Excited-state intramolecular proton transfer in 2-(2′-tosylaminophenyl)benzimidazole

Article abstract of DOI:10.1021/jp0204915

The spectroscopic properties of 2-(2′-tosylaminophenyl)benzimidazole (TPBI) have been studied in a series of different solvents. As revealed by absorbance, steady-state, and time-resolved emission spectroscopy, the molecule undergoes fast excited-state in

Full text of DOI:10.1021/jp0204915

J. Phys. Chem. A 2002, 106, 7655-7663
7655
ARTICLES
Excited-State Intramolecular Proton Transfer in 2-(2′-Tosylaminophenyl)benzimidazole
Christoph J. Fahrni,* Maged M. Henary, and Donald G. VanDerveer
School of Chemistry of Biochemistry, Georgia Institute of Technology, 770 State Street, Atlanta, Georgia 30332
ReceiVed: February 21, 2002; In Final Form: April 30, 2002
The spectroscopic properties of 2-(2′-tosylaminophenyl)benzimidazole (TPBI) have been studied in a series
of different solvents. As revealed by absorbance, steady-state, and time-resolved emission spectroscopy, the
molecule undergoes fast excited-state intramolecular proton transfer (ESIPT) to yield emission of the
corresponding tautomeric species with a large quantum yield (0.5). The fluorescence emission shows
monoexponential decay kinetics (τ ) 4.5 ns) regardless of the nature of the solvent. The ground-state
equilibrium is dominated by a single rotamer and small amounts of the deprotonated anion. The X-ray structure
of TPBI shows a substantial out-of-plane twist along the aryl-benzimidazole bond axis, which is presumably
due to π-stacking interactions between the tosylamide and benzimidazole rings. Ab initio calculations suggest
a different structure in the gas phase without π-stacking interactions and a substantially reduced twist angle.
A large energy barrier for interconversion of the cis- and trans-rotamers in the ground and excited state has
been predicted on the basis of DFT calculations, which is in agreement with all experimental data. The ground-
state equilibrium and ESIPT process of TPBI are essentially unaffected by the nature of the solvent, which
is of particular interest to sensing applications in cell biology.
Introduction
cis-enol form Ec is able to undergo fast proton transfer in the
excited state, resulting in a highly Stokes’ shifted emission of
Excited-state intramolecular proton transfer can be observed
in a wide range of molecules and has led to various applications
2
6
the keto tautomer (K*). The observed normal emission bands
at higher energy are due to all other species present such as the
trans-rotamer Et, the phenolate anion, and the open cis-enol
form, in which the OH group is hydrogen bonded to a solvent
1
,2
3-5
in the form of laser dyes, high-energy radiation detectors,
UV photostabilizers,6,7 or fluorescent probes.8-10 In the case of
the adiabatic, barrierless excited-state intramolecular proton
transfer (ESIPT), a covalently attached proton, typically of a
hydroxyl or amino group, in the electronically excited state
migrates to a neighboring hydrogen-bonded atom less than 2 Å
away. The phototautomer formed emits light and thermally
equilibrates back to the ground state with the proton bound to
its original atom. This process can proceed extremely fast and
several studies have found subpicosecond reaction rates for a
17
molecule rather than to the imidazole nitrogen. The equilibrium
position is strongly influenced by the polarity of the solvent,
which in turn determines the ratio of normal and ESIPT emission
intensities. For example, in aqueous solution, the high solvent
polarity combined with the capacity to form hydrogen bonds
interferes with the ESIPT process such that only a weak
tautomer emission is observed and the normal emission con-
2
7
variety of molecules.1
is more stable in the excited state but not the ground state, the
observed Stokes’ shift of the fluorescence emission is unusually
large.14
1-13
stitutes the major band. This property has been favorably
applied to probing of the microenvironment of large molecules,
as described for the interaction of HPBI with a protein.8
Because the formed phototautomer
,9
The emission of ESIPT molecules is also responsive toward
metal coordination, a property that is of interest for the
Among the compound classes exhibiting ESIPT, the photo-
physics of benzazole derivatives has been investigated ex-
27
development of cation sensitive ratiometric fluorescent probes.
1
5-25
tensively.
zothiazole (HPBT), -benzoxazole (HPBO), and -benzimidazole
HPBI) revealed dual emission in polar solvents,17 which has
Several studies on 2-(2′-hydroxyphenyl)ben-
For such applications, the strong environmental dependence of
the emission is undesirable because the probe should only reflect
changes in the metal concentration. This is particularly the case
for biological applications, in which the probe is exposed to a
large range of environments with different polarities, including
the cytoplasm, intracellular compartments, or partitioning into
membrane bilayers.
(
been attributed to the presence of different species in the ground
and excited state. For example, the ground-state equilibrium of
HPBI in polar media includes the cis- and trans-rotamers of
the enol form (Ec and Et) as well as the keto tautomer (K)
(Scheme 1). In aqueous solution or polar protic solvents the
Because the polarity responsiveness of HPBI is due to the
presence of several species in the ground-state equilibrium, the
problem could be eliminated with a molecule that exists only
as a single rotamer which can undergo ESIPT. Furthermore,
the efficiency of the ESIPT process is influenced not only by
equilibrium is further complicated by the pH-dependent forma-
tion of the phenolate anion. It has been shown that only the
*
To whom correspondence should be addressed. Phone: 404-385-1164/
fax: 404-894-2295. E-mail address: fahrni@chemistry.gatech.edu.
