Fluorescence quenching and laser photolysis of dipyrrolylbenzenes in the presence of chloromethanes

Article abstract of DOI:10.1134/S1070363207080166

Fluorescence quenching of 1,4-bis(1H-pyrrol-1-yl)benzene, 1-(1H-pyrrol-2-yl)-1-(1-vinyl-1H-pyrrol-1-yl)benzene, and 1,4-bis(1-vinyl-1H- pyrrol-2-yl)benzene with chloromethanes (methylene chloride, chloroform, and carbon tetrachloride) in solvents with dif

Full text of DOI:10.1134/S1070363207080166

ISSN 1070-3632, Russian Journal of General Chemistry, 2007, Vol. 77, No. 8, pp. 1386 1394.
Original Russian Text I.K. Petrushenko, V.I. Smirnov, K.B. Petrushenko, E.Yu. Shmidt, N.V. Zorina, Yu.Yu. Rusakov, A.M. Vasil’tsov, A.I. Mikhaleva,
B.A. Trofimov, 2007, published in Zhurnal Obshchei Khimii, 2007, Vol. 77, No. 8, pp. 1307 1316.
Pleiades Publishing, Ltd., 2007.
Fluorescence Quenching and Laser Photolysis
of Dipyrrolylbenzenes in the Presence of Chloromethanes1
I. K. Petrushenko, V. I. Smirnov, K. B. Petrushenko, E. Yu. Shmidt, N. V. Zorina,
Yu. Yu. Rusakov, A. M. Vasil’tsov, A. I. Mikhaleva, and B. A. Trofimov
Favorskii Irkutsk Institute of Chemistry, Siberian Division, Russian Academy of Sciences,
ul. Favorskogo 1, Irkutsk, 664033 Russia
e-mail: boris_trofimov@irioch.irk.ru
Received November 30, 2006
Abstract Fluorescence quenching of 1,4-bis(1H-pyrrol-1-yl)benzene, 1-(1H-pyrrol-2-yl)-1-(1-vinyl-1H-
pyrrol-1-yl)benzene, and 1,4-bis(1-vinyl-1H-pyrrol-2-yl)benzene with chloromethanes (methylene chloride,
chloroform, and carbon tetrachloride) in solvents with different polarities follows electron-transfer mechanism.
The occurrence of an electron-transfer step is confirmed by formation of short-lived pyrrolylbenzene radical
cations. An exception is quenching of fluorescence of 1,4-bis(1-vinyl-1H-pyrrol-2-yl)benzene in n-hexane in
the presence of CCl4 and CHCl3 and in pure CCl4. In this case, neutral 1,4-bis(1-vinyl-1H-pyrrol-2-yl)ben-
zene Cl radical is formed via recombination of 1,4-bis(1-vinyl-1H-pyrrol-2-yl)benzene radical cation and
chloride anion. A relation was found between the nature of the short-lived species detected by laser photolysis
and stable product obtained by stationary photolysis.
DOI: 10.1134/S1070363207080166
Fluorescent dipyrroles in which electron-donor
pyrrole rings are linked through an aromatic or other
electron-acceptor bridge attract interest as precursors
of extended systems that are widely used in the field
of optical technologies (as sensors, optical switchers,
light-emitting diods, semiconductors with a narrow
band gap, molecular crystals for frequency transduc-
tion) [1 5]. Due to their high hyperpolarizability ( ),
1,4-bis(1H-pyrrol-2-yl)benzenes and their analogs are
also promising as materials for computing and signal-
processing devices operating in the optical frequency
range [6]. Optical and electrical parameters of ma-
terials are largely determined by dynamics of excited
states of monomeric units, their ability to generate
charged species, and properties of radical cations.
pyrrolylbenzenes), mechanism of their fluorescence
quenching with chloromethanes (CH2Cl2, CHCl3,
CCl4), and effect of the medium on the nature and
spectral properties of short-lived intermediates
generated in the above systems by nanosecond laser
photolysis (NLP).
Synthesis of dipyrrolylbenzenes. 1,4-Bis(1H-pyr-
rol-2-yl)benzene (I) and its substituted derivatives
were synthesized previously according to laborious
multistep procedures [5 7] in an overall yield of
10 [7] or 20% [5], by the Negishi cross coupling of
protected N-(trimethylsilyl)ethoxymethylpyrrole with
1,4-dibromobenzene in the presence of NiCl2dppp as
catalyst [5], or by cyclization of N,N -diallyltere-
phthalimidoyl dichloride by the action of t-BuOK in
THF/DMF [7]. On the other hand, ten years ago we
reported [8 10] on one-pot synthesis of 1,4-bis(1-
vinyl-1H-pyrrol-2-yl)benzene (III) in 30% yield from
1,4-diacetylbenzene dioxime and acetylene (Trofimov
reaction [11]), and methods for deprotection of N-
vinylpyrroles were developed [12, 13].
