Charge transfer fluorescence of trans-stryrylpyridinium iodides

Article abstract of DOI:10.1016/j.jphotochem.2010.12.019

The photoprocesses of trans-1-methyl-4-[4-R-styryl]pyridinium iodide (R = H, P1) and derivatives with a cyano, a nitro and a methoxy group at the phenyl moiety, P2-P4, respectively, were studied in solution. In solvents of relatively low polarity, e.g. tetrahydrofuran, where contact ion pairs are present, the fluorescence spectrum of the styrylpyridinium is significantly red-shifted and the quantum yield is strongly enhanced. These findings are due to photoinduced electron transfer from I^- to the excited singlet state of the cation. The features of complementary trans-styrylquinolinium iodides are in good accordance.

Full text of DOI:10.1016/j.jphotochem.2010.12.019

Journal of Photochemistry and Photobiology A: Chemistry 218 (2011) 199–203
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Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Charge transfer fluorescence of trans-stryrylpyridinium iodides
Helmut Görner
Max-Planck-Institut für Bioanorganische Chemie, D-45470 Mülheim an der Ruhr, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
The photoprocesses of trans-1-methyl-4-[4-R-styryl]pyridinium iodide (R = H, P1) and derivatives with a
cyano, a nitro and a methoxy group at the phenyl moiety, P2–P4, respectively, were studied in solution.
In solvents of relatively low polarity, e.g. tetrahydrofuran, where contact ion pairs are present, the fluo-
rescence spectrum of the styrylpyridinium is significantly red-shifted and the quantum yield is strongly
enhanced. These findings are due to photoinduced electron transfer from I− to the excited singlet state
of the cation. The features of complementary trans-styrylquinolinium iodides are in good accordance.
© 2011 Elsevier B.V. All rights reserved.
Received 19 November 2010
Received in revised form
22 December 2010
Accepted 30 December 2010
Available online 4 January 2011
Keywords:
CT
Fluorescence
Iodide
Styrylpyridinium
Styrylquinolinium
1. Introduction
Photoinduced electron transfer can play a decisive role when the
anion is iodide, in contrast to perchlorates or other innocent anions
The photophysical and photochemical properties of quaternary
azastilbenes (also denoted as stilbazolium salts) in solution are
the subject of various studies [1–9]. Several iodides of the struc-
tural type trans-1-methyl-4-[4-R-styryl]pyridinium (+At-R) with R:
H, cyano, nitro, methoxy (P1, P2, P3, P4, respectively) have been
investigated [4–7]. Quaternary dialkylaminoazastilbenes, such as
trans-1-methyl-4-[4-dimethylaminostyryl]pyridinium iodide (P-
NMe2), have frequently been applied as fluorescence sensors
[10–18]. UV irradiation of the trans-isomer of parent P1 yields the
cis isomer, but this does not take place for P-NMe2 [9]. The quan-
tum yield (˚c) of trans → cis isomerization of the iodides P1–P4 as
well as the perchlorates (Pꢀ1–Pꢀ4) in a polar solvent, such as ace-
tonitrile or water, is ˚c = 0.4–0.5 [4–7]. A similar mechanism has
been reported for styrylquinolinium salts [4–9].
[2,5]. The spectroscopy of specific pyridinium iodides is of special
interest, because these salts exhibit an intermolecular CT absorp-
tion band [25–30]. The CT interaction is also the subject of various
studies concerning salts with aromatic cations [31–33]. The inter-
molecular CT fluorescence is a rare phenomenon in the literature
[34].
Here, the photophysical CT properties of iodides of trans-
1-methyl-4-[4-R-styryl]pyridinium with R: H, CN, NO2 and
OMe, P1–P4, respectively, and trans-1-methyl-4-[4-R-styryl]
quinolinium iodides with R: H, NO2 and OMe (Q1, Q3, Q4) were
studied. The fluorescence maxima of most iodides in solvents of
low polarity are strongly red-shifted, especially for P3 and Q3. The
corresponding perchlorates (Pꢀ1–Pꢀ4) and (Qꢀ1, Qꢀ3, Qꢀ4) show no
CT effects and were used for the purpose of comparison, Chart 1.
