) NMP,
Acetic acid, cyclohexylamine; (c) N,N’-dimethylethylenediamine, 110oC, 8h; (d)
Imidazole, cyclohexylamine, 140oC, 4h; (e) ceric ammonium nitrate, H2SO4-HNO3
mixture, RT, 10 min; (f) Zn dust, CH3COOH, THF, RT, 24h.
Fig. 1 (a) DLS analysis of PDI-DMEA (10 μM) and (b) DLS analysis of PDI-DMEA (10 μM)
on addition of Cu(ClO4)2 (50 µM) in CH3CN solution.
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Fig. 3 UV-Vis absorbance and fluorescence spectra of PDI-DMEA, PDI-NH2 and PDI 2
recorded in CH3CN; λex = 490 nm; slit width (Ex = 2 nm/2 nm, Em = 1 nm/1 nm).
with electron deficient-PDI core to make stronger push-pull
electronic system.
Based on these observations, we speculate that broad CT
band of PDI-DMEA at 605 nm might have contribution of two
transition, i:e, from HOMO of the –NH group (2o amine of
lateral DMEA unit –NHCH2CH2N(CH3)2 to the PDI core and from
HOMO of the –N(CH3)2 group (3o amine of lateral DMEA unit –
NHCH2CH2N(CH3)2 to the PDI core.
The emission spectrum of PDI-DMEA and PDI-NH2, show
emission intensity at 537 and 521 nm respectively, when
recorded in CH3CN solution (λex = 490 nm). On the other hand,
emission spectrum of PDI 2 shows high emission intensity at
537 and 572 nm in CH3CN solution (λex = 490 nm) (Fig. 3).
Fig. 2 Self-assembly of PDI-DMEA (10 µM) and morphological changes in PDI-DMEA
upon the addition of Cu(ClO4)2 in CH3CN solution; (a,c) normal and (b,d) magnified view
of Nano rods of PDI-DMEA extending into several µm as observed in the SEM; (e, f)
SEM images of PDI-DMEA on addition of Cu(ClO4)2 showing the disintegration of Nano
rods.
Spectroscopic response of PDI-DMEA
The average diameter of the spherical aggregates is in the
range of 450-600 nm (Fig. 2e-f), in consonance with DLS data.
We attribute that DMEA unit on PDI interact intermolecularly
with each other to form a rod-like self-assembly and probably,
on addition of Cu2+ ions, the DMEA unit of PDI-DMEA interacts
with Cu2+ ions and results in breaking of self-assembled nano
rods into spherical structures (Fig. 2f and Fig. S7 shows
presence of few nanorods which are in mid-way of
disintegration).
The absorption spectrum of PDI-DMEA (25 µM) in CH3CN on
addition of Cu(ClO4)2 (as source of Cu2+ ions, 250 µM) shows
complete disappearance of ICT band at 605 nm with
concomitant increase (10 times) in the intensity of the normal
absorption band at 515 nm (Fig. 4a) and consequently exhibit
naked eye colour change from colourless to yellow (Fig. 4c),
whereas addition of all other metal ions (as their perchlorate
salt) viz. Fe2+, Hg2+, Zn2+, Ni2+, Pb2+, Ba2+, Sr2+, Cs2+ and Co2+
(250 µM) to the solution of PDI-DMEA causes only the blue
shift of the ICT band from 605 nm to 570 nm as well as
insignificant naked eye colour change (Fig. 4a,c and Fig. S8).
The emission spectrum of PDI-DMEA (5 x 10-8 M) on
addition of Cu2+ ions resulted in enhancement of fluorescence
intensity at 537 nm and consequently weakly emissive solution
of PDI-DMEA (25 µM) becomes fluorescent bright yellow
colour solution when illuminated using 365 nm UV lamp (Fig.
4b,d). Although as expected enhancement in the fluorescence
intensity was observed in the emission spectrum of PDI-DMEA
on addition of all other metal ions (250 µM) however, visibly
(under 365 nm UV lamp) weak enhancement was seen except
Fe2+ where light greenish fluorescence was observed (Fig. 4d).
Photophysical properties of PDI-DMEA
The unsubstituted PDI
2 shows well defined vibronic bands
with λmax at 525 nm due to S0-S1 electronic transition3b
whereas PDI-NH2 shows broad absorption band with λmax at
560 nm due to ICT transition from HOMO of the NH2 to the
LUMO of the PDI core. On the other hand PDI-DMEA shows
absorption bands that covers almost whole visible range (Fig.
