A fluorescent chemosensor for wide-range pH detection

Article abstract of DOI:10.1039/b508136j

Simple polyamines, L1-L3, bearing anthracene and benzophenone units at the respective ends, behave as a fluorescent pH sensor applicable to wide-range pH detection. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b508136j

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A fluorescent chemosensor for wide-range pH detection{
Go Nishimura, Yasuhiro Shiraishi* and Takayuki Hirai
Received (in Cambridge, UK) 9th June 2005, Accepted 5th September 2005
First published as an Advance Article on the web 23rd September 2005
DOI: 10.1039/b508136j
L1–L3 show distinctive fluorescence at 380–540 nm in water,
Simple polyamines, L1–L3, bearing anthracene and benzophe-
none units at the respective ends, behave as a fluorescent pH
sensor applicable to wide-range pH detection.
attributable to an emission from photoexcited AN (Fig. S1 and
S4{). Fig. 1A shows fluorescence spectra of L2 (lex 5 368 nm) for
instance. Polyamine bearing a single AN unit at one end (L4) and
polyamines bearing AN and benzene units at the respective ends
(L5–L7) show similar spectra (Fig. S5–S8{). The IF of L4–L7 at
416 nm, when plotted against pH, demonstrates a single sigmoidal
curve with pKa9 6.2–7.8 (Fig. S5–S8{); as shown in Fig. 1B (open
symbol), L6 shows the typical example with pKa9 6.7. In L4–L7
systems, the IF at pH 1–5 is almost constant (Fig. S5–S8{),
indicating that these can only detect pH 5–10. However, for L1–L3
(Fig. S1 and S4{), the pH–IF profile is a ‘‘gentle slope’’ over
pH 2–10 range (Fig. 1B, closed symbol; for example L2),
suggesting that L1–L3 allows a wide-pH range detection.
Design of supramolecular systems enabling a fluorimetric detection
of chemical species in solution has attracted much attention.1
Much effort has been made toward development of a fluorescent
pH sensor (H+ detection).2 The simplest pH sensing system
consists of a fluorophore (e.g., anthracene) covalently linked to a
polyamine.3 In this system, the fluorescence intensity (IF) of the
fluorophore decreases with an increase in pH of the solution. This
occurs because deprotonation of the nitrogen atom, associated
with a pH increase, leads to an electron transfer (ELT) from the
nitrogen atom to a photoexcited fluorophore.4 The ELT process
depends strongly on the distance from the nitrogen atom to the
fluorophore,5 such that the deprotonation of the ‘‘crucial’’ nitrogen
atom triggers a drastic IF decrease. The pH–IF profile, therefore,
usually demonstrates a single or double sigmoidal curve with pKa9
1–8.6 Most of the pH sensors therefore detect only in a limited
pH range.
Dotted lines in Fig. 1B denote mole fraction distribution of the
different protonated species of L2, calculated from the protonation
constants, which were determined potentiometrically.7 L6 shows
almost the same distribution as that of L2 (Fig. S7{). The
1
deprotonation sequence of L6 determined by H and 13C NMR
reveals that: (i) first deprotonation occurs on the third nitrogen
Here we report a family of polyamines bearing anthracene (AN)
and benzophenone (BP) moieties at the respective ends, L1–L3
(Scheme 1), as a new fluorescent pH sensor applicable to a wide-
range pH detection: the pH–IF plots of these molecules
demonstrate a ‘‘gentle slope’’ profile over the pH 2–10 range.
This function involves pH-controlled two consecutive intramo-
lecular ELT processes: (i) ELT from photoexcited AN to BP
[ELT(AN* A BP)]; and (ii) ELT from the nitrogen atom to the
photoexcited AN [ELT(N A AN*)].
Scheme 1 Structure of polyamines, L1–L8.
Research Center for Solar Energy Chemistry, and Division of Chemical
Engineering, Graduate School of Engineering Science, Osaka University,
Toyonaka, 560-8531, Japan. E-mail: shiraish@cheng.es.osaka-u.ac.jp;
Fax: +81 6 6850 6273; Tel: +81 6 6850 6271
{ Electronic supplementary information (ESI) available: Properties and
spectra of L1–L8 compounds (including: Fig. S1–S11, Scheme S1, Tables
S1–S10). See http://dx.doi.org/10.1039/b508136j
Fig. 1 (A) Change in fluorescence spectra (lex 5 368 nm) of L2 (40 mM)
in aqueous NaCl (0.15 M) solution with pH. (B) Change in fluorescence
intensity of L2 ($) and L6 (#) at 416 nm with pH and mole fraction
distribution of the protonation states of L2 (dotted line). The fluorescence
quantum yield (Wf) at pH 1.6 is 0.23 (L2) and 0.38 (L6).