Other photochemical studies deal with related betaines, e.g.
trans-1-methyl-4-[4-R-styryl]pyridinium, R = OH, for which, in
aqueous solution, an equilibrium between the hydroxide and the
deprotonated forms with pKa = 8.5 has to be considered [19–21].
Characteristic spectroscopic properties of styrylaromatic salts have
been reviewed [1,2]. Photodimerization is another photoprocess of
quaternary azastilbenes [22]. The non-linear optical properties of
styrylpyridinium ions are the subject of various studies [23,24]. The
properties of photochromic molecules and materials and the ion-
pair charge transfer (CT) complexes have been summarized [24].
2. Experimental
The salts were the same as used previously [3–9]. The molar
absorption coefficients in methanol solution are ε348 = 2.5 × 104
and ε370 = 2.4 × 104 M−1 cm−1 for P1 and P4, respectively [7].
The solvents (Aldrich, Merck) were of the purest spectroscopic
quality, methyltetrahydrofuran (MTHF); dioxane, tetrahydrofuran
(THF), dichloromethane, dimethyl sulfoxide (DMSO) and acetoni-
trile were Uvasol quality. The absorption spectra were monitored
on a diode array spectrophotometer (HP, 8453). The iodides were
dissolved in THF either by using a DMSO stock solution (<1%) or
with ultrasound. In the former case, the absorption spectra change
E-mail address: goerner@mpi-muelheim.mpg.de
1010-6030/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jphotochem.2010.12.019

200
H. Görner / Journal of Photochemistry and Photobiology A: Chemistry 218 (2011) 199–203
-
-
-
-
R
I
ClO4
R
I
ClO4
R: H
CN
NO2
P1
P2
P3
P'1
P'2
P'3
P'4
R: H
NO2
OCH3
CH3
Q1 Q'1
Q3 Q'3
Q4 Q'4
+
N
+
CH3
N
OCH3 P4
Chart 1.
Table 1
with time (in the 1–200 min. range) in the manner that the CT
absorption bands (ACT) at 280–300 and ca. 450 nm became max-
imum. On the other hand, a too long storage (>1 d) was found
to decrease both ACT and the fluorescence intensity markedly. A
spectrofluorimeter (Varian Cary, eclipse) was employed to measure
the fluorescence spectra. The quantum yield of fluorescence was
obtained using 9,10-diphenylanthracene in air-saturated cyclohex-
ane (˚f = 0.6) and rhodamine 101 in ethanol (˚f = 0.9) as references.
The quantum yield ˚c of trans → cis isomerization was obtained
using P1 in argon-saturated acetonitrile as reference, ˚c = 0.5 [7].
The ratio of absorption coefficients εt/εc and the quantum yields of
cis → trans and trans → cis photoisomerization are:
Maximum of trans isomer, isosbestic point, relative CT absorption and quantum
yield of trans → cis isomerization.a
b
Comp.
Solvent
ꢀt (nm)
ꢀi (nm)
A450/Amax
˚
c
Pꢀ1
P1
THF
THF
344
344
357
343
344
344
343
345
338
360
375
374
372
305
310
302
290
310
310
303
310
300
<0.001
0.10
0.002
<0.001
<0.001
0.09
0.5
0.03
0.012
0.5
Dichloromethane
Acetonitrile
THF
THF
THF
Dichloromethane
Acetonitrile
THF
THF
THF
Pꢀ2
P2
P3
0.5
0.03
0.02
0.07
0.4
0.4
0.4
0.10
0.002
<0.001
0.09
0.10
0.09
P4
Q1
Q3
ꢀ
ꢁ
cis
εt × ˚c
=
(I)
313
314
0.005
0.4
trans
εc × ˚t
0
Acetonitrile
<0.001
Electrochemical measurements were performed in an air-
a
In air-saturated solution.