3, Inset). The characteristic absorption bands present in the
visible region with λmax at 478 and 515 nm are due to perylene
dimide moiety, whereas absorption band extended to red
region with λmax 605 nm is due to ICT transition from HOMO of
the N,N’-dimethylethylenediamine (DMEA) to the LUMO of the
PDI core indicating interaction of electron rich DMEA moiety
4 | J. Name., 2012, 00, 1-3
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Fig. 5 UV-Vis spectra of PDI-DMEA (25 μM) recorded in CH3CN on addition of the Cu2+
ions; Inset: plot of absorbance vs conc. of Cu2+ ions at 515 and 605 nm; magnified view
of absorption spectra showing blue shift from 605 to 570 nm on addition of 0.7
equivalent of Cu2+ ions.
Fig. 4 (a) UV-Vis spectra of PDI-DMEA (25 μM) on addition of metal ions (250 μM); (b)
Fluorescence spectra of PDI-DMEA (5x10-8 M) on addition of metal ions (5x10-7 M); (c)
Photographs of naked eye colour change of PDI-DMEA (25 μM) and (b) fluorescence
images of PDI-DMEA (25 μM) after addition of various metal ions (250 μM) under
illumination at 365 nm UV lamp. All spectra were recorded in CH3CN. λex = 490 nm; slit
width (Ex/Em = 10/10 nm).
UV-Vis and Fluorescence spectroscopy
In the UV-Vis titration of PDI-DMEA (25 µM, CH3CN) with Cu2+
ions, we observed immediate blue shift of the CT band from
605 to 570 nm and then intensity of CT band gradually
increased to its limiting value up to 0.7 equivalent (17.5 µM) of
Cu2+ ions. On further addition of Cu2+ ions CT band at 570 nm
slowly decreases and finally it vanish at 2 equivalent (50 µM)
of Cu2+ ions and then it achieves the plateau. On the other
hand, normal absorbance band at 515 nm continuously
increase predominantly to its limiting value up to 2.0
equivalent (50 µM) of Cu2+ ions and then it achieves the
plateau (Fig. 5, Inset). The isosbestic point appeared at 533
nm. The spectral fitting of the titration data by using non-
linear regression analysis programme (SPECTFIT-32) shows log
βPDI(Cu2+) = 8.35 ±0.2 (PDI:Cu2+; 2:1).
Fig. 6 Fluorescence spectra of PDI-DMEA (5x10-8 M) recorded in CH3CN on addition of
the Cu2+ ions; Inset: plot of fluorescence intensity (FI) vs conc. of Cu2+ ions at 537 nm;
λex = 490 nm; slit width (Ex/Em = 10/10 nm).
2.6x10-7 M – 9.5x10-6 M concentration of Cu2+ ions (Fig. 6,
Inset). The detection limit is found to be 20 nM which is
sufficiently low to detect the nano-mloar concentration of
Cu2+. The association constant (K) of the PDI-DMEA with Cu2+
determined by Benesi-Hildebrand plot is found to be 2.1×105
M−1. The Job’s plot27b shows a maxima at the mole fraction of
0.4 with an unsymmetrical peak shape which suggests the
formation of 2:1 PDI-DMEA-Cu2+ complex along with other
complexes (Fig. S9, ESI).
