Excited-state proton and charge transfer in protonated amino and methylated derivatives of 2-(2′-hydroxyphenyl)benzimidazole

Article abstract of DOI:10.1021/jp507917u

We studied the excited-state behavior of a family of mono- and diprotonated derivatives of 2-phenylbenzimidazole in different solvents, using steady-state and time-resolved fluorescence spectroscopy. The species investigated were 2-(4′-amino-2′-hydroxyphenyl)benzimidazole (1), the diethylamino analogue 2-(4′-N,N-diethylamino-2′-hydroxyphenyl)benzimidazole (2) and its N-methylated derivative 1-methyl-2-(4′-N,N-diethylamino-2′-hydroxyphenyl)benzimidazole (3). The O-methoxy derivatives of 2 and 3 (2-OMe and 3-OMe), and the simpler models 2-phenylbenzimidazole (4) and its 4′-amino (5) and 4′-dimethylamino (6) derivatives were also studied. We found that the dications of 1, 2, and 3 (protonated at the benzimidazole N3 and at the amino group) were strong photoacids, which were deprotonated at the hydroxyl group upon excitation in aqueous solution (totally for 2 and 3) to give a tautomer of the ground-state monocation. In contrast, no photodissociation was observed for the monocations of these species. Instead, some of the monocations studied behaved as molecular rotors, for which electronic excitation led to a twisted intramolecular charge transfer (TICT) state. The monocations of 2, 3, 2-OMe, 3-OMe, and 6, protonated at the benzimidazole N3, experienced a polarity- and viscosity-dependent radiationless deactivation associated with a large-amplitude rotational motion. We propose that this process is connected to an intramolecular charge transfer from the dimethylaminophenyl or diethylaminophenyl moiety (donor) to the protonated benzimidazole group (acceptor) of the excited monocation, which yields a twisted charge-transfer species. No fluorescence from this species was detected except for 3 and 3-OMe in low-viscosity solvents.

Full text of DOI:10.1021/jp507917u

Article
pubs.acs.org/JPCB
Excited-State Proton and Charge Transfer in Protonated Amino and
Methylated Derivatives of 2‑(2′-Hydroxyphenyl)benzimidazole
Sonia Ríos Vazquez, J. Luis Perez Lustres, Flor Rodríguez-Prieto,* Manuel Mosquera,*
́
́
and M. Carmen Ríos Rodríguez*
́ ́
Departamento de Química Física and Centro Singular de Investigacion en Química Bioloxica e Materiais Moleculares (CIQUS),
Universidade de Santiago de Compostela, E-15782 Santiago de Compostela, Spain
ABSTRACT: We studied the excited-state behavior of a family
of mono- and diprotonated derivatives of 2-phenylbenzimidazole
in different solvents, using steady-state and time-resolved
fluorescence spectroscopy. The species investigated were 2-(4′-
amino-2′-hydroxyphenyl)benzimidazole (1), the diethylamino
analogue 2-(4′-N,N-diethylamino-2′-hydroxyphenyl)-
benzimidazole (2) and its N-methylated derivative 1-methyl-2-
(4′-N,N-diethylamino-2′-hydroxyphenyl)benzimidazole (3). The
O-methoxy derivatives of 2 and 3 (2-OMe and 3-OMe), and the simpler models 2-phenylbenzimidazole (4) and its 4′-amino (5)
and 4′-dimethylamino (6) derivatives were also studied. We found that the dications of 1, 2, and 3 (protonated at the
benzimidazole N3 and at the amino group) were strong photoacids, which were deprotonated at the hydroxyl group upon
excitation in aqueous solution (totally for 2 and 3) to give a tautomer of the ground-state monocation. In contrast, no
photodissociation was observed for the monocations of these species. Instead, some of the monocations studied behaved as
molecular rotors, for which electronic excitation led to a twisted intramolecular charge transfer (TICT) state. The monocations
of 2, 3, 2-OMe, 3-OMe, and 6, protonated at the benzimidazole N3, experienced a polarity- and viscosity-dependent
radiationless deactivation associated with a large-amplitude rotational motion. We propose that this process is connected to an
intramolecular charge transfer from the dimethylaminophenyl or diethylaminophenyl moiety (donor) to the protonated
benzimidazole group (acceptor) of the excited monocation, which yields a twisted charge-transfer species. No fluorescence from
this species was detected except for 3 and 3-OMe in low-viscosity solvents.
hydrogen bond in the ground state, an ultrafast excited-state
intramolecular proton transfer, ESIPT, from the acid to the
basic site may occur. ESIPT molecules have technological
applications as UV photostabilizers23 and are potential
components for photoswitches24 and organic LEDs.25
1. INTRODUCTION
Proton- and electron-transfer reactions, frequently coupled,
play a crucial role in many essential processes taking place in
living organisms, like DNA chemical damage and repair,1,2
photostability of DNA base pairs and proteins,3 photosyn-
thesis,4 and respiration.5 Although these reactions have been
extensively investigated, the detailed mechanisms of charge
transfer and proton motion, and in particular, the coupling
between them, are not completely understood.6−11
Photoinduced proton- and electron-transfer processes are
triggered by the change of acid−base and redox properties of
molecules upon electronic excitation. Molecules with both
electron donor and acceptor moieties often undergo in the
excited state a photoinduced intramolecular electron transfer.
These systems have applications as luminescence sensors,12 in
organic solar cells,13,14 or in light-driven molecular machines.15
Molecules with increased acidity in the excited state are
called photoacids, as they have the tendency to be
deprotonated upon excitation in solvents capable of accepting
the proton or in the presence of basic species. Recent research
on photoacids has greatly contributed to our understanding of
the elementary steps in proton-transfer processes.16−19 Photo-
acids have also found applications as polymerization initiators,20
in molecular machines21 and in pH-jump experiments.22 If a
molecule has an acid group and a basic site linked by a
2-(2′-Hydroxyphenyl)benzimidazole (HBI),26−40 2-(4′-
amino-2′-hydroxyphenyl)benzimidazole (1),36,41,42 2-(4′-N,N-
diethylamino-2′-hydroxyphenyl)benzimidazole (2),36,41 1-
methyl-2-(4′-N,N-diethylamino-2′-hydroxyphenyl)-
benzimidazole (3),41 and 2-(3′-hydroxy-2′-pyridyl)-
benzimidazole43,44 are examples of ESIPT molecules studied
in our group (see Chart 1). A solvent-modulated ground-state
rotameric and tautomeric equilibrium was observed for these
benzimidazole derivatives. In apolar aprotic solvents, these
compounds exist in the ground state as the planar syn normal
form Nsyn (Chart 1), with an intramolecular hydrogen bond
N···H−O. This species undergoes in the first-excited singlet
state an intramolecular proton transfer from the hydroxyl group
to the benzimidazole N3 to yield the excited T tautomer (see in
Special Issue: Photoinduced Proton Transfer in Chemistry and
Biology Symposium
Received: August 5, 2014
Revised: September 13, 2014
© XXXX American Chemical Society
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Chart 1. Molecular Structures of the Neutral Isomers of HBI, 1, 2, and 3; Normal Forms (Nsyn, Nanti, Nnp), and the Tautomer T
Obtained after ESIPT Are Shown
Chart 2. Molecular Structures of the Monocations (MC) Studied in this Work, Ordered as a Function of the Electron-Donor
Strength of the Phenyl Moiety and the Steric Interactions of the Aromatic Rings
Chart 1 the limiting resonance structures of this species). In
protic solvents, a ground-state rotameric equilibrium between
Nsyn and its planar anti rotamer Nanti was detected for
HBI,31,32,34 1, and 2, and between Nsyn and a nonplanar Nnp
rotamer for the sterically hindered compound 3 (Chart 1).41
Nanti and Nnp, unable to undergo ESIPT, yield normal N*
emission upon excitation. Water stabilizes the ground-state T
tautomer, which was detected by its red-shifted absorption in
aqueous solutions of HBI, 1, 2, and 3.34,41
Whereas the fluorescence quantum yield and lifetime of T*
showed for HBI no dependence on solvent or temperature,45
those of the tautomers of 2 and 3 revealed a temperature-,
polarity-, and viscosity-dependent radiationless deactivation
connected with a large-amplitude conformational motion
occurring for these HBI derivatives.41 We have shown that
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this conformational change is associated with a charge transfer
experienced by T* from the deprotonated dialkylaminophenol
group (donor) to the protonated benzimidazole group
(acceptor), affording a nonfluorescent charge-transfer structure
Tc*t.41 This excited-state intramolecular proton-coupled charge-
transfer process was reported by us for 2-(2′-hydroxyphenyl)-
benzoxazole, 2-(2′-hydroxyphenyl)benzothiazole, and other
hydroxyarylbenzazoles in solution, but we did not observe it
for HBI.45 We demonstrated that the efficiency of this process
increased with the electron-donor strength and with the steric
hindrance caused by N1-methylation, and decreased as the
solvent viscosity was increased. Theoretical calculations
performed later by Tsai et al.36 on HBI, 1, and 2 predict that
two different charge-transfer states are stable in ethanol and
cyclohexane for these compounds. Recently reported inves-
tigations37 on ESIPT in crystal structures of HBI polymorphs
(α and β) showed that the tautomer form of polymorph β
undergoes in the excited state an intramolecular charge transfer
process which does not occur for polymorph α, revealing the
importance of intermolecular interactions for the relative
stability of T* and its charge-transfer form. In a poly(methyl
methacrylate) film, the polarizability of HBI tautomer was
found to be smaller in S1 than in S0, suggesting a twisted
geometry for T in S1.38 Molecules showing a viscosity-
dependent fluorescence lifetime due to an intramolecular
twisting occurring in the excited state are molecular rotors.