1
0.1021/jp0204915 CCC: $22.00 © 2002 American Chemical Society
Published on Web 07/31/2002

7656 J. Phys. Chem. A, Vol. 106, No. 34, 2002
Fahrni et al.
SCHEME 1
the solvent polarity but also by the pK value of the proton, which
is shifted in the excited state. With increasing pK, the thermo-
dynamics of the excited-state proton transfer become less
by flash chromatography on silica gel (EtOAc-hexane 1:2 f
1:1) providing 625 mg of the sulfonamide as a colorless solid
(1.72 mmol, 72%).
2
8-30
1
favorable, resulting in an increased normal emission band.
Mp: 164-165 °C. R : 0.3 (1:2 hexane:EtOAc). H NMR
f
Therefore, the pK value of the probe should be as low as
possible, but still sufficiently high to guarantee protonation of
the ground-state species at the pH at which the probe is used.
In this work, we have studied the excited-state intramolecular
proton-transfer reaction of 2-(2′-tosylaminophenyl)benzimida-
zole (TPBI, 2), which contains a sulfonamide group as the
proton donor. In contrast to HPBI, the cis- and trans-rotamers
of TPBI (the “enamine form” Ec and Et) are expected to be
very different in energy (Scheme 1). A simple rotation around
the carbon-oxygen bond is sufficient to position the lone pair
for the formation of the intramolecular hydrogen bond in the
trans-rotamer of HPBI (1). The analogous structure for TPBI
requires rehybridization of the sulfonamide nitrogen, which is
accompanied by the loss of resonance stabilization energy
between the nitrogen lone pair and the sulfone moiety.
Conclusively, the cis-rotamer Ec is substantially favored ther-
modynamically over the trans-rotamer Et, and therefore the
ground-state equilibrium is expected to be dominated by a single
rotamer (Ec). Because the pK value of aryl-substituted sulfon-
amides are comparable to their phenol analogues, the ESIPT
process in TPBI is expected to be similarly efficient as for HPBI.
(CDCl , 300 MHz): δ 2.25 (s, 3H), 6.98-7.08 (m, 3H), 7.18-
3
7.32 (m, 3H), 7.30 (m, 2H), 7.58-7.72 (m, 6H), 9.85 (s, broad,
+
1H), 13.05 (s, broad, 1H). MS (70 eV): 363 (M , 36), 299
(20), 208 (100), 91 (26), 65 (25). EI-HRMS, m/e: calculated
+
for (M ) C H N O S 363.10415, found 363.10279.
2
0
17
3
2
(b) 2-(2′-Mesylaminophenyl)benzimidazole (3). Prepared as
described for compound 2. Mp: 213-215 °C. Rf: 0.3 (1:2
1
hexane:EtOAc). H NMR (DMSO-d6, 300 MHz): δ 3.17 (s,
3
7
2
H), 7.27-7.35 (m, 3H), 7.54 (td, J ) 7.7, 1.1 Hz, 1H), 7.62-
.72 (m, 3H), 8.18 (dd, J ) 8.24, 1.1 Hz, 1H), 13.09 (s, broad,
+
H). MS (70 eV): 287 (M , 45). 208 (100), 181 (15), 118 (10),
+
92 (9), 65 (13). EI-HRMS, m/e: calculated for (M ) C14H13N3O2S
2
87.07285, found 287.07237.
Both compounds were purified by semipreparative reversed-
phase HPLC (Varian ProStar system with UV detector), 10 mm
RP 18 column (acetonitrile-water, gradient 30% f 2% water),
eliminating minor impurities (<1%) that were not detectable
by TLC or NMR.
Steady-State Absorption and Fluorescence Spectroscopy.
All sample stock solutions and buffer solutions were filtered
through 0.2 µm Teflon membrane filters to remove interfering
dust particles or fibers. UV-vis absorption spectra were
recorded at 25 °C using a Varian Cary Bio50 UV-vis
spectrometer with a constant-temperature accessory. Steady-
state emission and excitation spectra were recorded with a PTI
fluorometer and FELIX software. For all measurements the path
length was 1 cm with a cell volume of 3.0 mL. All fluorescence
spectra have been corrected for the spectral response of the
detection system (emission correction file provided by instru-
ment manufacturer) and for the spectral irradiance of the
excitation channel (via calibrated photodiode). Quantum yields
were determined using quinine sulfate dihydrate in 1 N H2SO4
Experimental Section
Materials and Reagents. 2-(2′-Aminophenyl)benzimidazole
(Alfa-Aesar, 98%), p-toluenesulfonyl chloride (Aldrich, 98%),
methanesulfonyl chloride (Aldrich, 98%). NMR: δ in ppm vs
1
SiMe4 (0 ppm, H, 300 MHz). MS: selected peaks, m/z. Melting
points are uncorrected. Flash chromatography (FC): Merck
silica gel (230-400 mesh). TLC: 0.25 mm, Merck silica gel
60 F254, visualizing at 254 nm or with 2% KMnO4 solution.
All solvents used for absorption and fluorescence measurements
were of spectroscopic grade.
Synthesis. (a) 2-(2′-Tosylaminophenyl)benzimidazole (2).
