An acridinium-based sensor as a fluorescent photoinduced electron transfer probe for proton detection modulated by anionic micelles

Article abstract of DOI:10.1039/c4ob00559g

A water-soluble fluorescent pH sensor of 9-amino-10-methylacridinium chromophore with the 2-(diethylamine)ethyl chain as a receptor shows an off-on response going from basic to acidic solution. Photoinduced electron transfer has been directly demonstrated to be the quenching mechanism by the observation of the long-lived acridinyl radical. The interaction of the protonated sensor with anionic micelles causes a significant increase in the detection sensitivity of pH. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4ob00559g

Organic &
Biomolecular Chemistry
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PAPER
An acridinium-based sensor as a fluorescent
photoinduced electron transfer probe for proton
detection modulated by anionic micelles†
Cite this: Org. Biomol. Chem., 2014,
2, 6677
1
Stefano Basili, Tiziana Del Giacco,* Fausto Elisei and Raimondo Germani
A water-soluble fluorescent pH sensor of 9-amino-10-methylacridinium chromophore with the
2-(diethylamine)ethyl chain as a receptor shows an “off–on” response going from basic to acidic solution.
Photoinduced electron transfer has been directly demonstrated to be the quenching mechanism by the
observation of the long-lived acridinyl radical. The interaction of the protonated sensor with anionic
micelles causes a significant increase in the detection sensitivity of pH.
Received 13th March 2014,
Accepted 30th June 2014
DOI: 10.1039/c4ob00559g
www.rsc.org/obc
promotes the complexation ability with cations, and the fluore-
scence enhancement is favored by the less polar region.
Introduction
5
The design of fluorescent chemosensors is of great interest due
to their wide application in analytical chemistry, cell biology, fluorophores of ionic nature reported in the literature, we
biomedicine and environment survey. Various chemical and have extended our investigation by using 9-amino-10-methyl-
biochemical analytes can be detected by these devices with acridinium ion (Acr ) as a fluorescent chromophore for the
high sensitivity and low detection limits. The most extensively construction of a pH sensor. The Acr -unit confers to all the
Given the limited number of investigations of sensors with
6
+
+
used mode of fluorescence modulation in the recognition derivatives synthesized so far water solubility, contrary to
process of chemosensors is the photoinduced electron transfer many pH-indicators with uncharged fluorophore that generally
1
+
(
PET). Such controllable “on/off” characteristics of fluore- show a poor water solubility. Moreover, the Acr -unit shows
scence emission by PET are of immediate relevance for the particular advantages to be used in sensory systems, such as a
7
design of not only molecular sensors, but also molecular high fluorescence quantum yield (Φ
F
= 0.96), which guaran-
digital systems, which are promising candidates for the tees a strong emission signal and a long fluorescence lifetime
7
achievement of innovative materials for information techno- (τ
F
= 18.2 ns), which allows the detection of relatively slow
logy. So far, an extensive number of fluorescence-based pH quenching processes. further advantage is the high
sensors have been reported. Most of them are “fluorophore- reduction potential of the singlet excited state (E*(S )
2
A
3
=
red
1
+
8
H amine acceptor” assemblies, separated by a spacer or not.
2.43 V), which makes the PET process, and therefore the
Recent chemosensor research is aiming at tuning the fluorescence quenching, thermodynamically and kinetically
sensor ability by a simple way rather than by synthesizing new more favorable. The attractive results presented in this paper
water soluble sensor molecules. In this context, studies on the suggest that the pH sensor 9-(2-(diethylamine)ethylamino)-10-
+
−
effect of micellar aggregates in improving the sensing ability of methylacridinium chloride (Acr -A Cl ) operates in an “off–on”
4
fluorescent sensors are of growing interest. In particular, it fashion, whereas in the “off” state (amine receptor free from
+
has been reported that neutral sensors for both protons and H ) a PET mechanism has been unequivocally demonstrated.
cations increase considerably their efficiency up to the critical The further increase of fluorescence quantum yield in the “on”
+
micellar concentration (c.m.c.) because the micelles provide a state (amine receptor linked to H ) observed in the presence
hydrophobic local environment to solubilize the sensor that of anionic surfactants has evidenced a newsworthy role of
micelles.
