The impact of the extended π-conjugation in photophysical, photochemical and aggregation behavior of new phthalocyanine–naphthalocyanine hybrids

Article abstract of DOI:10.1016/j.jphotochem.2017.02.025

We report the synthesis and characterization of phthalocyanine–naphthalocyanine hybrids and the impact of the extended π-conjugation on the macrocycles aggregation, singlet oxygen production, fluorescence lifetime and quantum yields in solution. We observed that the addition of naphthalol units in the structure causes a bathochromic shift in UV-vis, emission and excitation bands and decreases the quantum yield of singlet oxygen in solution.

Full text of DOI:10.1016/j.jphotochem.2017.02.025

Accepted Manuscript
Title: The impact of the extended ␲-conjugation in
photophysical, photochemical and aggregation behavior of
new phthalocyanine–naphthalocyanine hybrids
Authors: Thalita F.M.de Souza, Anderson O. Ribeiro
PII:
S1010-6030(16)30801-2
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.jphotochem.2017.02.025
JPC 10551
To appear in:
Journal of Photochemistry and Photobiology A: Chemistry
Received date:
Revised date:
Accepted date:
26-9-2016
22-2-2017
26-2-2017
Please cite this article as: Thalita F.M.de Souza, Anderson O.Ribeiro, The impact of
the extended ␲-conjugation in photophysical, photochemical and aggregation behavior
of new phthalocyanine–naphthalocyanine hybrids, Journal of Photochemistry and
Photobiology A: Chemistry http://dx.doi.org/10.1016/j.jphotochem.2017.02.025
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The impact of the extended π-conjugation in photophysical,
photochemical and aggregation behavior of new phthalocyanine–
naphthalocyanine hybrids
Thalita F. M. de Souza, Anderson O. Ribeiro*
Center for Natural Sciences and Humanities, Federal University of ABC (UFABC), Avenida dos
Estados, 5001, Bloco B, Santo André, SP, Brazil
* Corresponding author. anderson.ribeiro@ufabc.edu.br
1

Graphical Abstract
400
500
600
700
800
Wavelength (nm)
2

Research Highlights
1. In phthalocyanine-naphthalocyanine hybrids, the Q-band is red-shifted as much as new aromatic π-
system is added to the macrocyclic structure.
2. The addition of extra π aromatic system to the periphery of the phthalocyanine macrocycle reduces
the quantum yield of fluorescence in solution.
3. Higher extended π-conjugation in phthalocyanine–naphthalocyanine hybrids results in lower values
for singlet oxygen generation in solution.
3

Abstract
We report the synthesis and characterization of phthalocyanine–
naphthalocyanine hybrids and the impact of the extended π-conjugation on the
macrocycles aggregation, singlet oxygen production, fluorescence lifetime and
quantum yields in solution. We observed that the addition of naphthalol units in
the structure causes a bathochromic shift in UV-vis, emission and excitation
bands and decreases the quantum yield of singlet oxygen in solution.
Keywords: phthalocyanine, naphthalocyanine, hybrids, synthesis, singlet
oxygen, photosensitizer
1. Introduction
Phthalocyanines (Pc) are highly stable synthetic macrocyclic compounds
with intense absorption bands in the visible region (600–750 nm)[1,2,3]
.
Naphthalocyanines (Nc) are homologues of these compounds with extended π-
conjugated system, which causes a bathochromic shift to longer absorption
wavelengths (750–900 nm)[4,5,6] compared to the phthalocyanines. Both
macrocycles are widely applied as industrial dyes, in solar cells[7,8], nonlinear
optics[9], electrochromic devices[10], photodynamic therapy (PDT)[4] and
chemical sensors[6].
Several works demonstrated that the photophysical and photochemical
properties of these macrocycles can be changed by the introduction of different
substituents at the peripheral position[11], varying the central metal[12] and/or
alternating the metal axial ligand[13].
4

Also, structure–activity relationships of phthalocyanines and
naphthalocyanines in biological systems, considering the influence of peripheral
substituents or the central metal ion have been reported[4,14,15]. However,
detailed studies on the photochemical and photophysical studies of
phthalocyanine–naphthalocyanine hybrids are lacking[1617]
.
In the current study we investigate the impact of the extended π-
conjugation in new phthalocyanine–naphthalocyanine hybrids. We report the
synthesis and the photophysical and photochemical properties of theses
analogues related to their solubility and aggregation in solution.
2. Experimental
2.1. Materials
All reagents and solvents were used as received. Zinc-phthalocyanine
(ZnPc) was purchased from Sigma-Aldrich. Compounds (2) and (3) were
synthesized like described on our published papers[3]. The Zinc-
naphthalocyanine (ZnNc) was synthesized following published methods[18,19,20]
.
2.2. Equipment
FT-IR spectra were obtained on a Varian FTIR-660 spectrophotometer
1
with the sample prepared as KBr pellet. H NMR spectra were performed with
CDCl3 or DMSO-d6 as solvent and tetramethylsilane (TMS) as internal standard.
The mass spectra were obtained on a Bruker UltrafleXtreme by MALDI (Matrix
Assisted Laser Desorption Ionization)-TOF technique using α-cyano-4-
hydroxycinnamic acid (HCCA) as matrix.
5