To a solution of 2-(2′-aminophenyl)benzimidazole (500 mg, 2.39
mmol) in pyridine (7 mL) was added p-toluenesulfonyl chloride
3
1
as fluorescence standard (Φf ) 0.54 ( 0.05).
Fluorescence Lifetime Measurements. Fluorescence lifetime
data were acquired with a Photon Technology International (PTI)
fluorescence lifetime instrument. The system uses a nitrogen
laser (model GL-3300) as the excitation source combined with
a stroboscopic detector. All samples were excited at 337 nm
with a repetition rate of 10 Hz (pulse width 3-4 ns), and the
fluorescence signal was analyzed after passing through a
monochromator set at the peak emission of the corresponding
(
478 mg, 2.51 mmol) at room temperature, and the resulting
mixture was stirred for 6 h. The mixture was poured into H2O
10 mL), neutralized with HCl (1M), and extracted with ethyl
(
acetate (3 × 30 mL). The combined organic phase was washed
with H2O (2 × 10 mL), dried with MgSO4, filtered, and
concentrated in vacuo, affording a brown oil, which was purified

Intramolecular Proton Transfer in TSPBI
J. Phys. Chem. A, Vol. 106, No. 34, 2002 7657
TABLE 1: Crystallographic Data for
2
-(2′-Tosylaminophenyl)benzimidazole (2)
formula
20 17 3 2
C H N O S
363.43
colorless
molecular weight
color, habit
crystal size (mm)
crystal system
space group
a (Å)
1.02 × 0.07 × 0.07
monoclinic
P2(1)/n
7.3393(7)
17.8290(17)
13.7468(13)
99.842(2)
1772.3(3)
4
b (Å)
c (Å)
â (deg)
3
V (Å )
Z
3
calculated density (g/cm )
1.362
F(000)
temp (K)
760
198(2)
2
θ range (deg)
3.78-50.20
no. of reflns collected
no. of indep rflns
final R indices (obs data) (%): R, R
R indices (all data) (%): R, R
goodness of fit on F
3149
1980
a
b
w
3.95, 9.02
7.40, 10.06
0.932
a
b
w
2
Figure 1. ORTEP plot (50% probability) and atom numbering scheme
for the X-ray structure of 2-(2′-tosylaminophenyl)benzimidazole (2).
a
b
2
|2)1/2.
R ) ∑||F
o
| - |F
c
||/∑|F
o
|.
R
w
) (∑w(|F
o
| - |F
c
|) /∑w|F
o
TABLE 2: Selected Experimental and Calculated Structural
sample. The data were analyzed with the TimeMaster Pro
software package provided with the spectrometer.
Data for 2-(2′-Tosylaminophenyl)benzimidazole (2)
rotamer tautomer
Quantum Chemical Calculations. All calculations were
carried out with the Q-Chem electronic structure calculation suite
of programs.32 The geometries of the ground-state structures
were optimized by the density functional method using the
hybrid B3LYP functional with the split-valence polarized
parametera
X-ray struct
E
b
c
c
1
I* (S )
N(1)-C(7) (Å)
1.324(2)
1.362(3)
1.444(3)
1.465(3)
1.397(3)
1.379(3)
1.378(3)
1.385(4)
1.374(3)
1.389(3)
106.59(9)
1.324
1.383
1.408
1.465
1.424
1.404
1.390
1.397
1.388
1.407
106.048
1.326
1.348
1.339
1.438
1.429
1.427
1.361
1.410
1.358
1.414
106.525
126.525
107.326
126.496
120.309
N(2)-C(7) (Å)
N(3)-C(1) (Å)
C(2)-C(7) (Å)
6
-31G* (6-31G(d)) basis set. Excited-state geometries were
C(1)-C(2) (Å)
optimized in the first excited singlet state (S1) using configu-
C(1)-C(6) (Å)
3
3
ration interaction with all singly excited determinants (CIS)
C(5)-C(6) (Å)
C(4)-C(5) (Å)
and the 6-31G* basis set. To obtain estimates of the vertical
electronic excitation energies that include some account of
electron correlation, time-dependent density functional theory
C(3)-C(4) (Å)
C(2)-C(3) (Å)
C(14)-S(1)-N(3) (deg)
S(1)-N(3)-C(1) (deg)
N(1)-C(7)-N(2) (deg)
N(1)-C(7)-C(2) (deg)
C(1)-C(2)-C(7) (deg)
C(2)-C(1)-N(3) (deg)
34
(
TD-DFT) calculations with the B3LYP functional and 6-31G*
116.11(13) 124.816
112.41(18) 111.332
124.9(2)
120.8(2)
basis set were performed. B3-LYP corresponds to the combina-
35
125.796
121.735
tion of Becke’s three parameter exchange functional (B3) with
3
6
a Lee-Yang-Parr fit for the correlation functional (LYP).