In the present study we used model 1,4-bis(1H-
pyrrol-2-yl)benzenes to elucidate mechanism of the
reaction between photochemically excited pyrroles
and electron-acceptor molecules. For this purpose, we
examined electronic absorption and luminescence
spectra of 1,4-bis(1H-pyrrol-2-yl)benzene (I), 1-(1H-
pyrrol-2-yl)-4-(1-vinyl-1H-pyrrol-2-yl)benzene (II),
and 1,4-bis(1-vinyl-1H-pyrrol-2-yl)benzene (III) (di-
In the present work we synthesized dipyrrolylben-
zenes I III according to a modified Trofimov proce-
dure with the use of 1,4-diacetylbenzene dioxime
cesium salt. In such a way we succeeded in obtaining
three target products I III in an overall yield of
28.6% in one preparative step (Scheme 1).
1
Dedicated to M.G. Voronkov on the 85th anniversary of
his birth.
1386

FLUORESCENCE QUENCHING AND LASER PHOTOLYSIS
1387
Scheme 1.
Me
~~~
Me
HC CH (14 25 atm)
DMSO, 110 C, 2 h
N
H
N
H
OCs
~~~
N
CsO
N
I, 15.7%
+
+
N
N
N
N
H
II, 6.6%
III, 6.3%
1,4-Bis(1H-pyrrol-2-yl)benzene (I) was prepared
in 83% yield by deprotection of divinyl derivative III
via successive treatment with Hg(OAc)2 and NaBH4
according to improved procedure [12, 13] (Scheme 2).
Scheme 2.
(1) Hg(OAc)2, MeCN,
20 C, 30 min
(2) NaBH4
N
N
H
N
N
H
III
I, 83%
Electronic absorption and luminescence spectra
of compounds I III. The long-wave absorption band
in the electronic absorption spectra of compounds I
III is displaced toward shorter wavelengths in going
from I to II and then to III (Fig. 1, Table 1). Simul-
taneously, its intensity decreases. Most probably, this
is the result of weakening of conjugation in the
system due to increase in dihedral angles between the
benzene ring and initially one and then two pyrrole
rings upon successive replacement of hydrogen at the
pyrrole nitrogen atoms by bulky vinyl groups.
reduction of
indicates that the main reason for the
f
latter is increase in the radiative fluorescence lifetime
( ) in the series I III. Clearly defined vibrational
r
structure in the fluorescence spectra of these com-
pounds (which is most pronounced in n-hexane solu-
tions) and the lack of vibrational structure in the
absorption spectra (Fig. 1) are typical of nonrigid
Ifl
A
1.2
1.0
0.8
0.6
1.2
1.0
0.8
0.6
0.4
0.2
0
I
I
The equilibrium structures of molecules I III in
the ground state, calculated by the B3LYP/6-3111G*
method, are shown in Fig. 2. For all compounds I III,
trans conformations of the pyrrole rings are preferred.
The pyrrole rings decline from the benzene ring plane
through a dihedral angle of 28 29 for unsubstituted
dipyrrolylbenzene and 43 45 for N-vinylpyrrole
fragments.
II
III
II
III
0.4
0.2
0
200 250 300 350 400 450 500
, nm
Unlike absorption spectra, the fluorescence maxima
of compounds I III are less sensitive to substitution.
Introduction of vinyl groups makes the vibrational
structure less resolved and reduces the fluorescence
quantum yield ( f) in going from compound I to III
(Fig. 1, Table 1). The observed parallelism in the
hypochromic effect in the absorption spectra and
Fig. 1. Electronic absorption and fluorescence spectra of
dipyrrolylbenzenes I III in acetonitrile. The intensities
are proportional to the molar absorption coefficients in
the absorption spectra and to the quantum yields in the
fluorescence spectra. The intensities in the spectra of I
are normalized by unity.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 77 No. 8 2007

1388
PETRUSHENKO et al.
