Photophysical and non-linear optical behavior of novel tetra alkynyl terminated indium phthalocyanines: Effects of the carbon chain length

Article abstract of DOI:10.1016/j.poly.2014.12.020

We report on the synthesis, photophysical and nonlinear optical behavior of tetra-substituted alkynyl indium phthalocyanine complexes (3a and 3b). Both complexes showed large triplet quantum yields. Nonlinear optical properties were also evaluated for the

Full text of DOI:10.1016/j.poly.2014.12.020

Polyhedron 88 (2015) 73–80
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Photophysical and non-linear optical behavior of novel tetra alkynyl
terminated indium phthalocyanines: Effects of the carbon chain length
⇑
Owolabi M. Bankole, Jonathan Britton, Tebello Nyokong
Department of Chemistry, Rhodes University, Grahamstown 6140, South Africa
a r t i c l e i n f o
a b s t r a c t
Article history:
We report on the synthesis, photophysical and nonlinear optical behavior of tetra-substituted alkynyl
indium phthalocyanine complexes (3a and 3b). Both complexes showed large triplet quantum yields.
Nonlinear optical properties were also evaluated for the two complexes at a wavelength of 532 nm using
nanosecond Z-scan technique in dimethylsulfoxide. We observed two-photon absorption (2PA) and strong
reverse saturable absorption (RSA) as the dominant mechanisms at nanosecond laser excitation. The
underlining 2PA and observed RSA were subjected to further scrutiny by comparing the analytical absorp-
tion model to the transmittance optical absorption theory. The theoretical results were in good agreement
Received 4 November 2014
Accepted 20 December 2014
Available online 30 December 2014
Keywords:
Nonlinear optical absorption
Alkynyl indium phthalocyanine
Hyperpolarizability
Third order susceptibility
Reverse saturable absorption
ꢁ42
to the observed RSA and the 2PA mechanism. Large two-photon absorption cross-section (1.29 ꢀ 10
ꢁ
42
4
ꢁ14
ꢁ14
and 1.15 ꢀ 10
cm s/photon), third-order susceptibility (2.10 ꢀ 10
and 2.15 ꢀ 10
esu) and hyper-
ꢁ32
ꢁ32
polarizability (2.70 ꢀ 10
and 3.19 ꢀ 10
esu) were estimated for complex 3a and 3b, respectively.
Ó 2014 Elsevier Ltd. All rights reserved.
1
. Introduction
and nonlinear properties of indium phthalocyanines functionalized
with alkynyl-containing moieties at the peripheral positions of the
phthalocyanine ring. The rationale behind this novel idea as con-
tained in this research is based on: (1) the use of heavy metal atom
at the centre of the phthalocyanines and (2) presence of chromo-
phore containing an acetylene unit.
Indium is used as the central metal to increase the rate of inter-
system crossing via spin–orbit coupling and also to increase the
hyperpolarizability of the phthalocyanines [25]. The axial ligation
(Cl used in this case) minimizes intermolecular interactions which
cause aggregation of phthalocyanines in solution [26]. It has been
reported that the Introduction of groups containing triple bonds
enhances the intersystem crossing rate [27], hence we employed
these groups in this work.
To the best of our knowledge, placing terminal alkynyl moieties
on the peripheral positions of indium phthalocyanines has not been
reportedbefore. Thus, wereportfor thefirsttimethesynthesis, char-
acterization and nonlinear optical properties of In-phthalocyanine
tetra-substituted 5-hexyl-1-ol and 2-propyl-1-ol on the peripheral
positions, Scheme 1. The effect of the chain length is assessed. NLO
behavior of alkynyl substituted phthalocyanines have been reported
for complexescontainingZn and Co [28] and withno spacer between
the terminal group and the Pc ring. Low values of hyperpolarizability
were obtained [28]. In this work, spacers are placed between the Pc
ring and the terminal alkynyl group, with improved hyperpolariz-
ability. The presence of the alkynyl group allows for future modifica-
tion of the molecules using click chemistry.
Phthalocyanines (Pcs) have been used for a diverse range of
applications including in liquid crystals, photodynamic therapy
PDT), Langmuir–Blodgett films, chemical sensors, semiconductors,
(
electrochromic devices, ink-jet printing, electrophotography and
electrocatalysis [1–8]. The aforementioned applications rely on
the Pcs’ degree of aromaticity, unique electronic properties, high
thermal- and photo-stability and the architectural flexibility [9].
One of the emerging applications of these molecules as dis-
cussed in this work is their use as potential nonlinear optical lim-
iters [10–13]. The application of phthalocyanines as potential
nonlinear optical (NLO) materials is partly based on the mecha-
nism known as reverse saturable absorption (RSA) [14–18]. The
other mechanism responsible is two-photon absorption. NLO
materials protect sensors and human eyes from laser radiation
by effectively limiting the energy output of an incident light
[
19,20]. Modifications of phthalocyanine structures for improved
optical limiting purposes include the use of a variety of central
metal atoms, axial ligands and/or peripheral substituents [21].
Intense investigation into photophysical behavior of indium,
gallium and lead phthalocyanine derivatives as potential nonlinear
optical materials, have been carried out [22–24]. This paper
describes for the first time, the synthesis, photophysical studies
⇑
Corresponding author. Tel.: +27 46 6038260; fax: +27 46 6225109.
