The role of ion pairs in the second-order NLO response of 4-X-1-methylpiridinium salts

Article abstract of DOI:10.1002/cphc.200900696

A series of 4-X-1-methylpyridinium cationic nonlinear optical (NLO) chromophores (X=(E)-CH=CHC₆H₅; (E)-CH=CHC₆H₄-4′-C(CH₃)₃; (E)-CH=CHC₆H₄-4′-N(CH₃)₂; (E)-CH=CHC₆H₄-4′-N(C₄H₉)₂; (E,E)-(CH=CH)₂C₆H₄-4′-N(CH₃)₂) with various organic (CF₃SO₃, p-CH₃C₆H₄SO₃^-), inorganic (I^-, ClO₄^-, SCN^-, [Hg₂I₆]^2-) and organo-metallic (cis-[Ir(CO)₂I₂]) counter anions are studied with the aim of investigating the role of ion pairing and of ionic dissociation or aggregation of ion pairs in controlling their second-order NLO response in anhydrous chloroform solution. The combined use of electronic absorption spectra, conductimetric measurements and pulsed field gradient spin echo (PGSE) NMR experiments show that the second-order NLO response, investigated by the electric-field-induced second harmonic generation (EFISH) technique, of the salts of the cationic NLO chromophores strongly depends upon the nature of the counter anion and concentration. The ion pairs are the major species at concentration around 10 ^-3 m, and their dipole moments were determined. Generally, below 5×10 ^-4 m, ion pairs start to dissociate into ions with parallel increase of the second-order NLO response, due to the increased concentration of purely cationic NLO chromophores with improved NLO response. At concentration higher than 10 ^-3 m, some multipolar aggregates, probably of H type, are formed, with parallel slight decrease of the second-order NLO response. Ion pairing is dependent upon the nature of the counter anion and on the electronic structure of the cationic NLO chromophore. It is very strong for the thiocyanate anion in particular and, albeit to a lesser extent, for the sulfonated anions. The latter show increased tendency to self-aggregate.

Full text of DOI:10.1002/cphc.200900696

DOI: 10.1002/cphc.200900696
The Role of Ion Pairs in the Second-Order NLO Response
of 4-X-1-Methylpiridinium Salts**
Francesca Tessore,*[a] Elena Cariati,[a] Franco Cariati,[a] Dominique Roberto,[a] Renato Ugo,[a]
Patrizia Mussini,[b] Cristiano Zuccaccia,[c] and Alceo Macchioni[c]
A series of 4-X-1-methylpyridinium cationic nonlinear optical
(NLO) chromophores (X=(E)-CH=CHC6H5; (E)-CH=CHC6H4-4’-
C(CH3)3; (E)-CH=CHC6H4-4’-N(CH3)2; (E)-CH=CHC6H4-4’-N(C4H9)2;
(E,E)-(CH=CH)2C6H4-4’-N(CH3)2) with various organic (CF3SO3ꢀ, p-
ter anion and concentration. The ion pairs are the major spe-
cies at concentration around 10ꢀ3 m, and their dipole moments
were determined. Generally, below 5ꢀ10ꢀ4 m, ion pairs start to
dissociate into ions with parallel increase of the second-order
NLO response, due to the increased concentration of purely
cationic NLO chromophores with improved NLO response. At
concentration higher than 10ꢀ3 m, some multipolar aggregates,
probably of H type, are formed, with parallel slight decrease of
the second-order NLO response. Ion pairing is dependent
upon the nature of the counter anion and on the electronic
structure of the cationic NLO chromophore. It is very strong
for the thiocyanate anion in particular and, albeit to a lesser
extent, for the sulfonated anions. The latter show increased
tendency to self-aggregate.
ꢀ
CH3C6H4SO3 ), inorganic (Iꢀ, ClO4ꢀ, SCNꢀ, [Hg2I6]2ꢀ) and organo-
metallic (cis-[Ir(CO)2I2]ꢀ) counter anions are studied with the
aim of investigating the role of ion pairing and of ionic dissoci-
ation or aggregation of ion pairs in controlling their second-
order NLO response in anhydrous chloroform solution. The
combined use of electronic absorption spectra, conductimetric
measurements and pulsed field gradient spin echo (PGSE)
NMR experiments show that the second-order NLO response,
investigated by the electric-field-induced second harmonic
generation (EFISH) technique, of the salts of the cationic NLO
chromophores strongly depends upon the nature of the coun-
1. Introduction
The structural features of organic and organometallic mole-
cules necessary to produce significant second-order nonlinear
optical (NLO) properties are well known; for instance, in the
last two decades many different architectures of the so-called
one-dimensional (1D) push–pull NLO chromophores have been
investigated.[1]
line structure[3,4] and also in self-assembled thin films[5] of some
of their salts.
To investigate the role of ion pairing, we explored the effect
of the nature of the counter anion on the second-order NLO
response of various 4-X-1-Methylpyridinium salts (X=(E)-CH=
CHC6H5 (1); (E)-CH=CHC6H4-4’-C(CH3)3 (2); (E)-CH=CHC6H4-4’-
N(CH3)2 (3); (E)-CH=CHC6H4-4’-N(C4H9)2 (4); (E,E)-(CH=CH)2-C6H4-
4’-N(CH3)2 (5); see Table 1) in a solvent of low dielectric con-
In 1D push–pull cationic NLO chromophores the positive
charge lowers the energy of intramolecular charge transfer
(ICT), and thus leads to an increase in the second-order NLO
response. A detailed investigation on the second-order NLO re-
sponses of a series of N-methyl and N-aryl pyridinium salts was
done by Coe et al,[2] who showed that their ICT is characterized
by a significant increase in dipole moment upon excitation
from the ground to the excited state (Dmeg), and therefore they
are the origin of a considerable static quadratic hyperpolariza-
bility b0. The second-order NLO response was measured by the
hyper Rayleigh scattering (HRS) technique and by an indirect
method involving the Stark effect, with acetonitrile as solvent
[a] Dr. F. Tessore, Prof. E. Cariati, Prof. F. Cariati, Prof. D. Roberto, Prof. R. Ugo
Dipartimento di Chimica Inorganica
Metallorganica e Analitica “Lamberto Malatesta”
dell’Universitꢀ degli Studi di Milano
and Unitꢀ di Ricerca di Milano dell’INSTM
Via G. Venezian, 21-20133 Milano (Italy)
Fax: +39-02-50314405
E-mail: francesca.tessore@unimi.it
[b] Prof. P. Mussini
Dipartimento di Chimica Fisica ed Elettrochimica
dell’Universitꢀ di Milano
Via C. Golgi, 19-20133 Milano (Italy)
ꢀ
or in butyronitrile glasses at 77 K and with PF6 as counter
anion in order to avoid ion pairing as much as possible.
[c] Dr. C. Zuccaccia, Prof. A. Macchioni
Dipartimento di Chimica
Using solvents of lower dielectric constant or counter anions
of higher polarizability could introduce a relevant role of ion
pairing in controlling the second-order NLO response of these
cationic 1D push–pull NLO chromophores, since the nature of
the counter anion may play a significant role in controlling the
alignment of these cationic NLO chromophores in the crystal-
dell’Universitꢀ di Perugia
Via Elce di Sotto, 8-06123 Perugia (Italy)
[**] The Relevant Role of Ion Pairs and of their Dissociation and Aggregation in
Chloroform Solution on the Second-Order Nonlinear Optical Response of
Salts of Various 4-X-1-Methylpyridinium Chromophores: A Spectroscopic,
EFISH, Conductimetric and PGSE NMR Investigation (EFISH: electric-field-in-
duced second harmonic generation, PGSE: pulsed field gradient spin echo)
ChemPhysChem 2010, 11, 495 – 507
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495

F. Tessore et al.
dent wavelength of 1.907 mm)
of ZnII complexes with push–
pull stilbazole ligands such as c
and d (see Figure 1) and a sulfo-
nated anion such as trifluoro-
methanesulfonate (triflate) as
ancillary ligand. At concentra-
tions below 10ꢀ4 m, the value of
mb1.91 remarkably increases with
decreasing concentration and
reaches very large values. Given
the low nucleophilicity of the
triflate anion, this behaviour
was explained by solvolysis of
the triflate ligand to produce
first a kind of ion pair, which
then dissociates by further dilu-
tion to finally generate a cation-
ic ZnII complex, characterized by
an enhanced second-order NLO
response due to the presence
Table 1. Salts of the 4-X-1-methylpyridinium NLO chromophores investigated herein.
X
ꢀ
Yꢀ
Iꢀ
Iꢀ
Iꢀ
CF3SO3
Iꢀ
Iꢀ
ꢀ
ClO4
SCNꢀ
pTSꢀ[a]
[Hg2I6]2ꢀ
[cis-Ir(CO)2I2]ꢀ
[a] pTSꢀ: p-toluensulphonate
ꢀ
stant such as anhydrous CHCl3. The effect of organic (CF3SO3
,
of a positive charge. Careful investigations of the electrical
conductivity of anhydrous CHCl3 solutions of these sulfonated
complexes were in agreement with progressive ionic dissocia-
tion.[10,11]
p-CH3C6H4SO3ꢀ), inorganic (Iꢀ, ClO4ꢀ, SCNꢀ, [Hg2I6]2ꢀ) and or-
ganometallic (cis-[Ir(CO)2I2]ꢀ) counter anions of quite different
polarizability and size was also investigated.