120.40(19) 120.091
117.471
Transition energies were also calculated on the basis of the
semiempirical ZINDO method37 using configuration interactions
with an active space of 15 occupied and 15 unoccupied
molecular orbitals. The calculation of ground-state potential
curves is based on relaxed geometries with the corresponding
constraints of dihedral angles (for rotations) or bond lengths
C(1)-C(2)-C(7)-N(1) (deg) -28.637
-14.425 -0.633
a
Atom numbering scheme shown in Figure 1. b Geometry optimized
c
ground-state structure of the cis-rotamer E (density functional method
with B3LYP functional and 6-31G* basis set). c Geometry optimized
structure of the sulfonimino tautomer I in the lowest excited singlet
state S (Hartree-Fock method with configuration interaction using
1
(reaction coordinate). Excited-state potential curves were com-
all singly excited determinants (CIS) and 6-31G* basis set).
puted with the ground-state geometries via TD-DFT calculations
as described above (B3LYP/6-31G* basis set).
X-ray Structure Analysis. Crystals of 2 suitable for X-ray
structural analysis were grown from dichloromethane solution
by isothermal distillation with hexane over the period of 1 week.
A summary of the crystallographic details is given in Table
ride in anhydrous pyridine provided sulfonamide 2 in 72% yield.
The compound was crystallized from dichloromethane/hexane
and characterized via X-ray analysis. An ORTEP plot of the
molecule is shown in Figure 1, and all relevant geometrical
parameters are compiled in Table 2. The most striking feature
of the structure is the parallel orientation of the tosyl and
benzimidazole rings, leading to significant intramolecular
π-stacking interactions. The two planes deviate only by 6.6°
from a parallel orientation with a distance of approximately 3.3
Å. The unusual geometry is best revealed with a projection along
the C2-C7 bond axis (Figure 2), which shows a dihedral angle
of 29° between the benzimidazole ring and the attached phenyl
ring. The space filling representation in Figure 2 illustrates the
close contacts of the two aromatic rings. The substantial out-
of-plane twist of the aryl-benzimidazole unit is presumably a
result of the π-stacking interaction, which is most effective with
a planar orientation of the aryl rings. This assumption is further
supported by the geometry reported for 2-(2′-tosylamino-5′-
1. The X-ray data were collected on a Siemens SMART 1K
CCD diffractometer with graphite-monochromated Mo KR
radiation (λ ) 0.710 73 Å). The programs SADABS (Sheld-
3
8
39
rick) and SAINT 6.22 (Bruker) were used for absorption
corrections. The structure was solved by direct methods and
refined by least-squares calculations with the SHELXTL 5.10
3
9
software package. The hydrogen atoms were added by using
ideal geometries with a fixed C-H bond distance of 0.96 Å.
Results and Discussion
1
. Structural Studies. X-ray Crystallography. Reaction of
2-(2′-aminophenyl)benzimidazole with p-toluenesulfonyl chlo-

7658 J. Phys. Chem. A, Vol. 106, No. 34, 2002
Fahrni et al.
Figure 2. Molecular geometry of 2: (a) projection along the C2-C7
bond axis; (b) space-filling model showing the π-stacking interaction
between the toluene and benzimidazole rings.
Figure 4. Geometrically relaxed potential energy curves for the ground
(S
0
1
) and lowest excited singlet state (S ) of 2 as a function of the
dihedral angle Φ ) C1-C2-C7-N1.
was calculated by DFT with the B3LYP hybrid functional (6-
1G* basis set) by successive rotation around the C2-C7 bond
3
axis (Figure 4). As expected, the minimum energy conformation
with a dihedral angle of -14.4° is substantially more stable
than any other rotamer. The calculation revealed only one
additional local minimum with significant stability, which
corresponds to the conformer with a dihedral angle of 180°
Figure 3. Geometry optimized ground-state structures (B3LYP/6-
-1
(Figure 3B). This structure is 22.2 kJ mol less stable compared
3
1G*) for two rotamer conformations of 2: (a) most stable geometry;
to the minimum energy conformation, which can mostly be
attributed to the loss of resonance stabilization energy upon
pyramidalization of the sulfonamide nitrogen atom. The esti-
mated activation energy for the interconversion between the two
(
b) geometry with a local energy minimum and a dihedral angle (C1-
C2-C7-N1) of 180°.
methylphenyl)benzoxazole, in which the two aryl rings are
4
0
-1
essentially coplanar with a small dihedral angle of 2.5°.
conformers is 36.9 kJ mol . On the basis of the Maxwell-
Additional geometric constraints induced by the π-stacking
interaction can be observed along the sulfonamide backbone.
Whereas the bond angle C14-S1-N3 of 106° is in the typical
range for aromatic sulfonamides, the S1-N3-C1 angle of 116°
is considerably more acute than 124-125°, which is typically
found for similar compounds.4
Boltzmann distribution function, the populations of the cis- and
trans-rotamers at 298 K in the gas phase can be calculated to
be 0.99987/0.00013. Even though the absolute energies esti-
mated for the gas-phase structures might vary in solution due
to dipolar solvent stabilization, the stability difference between
the cis- and trans-isomers is considerable and is expected to
simplify the ground-state equilibrium.
2. Photophysical Studies. Absorption Spectra. The absorption
spectra of compound 2 were measured in different solvents
(Figure 5). Regardless of the solvent polarity, all spectra show
a distinct vibrational structure, which is indicative of a consider-
ably rigid molecular framework. Presumably, the two aromatic
rings of the phenyl-benzimidazole unit are in a coplanar
orientation, which is effectively stabilized by the strong in-
tramolecular hydrogen bond between the sulfonamide NH group
and the imidazole nitrogen.
1,42
Ab Initio Calculations. The observed π-stacking interaction
cannot necessarily be translated to the solution phase (or gas
phase) and could be due to a packing effect in the solid state.