Under argon using ꢀirr = 366 nm.
b
tight cell under an argon atmosphere in anhydrous acetonitrile
with Bu4NPF6 as the supporting electrolyte, using a potentiostat
(EG&G, 273A) with a 0.01 N AgNO3/Ag in acetonitrile as refer-
ence electrode. Glassy carbon disk electrodes served as working
and counter electrodes. The potentials are given as peak potentials
of square wave voltammograms vs. ferrocene/ferrocenium values:
−E◦ꢀ = 1.34, 1.19, 1.14 and 1.40 V for P1–P4, respectively. The mea-
surements refer to 24 ◦C.
a few h. As a measure of this CT absorption, the relative contribu-
tion A450/Amax is given in Table 1; this ratio is 0.002–0.1 in THF and
much smaller in dichloromethane. An example of the concentra-
tion effects is shown in the inset of Fig. 2, indicating the absence of
aggregation.
Photolysis of a trans isomer leads to the cis isomer and eventually
a photostationary state, the latter of which results from a further
decay pathway, i.e. of the singlet-excited cis isomer leading to the
trans and cis isomers. For a given trans-styrylpyridinium iodide ˚c
is substantial in acetonitrile but smaller in dichloromethane and
very small in THF. The time dependence of A350 indicates the effi-
cient trans → cis photoisomerization of the perchlorate in contrast
to the iodide (Fig. 3, inset). Examples of the absorption changes vs.
time of irradiation at 366 nm are shown in Fig. 3. The absorption
spectra of Pꢀ1 in THF upon irradiation at 366 nm show an isosbestic
point at ꢀi = 300 nm.
3. Results
3.1. Absorption
The absorption spectra of trans-styrylpyridinium salts in
polar solvents, e.g. acetonitrile or ethanol, have maxima (ꢀt) at
340–380 nm. The peak at ꢀt = 344 nm is the same for either Pꢀ1 or P1,
but the iodide absorbs weekly at 450 nm, in contrast to the perchlo-
rate. An example of the thermal absorption changes in THF is shown
in the inset of Fig. 1. The absorption at 280 and 450 nm increases
with time and at ꢀt decreases. These changes become minor after
1.0
2
I
f
1.0
3
A
1
1
A
I
0.5
1.0
1
0
2
1
f
λ
300
350 (nm) 400
0.5
2
300
350 (nm4) 00
λ
0.5
0.0
3
4
4
400
0.0
300
400
500
600
300
500
600
λ
λ
(nm)
(nm)
Fig. 1. Fluorescence emission (ꢀex = 450 nm) and excitation (ꢀf = 530 nm) spectra in
THF at 10 and 100 min after mixing of 0.5% P4 in DMSO; inset: absorption spectra
at 1, 10 and 100 min (1–3), respectively.
Fig. 2. Fluorescence emission (ꢀex = 450 nm) and excitation (ꢀf = 600 nm) spectra of
Q3 in THF-DMSO (100:1) upon each 50% dilution by THF (1–4, respectively); inset:
corresponding absorption spectra.

H. Görner / Journal of Photochemistry and Photobiology A: Chemistry 218 (2011) 199–203
201
Table 4
CT fluorescence emission and excitation maxima and quantum yield in THF.a
A
λ
2
1
0
CT
ex
CT
Comp.
ꢀ
(nm)
ꢀCf T (nm)
˚
f
1.5
P1
P2
P3
P4
Q1
Q3
440
440
430
450
450
440
485, 520sh
510, 540sh
590
490, 525sh
490, 520sh
590
0.10
0.09
0.1
0.08
0.1
1
4
3
A
1.0
0.08
0
50
Time (s)
a
In air-saturated solution using ꢀexc = 450 nm, ꢀf = 530 nm, 600 nm for P3/Q3.