The second-order NLO response was measured by means of
the electric-field-induced second harmonic generation (EFISH)
technique,[6] usually considered off-limits for ionic species.
However, a recent paper[7] showed that this technique could
be applied for determination of the second-order NLO re-
sponse of the iodide of a 1-methylpyridinium chromophore in
CHCl3, a solvent of low dielectric constant which should favour
ion pairs. The use of the EFISH technique, specially suitable for
1D NLO chromophores, instead of HRS[8] can avoid possible
overestimation of the value of the quadratic hyperpolarizability
due to multiphoton fluorescence[9] when a non-resonant
1.907 mm incident wavelength or a femtosecond laser is not
used.[2]
Herein we investigate whether some salts of the 1D cationic
NLO chromophores presented in Table 1 show in anhydrous
CHCl3 solution a second-order NLO response which is strongly
dependent on the nature of the counter anion and on concen-
tration, as expected for progressive dissociation of ion pairs by
dilution or for their aggregation by increasing concentration.
The EFISH investigation is also supplemented by a careful con-
ductivity investigation, which to our knowledge is the first
comprehensive case study of the conductimetric behaviour of
organic salts in a low-polarity solvent such as anhydrous CHCl3,
and by pulsed field gradient spin echo (PGSE) NMR experi-
ments, which have proven to be a useful tool for evaluating
the degree of aggregation of molecular species in solution.[12]
Previously, some of us reported[10,11] on the strong depend-
ence upon concentration in CHCl3 solution of the product
mb1.91 measured by the EFISH technique (b1.91 is, for the sake of 2. Results and Discussion
simplicity, the EFISH quadratic hyperpolarizability at an inci-
2.1. Synthesis of Salts of Pyridinium 1D Second-Order NLO
Chromophores with Various Counter Anions
The various salts of the 1D pyridinium second-order NLO chro-
mophores investigated herein are reported in Table 1. While
the iodides of 3 and 4 are commercially available, the iodides
of 1, 2 and 5 were synthesised by reaction of the correspond-
ing stilbazolic precursor (Figure 1) with an excess of iodome-
thane in toluene or CH2Cl2 solution at room temperature for
2 h. The iodides were obtained in satisfactory yield (60–75%)
by crystallization of the crude products from CH3CN/Et2O (1) or
CH2Cl2/n-hexane (2 and 5). The triflate salt of 4 was synthe-
sized by reaction of the commercially available iodide with a
stoichiometric amount of AgCF3SO3 in CH3CN solution at room
temperature, followed by crystallization of the crude product
from CH2Cl2/n-hexane (88% yield). In a similar way, reaction in
Figure 1. Stilbazolic precursors of the cationic NLO chromophores investigat-
ed herein.
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ChemPhysChem 2010, 11, 495 – 507

4-X-1-Methylpyridinium NLO Chromophores
CH3CN solution of the iodide of 4 with the appropriate silver
salt [Ag(4-CH3C6H4SO3), AgClO4, AgSCN] in a stoichiometric
ratio at room temperature for 2 h, followed by crystallization
of the crude product from CH3CN or CH2Cl2/n-hexane, afforded
the corresponding salts of 4 in satisfactory yield (60–80%).
The salt of 4 with [Hg2I6]2ꢀ was obtained in EtOH solution by
reaction of mercury(II) iodide with two equivalents of the
iodide of 4 at room temperature. The crude product was crys-
tallized from CH2Cl2/n-hexane (47% yield). Finally, the salt of 4
with cis-[Ir(CO)2I2]ꢀ was synthesized by a slight modification of
that reported in the literature.[13] The black polymeric species
[Ir(CO)2Cl]n, obtained by reaction of [{Ir(COT)2Cl}2] (COT=cyclo-
octene) with CO, was treated with an excess of KI, affording
K{cis-[Ir(CO)2I2]}, which was finally added as an aqueous suspen-
sion to a stirred solution of the iodide of 4 in CH2Cl2. The final
product was obtained in 40% yield by appropriate work-up of
the CH2Cl2 layer (see the Experimental Section).
Table 2. Electronic absorption spectra, dipole moments, EFISH mb1.91
values and molar conductivities L of various 4-X-1-methylpiridinium io-
dides, measured in anhydrous CHCl3 solution.
Cation
1
lmax [nm] m [D] EFISH mb1.91 [10ꢀ48 esu] L [Wꢀ1 cm2 molꢀ1
]
800[a]
0.22[a]
0.12[b]
0.10[c]
0.33[a]
0.17[b]
0.12[c]
0.63[a]
0.34[b]
0.34[c]
357
9.1
10.1
14.4
300[b]
170[c] (15.8[e]
810[a]
)
)
2
3
377
600[b]
205[c] (16.2[e]
1700[a]
503[d]
1000[b]
1000[c] (46.3[e]
1900[a]
)
1400[b]
0.63[a]
0.33[b]
0.28[c]
4
519
15.8
1090[c] (45[e]
1040[f]
)
925[g]
1860[a]
0.55[a]
0.27[b]
0.22[c]
5
532
n.d. 1600[b]
1450[c]
[a] Concentration 10ꢀ4 m. [b] Concentration 5ꢀ10ꢀ4 m. [c] Concentration
10ꢀ3 m. [d] Emission of 3 was studied in CHCl3 at different concentrations.
2.2. Spectroscopic Investigations and Measurements of
Dipole Moments
lem =577 nm at 5.5ꢀ10ꢀ6
m
and lem =603 nm at 5.5ꢀ10ꢀ4 m. [e] b0
(ꢀ10ꢀ30 esu) calculated by the two-level model from the b1.91 value at
10ꢀ3 m.[36] [f] Concentration 5ꢀ10ꢀ3 m. [g] Concentration 10ꢀ2 m.
2.2.1. 1H NMR Spectra
1
In the H NMR spectra in CDCl3 (see Experimental Section) of
the iodide salts, the signals of hydrogen atoms in a position to
the pyridine nitrogen atom at around 8 ppm are shifted to
lower fields compared to the corresponding stilbazolic precur-
sor (for example, Dd=0.87 ppm for 1; Dd=0.83 ppm for 2;
Dd=0.47 ppm for 5), as expected for enhanced electron-with-
drawing character of the positively charged nitrogen atom of
the pyridine ring.
Table 3. Electronic absorption spectra, dipole moments, EFISH mb1.91 and
molar conductivity L of various salts of (E)-4-(4’-dibutylaminostyryl)-1-
methylpiridinium, measured in CHCl3 solution.
For the various salts of 4 (see Table 1), the signals of the hy-
drogen atoms in a position to the pyridinic nitrogen atom are
at only slightly higher fields compared to those of the iodide
(Dd is in the range 0.02–0.4 ppm), varying from 8.33 ppm for
Anion
lmax [nm] m [D] EFISH
mb1.91 [10ꢀ48 esu]
L [Wꢀ1 cm2 molꢀ1
]
ꢀ
CF3SO3
515
8.5
1150[a]
0.58[a]
0.34[b]
0.28[c]
0.63[a]
0.33[b]
0.28[c]
710[b]
the salt with cis-[Ir(CO)2I2]ꢀ to 8.71 ppm for the salt with
225[c] (17.7[e]
)
Iꢀ
519
15.8 1900[a]
1400[b]
ꢀ
4-CH3C6H4SO3
.
In all the salts of 1–5, the coupling constant of the protons
of the double bond is 15-16 Hz, as expected for the E geome-
try around the CꢀC double bond.
1090[c] (45[e]
)
1040[f]
925[g]
ꢀ
ClO4
520
515
509
532
523
7.3
1950[a]
0.61[a]
0.33[b]
0.27[c]
0.79[a]
0.39[b]
0.27[c]
0.14[a]
0.10[b]
0.10[c]
2.37[a]
0.92[b]
0.64[c]
1.10[a]
1200[b]
250[c] (22.1[e]
)
2.2.2. Dipole Moments and Electronic Absorption Spectra
SCNꢀ
13.0 1800[a]
1800[b]
Dipole moments m of the various salts (Tables 2 and 3) were
measured in anhydrous CHCl3 solution by the Guggenheim
method[14] at concentration around 10ꢀ3 m. The dipole mo-
ments of ion pairs of some quaternary salts in a series of differ-
ent solvents, including CHCl3, in a range of concentrations
around 10ꢀ3 m were carefully investigated by Grunwald and
co-workers.[15] They reported that, in this range of concentra-
tions, electrolytic dissociation in CHCl3 is relatively small. There-
fore, the correction of the linear plots of dielectric constant
versus concentration due to ionic dissociation is negligible, in
accordance with the very good linear plots of capacitance
versus concentration that we have experimentally obtained
(see Experimental Section). Interestingly, the Guggenheim
method seems to be appropriate for determination of the
1790[c] (90.6[e]
)
pTSꢀ[d]
13.2 1150[a]
780[b]
690[c] (34.5[e]
6050[a]
)
[Hg2I6]2ꢀ
n.d.