The fluorescent titration of PDI-DMEA (5x10-8 M) shows
that on addition of Cu2+ ions, the emission bands at 537 and
573 nm slowly increase with increase in the concentration of
Cu2+ ions and then achieve the plateau at 200 equivalent (10
µM) of Cu2+ ions. This constitutes a ‘turn-on’ sensor for
detection of Cu2+ ions with nearly 310% increase in the
emission intensity (Fig. 6). These results could be attributed to
inhibition of electron transfer (PET) from DMEA to PDI due to
Cu2+ complexation. The fluorescence quantum yield27a
increases from 0.035 (for PDI-DMEA alone) to 0.29 (PDI-
DMEA-Cu2+ complex). The spectral fitting of these data shows
Mechanism of interaction of PDI-DMEA toward Cu2+ and other
metal ions
the formation of 1:1 and 2:1
complexes with stepwise stability constant of log βPDI(Cu2+)
(
PDI:Cu2+) stoichiometric
After close inspection of structure of PDI-DMEA, we could
argue that in our PDI-DMEA, the 3o nitrogen atom at lateral
appendage is more basic in comparison to 2o nitrogen atom
which is directly attached with electron deficient PDI core. So,
the probability of 3o nitrogen to undergo complexation or
=
5.81 ± 0.11 (PDI:Cu2+, 1:1) and log βPDI(Cu2+) = 13.6 ± 0.18
(L:Cu2+, 2:1). The fluorescence intensity plot demonstrates
two steps of linear change between 1x10-8 M - 2.5x10-7 M and
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protonation is much higher in comparison to 2o nitrogen and addition of all other metal ions, however observance of blue
accordingly we proposed binding mode II and III as more shift in ICT band from 605 to 570 nm on addition of >125 µM
probable form of complexes for interaction of Cu2+ with PDI- (in comparison to 17 µM of Cu2+) and ‘turn-on’ emission profile
DMEA in the UV-Vis titration (Fig. 7).
with 0.5 µM to 1 µM (in comparison to 10 µM of Cu2+ ions)
Initially in the binding mode II, probably only 3o nitrogen concentrations of other metal ions prompted us to investigate
atoms interact with Cu2+, while 2o nitrogen atoms are still the PDI-DMEA response for other metal ions and perchloric
capable of undergoing ICT process and as a consequence PDI- acid. On gradual titration of PDI-DMEA (25 µM, CH3CN) with
DMEA show blue shift from 605 to 570 nm. This assumption Fe2+, Hg2+ (taken as representative metal ions) and perchloric
was further verified by looking at the UV-Vis spectrum of PDI- acid we observed that CT band undergoes immediate blue
NH2 which shows absorption maxima at 560 nm (Fig. 3). So, as shift from 605 to 570 nm and then it achieves the plateau.
the contribution of ICT from 3o nitrogen to PDI core decreases, Interestingly, the weak absorbance of normal band of PDI-
the ICT from 2o nitrogen to PDI core increases and thus DMEA at 515 nm was not affected to greater extent. Similarly,
justified concomitant increase in the absorbance of CT band at the fluorescence titration of PDI-DMEA (5x10-8 M) with these
570 nm. As soon as the concentration of Cu2+ ions reaches >0.7 metal ions and perchloric acid caused increase in the emission
equivalent, the binding mode III becomes more preferred one intensity (Fig. S12-15).
which involves participation of both nitrogen atoms and
Surprisingly, when optical titrations of PDI-NH2 were
accounts for drastic decrease of ICT band at 570 nm and performed with these metal ions (Fe2+, Hg2+) and perchloric
appearance of normal absorption band of PDI core at 515 nm. acid, we observed insignificant changes in the absorbance and
This complexation of PDI-DMEA with Cu2+ was further emission spectrum of PDI-NH2 (Fig. S16-17). Based on these
confirmed by UV-Vis titration of PDI-NH2 (taken as control results, we propose that most of these metal ions under
compound) with Cu2+ ions. We observed that when solution of studies interact with more basic 3o amine via complexation
Cu2+ ions was added into the solution of PDI-NH2 (25 µM, (due to the high lewis acidity of some of the transition metals)
CH3CN), the ICT absorption band at 560 nm continuously or due to protonation to inhibit the ICT or PET from 3o amine
decreased up to 3 equiv. (75 µM) concentration of Cu2+ ions to PDI core and consequently blue shift from 605 to 570 nm
along with slight increase in the absorption intensity of normal was observed whereas, due to least basicity of –NH group of
band at 511 nm (Fig. S10, ESI).