2
1.0
P3
P2
300
350
400
450
λ
(nm)
I
f
Fig. 3. Absorption spectra of Pꢀ1 in argon-saturated THF at 0 (1) and 20 s irradiation
at 366 nm (2) and P1 in THF at 0 and 200 s (3 and 4); inset: time dependences at
350 nm of Pꢀ1 (ꢁ) and P1 (o).
0.5
3.2. Fluorescence
The fluorescence of trans-styrylpyridinium iodides P1–P3 in
acetonitrile has a maximum (ꢀf) at 420–440 nm (Table 2). The flu-
orescence excitation spectrum with peaks at ꢀex = 340–360 nm is
similar to the absorption spectrum. The quantum yield ˚f of any
styrylpyridinium iodide is very low in THF, dichloromethane or ace-
tonitrile, if excited at ꢀt = 340–360 nm. However, upon excitation
of P1–P4 in THF at 450 nm, a new fluorescence emission spec-
trum appears which has a red-shifted maximum (ꢀCf T). This peak
is 490 nm for P1/Q1 and 590 nm for P3/Q3. The fluorescence exci-
tation spectrum has likewise a red-shifted maximum (ꢀCexT) with
0.0
400
500
600
λ
(nm)
Fig. 4. Fluorescence emission (ꢀex = 450 nm) and excitation (ꢀf = 530 nm) spectra of
P2 and P3 in THF.
P4: OCH3
respect to ꢀt. The ꢀCT and ꢀeCxT values are compiled in Tables 3 and 4
f
2.1
together with ˚Cf T which in THF is 0.08–0.1. Examples of the fluo-
rescence emission and excitation spectra are shown in Figs. 1 and 4.
For Q3 in THF it can be seen that ꢀCexT is independent of the dye con-
centration using a ca. 10-fold variation up to 50 ␮M (Fig. 2). The
styrylpyridinium iodides exhibit a one-electron reduction at poten-
tials influenced by the substituent. Fig. 5 shows a plot of 1/ꢀCf T of
1/λfCT
(104 cm-1)
P2: CN
P1: H
P1–P4 in THF, corresponding to ꢀCf T = 485–590 nm, as a function of
−E◦ꢀ, indicating a linear relationship of the excited singlet state CT
level on the anionic peak potential.
1.8
P3: NO2
Table 2
-E0' (V)
1.2
1.3
1.4
Fluorescence maximum and quantum yield.a
b
Comp.
Pꢀ1
Solvent
ꢀex (nm)
ꢀf (nm)
˚
f
Fig. 5. Plot of the reciprocal fluorescence maximum of the styrylpyridinium iodides
in THF (ꢀex = 450 nm) as a function of the anionic peak potential.
THF
345
358
345
345
340
364
424
425
428
440
440
503
0.0006
0.0006
0.001
0.005
0.005
0.004
Dichloromethane
Acetonitrile
THF
Acetonitrile
THF
Pꢀ3
4. Discussion
Pꢀ4
In polar solvents +At and X− are free ions [3]. In solvents of low
polarity the anion is in close vicinity of the cation and via solvent
separated ion pairs (+At/I−) contact ion pairs (+At X−) are formed,
see Scheme 1. To characterize the solvent polarity, the emperical
parameter ETN, based on a pyridinium N-phenolate betaine, has fre-
quently been used [35]. The ground state properties of stilbazolium
salts are determined by the solvent polarity in the manner that
the ions are free for ETN > 0.4 and present as contact pairs for
0.15 < ETN < 0.3 [3].
a
In air-saturated solution.
ꢀexc = 350 nm.
b
Table 3
CT fluorescence emission and excitation maximum and quantum yield.a
CT
CT
˚
f
Solvent
ETN
P1: ꢀCf T (nm)
˚
P3: ꢀCf T (nm)
f
Dioxane
Trichloroethylene
MTHF
0.16
0.16
0.18
0.21
0.26
0.31
590
590
590
590
590
590
0.08
0.1
<0.01
<0.001
4.1. Photoprocesses of ions
THF
485
485
485
0.08
0.003
<0.001
Chloroform
Dichloromethane
The absorption spectra of various trans-stilbazolium salts in
polar solvents have maxima at ꢀt = 340–375 nm which are inde-
a
In air-saturated solution using ꢀexc = 450 nm, ꢀf = 530 nm, 600 nm for P3/Q3.