4200[b]
3850[c]
cis-
[Ir(CO)2I2]ꢀ
19.6 2020[a]
1250[b]
0.60[b]
0.43[c]
1180[c] (38.8[e]
)
[a] Concentration 10ꢀ4 m. [b] Concentration 5ꢀ10ꢀ4 m. [c] Concentration
10ꢀ3 m. [d] pTSꢀ =p-toluenesulfonate. [e] b0 (ꢀ10ꢀ30 esu) calculated by the
two-level model from the b1.91 value at 10ꢀ3 m.[36] [f] Concentration
5ꢀ10ꢀ3 m. [g] Concentration 10ꢀ2 m.
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497

F. Tessore et al.
dipole moment of ion pairs of these pyridinium salts (Table 1),
although Grunwald and co-workers usually measured the
dipole moment of ion pairs of some other quaternary salts by
applying the more sophisticated Onsager–Kirkwood method.[16]
The dipole moments of the ion pairs are, as expected, quite
high. For example, the dipole moment of the iodide of 2 is
10.1 D (Table 2), while that of the neutral compound obtained
by coordination of the related stilbazole b to the [Os3(CO)11]
moiety was reported to be only 5.1 D.[17a] Accordingly, for the
iodide of 3, the dipole moment is 14.4 D (Table 2), while those
of the neutral complexes of the corresponding stilbazole c, co-
ordinated to cis-[Ir(CO)2Cl], cis-[Ir(COT)2Cl], cis-[Rh(CO)2Cl],
[Rh(COD)Cl] and fac-[Os(CO)3Cl2] were reported to be signifi-
cantly lower (6.0, 8.1, 7.0, 6.7, and 7.4 D respectively).[17b]
Interestingly, for the various salts of 4, the shift of the ICT
band (Table 3) is dependent on the size and polarizability of
the counter anion, as expected for ion pairing. For instance,
the red shift when the counter anion is large, for example,
[Hg2I6]2ꢀ or cis-[Ir(CO)2I2]ꢀ, is higher (0.83–0.85 eV), respectively)
than when the counter anion is of smaller size (around 0.77–
0.80 eV). In fact, in the former case a higher positive charge on
the nitrogen atom of the pyridine ring is expected, due to the
increased average interionic distance in the structure of the
ion pair.
Evidence for further aggregation of ion pairs is given when
the electronic absorption spectra of the iodide of 3 are record-
ed at different concentrations in various solvents. In CHCl3 (die-
lectric constant e=4.8, the lowest among those considered),
the absorption maximum of the ICT shows a slight blue shift
with increasing concentration only at concentrations above
10ꢀ3 m, while in CH2Cl2 (e=8.9) and 1,2-dichloroethane (e=
10.65), already at lower concentrations (about 10ꢀ4 m), blue
shifts of up to 0.07 and 0.04 eV, respectively, occur with in-
creasing concentration.
The dipole moments of salts of 4 with other counter anions
are still quite high and dependent upon the nature of the
counter anion. For instance, salts with relatively small anions of
ꢀ
ꢀ
low polarizability such as CF3SO3 (8.5 D) and ClO4 (7.3 D)
show the lowest values (Table 3).
The significant differences in the dipole moments in CHCl3
solution of ion pairs of 4 with various counter anions can be
partially attributed, according to Grunwald et al.,[15] to a
change in the average interionic structure of the ion pairs, due
to the different contributions to the total dipole moment of
the various counter anions. This contribution is large for large
counter anions like cis-[Ir(CO)2I2]ꢀ (19.6 D) and p-toluenesulfo-
nate (13.2 D), as opposed to the smaller contribution of tri-
fluoromethanesulfonate (8.5 D). Besides the additional contri-
butions of the anion and cation to the dipole moment (m) of
the ion pair, m could also depend on the average interionic dis-
tance, which explains, for instance, the increase in dipole
moment in CHCl3 compared to other solvents with negligible
dipole contributions (e.g., anisole or chlorobenzene).[15] This
effect may explain the unexpectedly large dipole moments of
the iodides of 3 and 4 (14.4 and 15.8 D, respectively).
Electronic absorption spectra were recorded in CHCl3 solu-
tion (Tables 2 and 3). For the iodides, the major absorption
band is red-shifted in comparison with that of stilbazolic pre-
cursors a–e.[17] This shift depends upon the nature of the elec-
tronic transition associated with the absorption. For example,
for the iodides of 1 and 2, the absorption bands at 357 nm
(3.47 eV) and 377 nm (3.29 eV), respectively, are red-shifted by
only 0.55 and 0.13 eV, since these absorptions can be attribut-
ed to internal p–p* transitions that are less affected by the
quaternization process. On the contrary, in the absorption
spectra of the iodides of NLO chromophores carrying the
strongly electron donating dibutylamino or dimethylamino
groups, such as 3 and 5, the major absorption band at about
500–530 nm (2.48–2.34 eV), which involves charge transfer
from the HOMO to the LUMO, the former of which is a p-
bonding molecular orbital located primarily on the amino-
phenyl unit,[2b,18,19a] is quite strongly affected by the quaterniza-
tion process, due to significant lowering of the p* energy level
of the acceptor pyridine ring. In accordance, the red shift of
these absorptions is significant (0.85 and 0.8 eV for 3 and 5, re-
spectively).
This blue shift suggests that in solvents of low or medium
permittivity such as CHCl3, CH2Cl2 and 1,2-dichloroethane pro-
gressive formation of H aggregates of the ion pairs occurs.[20,21]
These shifts are more significant in emission spectra.[21] For in-
stance, significant red shift of the emission band related to the
ICT of the iodide of 3 was observed with increasing concentra-
tion already in the range 10ꢀ6–10ꢀ4 m (at 5.5ꢀ10ꢀ6 m, lem
=
577 nm, while at 5.5ꢀ10ꢀ4 m, lem =603 nm, with a shift of
0.09 eV).
2.3. EFISH Investigation of the Second-Order NLO Respons-
es in CHCl3 of Various Salts of 4-X-1-Methylpyridinium
The EFISH technique[6] was used to investigate the second-
order NLO responses of the salts of NLO chromophores 1–5.
Measurements were carried out in anhydrous CHCl3 at an inci-
dent wavelength of 1.907 mm (1.91 mm for the sake of simplici-
ty), chosen to generate a second harmonic (at 955 nm) far
enough from the major ICT absorption band typical of these
NLO chromophores, and thus to minimize dispersive enhance-
ment of the NLO response.[22] The experimental error is about
10%.
The mb1.91 values were plotted versus concentration for the
iodides of 1–5 (Figure 2A) and for the various salts of 4 (Fig-
ure 2B, where pTSꢀ denotes p-toluenesulfonate). In all cases,
decreasing the concentration from 10ꢀ3 to 10ꢀ4 m increases the
mb1.91 value by a factor varying from 1.3-fold (for the iodide of
5) to 7.8-fold (for the perchlorate of 4). With increasing dilution
a further strong increase of mb1.91 occurs, but below a concen-
tration of 10ꢀ4 m the EFISH signal-to-noise ratio becomes too
low to obtain sufficiently reproducible quantitative mb1.91 meas-
urements. The only exception to this trend is the thiocyanate
of 4, which does not show an increase in mb1.91 on dilution
(Figure 2B).
For the iodide of 4 a mb1.91 value of 840ꢀ10ꢀ48 esu, mea-
sured in CHCl3 by the EFISH technique, was reported, but with-
out details on concentration.[7] Such a value is similar to that of
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4-X-1-Methylpyridinium NLO Chromophores
chromophores (at 5ꢀ10ꢀ4 m, the iodide of 5 shows a mb1.91
value of 1600ꢀ10ꢀ48 esu, as opposed to 1000ꢀ10ꢀ48 esu for
the iodide of 3).
At the lowest concentration reliably investigated (10ꢀ4 m),
the value of mb1.91 reaches rather comparable values for the
various iodides of the two groups (800ꢀ10ꢀ48 esu for 1 and
810ꢀ10ꢀ48 esu for 2 in the first group and 1700ꢀ10ꢀ48 esu for
3, 1900ꢀ10ꢀ48 esu for 4 and 1860ꢀ10ꢀ48 esu for 5 in the
second group), that is, at higher dilutions the second-order
NLO response starts to be mainly controlled by the cationic
part of the NLO chromophores, since the role of the counter
anion no longer seems to be so relevant.
This behaviour is confirmed by the series of salts of 4
(Table 3 and Figure 2B). At 10ꢀ4 m, the mb1.91 value reaches a
common mean value around 1900ꢀ10ꢀ48 esu, independent of
the nature of the counter anion. However, sulfonated anions,
as expected for stronger ion pairing or for facile further aggre-
gation of ion pairs, show a lower limiting mb1.91 value of only
1150ꢀ10ꢀ48 esu. Major exceptions are provided by the salt
with [Hg2I6]2ꢀ, with a limiting mb1.91 value of 6050ꢀ10ꢀ48 esu at
10ꢀ4 m, about three times that of the other salts (the high
value is justified by the stoichiometry of the salt, which con-
tains two NLO chromophores), and by the thiocyanate anion,
with a high and constant mb1.91 value of about 1850ꢀ10ꢀ48 esu
that is not much dependent on concentration. We thus carried
out some measurements on the iodide of 4, at a concentration
of about 10ꢀ2 m. As expected for increased H-type aggregation,
as suggested by absorption spectroscopy, the mb1.91 value de-
creases, reaching 1040ꢀ10ꢀ48 esu at 5ꢀ10ꢀ3 m and 925ꢀ
10ꢀ48 esu at 10ꢀ2 m. (Tables 2 and 3).