Dipartimento di Chimica, Biologia e Biotecnologie and Centro di Eccellenza
Materiali Innovativi Nanostrutturati (CEMIN), Università di Perugia, Via Elce di
sotto 8, 06123 Perugia, Italy. E-mail: tiziana.delgiacco@unipg.it;
Results and discussion
Fax: +39 0755855560; Tel: +39 0755855539
1
13
†
Electronic supplementary information (ESI) available: H and C NMR spectra
+
−
+
−
Acr -A Cl was synthesized in good yield from reaction of N,N-
diethylethane)-1,2 diamine and 9-chloro-10-methylacridinium
of compounds, HPLC-HRMS analysis of Acr -A Cl , absorption and emission
spectra, relative fluorescence emission as a function of pH plots, laser flash
photolysis experiments. See DOI: 10.1039/c4ob00559g
(
chloride salts in turn prepared from the commercially avail-
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Scheme 2 Chemical structures at various pH values of the sensor.
+
−
Scheme 1 Synthesis of Acr -A Cl .
+
−5
Fig. 2 Fluorescence emission spectra of Acr -A (1.5 × 10 M) in water
recorded at different pH values, λexc = 445 nm. Inset: intensity recorded
at 490 nm as a function of pH; the full line is the fitting curve obtained
using eqn (1).
+
−5
Fig. 1 Absorption spectra of Acr -A (7.5 × 10 M) recorded in water at
different pH values. Inset: plot of absorbance at 385 and 445 nm vs. pH.
able 10-methyl-9(10H)-acridone and SOCl (Scheme 1). The
2
+
absorption spectra of Acr -A are essentially unchanged at
different pH (2–8) values in water as shown in Fig. 1. Thus,
+
+
+
protonated (Acr -AH ) and unprotonated (Acr -A) tertiary
amine species absorb at the same wavelength, as expected
being the structural difference away from the chromophore. In
more alkaline solutions (pH > 8) the absorption bands shift to
smaller wavelengths; in particular the visible band shifts Fig. 3 Fluorescence emission spectra of Acr -A (1.5 × 10 M) in water
hypsochromically by 40 nm (from 445 to 385 nm). This effect recorded at pH 2.91 (solid line), 6.00 (dashed line), 8.84 (dashed and
+
−5
dotted line) and 11.11 (dotted line), λexc = 400 nm.
is evident from the inset of Fig. 1.
The presence of isosbestic points (250, 280, 304, 336 and
4
00 nm) indicates the coexistence of two species in equili-
brium. The hypsochromic shift is explainable on the basis of at 445 nm, where (Acr-H)-A does not absorb (Fig. 2). The exci-
the less aromatic structure (Acr-H)-A, a derivative of 9-imine- tation at 400 nm (isosbestic point), where all three species
+
1
0-methylacridane, in equilibrium with Acr -A (Scheme 2), absorb (Scheme 2), produced conversely different emission
formed by deprotonation of the 9-amine group in very basic spectra depending on the pH (Fig. 3). In acidic solution, as
solution, in line with the well-known strong electrophilicity of expected, the emission shape is identical to that observed at
9
the 9-position of the acridinium ring. The (Acr-H)-A species λexc = 445 nm, while at alkaline pH a spectral shift to smaller
1
was characterized by H-NMR spectroscopy (see the Experi- wavelengths and a band structured in two maxima at 430 and
mental section).
450 nm, attributed to the emission of the less aromatic struc-
+
+
+
As the absorption spectra of Acr -AH and Acr -A are essen- ture (Acr-H)-A, were observed. At intermediate pH (6.00) the
tially unchanged with pH, the same is observable for the spectrum is clearly resulting from the emission of all the
shape and position of the emission spectra, recorded exciting species present in solution.
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In order to avoid the interference of (Acr-H)-A in the investi-
+
gation of the pH-responsive fluorescence properties of Acr -A
−
Cl , the photophysical characterization was performed exciting
at 445 nm. Under these conditions, the fluorescence intensity
shows an increase upon protonation (Fig. 2). A 17-fold
enhancement in fluorescence quantum yield Φ
.30) was measured. It has to be noted that this value is close
to that shown by N,N-diethylaminoethyl bonded to the aniline
F
(from 0.017 to
0
1
0
nitrogen of 4-aminonaphthalimide chromophore. The fluo-
rescence intensity (I ) at the maximum wavelength (490 nm) is
F
reported as function of pH ranged from 3 to 12 (inset in Fig. 2,
the corresponding relative fluorescence emission plot is
reported in Fig. S1†). The emission intensity–pH profile was
+
−4
2 2
Fig. 4 Absorption spectra of Acr -A (5.5 × 10 M) in N -saturated H O
1
1
accurately analyzed according to eqn (1), and the obtained
acidity constant is pK
than those of the analogous sensor 4-(2-(diethylamine)ethyl-
recorded 1.1 μs after the laser pulse at pH 3.0 (□), 7.6 (●), 9.0 (△) and 13 (▾);
a
= 7.0. This value is significantly smaller λexc = 355 nm.