PDI-DMEA (in line with PDI-NH2 derivatives), it shows least
Although fluorescence experiments were performed at interaction via protonation. Moreover, based on hard soft acid
much lower concentration in comparison to absorption base (HSAB) principle, Fe2+, Hg2+ etc. are incompetent to bind
experiments, however initially we propose binding mode III as the soft ligating sites of DMEA as effectively as Cu2+ can do. So,
the probable mode for complexation where both nitrogen the inhibition of ICT band and fluorescence enhancement on
atoms of two PDIs undergo interaction with Cu2+ and results in addition of Cu2+ ions is due to the true complexation of PDI-
sharp increase in fluorescence intensity (FI) which corresponds DMEA with Cu2+ ions rather than protonation.
to 22% of the total increase in the FI (first step of linear change
Molecular switches and Logic functions
highlighted with circle in Fig. 6). Later on the contribution of
binding mode
I simultaneously increase and fluorescence As described above, the charge transfer (CT) band of PDI-
enhancement effectively slows down as the conc. of Cu2+ DMEA undergoes distinctive changes on gradual addition of
increases from 2.5x10-7 to 9.5x10-6 M which accounts for 73% Cu2+ ions in the absorption spectrum (Fig. 5) and thus PDI-
of the total increase in FI (second step of linear change) (Fig. DMEA could represent rare example of “OFF-ON-OFF”
6). This fact was further supported by looking at fluorescence molecular switch at 570 nm, if the initial absorbance value at
titration of PDI-NH2 with Cu2+, where the emission band at 521 570 nm considered as thresh hold value for this particular
nm progressively increased showing 62% fluorescence case. Initially the absorbance at 570 nm for PDI-DMEA is in the
enhancement up to addition of 10 equivalents of Cu2+ ions (Fig. low state (‘OFF’) when [Cu2+] = 0 µM. The absorbance at 570
S11, ESI).
Although we have seen insignificant naked eye colour indicating high state (‘ON’) and again the absorbance at 570
nm rapidly increased with the addition of [Cu2+] = 17.5 µM,
change (absorbance) and ‘turn-on’ (emission) response on
nm rapidly decreased with [Cu2+] = 50 µM (representing low
state, ‘OFF’). Interestingly, we can visualize ‘ON-OFF-ON’
molecular switch, if the absorbance were monitored at 570
and 515 nm. For the working of this ‘ON-OFF-ON’ molecular
switch, we shall considered absorbance at 570 nm as high
(‘ON’ state) at [Cu2+] = 0 µM and then after addition of [Cu2+] =
50 µM, although the absorbance at 570 nm decreases (‘OFF’
state), however the absorbance at 515 nm concomitantly
increased (‘ON’) state. Alternatively, ‘ON-OFF-ON’ molecular
switch can be imagined as combination of ‘ON-OFF’ switch at
570 nm and ‘OFF-ON’ switch at 515 nm and thus PDI-DMEA
offer dual signalling windows for Cu2+ ions.28 Conversely, PDI-
DMEA behave only as ‘OFF-ON’ molecular switch at 570 nm on
Fig. 7 The probable binding modes of PDI-DMEA with Cu2+ as observed in the UV-visible
and fluorescence titrations.
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addition of [Fe2+] = 175 µM in the absorbance mode and thus The absorption band at 610 nm shift to 515 nm in the presence
provide an advantage to distinguish between different of the Cu2+ (50 µM) alone or in the presence of Cu2+ and Fe2+
transition metals.
(50 µM each), whereas the absorption band at 610 nm shift to
Besides using absorption channel as output, there is a 570 nm in the presence of the Fe2+ (50 µM) and thus all these
possibility to check functioning of molecular switch using outputs could be labelled as an ‘OFF’ state at 610 nm. We also
emission channel as output. As we also observed ‘turn-on’ observed that solution of PDI-DMEA exhibit new ICT band at
response in the emission spectrum on addition of either Cu2+, 570 nm on addition of Fe2+ ions (50 µM). However the
and Fe2+ ions, so PDI-DMEA represent ‘OFF-ON’ switch when presence of Cu2+ ions (50 µM) to the solution of PDI-
fluorescence was monitored at 537 nm. As molecular switches DMEA+Fe2+ or PDI-DMEA alone shows an output signal 515
and logic gates obey close analogy due to similar intrinsic nm and thus inhibits the outputs signal caused due to Fe2+ ions
protocols for conversion of input stimulation into output in the absorbance mode. So an INHIBIT function (AND gate
signals, so these molecular switches discussed above could be when one input is reversed) at 570 nm (absorbance) was
transformed into following logic gates.