E-mail address: t.nyokong@ru.ac.za (T. Nyokong).
http://dx.doi.org/10.1016/j.poly.2014.12.020
277-5387/Ó 2014 Elsevier Ltd. All rights reserved.
0

7
4
O.M. Bankole et al. / Polyhedron 88 (2015) 73–80
The optical limiting behavior of phthalocyanines molecules
tem). The liquid samples were placed in a cuvette (internal dimen-
sions: 2 mm ꢀ 10 mm ꢀ 55 mm, 0.7 mL) and a path length of
2 mm (Starna 21-G-2).
The nanosecond flash photolysis set-up to investigate the triplet
state behavior comprised of coupled laser systems, a Nd-YAG laser
have been assessed by using different techniques including tran-
sient absorption and Z-scan methods [29] and third harmonic gen-
eration (THG) [30]. The nonlinear behavior of the synthesized
complexes are measured using the Z-scan in this work.
(
(
already described above) pumping Lambda-Physik FL3002 dye
Pyridin 1 dye in methanol) laser. The analyzing beam source
2
. Experimental
.1. Materials
-Hexy-1-ol, propargyl alcohol, dichloromethane (DCM), 4-
was a Thermo Oriel 66902 xenon arc lamp. Details have been pro-
vided before [22].
2
2
2
.3. Synthesis
5
nitrophthalonitrile, 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), zinc
phthalocyanine (ZnPc), n-pentanol, diethylether, potassium car-
bonate and indium (III) chloride were obtained from Sigma Aldrich.
.3.1. 4-Hexy-5-ynyloxy-phthalonitrile (2a) (Scheme 1)
4
-Nitrophthalonitrile (1) (2 g, 11.5 mmol) was dissolved in dry
DMSO (20 mL) and 5-hexyn-1-ol (1.69 g, 11.5 mmol) added under
a nitrogen atmosphere. After stirring for 10 min, finely ground
6
Deuterated dimethyl sulfoxide (DMSO-d ), n-hexane, methanol,
dimethyl formamide (DMF) and dimethyl sulfoxide (DMSO) were
purchased from Merck. All solvents were of reagent grade and
were freshly distilled before use. The progress of the syntheses
anhydrous K
2
4
2
CO
h, with stirring. The reaction mixture was stirred at 40 °C for
8 h under nitrogen. Then the mixture was poured into 400 mL
3
(2.16 g, 15.6 mmol) was added in portions over
was monitored using SiO
.2. Equipment and methods
Infra-red spectra were collected on a Perkin-Elmer Universal
2
thin layer chromatography (TLC).
of cold water. The precipitate was filtered off and washed with
water, until the filtrate was neutral. Recrystalization from hexane
and methanol gives the desired compound as a yellow crystalline
2
ꢁ1
powder. Yield: 1.74 g (60%). IR (KBr)
2228 (CN); 2222 (C„C); 1557, 1488 (C@C phenyl); 1287 (Ar–O–
C); 1308 1251, 1003, 840, 660. Anal. Calc. for C14 O: C,
mmax/cm : 3282 („CAH);
1
ATR Sampling accessory spectrum 100 FT-IR spectrometer.
NMR spectra were obtained using a Bruker AVANCE 400 MHz
NMR spectrometer in DMSO-d . Elemental analyses were done
H
12 2
H N
1
7
4.98; H, 5.39; N, 12.49. Found: C, 74.56; H, 5.23; N, 12.36%. H
6
NMR (600 MHz, DMSO-d
6
): d, ppm: 8.02 (Ar-H, s, 1H), 7.75 (Ar-H,
–O–, t, 2H), 2.77 (C„CH, s,
, m, 2H), 1.58 (CH , m, 2H).
using a Vario-Elementar Microcube ELIII, while mass spectra data
were collected on a Bruker AutoFLEX III Smart-beam TOF/TOF mass
spectrometer using a-cyano-4-hydrocinnamic acid as the matrix in
d, 1H), 7.43 (Ar-H, d, 1H), 4.15 (CH
H), 2.22 (CH , t, 2H), 1.81 (CH
2
1
2
2
2
the positive ion mode.
Ground state electronic absorption spectra were recorded on a
Shimadzu UV-2550 spectrophotometer. Fluorescence emission
spectra were recorded on a Varian Eclipse spectrofluorimeter. Time
Correlated Single Photon Counting (TCSPC) setup (FluoTime 200,
Picoquant GmbH) was used for the fluorescence decay studies.
The excitation source was a diode laser (LDH-P-670 driven by
PDL 800-B, 670 nm, 20 MHz repetition rate, 44 ps pulse width,
Picoquant GmbH).