These fluorescent molecular rotors can be employed as
reporters of the local microviscosity, in particular in live cells,
by fluorescence lifetime imaging.46,47
solvents of various viscosities and polarities and compare it with
the fluorescence of the methoxy derivatives 2-OMe and 3-OMe
(Chart 2), unable to deprotonate under excitation, but prone to
undergo a charge-transfer process in the excited state.
Moreover, we investigated the fluorescence of the monocations
of simple models phenylbenzimidazole (4, Chart 2), p-
aminophenylbenzimidazole (5), and p-dimethylaminophenyl-
benzimidazole (6), sharing the same electron acceptor, but with
different electron donors.
The aims of the present study were (1) to find out if the
monocations and dications of 1, 2, and 3 deprotonate in the
excited state in protic solvents and (2) to examine if these
compounds and the related species shown in Chart 2 undergo
in the excited state a large-amplitude conformational motion
connected to an intramolecular charge migration analogous to
that observed for the neutral tautomer T*.
2. EXPERIMENTAL SECTION
2.1. Materials. HBI34 and 1, 2, 3, 2-OMe, and 3-OMe41
were synthesized as described elsewhere. Compound 4 was
provided by Aldrich.
Compound 5 was prepared by condensation of 20 mmol of
1,2-benzenediamine (Aldrich) with 20 mmol of 4-amino-
benzoic acid (Aldrich) at ∼100 °C for 24 h. The solid obtained
was dissolved in acetone and the product precipitated by
adding water. 1H NMR (300 MHz, DMSO), δ (ppm): 5.58 (s,
2H), 6.65 (d, 2H, J = 8.8 Hz), 7.10 (dd, 2H, J = 6.1 Hz, J = 3.4
Hz), 7.46 (m, 2H), 7.83 (d, 2H, J = 8.8 Hz), 12.40 (s, 1H). MS,
m/z (rel. intensity): 209 (100.0, M), 118 (7.6, M − 91).
To obtain 6, 15 mmol of 1,2-benzenediamine (Aldrich) were
added to a stirred solution of 15 mmol of 4-dimethylamino-
benzoic acid (Aldrich) in 25 mL of toluene at 90 °C. A total of
18 mmol of PCl3 were then added dropwise, and the mixture
was kept refluxing for 12 h. The solid obtained was repeatedly
washed with toluene and the product recrystallized from
ethanol. 1H NMR (300 MHz, DMSO), δ (ppm): 3.05 (s, 6H),
6.90 (d, 2H, J = 8.7 Hz), 7.35 (dd, 2H, J = 3.1 Hz, J = 5.9 Hz),
7.64 (dd, 2H, J = 3.1 Hz, J = 5.9 Hz), 8.06 (d, 2H, J = 8.7 Hz).
MS, m/z (rel. intensity): 237 (100.0, M), 222 (9.2, M − 15),
194 (9.2, M − 43), 119 (11.3, M − 118).
2.2. Methods. Solutions were made up in double-distilled
water and spectroscopy-grade solvents, and were not degassed.
Acidity was varied with HClO4 in nonaqueous solutions, and
with HClO4, NaOH or NaH2PO4/Na2HPO4 buffer (all Merck
p.a. products) in aqueous solutions. Nonaqueous solutions
were either slightly acidified or strongly acidified to have the
compounds in the ground state as the monocations or the
dications, respectively, as confirmed by UV−vis absorption
spectroscopy and comparison with the well-known spectra of
the cations in aqueous solutions. Sample concentrations of
∼10−5 mol dm−3 for absorption and ∼10−6 mol dm−3 for
fluorescence were employed. All experiments were carried out
at 25 °C.
We found that the monocations of HBI,34 2-(3′-hydroxy-2′-
pyridyl)benzimidazole,48 and 2-(1′-hydroxy-2′-naphthyl)-
benzimidazole,19,49 protonated at the benzimidazole N3, are
strong photoacids, which become completely deprotonated at
the OH group in aqueous solution and to a great extent in
short-chain alcohols. It is interesting to investigate the effect of
the electron-donating substituents at the phenol group on the
photoacid properties of this class of compounds. The amino
groups of 1, 2, and 3 act as electron donors, but this effect
disappears when they are protonated. We will study the
influence of a protonated or nonprotonated amino group on
the photoacid behavior for the monocations (MC, see Chart 2)
and dications (DC) of 1, 2, and 3.
If we compare the monocations of 1, 2, and 3 (Chart 2) with
the structure of the neutral tautomer T (Chart 1), we observe
that they have the same electron acceptor (protonated
benzimidazole), but the monocations have a weaker electron
donor (aminophenol instead of aminophenoxy group). There-
fore, it is also possible that the excited monocations undergo a
charge-transfer process from the aminophenol group to the
protonated benzimidazole moiety. This process is also possible
for the monocations of 2-OMe and 3-OMe (Chart 2). In
contrast, the dications of 1, 2, 3, 2-OMe, and 3-OMe,
protonated both at the benzimidazole N3 and at the amino
group, are not expected to undergo the photoinduced charge
transfer process, as the protonated aminophenol moiety is not a
good electron donor. Therefore, it is interesting to compare the
behavior of monocations and dications of the mentioned
species, to investigate the existence of the intramolecular
charge-transfer process.
pH was measured with a Radiometer PHM 82 pH meter
equipped with a Radiometer Type B combined electrode.
Reported pKa are practical values obtained from pH measure-
ments and concentrations of acid and base, measured from the
absorption spectra. They are not corrected to obtain the true
thermodynamic values.