observed (Fig. 8). According to the truth table given in figure 8,
We visualized that PDI-DMEA behave as NOT gate at 605 the absorbance at 515 nm corresponds to ‘TRANSFER’ logic
nm (absorbance) and becomes Cu2+ driven YES logic at 515 nm because the output is low when either both inputs are low or
(absorbance). The emission spectrum of PDI-DMEA exhibit 3.5 one input is low and output is high when either both inputs are
and 2.3 times increase in the intensity of emission band at 537 high or one input is high. In our case, the absence of Cu2+ and
nm on addition of 10 µM concentrations of Cu2+ or 0.5 µM Fe2+ inputs (entry 1, Fig 8a) or presence of Fe2+ (entry 3), the
concentrations of Fe2+ ions, or even when both Cu2+ and Fe2+ absorbance at 515 nm was low, whereas in the presence of
were added, we also observed 3.5 time increase in the Cu2+ and Fe2+ (entry 4) or presence of Cu2+ (entry 2), the
emission intensity of PDI-DMEA at 537 nm and thus these absorbance at 605 nm vanish and high absorbance value was
inputs can be used for developing two inputs based OR logic observed at 515 nm.
gate at 537 nm (Fig 8). Moreover, the molecular computation
By operating the OR and the NOR logic functions at 537 nm
in present scenario rely on multiple inputs-one output (Fluorescence mode) and 610 nm (absorbance mode) in
combination, so we can extend the present two inputs based parallel, a sequential logic circuit such as complementary
OR gate into four inputs based OR gate using Cu2+, Hg2+, Fe2+ output logic circuit can be fabricated. The complementary logic
ions and perchloric acid as four inputs and measuring the circuit is based on the phenomena of two parallel working
output at 537 nm.
logic gates that have output exactly opposite to each other as
The other demonstration of molecular information was a response to the two working inputs. Such combinations that
based on a photonic NOR logic function. The absorbance at have been used in electronic devices are the combinations of
610 nm could be at a high value only when inputs of Fe2+, Cu2+ IMPLICATION/INHIBT, YES/NOT, OR/NOR and AND/NAND
or both are absent (Fig 8).
gates.29,30 This sequential logic circuit has added advantages of
less power requirement, higher working speed, a lower
physical space requirement for fabrication and negligible time
lag between two outputs with respect to each other as two
optical outputs can be recorded at the same time.
Fluorosolvatochromism for differentiating organic solvents
We have also explored ICT and PET properties of PDI-DMEA
and their modulation in the presence of Cu2+ ions (PDI-DMEA
+
Cu2+ complex) to develop colour array to differentiate organic
solvents based on its response with organic solvents in terms
of physical or chemical interactions.31 A pattern of colour
changes were generated through comparison of colour as
observed in different flask in paint software (Fig. 9).
PDI-DMEA exhibit naked eye bluish and brownish colour in
toluene and DCM respectively; brick red in CHCl3; greenish
colour in DMSO and PDI-DMEA is colourless in CH3CN solution.
However, PDI-DMEA+Cu2+ complex shows brown and yellow
colour in THF and CH3CN solvents respectively; orange colour
in DCM and light brown colour in acetone.
Similarly, PDI-DMEA shows fluorescent pink and blue
colour in CHCl3 and ethanol respectively; light green in DMSO
when visualized under 365 nm UV lamp, whereas PDI-
DMEA+Cu2+ complex shows fluorescent bright yellow and dark
Fig. 8 (a) Truth table for various logic functions (NOR, INH, OR) fabricated for PDI-
DMEA using Cu2+ and Fe2+ ions as inputs and absorption/emission as outputs at
different wavelengths; (b) Pictorial representation of combinatorial circuit from NOR, green colour when visualized under 365 nm UV lamp (Fig. S18,
OR and INH logic gate using absorption/emission as output mode.
ESI).
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1H NMR titration of PDI-DMEA with Cu2+
The complexation behaviour of PDI-DMEA with Cu2+ was also
proved through 1H NMR titration experiments. Due to less
solubility of PDI-DMEA in CD3CN or DMSO-d6 at such high
concentration (5 mM), we prefer CDCl3 solvent for 1H NMR
titration (Fig. S19). Upon addition of Cu2+, the proton signals of
the perylene aromatic core shows ~0.02-0.03 ppm downfield
shift along with signal broadness. Moreover, we also observed
~0.015-0.02 ppm downfield shift of the DMEA protons (-CH2
and -NH). These results could be attributed to complexation of
PDI-DMEA with Cu2+ ions.