All Z-scan analyses were performed using a frequency-doubled
Nd:YAG laser (Quanta-Ray, 1.5 J/10 ns fwhm pulse duration) as the
excitation source. The laser was operated in a near Gaussian trans-
verse mode at 532 nm (second harmonic), with a pulse repetition
2.3.2. 4-Propy-2-ynyloxy-phthalonitrile (2b) (Scheme 1)
Compound 2b, Scheme 1, was synthesized following the
method described above for compound 2a, using propargyl alcohol
(b) (0.64 g, 11.5 mmol) in place of 5-hexyn-1-ol. Yield: 1.42 g
ꢁ
1
(75%). IR (KBr)
(C„C); 1557, 1488 (C@C phenyl); 1288 (Ar–O–C); 1306, 1256,
1011, 829, 670. Anal. Calc. for C11 O: C, 72.52; H, 3.32; N,
15.38. Found: C, 71.77; H, 3.14; N, 15.19%. H NMR (600 MHz,
DMSO-d ): d, ppm: 8.09 (Ar-H, d, 1H), 7.80 (Ar-H, d, 1H), 7.49
(Ar-H, dd, 1H), 5.00 (CH –O–, t, 2H), 3.70 (C„CH, s, 1H).
mmax/cm : 3286 („CAH); 2229 (CN); 2222
6 2
H N
1
6
2
2.3.3. 2,9(10),16(17),23(24)-Tetrakis(hex-5-
rate of 10 Hz and energy range of 0.1
l
J–0.1 mJ, as limited by the
ynoxy)phthalocyanatoindium(III) chloride (3a) (Scheme 1)
A mixture of 4-hexy-5-ynyloxy-phthalonitrile (2a) (0.25 g,
1.11 mmol), indium (III) chloride (0.25 g, 1.11 mmol) and few
drops of catalytic DBU in n-pentanol (1.5 mL) was stirred under
reflux at 140 °C for 20 h. The reaction was carried out under argon.
The crude dark blue product was cooled to room temperature and
washed with methanol, ethanol, 1 M HCl and acetone in succes-
energy detectors (Coherent J5-09). The low repetition rate of the
laser prevents cumulative thermal nonlinearities. The beam was
spatially filtered to remove the higher order modes and tightly
focused with a 15 cm focal length lens. The Z-scan system size
(
l ꢀ w ꢀ h) used was 600 mm ꢀ 300 mm ꢀ 350 mm (excluding
the computer, energy meter, translation stage driver and laser sys-
OR
Cl
N
N
N
N
N
CN
CN
CN
InCl , n-pentanol
3
OR
ROH, DMSO
K CO , RT
RO
CN DBU, 140 0C, 20 hrs
N
In
N
N
O
2
N
2
3
RO
3
2
1
OR
a
R =
b
Scheme 1. Synthetic pathway for complexes 3a and 3b.

O.M. Bankole et al. / Polyhedron 88 (2015) 73–80
75
sion. Further purification of the compound was achieved by Soxh-
let extraction using ethanol–water (1:1) for 24 h to remove the
traces of DBU from the compound. Column chromatography on sil-
ica gel as stationary phase and tetrahydrofuran (THF):methanol as
eluent was also performed. The blue-green indium phthalocyanine
obtained was thereafter concentrated and dried.
where I00 is the intensity of the light on focus, b is the two pho-
2
ton absorption (2PA) coefficient, Leff , z and z are the effective path
0
length in the sample of length L, translation distance of the sample
relative to the focus and Rayleigh length, respectively. Rayleigh
2
o
length is defined as
p
w =k, where k is the wavelength of the laser
and w
o
is the beam waist at the focus, (z ¼ 0). Leff is given by Eq. (3):
Yield: 64.6%, UV–Vis (DMSO): kmax/nm (log e): 696 (5.07), 629
ꢁaL
ꢁ
1
1 ꢁ e
(
2
1
H
4
1
4.40), 358 (4.83). FT-IR [KBr disc (
m
max/cm )]: 3285 („C-H);
934 (CH aliphatic); 3089 (Ar-H); 2222 (C„C); 1228, 1118, 1081,
045 (C–O–C), 966, 824, 741, 650 (Pc-Skeleton). Anal. Calc. for C56-
InCl.H O: C, 63.31; H, 4.70; N, 10.52. Found: C, 63.26; H,
.91; N, 10.68%. MS (MALDI-TOF) m/z: Calcd. 1046.36; Found:
L
eff
¼
ð3Þ
a
where
a is the linear absorption coefficient.
48
N
8
O
4
2
The molecular two-photon absorption (MTPA) cross-section
(cm s/photon) can be calculated from the known two-photon
r
2
4
+
1
049.22 [M+3H] . H NMR (600 MHz, DMSO-d
6
): d, ppm:8.53
absorption coefficient (b Þ using Eq. (4) [36]:
2
(
Ar-H, d, 4H), 7.62 (Ar-H, s, 4H), 7.18 (Ar-H, dd, 4H), 4.09 (CH
2
–
2
2
r2 ¼ 1
000h
N C
A
tb2
ð4Þ
O–, t, 8H), 2.11 (C„CH, s, 4H), 1.75 (CH , m, 8H), 1.48 (CH , m,
8
H), 1.09 (CH
2
, t, 8H).
where, h
t
is the incident photon measured in joules, N
A
is the
2
.3.4. 2,9(10),16(17),23(24)-Tetrakis(prop-2-
Avogadro constant, C is the concentration of the sample.