250
28.8
1 5
200
150
28.8
28.4
I
24.3
100
43.4
50
0
II
340
380
420
460
500
, nm
23.3
44.6
44.6
23.3
6
III
Fig. 3. Fluorescence spectra of 2 10 M solutions of
compound II in n-hexane ( = 334 nm) in the
Fig. 2. Equilibrium geometric structures of the ground
states of molecules I III, calculated by the B3LYP/6-
311G* method.
exc
presence of CHCl ; concentration of CHCl , M: (1) 0,
(2) 0.12, (3) 0.25, (4) 0.38, and (5) 0.5.
3
3
molecules; this pattern suggests flattening of mole-
cules I III upon excitation.
companied by variation of the shape of emission
bands or appearance of new bands in the long-wave
region of the spectrum. As an example, Fig. 3 shows
the fluorescence spectra of compound II in n-hexane
at different concentrations of chloroform.
Fluorescence quenching of dipyrrolylbenzenes
with chloromethanes. Addition of a small amount of
CHCl3 or CCl4 to a solution of compound I III in
acetonitrile or hexane appreciably reduces its fluores-
cence intensity. Fluorescence quenching is not ac-
For all the examined compounds, the dependence
of I0/I on the concentration of CHCl3 is linear and is
well described by Stern Volmer type Eq. (1).
Table 1. Absorption and luminescence spectra of dipyrrol-
ylbenzenesa
I0/I = 1 + Kd[CHCl3].
(1)
Here, I0 and I are the fluorescence intensities in the
absence and in the presence of quencher, respectively.
If quencher is carbon tetrachloride, the dependences of
I0/I versus [CCl4] for compounds I III both in polar
and in nonpolar solvents are nonlinear (Fig. 4); in the
examined range of quencher concentrations, these
dependences are well described by Eq. (2):
Absorption
Fluorescence
max, nm
Comp.
no.
,
max
l mol 1 cm
max, nm
f
1
MeCN
40000
33300
I
II
III
328.4
317.8
307.3
377
385
383
1.00
0.92
0.70
I0/I = 1 + (Kd + Kx)[CCl4] + KdKx[CCl4]2. (2)
23300
n-Hexane
Methylene chloride weakly quenches fluorescence
of compounds I III. These compounds show fluores-
cence even when methylene chloride is used as
solvent. By contrast, no fluorescence of dipyrrolyl-
benzenes I III is observed in chloroform and carbon
tetrachloride. The fluorescence quenching constants
in methylene chloride were not determined on a quan-
I
II
III
326.5
314.8
306.8
372
376
377
a
Insofar as compounds I III are poorly soluble in n-hexane,
the corresponding and values were not measured.
f
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 77 No. 8 2007

FLUORESCENCE QUENCHING AND LASER PHOTOLYSIS
1389
titative level, for their reliable determination requires
large concentrations of CH2Cl2, which could result
in change of the physical properties of the medium.
The relative fluorescence quantum yields of com-
pounds I III in methylene chloride, as well as the
quenching constants of CHCl3 and CCl4, calculated
by Eqs. (1) and (2), are given in Table 2.
Table 2. Fluorescence quenching constants of dipyrrolyl-
benzenes with chloromethanes in different media and
fluorescence quantum yields in methylene chloride
CCl4
CHCl3
CH2Cl2
Comp.
no.
Kd,
l mol
Kx,
l mol
Kd,
l mol
a
/
1
1
1
0
Positive deviations from the Stern Volmer equa-
tion in the fluorescence quenching of I III with CCl4
indicate that the quenching process simultaneously
follows two mechanisms, one of which is related to
dynamic quenching (Kd). The other mechanism (Kx)
involves either static quenching via formation of non-
fluorescent complex by the fluorophore and quencher
molecule in the ground state or the existence of a
quenching sphere that reduces the fraction of fluoresc-
ing molecules [14]. The larger fluorescence quenching
constant of CCl4 (Table 2) was attributed to the
dynamic constant, taking into account its dependence
upon solvent viscosity. The ratio of Kd values for
compound I in acetonitrile and 1,4-dioxane is 3.7,
which is similar to the viscosity ratio of 1,4-dioxane
(1.31 cP [15]) and acetonitrile (0.34 cP [15]), equal to
3.9. By contrast, Kx does not depend on the viscosity,
in keeping with both nonfluorescent complex and
quenching sphere models.
MeCN
7.0
1.8
I
II
III
40.3
41.6
23.9
15.8
5.5
1.7
0.55
0.77
0.89
3.6
1,4-Dioxane
5.7
n-Hexane
4.3
1.7
I
11.0
7.2
0.63
II
III
38.0
26.6
4.3
0.25
0.89
1.00
a
Fluorescence quantum yields of compounds I III in MeCN,
n-hexane, and 1,4-dioxane are assumed to be equal to unity.