Figure 2. Plot of mb1.91 versus molar concentration c of the iodides of 1–5 (A)
and various salts of 4 (B).
We emphasise that the increase in mb1.91 evidenced on dilu-
tion is not a result of inappropriate EFISH measurements, since
the mb1.91 value of N-(4-nitrophenyl)-l-prolinol (NPP), taken as
reference remains unchanged even at 10ꢀ4 m concentration
(see Figure 2B).
1090ꢀ10ꢀ48 esu (Table 3) obtained by us at a concentration of
10ꢀ3 m, as is usually adopted for EFISH measurements.
The increase in mb1.91 with decreasing concentration may be
associated with increased ionic dissociation of ion pairs on di-
lution, to generate a higher concentration of weakly or even
non-associated cationic NLO chromophores. These should be
characterized by an increased second-order NLO response
compared to the corresponding ion pair, due to the increased
positive charge on the pyridine ring, which lowers the energy
of the p* orbitals.[10,11]
Starting from the evidence, obtained from dipole moment
measurements, that at about 10ꢀ3 m the cationic NLO chromo-
phores investigated herein are mainly associated as ion pairs,
we calculated at this concentration the corresponding b1.91
values and then the b0 values for the iodides of 1–4 (Table 2)
and for the various salts of 4 (Table 3). We can thus compare
the EFISH b0 value of the iodide of 3 with the b0 values mea-
sured by the Stark-derived method on cationic NLO chromo-
phore 3 in acetonitrile by Coe et al.[2] The EFISH b0 value of
46.3ꢀ10ꢀ30 esu is, as expected for an ion pair, lower than the
b0 value of 90ꢀ10ꢀ30 esu reported by Coe et al. for the cationic
chromophore using the Stark-derived method (the HRS value
of 25ꢀ10ꢀ30 esu is probably underestimated).[2] Due to the dif-
ferent strength and size of the ion pairs formed by NLO chro-
mophore 4 with various counter anions, the EFISH b0 values of
the various salts of 4 are quite dependent on the nature of the
counter anion. These values are around (17–22)ꢀ10ꢀ30 esu for
Figure 2A evidences that the EFISH data of the iodides of
1–5 can be divided into two groups: those with cationic NLO
chromophores characterized by the absence of the ICT band,
such as 1 and 2, and those with cationic NLO chromophores
having a significant ICT band, such as 3, 4 and 5. At each con-
centration, the mb1.91 values of the first group (iodides of 1 and
2) are considerably lower (5–7 times) than those of the second
group (iodides of 3, 4 and 5), as expected for the lack of an
ICT transition at lower energy.
ꢀ
the salts with rather small counter anions such as CF3SO3 and
ClO4ꢀ, and higher (around (35–38)ꢀ10ꢀ30 esu) for salts with
ꢀ
Lengthening of the p-delocalised bridge between the donor
and acceptor groups of the NLO chromophore increases[17b]
the second-order NLO response, as reported for other NLO
larger counter anions such as 4-CH3C6H4SO3 and cis-[Ir(CO)2I2]ꢀ
(Table 3). Interestingly, for the salt of 4 with the thiocyanate
counter anion, the EFISH b0 value at 10ꢀ3 m is unexpectedly
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499

F. Tessore et al.
high (90.6ꢀ10ꢀ30 esu, Table 3). This high value would suggest a
relatively high positive charge on the nitrogen atom of the
pyridine ring, as confirmed by the large dipole moment (13 D)
of the ion pair and by conductimetric measurements (see
below).
In summary, the EFISH investigation has shown that in CHCl3
solution the second-order NLO response of the salts of the cat-
ionic NLO chromophores 1–5 is controlled at about 10ꢀ3
concentration by ion pairing, but on dilution to about 10ꢀ4
m
m
or less it increases, as expected for progressive ionic dissocia-
tion of the ion pairs.[23] The slight decrease of the NLO re-
sponse upon increasing the concentration above 10ꢀ3 m sup-
ports further aggregation of ion pairs to form H aggregates, as
suggested by the effect of concentration on the electronic ab-
sorption spectra in solvents of low permittivity and confirmed
by conductimetric and PGSE NMR investigations (see below). It
follows that in a solvent of low dielectric constant like CHCl3,
the second-order NLO response of ionic NLO chromophores
such as those investigated herein is strongly dependent upon
concentration and upon the nature of the counter anion.
2.4. Conductimetric Investigation
Figure 3. Plot of L versus molar concentration c for the iodides of 1–5 (A)
and for the series of salts of 4 with various anions (B).
To confirm that the increase in the second-order NLO response
on dilution is due to increased ionic dissociation of the ion
pairs, we carried out a careful conductimetric investigation in
anhydrous CHCl3 solution for at least six concentrations rang-
ing from 10ꢀ5 m to 10ꢀ3 m. Tables 2 and 3 report the molar con-
ductivities L.
tion comparable to those of the iodides of 3 and 5. The thio-
cyanate of 4 also starts to dissociate into ions, but only at con-
centrations well below 10ꢀ4 m (Figure 3B).
Plots of the L values [obtained through Eqs. (8) and (9); see
the Experimental Section] versus concentration for the iodides
of 1–5 (Figure 3A) and the series of salts of 4 with various
counter anions (Figure 3B) give information about the degree
of dissociation. At concentrations between 10ꢀ4 m and 10ꢀ3 m,
the L versus concentration plots for all salts are rather flat and
with low values of L, which increase significantly with increas-
ing dilution below 10ꢀ4 m. This trend, typical of extremely
weak electrolytes, is evidence for an increased degree of ionic
dissociation of ion pairs with increasing dilution.
The conductimetric data were analysed by applying the
Kraus and Bray function [Eq. (1)],[24] useful to describe the be-
haviour of a weak electrolyte in solution:
1
L
1
L0
cL
KaðL0Þ
¼
þ
ð1Þ
2
where Ka is the dissociation constant, and L0 the molar con-
ductivity at infinite dilution. For a weak uni-univalent electro-
lyte, the plot of 1/L versus cL is a straight line, with intercept
1/L0. The degree of dissociation a of the weak electrolyte is
given by Equation (2):
The iodides of 1 and 2 are less conductive than the iodides
of 3, 4 and 5 over the entire range of concentrations (Fig-
ure 3A), since for the former the lack of a strongly electron-do-
nating substituent probably results in a higher positive charge
on the nitrogen atom of pyridine ring, so that ion pairing is
easier. The slightly lower L values over the entire range of
concentrations of the iodide of 5 compared with those of 3
and 4 may indicate that lengthening of the p-delocalized
bridge produces a decrease in mobility of the cation, due to its
increased size.
L
L0
ð2Þ
a ¼
For all the salts investigated, the plot of the Kraus and Bray
function (see Figure 4) in the range of the lower concentra-
tions (between 10ꢀ5 m and 10ꢀ4 m) is a straight line, with very
good linear correlation coefficients (between 0.965 and 0.999).
Only at higher concentrations (more than 10ꢀ3 m) did we ob-
serve a deviation from linearity, suggesting the presence of ag-
gregates of higher nuclearity.
Among the various salts of 4, that with the [Hg2I6]2ꢀ anion
shows the highest molar conductivity L and that with the
ꢀ
anion 4-CH3C6H4SO3 (pTSꢀ) the lowest over the entire range
In the range of concentrations typical for EFISH measure-
ments (10ꢀ3–10ꢀ4 m), the conductimetric data were also ana-
lysed by applying the Kraus and Fuoss function,[25] by plotting
lgL versus lgc. For ion pairs which dissociate into ions, the
plot of lgL versus lgc should be a straight line with slope
ꢀ0.5, while when the ion pairs associate into larger aggregates
of concentrations (see Figure 3B). For the former the high
value of L is obviously due to its stoichiometry, while for the
latter the low value of L suggests more facile ion pairing or
further aggregation, in accordance with the EFISH investiga-
tion. The other salts of 4 show a trend of L versus concentra-
500
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ChemPhysChem 2010, 11, 495 – 507

4-X-1-Methylpyridinium NLO Chromophores
Figure 4. Plot of the Kraus and Bray function of the iodides of 1–5 (A) and
various salts of 4 (B).
Figure 5. Plot of the Kraus and Fuoss function for the iodides of 1–5 (A) and
various salts of 4 (B).
the slope of the plot decreases in absolute value and becomes
positive. The Kraus and Fuoss plots for the iodides of 1–5 and
for the various salts of 4 are reported in Figures 5A and B, re-
spectively.
All plots show deviations of the slopes from the decisive
value of ꢀ0.5 at higher concentrations that confirm the pres-
ence of higher aggregates with increasing concentration. For
the series of iodides (see Figure 5A), a larger deviation is ob-
served for 1 and 2 (the slope is about ꢀ0.34). This could be
due to the lack of a substituent in the para position of the aro-
matic ring, which results in less steric hindrance and facilitates
further aggregation.
