Surfactant dependent pH controlled "off-on", "off-on-off" and "on-off" fluorescent switches exhibited by N-benzylidenenaphthalen-1-amine

Article abstract of DOI:10.1007/s10895-015-1633-y

N-benzylidenenaphthalen-1-amine (L) acts as pH dependent "off-on", "off-on-off" and "on-off" fluorescent switch in 1:1 (v/v) CH₃CN:H₂O depending on the presence of anionic sodiumdocdecyl sulphate (SDS), neutral triton X-100 (TX-100)

Full text of DOI:10.1007/s10895-015-1633-y

J Fluoresc (2015) 25:1431–1435
DOI 10.1007/s10895-015-1633-y
ORIGINAL ARTICLE
Surfactant Dependent pH Controlled Boff-on^,
Boff-on-off^ and Bon-off^ Fluorescent Switches Exhibited
by N-Benzylidenenaphthalen-1-Amine
Jutika Kumar1 & Manas Jyoti Sarma1 & Prodeep Phukan1 & Diganta Kumar Das1
Received: 26 May 2015 /Accepted: 26 July 2015 /Published online: 19 August 2015
#
Springer Science+Business Media New York 2015
Abstract N-benzylidenenaphthalen-1-amine (L) acts as pH
dependent Boff-on^, Boff-on-off^ and Bon-off^ fluorescent
switch in 1:1 (v/v) CH3CN:H2O depending on the presence
of anionic sodiumdocdecyl sulphate (SDS), neutral triton X-
100 (TX-100) and cationic cetyltrimethylammonium bromide
(CTAB) surfactants respectively. DFT calculation shows the
possibility of formation of L.H+ due to protonation at immine
N and L.OH− due to introduction of OH− group at immine C.
The relative stability of these two cationic and anionic species
depends on the charge environment provided by surfactants.
This influences the photoinduced electron transfer (PET) pro-
cesses involved and results in different switch behaviour.
A number of pH responsive fluorescent Bon-off^ or Boff-
on^ type switches have been reported. Anthracene as
fluorophore joined to aromatic iminodiacetate as receptor
through the spacer -CH2- acts as Bon-off^ fluorescent pH
switch [4]. Bis-azamacrocyclic anthracene having two 12-
membered cyclic tetraamine connected through a 9,10-
dimethylanthracene spacer shows fluorescent Bon^ switch be-
haviour at pH<7 and Boff^ behaviour at pH>10 [5].
Polyamines bearing anthracene and benzophenone moiety at
the two ends is another reported fluorescent Bon-off^ switch
with Bon^ window between pH 2.0 to pH 9.0 [6]. Anthracene
moiety attached to diethylenetriamine behaves as Boff-on^
switch for 416 nm emission and Bon-off^ switch for 510 nm
emission when excited with wavelength of 402 nm [7]. When
anthracene was replaced by pyrene it showed fluorescence
Bon-off^ and Boff-on^ at emission wavelengths 376 and
480 nm respectively on excitation at 316 nm [8].
.
.
Keywords Benzylidenenaphthalen-1-amine Fluorescence
.
.
.
Molecular switch Off-on-off Surfactant DFT
A three components systems comprising - Cu2+, a
tetraaza ligand and the fluorophore Coumarin reported as
Bon-off-on^ fluorescent switch [9]. A series of compounds
synthesised by linking methylnaphthalene fragments at
both ends of a variety of polyamine chains showed fluo-
rescence Boff-on-off^ behaviour for fluorescence emission
418 nm on excitation wavelength of 287 nm [10]. pH
dependent Fluorescent Boff-on-off^ switch is also reported
for supramolecular system based on benzocrown ether
[11]; natural products anabasin and coumarin [12]; dipod
1,2-bis(8-quinolinoxymethyl)benzene and tetrapod 1,2,4,5-
tetrakis(8-quinolinoxymethyl)benzene [13] etc. Most of the-
se switches involve complicated organic synthesis.
Introduction
Chemically driven molecular switches are the small tools
based on molecules or molecular systems which can transmit
events occurring in molecular level to the macroscopic world
[1]. Fluorescence molecular switches gained recent interest
due to very high sensitivity and relatively low costs [2]. H+
in the form of pH as the stimulus is especially convenient
because it causes mild perturbation, does not cause degrada-
tion of the system and its operation can be repeated at will [3].
* Diganta Kumar Das
Fluorescent switch of Boff-on-off^ type involving surfac-
tant sodium dodecyl sulfate (SDS) micelle, pyrene, poly(-
acrylic acid) (PAA) and Cu2+ reported [14]. The condensation
product of salicylaldehyde and 2,4-di-nitrophenylhydrazine
showed different fluorescent switching behaviour based on
digkdas@yahoo.com
1
Department of Chemistry, Gauhati University, Guwahati, Assam,
India 781 014

