Diaza-18-crown-6 based chromophores for modulation of two-photon absorption cross-section by metal ions

Article abstract of DOI:10.1016/j.jorganchem.2008.01.019

Two chromophores with diaza-18-crown-6 as receptor have been synthesized in high yields. The electronic structure, one-photon absorption (OPA) spectra, and two-photon absorption (TPA) properties have been studied in detail. When no metal ion is added as i

Full text of DOI:10.1016/j.jorganchem.2008.01.019

Available online at www.sciencedirect.com
Journal of Organometallic Chemistry 693 (2008) 1186–1194
www.elsevier.com/locate/jorganchem
Diaza-18-crown-6 based chromophores for modulation of
two-photon absorption cross-section by metal ions
Atanu Jana, Arijit Kumar De, Amit Nag, Debabrata Goswami, Parimal K. Bharadwaj *
Department of Chemistry, Indian Institute of Technology, Kanpur 208016, India
Received 10 October 2007; received in revised form 28 December 2007; accepted 8 January 2008
Available online 17 January 2008
Abstract
Two chromophores with diaza-18-crown-6 as receptor have been synthesized in high yields. The electronic structure, one-photon
absorption (OPA) spectra, and two-photon absorption (TPA) properties have been studied in detail. When no metal ion is added as
input, both show negligible TPA cross-section (r2). However, in the presence of Zn(II)/Cd(II)/Mg(II)/Ca(II) ion, each exhibits large
TPA cross-section value. Binding of metal ion in the receptor increases the symmetric charge transfer leading to large r2 values. Theo-
retical calculations at the B3LYP functional with 6-31G* and LanL2DZ mixed basis set under DFT formalism support experimental
results.
Ó 2008 Elsevier B.V. All rights reserved.
Keywords: Aza-crown ether receptor; Charge transfer; DFT; OPA; TPA
1. Introduction
however, that TPA enhancement rather than its lowering
in the presence of a metal ion will be better understood.
With this design principle in mind, we have attached 4-
(dimethylamino)benzene or ferrocene group to diaza-18-
crown-6 to have two D–p–D0 moieties symmetrically dis-
posed (Scheme 1).
Crown ethers can bind biologically relevant alkali and
alkaline earth metal ions while mixed aza-oxa crown ethers
can complex other types of metal ions as well. Diaza-18-
crown-6 is a good receptor for Zn(II), Cd(II) and Mg(II)
ions in addition to alkali metal ions such as Na(I) and
K(I). Zinc is an essential nutrient required in normal
growth and development [5] and for cellular processes such
as DNA repair [6] and apoptosis [7]. This metal plays a key
role in the synthesis of insulin and the pathological state of
diabetes [8]. On the other hand, many enzymatic reactions
are mediated by Mg(II) while Ca(II) acts as a universal sec-
ond messenger in cells [9].
Molecules that exhibit optical nonlinearity are presently
in demand as they are potentially useful in opto-electronics
as well as all-optical data processing technologies. Third-
order optical nonlinearity of molecules measured in terms
of two-photon absorption cross-section (r2) is particularly
important in bio-photonics and materials science such as
photodynamic therapy [1], optical power limiting [2],
three-dimensional optical data storage [3], two-photon
up-conversion lasing [4], and so on. Besides, modulation
of TPA activity of molecules in the presence of certain ions
are important from the perspectives of detection and deter-
mination of static concentration of these ions in bio-sys-
tems which is important in understanding of biological
processes. An important class of compounds capable of
exhibiting large TPA cross-section are organic molecules
with symmetrical charge transfer possibilities. When a
metal ion influences this charge transfer, modulation of
the TPA activity can be achieved. It should be noted,
Metal ions are excellent 3D templates and can assemble
simple organic NLO-phores around with concomitant tun-
ing of the molecular nonlinear optical properties by virtue
of inducing a strong intra-ligand charge transfer (ILCT)
transition. There are very few examples on the metal ion
*
Corresponding author.