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atom from the AN unit; (ii) the second deprotonation occurring on
the second nitrogen atom from the AN unit leads to a partial
emission quenching (ca. 30%); and (iii) total emission quenching
occurs upon removal of the third proton from the first nitrogen
atom from AN. These indicate that the ELT(N A AN*) process
within L6 is triggered by the second deprotonation. The
deprotonation sequence of L2 is the same as that of L6, suggesting
that L2 involves other emission quenching processes at pH 2–5,
where H3L23+ species exist predominantly.
The molar extinction coefficient of L4 bearing a single AN end,
measured at 368 nm (pH 2.6), is 16-fold higher than that of L8
bearing a single BP end (Fig. S9{). This indicates that, for L2
bearing AN and BP ends, excitation light (lex 5 368 nm) is mostly
absorbed by the AN moiety, and hence, excitation of the BP
moiety is suppressed. Singlet excitation energy of AN (E0–0AN) and
reduction potential of AN [E(AN/AN2)] are 3.28 eV and 21.92 V
(vs. SCE in MeCN), respectively,8 and oxidation potential of BP
[E(BP+/BP)] is +2.65 V.9 Hence, free energy change in ELT from
BP to excited AN, DGELT(BP A AN*) (5 2[E0-0AN + eE(AN/AN2)
2 eE(BP+/BP)]),8 shows positive value (+1.29 eV), indicating that
the process is not favored thermodynamically. In contrast, the free
Scheme 2 Decay kinetics for (A) L6 and (B) L2 species (1/tN: rate
constant due to natural decay, kELT(N
AN*): rate constant due to
A
ELT(N A AN*), kELT(AN* A BP): rate constant due to ELT(AN* A BP)).
equilibrium exists between species of higher (HnLn+) and lower
(n21)+
(H L
n21
) protonation degree. Simultaneous excitation of both
(n21)+
species leads to a formation of excited HnLn+* and H
L
n21
*
species. In the case of L6, the fully protonated H4L64+* decays
with a rate constant equal to the reciprocal of tN, while H3L63+*
involves an additional quenching process due to ELT(N A AN*)
with a rate constant, kELT(N A AN*); the overall decay rate constant
of HnL6n+*, kL6, is expressed as 1/tN + kELT(N
energy change in ELT from excited AN to BP,10 DGELT(AN* A BP)
shows a negative value (20.36 eV),11 indicating that the process is
,
AN
BP
allowed thermodynamically. E0–0
is higher than E0–0
(3.22 eV)9 and hence also allows energy transfer (ENT) from
singlet excited-state AN to BP. However, the free energy change in
the process (20.05 eV) is much lower than that of ELT(AN* A
BP) (20.36 eV). These findings strongly indicate that the
ELT(AN* A BP) is involved in the L2 fluorescence quenching
at pH 2–5 (Fig. 1B, closed symbol). In the case of L6, free energy
changes in both ELT(benzene A AN*) and ELT(AN* A
A
AN*)
(Scheme 2A).4a,13b In the L2 system, the rate constant due to
ELT(AN*
(Scheme 2B). Hence, the overall decay rate constant of HnL2n+*,
kL2, is expressed as 1/tN + kELT(N A AN*) + kELT(AN* A BP)
A
BP), kELT(AN*
, must be considered
BP)
A
.
The 1/tN value for all of the HnL6n+* and HnL2n+* species is
constant, and the kELT(N A AN*) value for respective HnL6n+* and
HnL2n+* species, of the same protonation degree, should be equal.
Hence, the kELT(AN* A BP) value for respective HnL2n+* species is
obtained by subtracting the overall decay rate constant of HnL6n+*
from that of HnL2n+*, kL2–kL6.16 As summarized in Table 1, the
contribution of the ELT(AN* A BP) quenching to the overall
decay (kELT(AN* A BP)/kL2) for H3L23+* (75%) is much higher than
that for H4L24+* (33%) and that for species of lower protonation
degree (,6%). This suggests that the first deprotonation of L2
triggers ELT(AN* A BP) (Scheme 3). The contribution of
ELT(N A AN*) quenching is increased to 0% (H3L23+*), 15%
(H2L22+*), 66% (HL2+*), and 84% (L2*), indicating that the
second deprotonation (H2L22+ formation) triggers ELT(N A
AN*), and further deprotonations lead to complete fluorescence
quenching. These indicate that the ELT(AN* A BP) and
ELT(N A AN*) processes, occurring sequentially with L2
deprotonation (Scheme 3), leads to the ‘‘gentle slope’’ response.
