Blue to green shifted fluorescence in inter- and intramolecular hydrogen bonded di(benzimidazol-2-yl)benzene

Article abstract of DOI:10.1039/b907745f

Interconversion of intermolecular H-bonded di(benzimidazol-2-yl)benzene to an intramolecular H-bonded isomer shows a strong blue to green shifted fluorescence. The Royal Society of Chemistry 2009.

Full text of DOI:10.1039/b907745f

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Blue to green shifted fluorescence in inter- and intramolecular hydrogen
bonded di(benzimidazol-2-yl)benzenew
Bijayalaxmi Jena and S. Sundar Manoharan*
Received (in Cambridge, UK) 17th April 2009, Accepted 19th May 2009
First published as an Advance Article on the web 15th June 2009
DOI: 10.1039/b907745f
Interconversion of intermolecular H-bonded di(benzimidazol-2-yl)-
benzene to an intramolecular H-bonded isomer shows a strong blue
to green shifted fluorescence.
benzene (DIB) using B3PW91 functionals and 6–31G** as the
basis set (Fig. 1). Structure 1 (str. 1) projects two N–H groups
of the two adjacent benzimidazole rings pointing outward.
Structure 2 (str. 2), on the other hand, shows one of the N–H
groups (N1–H) projecting towards the N2 of the adjacent
benzimidazole ring. The optimized ground state geometry
indicates the presence of an intramolecular hydrogen bonding
in str. 2. Further optimization of the ground state geometry
suggests that (i) both structures are non planar (ii) str. 1 has
The presence of intramolecular H-bonding leads to (i) excited
state proton transfer (ii) excited state photo tautomerization.
The net effect is that, they serve as efficient electron sources for
1
–3
tunable stimulated emission. The first observation of the blue
0
green proton transfer fluorescence of 2-(2 -hydroxyphenyl)-
0
benzimidazole in crystalline form was reported by William
4
and Heller. Structural properties of benzimidazole molecules
a dihedral angle of F = 49.45 for both benzimidazole
rings with respect to the central benzene ring along
are important because they are largely governed by H-bonding,
unlike the benzoxazole and benzothiazole conjugated p-structures.
A large effect is observed due to the environment caused by the
non-bonding electron pair on nitrogen which plays an impor-
–C1QC2–C4QN2 and –C2QC1–C3QN1 atoms and str. 2 shows
0
a dihedral angle F = 22.60 along C2QC1–C3–N1(H) and a
0
dihedral angle F = 40.5 along –C1QC2–C4QN2 atoms.
The synthetic route to obtain both isomers involves a simple
condensation reaction followed by sublimation (see ESIw). In
the first step, a reaction involving phthalic anhydride and
phenylenediamine yields a white product showing blue emission
(DIB-b), which on vacuum sublimation gave a greenish yellow
product showing green emission (DIB-g). Theoretical calculations
suggest that the energy difference between DIB-g and DIB-b is
5
–8
tant role in the hydrogen bonding.
Investigation of the
small conjugated organic molecules with H-bonding between
9
acidic and basic moieties is an active field of research.
–12
Photoexcitation in these systems therefore shows a marked
change in the electron density of the H-bonded framework.
We report a simple conversion of blue to green emission in
hydrogen bonded di(benzimidazol-2-yl)benzene i.e., DIB-b
showing blue and DIB-g showing green emission by a sub-
limation route. The thermal and photophysical properties are
studied and the theoretical spectroscopic values in comparison
with the experimental results show the conversion of inter to
intramolecular H-bonding in these two isomers, affecting the
photophysical properties to a significant extent. In an earlier
ꢀ1
ꢀ94.699 Kcal mol indicating that, during the gas phase
condensation of the sublimated DIB-b compound, the struc-
ture gets oriented to the most stable conformer, namely DIB-g.
The infrared spectra of both isomers are shown in Fig. 2
where we observe a characteristic fingerprint feature for the
N–H and C–H stretching. A broad, strong vibrational band
ꢀ
1
between 2200–3200 cm
is attributed to intermolecular
13,14
study, Jalkanen and Suhai
reported the optimized structure
H-bonding on benzimidazole ligand in well documented reports
20
1
from earlier studies by Xue et al. and Yurdakul et al.