In the fluorescence quenching with chloroform, the
dynamic quenching constant increases in going from
di- to monovinyl derivative, and it has the maximal
value for unsubstituted dipyrrolylbenzene (Table 2).
Insofar as the intrinsic lifetimes of dipirrolylbenzenes
are likely to change in the reverse order, even stronger
increase of kq (as compared to Kd) may be expected
in the series III < II < I. Although the dynamic
quenching constants for methylene chloride were not
determined specially, the fluorescence quantum yields
in this solvent (Table 2) show that kq(I) > kq(II) >
kq(III).
Dynamic quenching is quantitatively characterized
by the bimolecular quenching rate constant (kq) which
is related to the dynamic quenching constant through
the equation kq = Kd/ , where
is the fluorophore
0
0
lifetime in the absence of quencher. Insofar as ex-
perimental values of for compounds I III are
0
lacking, the effects of particular dipyrrolylbenzene and
chloromethane on kq can be analyzed only on
a qualitative level. The data in Table 2 show that kq
9
8
7
for the same dipyrrolylbenzene ( does not change)
0
increases in the order CH2Cl2 < CHCl3 < CCl4, i.e.,
in parallel with variation of the electron affinity of
these chloromethanes.
6
5
Among the examined chloromethanes, carbon tetra-
chloride quenches fluorescence of dipyrrolylbenzenes
I III most effectively. The dynamic quenching
constants for compounds I and II in acetonitrile and
compound II in n-hexane have the maximal values and
almost coincide with each other. The values of Kd for
compound I in 1,4-dioxane and acetonitrile are
proportional to the reciprocal viscosity of these sol-
vents, indicating diffusion-controlled fluorescence
quenching of I and II in CCl4. Presumably, the fluo-
rescence quenching rate constant of compound III
also approaches the diffusion-controlled value, for Kd
decreases only slightly in going from n-hexane to
acetonitrile, in keeping with the solvent viscosities.
1
2
4
3
2
1
0
0
0.05
0.1
0.15
c, M
Fig. 4. Plots of I /I for compound II in (1) acetonitrile
and (2) n-hexane versus concentration of CCl .
0
4
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 77 No. 8 2007

1390
PETRUSHENKO et al.
Table 3. Fluorescence lifetimes of dipyrrolylbenzenes
I III, fluorescence quenching rate constants, and diffusion
rate constants of solventsa
tion of the corresponding radical cations and neutral
molecules in the most favorable conformations. The
calculated values are 6.50, 6.62, and 6.78 eV for
compounds I, II, and III, respectively. Therefore,
they may be ranked as follows according to their
ionization potentials: I < II < III.
CCl4
CHCl3
CH2Cl2
Comp.
no.
109,
s
0
10
10
10
kq 10
,
kq 10
,
kq 10
,
Taking into account the above considerations, the
data in Tables 2 and 3 indicate that the efficiency of
fluorescence quenching increases in parallel with the
donor power of the substrate and acceptor power of
the quencher. Such a relation is typical of reactions
following electron transfer mechanism. Our results
show that fluorescence quenching of compounds I III
is most likely to involve electron transfer from the
molecule of electronically excited dipyrrolylbenzene
to chloromethane (in polar solvents) or formation of
exciplex (in nonpolar solvents).
1
1
1
mol
mol
mol
1
MeCN (kdif 2.0 1010, l mol 1 s )
I
II
III
2.5
2.8
3.0
1.6
1.5
0.8
0.63
0.20
0.06
0.003
0.001
0.0004
1
1,4-Dioxane (kdif 0.5 1010, l mol 1 s )
I
0.44
0.29
0.002
1
n-Hexane (kdif 2.3 1010, l mol 1 s )
II
III
1.4
0.9
0.15
0.08
0.004
0.0001
Laser photolysis of dipyrrolylbenzenes. Photo-
excitation ( = 337 nm) of oxygen-free solutions of
compound I in dioxane and of compounds II and III
in hexane gives rise to short-lived products whose ab-
a
Calculated by the equation k = 8RT/3000 , where is the
dif
solvent viscosity.
sorption spectra contain maxima at
460, 425 and
The lifetimes
for compounds I III were es-
0
4
480, 410, and 530 nm, respectively. Their disap-
pearance follows the first-order kinetics. The kinetic
pattern and the rate constant of disappearance of the
short-lived products (kT) derived from compounds
I III do not depend on the exciting wavelength. This
means that absorbing species of only one type are
timated using the formulas [16] = 10 / and
=
0
r, where
is the intensity ofrthe most mlong-wave
band in the elmectronic absorption spectrum, and
is
f
f
the absolute fluorescence quantum yield of the cor-
responding dipyrrolylbenzene (Table 1). The values
of
determined in such a way and kq values cal-
0
generated in the above solvents. The lifetimes (
=
T
culated therefrom are given in Table 3.
