Two-photon absorption properties of Zn(II) complexes: Unexpected large TPA cross section of dipolar [ZnY2(4,4′-bis(para-di-n-butylaminostyryl)-2,2′-b ipyridine)] (Y = Cl, CF3CO2)

Article abstract of DOI:10.1016/j.cplett.2009.05.033

The two-photon absorption (TPA) properties of 4,4′-bis(para-di-n-butylaminostyryl)-2,2′-bipyridine (NBu₂bipy) and related dipolar ([ZnY₂(NBu₂bipy)] (Y = Cl, CF₃CO₂), [Zn(2,2′-bipyridine)₂(N

Full text of DOI:10.1016/j.cplett.2009.05.033

Chemical Physics Letters 475 (2009) 245–249
Contents lists available at ScienceDirect
Chemical Physics Letters
journal homepage: www.elsevier.com/locate/cplett
Two-photon absorption properties of Zn(II) complexes: Unexpected large TPA
cross section of dipolar [ZnY2(4,40-bis(para-di-n-butylaminostyryl)-2,20-bipyridine)]
(Y = Cl, CF3CO2)
Claudia Dragonetti a, Marcella Balordi a, Alessia Colombo a, Dominique Roberto a,b,
,
*
Renato Ugo a, Ilaria Fortunati c, Eleonora Garbin c, Camilla Ferrante c, , Renato Bozio ,
c
*
Alessandro Abbotto d, Hubert Le Bozec e
a Dipartimento di Chimica Inorganica, Metallorganica e Analitica ‘Lamberto Malatesta’ dell’Università di Milano, UdR INSTM di Milano, Italy
b ISTM-CNR, via Venezian 21, 20133 Milano, Italy
c Dipartimento di Scienze Chimiche, Università di Padova, UdR INSTM di Padova, via Marzolo 1, 35131 Padova, Italy
d Dipartimento di Scienze dei Materiali, Università di Milano-Bicocca, UdR INSTM, via Cozzi 53, 20125 Milano, Italy
e UMR 6226 CNRS-Université de Rennes 1, Campus de Beaulieu, 35042 Rennes, France
a r t i c l e i n f o
a b s t r a c t
The two-photon absorption (TPA) properties of 4,40-bis(para-di-n-butylaminostyryl)-2,20-bipyridine
Article history:
(NBu2bipy)
and
related
dipolar
([ZnY2(NBu2bipy)]
(Y = Cl,
CF3CO2),
[Zn(2,20-bipyri-
Received 23 February 2009
In final form 15 May 2009
Available online 20 May 2009
dine)2(NBu2bipy)][PF6]2) and octupolar ([Zn(NBu2bipy)3][PF6]2) complexes were investigated by the
two-photon emission (TPE) technique in a femtosecond regime, working in the 730–930 nm spectral
range. We found, in contrast with previous literature data, that the TPA enhancement upon coordination
of a TPA active ligand to a metal center may be larger in the dipolar rather than the corresponding octu-
polar complex, the response being easily modulated by the choice of the ancillary ligands.
Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
(NMe2bipy) increases upon coordination to a Zn(II) center, the
enhancement being huge in the octupolar complex [Zn(NMe2bi-
Molecular nonresonant two-photon absorption (TPA) has at-
tracted growing interest over recent years due to the many appli-
cations in material science and biological imaging [1–8]. For such
reason, organic molecular engineering was directed towards TPA
optimization leading to a range of molecules of various symmetries
including dipoles [9], quadrupoles [10–18], octupoles [19–22], and
branched structures [23–31]. As a general trend, it was evident
that dipolar chromophores usually exhibit a smaller TPA cross-sec-
tion with respect to quadrupolar and octupolar derivatives. How-
ever, the judicious combination of dipolar branches within a
three-branched octupolar structure can induce a very intense
TPA response [7]. Surprisingly, although coordination chemistry
is a powerful tool to build up various symmetries including octu-
polar arrangements, using a wide range of metals with different
oxidation states and ligands, only recently the potential of organo-
metallic and coordination compounds in the design of TPA materi-
als was highlighted [32–41]. Theoretical calculations (ZINDO-SOS)
predicted that, working in the long wavelength region, the TPA re-
py)3][PF6]2 but lower in a dipolar complex such as [ZnCl2(NMe2bi-
py)] (Table 1) [38,39]. In contrast, experimentally, both Z-scan and
two-photon emission (TPE) measurements evidenced that the TPA
cross section of the structurally related octupolar [Zn(NBu2bi-
py)3][PF6]2 (NBu2bipy = 4,40-bis(para-di-n-butylaminostyryl)-2,20-
bipyridine) complex is not as high as expected (Table 1) [40,41].