Starting with the coordinates of the X-ray structure, a full DFT
geometry optimization with the B3LYP hybrid functional and
a 6-31G* basis set was performed. The resulting calculated gas-
phase structure revealed a more relaxed geometry without the
π-stacking interaction of the two aryl units (Figure 3A). A
compilation of the most important geometrical parameters is
given in Table 2. In general, the bond lengths and angles of the
aromatic rings are in very good agreement with the data obtained
from the X-ray structure. The considerably wider angle of 124.8°
for S1-N3-C1 reflects the more relaxed geometry that is now
The peak absorption wavelength undergoes only a slight
hypsochromic shift from 319 to 314 nm with increasing solvent
polarity (Table 3). The close similarity of all spectra indicates
that the ground-state structure of 2 must be very similar in all
solvents. Even in protic and very polar solvents such as MeOH
or EtOH, no significant changes were observed that might imply
the presence of an additional ground-state conformation or
rotamer. Apparently, the cis-rotamer Ec is the most stable
structure not only in the gas phase, as suggested by the DFT
prediction, but also in solution phase. The molar absorption
within the typical range of arenesulfonamides based on pub-
lished X-ray structures.4
1,42
In addition, the intersecting ben-
zimidazole and phenyl ring planes are substantially less twisted,
as reflected by the smaller dihedral angle of -14.4° for C1-
C2-C7-N1. Conclusively, the π-stacking interaction might
only be important in the solid state, and not in the gas or solution
phase.
-
1
-1
The topology of the potential energy surface in the ground
state has important implications for the emission properties of
the molecule. Because only the cis-rotamer Ec undergoes ESIPT,
any other rotamer that is present in the ground-state equilibrium
will decrease the population of Ec and therefore also decrease
the efficiency of the ESIPT process. Whereas in the case of
HPBI, several rotational isomers are in thermal equilibrium in
coefficients range between 14 000 to 20 000 cm M , which
is in the typical range for allowed π-π* transitions. The nature
of the S0 f S1 electronic transition was further confirmed by
quantum chemical calculations (vide infra). Another interesting
feature is the weak but significant absorption band above 350
nm, which was only observed in polar protic solvents as well
as in acetonitrile (Figure 5a). Mosquera et al. observed a similar
weak band in the absorption spectra for HPBI (1) and suggested
the presence of the keto-tautomer K in the ground-state
17
the ground state, the sulfonamide analogue TPBI 2 is expected
to show significant differences for the stability of the cis- and
trans-rotamers (vide supra). A relaxed potential energy surface
1
7
equilibrium. Another possibility might include the presence

Intramolecular Proton Transfer in TSPBI
J. Phys. Chem. A, Vol. 106, No. 34, 2002 7659
Figure 6. Normalized fluorescence emission spectra for 2-(2′-
tosylaminophenyl)benzimidazole (2) in various solvents at 298 K.
red-shifted absorption band with a maximum at 340 nm (Figure
5
a). The weak low-energy band observed in neutral ethanol
matches well with this absorption band and therefore is
consistent with the presence of small amounts of deprotonated
anion. Further evidence for this interpretation was found by the
steady-state fluorescence data, as illustrated below.
Fluorescence Steady-State Spectra. The fluorescence spectral
data of TPBI (2) were measured in the same solvent set used
for the absorption spectra and are compiled in Table 4. In all
solvents, a strongly Stokes’ shifted emission band was observed,
which can presumably be attributed to the sulfonimino tautomer
I* (Scheme 1). Initial measurements showed a weak emission
band at higher energy in most solvents, which we interpreted
with the presence of the trans-rotamer Et. Nevertheless, as
quantum chemical calculations suggested, the ground-state
equilibrium should be exclusively dominated by the cis-rotamer
Ec, and therefore the normal emission band was a surprising
observation. Careful analysis of the compound by analytical
reversed-phase HPLC revealed a small amount of impurity
Figure 5. UV-vis absorption spectra for 2-(2′-tosylaminophenyl)benz-
imidazole (2) in various solvents at 298 K.