1
0
amino)-naphthalimide (pKa = 8.7) and the parent triethyl-
amine (pK = 10.7). The higher basicity of triethylamine
with respect to 2-(diethylamine)ethylamino sensors can be oxygen through an almost diffusional process (kox = 6.4 × 10
a
9
−
1
−1
explained on the basis of the electron withdrawing aniline
nitrogen in the more complex structure,
M
s ). This species is the only one visible at very alkaline
pH, confirming its formation from (Acr-H)-A. The spectrum at
pH 8 recorded 1.1 μs after the laser pulse in a wider wave-
length scale (Fig. S3†) shows a broad band from 650 to 900 nm
that disappears at pH 12. This absorption can be reasonably
attributed to the formation of a π-dimer between the electron
þ
þ
þ
ðpKaꢀpHÞ
IFðAcr -AH Þ þ IFðAcr -AÞ10
IF ¼
ð1Þ
ðpKaꢀpHÞ
1
þ 10
•
•+
+
even more effectively by the peculiar positive charge of the acri- transfer (ET) state, Acr -A , and the Acr -A ground state.
+
−
dinium chromophore. The pK
of 7.0 makes Acr -A Cl a good A similar broad absorption band has already been observed
a
candidate as a biologically relevant probe to monitor small pH by Fukuzumi and coworkers with both 9-(1-naphthyl)- and
1
6
fluctuations near to neutral physiological values in aqueous 9-mesityl-10-methylacridinium in MeCN, though shifted to
solution and in vivo.
greater wavelengths (880–1100 nm), maybe due to the more
The switching on/off of fluorescence is indicative of an aromatic system.
1
7
intramolecular fluorescence quenching by PET. According to
The decays recorded at 510 and 750 nm in N -saturated
2
this mechanism, one electron of the most basic nitrogen (ali- solution at pH 8 obey first-order kinetics with similar rate con-
5
−1
5
−1
phatic nitrogen) is transferred to the excited amino-acridinium stants (1.2 × 10
moiety. When the amine is protonated, the fluorescence is with the intramolecular back electron transfer of the ET state
restored.
(insets of Fig. S3†). These results clearly show that a rapid
The occurrence of an intramolecular PET process from ter- equilibrium is established between a monomer and a dimer.
s
and 1.0 × 10 s , respectively), in line
1
+
tiary amine to Acr * has been unequivocally confirmed by The long-lived ET state suggests that the thermal back electron
flash photolysis investigation. Indeed, upon laser excitation transfer (BET) from Acr to A moieties is slower than the
-saturated aqueous solutions of Acr -A forward photoinduced electron transfer, although both pro-
Cl , time-resolved absorption spectra were recorded at pH cesses are strongly exergonic (the free energy change was deter-
values of 3.0, 7.6, 9.0 and 13. The absorption spectra mined to be −1.47 eV for the PET and −1.44 eV for the BET).
obtained 1.1 μs after the laser pulse are shown in Fig. 4. It is This behavior is explainable assuming that the free energy
interesting to note that at pH 3.0, where fluorescence is the change of BET is in the Marcus inverted region. On the basis
•
•+
+
(
λ
exc = 355 nm) of N
2
−
1
2
18
2
0
+
+
predominant deactivation channel of the S
state of Acr -AH , of this model the BET rate decreases with increasing the
1
no transient was recorded. At pH 7.6 an absorption centered at thermodynamic driving force in the exergonic region.
5
20 nm, assigned to the amino-acridinyl radical moiety, was
It should be noted that the PET process from the diethyl-
This evidence endorses the PET hypothesis, amine to the 9-amino-10-methylacridinium fluorophore across
1
3
detected.
although the absorption of the resulting amine radical cation the dimethylene spacer is rather more exergonic with respect
was not detected, most likely because of the detectable wave- to the system with the amine receptor bonded, by the same
1
4
length region. The charge separation could be promoted spacer, to the aniline nitrogen of 4-aminonaphthalimide
1
5
10,21
both by anion-complexation and a polar solvent. At higher (ΔGPET ≈ 0, with triethylamine taken as the model).