Reversibility and dual sensing
To check the reversibility of the Cu2+ recognition process, the
EDTA solution was added to the solution of PDI-DMEA-Cu2+
complex and optical measurements were carried out. We
observed that the fluorescence intensity of PDI-DMEA-Cu2+
complex decreases slowly however it has not reached to its
initial value. Similarly, addition of EDTA to the solution of PDI-
DMEA-Cu2+ (1:2) complex only marginally regenerate the ICT
band at 605 nm, however we observed drastic decrease in the
normal absorption band at 515 nm. We also observed colour
change from yellow to very light-yellow (visible) and bright
fluorescent parrot colour to weekly emissive sea green colour
(under 365 nm UV lamp) on addition of EDTA (Fig. S20-22, ESI).
We also exploited PDI-DMEA-Cu2+ complex for sensing of CN-
ions via displacement assay, however we observed similar
response as observed with EDTA (only 50% recovery of
fluorescence at 537 nm and absorption band at 515 nm
without affecting the ICT band) (Fig. S20,22 ESI). Therefore, the
aggregates of PDI-DMEA forms very strong complex with Cu2+,
which finds difficulty in decomplexation on addition of EDTA or
CN- ions.
Fig. 9 Patterns of (A) PDI-DMEA (25 µM) and (B) PDI-DMEA-Cu2+ (1:10) complex
obtained in response to various organic solvents for application to develop
colourimetric (upper) and fluorometric (lower) colour array. For visualization colour
range as observed in different flask were transformed to different colour range in paint
software.
Confocal Laser Scanning Microscopic Studies
Thin films of PDI-DMEA (1x10-5 M) were prepared by drop cast
method and analyzed by using confocal laser scanning
microscope (CLSM). Thin films of PDI-DMEA do not show any
fluorescence in CLSM which could be attributed due to
aggregation induced quenching via π-π stacking of PDI
chromophore in solid state.1 However, when 20 µL solution of
PDI-DMEA (1x10-5 M) in CH3CN was placed on glass slide and
imaged under CLSM, we observed weak emission in solution
phase (Fig. 10, a-b) which changes to bright green fluorescence
on addition of Cu2+ ions (5 equiv.) which truly support
complexation of PDI-DMEA with Cu2+ (Fig. 10, c-d).
Effect of coexisting ions
To evaluate the selectivity and sensitivity of PDI-DMEA
towards Cu2+, we also carried out several competition
experiments. We observed that the optical changes which
were observed in PDI-DMEA in the presence of Cu2+ (inhibition
of ICT band and fluorescent enhancement) were retained even
in the presence of other metal ions (selectivity). On the other
hand, no change in optical properties of PDI-DMEA were
observed in the presence of other competitive metal ions,
however addition of Cu2+ in the same solution immediately
cause the disappearance of CT band and fluorescence
enhancement (sensitivity) (Fig. S23, ESI).
Conclusions
In conclusion, we report Nano rods self-assembly of bay
functionalized perylenediimide derivative (PDI-DMEA) and its
response towards Cu2+ in solid and solution form. On
interaction with Cu2+, the Nano rods disintegrate to form
spherical nanostructures whereas in solution the Cu2+
complexation results in restriction of ICT absorption band at
605 nm (absorbance) and significant enhancement of the
Fig. 10 Images from confocal laser scanning microscopy (CLSM) of PDI-DMEA (10 µM);
(a-b) non-fluorescent nature of PDI-DMEA (c-d) addition of 5 equivalents of copper
perchlorate to the solution of PDI-DMEA results in ‘switching on’ of fluorescence.
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emission intensity at 537 nm (fluorescence and CLSM studies). 14. a) L. Zhang, Y. K. Che, J. S. Moore, Acc. Chem. Res., 2008, 41
The multiple outputs obtained using Cu2+ and Fe2+ as inputs
were elaborated for developing various logic functions and
,
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phenomena through modulation of its ICT/PET properties in
the absence and presence of Cu2+ to develop the colour array
as observed under sunlight or after 365 nm UV illumination for
differentiating the organic solvents.
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Acknowledgements
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This work was supported by the DST-SERB fast track (SB/FT/CS-
080/2012) and CSIR (02/0084/12/EMR-II) grants. We thank
DST, New Delhi for FIST programme and UGC, New Delhi, for
CAS-II and UPE programme. KK is thankful to the CSIR and DST
for funding the fellowship.
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