E-mail address: pkb@iitk.ac.in (P.K. Bharadwaj).
0022-328X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2008.01.019

A. Jana et al. / Journal of Organometallic Chemistry 693 (2008) 1186–1194
1187
NO2
O
O
O
O
O
O
O
O
F
NH
HN
O2N
N
N
NO2
DMSO
Na CO
2
3
L1
L0
NH NH .H O
2
2
2
Pd / C
Ethanol, Reflux
CHO
Fe
in Ethanol
O
O
O
O
H2N
N
N
NH2
N
N
N
N
O
O
O
O
Fe
Fe
L2
L3
CHO
N
in Ethanol
O
O
O
O
N
N
N
N
N
N
L4
Scheme 1. Synthetic scheme for the chromophores L3 and L4 studied in this work.
induced TPA cross-section reported earlier in the literature
[10]. Herein, the NLO-phores L3 and L4 are designed such
that in absence of a metal ion, the two D–p–D0 units pres-
ent in each compound, are electronically independent.
When a metal ion occupies the cavity, it can electronically
connect the two D–p–D0 moieties into D–p–A–p–D leading
to increased symmetry of charge transfer and hence larger
TPA cross-section. Theoretical calculations were carried
out to support the experimental findings.
zene groups can be grafted by simple aromatic nucleophilic
substitution (ArSN) reaction that on reduction affords the
corresponding amines that can readily undergo Schiff base
condensation with aldehydes leading to the target
compounds.
2.2.1. Synthesis of N,N0-bis(4-nitrophenyl)-4,13 diaza-18-
crown-6 derivative (L1)
To a solution of 1,4,10,13-tetraoxa-7,16-diaza cyclooc-
tadecane, L0 (1 g; 3.8 mmol) in dry DMSO (15 mL) was
added anhydrous Na2CO3 (1.0 g; 9.5 mmol). Subse-
quently, 1-fluoro-4-nitrobenzene (0.9 mL; 8.7 mmol) in
dry DMSO (15 mL) was added drop wise in 30 min and
the reaction mixture was allowed to stirred at 80 °C for
72 h. It was then poured into ice water (250 mL). The
separated yellow solid was collected by filtration and
washed thoroughly with water (5 ꢀ 100 mL). This yellow
solid contains a mixture of mono- as well as bis-deriva-
tive with some excess of 1-fluoro-4-nitrobenzene and
was separated by column chromatography (neutral alu-
mina). After removing the impurities, the bis-derivative
(L1) was eluted first using hexane: EtOAc (55:45 v/v) as
the eluent. After evaporating the solvent, the yellow solid
was recrystallized from MeCN to obtain a bright yellow
crystalline solid. Yield: ꢁ60%; m.p. 205 °C. 1H NMR
spectra (400 MHz, CDCl3, TMS, 25 °C) d: 3.41–3.94
(m, 24H), 6.02 (d, J = 9.5 Hz 6H), 8.08 (d, J = 9.4 Hz,
4H); 13C NMR spectra (100 MHz, CDCl3) d: 51.7, 68.7,
71.1, 110.3, 126.2, 137.6, 152.7; ESI-MS (m/z): 527
(100%) [M+Na]+. Anal. Calc. for C24H32N4O8: C,
57.13; H, 6.39; N, 11.10. Found: C, 56.98; H, 6.31; N,
11.19%.
2. Experimental
2.1. Materials
Reagent grade 1,4,10,13-tetraoxa-7,16-diazacycloocta-
decane, 1-fluoro-4-nitrobenzene, 4-(dimethylamino)benzal-
dehyde, ferrocene carboxaldehyde, 10% Pd in activated
charcoal, Rhodamine-6G and all the metal perchlorate
salts were acquired from Aldrich (USA) and used as
received. Reagent grade hydrazine hydrate, Na2CO3 and
the solvents were purchased from SD Fine Chemicals
(India). The solvents were purified before use following
established methods. All the reactions were carried out
under dinitrogen atmosphere unless otherwise mentioned.