In this paper, we present a study of the ground- and excited-
state behavior of the monocations and dications of 1, 2, and 3,
in different acidified protic and aprotic solvents. We report the
room-temperature fluorescence of 1, 2, and 3 in acidified
UV−vis absorption spectra were recorded in a Varian Cary
3E spectrophotometer. Fluorescence excitation and emission
spectra were recorded in a Spex Fluorolog-2 FL340 E1 T1
spectrofluorometer, with correction for instrumental factors by
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means of a Rhodamine B quantum counter and correction files
supplied by the manufacturer. Fluorescence quantum yields
were measured using quinine sulfate (<3 × 10−5 mol dm−3) in
aqueous H2SO4 (0.5 mol dm−3) as standard (ϕ = 0.546).50,51
The estimated standard uncertainty of the fluorescence
quantum yields measurements was 5%. Fluorescence lifetimes
were determined by single-photon timing in an Edinburgh
Instruments FL-900 spectrometer equipped with a hydrogen-
filled nanosecond flashlamp and the reconvolution analysis
software supplied by the manufacturer.
Model equations were fitted to the experimental data by
means of a nonlinear weighted least-squares routine based on
the Marquardt algorithm. The reported uncertainties represent
the statistical standard deviations obtained in the fits.
3. RESULTS
3.1. Absorption Spectra of 1, 2, 3, 2-OMe, and 3-OMe
in Aqueous Solutions. The absorption spectra of 1 in
aqueous solutions under neutral and acidic conditions are
shown in Figure 1a. The absorption spectrum of 1 in neutral
medium showed an intense band at 31000 cm−1 and a weaker
band at ∼27000 cm−1. On decreasing the pH from neutral
solution, the absorption spectrum shifted to the red and its
molar absorption coefficient increased with respect to that
observed under neutral conditions. A new spectrum, peaking at
29320 cm−1, was observed at pH 3.02. A further decrease of pH
strongly shifted the absorption spectrum to the blue, the molar
absorption coefficient decreasing to less than half its value at
pH ∼ 3. The absorbance−pH data of 1 at 35710 cm−1 and at
27780 cm−1 are given in the inset of Figure 1a.
The absorption spectra obtained for 2 and the methoxy
derivative 2-OMe in aqueous solutions at different acidities
(Figure 1b,c) showed the same general features as those of 1.
The absorption spectrum recorded for 2-OMe at pH ∼ 10
showed, however, only one band similar in shape and position
to the intense absorption band obtained for 1 and 2 under
neutral conditions. The absorbance−pH curves were similar for
1, 2, and 2-OMe (insets of Figure 1).
The absorption spectra of 3 and the methoxy derivative 3-
OMe in aqueous solutions at different acidities are shown in
Figure 2. The absorption spectrum of 3 under neutral
conditions, peaking at 30500 cm−1, was very broad (Figure
2a). Upon acidifying the solution (pH ∼ 5), the spectrum
shifted to the red (maximum at 28740 cm−1) and the molar
absorption coefficient significantly increased. A further increase
of acidity strongly shifted the spectrum to the blue, and a new
band, peaking at 35460 cm−1, was obtained at [HClO4] ∼ 0.7
mol dm−3, its maximum molar absorption coefficient decreasing
to about half the value obtained for the absorption spectrum
recorded at pH ∼ 5. Furthermore, increasing the pH from
neutral conditions caused a blue shift of the spectrum, a new
band being obtained at pH > 10 (results not shown). On the
other hand, the absorption spectrum recorded for 3-OMe in
aqueous solutions under strongly ([HClO4] = 0.1 mol dm−3)
and mildly (pH ∼5) acidic conditions were very similar to
those obtained for 3. Upon increasing the pH from pH 5, the
absorption spectrum of 3-OMe strongly shifted to the blue, and
a new band (peaking at 32150 cm−1) was obtained at pH > 8,
this absorption spectrum being ∼1600 cm−1 blue-shifted with
respect to that measured for 3 at pH ∼ 8. The insets of Figure
2a,b show the absorbance−pH data for compounds 3 and 3-
OMe.
Figure 1. Absorption spectra of (a) 1, (b) 2, and (c) 2-OMe in
neutral, mildly acidic, and strongly acidic aqueous solution. The insets
show the pH dependence of the absorbance in aqueous solution (a)
for 1 at 27780 cm−1 (purple circle) and at 35710 cm−1 (gray circle);
(b) for 2 at 27030 cm−1 (purple circle) and at 31250 cm−1 (gray
circle); and (c) for 2-OMe at 27030 cm−1 (purple circle) and at 31250
cm−1 (gray circle).
3.2. Fluorescence Spectra, Quantum Yields, and
Lifetimes of 1, 2, 3, 2-OMe, 3-OMe, 5, and 6 in Various
Solvents under Strongly Acidic Conditions (Ground-
State Diprotonated Species). The fluorescence spectra of 1,
2, 2-OMe, 3, and 3-OMe in strongly acidified aqueous solution
and acetonitrile (where the ground-state species are the
dications DC, see later) are shown in Figure 3. The
fluorescence spectra of 1, 2, and 3 exhibited the same general
features, and the excitation spectra matched the first absorption
band measured under the same conditions. The emission
spectra of 2 and 3 in water showed only one band (at about
23000 cm−1 and 22000 cm−1, respectively) strongly red-shifted
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Figure 2. Absorption spectra of (a) 3 and (b) 3-OMe in neutral,
mildly acidic, and strongly acidic aqueous solution. The insets show
the pH dependence of the absorbance (a) for 3 at 28570 cm−1 (purple
circle) and at 34480 cm−1 (gray circle), and (b) for 3-OMe at 27030
cm−1 (purple circle) and at 33330 cm−1 (gray circle).
Figure 3. Normalized fluorescence excitation and emission spectra in
strongly acidified solutions of (a) 1 in acetonitrile (vexc = 31060 cm−1,
̃
v
= 27250 cm−1) and water (v = 31150 cm−1, vem = 22990 cm−1),
̃
̃
̃
em
exc
(b) 2 in acetonitrile (v = 30770 cm−1, vem = 27400 cm−1) and water
̃
̃
exc
(v = 30770 cm−1, v = 23260 cm−1), and 2-OMe in water (v
=
̃
̃
̃
exc
em
exc
30860 cm−1, v = 27170 cm−1), and (c) 3 in acetonitrile (vexc = 35650
̃
̃
em
cm−1, v = 27030 cm−1) and water (v = 35340 cm−1, v = 22200
with respect to its excitation spectrum. However, for 1 the main
emission band (at 23000 cm−1) was accompanied by a shoulder
at about 27000 cm−1, which overlapped its excitation spectrum.
In acidified acetonitrile, the emission spectra of 1, 2, and 3,
located at about 27000 cm−1, were very similar to those
recorded for 2-OMe and 3-OMe in acidified aqueous solution,
except for a weak emission observed at ∼22000 cm−1, not
detected for the methoxy derivatives. The fluorescence
quantum yields ΦDC of the species studied in this work in
different strongly acidified solvents are given in Table 1.
The fluorescence of 2-OMe exhibited monoexponential
decays both in aqueous solution and acetonitrile, with the same
lifetime (1.7 ns, Table 2). A monoexponential decay was also
observed for 2 in aqueous solution at 23260 cm−1, with a
lifetime of 3.8 ns. In acetonitrile, the fluorescence decay was
monoexponential at the main emission band (in the range
24390 cm−1 to 28570 cm−1) with a lifetime of 1.5 ns and
became biexponential at lower wavenumbers with lifetimes of
1.5 and 4.0 ns (at 23260 cm−1, see Table 2).
̃
̃
̃
em
exc
em
cm−1), and 3-OMe in water (v = 35460 cm−1, v = 27030 cm−1).
̃
̃
exc
em
The absorption spectra of 1, 2, and 3 in aqueous solution of the same
pH as that of the fluorescence spectra are also shown (gray band).
emission spectra of 1 were independent of the monitoring
wavenumbers both in acidified aqueous solution and
acetonitrile. The emission spectrum in aqueous solution of
pH 3.71 showed only one band, located at 25220 cm−1, which
overlapped its excitation spectrum. This spectrum, peaking at
28450 cm−1, coincided with the absorption spectrum of 1
measured under the same acidity conditions. The fluorescence
excitation spectrum of 1 measured in slightly acidified
acetonitrile overlapped its emission band and the emission
spectrum almost matched that observed in acidic aqueous
solution.