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J Fluoresc (2015) 25:1431–1435
2000
1600
1200
800
400
0
2
4
6
8
10
12
pH
Fig. 2 Changes in fluorescent intensity I of L at different pH in 1:1 v/v
CH3CN:H2O containing 3 % w/v SDS
Synthesis and Characterisation
of N-Benzylidenenaphthalen-1-Amine (L)
L (Scheme 1) was already synthesised and reported as fluo-
rescent sensor for Al3+ ion [17]. Briefly: 1-naphthylamine
(0.136 g, 1.0 mmol) was dissolved in dichloromethane
(20 mL). Benzaldehyde (0.106 g, 1.0 m mol) was added and
the reaction mixture was refluxed for 10 h. After completion
of reflux, the reaction mixture was diluted with dichlorometh-
ane and extracted successively with water and NaHCO3 solu-
tion (10 % aqueous). The organic extracts collected were dried
over anhydrous Na2SO4 and the solvent was evaporated to
obtain a liquid product of brown colour. Yield: 70 %.
ESI-MS: [M+H]+, m/z, 232.11 (C17H13N, 100 % abun-
dance); FT-IR: (KBr, cm−1): 1597.08 (νC=N), 3045 (νC−H),
1409 (νC=C); 1H NMR: (CDCl3, δ ppm, TMS): 8.57 (s, 1H);
8.38 (m 1H); 8.06 (m, 2H); 7.89 (m, 1H); 7.76 (d, 1H, J=8.4);
7.57 (m, 6H); 7.06 (m, 1H); 13C NMR: (CDCl3, δ ppm, TMS):
160.4, 149.2, 136.3, 133.8, 131.4, 128.9, 128.8, 128.5, 127.6,
126.3, 126.0, 123.9, 112.7.
Scheme 1 N-benzylidenenaphthalen-1-amine (L)
cationic, neutral or anionic surfactant micelles present in the
solution [15]. Charge nature of surfactant micelle was
employed to influence the keto-enol tautomerism of
salicylaldehyd-aniline condensation product and hence the
pH dependent fluorescent switch behaviour [16].
In this paper we report that pH dependent fluorescent
switch behaviour of N-benzylidenenaphthalen-1-amine is
Boff-on^, Boff-on-off^ and Bon-off^ type when anionic SDS,
neutral TX-100 or cation CTAB surfactant molecules are pres-
ent in the medium.
Experimental
DFT Optimisation Studies
All the chemicals were purchased from Loba, Chemie.
Fluorescence spectra were recorded in Hitachi 2500 spectro-
photometer at room temperature by using 1.0 cm quartz cu-
vette. pH values were measured using a Merck digital pH
metre. NMR spectra were recorded on a Bruker Ultra shield
300 MHz spectrophotometer. FT-IR spectra were recorded on
Perkin-Elmer spectrophotometer (RX1). High Resolution
Mass Spectra were recorded in Agilent spectrometer using
HPLC methanol as solvent.
DFT calculations were performed with the Gaussian 09 pack-
age [18]. The structure of L, L.H+ and L.OH− were optimised
using Becke’s three-parameter hybrid functional combined
with the Lee-Yang-Parr correlation functional (B3LYP) [19,
20] level of DFT with 6-311G++ besis set for all calculations.
2000
1600
1200
800
400
0
10000
8000
6000
4000
2000
0
2.5 3.5 4.5 5.5 6.5 7.5 8.5 9.5 10.5 11.5
pH
2
3
4
5
6
7
8
9
10 11 12
pH
Fig. 1 Changes in fluorescent intensity I of L at different pH in 1:1 v/v
Fig. 3 Changes in fluorescent intensity I of L at different pH in 1:1 v/v
CH3CN:H2O
CH3CN:H2O containing 3 % w/v CTAB