Chromatographic separations were achieved by column
chromatography using neutral alumina from Acme Chem-
icals (India).
2.2. Synthesis of ligands
The synthesis of the compounds was achieved in several
stages as illustrated in Scheme 1. One or two p-nitroben-

1188
A. Jana et al. / Journal of Organometallic Chemistry 693 (2008) 1186–1194
2.2.2. Synthesis of N,N0-bis(4-aminophenyl)-4,13 diaza-18-
crown-6 derivative (L2)
(400 MHz) and 13C NMR spectra (100 MHz) of the com-
pounds were recorded on a JEOL JNM-LA400 FT spec-
trometer in CDCl3 with tetramethylsilane as internal
standard. Melting points were measured with an electrical
melting point apparatus by PERFIT, India and were
uncorrected. Elemental analyses were recorded in an Ele-
mentar Vario EL III Carlo Erba 1108 Elemental Analyser.
The ESI-MS data were obtained from a MICROMASS
QUATRO Quadruple Mass Spectrometer. Each sample
dissolved in acetonitrile and introduced into the ESI source
through a syringe pump at the rate of 5 ll/min. The ESI
capillary was set at 3.5 kV and the cone voltage was
40 V. The spectra were collected in 6 s scans and the print-
outs were averaged of 6–8 scans. Microanalyses for the
compounds were obtained from CDRI, Lucknow, India.
UV–vis spectra were recorded on a JASCO V570 UV–
vis–NIR spectrophotometer in dry acetonitrile at 298 K.
The nitro-compound obtained above was reduced to the
corresponding amine by hydrazine hydrate in the presence
of Pd–C catalyst in refluxing ethanol [11]. Typically, the
bis-nitro derivative (L1) (0.8 g; 1.6 mmol) was taken in a
two necked round bottom flask with 50 mL absolute etha-
nol, added 0.015 g of 10% Pd in activated charcoal fol-
lowed by hydrazine hydrate (2.6 mL) over a period of
30 min and the mixture was refluxed for 3 h. It was filtered
hot and then evaporated almost to dryness. The mixture
was shaken with 100 mL water to remove excess hydrazine
hydrate and the desired amine extracted with chloroform,
dried over anhydrous Na2SO4 and finally evaporated to
obtain the product as a colorless semi-solid. Yield:
1
ꢁ65%; H NMR spectra (400 MHz, CDCl3, TMS, 25 °C)
d: 3.4–3.67 (m, 24H), 3.89 (b, 4H), 6.55–6.71 (m, 8H);
13C NMR spectra (100 MHz, CDCl3) d: 52.8, 69.1, 70.7,
116.2, 117.2, 137.4, 141.3; ESI-MS (m/z): 445 (40%)
[MH]+. Anal. Calc. for C24H36N4O4: C, 64.84; H, 8.16;
N, 12.60. Found: C, 64.73; H, 7.91; N, 12.48%.
2.3.1. Measurement of two-photon absorption cross-section
(r2)
Two-photon absorption cross-section measurements
were carried out by open-aperture Z-scan technique [12]
in the wavelength range of 750–850 nm in 10ꢂ4 M dry ace-
tonitrile solution of the chromophores and their metal
complexes.
2.2.3. Synthesis of the chromophores (L3 and L4)
The NLO-phores were synthesized by Schiff base con-
densation of the amine with an equivalent amount of either
4-(dimethylamino)benzaldehyde or ferrocene carboxalde-
hyde in dry ethanol. The desired product precipitated out
on stirring at RT for 24 h. This was collected by filtration,
washed twice with dry ethanol followed by diethyl ether
and finally dried in vacuo. These compounds were directly
used for analysis and further studies. All attempts to make
single crystals of any of these compounds remained
unsuccessful.