For the various salts of 4, on the other hand, a more signifi-
cant deviation from a value of ꢀ0.5 is observed for the salt
with the p-toluenesulfonate anion (pTSꢀ, slope=ꢀ0.39), while
the slope for the salt with the thiocyanate anion is always
quite near to the decisive value, thus confirming, in accord-
ance with EFISH measurements, for the latter a strong and
stable ion pair and for the former a significant tendency to ag-
gregate to higher nuclearity species.
Figure 6 plots the ratio mb1.91/c versus a for all salts at con-
centrations below 10ꢀ3 m, in order to evidence a correlation
between the second-order NLO response and the degree of
dissociation. The plots for the iodides of 1–5 (Figure 6A) and
the various salts of 4 (Figure 6B) show a good linear correla-
tions mb1.91/c versus a for each salt, confirming a direct relation
between the second-order NLO response and the degree of
ionic dissociation.
Figure 6. Correlation between second-order NLO response mb1.91 and degree
of dissociation a for the iodides of 1–5 (A) and the series of various salts of
4 (B).
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501

F. Tessore et al.
2.5. PGSE NMR studies
dissociated into free ions. By positioning the iodide anion in
front of positively charged nitrogen atom of the pyridinium
ring in the ion pair structure, a P value for the ion pair (PIP) of
33.46 in CDCl3 was calculated (2a=25.4, 2b=9.2 and 2d=
5.4 ꢂ; ionic radius of Iꢀ =2.11 ꢂ[27]).
Pulsed field gradient spin echo NMR measurements were car-
ried out for the iodide and the p-toluenesulfonate of 4 in
CDCl3, as a function of concentration, in order to investigate
the dissociation and aggregation processes in depth. Molecular
hydrodynamic dimensions were derived from the experimen-
tally measured translational self-diffusion coefficient Dt by
using a modification of the Stokes–Einstein equation (3), which
takes into account the pronounced elliptical shape of the pyri-
dinium cation and of the ion pairs (Table 4):
Comparison of the experimental P values in CD3Cl reported
in Table 4 with the calculated PIP for the iodide of 4 indicates
that only at the two lower concentrations (7.7ꢀ10ꢀ5 and 2.64ꢀ
10ꢀ4 m) are the values of P smaller than PIP, that is, at these
concentrations ion pairs start to be sufficiently dissociated into
free ions. In agreement with the previous conductivity investi-
gation, at concentrations on the order of 10ꢀ3 m or higher, the
experimental P value becomes greater than PIP, and this con-
firms that ion pairs start to associate to form higher aggre-
gates. This self-aggregation of the iodide of 4 can be thus eval-
uated by introducing the aggregation number N, defined as
the average number of ion pairs per aggregate. To evaluate N
some hypotheses must be made concerning the structures of
the aggregates. Two limit models can be taken into account:
ion pairs stack as H aggregates (AG1) or form a head to tail
polymer (AG2; Figure 7B). In the AG1 model, 2a and 2b pa-
rameters of the ellipsoid remain constant while 2d increases
(up to L1) as a consequence of aggregation. Instead, only 2a
changes (up to L2) in the aggregation process for model AG2.
Thus, P values were calculated for the AG1 and AG2 models
while varying only the 2d and 2a parameters, respectively, so
that N values could be obtained as the ratios of L1/2d and L2/
2a for AG1 (N+AG1) and AG2 (N+AG2), respectively (Table 4).
N+AG1 and N+AG2 span from 1 to 3.8 and from 1 to 1.9, respec-
tively. It is not surprising that N+AG1 >N+AG2, since the former
has a more spherical shape.
kT
pffiffiffiffiffiffiffiffi
Dt ¼
ð3Þ
fcph3 abd
pffiffiffiffiffiffiffiffi
3
In Equation (3), abd replaces the hydrodynamic radius rH, k
is the Boltzman constant, T the absolute temperature, h the
fluid viscosity, c the “size factor”, f the “shape factor”, and a, b
and d are the semi-axes of the ellipsoid (a>b>d).[12] From the
Dt values the structural parameter P, defined as kT/phDt =
pffiffiffiffiffiffiffiffi
fsc3 abd, was obtained (Table 4). Since both f and c can be ex-
pressed as a function of a, b, and d,[26] the structural parameter
P for the cation (P+) was computed by using a, b and d param-
eters of the ellipsoid that better fit the modelled van der Waals
surface of cation 4 (2a=21.2, 2b=9.2, 2d=5.4 ꢂ, Figure 7A)
and the hydrodynamic radius of the solvent (2.60 and 2.67 ꢂ
for CDCl3 and DMSO, respectively). The choice of these struc-
tural parameters was confirmed by the calculated value of P
(29.63 ꢂ), in excellent agreement with the experimental value
obtained in DMSO (29.46 ꢂ, average value of entries 12 and 13
in Table 4). In this solvent it is known that salts are prevalently
In the case of the p-toluene-
sulfonate of 4, it can be reason-
ably assumed that in the ion-
Table 4. Diffusion coefficient (10ꢀ10 Dt [m2 sꢀ1]), structural parameter P [ꢂ], and aggregation numbers N for the
iodide and the p-toluenesulfonate of 4 in CDCl3 and DMSO as a function of concentration C [m].
ꢀ
pair structure the polar SO3
head of the anion approaches
the positively charged pyridine
nitrogen atom of the cation, as
suggested for similar ion pairs
by Grunwald and co-work-
ers.[15,16] Two limit situations can
thus occur: the methyl group of
the p-toluenesulfonate anion is
oriented toward the NBu2 sub-
stituent (structure A) or in the
Entry
Solvent
C
Dtþ
Dtꢀ
P+
Pꢀ
N+AG1
NꢀAG1
N+AG2
NꢀAG2
iodide
32.65
33.19
33.73
35.00
37.83
38.89
39.76
42.44
44.61
47.97
52.04
29.35
29.58
1
2
3
4
5
6
7
8
CDCl3
CDCl3
CDCl3
CDCl3
CDCl3
CDCl3
CDCl3
CDCl3
CDCl3
CDCl3
CDCl3
DMSO
DMSO
7.7ꢀ10ꢀ5
7.07
6.94
6.83
6.59
6.10
5.93
5.80
5.44
5.17
4.81
4.43
2.08
2.06
0.9
0.9
1.0
1.1
1.5
1.7
1.8
2.2
2.5
3.1
3.8
0.7
0.9
0.9
0.9
1.0
1.0
1.2
1.2
1.3
1.4
1.5
1.7
1.9
0.7
0.9
2.64ꢀ10ꢀ4
7.93ꢀ10ꢀ4
1.76ꢀ10ꢀ3
4.4 ꢀ10ꢀ3
6.71ꢀ10ꢀ3
9.12ꢀ10ꢀ3
1.53ꢀ10ꢀ2
2.79ꢀ10ꢀ2
4.73ꢀ10ꢀ2
9.31ꢀ10ꢀ2
1.76ꢀ10ꢀ4
5.55ꢀ10ꢀ3
9
10
11
12
13
opposite
direction
(struc-
ture B).[28–31] To discriminate be-
tween these two structures, the
inter-ionic structure of the ion
pair of the p-toluenesulfonate
of 4 was investigated by record-
ing a series of 1H NOESY NMR
spectra[32] at different concen-
trations (1.02ꢀ10ꢀ3, 6.94ꢀ10ꢀ3
and 1.54ꢀ10ꢀ2 m) in CDCl3. Very
weak interionic contacts could
be detected only for the more
p-toluenesulfonate
14
15
16
17
18
19
20
21
22
CDCl3
CDCl3
CDCl3
CDCl3
CDCl3
CDCl3
CDCl3
CDCl3
DMSO
6ꢀ10ꢀ6
6.76
6.40
6.27
5.92
5.83
5.78
5.60
5.09
2.06
7.10
6.68
6.34
5.94
5.82
5.78
5.52
5.05
3.59
32.83
34.57
35.49
37.37
38.10
39.66
41.46
45.31
29.629
31.23
33.15
35.08
37.21
38.17
39.68
42.09
45.72
16.967
0.8
0.8
0.9
1.0
1.1
1.3
1.5
2.0
0.5
0.7
0.8
1.0
1.1
1.3
1.5
2.1
0.8
0.8
0.9
1.0
1.0
1.1
1.1
1.3
0.5
0.7
0.8
1.0
1.0
1.1
1.2
1.3
3.9ꢀ10ꢀ5
3.09ꢀ10ꢀ4
1.02ꢀ10ꢀ3
1.86ꢀ10ꢀ3
3.62ꢀ10ꢀ3
6.94ꢀ10ꢀ3
1.54ꢀ10ꢀ2
2.91ꢀ10ꢀ3
502
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4-X-1-Methylpyridinium NLO Chromophores
Figure 8. Aggregation numbers for the iodide and the p-toluenesulfonate of
4 as a function of concentration in CDCl3.
