Photophysical processes of some benzimidazole derivatives

Article abstract of DOI:10.1016/j.saa.2005.01.028

The photophysical properties of N-(α-naphthyl)-benzimidazole (α-NABI), N-(β-naphthyl)-benzimidazole (β-NABI) and N-(α-pyridyl)-benzimidazole (α-PYBI) were studied and α-NYBI exhibit intramolecular charge transfer fluorescence in polar solvents. The fluorescence of benzimidazoles can be quenched by acetic acid and the existence of exciplexes was observed between the benzimidazole derivatives and acetic acid. Particularly, the maximum emission peak of solution of α-PYBI in mixed solvent, ether and acetic acid, presents obvious red-shift with the increase of concentration of acetic acid in the mixed solvent.

Full text of DOI:10.1016/j.saa.2005.01.028

Spectrochimica Acta Part A 62 (2005) 592–595
Photophysical processes of some benzimidazole derivatives
∗
Zhaobin Chen , Caihong Zhang, Liheng Feng
School of Chemistry and Chemical Engineering, Shanxi University, Taiyuan 030006, PR China
Received 7 December 2004; accepted 21 January 2005
Abstract
The photophysical properties of N-(␣-naphthyl)-benzimidazole (␣-NABI), N-(␤-naphthyl)-benzimidazole (␤-NABI) and N-(␣-pyridyl)-
benzimidazole (␣-PYBI) were studied and ␣-NYBI exhibit intramolecular charge transfer fluorescence in polar solvents. The fluorescence
of benzimidazoles can be quenched by acetic acid and the existence of exciplexes was observed between the benzimidazole derivatives and
acetic acid. Particularly, the maximum emission peak of solution of ␣-PYBI in mixed solvent, ether and acetic acid, presents obvious red-shift
with the increase of concentration of acetic acid in the mixed solvent.
©
2005 Elsevier B.V. All rights reserved.
Keywords: Benzimidazole derivatives; Photophysical processes; Quenching; C60; Acetic acid
1
. Introduction
2. Experimental
Benzimidazole groups exist extensively in biologically ac-
The reagents and chemicals for preparation these ben-
zimidazole derivatives were used as received unless noted
otherwise. Cyclohexane, ethyl acetate, chloroform, ethanol,
benzene, diethyl ether, acetic acid and methanol were analyt-
ical grade reagents and treated according to standard method
used before, which were applied to measurement of the light-
emitting properties.
tive substance and pharmaceutical molecule. The structures
of cyanocobalamin (Vitamin B12), as an example, contain a
benzimidazole group [1]. Moreover, the derivatives of benz-
imidazole have been widely used in composition of optoelec-
tronic functional materials, which due to their heteroatom-
containing ␲-conjugated structure [2]. Rettig have found that
the excited state of N-aryl-substituted derivatives of nitrogen-
containing heterocycle, such as carbazole, indole, benzimida-
zole, etc., exhibitintramolecularchargetransfer(ICT)withan
increase in electron-accepting ability of the N-aryl-asmoiety
The compounds, ␣-NABI, ␤-NABI and ␣-PYBI were syn-
thesized by reaction of carbon-nitrogen bond forming of ben-
zimidazole with ␣-bromonaphthalene, ␤-bromonaphthalene
and ␣-bromopyridine, respectively. The structures were char-
1
[
3,4].
acterized by IR, HNMR and elementary analysis. The syn-
In this paper, the photophysical processes of N-(␣-
thesis route was outlined in Scheme 1. [6]
naphthyl)-benzimidazole (␣-NABI), N-(␤-naphthyl)-benzi-
midazole (␤-NABI) and N-(␣-pyridyl)-benzimidazole (␣-
PYBI) were investigated using fluorescence quenching tech-
nique, etc., in which the formation conditions and mechanism
of exciplex were especially investigated since the complexes
of excited and ground states was often a middle state of pho-
tochemical, photophysical and photobiological primary pro-
cesses [5].
The measurement of fluorescence was performed on a
Perkin-Elmer LS-50B fluorometer with an excitation wave-
length of 270 nm. Both excitation and emission bonds were
set at 5 nm on these experimental conditions. All the experi-
◦
ments were carried out at 25 ± 1 C.
3. Results and discussion
3.1. Absorption and fluorescence emission spectra
∗
The UV–vis absorption spectra of ␣-NABI, ␤-NABI and
Corresponding author. Tel.: +86 351 7011492; fax: +86 351 7011688.
E-mail address: zchen@sxu.edu.cn (Z. Chen).
␣-PYBI in chloroform were shown in Fig. 1. It can be seen
1
386-1425/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2005.01.028