ð3Þ
ynoxy)phthalocyanatoindium(III) chloride (3b) (Scheme 1)
A mixture of 4-propy-2-ynyloxy-phthalonitrile (2b) (0.3 g,
.64 mmol), indium (III) chloride (0.36 g, 1.64 mmol) and 20 l of
The imaginary third-order susceptibility (I ½
v
ꢂ) is directly
m
proportional to the two-photon absorption coefficient, b as shown
2
1
l
in Eq. (5) [37]:
catalytic DBU in n-pentanol (1.7 mL) was heated to 140 °C under
nitrogen for 20 h. The crude dark product was cooled to room tem-
perature and washed with methanol, ethanol, 1 M HCl and acetone
in succession. Further purification of the compound was achieved
only by Soxhlet extraction using ethanol–water (1:1) for 24 h to
remove the traces of DBU from the compound. The blue-green
indium phthalocyanine obtained was thereafter concentrated and
dried.
I
m
½
vð3Þꢂ ¼ n2
e
0
ckb =2
p
ð5Þ
2
where, n is the linear refractive index, c is the speed of light
respectively, is the permittivity of free space and k is the wave-
length of the laser.
e
0
The relationship between second order hyperpolarizability (
c)
ð3Þ
and third-order susceptibility (I
Eq. (6) [12]:
m
½
v
ꢂ) of the material is given by
Complex 3b yield: (75%). UV–Vis (DMSO): kmax/nm (log
e
): 693
ꢁ1
I ½ ꢂ
vð3Þ
4
(
(
(
5.01), 625 (4.35), 360 (4.75). FT-IR [KBr disc (
m
max/cm )]: 3286
m
c
¼
ð6Þ
„CAH); 2929 (CH aliphatic); 3077 (Ar-H); 2161 (C„C); 1604
Ar C–C), 1280, 1215, 1084, 1011 (C–O–C), 939, 825, 742, (Pc-Skel-
f Cmol
N
A
where Cmol is the molar concentration of the active species in
eton). Anal. Calc. for C44
24 8 4
H N O InCl: C, 60.12; H, 2.75; N, 12.75.
the triplet state, f is the Lorentz local field enhancement factor,
defined as f = (n + 2)/3; n is the refractive index of the sample
and N
Found: C, 60.01; H, 2.74; N, 12.01%. MS (MALDI-TOF) m/z: Calcd.
2
+
1
8
78.86; Found: 879.36 [M+H] . H NMR (600 MHz, DMSO-d
6
): d,
A
as defined above.
ppm: 8.44 (Ar-H, d, 4H), 7.78 (Ar-H, s, 4H), 7.36 (Ar-H, dd, 4H),
4
.45 (CH
2
–O–, t, 8H), 2.70 (C„CH, s, 4H).
3
. Results and discussion
2.4. Fluorescence and triplet quantum yields
3.1. Synthesis and characterization
Fluorescence (
mined using established methods [31–33] using ZnPc as a standard
in DMSO ( = 0.2 [33] and = 0.65) [32].
Quantum yields of internal conversion (
F T
U ) and triplet (U ) quantum yields were deter-
The novel In-phthalocyanines (3a and 3b) containing terminal
alkyne at the peripheral positions Pcs are reported for the first time
in this work.
U
F
U
T
U
IC) were obtained
from Eq. (1), which assumes that fluorescence, intersystem cross-
ing and internal conversion are the only three processes which
are jointly responsible for the deactivation of the excited singlet
state of both indium (III) phthalocyanine molecules (3a and 3b):
Modification of phthalocyanine molecules to further improve
their architectural flexibility with terminal alkynes is now becom-
ing an active area of research [38–41]. The compounds are readily
soluble in organic solvents such as DMF, DCM, THF, chloroform and
DMSO. The alkyne chain-length of compounds 3a and 3b were
carefully chosen to determine the effect of the chain length on both
the photophysical and non-linear optical properties of the new
phthalocyanines.
U
IC ¼ 1 ꢁ ð
The triplet lifetimes were determined by fitting the data
obtained to single exponential using OriginPro 8 program.
U
F
þ
U
T
Þ
ð1Þ
Targeted alkynyl-phthalonitriles (2a and 2b) were synthesized
via a based-catalyzed nucleophilic aromatic displacement between
2.5. Nonlinear optical parameters
4
-nitrophthalonitrile (1) and propargyl alcohol and 5-hexy-1ol,
The nonlinear optical properties of phthalocyanines are evalu-
Scheme 1. Cyclotetramerization of 2a and 2b (to form 3a and 3b)
ated using the Z-scan technique that has been previously described
34,35]. The technique relies on the total transmittance passing
occurred in the presence of the desired metal salt (InCl ) and a cat-
alytic amount of DBU. Since a large atom like indium requires high
3
[
through the sample as a result of the incident laser pulses. The gen-
eral equation for normalized transmittance (Tnorm(Z)) can be written
as Eq. (2).
energy to insert the metal ion into the phthalocyanine cavity, high-
boiling solvent, n-pentanol, was employed in order to obtain the
desired complexes, 3a and 3b [42]. The compounds were obtained
in satisfactorily high yields and were characterized by elemental
analysis, UV–Vis, FT-IR, H NMR spectroscopies and MALDI-TOF.
The characterization results are consistent with the predicted struc-
tures as shown in the experimental section. The elemental analyses
1
1
T
normðZÞ
¼
ð2Þ
2
1
þ b2Leff ðI00=ð1 þ ðz=z
0
Þ ÞÞ

7
6
O.M. Bankole et al. / Polyhedron 88 (2015) 73–80
gave percentage carbon values that were within 1% in all cases,
which are within acceptable range for phthalocyanine complexes.