The N values as a function of concentration for the iodide
and the p-toluenesulfonate of 4 follow substantially the same
trends (Figure 8), although the p-toluenesulfonate shows a
slightly larger tendency to aggregate. At least in model AG1, it
seems that self-aggregation proceeds beyond ion quadru-
ples;[33] the AG1 model corresponds to the more probable H-
type aggregation, as suggested by EFISH measurements and
solvent effects on the electronic absorption spectra. Assuming
that ion pairs undergo indefinite self-aggregation following
the isodesmic model,[34] an estimate of the equilibrium aggre-
gation constant KAGG can be obtained by the relationship:
N(Nꢀ1)=KAGGC.[35] Taking into account concentrations higher
than 5ꢀ10ꢀ4 m, where the concentration of free ions becomes
small, the following KAGG values are obtained: iodide of 4
(AG2): 19ꢁ2, iodide of 4 (AG1): 118ꢁ6, p-toluenesulfonate of
4 (AG2): 32ꢁ1, p-toluenesulfonate of 4 (AG1): 154ꢁ6mꢀ1.
The greater tendency of the p-toluenesulfonate of 4 to form
aggregates compared to the iodide of 4 is in agreement with
the evidence provided by the Krauss and Fuoss plots of
Figure 5.
Figure 7. Lateral and top views of the van der Waals surfaces of cationic
NLO chromophore 4 with structural parameters [ꢂ] of the ellipsoid used as
geometrical model for PGSE NMR data analysis (A) and two models for the
aggregation of monomers (black) into dimers, trimers and higher aggregates
(red) (B).
3. Conclusions
We have reported evidences based on dipole-moment meas-
urements and conductimetric and PGSE NMR investigations
that 4-X-1-methylpyridinium salts in anhydrous CHCl3 solution
are associated as ion pairs in the range of concentration
around 5ꢀ10ꢀ4–10ꢀ3 m. At lower concentrations, significant
dissociation into ions takes place, while at higher concentra-
tion ion pairs become associated into larger aggregates, prob-
ably of H type. For the second-order cationic NLO chromo-
phore 4, dissociation into free ions and association to form
higher aggregates has been shown to be controlled by the
nature of the counter anion.
concentrated solution. In particular, the hydrogen atoms in
ꢀ
ortho position to the SO3 group of the anion interact with the
methyl protons of the methylpyridinium group and with those
in the 2,6-positions of the pyridine ring (see Figure 1 for the
numbering scheme). Interionic contacts were not observed for
any other resonance of the cation, and thus structure B is con-
firmed. A value of 37.24 ꢂ (2a=30, 2b=9.2, 2d=5.4 ꢂ) was
thus calculated for PIP of the ion pair with structure B. The
N+AG1 and N+AG2 values for the two models of aggregation of
ion pairs, reported in Figure 8, were calculated as described
above. Since in the case of the p-toluenesulfonate of 4, also
the counter anion has some protons, it was possible to evalu-
ate Dt, P and N values for both the cation and the anion
(Table 4). Interestingly, for concentrations higher than 3.09ꢀ
10ꢀ4 m, cation and anion translate, as expected for an ion pair,
as a whole object, since Dþt and Dtꢀ, P+ and Pꢀ and N+ and Nꢀ
show similar values.
It follows that the second-order NLO response of the pyridi-
nium salts investigated here strongly depends upon the nature
of the counter anion and also upon concentration. Below
5ꢀ10ꢀ4 m, the ion pairs start to dissociate into ions with a
strong increase in second-order NLO response mb1.91, measured
by the EFISH technique. An unexpected exception (a rather
constant mb1.91 value) was found when thiocyanate is the coun-
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503

F. Tessore et al.
the Dipartimento di Chimica Inorganica Metallorganica e Analitica
“Lamberto Malatesta” of the Universitꢃ di Milano, onCHCl3 solu-
tions of different concentrations (usually 10ꢀ3, 5ꢀ10ꢀ4 and 10ꢀ4 m)
at a non-resonant incident wavelength of 1.907 mm (simplified as
1.91 mm), obtained by Raman-shifting the fundamental 1.064 mm
wavelength produced by a Q-switched, mode-locked Nd3+:YAG
laser.
ter anion of 4, but conductivity measurements in anhydrous
CHCl3 confirmed that the thiocyanate of 4 shows in the range
of concentration investigated (10ꢀ4–10ꢀ3 m) trends typical of a
tight ion pair which does not easily dissociate. Salts of 4 with
ꢀ
sulfonated anions such as CF3SO3 and p-toluenesulfonate
show a significant tendency to form stronger ion pairs and
higher aggregates, while salts with other counter anions such
as Iꢀ, ClO4ꢀ, [Hg2I6]2ꢀ and cis-[Ir(CO)2I2]ꢀ show a minor tendency
to ion pairing. This group, with increasing dilution, shows in-
creased dissociation into free ions, with higher second-order
NLO response mb1.91, which finally becomes quite independent
of the nature of the counter anion.
Dipole-Moment Measurements: Dipole moments were evaluated in
CHCl3 solutions by means of the Guggenheim method.[14] Referring
to the solvent as 1 and to the solute as 2, the value of m (expressed
in Debye) is obtained from the orientational polarization of the
solute mP2 through Equation (5):
rffiffiffiffiffiffiffiffiffiffi
pffiffiffiffiffiffiffiffiffi
pffiffiffiffiffiffiffiffiffi
In conclusion, our multidisciplinary investigation has shown
that, when working in a solvent of low electrical permittivity
such as anhydrous CHCl3, the second-order NLO response of
4-X-1-methylpyridinium second-order cationic NLO chromo-
phores depends on concentration and on the nature of the
counter anion. Therefore, such dependence cannot be neglect-
ed and should be carefully taken into consideration.
9kT
4pNa
ð5Þ
m ¼
mP2T ¼ 0:012813 mP2T
The orientational polarization is determined by Equation (6):
3M2
mP2 ¼
2 ða ꢀ nÞ
ð6Þ
d1ðe1 þ 2Þ
where M2 is the molecular weight of the solute, d1 the density of
the solvent, e1 its dielectric constant and a and n are, respectively,
the slopes of the straight-line plots of dielectric constant versus
concentration and the square root of the refractive index versus
concentration of solutions at various concentration of the solute.
Experimental Section
General Comments: IrCl3 was purchased from Engelhardt. Iodome-
thane, methyl trifluoromethanesulfonate, all silver salts (AgClO4,
AgCF3SO3, Ag(4-CH3C6H4SO3), AgSCN), mercury (II) iodide, stilbazolic
ligands a and c (Figure 1), the iodides of 3 and 4 (Table 1) and all
solvents were purchased from Sigma-Aldrich and used without fur-
ther purification, whereas stilbazoles b[17a] and d[17b] (Figure 1) and
The dielectric constant is linearly related to the physically measura-
ble capacity C of the solution by Equation (7):
e ¼ a þ bC
ð7Þ
[13]
[Ir(COT)2Cl]2 (COT=cyclooctene) were synthesized according to
the literature. All the other salts investigated, with the exception of
the iodides of 3 and 4 (Table 1), were prepared as described below,
under a nitrogen atmosphere, by using flasks previously dried over
flame and cooled under nitrogen flow. Carefully dried CHCl3 stabi-
lized with amylene was used for measurements of dipole moment,
electronic absorption and emission spectra, EFISH and conductivity.
The coefficients a and b were obtained by calibration of the dipole
meter with a series of pure solvents (in our case CHCl3, chloroben-
zene and ethyl acetate) of known dielectric constant e.
For each salt five solutions in CHCl3 were prepared and measured
working in a concentration range of 10ꢀ4 –10ꢀ3 m, characterized by
strong ion pairing. In all cases extremely good linear plots were
obtained, confirming that ionic dissociation of strong ion pairs is
not relevant in the range of concentrations investigated. For exam-
ple, Figure 9 shows the linear plots of dielectric constant versus
concentration and square of the refractive index versus concentra-
tion for the salt of 4 with the triflate anion.
1
All products were characterized by H NMR (Bruker DRX-300 spec-
trometer; see Figure 1 for atom numbering) and elemental analy-
sis. Electronic absorption spectra were recorded on a Jasco V-530
spectrophotometer, and emission spectra on a JASCO FP-777 spec-
trofluorimeter. Dipole moments m were measured by using a WTW-
DM01 dipole meter (dielectric constant) coupled with an RX-5000
ATAGO Digital Refractometer (refractive index) according to the
Guggenheim method.[14] Elemental analyses were carried out at
the Dipartimento di Chimica Inorganica, Metallorganica e Analitica
“Lamberto Malatesta” of the Universitꢃ di Milano.