1/kT) of the short-lived intermediates in oxygen-free
solutions of compound I in 1,4-dioxane and of com-
pounds II and III in n-hexane are 23.9, 5.7, and
4.0 s, respectively. In the presence of oxygen, the
short-lived products are characterized by considerably
shorter lifetimes. Monoexponential kinetics of the
It is known that halomethanes, in particular chlo-
romethanes, act as electron acceptors in quenching of
electronically excited sites of many aromatic com-
pounds, amines, and some heterocyclic compounds.
The electron affinities (EA) of CCl4, CHCl3, and
CH2Cl2 are 2.12, 1.75, and 1.36 eV, respectively [17].
Dipyrrolylbenzenes I III are good electron donors.
This follows, e.g., from ready electrochemical oxida-
tion of compounds I [5] and III [18]. The ionization
potentials (IP) of I III should be relatively low. In
fact, attachment of even one pyrrole ring (IP 8.2 eV)
to benzene ring (IP 9.2 eV) gives a conjugated sys-
tem, 2-phenylpyrrole, whose ionization potential is
7.61 eV [19]. Introduction of one stronger electron-
donor pyrrole fragment into the para position of the
benzene ring should lead to even lower IP values for
molecules I III. Replacement of hydrogen at the
nitrogen atom of 2-phenylpyrrole by electron-with-
drawing vinyl group increases the ionization potential
to 7.76 eV [19]. An analogous effect of vinyl substitu-
tion should be observed for compounds I III. In
addition, we calculated the ionization potentials of
dipyrrolylbenzenes I III by the B3LYP/6-311G*
method as the differences between the heats of forma-
decay of photolysis products and shortening of
T
upon saturation of solutions with oxygen allowed us
to assign the short-lived products generated by laser
photolysis of compounds I III to their triplet states.
Low optical densities inherent to T T transitions must
be noted; they indicate low yield of intersystem cross-
ing and are consistent with high absolute fluorescence
quantum yields of the compounds under study.
Flash photolysis of solutions of compounds I III
in the presence of CCl4 and CHCl3 and in CH2Cl2 as
solvent gives rise to new intermediate species whose
disappearance does not follow first-order kinetics and
does not depend on the presence or absence of oxygen.
The lifetime of triplet states of molecules I III is not
sensitive to the presence of chloromethanes. Fluores-
cence quenching and the lack of triplet state quench-
ing indicate that the reactive electronically excited
state is singlet. The absorption spectra of short-lived
intermediates generated by reaction of the singlet
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 77 No. 8 2007

FLUORESCENCE QUENCHING AND LASER PHOTOLYSIS
1391
A
A
0.1
0.05
0
0.5
0.4
0.3
0.2
0.1
1
2
0
430 450 470 490 510 530 550
400
450
500
550
600
, nm
, nm
Fig. 5. Absorption spectra of the system III
Fig. 6. Absorption spectrum of the system IV
4
3
5
(5 10 M) CCl (0.1 M) during flash laser photolysis
(5 10 M) III (5 10 M) upon flash laser photolysis
in acetonitrile, recorded after complete disappearance of
the T T-absorption.
4
in (1) acetonitrile and (2) n-hexane, recorded in 800 ns
after exciting flash.
excited states of I III with chloromethanes are shown
in Fig. 5 (as an example), and their parameters are
collected in Table 4. The spectra were recorded in the
presence of oxygen with a view to avoid superposition
of T T absorption.
spectra of the short-lived products generated by reac-
tion of the electronically excited states of compounds
I III with chloromethanes are identical to the absorp-
tion spectra of the corresponding dipyrrolylbenzene
radical cations. An exception was quenching of fluo-
rescence of compound III with CHCl3 and CCl4 in
n-hexane. Here, unlike polar acetonitrile, no absorp-
To elucidate the nature of intermediate products
we recorded the electronic absorption spectra of radical
cations derived from dipyrrolylbenzenes I III.