The fluorescence behavior observed for 2 (Figure 4b) was
very similar to that of 1 (Figure 4a). The excitation and
emission spectra of 2-OMe in acidified water were also
independent of the monitoring wavenumbers, and, except for a
small red shift, showed the same features as the excitation and
emission bands recorded for 2 in the same solvent (Figure 4b).
The fluorescence spectra of 3 and 3-OMe in slightly acidified
acetonitrile are shown in Figure 4c. For both compounds, the
excitation spectrum virtually matched the absorption spectrum
measured under the same acidity conditions, and the emission
3.3. Fluorescence Spectra, Quantum Yields, and
Lifetimes of 1, 2, 3, 2-OMe, 3-OMe, 4, 5, and 6 in
Various Solvents under Mildly Acidic Conditions
(Ground-State Monoprotonated Species). The fluores-
cence spectra of 1 in mildly acidified aqueous solution and
acetonitrile (where the ground-state species exist as mono-
cations, see later) are shown in Figure 4a. The excitation and
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Table 1. Fluorescence Quantum Yields ΦDC of the Dications of the Species Studied in this Work at 298 K in Various Strongly
Acidified Solvents, for Which the Viscosity and the Relative Dielectric Permittivity Are Shown
ΦDC
a
b
solvent
dioxane
εr
η
/cP
5
6
1
2
2-OMe
3
3-OMe
2.21
1.44
0.24
0.45
0.55
0.36
10.64
4.21
1.20
0.55
1.00
19.90
1412
0.42
0.38
diethyl ether
ethyl acetate
tetrahydrofuran
acetonitrile
1-octanol
4.20
0.42
0.27
0.13
0.42
0.25
0.23
0.28
0.18
0.39
0.21
0.19
6.02
0.37
0.55
0.38
0.50
0.43
0.43
0.46
0.46
0.41
0.40
7.58
0.26
0.14
0.24
35.94
10.30
16.56
24.55
32.66
78.30
37.70
42.50
0.54
0.32
0.23
0.60
2-butanol
ethanol
0.26
methanol
water
ethylene glycol
glycerol
a
b
Values at 298.15 K from ref 75. Values at 298.15 K from ref 76.
Table 2. Fluorescence Lifetimes τ1 and τ2 and Associated Fractional Intensities (in Square Brackets) of 2 and 2-OMe in
Acetonitrile and Water under Excitation of Dication DC in Strongly Acidic Conditions at 298 K; The Species to Which Each
Lifetime Was Assigned Are Also Shown
compound
v
/cm−1
v
/cm−1
em
τ1 /ns
τ2 /ns
χ2
̃
̃
exc
Strongly Acidified Acetonitrile
1.522 0.005 (DC*)
2 DC
30770
28570
1.038
1.153
1.092
1.018
1.124
30770
30770
31250
31250
24390
23260
27030
23810
1.587 0.005 (DC*)
a
1.5 [52%] (DC*)
4.05 0.04 [48%] (TC*)
2-OMe DC
1.744 0.003 (DC*)
1.755 0.003 (DC*)
Strongly Acidified Aqueous Solution
3.812 0.002 (TC*)
2 DC
30770
33330
33330
23260
25974
24390
1.072
1.022
1.092
2-OMe DC
1.679 0.004 (DC*)
1.714 0.004 (DC*)
a
Fixed value, obtained at emission wavenumbers where the decay is monoexponential. Fixing this value, a more precise value of τ2 is obtained at this
wavenumber.
spectrum overlapped its excitation spectrum. The fluorescence
emission bands of 3 and 3-OMe were much broader than the
emission bands recorded for 1, 2, and 2-OMe in the same
solvent. The emission spectrum of 3-OMe in acidified glycerol
was however narrower than those recorded for 3 and 3-OMe in
acetonitrile (Figure 4c).
The absorption and fluorescence spectra of the model
compounds 4, 5, and 6 in slightly acidified acetonitrile (so as to
have all these compounds as the monocations) are shown in
Figure 4d. For all compounds, the excitation spectrum matches
the first absorption band and the emission spectrum overlaps its
excitation spectrum. Also, a red shift of both the excitation and
emission spectra is observed on going from 4 to 5 and from 5
to 6.
The fluorescence quantum yields ΦMC of the monocationic
species studied in this work, obtained in different mildly
acidified solvents, are compiled in Table 3. It is observed that
the quantum yields of 1 were similar to those of the model
compound HBI, without the amino group. However, the values
measured for 2 were in some cases lower than those found for 1
and HBI, and the effect was clearly stronger for 2-OMe.
Furthermore, the quantum yields of 2 and 2-OMe markedly
increased with the solvent viscosity (especially for 2-OMe) and
decreased as the dielectric permittivity of the solvent increased.
The fluorescence quantum yields of 3 and 3-OMe were
generally lower than those measured for 2 and 2-OMe. The
fluorescence quantum yields of the monocations of model
compounds 4, 5, and 6, with no hydroxyl or methoxy groups,
decreased in the series 4 ≥ 5 ≫ 6, the values decreasing up to
58 times (in acetonitrile) on going from 5 to 6.
Table 4 shows the fluorescence decays of 1, 2, 2-OMe, and
3-OMe, and the model compounds 5 and 6, recorded in
slightly acidified acetonitrile and 1-octanol. In acetonitrile, the
decay was monoexponential for 1 and 5 (lifetimes around 1.3
ns), but it became biexponential for the other derivatives
(lifetimes around 1 and 0.1 ns). The intensity fraction
corresponding to the short lifetime increased as the emission
wavenumber decreased. For 3-OMe, the biexponential decay
obtained at 23530 cm−1 with lifetimes of 1.9 (5%) and 0.216 ns
(95%) became monoexponential at 21740 cm−1 with a lifetime
of 0.164 ns. In slightly acidified 1-octanol, the fluorescence
decay of 2 became monoexponential (lifetime 1.28 ns), whereas
2-OMe, 3-OMe, and 6 exhibited biexponential decays (Table
4), with a long lifetime around 1 ns and a shorter one in the
range 0.3 to 0.6 ns.
4. DISCUSSION
4.1. Ground-State Acid−Base Equilibria of 1, 2, 3, 2-
OMe, and 3-OMe in Aqueous Solution: First Protonation
Occurs at the Benzimidazole Nitrogen. In a previous
paper, we showed that 1, 2, and 3 in neutral aqueous solution
exhibit tautomeric equilibria between the normal form N and
F
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wavenumbers was performed for each of the compounds
investigated (insets of Figures 1 and 2), yielding the pKa values
listed in Table 5. We include also in this Table the pKa values
obtained by the same method for 4, 5, and 6, some of which
were already measured by other authors.52,53 Our results
indicate that the species involved in the ground-state acid−base
equilibria for all compounds with an amino group in neutral-to-
acid media are the neutral form, the monocation MC, and the
dication DC (see Scheme 1, where for 1, 2, and 3, species T in
equilibrium with N has been omitted for clarity). Protonation
of the neutral form to give the monocation can take place at the
benzimidazole N3 or at the amino N. In agreement with
previous studies on 6,52 we propose that the first protonation
occurs for all the species at the benzimidazole N3, based on the
following facts:
(1) The absorption spectra of the neutral form and the
monocation of 1 and 2 were red-shifted (stronger for 2
than for 1) with respect to those of HBI (Figure 5a,b)
and showed a higher molar absorption coefficient (Figure
1) than the neutral form and the monocation of HBI.34
This reveals a more extended π-electron conjugation for
the neutral and monoprotonated forms of 1 and 2 than
for those of HBI, which must be due to the involvement
of the nitrogen lone pair at the −NH2 or −NEt2 group in
the resonance system. The stronger electron-donating
ability of diethylaniline (Eox = 0.34 V)54 compared to that
of aniline (Eox = 0.54 V)54 causes a more extended π-
conjugation in 2 than in 1. As the absorption spectra of
the monocations of 2-OMe, 3, and 3-OMe were very
similar to those of 1 and 2 (Figures 1 and 2), we propose
that the neutral form of all these species protonates also
at the benzimidazole N3.