TABLE 3: UV-vis Spectral Data (Absorption Maxima and
Extinction Coefficients) for
2
-(2′-Tosylaminophenyl)benzimidazole (2) in Various
Solvents at 298 K
λ
max
ꢀ
max
(M- cm-1
λ
max
ꢀ
max
1
(M cm-1
)
-1
solvent
(nm)
)
(nm)
cyclohexane (sat)
dioxane
300
300
299
300
299
299
300
302
300
299
302
318
319
318
318
317
315
318
316
315
314
341
15900
16600
15400
18400
16900
15600
12000
13300
16900
20700
17600
17800
15400
19700
17400
17000
12300
13900
17900
14100
diethyl ether
dichloromethane
ethyl acetate
acetonitrile
tetrahydrofuran
n-butanol
ethanol
methanol
EtOH/NaOEt
(<1%), which we were not able to detect by thin-layer
chromatography nor in the proton NMR spectrum. Rigorous
purification of a sample by semipreparative reversed-phase
HPLC yielded very pure material. As shown in Figure 6, the
spectra of the purified sample exhibit a single, clean emission
band in all solvents and are lacking the normal emission at
higher energy as observed for HPBI derivatives.1
7,27
Most
importantly, the normalized emission spectra were invariable
of the excitation energy in any given solvent. This further
supports the presence of a single species that is responsible for
the observed emission band. Furthermore, the long wavelength
tail in the absorption spectra in polar solvents was still found
with the rigorously purified sample and is therefore not due to
impurities. These results contrast the data reported for 2-(2′-
of small amounts of deprotonated ground-state anion, as
suggested by Douhal et al. for 2-(2′-hydroxyphenyl)imidazole
and 2-(2′-hydroxyphenyl)benzimidazole (1).43 In case of HPBI
the latter possibility was excluded on the basis of the absorption
spectrum of the deprotonated dye, which does not show an
absorption band in the low-energy wavelength region.17 Addition
of sodium ethoxide to a solution of TPBI in ethanol yielded a
30
acetamidophenyl)benzimidazole and 2-(2′-benzamidophenyl)-
TABLE 4: Fluorescence Spectral Data for 2-(2′-Tosylaminophenyl)benzimidazole (2) in Various Solvents at 298 K
excitation
emission
quantum
lifetimeb
τ (ns)
a
ø2 c
(10 s-1)d
8
nr (10 s-1)e
8
solvent
λ
max (nm)
λ
max (nm)
yield Φ
f
k
f
k
cyclohexane
dioxane
319
319
317
317
316
314
318
315
314
313
507
492
485
489
490
480
490
470
470
470
0.53
0.53
0.58
0.59
0.49
0.49
0.57
0.54
0.55
0.58
5.67
6.42
6.23
6.40
5.88
6.04
6.03
5.68
5.84
5.59
1.25
1.26
1.26
0.83
1.02
1.14
1.02
1.04
1.01
1.07
0.93
0.83
0.93
0.92
0.83
0.81
0.95
0.95
0.94
1.04
0.83
0.73
0.67
0.64
0.87
0.84
0.71
0.81
0.77
0.75
diethyl ether
dichloromethane
ethyl acetate
acetonitrile
tetrahydrofuran
n-butanol
ethanol
methanol
a
as reference. b Lifetime for monoexponential decay model. c Goodness of fit parameter for monoexponential
) Φ )/τ).
/τ). e Nonradiative deactivation rate constant (knr ) (1 - Φ
Quinine sulfate in 1 N H
2
SO
4
d
curve fit. Radiative deactivation rate constant (k
f
f
f

7660 J. Phys. Chem. A, Vol. 106, No. 34, 2002
Fahrni et al.
Figure 8. Normalized fluorescence excitation (a) and emission (b)
spectra for 2-(2′-tosylaminophenyl)benzimidazole (2) in neutral and
basic (EtOH/NaOEt) ethanol at 298 K. The indicated wavelengths refer
to the emission or excitation wavelength at which the spectra were
recorded.
Figure 7. Emission spectra for 2-(2′-tosylaminophenyl)benzimidazole
2) and 2-(2′-mesylaminophenyl)benzimidazole (3) in solution (dichlo-
romethane) and solid state at 298 K.
(
benzimidazole,29 both of which exhibit dual fluorescence
emission in polar solvents.
emission spectra of 2 and 3 in the solid-state both exhibit a
hypsochromic shift, but are not identical (Figure 7). The
emission maximum of 2 appears at a longer wavelength with a
shift of 10 nm compared to 3. This indeed would be consistent
with an excited-state interaction of the benzimidazole chro-
mophore with the arenesulfonamide group, which results in a
more stabilized excited state and therefore a lower emission
energy. Because the ESIPT process generates a tautomeric
species with an abnormally large Stokes’ shifted emission,
excited-state interaction via excimer formation is not expected
to produce an additional dramatic shift in the emission fre-
quency. Therefore, the observed small differences in the
emission spectra support excited-state interactions and are
consistent with a π-stacking interaction in the solid state, but
not the solution phase.
The steady-state fluorescence data clearly suggest the presence
of a single emissive species regardless of the nature of the
solvent. Nevertheless, the UV absorption spectra in protic
solvents as well as in acetonitrile show an additional weak band
above 330 nm, which could be due either to the presence of
small amounts of the imino tautomer (I, Scheme 1) or the
deprotonated sulfonamide anion in the ground-state equilibrium
(vide supra). If this weak absorption band originates from the
sulfonamide anion, excitation at this wavelength should give
rise to normal emission at higher energy. Indeed, when a solution
of compound (2) in ethanol was excited at 360 nm a very weak
blue-shifted emission band with a maximum at 411 nm was
observed (Figure 8b). The slight broadening of the band toward
longer wavelengths can be attributed to a small contribution of
the tautomer emission with a maximum at 470 nm. Addition of
excess sodium ethoxide strongly enhances the higher energy
band, resulting in a spectrum with an identical maximum at
411 nm. The identification of this species as the deprotonated
anion is further supported by the excitation spectrum acquired
at 411 nm. In this energy region the intensity of the tautomer
emission is approaching zero, therefore, the excitation spectrum
should be indicative for the species with an emission maximum
at 411 nm. The corresponding normalized excitation spectra
measured in neutral and basic ethanol are virtually identical,
and their maxima closely match the UV absorption spectrum
of the sulfonamide anion (Figure 8a). These data strongly
support the presence of small amounts of the deprotonated
sulfonamide. Given the close similarity of the weak low-energy
band observed not only in ethanol but also in methanol, butanol,
and acetonitrile, it is reasonable to assume that in all of these
solvents small amounts of deprotonated ligand are present in
the ground-state equilibrium.