Therefore, in order to get a detailed experimental evidence on
the effect of various kinds of coordination on the TPA properties
of NBu2bipy (1), we prepared and characterized not only such li-
gand and its octupolar Zn(II) complex (2) but also the correspond-
ing dipolar [ZnCl2(NBu2bipy)] (3), [Zn(CF3CO2)2(NBu2bipy)] (4),
and [Zn(2,20-bipyridine)2(NBu2bipy)][PF6]2 (5) complexes and we
investigated their TPA properties (Fig. 1).
2. Experimental
Ligand 1 and complexes 2 and 3 were prepared according to the
literature [42]; the new complexes 4 and 5 were obtained by
reaction of NBu2bipy with Zn(CF3CO2)2ꢀnH2O and [Zn(2,20-bipyri-
dine)2][PF6]2, respectively. All compounds were fully characterized
by elemental analyses, 1H NMR, UV–vis, and emission spectrosco-
pies (Table 1).
sponse
of
4,40-bis(para-dimethylaminostyryl)-2,20-bipyridine
* Corresponding authors. Address: Dipartimento di Chimica Inorganica, Metall-
organica e Analitica ‘Lamberto Malatesta’ dell’Università di Milano, via G. Venezian,
21, 20133 Milano, Italy. Fax: +39 02 50314405 (D. Roberto).
Synthesis of [Zn(CF3CO2)2(NBu2bipy)] (4, NBu2bipy = 4,40-bis(para-
di-n-butylaminostyryl)-2,20-bipyridine): Zn(CF3COO)2ꢀ2H2O (36 mg,
E-mail address: dominique.roberto@unimi.it (D. Roberto).
0009-2614/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2009.05.033

246
C. Dragonetti et al. / Chemical Physics Letters 475 (2009) 245–249
Table 1
Absorption and emission properties of bipyridines and related Zn(II) complexes.
Compound
kabs
kem
FQY
kTPA
rTPA
EFa
Method
Refs.
(nm)
(nm)
(nm)
(GM)
NMe2bipy
[Zn(NMe2bipy)3]Y2
Y = PF6
ZnCl2(NMe2bipy)
1
2
395
463
–
–
–
–
736.2
949.2
822.0
830.6
780
965
765
900
920
493.6
4132.1
9932.9
770.3
–
ZINDO-SOS
ZINDO-SOS
[38]
[38]
2.8b
6.7b
1.6b
–
445
400
474
–
493
644
–
ZINDO-SOS
TPEd
[38]
This work
[40]
0.087 0.015
–
278 57c
860 20
1700 500
1018 160c
602 99
637 78
299 84
–
Z-scand
2
3
4
5
473
462
463
476f
640c
628
633
640
0.18 0.02
0.36 0.04
0.26 0.02
0.12 0.03
1.2e
2.2e
2.3e
1.1e
TPEd
TPEd
TPEd
TPEd
This work
This work
This work
This work
920
920
a
Enhancement factor = (rTPA of the complex)/(rTPA of the NR2bipy ligand)(number of NR2bipy ligands in the complex).
With respect to NMe2bipy at 736 nm.
Data are different from those previously reported due to a defective calibration of the fluorescence spectrometer in Ref. [41].
In CH2Cl2.
With respect to NBu2bipy at 780 nm.
There is a shoulder at 571 nm.
b
c
d
e
f
NBu2
NBu2
Bu2N
Bu2N
N
2+
N
N
N
N
N
N
Y
-
Zn
N
(PF6 )2
Zn
Y
Bu2N
3 (Y = Cl)
NBu2
Bu2N
4 (Y = CF3CO2)
NBu2
2
NBu2
N
2+
N
N
N
-
Zn
N
(PF6 )2
N
5
NBu2
Fig. 1. Chemical Structures of Zn(II) complexes.