Another interesting aspect of the L1–L3 molecules bearing AN
and BP moieties is that the slope of the pH–fluorescence intensity
profile changes depending on the chain length of the polyamine
(see graphical abstract): the slope tends to be ‘‘gentler’’ with longer
chain length: L3 . L2 ¢ L1. The fluorescence quenching behavior
of L1 and L3 is analogous to that of L2: the first deprotonation
triggers ELT(AN* A BP) and the second (and later) deprotona-
tion triggers ELT(N A AN*) (Fig. S11 and Tables S8 and S9{).
The gentler slope of L3 is explained as follows: on the mono-
deprotonated L3 species, ELT(AN* A BP) occurs more slowly
(2.15 6 108 s21) than on the corresponding L2 species (Table S9{),
benzene) processes show a positive value (+0.94 and +1.34 eV).12
benzene
E0–0
(4.76 eV)8 is higher than E0–0AN, such that ENT(AN*
A benzene) does not occur, resulting in the constant IF at pH 2–5
(Fig. 1B, open symbol). When a mixture of L4 and L8 was used
for fluorescence measurement, the obtained pH–IF profile is
almost the same as that obtained using only L4 (Fig. S5{). This
indicates that ‘‘intermolecular’’ ELT(AN* A BP) does not occur;
‘‘intramolecular’’ ELT(AN* A BP) within L1–L3 is then the
crucial factor triggering the ‘‘gentle slope’’ response.
As shown in Fig. 1B, at strongly acidic pH (,3) where H4L24+
species exist predominantly, contribution of the ELT(AN* A BP)
process to the IF of L2 is minor. This may be ascribed to a large
electrostatic repulsion of the protonated amines, as reported for
the related polyamines,13 which suppresses the required bending
movement of the polyamine chain for the ELT. UV-vis
measurement revealed a pH-induced red-shift of the absorption
spectra of L2 (Fig. S2{) attributable to a dipole–dipole interaction
between AN and BP,14 while no change was observed for L6 (Fig.
S7{). 1H NMR titration of L2 in D2O/CD3CN (80/20 v/v) revealed
a corresponding upper-field shift of AN and BP resonances with a
pH increase (Fig. S3{).15 These suggest that the pH increase
actually brings these moieties closer.
To clarify the mechanism of the ‘‘gentle slope’’ response of
L1–L3, time-resolved fluorescence measurement was employed.
Broad analysis over pH 1–12 indicates that fluorescence decays of
L1–L7 are explained with the sums of two or three exponentials
(Fig. S10 and Tables S2–S7{). The decay kinetics of L6 and L2 can
be interpreted as shown in Scheme 2. In both cases, ground-state
5314 | Chem. Commun., 2005, 5313–5315
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Table 1 Fluorescence quenching rate constants for respective L2 species of different protonation degreea
H4L24+
H3L23+
H2L22+
HL2+
L2
kL2/108 s21
1.28
0 (0)
0.43 (33)
c
3.47
0.01 (0)
2.60 (75)
1.09
0.17 (15)
0.07 (6)
2.69
1.77 (66)
0.06 (2)
5.68
4.79 (84)
0.03 (1)
kELT(N A AN*)/108 s21 [contribution (%)]b
kELT(AN* A BP)/108 s21 [contribution (%)c]
a
b
1/tN: 8.55 6 107 (s21). kELT(N A AN*)/kL2 6 100. kELT(AN* A BP)/kL2 6 100.
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Scheme 3 Schematic representation for the mechanism of ‘‘gentle slope’’
fluorescence response of L2.
because of a longer distance between the AN and BP moieties due
to a low angular bending of the long polyamine chain.13 ELT(N A
AN*) also occurs more slowly on the L3 species of lower
protonation degree, because of delocalization of positive charges
along the polyamine chain.14 On respective L1 species,
ELT(AN* A BP) and ELT(N A AN*) occur more rapidly
(Fig. S11 and Table S8{), but the slope of L1 is nearly the same as
that of L2. This is because deprotonation of L1 occurs at higher
pH than that of L2 (i.e. L1 has higher protonation constants than
L2) because of a smaller number of nitrogens.
logK(H2L2/HL2?H)
5 7.12, logK(H3L2/H2L2?H) 5 5.01,
In summary, we have demonstrated that the simple-structured
polyamines, L1–L3, bearing AN and BP moieties at respective
ends, behave as a fluorescent pH sensor applicable to a wide-range
pH detection. The concept for molecular design presented here,
based on sequential electron transfer, may contribute to the
development of a more convenient fluorescent chemosensor.
This work was supported by the Grant-in-Aid for Scientific
Research (No. 15360430) and on Priority Areas (417) (No.
17029037) from the Ministry of Education, Culture, Sports,
Science and Technology, Japan (MEXT). G. N. acknowledges the
Research Fellowship of the Japan Society for the Promotion of
Science (JSPS) for Young Scientists.
logK(H4L2/H3L2?H) 5 3.12. The constants for the other materials:
see Table S1{.
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AN
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