9
of dipeptides and studied the effect of hydration on N-acetyl-L-
alanine-N-methylamide’s geometries, relative energies and
vibrational properties. In another report, experimental and
ab initio theoretical vibrational Raman optical activity spectra
is presented for N-acetyl-L-alanine-N-methylamide in order to
determine the prominent conformation of the molecule in
Among the two isomers of DIB, such a broad feature is
observed for the DIB-b isomer. In other words, the presence
of extensive intermolecular H-bonding is indicated for DIB-b,
which has a structure similar to str. 1 (see in Fig. 1) resulting in
extensive H-bonding because of the two free N–H groups
1
5
solution phase. Furthermore, other reports suggested that
short polypeptides can adopt secondary structures in aqueous
16–18
solution,
emphasizing the need for structural determination
of these molecules.
To start with, we show the optimized ground state
geometries of two possible structures of di(benzimidazol-2-yl)-
Department of Chemistry, Indian Institute of Technology Kanpur,
Kanpur-208016, India. E-mail: ssundar@iitk.ac.in;
Fax: +91-512-2597436; Tel: +91-512-2597425
w Electronic supplementary information (ESI) available: Synthesis of
DIB-b and DIB-g, frontier molecular orbitals, UV-Visible absorption
spectra (experimental and theoretical), theoretical IR and Raman
spectra and experimental and calculated vibrational frequencies. See
DOI: 10.1039/b907745f
Fig. 1 The optimized ground state geometries of two isomers of
di(benzimidazol-2-yl)benzene using B3PW91 functionals and 6–31G**
as the basis set. Both structures indicate C
1
point group symmetry.
4
426 | Chem. Commun., 2009, 4426–4428
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in DIB-b, there is a one step sharp fall at 500 1C. In both cases,
the decomposition shows an endothermic DTA peak as shown
in the inset of Fig. 3.
We now show the influence of inter and intramolecular
H-bonding on absorption and photophysical properties. In the
inset to Fig. 4 we show the absorption spectrum of DIB-b in
chloroform having a peak maximum at 305 nm with an
absorption edge at 340 nm. The absorption spectrum of
DIB-g shows a peak maximum at 360 nm, absorption edge
at 405 nm with a substantial tailing up to 490 nm. The peaks at
3
05 nm for DIB-b and at 360 nm for DIB-g correspond to a
n–p* transition. There is a shift of 55 nm in the n–p* transition
between the isomers. The photoluminescent spectrum (PL) of
the compounds in the solid state (lexc at 325 nm) shows a
sharp blue emission for DIB-b with a peak maximum at
around 390 nm (full width half maximum of B90 nm). On
the other hand, DIB-g shows (lexc at 375 nm), a green emission
having a peak maximum at around 510 nm (full width half
maximum of B75 nm). A shift of B120 nm is observed
between the PL peak maximum of the two isomers i.e., DIB-b
and DIB-g (Fig. 4). However the PL spectrum of DIB-b
Fig. 2 The IR spectra for both isomers, DIB-b having a strong and
ꢀ1
broad peak and DIB-g having a weak peak between 2200–3200 cm
.
3
isomer recorded in CHCl , shows that the torsional strain in
facing outwards. On the other hand, DIB-g shows a comparatively
the lattice is considerably reduced in solution and a shift of
about 30 nm is observed from the solid PL spectrum to the
solution spectrum of DIB-b. This shift is apparently absent for
the DIB-g isomer. This may be attributed to the fact that DIB-b
has intermolecular H-bonding in the solid state and this
weakens in the solution phase leading to B30 nm red shifted
emission. In contrast, this shift is less in DIB-g due to
intramolecular hydrogen bonding.
ꢀ1
weaker C–H stretching frequency between 2200–3200 cm
indicating the absence of extensive intermolecular hydrogen
bonding. This is in line with the optimized structural geometry
for str. 2 as shown in Fig. 1 indicating the presence of
intramolecular H-bonding. Such conversion occurs during
the sublimation process of DIB-b (str. 1) where heating leads
to a shift from intermolecular to intramolecular H-bonding.
Nevertheless, in both isomers we see a clear signature for N–H
streching, DIB-g showing a more intense band compared to
DIB-b. The presence of H-bonding can be further substan-
tiated from thermogravimetric analysis (TGA). The TGA
curve for DIB-b shown in Fig. 3 indicates a one step decom-
position, occurring at 500 1C accounting for melting accompanied
by decomposition in one step. In contrast, the DIB-g isomer
shows a two step decomposition showing melting followed by
decomposition between a wide temperature range (200–700 1C).