The femtosecond experimental scheme involves mode-
locked Coherent Mira titanium: sapphire laser (Model
900) which is pumped by Coherent Verdi frequency dou-
bled Nd: vanadate laser. The model 900 Mira is tunable
from 740 to 900 nm and its repetition rate is 76 MHz.
The duration of the pulse was 150 fs as measured by auto-
correlation technique. Using a 20 cm focal length lens, the
beam was focused into a 1 mm long cell filled with sample,
where it easily produces GW-level intensity at the focal
point of the lens. The sample was scanned through the
focal point using a motorized translation stage (model
ESP 300), which can step with a minimum resolution of
0.1 lm. This allows a smooth intensity scan for the samples
in this wavelength. The transmitted beam, after passing
through the sample was focused into the aperture of a
UV-enhanced amplified silicon photo detector (Thorlabs
DET 210) by using a 7.5 cm focal length lens. We measured
the signal in an oscilloscope (Tektronix TDS 224), which
was triggered by the chopper frequency. The signal was
measured in an oscilloscope (Tektronix TDS 224), which
is finally interfaced with the computer using GPIB card
(National Instruments). The data was acquired using Lab-
VIEW programming. The nonlinear absorption coefficient
b was obtained [13], by fitting our measured transmittance
values to the following formula:
L3: Yield ꢁ85%; m.p. 165 °C; 1H NMR spectra
(400 MHz, CDCl3, TMS, 25 °C) d: 3.58–3.71(m, 24H),
4.17(s, 10H), 4.4(d, J = 10.6 Hz, 4H), 4.71(d, J = 9.0 Hz,
4H), 6.63(d, J = 9.0 Hz, 2H), 7.02–7.09 (m, 6H), 8.26 (d,
J = 5.9 Hz, 2H); 13C NMR spectra (100 MHz, CDCl3) d:
48.8, 51.4, 68.6, 69.0, 69.1, 69.2, 69.6, 70.1, 70.2, 70.8,
71.3, 112.0, 121.3, 121.9, 122.2, 141.6, 161.4; ESI-MS (m/
z): 837(30%) [M]+, 878 (100%) [M+MeCN]+. Anal. Calc.
for C46H52N4O4Fe2: C, 66.04; H, 6.26; N, 6.69. Found:
C, 65.93; H, 6.48; N, 6.43%.
L4: Yield ꢁ65%; m.p. 150 °C; 1H NMR spectra
(400 MHz, CDCl3, TMS, 25 °C) d: 3.01 (s, 6H), 3.03 (s,
6H), 3.64–3.76 (m, 24H), 6.66–6.72 (m, 6H), 7.06–7.18
(m, 6H), 7.71–7.74 (m, 4H), 8.27 (d, J = 4.6 Hz, 2H); 13C
NMR spectra (100 MHz, CDCl3) d: 40.2, 51.5, 69.0, 70.7,
70.9, 111.6, 112.1, 121.6, 122.2, 129.9, 130.4, 146.0, 151.9,
160.3; ESI-MS (m/z): 707 (25%) [M]+, 748 (100%)
[M+MeCN]+. Anal. Calc. for C42H54N6O4: C, 71.36; H,
7.69; N, 11.89. Found: C, 71.23; H, 7.39; N, 11.99%.
TðzÞ ¼ 1 ꢂ bI0L=ð2ð1 þ z2=z02ÞÞ
where b = nonlinear absorption coefficient, I0 = on-axis
electric field intensity at the focal point in absence of the
sample, L = sample thickness, z0 = Rayleigh range =
pw0 /k, w0 is the minimum spot size at the focal point.