(2) The absorption spectra of the dications of 1 and 2, for
which the amino group is protonated (and therefore
unable to act as an electron donor), coincided in shape
and position with the absorption spectrum of the
monocation of HBI (Figure 5c), suggesting a similar
structure for the species involved.
(3) For both 1 and 2, the molar absorption coefficients of the
monocations were about 1.5 times higher than those of
the neutral forms. In contrast, no difference was observed
for HBI.34 This must be due to the presence in the
monocations of 1 and 2 of a positively charged
benzimidazolium ring (better electron acceptor than
the benzimidazole moiety of the neutral form), its charge
withdrawing ability increasing the electron conjugation
with the aminophenyl group.
Figure 4. Absorption spectra and normalized fluorescence excitation
and emission spectra of the monoprotonated forms of the indicated
species in mildly acidic solutions. (a) 1 in acetonitrile (v = 27930
̃
exc
cm−1, v = 25190 cm−1) and water at pH 3.71 (v = 28410 cm−1, v
̃
̃
̃
em
exc
em
= 25190 cm−1), (b) 2 in acetonitrile (v = 27400 cm−1, v = 25160
̃
̃
exc
em
cm−1) and water at pH 4.71 (v = 26820 cm−1, v = 24970 cm−1)
̃
̃
exc
em
and 2-OMe in acetonitrile (v = 26320 cm−1, vem = 24440 cm−1), (c)
̃
̃
exc
3 in acetonitrile (v = 28010 cm−1, vem = 23530 cm−1) and 3-OMe in
̃
̃
exc
acetonitrile (v = 28010 cm−1, v = 23150 cm−1) and glycerol (v
=
̃
̃
̃
exc
em
exc
The involvement of the amino nitrogen lone pair in the π
system is also responsible for the differences between the pKa
values of HBI and those of its amino derivatives. It is observed
in Table 5 that the pKa2 value increased in the order HBI < 1 <
2. This is explained by the fact that the amino group at C4′
causes an increase of the electron density (and therefore the
basicity) at the benzimidazole N3, stronger for −NEt2 than for
−NH2. The same effect explains the pKa increase for
monocation deprotonation in the series 4 < 5 < 6 (Table 5).
Besides, it is also observed that the pKa value corresponding to
protonation of the amino group was more than 2 units lower
for 1 and 2 (pKa1) than for aniline (pKa2 in Table 5). This must
be due to the fact that the electron density at the amino N is
much higher for aniline than for protonated 1 and 2, as the
23920 cm−1), and (d) 4 (v = 33330 cm−1, v = 27620 cm−1), 5 (v
̃
̃
̃
exc
em
exc
= 28570 cm−1, v = 25380 cm−1), and 6 (v = 26740 cm−1, v
=
̃
̃
̃
em
exc
em
24040 cm−1) in acetonitrile.
the tautomer T (see Chart 1), the last species being responsible
for the red-shifted absorption band appearing at ∼27000
cm−1.41 This equilibrium is impossible for 2-OMe and 3-OMe,
which exist only as normal form N and lack this absorption
band.
The absorption spectra of 1, 2, and 2-OMe (Figure 1), as
well as those of 3 and 3-OMe (Figure 2) in aqueous solutions
at different acidities indicate the existence of two acid−base
equilibria for all these compounds in the pH ranges studied. A
global analysis of the absorbance−pH data at different fixed
G
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Table 3. Fluorescence Quantum Yields ΦMC of the Monocations of the Species Studied in this Work at 298 K in Various Mildly
Acidified Solvents, for Which the Viscosity and the Relative Dielectric Permittivity Are Shown
ΦMC
a
b
solvent
εr
η
/cP
4
5
6
HBI
1
2
2-OMe
3
3-OMe
dioxane
2.21
1.44
0.24
0.45
0.55
0.36
0.92
1.99
10.64
4.21
1.20
0.55
1.00
19.90
1412
0.35
0.28
0.30
0.10
0.27
0.09
0.09
0.007
0.01
0.015
0.21
0.12
0.03
0.01
0.005
0.06
0.15
0.19
0.14
diethyl ether
ethyl acetate
tetrahydrofuran
acetonitrile
4.20
6.02
0.36
0.26
0.32
0.21
0.27
0.07
0.14
0.09
0.01
7.58
0.48
0.41
0.42
0.47
0.24
0.24
0.07
0.01
35.94
36.71
46.45
10.30
16.56
24.55
32.66
78.30
37.70
0.47
0.01
0.02
N,N-dimethylformamide
dimethyl sulfoxide
1-octanol
0.24
0.27
0.33
0.25
0.29
0.10
0.12
0.37
0.49
0.26
0.14
0.02
0.15
0.32
0.02
0.27
0.21
0.06
0.03
0.008
0.07
0.26
0.01
0.16
2-butanol
ethanol
0.47
0.42
0.52
0.11
0.03
methanol
water
ethylene glycol
glycerol
42.50
0.14
a
b
Values at 298.15 K from ref 75. Values at 298.15 K from ref 76.
Table 4. Fluorescence Lifetimes τ1 and τ2 and Associated Fractional Intensities (in Parentheses) of the Indicated Species in
Slightly Acidified Acetonitrile and 1-Octanol under Excitation of Monocation MC at 298 K
compound
v
/cm−1
v
/cm−1
em
τ1 /ns
τ2 /ns
χ2
̃
̃
exc
Slightly Acidified Acetonitrile
1.340 0.007
1 MC
2 MC
27030
24390
1.088
1.052
1.055
1.084
1.003
1.019
1.128
1.018
1.009
1.009
1.041
1.072
26320
26320
26320
26320
26320
26320
28170
28170
27030
26320
26320
25000
24390
23260
25320
24690
23810
23530
21740
25000
25000
23810
1.125 0.008 (74%)
1.112 0.007 (72%)
1.148 0.008 (66%)
1.16 0.04 (10%)
1.18 0.04 (6%)
0.22 0.02 (26%)
0.15 0.02 (28%)
0.17 0.01 (34%)
0.124 0.006 (90%)
0.121 0.005 (94%)
0.092 0.004 (95%)
0.216 0.008 (95%)
0.164 0.007
2-OMe MC
3-OMe MC
1.13 0.04 (5%)
1.9 0.1 (5%)
5 MC
6 MC
1.264 0.002
a
a
0.9 0.1 (29%)
0.08 0.01 (71%)
a
a
0.9 0.1 (13%)
0.08 0.01 (87%)
Slightly Acidified 1-Octanol
1.275 0.003
2 MC
25970
25970
25640
25640
27780
27780
26320
26320
26320
24390
22220
24100
22730
25320
22220
25000
23810
22470
1.024
1.123
1.001
1.084
1.040
1.186
1.185
1.022
1.039
1.294 0.003
2-OMe MC
3-OMe MC
6 MC
1.078 0.006 (86%)
1.098 0.008 (83%)
1.20 0.01 (55%)
1.22 0.01 (78%)
0.31 0.03 (14%)
0.37 0.04 (17%)
0.47 0.02 (45%)
0.54 0.05 (22%)
a
a
1.1 0.1 (51%)
0.6 0.1 (49%)
a
a
1.1 0.1 (44%)
0.6 0.1 (56%)
a
a
1.1 0.1 (45%)
0.6 0.1 (55%)
a
Global fit.
electron density is displaced toward the protonated benzimi-
dazole ring.