Z. Chen et al. / Spectrochimica Acta Part A 62 (2005) 592–595
593
Scheme 1. The synthesis route of the compounds.
3.2. Solvent effect on photoluminescence
N-aryl-substituted benzimidazole, ␣-NABI, ␤-NABI and
␣
-PYBI, have good solubility in common organic solvents,
such as cyclohexane, diethyl ether, ethyl acetate, chloroform,
ethanol, benzene, etc. The emission spectra of ␣-NABI, ␤-
NABI and ␣-PYBI were investigated in different solvents and
the results showed the peak positions of emission spectra of
␣
-NABI and ␤-NABI in different solvents are similar with-
out an obvious change as a function of the solvent polarity.
But the emission spectra of ␣-PYBI exhibit a charge in dif-
ferent solvents (Fig. 3). In contrast, a red shift, loss of the
fine structure and broadened band in the emission spectra of
Fig. 1. Absorption spectra of ␣-NABI, ␤-NABI and ␣-PYBI in CHCl3.
␣
-PYBI are observed and the fluorescence halfwidth ꢀν are
increased with increasing of solvent polarity [8].
The spectral characteristics indicate the existence of a in-
tramolecular charge transfer (ICT) in the excited ␣-PYBI [9].
Rettig and Zander predicted that the ICT emission will be
observed if the molecular system satisfies
from Fig. 1 that the maximum absorptions of the ␣-NABI, ␤-
NABI and ␣-PYBI are at 284, 278 and 253 nm, respectively.
The fluorescence emission spectra of three compounds were
shown in Fig. 2. The maximum emission peaks position of
solution of ␣-NABI, ␤-NABI and ␣-PYBI in CHCl3 are at
E(ICT) − E(LE) < 0
E(ICT) = IP(D) − EA(A) + C + ꢀEsolv
where E(ICT) and E(LE) are the energies of the lowest ICT
state and the lowest locally excited state, respectively. IP(D)
and EA(A) are the ionization potential and electron affinity of
3
60, 375 and 388 nm, respectively. By comparison with ␣-
NABI and ␤-NABI, the emission peak of ␣-PYBI exhibit an
obvious red shift. Moreover, the fluorescence halfwidth ꢀν of
␣
-PYBI is larger compare with that of ␣-NABI and ␤-NABI
[7].
Fig. 2. Emission spectra of ␣-NABI, ␤-NABI and ␣-PYBI in CHCl3.
Fig. 3. Fluorescence spectra of ␣-PYBI in different solvents.

5
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Z. Chen et al. / Spectrochimica Acta Part A 62 (2005) 592–595
the donor and acceptor moieties of the molecule, respectively.
C is the coulomb energy and ꢀEsolv is the interaction between
the charge-separated molecule and the polar solvent. On go-
ing from ␣-NABI or ␤-NABI to ␣-PYBI, the donor moiety
is the some, but the electron-accepting ability of pyridyl is
enhanced. Therefore ICT emission of ␣-PYBI can be seen in
polar solvents [3,4].
3
.3. The quenching of fluorescence of α-NABI, β-NABI
and α-PYBI by C60
The interactions between N-aryl-substituted benzimida-
zole derivatives and C60 were examined. The results of fluo-
rescence quenching of ␣-PYBI, as a sample, were shown in
Fig. 4. As we have seen in Fig. 4, on progressive increase in
Fig. 5. Fluorescence spectra of solution of ␣-NABI in mixed solvents of
ether and acetic acid.
concentration of C , the emission intensity of fluorescence
60
are decreased. The quenching process of fluorescence is con-
formed to Stern-Volmer equation, which demonstrated the
strong interaction between ␣-PYBI and C60 happens in the
excited.
The interactions of the benzimidazole with C60 are mainly
coming from the photoinduced charge-transfer process of the
␲-electron system in ␣-NABI, ␤-NABI and ␣-PYBI with the
electron acceptor (C ).
60
3
.4. Interaction of α-NABI, β-NABI and α-PYBI with
acetic acid
Fig. 6. Fluorescence spectra of solution of ␤-NABI in mixed solvents of
The studies of interactions between N-aryl-substituted
ether and acetic acid.
benzimidazole derivative and acetic acid were helpful to un-
derstand the photo-electronic properties and intramolecular
charge transfer (ICT) processes of ␣-NABI, ␤-NABI and ␣-
PYBI.
molecule fluorescence of ␣-NABI and ␤-NABI are quenched
efficiently. At the same time, the new emission bands appear
at relatively longer wavelength. The emission intensity of the
new band increase with increase in the concentration of acetic
acid and the spectra all pass an isostilbic point, which indi-
cate the exciplex are formed between ␣-NABI or ␤-NABI
and acetic acid.
Similarly, the fluorescence of ␣-PYBI are also quenched
by acetic acid and the new emission band of exciplex appears
in longer wavelength region (Fig. 7). However, it should be
particularly indicated that the emission spectra of ␣-PYBI are
Figs. 5 and 6 give the fluorescence spectra of solutions of
␣
-NABI and ␤-NABI in mixed solvent of ether and acetic
acid, respectively. As we have seen in Figs. 5 and 6, on pro-
gressive increase in concentration of acetic acid, the simple
Fig. 4. The quenching of fluorescence of ␣-PYBI by C60 in CHCl3. Concen-
−
7
tration of ␣-PYBI is 8.63 × 10 mol/L; concentration of C60 (mol/L) (0)
Fig. 7. Fluorescence spectra of solution of ␣-PYBI in mixed solvents of
ether and acetic acid.
−7
−7
−6
−6
.
0
.00; (1) 2.38 × 10 ; (2) 6.35 × 10 ; (3) 1.43 × 10 ; (4) 2.22 × 10

Z. Chen et al. / Spectrochimica Acta Part A 62 (2005) 592–595
595
gated. ␣-PYBI exhibit intramolecular charge transfer fluo-
rescence in polar solvent. The existence of exciplexes were
observed between the benzimidazole derivatives and acetic
acid. Particularly, the maximum emission peak of ␣-PYBI
presents obvious red shift with the increase of concentration
of acetic acid in mixed solvent of ether and acetic acid.
Acknowledgements
The present work was supported by the Science Founda-
tion of Taiyuan City of China, Laboratory of Organic Solid,
Chinese Academy of Sciences and The Returned Student Sci-
ence Foundation of Shanxi Province of China.
Fig. 8. The linear dependence of the maximal emission wavelength with
mass percents of acetic acid.
obviously red shift with progressive increase in concentration
ofaceticacidinthemixedsolventofetherandaceticacid. The
plots of maximal emission wavelength versus concentration
of acetic acid in the mixed solvent show a linear dependence
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The photophysical properties of N-aryl-substituted ben-
zimidazoles, ␣-NABI, ␤-NABI and ␣-PYBI, were investi-