This is
pH (9.0) an additional band was detected at 560 nm that is in agreement with the lower oxidizing power of the latter fluoro-
+
−
likely due to triplet–triplet absorption of (Acr-H)-A because: (i) phore. Really, the difference in ΔGPET between Acr -A Cl and
it was formed within the laser pulse, (ii) it decayed by first the corresponding 4-amino-1,8-naphthalimide-based chemo-
order kinetics and (iii) its decay was accelerated by molecular sensor should be smaller, considering that the amino receptor
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in the latter should be more easily oxidizable on the basis of a profile similar to that in only water is obtained (insets in
pK values (see above). Fig. 5 and 6, the corresponding relative fluorescence emission
a
Cationic, anionic and nonionic surfactants, such as CTAB plots are reported in Fig. S8 and S9†), but with a shift of the
cetyltrimethylammonium bromide), SDS (sodium dodecyl “off–on” response window to the right along the pH axis. The
(
sulfate) and SOBS (sodium octyl benzene sulfonate), SB3-14 data correlation by eqn (1) produced pK of 9.2 and 9.4 for SDS
a
2
4
(
N-tetradecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate), and SOBS, respectively, values quite higher than that deter-
respectively, above the c.m.c. (8.62 × 10 , 8.30 × 10 , 1.42 × mined in water (7.0). This apparent increase of the basic
0 , and 2.88 × 10 M for CTAB, SDS, SOBS and SB3-14, strength of the amine moiety reveals that the positive sensor
respectively) have been explored in order to maximize the pH structure, localized on the negatively charged micellar surface
sensing efficiency of Acr -A Cl . No significant effect was in place of the counterion (Na ), is exposed to higher H con-
observed on the spectral properties of the sensor in the pres- centration in the micellar interface, producing a “proton
ence of cationic and zwitterionic surfactants (Fig. S4 and S5†). sponge effect”. Being the effective pH on the interface lower
Such a behavior is expected as the CTAB headgroups repel than that measured in the bulk, there is no evidence of the
both protons and the positive sensor, while the zwitterionic hypsochromic shift in the absorption spectra in basic solution
−
4
−3
−
2
−4
1
2
2
+
−
+
+
SB3-14 produces low electrostatic affinity.
(Fig. S5 and S6†) due to the deprotonation of the 9-amine
2
4
Instead, the addition of SDS and SOBS, though it did not group (see above). SDS and SOBS micelles not only move the
induce appreciable shift of the absorption (Fig. S6 and S7†) position of the pH response window, but also affect strongly
and emission maxima, resulted in a substantial enhancement the fluorescence enhancement (68 and 79 with SDS and SOBS,
of fluorescence emission in the pH detection as the solutions respectively, against 17 determined in water), in terms of
2
3
went from basic to acidic (Fig. 5 and 6). From the data of quantum yields (ΦF from 0.0088 to 0.60 and 0.0076 to 0.60,
fluorescence emission intensity at 490 nm as a function of pH, respectively), thus enhancing the sensor sensitivity. The
F
almost fourfold enhancement in Φ value has benefited from
+
+
the fact that the double protonated sensor (Acr -AH ) is less
free to rotate and vibrate, due to the electrostatic interaction
with the negatively charged surface of micelles, causing an
increase in fluorescence at the expense of internal con-
2
5
F
version. Another factor that favors the Φ increase is the pre-
vention of the otherwise efficient quenching by water, that is, a
water-associated non-radiative decay.
The interaction of the protonated sensor (Acr -AH ) with
SDS and SOBS in aqueous solution has also been investigated
+
+
+
+
in pre-micellar regions. The two systems, Acr -AH /SDS and
+
+
Acr -AH /SOBS, were studied by emission spectroscopy by
varying the surfactant concentration.
As shown in Fig. 7, the fluorescence intensity decreases
Fig. 5 Fluorescence emission spectra of Acr -A (1.5 × 10 M) in water with surfactant concentrations up to 2.5 mM and 5 mM for
+
−5
in the presence of SDS (0.01 M) at different pH; λexc = 445 nm. Inset:
intensity recorded at 490 nm as a function of pH; the full line is the
SDS and SOBS, respectively, while it increases up to plateau
values when SDS and SOBS concentrations approach the c.m.c.
values. Accordingly, the interaction of the sensor with the
fitting curve obtained using eqn (1).