Because of breaking of network of intermolecular H-bonding
Fig. 4 Photoluminescent spectra (PL) in the solid state and in
solution. DIB-b shows a sharp blue emission with a peak maximum
at around 390 nm (solid state) and in CHCl
with a peak maximum at around 420 nm. DIB-g emits at 510 nm in the
solid state with green emission and in CHCl , the peak maximum is at
3
shows a blue emission
Fig. 3 The TG curve of DIB-b shows one step melting accompanied
by decomposition at 500 1C and for DIB-g, two steps melting followed
by decomposition between 200–700 1C. The inset shows the DTA
curve indicating the endothermic peak for both isomers.
3
504 nm. The inset shows the absorption spectra of DIB-b and DIB-g
having peak maxima at 305 nm and 360 nm, respectively.
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Table 1 Some of the calculated excitation energies (E), wavelengths
l) and oscillatory strengths (f) for low lying singlet (Sn) states for
str. 1 and str. 2, which match well with the experimental values of
The IR spectral values from the experimental and theore-
tical calculations are shown in Table 2 along with the simu-
lated Raman spectral features. It is convincingly clear that the
N–H stretching, N–H in plane bending and N–H out of plane
bending frequency values for DIB-b and DIB-g are in
accordance with the theoretical values (see Table 2). A notable
feature for the inter and intramolecular H-bonding is seen
from the experimental and theoretical IR data, especially from
the C–H stretching frequency. As denoted earlier, DIB-b
(str. 1) has a dihedral angle similar for both benzimidazole
groups while the DIB-g isomer has two different dihedral
angles. As a result, the C–H stretching frequency is split for
DIB-g and serves as a good analytical tool to distinguish
between the two isomers (see ESIw for theoretical IR spectra).
A comparison between the C–H stretching values in plane
bending and out of plane bending is listed in Table 2 (also see
ESIw for the simulated spectra and the table comparing all the
IR and Raman frequencies obtained experimentally and
theoretically).
(
DIB-b and DIB-g, respectively
Excited Molecular orbital
(DE/eV)
l/nm
(calc., exp.)
Molecule states
f
DIB-g
S4
LUMO – HOMO (3.509)
LUMO+1 – HOMO
0.2835 (353, 360)
S10
S14
LUMO – HOMOꢀ2 (4.11) 0.1898 (301, 305)
LUMO+1 – HOMO
LUMO – HOMOꢀ2 (4.32) 0.1701 (282, 293)
LUMO+1 – HOMOꢀ1
LUMO+1 – HOMO
LUMO – HOMOꢀ3 (4.42) 0.2688 (276, 273)
LUMO – HOMOꢀ2
S16
LUMO+1 – HOMOꢀ1
LUMO+1 – HOMO
LUMO – HOMO (3.89)
DIB-b
S5
S14
0.2333 (318, 305)
LUMO – HOMOꢀ1 (4.43) 0.3585 (279, 260)
LUMO+1 – HOMO
LUMO+2 – HOMO
Table 2 Experimental and calculated vibration frequencies and their
tentative assignments
In summary, we have shown the effect of H-bonding on the
photophysical properties of benzimidazole derivatives. The
presence of intra or intermolecular H-bonding brings about a
rapid change in the electron charge density distribution of the
low lying singlet states, which affects the photoluminescence to
a large extent as demonstrated by both the experimental and
theoretical data.
DIB-g
DIB-b
Raman
Raman
FTIR exp./calc. calc.
Assignment FTIR exp./calc. calc.
b
N–H str
C–H str
3442/3543
3164, 3055,
2
1563/1593
1138/1140
3573
3202,
899/3118, 3074 3143
3449/3552
3056, 2669,
3581
3135
b
a
a
a
2426 /3180
The authors thank CSIR, India, for financial help.
c
N–H ipbc
C–H ipb
1524
1153,
1138
1060
1521/1521
1124/1171
1524
1153,
1110
967,
934
—
Notes and references
d
C–H opb
946/934
973/997
754/751
1
A. U. Acuna, A. Costela and J. M. Munoz, J. Phys. Chem., 1986,
0, 2807.