In both cases a mixture of four possible structural isomers are
obtained for tetra-substituted phthalocyanines. The four probable
1.2
1
3
a
0.8
3b
isomers can be designed by their molecular symmetry as C4h, C2v
,
0
0
.6
.4
C
s
and D2h [43]. In this study, synthesized tetra-substituted phtha-
locyanine compounds are obtained as isomer mixtures as
expected. No attempt was made to separate the isomers for 3a or
0.2
0
3
b.
After conversion of the substituted phthalonitriles to indium
phthalocyanines (3a and 3b), the characteristics intense IR peak
300 350 400 450 500 550 600 650 700 750 800
ꢁ1
Wavelength (nm)
(Fig. 1) around 2230 cm , which is associated with C„N stretch
in phthalonitriles, disappeared to confirm the formation of metal-
lo-phthalocyanine complexes. Other notable signals at 3285 and
Fig. 2. Absorption spectra of 3a (solid line) and 3b (dotted line) in DMSO.
ꢁ1
3
286 cm are due to C„CAH, for 3a and 3b, respectively. The
complexes showed characteristic vibrations due to ether groups
could be due to the electron donating abilities of alkynyl groups.
Electron donating groups are known to result in the red shifting
of the Q band [48].
ꢁ1
(
3
C–O–C) 1011–1279 cm
076 cm and aliphatic CH stretching at ca. 2928–2934 cm
,
aromatic CH stretching at ca.
ꢁ1
ꢁ1
.
The mass spectral studies on the newly synthesized phthalocy-
anines was definitive and gave expected masses of the complexes
a and 3b. The observed molecular ion peaks for complex 3a was
Fluorescence behavior of the synthesized ClInPc derivatives (3a
and 3b) in DMSO were similar to each other. Fluorescence is weak
for both complexes due to the heavy atom effect of In, Fig. 3. The
excitation spectra was slightly red-shifted compared to the absorp-
tion spectra for both complexes. Also the excitation spectrum was
broad compared to absorption spectra. This indicates change in
nuclear configuration following excitation. The loss of symmetry
is possible due to the large size of indium metal which is more dis-
placed from the cavity of the phthalocyanines ring and become
more pronounced upon excitation [49]. The Stokes shifts are small
as typical of phthalocyanines [47].
3
+
found at m/z = 1049.22 which is attributed to [M+3H] and m/
z = 879.36 [M+H] for 3b. Fragmentation of molecular ion peaks
+
+
+
ꢁ
as [M] , [M+nH] , or [MꢁnH] have been reported [44], hence the
1
observed mass spectral pattern for 3a. The mass and H NMR spec-
tra and the elemental analysis confirm the purity of the synthe-
sized complexes.
3.2. Ground state electronic absorption and fluorescence spectra
The aggregation behavior of the synthesized phthalocyanines
was investigated using absorption studies in DMSO at different
concentrations. As the concentration was increased, the intensity
of the absorption of the Q-band also increased without formation
of new bands due to the aggregated species for both complexes
The Q-bands for 3a and 3b (Fig. 2) in DMSO were observed at
96 nm (3a) and 693 (3b) (Table 1). Alkyne substituted phthalocy-
anines are known for metal free or metals such as Zn and Co [45],
with not much change in the Q band with change in central metal.
The Q-band of complex 3a was red-shifted by 3 nm, compared to
6
(
3a and 3b) [50]. The Beer–Lambert law was obeyed for both com-
ꢁ6
plexes in DMSO in the concentrations ranging from 1.8 ꢀ 10 to
3
b. This red shift is due to increase in carbon chain-length for 5-
hexy-1-ol (3a) which shifts the peak to a slightly longer wave-
length due to the interaction between bonded electrons of the
alkynyl group and the bond electrons in the triple bonds [46].
ꢁ6
3
.0 ꢀ 10 M.
r
p
3
3
.3. Photophysical studies
This leads to slight destabilization of highest occupied molecular
orbital (HOMO) level in 3a and result in the shift to the longer
wavelength [46]. Both complexes 3a and 3b show red-shifted Q
band when compared to unsubstituted InPc [47], Table 1. This
.3.1. Fluorescence lifetimes and quantum yields
The fluorescence quantum yields (
tent with MPc complexes containing heavy metals. The values
obtained are lower than that of unsubstituted ClInPc ( = 0.018
47], Table 1, suggesting quenching of fluorescence by alkynyl
groups. Compound 3a has a larger value, this suggest less fluores-
cence quenching of the excited singlet state in 3a compared to 3b.
The fluorescence lifetimes for the compounds were
F
U ) for 3a and 3b were consis-
U
F
[
4
000 3500 3000 2500 2000 1500 1000
1
00
U
F
80
F
(s )
(2a)
obtained from TCSPC technique by fitting the fluorescence decay
data to a bi-exponential function (Fig. 4). The presence of two life-
times for phthalocyanines may be attributed to the presence of
aggregates which quench fluorescence to give quenched (shorter)
and unquenched (longer) lifetimes [51]. The average lifetimes
3
282
60
2228
4
8
0
0
(savF) reported for the compounds are within the range of reported
values of phthalocyanines [52]. avF values for both compounds 3a
s
6
4
2
0
0
0
and 3b are low compared to the unsubstituted indium phthalocy-
anines [47]. Again, this suggests quenching of fluorescence by the
alkynyl substituents.