Electrical conductivity measurements were carried out in CHCl3 sol-
utions at concentrations ranging from 10ꢀ5 to 10ꢀ3 m, the upper
value being determined by the solubility of the salts, and the
lower by the instrumental limits. All measurements were carried
out at 298 K with an AMEL 160 conductimeter, equipped with an
immersion cell with platinated platinum electrodes and glass insu-
lating parts, characterized by a cell constant of ca. 1 (exactly estab-
lished at the beginning of the experimental work by using an
aqueous 0.01m KCl solution as standard). The conductance G of
EFISH Measurements: The molecular quadratic hyperpolarizabilities
of the 4-X-1-methylpyridinium salts investigated in this work
(Table 1) were obtained by the solution-phase dc electric field in-
duced second harmonic (EFISH) generation technique,[6] which can
provide direct information on the intrinsic molecular second-order
NLO properties through Equation (4):
the pure solvent (5ꢀ10ꢀ9
W
ꢀ1) was determined before each set of
experiments and subsequently subtracted from the conductance
measured for the solutions.
gEFISH ¼ ðmbl=5 kTÞ þ gðꢀ2 w; w, w, 0Þ
ð4Þ
Molar conductivity L is given by Equation 8:
where mbl/5kT represents the dipolar orientational contribution
and g(ꢀ2w; w, w, 0), a third-order term at frequency w of the inci-
dent wavelength, is the electronic contribution to gEFISH, which is
negligible for the kind of molecules investigated herein.[17b,19] bl is
the projection along the dipole moment axis of the vectorial com-
ponent bVEC of the tensor of the quadratic hyperpolarizability at in-
cident wavelength l. All EFISH measurements were carried out at
1000k
c
ð8Þ
L ¼
where c is the concentration of the solution (in moldmꢀ3) and k
the specific conductivity (in Wꢀ1 cmꢀ1), directly obtained by the ex-
perimental measure of the conductance G through Equation (9):
504
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ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemPhysChem 2010, 11, 495 – 507

4-X-1-Methylpyridinium NLO Chromophores
(E)-4-Styryl-1-methylpyridinium Iodide: A solution of iodomethane
(2.49 mL; 40 mmol) in toluene (2 mL) was added dropwise to a so-
lution of stilbazole a (200 mg; 1.1 mmol) in toluene (6 mL). Vigo-
rous stirring for 2 h at room temperature in the dark (aluminium
foil) afforded a yellow solid, which was dissolved in CH3CN and
precipitated by addition of Et2O at room temperature to give
230 mg of the iodide of 1 (65%) as a yellow powder. 1H NMR
3
(300 MHz, CDCl3, 258C, TMS): d =9.02 (d, J(H,H)=6.6 Hz, 2H; H2,
3
3
H6), 8.04 (d, J(H,H)=6.5 Hz, 2H; H3, H5), 7.72 (d, J(H,H)=16.3 Hz,
1H; H7), 7.66 (m, 2H; H10, H14), 7.48 (m, 3H; H11, H12, H13), 7.18 (d,
3J(H,H)=16.1 Hz, 1H; H8), 4.58 (s, 3H; NCH3): elemental analysis (%)
calcd (found): C 52.03 (52.05), H 4.37 (4.33), N 4.33 (4.40).
(E)-4-(4’-tert-Butylstyryl)-1-methylpyridinium Iodide: A solution of
iodomethane (0.66 mL; 10.6 mmol) in CH2Cl2 (2 mL) was added
dropwise to a solution of stilbazole b (50 mg; 0.21 mmol) in CH2Cl2
(18 mL). Vigorous stirring for 2 h at room temperature in the dark
(aluminium foil) afforded a yellow solid, which was dissolved in
CH2Cl2 and precipitated by addition of n-hexane at room tempera-
ture to give 59.7 mg of the iodide of 2 (75%) as a yellow powder.
Figure 9. An example of the good linear correlation of the dielectric con-
stant and square of the refractive index versus concentration (weight ratio
of solute to solvent) for the salt of 4 with triflate counter anion.
3
1H NMR (300 MHz, CDCl3, 258C, TMS): d =8.98 (d, J(H,H)=6.7 Hz,
3
3
2H; H2, H6), 7.99 (d, J(H,H)=6.8 Hz, 2H; H3, H5), 7.70 (d, J(H,H)=
16.2 Hz, 1H; H7), 7.60 (d, 3J(H,H)=8.4 Hz, 2H; H10, H14), 7.50 (d,
3J(H,H)=8.5 Hz, 2H; H11, H13), 7.13 (d, 3J(H,H)=16.2 Hz, 1H; H8),
4.57 (s, 3H; NCH3), 2.19 (s, 9H; C(CH3)3): elemental analysis (%)
calcd (found): C 57.00 (57.03), H 5.85 (5.78), N 3.69 (3.72).
1 L
ð9Þ
G ¼
k S
where L/S is the cell constant.
(E,E)-4-[4’-(Dimethylaminophenyl)-1,3-butadienyl]-1-methylpyridini-
um Iodide: A solution of iodomethane (1.86 mL; 29.9 mmol) in
CH2Cl2 (2 mL) was added dropwise to a solution of stilbazole d
(150 mg; 0.6 mmol) in CH2Cl2 (68 mL). Vigorous stirring for 2 h at
room temperature in the dark (aluminium foil) afforded a red solid,
which was dissolved in CH2Cl2 and precipitated by addition of n-
hexane at room temperature to give 145 mg of the iodide of 5
(62%) as a pale violet powder. 1H NMR (300 MHz, CDCl3, 258C,
TMS): d =8.97 (d, 3J(H,H)=6.5 Hz, 2H; H2, H6), 7.77 (d, 3J(H,H)=
PGSE Diffusion NMR Measurements: PGSE NMR measurements
were performed on a Bruker Avance DRX 400 spectrometer
equipped with a direct QNP probe and a z-gradient coil controlled
by a Great 1/10 gradient unit, by using the standard stimulated
echo pulse sequence[36] at 295 K without spinning. The depend-
ence of the resonance intensity I on a constant waiting time and
on a varied gradient strength G is described by Equation (10):
3
3
6.5 Hz, 2H; H3, H5), 7.52 (dd, J(H,H)=10.7 Hz, J(H,H)=15.1 Hz, 1H;
ꢀ
ꢁ
I
I0
d
2
ln ¼ ꢀðdgÞ Dt Dꢀ G2
ð10Þ
3
3
H8), 7.46 (d, J(H,H)=8.9 Hz, 2H; H12, H16), 7.04 (d, J(H,H)=15.2 Hz,
3
1H; H10), 6.90 (dd, J(H,H)=10.7 Hz, J(H,H)=15.2 Hz, 1H; H9), 6.81
(m, 2H; H13, H15), 6.55 (d, 3J(H,H)=15.2 Hz, 1H; H7), 4.54 (s, 3H;
NCH3), 3.08 (s, 6H; N(CH3)2): elemental analysis (%) calcd (found): C
55.11 (55.36), H 5.39 (5.25), N 7.14 (7.44).
3
3
where I is the intensity of the observed spin echo, I0 the intensity
of the spin echo without gradients, Dt the translational self-diffu-
sion coefficient, D the delay between the midpoints of the gradi-
ents, d the length of the gradient pulse and g the magnetogyric
ratio. The shape of the gradients was rectangular, their duration d
was 4 ms and their strength G was varied during the experiments.
1H PGSE NMR spectra were acquired with 32 K points, 16, 32, 64,
128, 256, 512 or 3072 scans, depending on concentration, and a
spectral width of 5000 Hz. All spectra were processed with a line
broadening of 1.0 Hz. The semi-logarithmic plots of ln(I/I0) versus
G2 were fitted by using a standard linear regression algorithm to
give an R factor always better than 0.99. Gradients were calibrated
on the diffusion of HDO in D2O;[37] data at different temperatures
were estimated by interpolation of the data reported by Mills,
giving DHDO =1.748ꢀ10ꢀ9 m2 sꢀ1 at 295 K. The residual resonance of
CHCl3 was used as internal standard. The uncertainty in measure-
ments of Dt was estimated by determining the standard deviation
of Dt by performing experiments with different diffusion times D.
The standard propagation of error analysis gave a standard devia-
tion of approximately 3–4% for Dt and a consequent error of 9–
12% for the value of the aggregation number N. Only four gradi-
ents were used for the measurements carried out for the p-tolue-
nesulfonate of 4 at the lowest concentration (ca. 6ꢀ10ꢀ6 m); conse-
quently, the error in the corresponding Dt is slightly higher (around
5–8%).
(E)-4-[4’-(Dibutylamino)styryl)]-1-methylpyridinium Trifluorometha-
nesulfonate: A solution of Ag(CF3SO3) (114 mg, 0.444 mmol) in
CH3CN (10 mL) was added dropwise at room temperature to a so-
lution of the iodide of 4 (200 mg, 0.444 mmol) in CH3CN (40 mL).
The reaction mixture was stirred at room temperature in the dark
(aluminium foil) for 1 h, and then AgI was filtered off. Evaporation
of the filtrate gave a red powder, which was dissolved in CH2Cl2
and precipitated by addition of n-hexane at room temperature to
1
afford 185 mg of the triflate of 4 (88%) as a red powder. H NMR
3
(300 MHz, CDCl3, 258C, TMS): d =8.44 (d, J(H,H)=6.8 Hz, 2H; H2,
3
3
H6), 7.74 (d, J(H,H)=6.8 Hz, 2H; H3, H5), 7.57 (d, J(H,H)=15.8 Hz,
1H; H8), 7.49 (d, 3J(H,H)=8.8 Hz, 2H; H10, H14), 6.79 (d, 3J(H,H)=
15.8 Hz, 1H; H7), 6.66 (d, J(H,H)=8.8 Hz, 2H; H11, H13), 4.25 (s, 3H;
NCH3), 3.36 (t, J(H,H)=7.3 Hz, 4H; NCH2), 1.61 (m, 4H; CH2), 1.32
(m, 4H; CH2), 0.92 (t, J(H,H)=7.2 Hz, 6H; CH3): elemental analysis
3
3
3
(%) calcd (found): C 58.46 (58.52), H 6.61 (6.44), N 5.93 (6.04).