J Fluoresc (2015) 25:1431–1435
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2000
1600
1200
800
400
0
0
2
4
6
8
10
12
14
pH
HOMO of L
LUMO of L
Fig. 4 Changes in fluorescent intensity I of L at different pH in 1:1 v/v
CH3CN:H2O containing 3 % w/v TX-100
Fig. 5 DFT optimised structure of L and its HOMO and LUMO in
singlet
Preparation of Solutions
L was dissolved in 1:1 v/v CH3CN:H2O. Appropriate surfac-
tant was added so that the final surfactant concentration is 3 %
(w/v). The solutions were allowed to stand for 3 h before
fluorescence spectra were recorded.
pH 10.0 which started to decreased thereafter and became
fully Boff^ at pH 12.0. Hence L in 1:1 v/v CH3CN:H2O be-
haves fluorescence Boff-on^ switch in presence of anionic
surfactant SDS, Bon-off^ switch in presence of cationic sur-
factant CTAB and Boff-on-off^ when neutral surfactant TX-
100 is present.
Result and Discussions
L shows both cis and trans isomers out of which the trans
form is the stable one [21]. The pKa of free naphthylamine and
benzaldehyde are 3.92 and 14.9 respectively. In TX-100 when
pH was decreased from pH 7.0 the fluorescence intensity
started to decrease at pH 4.0 and becomes minimum at
pH 2.0. This is due to protonation of the imine N atom and
due to this protonation the electron density at the benzalde-
hyde ring decrease and secondary PET generated from naph-
thylamine ring to benzaldehyde ring as shown in Scheme 2.
Such type of PET processes as well reported [22]. On the other
hand when we increased pH from pH 7.0 the fluorescence
intensity started to decrease from pH 9.8 and attained mini-
mum at pH 12.0. The OH− attacks the immine C atom forming
anion with negative charge on the N atom. This negative
charge on N will be involved in PET process from it into the
naphthylamine ring as shown in Scheme 1. Hence fluo-
rescence intensity decreases at pH 9.8 and above. Thus
we obtained bell shaped fluorescence intensity versus
pH profile leading to fluorescent Boff-on-off^ switch behav-
iour in TX-100.
L in 1:1 v/v CH3CN:H2O showed fluorescence emission in
the range 370 to 570 nm with λmax at 430 nm when excited
with wavelength 310 nm [17]. Figure 1 shows the fluores-
cence intensity (I) of L as a function of pH in 1:1 (v/v)
CH3CN:H2O. A sharp increase in intensity was observed at
pH ca. 3.0. Another hump in fluorescence intensity was ob-
served at pH 5.6.
Figure 2 shows the fluorescence intensity of L as a function
of pH when the solution contain 3 % (w/v) SDS. The fluores-
cence intensity increased from pH 3.0 and attained maximum
at pH 6.0 resulting in fluorescent Boff-on^ switch. When the
solution contained 3 % (w/v) CTAB fluorescence was Bon^
till pH 9.0 (Fig. 3). Further increase in pH led to decrease in
fluorescence intensity attaining full Boff^ state at pH 11.0.
Presence of 3 % (w/v) TX-100 in 1:1 v/v CH3CN:H2O solu-
tion of L resulted fluorescence Boff-on-off^ switch behaviour
(Fig. 4). The fluorescence intensity was found to increase
from pH 2.0 till pH 4.0 and remained unchanged till
Scheme 2 PET process in L.H+
(at pH 2.0) and L.OH−
(at pH 12.0)

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J Fluoresc (2015) 25:1431–1435
stability of L.H+ and L.OH− in presence of surfactant of different
charges and (ii) Presence of secondary PET from naphthylene
ring to banzaldehyde ring in L.H+ and primary PET from neg-
ative charge of immine N to naphthylene ring in L.OH−.
Acknowledgments We thank DST, New Delhi and UGC, New Delhi
for financial assistance to the department through FIST and SAP respec-
tively. UGC is also thanked for journal excess through INFLIBNET. IIT-
Guwahati is thanked for HRM spectra.
L.H+
HOMO of L.H+
LUMO of L.H+
Fig. 6 DFT optimised structure of L.H+ and its HOMO and LUMO in
singlet
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In SDS medium the protonation of immine N takes place
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LUMO. DFT calculation also confirmed that OH− can get
associated with the immine C atom (Fig. 7).
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