(
3a)
3
285 Aromatic CHAliphatic CH
The fluorescence rate constant (k
the fluorescence quantum yield ( ) of the Pcs. Complex 3b has
relatively lower k than 3a, this suggests more quenching in 3b
than 3a as observed for values. The larger rate constant for
F
) is directly proportional to
U
F
F
0
U
F
4
000 3500 3000 2500 2000 1500 1000
-1
intersystem crossing (kISC) for complexes 3a and 3b than unsubsti-
tuted ClInPc, Table 1, suggest that the presence of alkynyl moieties
on the peripheral positions encourages rate of intersystem crossing
Wavenumber (cm )
Fig. 1. FT-IR spectra of 2a and 3a.

O.M. Bankole et al. / Polyhedron 88 (2015) 73–80
77
Table 1
Spectral and photophysical data for complexes 3a and 3b in DMSO.
Complex akabs nm,
log
akem
akexc
U
b
s
(ns)
savF (ns)
s
T
(l
s)
U
sISC(av) (ns)
k
c
(s
ꢁ1
)
k
ISC (s
d
ꢁ1
)
F
F
T
F
7
9
(
e)
nm
nm
(10 )
(10 )
ꢁ
3
3
3
a
696 (5.01)
693 (5.07)
686 (4.46)
710
710
700
702
706
689
0.012
0.009
0.018e
0.31 ± 4.0 ꢀ 10 (97.57%)
0.37 ± 0.04 75 ± 0.9 0.82
0.36 ± 0.06 66 ± 0.8 0.84
0.45 ± 0.11 3.2 ± 0.1
0.43 ± 0.17 2.5 ± 0.2
1.99e
2.22 ± 0.11
2.33 ± 0.17
1.01e
ꢁ
2
2
.85 ± 8.0 ꢀ 10 (2.43%)
ꢁ3
b
0.32 ± 4.0 ꢀ 10 (98.39%)
2
2
–
.72 ± 13.0 ꢀ 10ꢁ (1.61%)
ClInPc
0.90e
50c
0.91e
a
b
c
k
abs, kem and kexc represent absorbance, emission and excitation wavelengths, respectively.
Abundances in brackets.
k
k
F
=
U
F
/s
F
avF, k is the fluorescence rate constant.
d
e
ISC
= U /savF, kISC is the rate constant for intersystem crossing.
T
Values from Ref. [47].
(
A) 1
.01
1
.99
.98
.97
.96
.95
.94
1.2
0
0
0
0
0
0
1
(A)
0
0
0
0
.8
.6
.4
.2
0
emission
Experimental Z-scan
data
excitation
absorption
Theoreꢀcal fit
-4
-3
-2
-1
0
1
2
3
4
500
550
600
650
700
750
800
Z (cm)
Wavelength (nm)
1
.9
.8
.7
.6
(
B)0
1
.2
0
0
0
(
B)
1
0.8
0.6
0.4
0.2
0
0.5
Absorpꢀon
3
b in DMSO
0
0
0
0
.4
.3
.2
.1
0
excitaꢀon
emission
3a in DMSO
-4
-2
0
2
4
Z (cm)
500
550
600
650
700
750
800
Fig. 5. (A) Open aperture Z-scan for 3b in DMSO showing the fitting. The black solid
Wavelength (nm)
ꢁ
2
curve is the theoretical fit, I00 ꢃ 60.0 MW cm . (B) Representative open-aperture Z-
ꢁ
2
scans of the compounds 3a and 3b. Peak input intensity (I00) ꢃ 130 MW cm
.
Fig. 3. Absorption, fluorescence emission and excitation spectra of (A) 3a and (B) 3b
in DMSO. Excitation wavelength = 610 nm.
from excited singlet to triplet state. The rate of intersystem cross-
ing in complex 3b is more pronounced than that of 3a, this can also
be attributed to the lower intersystem crossing lifetime observed
in compound 3b compared to 3a.
10000
8
000
000
000
000
0
6
4
2
3.3.2. Triplet quantum yields and lifetimes
T
In Table 1, large triplet quantum yields (U ) values are observed
for 3a and 3b in DMSO due to the presence of heavy atom effect of
indium. The triplet lifetimes ( ) for the two complexes ranged are
low at 66 s (3b) and 75 s (3a), corresponding to the large triplet
quantum yields. The values were slightly longer than for unsubsti-
tuted InPc, Table 1.
s
T
0
0
2
2
4
4
6
6
8
8
10
10
8
6
4
2
0
l
l
-
-
-
-
2
4
6
8
3.4. Nonlinear optical (NLO) studies
3.4.1. Z-scan measurements
Fig. 5(A) shows a representative open aperture Z-scan for com-
Time/ns
plex 3b recorded at 532 nm and 10 ns pulses using peak input
ꢁ2
Fig. 4. Fluorescence decay profile of 3a in DMSO.
intensity of ca. 60.0 MW cm . For irradiance intensities at ca.

7
8
O.M. Bankole et al. / Polyhedron 88 (2015) 73–80
Table 2
Nonlinear optical properties of samples 3a and 3b in DMSO at 532 nm wavelength and 10 ns pulses. The peak intensity for each measurement was ꢃ130 MW cmꢁ2.