(E)-4-[4’(-Dibutylamino)styryl]-1-methylpyridinium Perchlorate: A so-
lution of AgClO4 (92 mg, 0.444 mmol) in CH3CN (20 mL) was added
dropwise at room temperature to a solution of the iodide of 4
(200 mg, 0.444 mmol) in CH3CN (80 mL). The reaction mixture was
stirred at room temperature in the dark (aluminium foil) for 1 h,
ChemPhysChem 2010, 11, 495 – 507
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemphyschem.org
505

F. Tessore et al.
and then AgI was filtered off. Evaporation of the filtrate gave an
orange powder, which was dissolved in CH2Cl2 and precipitated by
addition of n-hexane at room temperature to afford 150 mg of the
perchlorate of 4 (80%) as an orange powder. 1H NMR (300 MHz,
(E)-4-[4’-(Dibutylamino)styryl]-1-methylpyridinium
cis-dicarbonyl-
diiodoiridate(I): A stream of CO was bubbled into a solution of [{Ir-
(COT)2Cl}2] (417 mg, 0.466 mmol) in degassed EtOH (80 mL), lead-
ing to precipitation of [Ir(CO)2Cl]n as a black powder. KI (298.5 mg,
1.86 mmol) was added and the reaction mixture was heated to
reflux for 90 min. The yellow-green solution was cooled to room
temperature and KCl filtered off. After removal of the solvent in
vacuo, K[Ir(CO)2I2] was recovered as a dark brown powder. A sus-
pension of K[Ir(CO)2I2] (480 mg, 0.888 mmol) in degassed distilled
H2O (80 mL) was then added to a solution of the iodide of 4
(400 mg, 0.888 mmol) in CH2Cl2 (50 mL) and the mixture was left at
room temperature with vigorous stirring for 24 hrs. The layers
were separated and the organic layer washed with H2O (3ꢀ25 mL),
dried over Na2SO4 and evaporated, affording 300 mg of the cis-di-
carbonyldiiodoiridate(I) of 4 (41% with respect to the iodide of 4)
3
CDCl3, 258C, TMS): d =8.37 (d, J(H,H)=6.8 Hz, 2H; H2, H6), 7.75 (d,
3
3J(H,H)=6.8 Hz, 2H; H3, H5), 7.58 (d, J(H,H)=15.9 Hz, 1H; H8), 7.50
3
3
(d, J(H,H)=8.8 Hz, 2H; H10, H14), 6.80 (d, J(H,H)=15.9 Hz, 1H; H7),
6.66 (d, 3J(H,H)=8.4 Hz, 2H; H11, H13), 4.27 (s, 3H; NCH3), 3.36 (t,
3J(H,H)=7.8 Hz, 4H; NCH2-), 1.61 (m, 4H; CH2), 1.40 (m, 4H; CH2),
0.99 (t, 3J(H,H)=7.2 Hz, 6H; CH3): elemental analysis (%) calcd
(found): C 62.47 (62.53), H 7.39 (7.41), N 6.42 (6.37).
(E)-4-[4’-(Dibutylamino)styryl]-1-methylpyridinium
Thiocyanate:
Solid AgSCN (74 mg, 0.444 mmol) was added at room temperature
to a solution of the iodide of 4 (200 mg, 0.444 mmol) in CH3CN
(40 mL). The reaction mixture was stirred at room temperature in
the dark (aluminium foil) for 5 d, and then AgI was filtered off.
Evaporation of the filtrate gave a red powder, which was dissolved
in CH3CN and precipitated by addition of n-hexane at room tem-
perature to afford 129 mg of the thiocyanate of 4 (76%), as a red
1
as a dark orange powder. H NMR (300 MHz, CD2Cl2, 258C, TMS): d
3
3
=8.33 (d, J(H,H)=6.8 Hz, 2H; H2, H6), 7.82 (d, J(H,H)=6.8 Hz, 2H;
H3, H5), 7.70 (d, J(H,H)=15.8 Hz, 1H; H8), 7.57 (d, J(H,H)=9.0 Hz,
3
3
2H; H10, H14), 6.91 (d, 3J(H,H)=15.8 Hz, 1H; H7), 6.72 (d, 3J(H,H)=
9.0 Hz, 2H; H11, H13), 4.27 (s, 3H; NCH3), 3.40 (t, J(H,H)=7.8 Hz, 4H;
3
NCH2), 1.61 (m, 4H; -CH2-), 1.43 (m, 4H; CH2), 1.01 (t, 3J(H,H)=
7.3 Hz, 6H; CH3): elemental analysis (%) calcd (found): C 34.92
(34.88), H 3.78 (3.81), N 3.39 (3.32).
1
3
powder. H NMR (300 MHz, CDCl3, 258C, TMS): d=8.52 (d, J(H,H)=
6.4 Hz, 2H; H2, H6), 7.81 (d, 3J(H,H)=6.5 Hz, 2H; H3, H5), 7.64 (d,
3J(H,H)=15.9 Hz, 1H; H8), 7.51 (d, 3J(H,H)=8.9 Hz, 2H; H10, H14),
3
3
6.83 (d, J(H,H)=15.8 Hz, 1H; H7), 6.66 (d, J(H,H)=9.0 Hz, 2H; H11,
H13), 4.40 (s, 3H; NCH3), 3.37 (t, J(H,H)=7.5 Hz, 4H; NCH2), 1.63 (m,
3
3
4H; CH-), 1.40 (m, 4H; CH2), 0.99 (t, J(H,H)=7.3 Hz, 6H; CH3): ele-
Acknowledgements
mental analysis (%) calcd (found): C 72.40 (72.35), H 8.19 (8.24), N
11.01 (11.05).
This work was supported by the Ministero dell’Istruzione, dell’Uni-
versitꢀ e della Ricerca (research project FIRB, year 2001, title:
“Nano-Organization of Hybrid Organo-Inorganic Molecules with
Magnetic and Non Linear Optical Properties” and project PRIN,
year 2007, title: “Nuove strategie per il controllo di reazioni met-
allo assistite: interazioni non convenzionali di frammenti moleco-
lari”). We deeply thank Dr. Stefania Righetto for help in EFISH
measurements.
(E)-4-[4’-(Dibutylamino)styryl]-1-methylpyridinium p-Toluenesulfo-
nate: A solution of Ag(4-CH3C6H4SO3) (152 mg, 0.546 mmol) in
CH3CN (10 mL) was added dropwise at room temperature to a so-
lution of the iodide of 4 (246 mg, 0.546 mmol) in CH3CN (40 mL).
The reaction mixture was stirred at room temperature in the dark
(aluminium foil) for 3 h, and then AgI was filtered off. Evaporation
of the filtrate gave a red powder, which was dissolved in CH3CN
and precipitated by addition of n-hexane at room temperature to
afford 150 mg of the p-toluenesulfonate of 4 (68%), as a red
1
3
powder. H NMR (300 MHz, CDCl3, 258C, TMS): d =8.71 (d, J(H,H)=
Keywords: aggregation · cations · chromophores · ion pairs ·
nonlinear optics
3
6.5 Hz, 2H; H2, H6), 7.82 (d, J(H,H)=8.1 Hz, 2H; H ortho to CH3 of
the anion), 7.74 (m, 2H; H3, H5), 7.49 (m, 3H; H8, H10, H14), 7.14 (d,
3J(H,H)=7.9 Hz, 2H; H ortho to SO3 of the anion), 6.75 (brm, 3H;
3
H7, H11, H13), 4.44 (s, 3H; NCH3), 3.40 (t, J(H,H)=6.9 Hz, 4H; NCH2-),
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(E)-4-[4’-(Dibutylamino)styryl]-1-methylpyridinium Hexaiododimer-
curate: A solution of HgI2 (100 mg, 0.222 mmol) in EtOH (20 mL)
was added dropwise at room temperature to a solution of the
iodide of 4 (200 mg, 0.444 mmol) in EtOH (70 mL). The reaction
mixture was stirred at room temperature in the dark (aluminium
foil) for 1 h, affording a red precipitate, which was dissolved in
CH2Cl2 and precipitated by addition of n-hexane at room tempera-
ture to give 189 mg of the hexaiododimercurate of 4 (47%), as a
red powder. 1H NMR (300 MHz, CDCl3, 258C, TMS): d =8.39 (d,
3J(H,H)=6.6 Hz, 2H; H2, H6), 7.82 (d, 3J(H,H)=6.6 Hz, 2H; H3, H5),
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3
3
7.68 (d, J(H,H)=15.8 Hz, 1H; H8), 7.54 (d, J(H,H)=8.8 Hz, 2H; H10,
H14), 6.86 (d, J(H,H)=15.8 Hz, 1H; H7), 6.68 (d, J(H,H)=8.2 Hz, 2H;
H11, H13), 4.34 (s, 3H; CH3), 3.38 (t, J(H,H)=7.6 Hz, 4H; NCH2), 1.61
3
3
3
3
(m, 4H; CH2), 1.40 (m, 4H; CH2), 0.99 (t, J(H,H)=7.2 Hz, 6H; CH3):
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Received: September 2, 2009
Published online on December 22, 2009
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