The experimental pKa values allow us to arrange the
substituent groups of the phenyl moiety according to their
electron-donor capacity to the benzimidazole unit. This is
depicted in Chart 2, together with the relative steric
interactions of the aromatic rings.
4.2. Excited-State Behavior of 1, 2, 3, 2-OMe, and 3-
OMe in Strongly Acidic Solutions: Photodissociation of
the Dication DC* at the Hydroxyl Group. The fluorescence
excitation spectrum of 1, 2, and 3 in strongly acidic aqueous
solution and acetonitrile (Figure 3) can be attributed to the
dication DC, as it matched for each compound the absorption
spectrum of DC recorded in the same acidity conditions.
Likewise, the fluorescence excitation spectrum of 2-OMe and
The basicity at benzimidazole N3 increases with OH
substitution at C2′, as shows the pKa increase on going from
4 to HBI and from 5 to 1 (pKa2 in Table 5). This effect is
stronger for the methoxy group, as evidenced by the higher
pKa2 values for the methoxy derivatives 2-OMe and 3-OMe
than for the hydroxy compounds 2 and 3 (Table 5). These
results indicate that the OH and OMe groups at the ortho
position of the phenyl ring increase the electron density at the
benzimidazole N, this effect being stronger for OMe than for
OH.
H
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Table 5. Acidity Constants Obtained for the Compounds
Studied in this Work and Some Simple Model Compounds
in Aqueous Solution at 298 K
compound
pKa1 DC ⇌ MC
pKa2 MC ⇌ N
1
2
1.50 0.01
2.29 0.01
2.34 0.02
3.25 0.01
2.85 0.06
6.11 0.02
6.31 0.02
6.74 0.02
6.44 0.03
6.60 0.01
2-OMe
3
3-OMe
a
HBI
5.48 0.09
4
4.95 0.05
5.95 0.02
6.11 0.02
5
1.48 0.01
1.50 0.01
6
b
benzimidazole
aniline
5.53
c
4.63
c
N,N-diethylaniline
5.15
a
b
c
Value from ref 34. Value from ref 77. Values from ref 76.
3-OMe in strongly acidic aqueous solution (Figure 3b,c) is
attributed to the dication, as it is very similar to the absorption
band of DC in the same solvent (Figures 1c and 2b). The
absorption and fluorescence excitation spectra shown in Figure
3 reveal a great similarity of the spectra for 1, 2, and 2-OMe,
and a blue shift of the spectra for 3 and 3-OMe with respect to
those of the other compounds. This behavior can be explained
by assuming that the methyl group at benzimidazole N1 in 3
and 3-OMe greatly enhances the steric hindrance between
adjacent ring systems, causing the dications of 3 and 3-OMe to
be nonplanar, DCnp. The loss of conjugation in DCnp would
result in a blue shift of its absorption and excitation spectrum
with respect to the planar form DC of 1, 2, and 2-OMe, and a
change in the shape of the spectrum. Similar results were
obtained for other methylated benzimidazole derivatives.55,56
The fluorescence spectra in strongly acidified solutions
shown in Figure 3 reveal a similar behavior for 1, 2, and 3 in
acetonitrile, and for 2-OMe and 3-OMe in water, only a greater
Stokes shift being detected for 3 and 3-OMe. A completely
different emission spectrum is nevertheless observed for 1, 2,
and 3 in water, which will be discussed later.
Under excitation of DC in acetonitrile for 1 and 2, and in
water for 2-OMe, a single emission band overlapping its
excitation spectrum was observed (Figure 3), which we assign
to DC*. For the dications of 3 in acetonitrile and 3-OMe in
water, the emission spectrum was located in the same position
as that detected for the dications of 1, 2, and 2-OMe, showing
only some loss of structure. This result contrasts with the
absorption and excitation spectra, which were clearly blue-
shifted for 3 and 3-OMe, as discussed above. This causes that
for 3 and 3-OMe the emission spectrum of the dication hardly
overlaps its excitation band, and the Stokes shift is very large.
Figure 5. Normalized absorption spectra in aqueous solution of (a)
the neutral forms, (b) the monocations of HBI, 1, and 2, and (c) the
dications of 1 and 2 and the monocation of HBI.
This indicates a great structural change from the ground to the
excited state for 3 and 3-OMe dication. As the emission
spectrum for these compounds appears in the same position as
those of the planar dications of 1, 2, and 2-OMe, we propose
that the ground-state nonplanar dication DCnp of 3 and 3-OMe
reaches a planar conformation in the excited state from which
fluorescence takes place. Similar behavior was found for related
systems.55,56
The fluorescence quantum yields of the dications of 1, 2, 3,
2-OMe, and 3-OMe (Table 1) were high and showed only a
weak dependence on the solvent properties. Similar results
were obtained for the model compounds 5 and 6 (Table 1).
Very different fluorescence behavior was observed for 1, 2,
and 3 on changing the solvent from strongly acidified
acetonitrile to water. In aqueous solution, excitation of the
dication led to a red-shifted emission spectrum, with maximum
Scheme 1. Acid−Base Equilibria of 1, 2, 2-OMe, 3, 3-OMe, 5, and 6 (R1 = H or CH3, R2 = H, OH, or OCH3, R3 = H, CH3, or
a
b
C2H5), Showing the Dication (DC), the Monocation (MC), and the Neutral Form (N)
a
b
For 3 and 3-OMe, DC has a nonplanar structure. For 1, 2, and 3, the neutral tautomer T exists in equilibrium with N in neutral aqueous solution.
N has a nonplanar structure for 3 and 3-OMe in aqueous solution (see ref 41).
I
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at 22000−23000 cm−1 and very large Stokes shift (Figure 3).
This indicates that the initially excited dication is converted in
the excited state to another species, whose fluorescence is
detected. This excited-state transformation does not take place
for the dications of the methoxy derivatives 2-OMe, and 3-
OMe (Figure 3b,c).
It is well-known that many phenol derivatives are stronger
acids in the excited state. The acidity increase upon excitation is
even stronger for cationic species like the monocations of
HBI,34 2-(3′-hydroxy-2′-pyridyl)benzimidazole,48 and 2-(1′-
hydroxy-2′-naphthyl)benzimidazole,19,49 protonated at the
benzimidazole N3. These species behave as strong photoacids,
which deprotonate upon excitation at the hydroxyl group,
totally in water and very efficiently in alcohols. We propose that
the dications of 1, 2, and 3 are also strong photoacids that
transfer their hydroxyl proton to water upon excitation to yield
the fluorescent tautomeric monocation TC*, protonated at the
benzimidazole N3 and at the amino group, and deprotonated at
the hydroxyl group (Scheme 2). This species is a tautomer of
dication of 3, DCnp, is followed by deprotonation and rotational
relaxation in the excited state to yield the planar TC* form with
intramolecular hydrogen bond O−···H−N (Scheme 3).
The dication of 1 seems to be a weaker photoacid than those
of 2 and 3, as the red-shifted emission of TC* is accompanied
by a weak shoulder at emission wavenumbers corresponding to
the undissociated dication (Figure 3a). This shoulder is not
detected in the fluorescence spectra of 2 and 3.
The emission spectrum of 1, 2, and 3 in acidified acetonitrile
shows a shoulder at lower wavenumbers that is not detected for
2-OMe and 3-OMe and is located at the same position as the
emission maximum of TC* in water. We suggest that this weak
emission corresponds to a small amount of TC* formed in
acetonitrile, this process being favored by minor amounts of
water in this solvent.