0.11 mmol) was dissolved in 3 ml of EtOH. The ligand NBu2bipy
(67 mg, 0.11 mmol) was dissolved in 15 ml of CH2Cl2 and this
solution was slowly added to the solution containing the metal
salt, at room temperature. The resulting red solution was stirred
at room temperature for 24 h. The solvent was partially removed
in vacuum and the solution was cooled at ꢁ18 °C overnight. A
red powder was filtered off, washed with n-hexane and cold EtOH
and then dried under vacuum. Yield: 32 mg (32%). C46H54O4N4F6Zn

C. Dragonetti et al. / Chemical Physics Letters 475 (2009) 245–249
247
estimated following the procedure by Xu et al. [46] and using Fluo-
rescein in water at pH > 10 as reference standard.
n-Bu
n-Bu
N
N
6
3
3. Results and discussion
3" 2"
5" 6"
α
N
n-Bu
n-Bu
N
Absorption and fluorescence emission spectra of 1, 3, and 5 in
CH2Cl2 are shown in Fig. 2, whereas for compound 2 and 4 are gi-
ven in the Supplementary material. Typical concentrations are
1 ꢂ 10ꢁ5 M for the one-photon absorption (OPA) spectra, 1 ꢂ
10ꢁ6 M for the one-photon fluorescence spectra, and between
2 ꢂ 10ꢁ5 M and 5 ꢂ 10ꢁ5 M for the TPA spectra.
β
5
Scheme 1. Structure of NBu2bipy ligand (1) with site numbering for NMR
assignment.
The one-photon absorption spectrum show a marked red shift
of the maximum corresponding to the intraligand charge transfer
(ILCT) transition upon coordination of 1 with Zn(II), whereas sligh-
ter changes are promoted by the different ancillary ligands. The red
shift, also observed in the emission spectrum, is attributed to an in-
crease of the electron-withdrawing ability of the bipyridine moiety
of 1 upon coordination to Zn(II). Table 1 collects absorption and
emission peaks and FQY values. The free ligand 1 shows the lowest
FQY, all the coordination compounds show a net increase of the
FQY, with respect to the free ligand, that follows the series:
5 < 2 < 4 < 3. Since an increase of the FQY can be interpreted as a
sign of a more rigid structure with less internal degrees of freedom,
we expect that the free ligand is less rigid than all the coordination
compounds, meaning that it can assume different conformations in
solution that can interchange rapidly among themselves. The
absorption and emission properties of compound 2 are almost
identical to those of 5, whereas 3 behaves similarly to 4. These
findings can be considered as an evidence of similar geometry
and electronic properties of ligand 1 in the Zn(II) complexes when
the complex is neutral (3 and 4) or cationic (2 and 5).
(906.38): calcd. C, 60.96; H, 5.96; N, 6.18; found: C, 60.76; H, 5.87;
N, 5.97. 1H NMR (CDCl3): d 8.70 (broad s, 2H, 3-H), 8.09 (broad s,
2H, 6-H), 7.55–7.48 (m, 6H, 5-H, 30-H and 50-H), 7.43 (d, 2H,
J = 16.1 Hz, b-H), 6.83 (d, 2H, J = 16.1 Hz,
a-H), 6.69 (d, 4H,
J = 8.9 Hz, 20-H and 60-H), 3.37 (t, 8H, J = 7.9 Hz, NCH2), 1.65
(quintet, 8H, J = 7.6 Hz, NCH2CH2), 1.42 (sextet, 8H, J = 7.6 Hz,
NCH2CH2CH2), 1.02 (t, 12H, J = 7.4 Hz, CH3) (see Scheme 1).
Synthesis of [Zn(2,20-bipyridine)2(NBu2bipy)][PF6]2 (5, NBu2bipy =
4,40-bis(para-di-n-butylaminostyryl)-2,20-bipyridine): The complex
was obtained in two steps. Step 1, synthesis of [Zn(2,20-bipyri-
dine)2][PF6]2: 2,20-bipyridine (70 mg, 0.45 mmol) was dissolved
in 7 ml of CH2Cl2. Zn(CH3COO)2 (49 mg, 0.23 mmol) was dissolved
in 3 ml of MeOH and added to the solution of 2,20-bipyridine at
room temperature. NaPF6 (378 mg, 2.26 mmol) was added and
the resulting solution was stirred overnight at room temperature.