Radical cations were generated by quenching of the
triplet state of a strong electron acceptor, tetrachloro-
p-benzoquinone (IV), with compounds I III accord-
ing to Scheme 3 [20].
tion of III+
(
490 nm) was observed in the NLP
max
spectrum, but a new absorption band appeared with
its maximum at 410 nm (Fig. 5, Table 4).
Thus, the formation of dipyrrolylbenzene radical
cations upon fluorescence quenching of compounds
I III with chloromethanes confirms the above con-
clusion on the quenching mechanism. The formation
of solvated radical ions and exciplexes for classical
fluorescing donor acceptor systems may be repre-
sented by Scheme 4 [21].
As solvent we used acetonitrile. Insofar as com-
pound IV and dipyrrolylbenzenes absorb at the wave-
length of exciting laser flash (
337.1 nm), the con-
ex
Scheme 3.
IV + h
1IV
3IV + I(II, III)
Table 4. Positions of absorption maxima ( , nm) of inter-
mediate products formed upon fluorescence quenching of
dipyrrolylbenzenes I III with chloromethanes and of the
triplet state of tetrachlorobenzoquinone (IV) with dipyr-
rolylbenzenes I III in solvents with different polaritiesa
1IV,
3IV,
IV + I+ (II+ , III+ ).
Comp.
no.
centrations of compounds IV and I III were selected
in such a way that more than 90% of light was ab-
sorbed by quinone IV. All compounds I III quench
the triplet states of quinone IV. Disappearance of the
absorption band corresponding to the triplet state of
CCl4
CHCl3
MeCN
CH2Cl2
IV
I
II
III
460, 500a 460, 500
480
470, 490
490
490
molecules IV (
tra is accompanmieadx by appearance of an absorption
band belonging to radical anion IV ( 450 nm
510 nm [20]) from the NLP spec-
470, 490 460, 490
1,4-Dioxane
460, 490
450, 490
max
I
460, 490
n-Hexane
[20]), as well as of bands with their maxima at 500,
490, and 490 nm for compounds I, II, and III, re-
spectively (Fig. 6). The latter were assigned to radical
cations of dipyrrolylbenzenes I III.
II
III
460, 480 (sh) 440, 480
410 410
a
The data given in Table 4 show that the absorption
The principal maxima are underscored.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 77 No. 8 2007

1392
PETRUSHENKO et al.
monomer nor gives rise to new emission band in the
Scheme 4.
long-wave region, which would indicate formation of
a fluorescing exciplex. In this case, radical cation salts
III+ Cl are not formed, but new species are revealed
kd + kfe
1(D+ A )
ki
D + A
kc
1D* + A
1D*
A
(
410 nm). The latter are formed during the
max
ks
kf
k
i
(2D+8 2A8 )
2D+s + 2A s
fluorescence lifetime of the monomer. These data
indicate high rate of the reaction between the
components in the exciplex formed by molecule III
and CCl4 or CHCl3, which competes with its fluores-
cence. It should be noted that excitation of III in
nonpolar CCl4 gives the same product as that formed
upon fluorescence quenching with CCl4 in hexane.
Excitation of III in more polar CHCl3 and CH2Cl2
produces radical cation III+ (Table 4).
D + A
1
Here, D* is an electron donor in the singlet ex-
cited state, A is an electron acceptor in the ground
1
1
state, D* A is encounter complex, (D+ A )* is
singlet exciplex, 2Ds+ 2As is solvent-separated
radical ion pair, Ds+ and As are solvated radical
2
2
ions, and k stands for the corresponding rate constants.
Radical cation III+ is the strongest electrophile
among radical cations I+ III+ . Presumably, the
product formed as a result of photoexcitation of com-
pound III in n-hexane in the presence of CCl4 and
CHCl3 and in pure CCl4 is neutral radical III Cl
originating from recombination of III+ and Cl in
nonpolar media. Analogous short-lived adducts were
proposed as intermediates in the photochemical reac-
tion of biphenyl with CCl4 [23] and reductive de-
chlorination of chlorobenzenes in their reactions with
electronically excited bichromophoric compounds [24].
In weakly polar solvents ( < 7) exciplexes cannot
dissociate into radical ions [22]. Relatively long-lived
radical cations generated in n-hexane and 1,4-dioxane
(Table 4) exist as salts that are formed as a result of
fragmentation of chloromethane radical anion in the
exciplex. In keeping with Scheme 2 (D = I, II; A =
chloromethane (R Cl), the process may be represented
as follows (D = I):
1(I+ R Cl )*
I+ Cl + R .