The peak emission wavelength and Stokes’ shift decrease with
increasing solvent polarity, suggesting an excited state with a
smaller dipole moment compared to the ground state. This
observation is consistent with various other ESIPT systems
30,44
reported in the literature.
As compared to the phenol
analogue HPBI (1), the emission is shifted to longer wavelength.
For example, in ethanol a peak emission of 450 nm was reported
for HPBI (1) whereas TPBI (2) emits with a maximum of 470
nm.17 The quantum yields of the tautomer emission in different
solvents range between 0.49 and 0.59, which is almost invariable
given the experimental error associated with quantum yield
measurements. The data are very similar compared to the
43
quantum yields reported for HPBI (0.50-0.65) or 2-(3′-
hydroxy-2′-pyridyl)benzimidazole (0.44-0.53),44 which are also
independent of the nature of the solvent.
The excitation spectra are insensitive toward alteration of the
emission wavelength at which the spectra were acquired. As
illustrated in Table 4, the peak excitation wavelengths are also
identical with the measured maxima of the UV spectra. This
again is consistent with a single ground-state rotamer, which is
responsible for the abnormally Stokes’ shifted emission band.
The X-ray structure reveals a π-stacking interaction between
the aromatic rings of toluene and benzimidazole, but ab initio
optimization of the gas phase geometry suggests a more
geometrically relaxed and open structure lacking the π-stacking
interaction. Aromatic hydrocarbons quite frequently exhibit
excimer fluorescence. Stabilizing interactions between two
aromatic rings are possible if one of the two moieties is in an
excited state and the HOMO and LUMO of the combined
system are only singly occupied. The interplanar separation for
excimers of aromatic molecules is typically between 300 and
3
50 pm and is thus in the same range as the separation of 330
45
pm observed in the X-ray structure of 2. If the π-stacking
interaction of 2 is also important in solution phase, it should be
possible to obtain experimental evidence via comparison of the
emission spectra of an aromatic vs an aliphatic sulfonamide
derivative. We therefore synthesized the methanesulfonamide
analogue 3, which is lacking the benzene ring required for
intramolecular excimer formation. As shown in Figure 7, the
solution phase emission spectra of 2 and 3 are virtually identical
with a maximum at 489 nm. The interaction of the arene-
sulfonamide group does not appear to be significant on the basis
of these data, which would support an open conformation
without π-stacking interaction. This interpretation was further
confirmed by the fact that monoexponential fluorescence decay
kinetics with very similar lifetimes were observed regardless
of the nature of the solvent (vide infra). Interestingly, the
Time-ResolVed Spectroscopy. The time-correlated single
photon counting data for the fluorescence decay of TPBI in

Intramolecular Proton Transfer in TSPBI
J. Phys. Chem. A, Vol. 106, No. 34, 2002 7661
Figure 10. Geometrically relaxed potential energy curves for the
ground (S ) and lowest excited singlet state (S ) of 2 as a function of
0 1
the N3-H atom distance (proton-transfer reaction coordinate). Absorp-
tion and emission energies are indicated for TD-DFT and CI-ZINDO
(in parentheses) calculations.
Figure 9. Fluorescence decay data of 2-(2′-tosylaminophenyl)ben-
zimidazole (2) in methanol (time-correlated single photon counting).
The curve fit is based on a monoexponential decay law (τ ) 5.59 ns,
2
ø ) 1.07).
To investigate the thermodynamics of the ESIPT process in
TPBI 2, the potential energy curves for the ground and first
excited singlet state were calculated as a function of the N-H
bond length (Figure 10). Whereas the interconversion between
the cis- and trans-rotamers is energetically unfavorable in the
ground state as well as excited state, the calculated potential
energy curve for proton transfer in the excited state is almost
methanol is shown in Figure 9. The emission signal fits well to
2
a simple monoexponential decay law with a ø value near unity
(1.07). Regardless of the nature of the solvent, the tautomer
emission of TPBI revealed monoexponential decay signals
without any sign of a biexponential component (Table 4).
-1
barrierless (3.9 kJ mol ). Thermal equilibration of the tautomer
to the ground-state amide tautomer also occurs with a low barrier
Because the only rotamer of TPBI (2) detected in the ground
state is the cis-conformer Ec, the imino tautomer I* must be
formed in the excited state via ESIPT from excited Ec*. Given
the instrument resolution of 0.2 ns, the conversion of the cis-
rotamer Ec* to I* must occur via ultrafast proton transfer, as
has been characterized for various other ESIPT systems includ-
-
1
of 11.7 kJ mol .
The optimized excited-state structure of the tautomer (I)
shows a distinctly different geometry compared to the computed
ground-state structure of 2 (Table 2). An interesting feature is
the coplanar orientation of the phenyl-benzimidazole rings with
a very small dihedral angle of -0.633°. The excited-state
potential energy curve of the enamine rotamer also reaches
minimum energy at a coplanar geometry, such that the rotation
around the C2-C7 bond axis might already occur prior to the
proton-transfer process. Whereas the ground-state structure of
ing HPBI.13,16,46,47
The estimated radiative rate constants for the
tautomer emission of (2) are virtually identical in all solvents
used. Similarly, the nonradiative rate constants vary little
between the various solvents. The fluorescence lifetime data
are consistent with the interpretation of the steady-state spectra
and additionally support the presence of a single emissive
species in the excited-state manifold.