(cm GWꢁ1)
ð3Þ
c
ðesuÞ
r
(cm4 s/photon)
k (
r
/r
I
lim (J cm
ꢁ2
)
Complex
b
2
Im
½
v
ꢂ /esu
2
ex
0
)
ꢁ
14
ꢁ32
ꢁ32
ꢁ42
3
3
a
b
427
438
2.10 ꢀ 10
2.15 ꢀ 10
2.70 ꢀ 10
3.19 ꢀ 10
1.29 ꢀ 10
1.15 ꢀ 10
5.17
3.46
0.55
0.48
ꢁ
14
ꢁ42
ꢁ
2
1
30.0 MW cm (Fig. 5B) the normalized transmittance of the two
3.4.2. Two photon absorption (2PA) and sequential photon (RSA)
mechanisms
The Jablonski diagram in Fig. 6, shows the contribution of the
excited states of the Pcs to the multi-photon absorption processes
observed in the Z-scans [26,56]. 2PA at 532 nm and ns pulsed laser
involves simultaneous absorption of two photons by the Pcs
through [S ? S ] transition, followed by the Pc relaxing to the S
0 n 1
state, undergoing intersystem crossing to the triplet state and
absorbing further singular photons whilst in this state. The RSA
complexes (3a and 3b) dropped below 0.5 making them good opti-
cal limiting materials [15,17,53]. The reduction in the linear trans-
mission about the focus of the lens at 532 nm exhibited by each
molecule is an indication of positive nonlinear absorption. Such
molecules are referred to as exhibiting reverse saturable absor-
bance (RSA) [15,34,35,53–55] due to the optical limiting possibly
being caused by sequential photon absorption. The other mecha-
nism which could be the cause of the optical limiting is two photon
absorption (2PA).
mechanism is when the transition to the S
sequential one photon absorption via a virtual level. Once in the
state the processes proceed like in the 2PA mechanism. There
is also a possibility that there may be relaxation from the virtual
level to the S state. The excited state of absorption cross-section
) being larger than the ground state absorption cross-section
) is a condition which is primarily responsible for RSA occurring
n
state occurs with
The Z-scan data obtained for complexes 3a and 3b were found to
perfectly fit the transmission equation (Eq. (2)) for two-photon
absorption (2PA) function. The estimated nonlinear absorption coef-
S
n
ꢁ1
ficients (b
4
2
) for 3a and 3b are respectively ꢄ 427 cm GW and
1
ꢁ1
38 cm GW , Table 2. It is believed that contributions from higher
(
(
r
r
T
mechanisms such as three-photon absorption (3PA) are also
expected for the nonlinearity of the complexes due to the ns pulses
and the high peak intensities used [10,15,22,37]. Further analysis to
show whether the dominant mechanism for the observed z-scan
spectra is from sequential photon reverse saturable absorption or
0
in the Pcs [57–59]. The incident pulses have duration of 10 ns com-
pared to the shorter lifetimes (<1 ns, Table 1, ISC) of the Pcs to pop-
ulate the triplet states T via intersystem system crossing. Also, the
high triplet quantum yields of the two compounds (ꢄ1) ensure a
large ISC to the triplet states as well as [T ? T ] transitions.
s
1
2
PA follows.
1
2
S
n
T
n
Internal
Conversion
Triplet absorpꢀon
Virtual
State
ISC
S
1
T
1
Phosphorescence
Fluorescence
S
0
Fig. 6. Jablonski energy level diagram for theorized RSA and/or simultaneous two-photon absorption (2PA) of 3a and 3b.

O.M. Bankole et al. / Polyhedron 88 (2015) 73–80
79
1
.9
.8
.7
.6
.5
.4
.3
.2
.1
0
1.2
1
0
0
0
0
0
0
0
0
0
3
3
b
a
0
.8
.6
0
Theoreꢀcal data fit
Experimental data fit
0.4
0.2
-4
-2
0
2
4
0
Z (cm)
0
0.5
1
1.5
2
I (J/cm )
Fig. 7. Representative open aperture (OA) Z-scan fits between experimental data
and theoretical analytical of absorption model for complex 3b in DMSO with
o
0
Fig. 8. Transmittance vs input fluence (I ) curves for 3a (red ) and 3b (green ) in
varying
T
r parameters.
DMSO. Peak input intensity (I00) ꢃ 130 MW cmꢁ for each sample. (Color online.)
2
Analytical absorption model Eq. (7) [60] was used to further
verify the existence of 2PA mechanism as opposed to the RSA
mechanism for the two complexes.
(
k) which is defined as the ratio of
that such that is greater than unity. The k values of
.17 and 3.46 (Table 2) for complexes 3a and 3b, respectively were
T 0 .
r to r [63] For RSA, it implies
r
T
>
r
0
T 0
r /r
5
T
In½1 þ qꢂ
obtained. The k value observed for the complexes showed that the
absorbance around the 532 nm region and 10 ns pulse radiation
will predominantly be due to triplet–triplet absorption
¼
ð7Þ
T
linear
q
a
h
r
v
T
[
22,37,63,64].
where q ¼ 2
I
0
L
eff
ð8Þ
where
as defined above.