The b values are obtained by curve fitting the measured
2.3. Methods
2
All the compounds were characterized by elemental
analysis, NMR, and ESI-MS spectra. Both 1H NMR

A. Jana et al. / Journal of Organometallic Chemistry 693 (2008) 1186–1194
1189
2.5
2.0
1.5
1.0
0.5
0.0
a
1.25
1.00
0.75
0.50
0.25
0.00
b
L4
L3
L4+Na(I)
L4+K(I)
L4+Mg(II)
L4+Ca(II)
L4+Zn(II)
L4+Cd(II)
L3+ Na(I)
L3+ K(I)
L3+ Mg(II)
L3+ Ca(II)
L3+ Cd(II)
L3+ Zn(II)
300
400
500
600
700
300
400
500
600
700
Wavelength (nm)
Wavelength (nm)
Fig. 1. (a) Single photon absorption spectra of chromophore L3 and their corresponding metal complexes in 10ꢂ5 M CH3CN solution. (b) Single photon
absorption spectra of L4 and their corresponding metal complexes in 10ꢂ5 M CH3CN solution.
0
0.5 × 10-7 M
1.0
0.8
0.6
0.4
0.2
0.0
0.5 × 10-6
0.3 × 10-5
0.4 × 10-5
0.6 × 10-5
0.7 × 10-5
0.9 × 10-5
1.0 × 10-5
0.2 × 10-4
0.3 × 10-4
0.4 × 10-4
0.8 × 10-4
1.0 × 10-4
1.0 × 10-3
M
M
M
M
M
M
M
M
M
M
M
M
M
0.025
0.020
0.015
0.010
a
b
0
[Zn(II)]
/A-
0
A
Y = 0.00921 + 1.51741 x 10-8
R = 0.99
X
[Zn(II)]
300
400
500
600
700
0
200000 400000 600000 800000 1000000
[M]-1
Wavelength (nm)
Fig. 2. (a) Absorption spectra of the chromophore L4 as a function of [Zn(II)]. The arrows indicate the trend for increasing [Zn(II)]. The [L4] is 1 ꢀ 10ꢂ5
M. (b) Plot of A0/A ꢂ A0 against [Zn(II)]ꢂ1 for binding constant determination. The absorption data yield logKs = 5.783.
open-aperture traces with the above equation. After getting
the value of b, the TPA cross-section r2 in GM unit of one
solute molecule is given by the following expression:
Table 1
Photophysical data for the chromophores L3 and L4 and their corre-
sponding metal complexes
Compounds
kmax (nm) e ꢀ 104
Max r2 (GM)
in the range
of 750–850 nm
r2 ¼ bhm=N ꢃ c ꢃ 10ꢂ3
(mol Lꢂ1 cmꢂ1
)
where m is the frequency of the incident laser beam, N is
Avogadro constant, c is the concentration of the com-
pound in CH3CN solvent. Rhodamine 6G was taken as
the reference to calibrate the measurement technique for
which the r2 value is known in the literature [14].
L3
L3 + Na(ClO4) ꢃ xH2O
L3 + K(ClO4) ꢃ xH2O
334, 546
336, 539
336, 540
5.736, 1.426
6.251, 2.545
7.091, 2.098
13.601, 4.101
8.782, 2.210
21.745, 4.989
7.707, 1.874
4.899
21
1399
1120
2567
2852
3223
2651
10
13
16
62
52
L3 + Mg(ClO4)2 ꢃ xH2O 352, 536
L3 + Ca(ClO4)2 ꢃ xH2O
L3 + Zn(ClO4)2 ꢃ xH2O
L3 + Cd(ClO4)2 ꢃ xH2O
L4
344, 527
354, 537
337, 538
365
2.3.2. Binding constant determination
The binding constants (Ks) for the complexes between
the chromophores and metal salts were determined by
UV–vis spectroscopic titration method. The concentration
of the ligands was 1 ꢀ 10ꢂ5 M and that of metal ions was in
the range of 1 ꢀ 10ꢂ3 M to 1 ꢀ 10ꢂ7 M. The linear fit of
the absorption spectral data at a particular wavelength
L4 + Na(ClO4) ꢃ xH2O
L4 + K(ClO4) ꢃ xH2O
362, 482
361, 481
4.562, 1.460
4.469, 2.174
1.997, 5.827
3.009, 3.905
8.854
L4 + Mg(ClO4)2 ꢃ xH2O 363, 455
L4 + Ca(ClO4)2 ꢃ xH2O
L4 + Zn(ClO4)2 ꢃ xH2O
L4 + Cd(ClO4)2 ꢃ xH2O
359, 462
446
67
54
449
7.124

1190
A. Jana et al. / Journal of Organometallic Chemistry 693 (2008) 1186–1194
Fig. 3. Open-aperture Z-scan traces for some metal complexes of the chromophore L3 in 10ꢂ4 M CH3CN solution. Solid lines are the best fits to the
experimental data.