The solvent was removed in vacuum and the resulting solid was
washed with H2O to remove the excess of NaPF6 and NaCl. The
resulting pink powder was then washed with n-hexane and dried
under vacuum. Yield: 81 mg (67%). C20H16N4P2F12Zn (667.638):
calcd. C, 35.98; H, 2.40; N, 4.20; found: C, 35.56; H, 2.28; N, 4.37.
1H NMR (CD3CN): d 8.53 (d, 4H, J = 8.2 Hz, 3-H), 8.27 (t, 4H,
J = 7.9 Hz, 4-H), 7.96 (d, 4H, J = 4.4 Hz, 6-H), 7.58 (t, 4H, J = 5.8 Hz,
5-H). 31P NMR(CD3CN): d ꢁ144.06 (2P).
OPA and TPA spectra in CH2Cl2 for all compounds are depicted
in Fig. 3, with the abscissa scale of the TPA spectra divided by
two to have a direct comparison with the OPA spectra. The
rTPA are expressed in Göppert-Mayer units (1 GM = 1 ꢂ
10ꢁ50 cm4 s photonꢁ1 moleculeꢁ1). The TPA maximum wave-
Step 2, synthesis of [Zn(2,20-bipyridine)2(NBu2bipy)][PF6]2:
[Zn(2,20-bipyridine)2][PF6]2 (52 mg, 0.078 mmol) was dissolved in
5 ml of CH3CN at room temperature. At the resulting pink solution
was added NBu2bipy (48 mg, 0.78 mmol) dissolved in 5 ml of
CH2Cl2. The resulting orange solution was heated for few minutes
in order to completely dissolve the ligand and the reaction mixture
was stirred overnight at room temperature and then dried under
vacuum. The resulting violet powder was washed with n-hexane
and diethyl ether. Yield: 78 mg (78%). C58H70N8P2F12Zn
(1282.553): calcd. C, 56.45; H, 5.67; N, 9.08; found: C, 56.17; H,
5.43; N, 8.71. 1H NMR (CD3CN): d 8.61 (broad s, 2H, 3-H, NBu2bipy),
8.52 (d, 4H, J = 8.4 Hz, 3-H, 2,20-bipyridine), 8.26 (t, 4H, J = 7.5 Hz,
4-H, 2,20-bipyridine), 8.07 (broad s, 2H, 6-H, NBu2bipy), 7.95 (d,
4H, J = 4.9 Hz, 6-H, 2,20-bipyridine), 7.70 (d, 2H, J = 16.1 Hz, b-H,
NBu2bipy), 7.58 (t, 4H, J = 5.8 Hz, 5-H, 2,20-bipyridine). 7.54–7.51
lengths together with the corresponding rTPA are collected in Table
1. In the TPA spectra of compounds 2, 3, 4, and 5 there is a shoulder
around 420–430 nm. This shoulder can be a consequence of the
presence of a similar feature in the reference TPA spectrum of fluo-
rescein in water at pH > 10 [46]. Therefore we will not discuss it
any further, whereas we concentrate our attention on the main
TPA maxima.
(m, 6H, 5-H, 30-H and 50-H, NBu2bipy), 7.02 (d, 2H, J = 16.1 Hz,
a-
H, NBu2bipy), 6.76 (d, 4H, J = 8.6 Hz, 20-H and 60-H, NBu2bipy),
3.38 (t, 8H, J = 7.9 Hz, NCH2), 1.61 (quintet, 8H, J = 7.6 Hz,
NCH2CH2), 1.40 (sextet, 8H, J = 7.6 Hz, NCH2CH2CH2), 1.00 (t, 12H,
J = 7.4 Hz, CH3). 31P NMR(CD3CN): d ꢁ144.06 (2P).
Linear absorption and fluorescence spectra were recorded with
a Cary 5 and FluoMax P (Jobin–Yvon), respectively. Fluorescence
quantum yields (FQY) were determined in CH2Cl2 following the
procedure of Demas and Crosby [43]. Fluorescein in water at
pH > 10 (FQY = 0.93) [44] and Coumarin 540A in ethanol
(FQY = 0.56) were used as reference standard [45]. The TPE tech-
nique was used to determine the TPA cross-section (rTPA) in the
730–930 nm excitation wavelength range employing ultrafast
(150 fs) laser pulses [46]. The instrumental set-up has been previ-
ously described [41]. Absolute values of the TPA cross section were
Fig. 2. Normalized linear absorption (full lines) and fluorescence (dashed lines)
emission spectra of 1 (no symbol), 3 (squares) and 5 (stars) in CH2Cl2.