Fluorescence decay measurements performed for 2 and 2-
OMe (Table 2) in strongly acidified solvents support the
previous assignments. As expected, the fluorescence decay was
monoexponential for 2-OMe. The same lifetime (1.7 ns) is
obtained in acetonitrile and in water, which is assigned to 2-
OMe DC*. The monoexponential decay obtained for 2 in
strongly acidified aqueous solution led to a lifetime value of 3.8
ns, which is assigned to the species TC* formed by dissociation
of the excited dication. Monoexponential decay was also
obtained for 2 in acetonitrile at higher emission wavenumbers,
with a lifetime of 1.55 ns attributed to DC*. This value is
similar to that of 2-OMe DC* (1.7 ns). Biexponential decay
was detected for 2 in acetonitrile in the lower-wavenumber
region. The second lifetime (4.05 ns) was very similar to the
fluorescence lifetime of TC* in water (3.8 ns). This result
confirms the assignment of the lower-wavenumber shoulder in
the emission spectrum of 2 in acetonitrile (Figure 3b) to the
fluorescence of TC*.
Scheme 2. Excitation and Deactivation Pathways of the
a
Dications of 1 and 2 in Aqueous Solution
4.3. Excited-State Behavior of 1, 2, 3, 2-OMe, 3-OMe,
4, 5, and 6 in Mildly Acidic Solutions: Deactivation of
the Monocation MC* by Intramolecular Charge Trans-
fer. For both 1 and 2 in slightly acidic aqueous solution, the
fluorescence excitation spectrum matched the absorption
spectrum recorded in the same conditions (Figure 4a,b), and
therefore, it must be due to the main species present at this
acidity, the monocation MC protonated at the benzimidazole
N3 (Scheme 1). This means that excitation of MC gives rise to
a
Fluorescence of DC* in water detected only for 1.
the monocation MC present in the ground state. As the
spectrum of the red-shifted emission of TC* is similar for 1, 2,
and 3, we assume that TC* has the same planar structure for all
three species. This implies that excitation of the nonplanar
Scheme 3. Excitation and Deactivation Pathways of the Dication of 3 in Aqueous Solution
J
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a
Scheme 4. Excitation and Deactivation Pathways of the Monocation of Compounds 6, 2, 2-OMe, 3, and 3-OMe
a
MCc*t fluoresces slightly for 3 and 3-OMe.
the one-band emission spectrum, which was independent of the
excitation wavenumber, and overlapped its excitation spectrum.
These results indicate that MC* is the fluorescent species. The
fluorescence excitation and emission spectra of 1 and 2 in
slightly acidified acetonitrile were practically coincident with
the spectra measured in water, and the spectra of 2-OMe were
very similar to that of 2 (Figure 4a,b). These results led us to
conclude that the monocation MC is the only absorbing and
fluorescent species for 1, 2, and 2-OMe in slightly acidified
acetonitrile and water.
The above results indicate that the monocations of 1 and 2
do not dissociate in the excited state in water. This behavior is
clearly different from that showed by the monocations of the
related compounds HBI,34 2-(3′-hydroxy-2′-pyridyl)-
benzimidazole,48 and 2-(1′-hydroxy-2′-naphthyl)-
benzimidazole.19,49 For all these compounds, the excited
monocation is a strong photoacid that deprotonates at the
hydroxyl group to yield the neutral tautomer T*, totally in
water and very efficiently in short-chain alcohols. The fact that
the monocations of 1 and 2 do not deprotonate in the excited
state must be related to the presence in these compounds of a
−NH2 or a −NEt2 group. The charge donation of these groups
to the phenol ring must inhibit dissociation at the hydroxyl
group, as a consequence of the increase in the excited-state pKa
value of the OH group of 1 and 2 with respect to those of HBI
and related compounds lacking the amino group.
The absorption and fluorescence spectra of 3 and 3-OMe in
acidified acetonitrile (Figure 4c) showed the same general
features as those of 1, 2, and 2-OMe (Figure 4a,b). The
excitation spectrum matched the absorption spectrum of MC,
and the emission spectrum overlapped its excitation spectrum.
This indicates that the emission band of 3 and 3-OMe is also
due to MC*. However, it is seen that the emission spectra of 3
and 3-OMe were much broader than those recorded for 1, 2,
and 2-OMe (Figure 4a−c). This suggests that the fluorescence
of MC* is accompanied by the emission of a second species for
3 and 3-OMe. The identity of this species will be discussed
later.
In spite of the similarity between the fluorescence spectrum
of monoprotonated 1, 2, and 2-OMe, a close look at the
fluorescence quantum yields (ΦMC, Table 3) and lifetime values
(τ1 and τ2, Table 4) reveals important differences between 1
and its derivatives 2 and 2-OMe. It is observed in Table 3 that
both HBI and its amino derivative 1 showed monocation
fluorescence quantum yields in the range 0.24 to 0.36 in all the
solvents studied (except 1 in water, ΦMC = 0.10). However,
ΦMC values for 2 and 2-OMe (with a −NEt2 group at C4′)
were lower than for 1 and HBI in solvents of high polarity and
low viscosity, this effect being stronger for the methylated
derivative 2-OMe.
The fluorescence quantum yields of 3 monocation were
lower than that of 2 (Table 3), and the same occurs for 3-OMe
and 2-OMe. As can be seen in Table 3, ΦMC values were
relatively high for 3-OMe in very viscous solvents (1-octanol,
glycerol, ...), but they decreased as solvent viscosity decreased
and solvent polarity increased, showing the same behavior as 2
and 2-OMe.
To investigate the effect of the hydroxyl or methoxy groups
of 1, 2, 3, and derivatives on their fluorescence behavior, we
studied the model compounds 4, 5, and 6, with no hydroxyl or
methoxy substituents at C4′ (Chart 2). The absorption and
fluorescence spectra as well as the pKa values of these
compounds in aqueous solution have already been re-
ported.52,53,57 It has been shown that the first protonation of
4, 5, and 6 takes place, like for HBI and its amino and
diethylamino derivatives, at the benzimidazole N3 to give
monocation MC. For 4, 5, and 6, the absorption spectrum of
MC in acidified acetonitrile and the excitation and emission
spectra in the same solvent are shown in Figure 4d. We
conclude from these spectra that for the three compounds
excitation of MC leads to MC* fluorescence. However, whereas
MC* fluorescence quantum yields were high for 4 and 5 and
virtually independent of solvent (as for HBI and 1, see Table
3), the values for 6 drastically decreased with respect to those of
4 and 5. Moreover, ΦMC values for 6 strongly decreased as the
solvent polarity increased and solvent viscosity decreased.
The above results provide compelling evidence that the
monocations of 6, 2, 2-OMe, 3, and 3-OMe undergo in the
excited state a radiationless deactivation process that increases
its efficiency as the solvent polarity increases and the solvent
viscosity decreases. Moreover, the strong dependence of the
process with the solvent viscosity suggests that it involves a
large-amplitude motion. Also, as neither the monocations of
HBI and 1, nor those of 4 and 5, experience that radiationless
deactivation, we conclude that the presence of the −NEt2 or
−NMe2 substituent at C4′ strongly favors the occurrence of the
process. To explain these results, we propose that the
radiationless deactivation undergone by the monocations of
K
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2, 2-OMe, 3, 3-OMe, and 6 is an intramolecular charge
migration, from the diethyl- or dimethylaminophenyl ring
(donor) to the protonated benzimidazole moiety (acceptor),
connected to a large-amplitude rotational motion (TICT
process), to give a nonfluorescent charge-transfer intermediate
MCc*t (Scheme 4). Moreover, as species 1 does not undergo
that process, we can conclude that the aminohydroxyphenyl
ring is not a good enough electron donor. An analogous charge-
transfer process has been previously reported by us41 for the
neutral tautomer T* of compounds 2 and 3 from the
deprotonated diethylaminophenol ring to the protonated
benzimidazole to give a nonfluorescent charge-transfer
intermediate. A similar polarity- and viscosity-dependent
photoinduced intramolecular charge transfer coupled to an
interannular torsional motion has been proposed for related
systems as, for example, hydroxyphenylbenzoxazole and
- b e n z o t h i a z o l e , 4 5 [ ( d i m e t h y l a m i n o ) p h e n y l ] -
methylpyridinium,58 phenol-pyridinium cation,59 and hydrox-
yphenylimidazopyridines.60,61 Different theoretical and exper-
imental approaches have been used to elucidate the nature of
the intramolecular charge transfer processes in these and other
donor−acceptor organic molecules.40,62−66
Lifetime τ2 (the shorter times are at the resolution limit of our
equipment) tends to be shorter, and its intensity fraction
higher, for compounds with lower ΦMC values, indicating its
relation to the process MC* → MC*ct. In the more viscous
solvent 1-octanol, the process MC* → MC*ct should be less
efficient than in acetonitrile. In agreement with this, τ2 values
(Table 4) significantly increased on going from acetonitrile to
1-octanol for 2, 2-OMe, 3-OMe, and 6, and its intensity
fraction significantly decreased. For 2, the fluorescence decay in
1-octanol was monoexponential with a τ1 value (1.28 ns) very
similar to that of 1 and 5 in acetonitrile, indicating that the
charge-transfer process is not efficient for 2 in this solvent.