202
H. Görner / Journal of Photochemistry and Photobiology A: Chemistry 218 (2011) 199–203
pentent of the type of anion [2,3]. The ground state of a cation is
characterized by an activation barrier between the trans and cis
geometries. The thermal cis → trans back-reaction is accessible at
room temperature and sensitive to structure and solvent polarity
[3]. The quantum yields ˚t and ˚c of isomerization in acetoni-
trile are substantial, based on values for P1–P4. The triplet pathway
for trans → cis photoisomerization, due to a high quantum yield
of intersystem crossing (˚isc), is a unique feature of arylethylenes
which are substituted by a nitro group [2,4–6]. Excitation of P3 in
a polar solvent, e.g. ethanol or acetonitrile, leads from the excited
singlet (1*+At) via the triplet state (3*+At) after further intersystem
crossing into both isomers [4–7]. In contrast to P3, excitation of
other cations not containing a nitro group leads from 1*+At into the
cis–trans mixture via singlet states [7]. A conceivable mechanism is
constituted by reactions (1ꢀ)–(6ꢀ).
lar excited singlet state in rigid media. The deactivation pathway of
1*+At at room temperature, besides fluorescence to the trans isomer,
is internal conversion to the +Ap transition state, either directly (P1,
P2, P4) or after intersystem crossing (P3). Here, ˛ is the probabil-
ity to react into the cis-isomer +Ac. The dimethylamino derivative
P-NMe2 does not follow this pattern since the excited singlet state
is too low in energy to overcome the barrier in the excited singlet
state, required for a path to the cis-isomer. Nevertheless, the major
deactivation is an activated internal conversion step at the trans
geometry due to intramolecular CT [9].
4.2. CT fluorescence
Excitation of the contact ion pair (+At I−) leads to the excited
singlet state with CT character, step (1), and to the emission of light,
step (2).
+At hꢁ → 1∗+At
1∗+At hꢁf → +At
(1ꢀ)
(2ꢀ)
(+AtI−) hꢁCT → 1 ∗ (+AtI−)
(1)
(2)
(
1∗AtI−) → hꢁCT → (+AtI−)
1∗+At
→
3∗+At
(3ꢀ)
f
1∗+At(or 3∗+At) → +Ap
+Ap → ˛+Ac + (1 − ˛)+At
(4ꢀ5ꢀ)
(6ꢀ)
The fluorescence emission and excitation CT spectra of the
styrylpyridinium iodides in THF show a mirror image relationship,
see Figs. 1, 2 and 4. The blue (P4), green (P2) or yellow (P3/Q3) emis-
sion is efficient, see the substantial ˚Cf T values in Tables 3 and 4.
Concerning these effects there are only a few cases to compare in
the literature [34] and no ˚Cf T values as yet. A related effect deal-
ing with emission and excitation CT spectra is phosphorescence of
pyridinium iodides at low temperatures [26].
•
1∗+At (or 3∗+At) + D → A + D•
(7ꢀ)
+
If the donor is iodide, a radical could be intermediate to the
reduced photoproduct. The electron transfer reaction (7ꢀ) from
iodide or amines to the excited singlet and triplet states of trans-
4-R-styrylpyridinium and corrresponding styrylquinolinium salts
in polar solvents have been presented earlier [5–8]. The triplet
state can be quenched by oxygen and/or is the observed key
intermediate for electron transfer from an appropriate donor,
when oxygen is removed. To account for the latter, reaction
(7ꢀ) is proposed. Generally, the radical pathway competes with
trans → cis photoisomerization. The potential energy surface of
any styrylpyridinium cation should have a minimum at the per-
pendicular geometry of the excited singlet state, from where
the radiationless transition to the perpendicular configuration
of the ground state (+Ap) takes place. Linear correlations have
been reported between solvent polarity and adiabatic and dia-
batic transition energies for the thermal cis → trans isomerization
of solvatochromic hydroxy-styrylpyridinium type merocyanines
[19–21]. A relationship between the rate of thermal cis → trans
isomerization and the solvent polarity is also known for related
systems [3].