248
C. Dragonetti et al. / Chemical Physics Letters 475 (2009) 245–249
Fig. 3. One-photon absorption (full line) and two-photon absorption (line + symbol) spectra of 1 (triangles), 3 (squares) and 5 (stars) in the upper panel, and of 2 (circle) and 4
(diamonds) in CH2Cl2 in the lower panel. The abscissa scale of the TPA spectra is divided by two to have a direct comparison with the OPA spectra.
The strong overlap of the OPA and TPA spectra of the ligand 1
(Fig. 3) is a clear evidence that the one- and two-photon allowed
excited electronic state is the same in both cases. Theoretical calcu-
lations for 1 in vacuum consider a C2h symmetry; therefore the cal-
culated OPA and TPA maxima are not coincident, as reported also
in Table 1 [38]. Our experimental data do not confirm this theoret-
ical prediction, and this can be a consequence of the ability of this
ligand to easily rotate around the 2,20-bipyridine bond, conse-
quently it does not have a rigid C2h structure in solution and is
preferentially arranged in a non-centrosymmetric structure. The
TPA spectrum of 2 shows a slight blue shift with respect to the
OPA. Assuming a D3 symmetry for this complex, one should expect
a first excited state both one- and two-photon allowed and a sec-
ond excited state which is only TPA active, as demonstrated by the-
as enhancement factor (EF) the maximum rTPA of the complex di-
vided by the product of the maximum rTPA of 1 times the number
of ligands 1 present in the complex. The EF shows that neutral
dipolar complexes 3 and 4 display a net increase of the TPA re-
sponse, whereas rTPA for the cationic complex 5 is almost equal
to that of 1. For the cationic octupolar complex 2, there is a slight
enhancement (EF = 1.2) of the TPA response with respect to ligand
1, but there is no significant enhancement if one, more appropri-
ately, compares the rTPA of 1 with that of complex 5 that has only
one ligand 1. This latter evidence confirms the absence of notice-
able intermolecular interactions between the ligands 1 in the octu-
polar complex 2, as above suggested discussing the spectral
features of the TPA maxima.
Comparison of the maximum rTPA values for compound 1, 2 and
oretical calculations for
a
similar compound and for other
3 with the one computed with ZINDO-SOS calculations for struc-
turally related compounds (see Table 1) leads to the following con-
siderations [38,39]. The smaller experimental value found for rTPA
of 1 can be attributed to a theoretical value computed just for the
C2h conformer, while, as above discussed, in solution 1 is present as
a mixture of conformers whose TPA cross section can be lower than
the one arising solely from the C2h conformer. For the dipolar com-
octupolar systems [20,38]. In the investigated spectral range
(730–930 nm) the TPA spectrum of the octupolar compound 2
shows a single maximum, shifted just of approximately 20 nm
with respect to the OPA maximum, suggesting that the interaction
among the ligands is weak. As expected for non-centrosymmetric
structures, the TPA maxima for all dipolar complexes: 3, 4 and 5
are almost coincident with the OPA one, although for compound
5 we could only approach it, because of instrumental limits. On
the other hand, since the TPA maximum is expected near to the
OPA maximum, the highest value of
to the maximum TPA cross section.
In order to compare the increased TPA properties of the dipolar
3, 4, and 5 and the octupolar 2 complexes with that of 1, we define
pound 3, instead, the experimental and calculated rTPA values can-
not be compared because they refer to different excited electronic
states. In our experimental data, corresponding to an excited state
at 920 nm, we observe a substantial coincidence of the OPA and
TPA maxima, whereas the calculated TPA values are referred to
an excited state that falls at 830 nm, which we do not observe in
the experimental TPA spectrum. The rTPA value observed for the
rTPA measured should be close

C. Dragonetti et al. / Chemical Physics Letters 475 (2009) 245–249
249
octupolar complex 2 is in complete contrast with the theoretical
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This work was supported by the Consorzio INSTM (PRISMA
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n.2006031511) and by CNR (PROMO 2006). COST Action D35 is
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Appendix A. Supplementary material
One photon absorption and emission spectra of the octupolar
complex 2 (Fig. A1) and of the dipolar complex 4 (Fig. A2) in
dichloromethane.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.cplett.2009.05.033.