+
−5
Fig. 6 Fluorescence emission spectra of Acr -A (1.5 × 10 M) in water
in the presence of SOBS (0.02 M) at different pH; λexc = 445 nm. Inset:
intensity recorded at 490 nm as a function of pH; the full line is the
fitting curve obtained using eqn (1).
+
×
10−5 M)
Fig. 7 Fluorescence emission intensity of Acr -A (1.5
recorded at 490 nm as a function of [SDS] (□) and [SOBS] (■) at pH 2.8;
exc = 445 nm.
λ
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surfactant is operating in the pre-micellar region. These data (Spex, F112AI) spectrofluorometer. The pH was monitored
can be rationalized as already reported for the 9-aminoacridi- using a pH meter (Hanna Instruments HI9023C).
2
6
nium/SDS system. At low surfactant concentrations ([SDS]
+
+
Synthesis of 9-(2-(diethylamine)ethylamino)-10-
methylacridinium chloride (Acr -A Cl )
<
2.5 mM, [SOBS] <5 mM), together with Acr -AH /surfactant
+
−
+
+
aggregates, there are some weakly bound dimers (Acr -AH )
2
7
+
−
sharing the same clusters, that contribute to quenching sig- Acr -A Cl was synthesized in two steps.
nificantly the emission intensity, being complexes less-emit-
(1) Synthesis of 9-chloro-10-methylacridinium chloride. It
ting species than monomers. In contrast, at higher surfactant was prepared by reaction of 10-methyl-9(10H)-acridone with
2
concentrations (2.5 mM < [SDS] < c.m.c., 5 mM < [SOBS] < SOCl in the presence of catalytic amounts of DMF according
3
0
c.m.c.) there is an enhanced fluorescence because the clusters to a literature procedure. The product was obtained as a light
reorganize into pre-micellar aggregates, where it is unlikely brown solid after concentration under vacuum at 50 °C for
2
that two sensor molecules occupy the same pre-micelle. At several hours to remove SOCl and DMF. It was characterized
surfactant concentrations above the c.m.c. values, the by the absorption spectrum and used without further purifi-
fluorescence intensity reaches the limiting value as all sensor cation. UV-Vis: λmax(MeCN)/nm 348 (sh), 363 (A 0.41), 402
3
1
molecules are adsorbed on the micellar surface basically in a (0.096), 426 (0.095), 448 (sh).
monomeric form.
2
8
+
−
(2) Synthesis of Acr -A Cl . A solution of 9-chloro-10-
methylacridinium chloride (1 mmol) and 2-(diethylamine)-
ethylamine (2 mmol) in anhydrous acetonitrile (3 ml) was kept
for 2 days under argon. The precipitated product was filtered,
washed with cold acetonitrile and dried under vacuum, yield-
Conclusions
In conclusion, a simple and efficient fluorescent pH sensor ing 300 mg of a pale yellow solid (90%). M.p.: 215 °C (dec.).
+
has been developed. The action of a PET process as a HPLC-HRMS: calcd for C20
H
26
N
3
[M] : 308.2136, found:
1
fluorescence quenching mechanism, in a few cases directly 308.2129. H-NMR (400 MHz, D O): δ (ppm) 8.00 (m, 4H, Ar),
2
demonstrated in the literature, is here endorsed by the nature 7.77 (d, 2H, Ar), 7.57 (t, 2H, Ar), 4.31 (t, 2H, ArNHCH
2
) 3.84 (s,
), 3.16 (q, 4H, 2CH CH
of anionic micelles increases significantly the signaling 1.18 (t, 6H, 2CH CH ). C NMR (100 MHz, D O): δ (ppm)
+
of the transients detected by pulsed techniques. The presence 3H, N CH
3
), 3.55 (t, 2H, CH
2
N(Et)
2
2
3
)
1
3
2
3
2
efficiency of the sensor, due to the particular immobilization 159.1 (C9 acridine ring), 140.7 (ipso ArNAr); 136.0 (C1, C8 acri-
of its protonated form on the charged micelle surface. More- dine ring); 124.8 (C4, C5 acridine ring); 123.7 (C3, C6 acridine
over, the micelle regulates the working pH interval of the ring), 116.9 (C2, C7 acridine ring); 113.0 (ipso ArCNHAr); 49.9
sensor. In particular, it moves the ON window, positioned at (CH
the physiological pH range (6.8–7.4) in water, to higher pH. (NCH
2
N(Et)) 47.9 (NCH
CH ).