A possible explanation for the above behavior is that MC*
emission takes place from different multistep relaxation
structures. Under this assumption, the observed biexponential
fluorescence decay would be a simplification of a nonexponen-
tial decay. The complex decay kinetics observed for the
monocations here studied has also been found for other charge-
transfer molecular rotors, as for example, p-(9-anthryl)-N,N-
dimethylaniline,71 auramine,72,73 and thioflavin-T.74 Nonexpo-
nential and wavelength-dependent kinetics is a common
characteristic for this type of molecules, and several theoretical
models were proposed to explain the complex decay.71−74
Taking into account that the charge-transfer process
undergone by MC* involves a conformational motion, we
would expect the process to be more efficient for 3 and 3-OMe
than for 2 and 2-OMe, because the steric hindrance of the
methyl group at benzimidazole N1 in 3 and 3-OMe would
facilitate the conformational motion needed for the process to
take place. A close look at the data in Table 3 reveals that, as
expected, ΦMC values were lower for 3 and 3-OMe than for 2
and 2-OMe. However, the emission spectrum of MC* in
acetonitrile was for 3 and 3-OMe much broader than that
recorded for the monocations of 1, 2, and 2-OMe (Figure 4),
but it became narrower in very viscous solvents like glycerol
(Figure 4c). This suggests that, for the N-methyl derivatives,
the charge-transfer species might be fluorescent, leading to a
broad emission spectrum, comprising the fluorescence of MC*
and MCc*t. However, in very viscous solvents, like glycerol,
where conformational motion cannot take place, and therefore
the process MC* → MC*ct is inhibited, emission is virtually due
only to MC*. Emission from an intramolecular charge-transfer
state has been reported for the related compounds
dimethylaminophenylimidazopyridines67−69 and also for the
well-known dimethylaminobenzonitrile and related species.70
Additionally, it is observed in Table 3 that ΦMC values were
clearly lower for 2-OMe than for 2. This is also in agreement
with our interpretation of the results, because the methoxy
group in 2-OMe slightly improves the strength of the electron
donor, and therefore the charge-transfer efficiency is higher for
2-OMe than for 2. As deduced from the values in Table 3, this
effect is not significant for 3-OMe, with quantum yields similar
to 3, probably due to the fact that the steric hindrance of the
methyl group at benzimidazole N1 in both compounds strongly
facilitates the torsional motion connected to the charge transfer.
From the proposed mechanism (Scheme 4), monoexponen-
tial fluorescence decays would be expected for the monocations
of the investigated compounds. However, fluorescence decays
were biexponential for all the compounds except 1 and 5, and 2
in 1-octanol (see Table 4). For 2, 2-OMe, 3-OMe, and 6 in
acetonitrile, the value of lifetime τ1 (∼1.1 ns) was very similar
to that obtained for 1 and 5 (∼1.3 ns), which do not experience
the intramolecular charge-transfer process MC* → MC*ct, and
its contribution increased with the emission wavenumber.
5. CONCLUSIONS
We have shown that the first protonation of compounds 1, 2, 3,
2-OMe, and 3-OMe in the ground state takes place at the
benzimidazole N3 to give monocation MC. The second
protonation occurs at the amino group to yield dication DC
(nonplanar for 3 and 3-OMe). In acetonitrile, under excitation
of DC, fluorescence from DC* was observed for all these
compounds. In aqueous solution, the excited dication DC* of
1, 2, and 3 behaves as a strong photoacid, deprotonating
(totally for 2 and 3) at the hydroxyl group to yield the
fluorescent tautomeric monocation TC*. This species,
protonated at the imidazole and amino nitrogens and
deprotonated at the hydroxyl group does not exist in significant
amounts in the ground state. After deactivation, TC protonates
again to regenerate DC.
Unlike HBI and some derivatives, the excited monocations of
1, 2, and 3 did not deprotonate at the hydroxyl group. The
fluorescence from MC* was detected for 1, 2, and 3, and their
methoxy derivatives in all the solvents investigated. On the
other hand, whereas the monocation fluorescence quantum
yield of 1 was virtually independent of solvent, those of 2, 3, 2-
OMe, 3-OMe, and the simple model compound 6 showed a
viscosity- and polarity-dependent radiationless deactivation
associated with a large-amplitude conformational motion
undergone by MC*. We have shown that this rotational
motion is connected to an intramolecular charge transfer
(TICT) from the phenyl unit to the protonated benzimidazole
moiety, to give a charge-transfer structure MC*ct. Fluorescence
from this charge-transfer species could only be detected for 3
and 3-OMe. The efficiency of this process, not observed for 1,
4, and 5, increased with the strength of the electron-donor
group and with the steric hindrance of the methyl group at
benzimidazole N1.
AUTHOR INFORMATION
Corresponding Authors
*E-mail: carmen.rios@usc.es.
*E-mail: manuel.mosquera@usc.es.
*E-mail: flor.rodriguez.prieto@usc.es.
■
L
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Article
and Diffusion-Controlled Regimes. J. Am. Chem. Soc. 2007, 129,
5408−5418.
Notes
The authors declare no competing financial interest.
(18) Veiga-Gutier
́
rez, M.; Brenlla, A.; Carreira Blanco, C.; Fernan
́
dez,
ez
B.; Kovalenko, S. A.; Rodríguez-Prieto, F.; Mosquera, M.; Per
́
ACKNOWLEDGMENTS
Lustres, J. L. Dissociation of a Strong Acid in Neat Solvents: Diffusion
Is Observed after Reversible Proton Ejection Inside the Solvent Shell.
J. Phys. Chem. B 2013, 117, 14065−14078.
■
We are indebted to the European Regional Development Fund,
the Spanish Ministry of Economy and Competitiveness (Grant
CTQ2010-17835), and the Xunta de Galicia (Grants
GPC2013/052, CN2012/314, and EM2012/091) for financial
support of our work. S.R. and J.L.P.L. are thankful for a MEC-
(19) Brenlla, A.; Veiga Gutier
Rodríguez-Prieto, F.; Mosquera, M.; Per
́
rez, M.; Ríos Rodríguez, M. C.;
ez Lustres, J. L. Moderately
́
Strong Photoacid Dissociates in Alcohols with High Transient
Concentration of the Proton-Transfer Contact Pair. J. Phys. Chem.
Lett. 2014, 5, 989−994.
FPU fellowship and a “Ramon
́
y Cajal” contract, respectively.
(20) Mansueto, E. S.; Wight, C. A. Excited-State Proton-Transfer
Polymerization of Amorphous Formaldehyde. J. Am. Chem. Soc. 1989,
111, 1900−1901.
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