2
shows almost equal bond lengths for all C-C bonds in the
central aryl ring, the excited-state tautomer I* exhibits significant
differences. The computed bond lengths of the phenyl ring differ
by approximately 7 pm and clearly reflect the cross-conjugated
nature of the delocalized π-electron system in I*. For the same
reason the C2-C7 and C1-N3 bonds are contracted by 4-6
pm compared to the ground-state structure, whereas the N1-
C7 is slightly elongated (Table 2).
3
. Quantum Chemical Calculations. Potential Energy
CurVes. Calculations of the ground- and excited-state potential
curves were carried out using the Q-Chem software package.
The ground-state (S0) geometries were optimized by the density
functional method using the B3LYP hybrid functional and a
-31G* basis set. Excited-state energies were calculated by time-
dependent density functional theory (TD-DFT) using the
optimized ground-state geometries. All potential energy curves
were constructed by variation of the corresponding structural
parameter (dihedral angle or bond length) including a full
geometry optimization.
3
2
6
The potential energy calculations are in agreement with all
experimental data, which suggest a single enamine rotamer (Ec)
in the ground state. Upon excitation, Ec undergoes ultrafast
intramolecular proton transfer to yield the emissive tautomeric
imino form I*.
As previously indicated in Figure 4, the ground-state potential
curve calculated for rotation around the phenyl-benzimidazole
bond axis reveals only two stable conformers. The trans-rotamer
Et, corresponding to a dihedral angle of 180°, is considerably
higher in energy such that in a thermal equilibrium at room
temperature the cis-rotamer Ec will be the predominant species.
The energy barrier and relative stability between rotamers can
drastically change in the excited state. This might potentially
lead to emission from rotamers that are energetically not
accessible in the ground-state equilibrium. The approximated
first excited-state potential energy curve for rotation around the
phenyl-benzimidazole bond axis shows an even greater activa-
S0 f S1 Vertical Excitation and S0 r S1 Emission Energies.
The gas-phase vertical excitation energy for the cis-rotamer Ec
of 2 was calculated by the TD-DFT method using the B3LYP
hybrid functional and the split-valence polarized 6-31G* basis
3
4
set. Recent work by Stratmann et al. has shown that time-
dependent density functional theory yields significantly better
results compared to Hartree-Fock based methods such as the
random-phase approximation (RPA) or CIS. A comparison of
the computed excitation energy shows a reasonably good
prediction with a deviation of 0.1 eV (Table 5). Using the
geometries obtained from the DFT optimization the excitation
energy was also computed with the configuration interaction
approach as implemented in the semiempirical method ZIN-
-
1
tion barrier of 74.6 kJ mol compared to that of the ground
state. Therefore, the interconversion between the cis- and trans-
rotamers via an excited-state pathway can be excluded.
3
7
DO. The CI-ZINDO results given in Table 5 demonstrate the

7662 J. Phys. Chem. A, Vol. 106, No. 34, 2002
Fahrni et al.
process of TPBI are essentially unaffected by the nature of the
solvent, which is of particular interest to sensing applications
in cell biology.
Acknowledgment. Financial support via startup funds from
the Georgia Institute of Technology is gratefully acknowledged.
Computations were supported by the Center for Computational
Molecular Science and Technology at the Georgia Institute of
Technology and partially funded through a Shared University
Research (SUR) grant from IBM and the Georgia Institute of
Technology. We also thank S. Shealy and D. Bostwick for mass
spectral data, Dr. David Sherrill for helpful discussions, Dr.
Mostafa El-Sayed for access to the fluorescence lifetime
spectrometer, and J. Cody and B. Ayres for a critical review of
the manuscript.
Figure 11. Molecular orbital surfaces of the HOMO and LUMO
responsible for the lowest energy transition in the UV-vis spectrum
of 2 (CI-ZINDO calculation).
TABLE 5: Experimental and Computed Vertical Excitation
Energy (eV) for the Cis-Rotamer E
c
and Emission Energy
(
eV) for the Tautomer I*
method
E
c
I*
TD-DFT (B3LYP/6-31G*)
CI-ZINDO
Expt
4.04
3.87
3.94-3.88
3.20
2.96
2.44-2.64
Supporting Information Available: Crystallographic data
for the structural analysis have been deposited with the
Cambridge Crystallographic Data Centre (CCDC 188405).
Copies of this information may be obtained free of charge from
The Director, CCDC, 12 Union Road, Cambridge CB21EZ, UK
usefulness of semiempirical methods. The predicted excitation
energy is virtually identical compared to the experimental values.
Similarly, the computed TD-DFT energies for the emission
of the tautomer I* formed upon proton transfer in the excited
state shows a lower but still reasonable correlation with the
experimental value. The CI-ZINDO result is in better agreement
by about 0.2 eV but still predicts an emission energy that is
blue-shifted by 0.4 eV compared to the experimental data (Table
(
Fax: +44-1223-336033; e-mail: deposit@ccdc.cam.ac.uk or
www: http://www.ccdc.cam.ac.uk).
References and Notes
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