2 3 2 3
CH ); 43.5 (ArNHCH ); 35.8 (NCH ); 7.9
2
3
Thus, this approach could be used to visualize whether the pH
Characterization of 9-imine-10-methylacridane, (Acr-H)-A
from a basic D O solution
amphiphilic nanoaggregates that could facilitate the self- (pH ≈ 13). The H-NMR analysis of the CDCl solution showed
of biological systems is shifting out of or returning into the
physiological range. We are now dealing with the use of (Acr-H)-A was extracted with CDCl
3
2
1
3
assembling of multicomponent fluorescent sensors.
the product, (Acr-H)-A, in mixture with the corresponding
hydrolysis products, identified as 10-methyl-9(10H)-acridone
and [2-(diethylamino)ethyl]amine. The fast hydrolysis of (Acr-
H)-A, even at low temperature, did not allow its purification. By
the comparison of the mixture H-NMR spectrum with that
recorded for the pure samples of 10-methyl-9(10H)-acridone
Experimental section
General information
1
All the reagents were obtained from commercial suppliers and [2-(diethylamino)ethyl]amine, the following signals were
1
(
3
Sigma-Aldrich) and used as received, except for the surfac- assigned to the (Acr-H)-A structure: H-NMR (CDCl ,
tants, which were crystallized twice from an acetone–methanol 400 MHz): δ 8.28 (d, 1H, Ar), 7.95 (d, 1H, Ar), 7.61–7.48 (m, 2H,
mixture before using them. Dimethylformamide (DMF, Ar), 7.34 (d, 1H, Ar), 7.28–7.12 (m, 3H, Ar), 4.27 (m, 2H,
2 3 2 2
Aldrich), anhydrous ethanol (Carlo Erba, RPE), anhydrous ArNCH ) 3.69 (s, 3H, NCH ), 3.20 (m, 2H, CH N(Et) ), 2.98 (m,
acetonitrile (MeCN, Aldrich) for synthesis and acetonitrile 4H, 2CH CH ), 1.26 (t, 6H, 2CH CH ).
2
3
2
3
(
Carlo Erba, for UV spectroscopy) for spectroscopic measure-
Photophysical measurements
ments were used as received. Milli Q water (18 MΩ, pH 6.32)
was used. H NMR and C NMR spectra were recorded on a Fluorescence quantum yields (experimental error of ca. 7%)
1
13
Bruker Avance III HD 400 MHz Smart Probe spectrometer were determined from the emission spectra of the samples
1
13
(
400 MHz for H NMR and 100 MHz for C NMR). LC/MS ana- recorded at an absorbance smaller than 0.1 at the excitation
lysis was performed on an Agilent 6540 UHD accurate mass wavelength, to avoid self-absorption effects, by the use of tetra-
32
Q-TOF LC/MSMS. Absorption spectra were recorded on an cene in cyclohexane as a reference (Φ
Agilent 8453 Diode Array UV-VIS spectrophotometer. Fluo- For pH-dependent fluorescence measurements, the solu-
rescence emission spectra were measured using a Fluorolog-2 tions were acidified with HClO down to a pH of ≈2 and basi-
F
= 0.17).
4
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fied with NaOH up to pH ≈ 12. The evolution of the fluore-
scence intensity between these two pH values was fully
reversible. Indeed, for the solution acidified with HClO down
4
to a pH of ≈2, basified with NaOH solution up to pH ≈ 12 and
subsequently acidified, the fluorescence intensity completely
revived. Both the emission and the absorption were measured
by putting 3 ml of water at a known pH in a quartz cuvette
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(1 cm optical path) together with appropriate microliters of the
mother solution of the sensor prepared in anhydrous ethanol.
The same experimental procedure was followed in the pres-
ence of a surfactant.
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Transient kinetics on the time scale of several micro-
seconds were collected using a nanosecond laser flash photo-
lysis setup previously described (Nd:YAG Continuum, Surelite II,
third harmonics, λexc = 355 nm, pulse width ca. 7 ns and
3
3
energy ≤1 mJ per pulse).