for 1:1 complexation was obtained by applying the follow-
ing equation [15].
sition. This band makes a slight red-shift (2–15 nm) and
becomes more intense in the presence of a metal ion. The
lower energy band at 546 nm does not shift to any signifi-
cant extent although increases in intensity along with the
appearance of another weak and broad band around
A0=ðA ꢂ A0Þ ¼ ½a=ðb ꢂ aÞꢄ½ð1=Ks½MꢄÞ þ 1ꢄ;
where A0 and A are the absorbance of the metal-free ligand
and the metal complex respectively. [M] is the concentra-
tion of various metal ions added. When A0/(A ꢂ A0) was
plotted against the metal ion concentration [M]ꢂ1, Ks was
directly obtained from the intercept/slop ratio. The details
of the theoretical analysis of the stoichiometries and corre-
sponding binding constant determination are available in
the literature [16]. Here the calculated values are in consis-
tence with good correlation coefficients (P0.99).
3500
L3+Mg(II)
L3+Ca(II)
L3+Zn(II)
L3+Cd(II)
3000
2500
2000
1500
1000
3. Results and discussion
3.1. One-photon absorption spectra of the chromophores and
their corresponding metal complexes
One-photon absorption spectra for L3, L4 and their dif-
ferent metal complexes, taken in acetonitrile (10ꢂ5 M) are
collected in Fig. 1. The ligand L3 exhibits two charge trans-
fer bands, one at 334 nm and the other at 546 nm consis-
tent with other ferrocenyl chromophores [17]. The
334 nm band is due to ligand-centered p–p* electronic tran-
740
760
780
800
820
840
Wavelength (nm)
Fig. 4. Experimentally determined TPA cross-section of few metal
complexes of chromophore L3 in 10ꢂ4 M CH3CN solution as a function
of wavelength.

A. Jana et al. / Journal of Organometallic Chemistry 693 (2008) 1186–1194
1191
440 nm in the presence of the metal ions. These bands are
assignable to donor–acceptor charge transfer transitions.
Metal free L4 exhibits a strong band around 365 nm along
with a shoulder near 330 nm. Both these bands are of intra-
molecular charge transfer [18] from the NMe2 donor end to
the diaza-18-crown-6 which is a weak acceptor. Upon addi-
tion of a metal ion, the macrocycle becomes a good accep-
tor and two bands are observed whose position as well as
emax values are sensitive to the nature of the metal. In
Fig. 1, the charge transfer bands are clearly generated for
L4 especially in the presence of Zn(II) and Cd(II). On the
other hand, such intense charge transfer bands are only
weakly observed for L3. It clearly indicates that donor
acceptor interaction induced is larger for L4/Zn(II) com-
pared to L3/Zn(II). However, these observations are not
directly related to the TPA cross-section data, where L3
in the presence of Zn(II) gives the largest value.