The plot of 1/ꢀCf T of the four trans-styrylpyridinium iodides vs.
−E◦ꢀ (Fig. 5) reveals a linear dependence of the excited singlet state
level on the potential. Note that the fluorescence bands of P1, P2
and P4 have a red-shifted shoulder and therefore the 1/ꢀCf T mid-
point would be somewhat lower. No differences in ꢀt, ꢀf or ˚f
were found for the methoxystyrylquinolinium iodide Q4 or per-
chlorate Qꢀ4 in THF (420, 560 nm or 0.01, respectively) in contrast
to the trans-methoxystyrylpyridinium salts Pꢀ4 and P4, indicating
the absence of CT interaction in the quinolinium case. The fluores-
cence of styrylpyridinium iodides frequently deals with free cations
of P-NMe2 type [9–18], but for these molecules no intermolecular
CT fluorescence is known as yet. The presence of such a dialky-
lamino group may cause a fast deactivation via internal conversion
at the trans geometry, thus competing with emission.
4.3. Deactivation pathways for ion pairs
The major deactivation of the trans isomer of the singlet-excited
cation 1*+At is trans → cis photoisomerization and fluorescence
plays a minor role. At low temperatures, however, the contribu-
tion of the two pathways interchange to low ˚c and large ˚f [4–8].
This is a consequence of inhibited twisting into the perpendicu-
In competition to fluorescence, intersystem crossing (3) occurs
and formation of a radial pair (4/5), which may react by back elec-
tron transfer or leads to products (6).
1∗(+AtI−) → 3∗(+AtI−)
(3)
1
-
1
+At /I
1
-
*
+At
+At I
*
*
-
- I
(4)
(3)
3
-
+
*
At I
h ν
•
•
CT
CT
(5)
A + I
h ν
(2)
-
f
a
(6)
-
-
+At + I
+At I
+At /I
product
Scheme 1.

H. Görner / Journal of Photochemistry and Photobiology A: Chemistry 218 (2011) 199–203
203
References
-
I
H
H
+
hνCT
N CH3
[1] A. Mishra, R.K. Behera, P.K. Behera, B.K. Mishra, G.B. Behera, Chem. Rev. 100
(2000) 1973.
R
N
CH3
R
R
•
-I•
+I •
[2] H. Görner, H.J. Kuhn, Adv. Photochem. 19 (1995) 1.
[3] H. Güsten, D. Schulte-Frohlinde, Z. Naturforsch. 34b (1979) 1556.
[4] H. Görner, D. Schulte-Frohlinde, Chem. Phys. Lett. 101 (1983) 79.
[5] H. Görner, D. Schulte-Frohlinde, J. Phys. Chem. 89 (1985) 4105.
[6] H. Görner, J. Phys. Chem. 89 (1985) 4112.
-
H
H
I
+
R
N
CH3
N
CH3
Ion pair
I
[7] H. Görner, A. Fojtik, J. Wróblewski, L.J. Currell, Z. Naturforsch. 40a
(1985) 525.
[8] (a) H. Görner, J. Phys. Chem. 91 (1987) 1187;
Scheme 2.
(b) H. Görner, Phys. Chem. Chem. Phys. 4 (2002) 482.
1∗(+AtI−) → (A• + I• )
3∗(+AtI−) → (A• +I• )
(4)
[9] H. Görner, H. Gruen, J. Photochem. 28 (1985) 329.
[10] H. Eberhardt, P. Fromherz, J. Phys. Chem. 93 (1989) 7717.
[11] M.J. Hinner, G. Hübener, P. Fromherz, J. Phys. Chem. B 108 (2004) 2445.
[12] B. Strehmel, H. Seifert, W. Rettig, J. Phys. Chem. B 101 (1997) 2232.