All measurements were carried out at room temperature;
the solutions were saturated by bubbling with nitrogen.
Ground-state absorption spectra were recorded before and
after the transient absorption measurements to check for
photodegradation.
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3
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1
6 S. Fukuzumi, H. Kotani and K. Ohkubo, Phys. Chem. Chem. 24 The slightly higher values of pK
a
and fluorescence
Phys., 2008, 10, 5159.
enhancement for SOBS with respect to SDS could indicate
a more intense negative charge on the surface of SOBS
micelles, in line with the sulfonate and sulfate nature of
SOBS and SDS, respectively.
1
7 Likely the π-dimer formation involves not only the acridi-
nium moiety but also the tertiary amino group held in
close proximity by the acridinium interaction; evidence of
radical cations of aliphatic amine dimers, absorbing from 25 Similarly, β-cyclodextrin encapsulation is known to
4
80 to 580 nm, has already been noted by Scaiano et al. in
enhance the phosphorescence of pH sensors based on
1-bromonaphthalene phosphor in anaerobic solutions.
Also this result was explained by the contribution of the
electrostatic interaction of the charge on the emissive proto-
nated sensor with the host macrocycle OH groups as an
inhibiting effect of the major phosphorescence quenching
(triplet–triplet annihilation). See: R. A. Bissel and A. P. de
Silva, J. Chem. Soc., Chem. Commun., 1991, 1148.
1
4
zeolite cavities.
1
8 ΔGPET was calculated according to the equation ΔGPET
ox − E*(S red − Ered. The oxidation potential (Eox) of the
amine is taken equal to that of triethylamine (0.96 V vs.
=
E
1
)
1
9
SCE in MeCN), and the energy of the first excited singlet
E*(S red) and the reduction potential of the chromophore
(
1
)
7
are equal to that of 9-aminoacridinium (2.91 eV in MeCN)
and 10-methylacridinium (−0.48 V vs. SCE in MeCN),
respectively. To calculate ΔGBET
7
26 R. V. Pereira and M. H. Gehlen, Spectrochim. Acta, Part A,
2005, 61, 2926.
, the same previous
equation was used, missing the E*(S1)red term. In this case 27 The charge repulsion between cationic sensors is shielded
Eox refers to the acridinium moiety (−0.48 V), while Ered to
by the anionic surfactants in the aggregate.
the amine group (0.96 V).
9 C. K. Mann and K. K. Barnes, in Electrochemical Reactions
in Non-aqueous Systems, Marcel Dekker, New York, N.Y.,
28 Really, c.m.c. values determined in acidic solution (6 and
13 mM for SDS and SOBS, respectively) are lower than
1
2
9
those in water in the absence of electrolytes. Therefore
same dimers survive as long as not enough micelles are
formed.
1
970.
2
2
0 R. A. Marcus, Annu. Rev. Phys. Chem., 1964, 15, 155.
1 R. M. Duke, E. B. Veale, F. M. Pfeffer, P. E. Krugerc and 29 E. Dutkiewicz and A. Jakubowska, Colloid Polym. Sci., 2002,
T. Gunnlaugsson, Chem. Soc. Rev., 2010, 39, 3936. 280, 1009.
2 P. Mukerjee and K. J. Mysels, in Critical Micelle Concen- 30 B. F. Cain, G. J. Atwell and W. A. Denny, J. Med. Chem.,
trations of Aqueous Surfactant System, Superintendent of 1976, 19, 772.
Documents, U.S. Government Printing Office, Washington 31 S. A. Jonker, F. Ariese and J. W. Verhoeven, Recl. Trav. Chim.
D.C., 1971. Pays-Bas, 1989, 108, 109.
3 The study of the photophysical properties in the presence 32 M. Montalti, A. Credi, L. Prodi and M. T. Gandolfi, in Hand-
2
2
of surfactants was always carried out by exciting at 445 nm,
where protonated (Acr -AH ) and unprotonated (Acr -A)
book of Photochemistry, CRC Press, Boca Raton, FL, 3nd
edn, 2006.
+
+
+
species absorb, although under these conditions there is 33 H. Gorner, F. Elisei and G. G. Aloisi, J. Chem. Soc., Faraday
little interference by (Acr-H)-A at the isosbestic point. Trans., 1992, 88, 29.
This journal is © The Royal Society of Chemistry 2014
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