3.2. Binding constant determination of metal complexes
The binding constants were determined from the varia-
tion of absorbance intensity at proper observation wave-
lengths for both the chromophores. A typical example of
spectral change in case of L4, upon addition of metal salt
is given in Fig. 2a, which clearly shows one isosbestic point
corresponding to only one equilibrium process. Here, for
the complexation of chromophores L3 and L4 with Zn(II)
salts, the logKs values were found to be 4.87 and 5.78,
Fig. 5. Contour surfaces of HOMO and LUMO for the chromophores L3 and L4 and their corresponding Mg(II) and Zn(II) complexes.

1192
A. Jana et al. / Journal of Organometallic Chemistry 693 (2008) 1186–1194
Fig 5. (continued)
respectively. And for Mg(II) complexes these values were
4.76 and 4.68, respectively.
L3. It is also very clear that there is an increasing order of
TPA cross-section value when we go through the alkali
metal to alkaline earth metal and finally to the transition
metal. It is certainly due to the effective nuclear charge of
the metal ion that is increased in the same trend with
greater extent of encapsulation in the aza crown ether
receptor and hence they can communicate with the two side
arms more effectively. Among the metal ions studied,
Zn(II) shows the highest value of r2 in the wavelength
range. The corresponding open-aperture Z-scan traces for
L3 with Mg(II), Ca(II), Zn(II) and Cd(II) are given in
Fig. 3.
We also recorded the change in TPA cross-section val-
ues as a function of wavelengths only for L3 (as this chro-
mophore gave some significant r2 values in practice) in the
presence of different metal ions. Interestingly, in every case
there is a gradual increase of r2 value up to a certain limit
3.3. Two-photon absorption
The TPA cross section (r2) measurements were carried
out in the wavelength range, 750–850 nm which is within
the window of minimum cell damage for any in vivo stud-
ies. The r2 values for L3, L4 alone and in the presence of
metal ions are collected in Table 1. Metal free L3 and L4
do not show any significant TPA cross-section. In the pres-
ence of Na(I) or K(I) ion as input, the r2 value increases
marginally in case of L4 but significantly in case of L3. This
is commensurate with the fact that while N,N0-dimethyl-
benzene is a strong donor group, ferrocene is a good elec-
tron transfer group [19]. Other metal ions studied also
corroborate this fact and show much higher r2 values with

A. Jana et al. / Journal of Organometallic Chemistry 693 (2008) 1186–1194
1193
followed by dropping to a minimum (Fig. 4). All the photo-
physical data for the chromophores L3 and L4 and their
different metal complexes are given in Table 1.
Mg(II) ion has no diffused electron cloud like Zn(II) the
LUMO is just shifted towards metal center rather than
residing upon it like the former.
Here, in every case, it is worth noting that a more obvi-
ous increased intramolecular charge transfer transition
triggered by metal ions is very beneficial to interpret the
enhancement of TPA cross-section from the free chro-
mophores [23]. Again from the energy level diagrams of
the ligands it is clear that the energy of both HOMO and
LUMO orbitals decrease significantly when both form
complexes with metal ions, which indicates that an elec-
tronic charge delocalization takes place after complexation.
As for example, we find from Fig. 6, the HOMO and
LUMO energy of Mg(II) complex of chromophore L3,
ꢂ8.43 eV and ꢂ5.35 eV (these are ꢂ8.46 eV and ꢂ6.19 eV
respectively for the Zn(II) complex) is more stable than
that of free chromophore (L3), which are ꢂ4.81 eV and
ꢂ1.00 eV, respectively. Furthermore, it also accompanied
with the decrease of HOMO–LUMO gap after complexa-
tion as expected, which are 3.81 eV for the chromophore
(L3) itself and 3.07 eV for the Mg(II) complex (this is
2.26 eV for the Zn(II) complex). The similar trend is fol-
lowed for the free chromophore L4 and its complex with
Mg(II) (and also for Zn(II) complex). Fig. 6. consists of
calculated frontier orbital energy data for 10 occupied
molecular orbitals and 10 unoccupied molecular orbitals
obtained by B3LYP/6-31G* and are plotted serially for
comparison with free chromophores.