[13] M.S. Antonious, Spectrochim. Acta A 53 (1997) 317.
[14] A. Mishra, G.B. Behera, M.M.G. Krishna, N. Periasamy, J. Lumin. 92 (2001) 175.
[15] Z. Tian, Y. Chen, W. Ynag, J. Yao, L. Zhu, Z. Shuai, Angew. Chem. Int. Ed. 43 (2004)
4060.
[16] S. Amoroso, V.V. Agon, T. Starke-Peterkovic, M.D. McLeod, H.-J. Apell, P. Sebban,
R.J. Clarke, Photochem. Photobiol. 82 (2006) 495.
[17] S.J. Yamada, N. Uematsu, K. Yamashita, J. Am. Chem. Soc. 129 (2007) 12100.
[18] (a) A.L. Lusk, P.W. Bohn, Langmuir 16 (2000) 9131;
(b) S.A. El-Daly, M.H. Abdel-Kader, R.M. Issa, E.-S.A. El-Sherbini, Spectrochim.
Acta A 59 (2003) 405;
(c) J. Kabatc, B. Je˛drzejewska, P. Orlin´ ski, J. Pa˛czkowski, Spectrochim. Acta A 62
(2005) 115;
(d) T.H.N. Pham, R.J. Clarke, J. Phys. Chem. B 112 (2008) 6513;
(e) H. Yao, M. Yamashita, K. Kimura, Langmuir 25 (2009) 1131.
[19] U.E. Steiner, M.H. Abdel-Kader, P. Fischer, H.E.A. Kramer, J. Am. Chem. Soc. 100
(1978) 3190.
(5)
(6)
A• +I• → (+AtI−) + products
Electron transfer steps (4) or (5) take place for iodides due to the
low ionization potential, but not where perchlorates are present.
For a possible reduction product, see Schemes 1 and 2. The major
reaction is electron back transfer to the undamaged ion pair. The
transient absorption spectra of the trans-nitrostyryl salts reveal a
triplet state after the 308 or 353 nm pulse for iodides (P3, Q3) in
polar solvents and for the perchlorates (Pꢀ3, Qꢀ3) in any medium
examined. The longer lived transient of P3 or Q3 in THF is ascribed
to the nitrostyrylaromatic radical, i.e. electron transfer occurs also
from the triplet state [6].
Fluorescence quenching of aromatic compounds in acetoni-
trile by iodide has been investigated [32]. For benzoquinolizinium
iodides a significant solvatochromism has been studied [28]. For a
bispyridinoethylene (BPE2+) in acetonitrile, electron transfer with
a quantum yield of 0.1, producing I2•− and the corresponding BPE•+
radical, has been reported [33]. It should be recalled that pyri-
dinium iodides are model compounds to reveal the degree of CT
character in a given solvent [25–30]. Interestingly, the CT absorp-
tion of a pyrylium iodide has been introduced by Kosower as an
emperical solvent polarity parameter: Z-value [29]. This Z-value
is in good agreement proportional to the ETN-value of numerous
solvents [35,36].
[20] (a) S.A. El-Daly, M.H. Abdel-Kader, R.M. Issa, E.A. El-Sherbini, Spectrochim. Acta
A 59 (2003) 405;
(b) T. Kolev, B.B. Koleva, S. Stoyanov, M. Spiteller, I. Petkov, Spectrochim. Acta
A 70 (2008) 1087.
[21] S.T. Abdel-Halim, M.H. Abdel-Kader, U.E. Steiner, J. Phys. Chem. 92 (1988) 4324.
[22] (a) F.H. Quina, D.G. Whitten, J. Am. Chem. Soc. 99 (1977) 877;
(b) D.G. Whitten, J. Am. Chem. Soc. 108 (1986) 7865.
[23] (a) C. Chen, J. Photochem. Photobiol. A: Chem. 109 (1997) 155;
(b) C.-L. Zhan, D.-Y. Wang, J. Photochem. Photobiol. A: Chem. 147 (2002) 93;
(c) X. Deng, Z. Xiao-Hong, Chin. Phys. Lett. (2009) 077806.