The existence of an additional path for deactivation
of exciplex is also responsible for the lack of fluores-
cent properties of exciplexes formed by dipyrrolyl-
benzenes I and II with chloromethanes; therefore,
they cannot be distinguished spectrally upon fluores-
cence quenching (Fig. 3). When D = III, fluores-
cence quenching in n-hexane (as well as in MeCN)
neither changes the shape of the emission band of the
The nature of intermediates (are they charged or
neutral radicals) is directly reflected in the final
products. For all systems where NLP produces de-
tectable radical cations, continuous irradiation of
solutions at a wavelength corresponding to absorption
of parent dipyrrolylbenzene leads to increase in the
absorption intensity at
600 nm due to a stable
max
product (Fig. 7). After the photolysis, a blue film and
a precipitate (presumably, oligomeric) deposit on the
walls and bottom of the cell. If no radical cations are
present (only neutral radicals are observed), the solu-
tion remains transparent and colorless, and no absorp-
tion bands appear in the long-wave region of the spec-
trum (Fig. 8).
A
0.6
1 4
0.5
0.4
0.3
Thus, the presence of chloromethanes gives rise to
an additional path of decay of the electronically ex-
cited state of dipyrrolylbenzenes I III, which involves
electron transfer from their singlet states to CH2Cl2,
CHCl3, or CCl4 molecule. The triplet states of I III
are inactive in one-electron transfer. Radical cations
derived from compounds I III in nonpolar and
moderately polar solvents exist as ion pairs where the
positive charge is compensated by negative charge of
chloride ion. In nonpolar solvents, such as n-hexane
and CCl4, radical cations of the weakest donor
(compound III) combine with Cl to form an adduct
like III Cl.
4 1
0.2
0.1
0
240 340 440 540 640 740
, nm
Fig. 7. Change of the electronic absorption spectrum of
5
compound III (c = 5 10 M) in acetonitrile in the
presence of CCl after irradiation (
334 nm) over a
4
exc
period of (1) 0, (2) 5, (3) 10, and (4) 20 s.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 77 No. 8 2007

FLUORESCENCE QUENCHING AND LASER PHOTOLYSIS
EXPERIMENTAL
1393
The IR spectra were recorded in KBr on a Bruker
A
1 4
1
IFS 25 spectrometer. The H and 13C NMR spectra
0.5
were obtained on Bruker DPX-400 and AV-400 in-
1
struments at 400.13 MHz for H and 100.61 MHz for
0.4
0.3
0.2
13C using HMDS as internal reference. Spectropho-
tometric measurements were performed on a Perkin
Elmer Lambda 35 spectrophotometer. The fluores-
cence spectra were measured on a Perkin Elmer LS
55 luminescence spectrometer. To avoid the effect of
photochemical reactions, the scan rate was no lower
4 1
0.1
0
1
than 500 nmmin . The fluorescence quantum yields
were determined relative to anthracene in n-hexane
240 340 440 540 640 740
, nm
(
0.3 [16]) and were calculated with account taken
of ft0he refractive indices of the solvents. Nanosecond
laser photolysis was performed with the aid of an
AIL3 N2 laser ( 337.1 nm, pulse energy 3 mJ, half-
height pulse duration 7 ns; samples were irradiated in
a 1 1 cm2 quartz cell). The experimental setup was
described previously [25]. A DRSh-500 mercury lamp
was used for stationary photolysis. The excitation
wavelength corresponded to the edge of the long-wave
absorption band of compound I III. A quartz cell
filled with water (cell path length 5 cm) was used as
a heat filter. Solutions were freed from oxygen by
purging with argon over a period of 20 min. All sol-
vents (purity 99.9%, Merck) were used without ad-
ditional purification.
Fig. 8. Change of the electronic absorption spectrum of
compound III (c = 5 10
presence of CCl after irradiation (
period of (1) 0, (2) 5, (3) 10, and (4) 20 s.
5
M) in n-hexane in the
334 nm) over a
4
exc
1H NMR spectrum (CDCl3), , ppm: 11.23 s (2H,
NH), 7.58 s (4H, o-H), 6.83 s (2H, 5-H), 6.48 s (2H,
4-H), 6.10 s (2H, 3-H). 13C NMR spectrum (CDCl3),
C, ppm: 131.03, 130.88 (C2, Ci), 123.54 (Co), 119.03
(C5), 109.03, 105.14 (C3, C4). IR spectrum (KBr),
1
, cm : 3439, 3389, 3102, 2926, 1665, 1653, 1639,
1601, 1580, 1504, 1461, 1406, 1302, 1242, 1188,
1113, 1035, 1011, 913, 881, 836, 795, 719, 668, 619,
546, 507, 472.