A. U. Acuna, F. Amat, J. Catalan, A. Costela, J. M. Figuera and
9
d
N–H opb
754/742
—
2
a
b
c
J. M. Munoz, Chem. Phys. Lett., 1986, 132, 567.
strong broadening. str = stretching. ipb = in plane bending.
opb = out of plane bending.
d
3 E. M. Kosover and D. Huppert, Annu. Rev. Phys. Chem., 1986, 37,
27.
D. L. William and A. Heller, J. Phys. Chem.,, 1970, 74, 4473.
1
4
Further evidence of the inter and intramolecular H-bonding
5 P. T. Chou, W. S. Yu, Y. C. Chen, C. Y. Wei and S. S. Martinez,
J. Am. Chem. Soc., 1998, 120, 12927.
comes from the theoretical calculations on str. 1 and str. 2, which
agree with DIB-b and DIB-g, respectively. The intermolecular
H-bonded DIB-b (str. 1) shows a dipole moment of 3.4624 D
while the intramolecular H-bonded DIB-g (str. 2) has a dipole
moment of 5.275 D, which indicates a more unsymmetrical charge
distribution in the ground state geometry for str. 2 (see ESIw for
the frontier molecular orbital picture). The point group symmetry
6
7
8
9
P. Chowdhury, S. Panja, A. Chatterjee, P. Bhattacharya and
S. J. Chakravorti, J. Photochem. Photobiol., A, 2004, 170, 131.
N. Kumar, S. Chakravorti and P. Chowdhury, J. Mol. Struct.,
2008, 891, 351.
P. Chowdhury, S. Panja, A. Chattetjee, P. Bhattacharya and
S. Chakravorti, J. Photochem. Photobiol., A, 2006, 173, 106.
R. G. Parr and W. Wang, Density Functional Theory of Atoms and
Molecules, Oxford University Press, Oxford, 1989.
was found to be C
1
for both structures i.e., str. 1 and str. 2.
10 F. Jensen, Introduction to Computational Chemistry, Wiley,
England, 1999.
The experimental and the calculated absorption energies as
well as the dipole moment and oscillatory strength are reported
in Table 1. Analysis of the TD-DFT wavefunction for DIB-g
shows the predicted absorption peaks at 353 nm (f = 0.2835),
1
1
1 J. Goodman and L. E. Brus, J. Am. Chem. Soc., 1978, 100, 7472.
2 J. L. Herek, S. Pedersen, L. Banares and A. H. Zewail, J. Chem.
Phys., 1992, 97, 9046.
1
1
3 K. J. Jalkanen and S. Suhai, Chem. Phys., 1996, 208, 81.
4 W. G. Han, K. J. Jalkanen, M. Elstner and S. Suhai, J. Phys.
Chem. B, 1998, 102, 2587.
3
01 nm (f = 0.1898), 282 nm (f = 0.1701) and 276 nm
(f = 0.2688), which matches well with the experimental values
1
1
1
1
1
5 Z. Deng, P. L. Polavarapu, S. J. Ford, L. Hetcht, L. D. Barron,
C. S. Ewig and K. J. Jalkanen, J. Phys. Chem., 1996, 100, 2025.
6 Z. S. Shi, K. Chen, Z. G. Liu and N. R. Kallenbach, Chem. Rev.,
of 360 nm, 305 nm, 293 nm and 273 nm, respectively. Similarly
for DIB-b the simulated absorption peaks at 318 nm (f = 0.233)
and 279 nm (f = 0.358) match with the experimental values at
2
006, 106, 1877.
7 P. Mukhopadhay, G. Zuber and D. N. Beratan, Biophys. J., 2008,
95, 5574.
8 C. D. Poon and E. T. Samulski, J. Am. Chem. Soc., 2000, 122,
3
05 nm and 260 nm, respectively (see ESIw). We observe the
experimental optical band gap to be in excellent agreement with
the theoretical values for different low lying singlet states, S1,
S10, S14, S16 of the DIB-g isomer and S6 and S14 states for the
DIB-b isomer.
5
642.
9 G. Xue, Z. Juenfong, G. Shi, Y. Wu and B. Shuen, J. Chem. Soc.,
Perkin Trans. 2, 1989, 33.
20 S. Yurdakul and M. Kurt, J. Mol. Struct., 2003, 650, 181.
4
428 | Chem. Commun., 2009, 4426–4428
This journal is ꢁc The Royal Society of Chemistry 2009