Theoretical (Fig. 7) OA Z-scan datasets were fitted to the exper-
imental transmittance datasets using least squares regression
r
T
is the excited state of absorption cross-section, I
0
; Leff and
a
3.4.4. Third-order nonlinear susceptibility (I
hyperpolarizability (
m
½
vð3Þꢂ) and second-order
c)
c
measures the interaction of incident photon with the perma-
method while treating
r
T
as a free parameter to obtain value of
nent dipole moments of the alkynyl Pcs. Good optical limiting
materials must have high absorbency as the intensity of the inci-
dent light is increased. Hence, the higher the
2
R ꢄ 0.9. The values of
r
T
correspond to T–T transitions obtained
ꢁ17
2
for 3a and 3b with the theoretical fits of 8.8 ꢀ 10
cm and
c values, the better
ꢁ
17
2
6
.8 ꢀ 10
cm , respectively. Each of these values exceed ground
the compounds are as optical limiters. Complex 3b gave better val-
ꢁ
17
ꢁ32
ꢁ32
state of absorption cross-section ð
r
0
¼ 1:7 ꢀ 10
for 3a and
ues than 3a at 3.19 ꢀ 10
and 2.7 ꢀ 10
esu, respectively,
ꢁ17
ð3Þ
1
.97 ꢀ 10
for 3b) by factors of 5 and 3. These values are a very
Table 2. I
m
½
v
ꢂ measures the fast response of a nonlinear optical
good indicator that RSA could be occurring for the Pcs.
material to the perturbation initiated by intense laser pulses. Esti-
ð3Þ
Saturation fluence (Fsat) curves of 3a and 3b are compared by
mated values for I
tions (
m
½
v
ꢂ and the two-photon absorption cross-sec-
plotting transmittance (T) against the input fluence (I
0
) (Fig. 8).
r
2
) for the two Pcs are high and consistent for organic
The plots showed rapid decrease in transmittance as the input flu-
ence increases. Similar observations ascribed to 2PA for organic
compounds have been previously reported [60,61].
molecules as nonlinear optical materials [15,22,62–64].
To show the merits and potentials of our molecules as nonlinear
ð3Þ
optical materials, the obtained values for I
m
½
v
ꢂ,
c, k and b in this
2
At I
0
values higher than 0.18 J cmꢁ2, more rapid decrease of
work were compared with different phthalocyanines (In-phthalo-
transmittance for 3b with respect to 3a was observed which can
be attributed to the difference in carbon chain-lengths of the alky-
nyl substituents on the peripheral positions of the Pcs. Thus, com-
plex 3b showed better NLO property than 3a and this further
corroborate our earlier result on its low fluorescence quantum
yields and lifetimes in (Table 1) due to self-quenching by alkynyl
moieties in 3a than propagyl chain length of 3b.
cyanines inclusive) that have been previously reported [65]. The
ð3Þ
values for I
for optical materials, while k and b
m
½
v
ꢂ and
c
reported in this work are on the high side
are within the range of values
2
reported for the studied phthalocyanines. This suggests that our
synthesized molecules have good prospects as optical limiters
against laser radiation.
1
.4
3
.4.3. Limiting intensity (Ilim
Limiting intensity curves of the two alkynyl phthalocyanines
3a and 3b) are presented in Fig. 9. An important parameter that
has been reported in the literature to measure the optical limiting
performance of materials is incident intensity threshold, (Ilim
which is defined as the input intensity at 50% linear transmittance
16]. Low value of Ilim is an indication of a good NLO response to
)
1.2
1
(
0
.8
)
0.6
[
0
0
.4
.2
0
the input intensity. From Table 2, complex 3b has lower Ilim value
hence exhibits larger optical limiting response than that of com-
plex 3a. In addition, values of Ilim obtained for both complexes
were found to be among the lowest reported for phthalocyanines
derivatives in recent literatures which ranged from 0.2 to
0
0.5
1
1.5
Io (J/cm2)
ꢁ
2
0
.8 J cm [13,15,22,62].
Fig. 9. Output fluence (Iout) vs input fluence (I
The black solid line represents
00) ꢃ 130 MW cm for each sample.
0
) for complexes 3a ( ) and 3b ( ).
Another measure of effective nonlinear response of optical
a case of linear transmission. Peak intensity
materials is excited to ground state absorption cross-sections ratio
ꢁ2
(I

8
0
O.M. Bankole et al. / Polyhedron 88 (2015) 73–80
[
[
17] P.K. Hegde, A.V. Adhikari, M.G. Manjunatha, P. Poornesh, G. Umesh, Opt. Mater.
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1180.
4
. Conclusions
3
In summary, two novel tetra-alkynyl indium phthalocyanines
have been synthesized and characterized. The two complexes
showed high triplet quantum yields (close to unity). This further
justified the high rate of intersystem crossing from excited singlet
state to first excited triplet state. We also present our results on
the nonlinear optical properties of the novel In-phthalocyanines
with open aperture Z-scan studies at 532 nm, 10 ns pulses in
dimethylsulfoxide. The results suggest that the nonlinear optical
processes could be occurring via either sequential photon absorp-
tion (RSA) or a two-photon absorption mechanism for the studied
In-phthalocyanines. Due to the fact that there was some uncer-
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