Synthesis of dipyrrolylbenzenes I III. A solution
of 2.00 g of 1,4-diacetylbenzene dioxime in 20 ml of
DMSO was added to a mixture of 1.98 g of cesium
fluoride and 0.31 g of lithium hydroxide in 10 ml of
methanol, and the mixture was stirred for 45 min at
45 50 C. The solvents (methanol and water) were dis-
tilled off, the residue (cesium oximate) was transferred
into a 0.25-l rotating steel high-pressure reactor,
50 ml of DMSO was added, and the mixture was
saturated with acetylene (initial pressure 14 atm). The
mixture was heated to 110 C and was kept for 2 h at
that temperature. It was then diluted with water to a
volume of 200 ml and extracted with diethyl ether
(5 15 ml). The extracts were washed with water
(3 10 ml) and dried over K2CO3, and the solvent was
distilled off. The residue, 1.67 g, was dissolved in a
small amount of diethyl ether, and the undissolved
material (compound I) was purified by column chro-
matography on Al2O3 using DMSO as eluent. Yield
0.34 g (15.7%). The ether solution was separated by
column chromatography on Al2O3 (hexane diethyl
ether, 3:1) to isolate 0.16 g (6.6%) of compound II
and 0.17 g (6.3%) of III.
1-(1H-Pyrrol-2-yl)-4-(1-vinyl-1H-pyrrol-2-yl)-
benzene (II). mp 128 130 C. 1H NMR spectrum
(CDCl3), , ppm: 8.42 s (1H, NH), 7.48 d (2H, m-H,
3J = 8.4 Hz), 7.36 d (2H, o-H, J = 8.4 Hz), 7.10 m
3
3
3
(1H, 5 -H), 6.91 d.d (1H, HX, JAX = 8.8, JBX
15.7 Hz), 6.87 m (1H, 5-H), 6.55 m (1H, 3-H),
6.30 m (2H, 4-H, 4 -H), 6.26 m (1H, 3 -H), 5.19 d
=
3
3
(1H, HB, JBX = 15.7 Hz), 4.75 d (1H, HA, JAX
=
8.8 Hz). 13C NMR spectrum (CDCl3), C, ppm:
134.03 (C2 ), 131.90 (C ), 131.66 (C2), 131.61 (Ci),
130.18 (Cp), 129.69 (Co), 123.68 (Cm), 118.99 (C5),
118.47 (C5 ), 110.17 (C4), 110.03 (C4 ), 109.98 (C3),
106.27 (C3 ), 99.02 (C ). IR spectrum (KBr), , cm :
1
3429, 3365, 1912, 1720, 1606, 1466, 1343. 1318,
1179, 1113, 1074, 1059, 1037, 975, 913, 890, 844,
803, 791, 740, 711, 673, 652, 632, 591, 678, 540,
515, 458, 409. Found, %: C 81.89; H 6.22; N 11.90.
C16H14N2. Calculated, %: C 82.02; H 6.02; N 11.96.
The spectral parameters and physical constants of
1,4-bis(1-vinyl-1H-pyrrol-2-yl)benzene (III) were
reported in [9].
1,4-Bis(1H-pyrrol-2-yl)benzene (I). mp 289
290 C (decomp.); published data [7]: mp 298 307 C.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 77 No. 8 2007

1394
PETRUSHENKO et al.
13. Trofimov, B.A., Korostova, S.E., Shevchenko, S.G.,
ACKNOWLEDGMENTS
Mikhaleva, A.I., and Matel’, T.L., Zh. Org. Khim.,
The authors thank Prof. L.B. Krivdin for his help
in performing quantum-chemical calculations.
1996, vol. 32, no. 6, p. 897.
14. Lakowicz, J.R., Principles of Fluorescence Spectros-
copy, New York: Plenum, 1983. Translated under the
title Osnovy fluorestsentnoi sSpektroskopii, Moscow:
Mir, 1986, p. 266.
This study was performed under financial support
by the Russian Foundation for Basic Research (project
no. 06-03-08051-ofi) and by the President of the
Russian Federation (program for support of leading
scientific schools, project no. NSh-544.2006.3).
15. Gordon, A.J. and Ford, R.A., The Chemist’s Com-
panion, New York: Wiley, 1972. Translated under the
title Sputnik khimika, Moscow: Mir, 1976, p. 266.
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