[24] F. Ito, T. Nagamura, J. Photochem. Photobiol. C: Rev. 8 (2007) 174.
[25] (a) A.T. Balaban, M. Mocanu, Z. Simon, Tetrahedron 20 (1964) 119;
(b) J.W. Verhoeven, I.P. Dirkx, Th.J. De Boer, Tetrahedron 25 (1969) 3395;
(c) K. Kalyanasundaram, T. Colassis, R. Humphry-Baker, P. Savarino, E. Barni, E.
Pelizzetti, M. Grätzel, J. Am. Chem. Soc. 111 (1989) 3300;
(d) A.M. Moran, S. Park, N.F. Scherer, J. Phys. Chem. B 110 (2006) 19771.
[26] (a) G. Briegleb, W. Jung, W. Herre, Z. Phys. Chem. NF 38 (1963) 253;
(b) G. Briegleb, W. Herre, W. Jung, H. Schuster, Z. Phys. Chem. NF 45 (1965) 229.
[27] (a) F. Lehmann, G.J. Mohr, P. Czerney, U.-W. Grummt, Dyes Pigments 29 (1995)
85;
(b) U.-W. Grummt, E. Birckner, H. Lindauer, B. Beck, R. Rotomskis, J. Photochem.
Photobiol. A: Chem. 104 (1997) 69.
[28] S. Arai, M. Hida, T. Yamagishi, Dyes Pigments 29 (1995) 263.
[29] (a) E.M. Kosower, J. Am. Chem. Soc. 80 (1958) 3253;
(b) E.M. Kosower, Can. J. Chem. 83 (2005) 1207.
[30] G.G. Aloisi, F. Elisei, L. Latterini, Gazz. Chim. Ital. 124 (1994) 181.
[31] (a) E. Marri, U. Mazzucato, C.G. Fortuna, G. Masumarra, A. Spaletti, Photochem.
Photobiol. A: Chem. 179 (2006) 314;
5. Conclusions
The effects of solvents on the ground and excited singlet states
of trans-4-R-styrylpyridinium and complementary styrylquinolin-
ium iodides with R = OCH3, H, CN and NO2 were examined. The CT
absorption and fluorescence features are due to photoinduced elec-
tron transfer from iodide to the excited singlet state of the cation.
They are most pronounced in THF, where contact ion pairs are
present. The excited singlet state level depends on the reduction
potential. A major finding is that the quantum yield of CT fluores-
cence is relatively large: ˚f strongly enhanced upon excitation of
the CT band centered at 440–450 (or 300) nm with respect to that
of the hypsochromicly shifted main absorption band.
(b) C.G. Fortuna, U. Mazzucato, G. Musumarra, D. Pannacci, A. Spalletti, J. Pho-
tochem. Photobiol. A: Chem. 216 (2010) 66.
[32] M. Mac, J. Wirz, J. Najbar, Helv. Chim. Acta 76 (1993) 1319.
[33] T.W. Ebbesen, C.M. Previtali, T. Karatsu, T. Arai, K. Tokumaru, Chem. Phys. Lett.
119 (1985) 489.
[34] (a) Z.-X. Li, C.-H. Xu, W. Sun, Y.-C. Bai, C. Zhang, C.-J. Fang, C.-H. Yan, New J.
Chem. 33 (2009) 853;
Acknowledgments
(b) H.-H. Lin, C.-C. Cheng, Dyes Pigments 83 (2009) 230.
[35] C. Reichardt, Chem. Rev. 94 (1994) 2319.
[36] (a) C. Reichardt, Org. Proc. Res. Dev. 11 (2007) 105;
(b) C. Reichardt, Green Chem. 7 (2005) 339.
The author thanks Professor Wolfgang Lubitz for his support and
Mrs. Petra Höfer and Mr. Leslie J. Currell for technical assistance.