3.4. Theoretical studies
Geometry optimization has been carried out for the
ligands (L3 and L4) and their Mg(II) and Zn(II) complexes.
All calculations were performed by the GAUSSIAN 03 pro-
gram [20] using the B3LYP functional [21] with 6-31G*
(for C, H, N, and O) and LanL2DZ [22] (for Fe, Zn and
Mg) basis set. When LanL2DZ basis set was used along
with the 6-31G* basis set then Gen was used to designate
this. This was chosen as a compromise between accuracy
and applicability to a larger molecular system. All the opti-
mized geometries possess the C1 symmetry. Experimen-
tally, we found that when free chromophores (L3 or L4)
coordinate with metal ions, the magnitude of the TPA
cross-section increased. In case of L3 the change is much
more extensive rather than the chromophore L4. The
TPA cross-section is directly correlated with the extent of
intramolecular charge transfer transition through p-bridge.
Generally, the frontier molecular orbitals, especially
HOMOꢂ1, HOMO, LUMO and LUMO+1 play the sig-
nificant role to electronic transition during two-photon
absorption [10]. For more intuitional analysis of the influ-
ence of the metal ion upon TPA property, we should con-
sider the contours of some correlative occupied and
unoccupied frontier orbitals (Fig. 5).
From the contour surface diagrams it is clear that in free
ligand both the HOMO and LUMO are situated in the side
arms of the ligands, indicating only a p ? p* electronic
transition (i.e., intra-ligand charge transfer, ILCT).
Whereas, in metal complexes the HOMO is mainly concen-
trated on the p-conjugated side arm and LUMO resides on
the metal atom (in case of Zn(II)) which clearly indicates
that a strong intraligand charge transfer transition acceler-
ated by metal atom, takes place after complexation. As
4. Conclusion
In this paper, the linear and nonlinear properties of two
new dipolar chromophoric systems were studied in detail.
The TPA properties have been also investigated using
femto-second laser. The results reveal that there is an
increasing order of TPA cross-section value from alkali
metal ions to transition metal ions followed by alkaline
earth metal ions depending upon the extent of binding with
aza crown ether receptor which not only depends on the
conjugation length, but also on the strength of the electron
donor or acceptor property, which provide a rational
design strategy for TPA molecules to fit in different appli-
cations. Overall, the effect of the alkali metals, alkaline
earth metals and transition metals on the TPA cross-sec-
tion was investigated. One crucial inference drawn from
the present work is that the dipolar molecules can serve
as a promising alternative TPA dyes for a variety of appli-
cations. Theoretical calculations at B3LYP functional with
6-31G* and LanL2DZ mixed basis set show clearly the
shifting of charge density in the chromophores by metal
ion incorporation into the cavity. Further studies on simi-
lar model systems are in progress in our laboratory.
2
0
LUMO
HOMO
LUMO
HOMO
-2
-4
LUMO
HOMO
LUMO
HOMO
LUMO
HOMO
-6
LUMO
HOMO
-8
-10
-12
L4
L3
L3+Mg(II) L3+Zn(II)
L4+Mg(II) L4+Zn(II)
Acknowledgements
Fig. 6. B3LYP/6-31G* predicted molecular orbital energy levels for
dipolar chromophores and their corresponding Mg(II) and Zn(II)
complexes.
Financial support received from CSIR, New Delhi,
India to P.K.B. is gratefully acknowledged. D.G. thanks

1194
A. Jana et al. / Journal of Organometallic Chemistry 693 (2008) 1186–1194
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DST, MCIT (India), and International SRF Program of
Welcome Trust (U.K.) for the financial grant. A.J. and
A.K.D. thank CSIR for their fellowship. We also thank
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´
Bredas, S.R. Marder, J.W. Perry, J. Am. Chem. Soc. 126 (2004) 9291;
Supplementary data associated with this article can be
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2008.01.019.
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