All absorption spectra were taken on a Varian Cary 50 UV-vis
spectrometer and fluorescence emission and excitation spectra using a Varian
Eclipse fluorescence spectrophotometer.
Fluorescence decay profiles were measured using a time-correlated
single photon count (TCSPC) spectrometer (Fluotime 300, PicoQuant) with a
laser diode emitting 634 nm (LDH-D-C-634, PicoQuant). The data was analyzed
with the program Fluofit, PicoQuant. All measurements were performed at room
temperature.
2.3. Synthesis
2.3.1. 2,3-Dicyano-naphthalene (1)
To a solution of 400μL (4 mmol) of o-xylene in 12.0 ml of dry CH2Cl2,
were added 2.5 g (14 mmol) of N-bromosuccinimide. The reaction mixture was
refluxed and irradiated with a 125-W mercury vapor lamp for 6 h. The mixture
was filtered while hot and the filtrate was evaporated to obtain a yellowish solid.
The crude product was dissolved in 12.0 ml of dry DMF, and 0.31 g (4 mmol) of
fumaronitrile and 3.0 g (20 mmol) of dry NaI were added to the solution. The
stirred solution was kept for 12h at 70-80°C. The dark reaction mixture was
added dropwise to a stirred solution of 3.0 g (20 mmol) of sodium thiosulfate in
300 ml of water. The yellow precipitate was filtered off, washed with water and
acetone, and then purified by a silica-gel column with hexane:ethyl acetate (2:1)
as eluent. (yield, 57%). IR (KBr pellet) ν (cm-1): 3097-3031 (Ar-H), 2971-2867
1
(Aliph. C-H), 2227 (C≡N), 1716, 1618, 1361, 1274, 1082 cm-1. H NMR (400
MHz, CDCl3), (δ): 8.37 (2H, s), 8.00 (2H, s), 7.81 (2H, s) ppm.
6

2.3.2. General Procedure for the Synthesis of Compounds (4) and (5)
0.22 g (1.2 mmol) of (1), 0.1 g (0.41 mmol) of (2) and 0.18 g (0.82 mmol)
of zinc(II) acetate were suspended in 3 ml dry pentanol at 150°C, under inert
gas. Then, 0.25 ml (1.6 mmol) of DBU was added and the solution was stirred
for 12h at 150ºC. After cooling, methanol was added and the resulted
precipitate was centrifuged. The crude product was purified by a silica-gel
column with toluene/THF (10-50%) as eluent.
Compound (4): (yield, 11mg). IR (KBr pellet) ν (cm-1): 3093-3014 (Ar-H),
2971-2813 (Aliph. C-H), 1718, 1606, 1484, 1371, 1338, 1230, 1088 cm-1.
1
MALDI-TOF-MS: calc.: 1016.28, found: m/z 1016.29 [M+]. H NMR (400 MHz,
DMSO-d6), (δ): 9.38 (5H, m), 8.59 (5H, m), 7.91 (5H, m), 4.75 (3H, m), 4.49 (9H,
m), 4.13 (3H, m), 1.62 (9H, m), 1.52 (9H, m) ppm.
Compound (5): (yield, 14mg). IR (KBr pellet) ν (cm-1): 3090-3016 (Ar-H),
2981-2800 (Aliph. C-H), 1708, 1606, 1488, 1361, 1263, 1089 cm-1. MALDI-
TOF-MS: calc.: 856.19, found: m/z 856.16 [M+]. 1H NMR (400 MHz, DMSO-d6),
(δ): 9.35 (4H, m), 8.54 (4H, m) 8.26 (4H, m), 7.84 – 7.66 (9H, m), 4.75 (1H, m),
4.44 (3H, m), 4.14 (1H, m), 1.66 (3H, m), 1.55 (3H, m) ppm.
2.4. Fluorescence quantum yields and lifetimes
Fluorescence quantum yields ( ) were determined in DMSO by the
comparative method using equation (1) [21,22]
.
퐹. 퐴푠푡푑 . 푛2
휙퐹 = 휙퐹ꢀ푡푑
퐹
ꢀ푡푑
. 퐴 . 푛ꢀ2푡푑
(1)
where F and FStd are the areas under the fluorescence emission curves of the
samples (3-6) and the standard, respectively. A and AStd are the relative
7

absorbance of the samples (3-6) and standard at the excitation wavelength,
n2
nS2td
are the refractive indices of solvents for the sample
respectively,
and
and standard, respectively. Unsubstituted ZnPc (in DMSO) (ΦF = 0.20)[21,22] was
employed as the standard. Both the sample and standard were excited at the
same wavelength. The absorbance of the solutions was ranged between 0.06
and 0.09 at the excitation wavelength.
The fluorescence lifetime ( ) values were directly determined in DMSO
by time correlated single photon counting (TCSPC) method.
2.5. Singlet oxygen quantum yields
Singlet oxygen quantum yield (휙 ) were carried out using the
experimental set-up described in literature[21,22], in DMSO, using ZnPc as
reference. DPBF (1,3-diphenylisobenzofuran) was used as chemical quenchers
for singlet oxygen. Briefly, a 3.0 mL portion of unsubstituted zinc phthalocyanine
(ZnPc) and samples (3-6) solutions (all solutions with same absorption at 630
nm) containing DPBF (absorption ~0.8 at 415 nm) was irradiated in the Q band
region with a red 630 nm LED lamp (23 mW) in cycles of 6 s each one. The 휙
values were calculated using equation (2):
ꢀ푡푑
.
푎푏푠
휙훥 = 휙훥ꢀ푡푑
ꢀ푡푑
.
푎푏푠
(2)
where 휙
is the singlet oxygen quantum yields for the standard unsubstituted
[22]
ZnPc (휙 = 0.67 in DMSO) . and
are the DPBF photo-bleaching rates
in the presence of the respective samples (3-6) and standard, respectively.
and
are the rates of light absorption by the samples and standards,
8

respectively. Solutions of sensitizer containing DPBF were prepared in the dark.
DPBF degradation at 415 nm was monitored.
3. Results and Discussion
3.1. Synthesis and characterization
New phthalocyanine–naphthalocyanine hybrids were synthesized (shown
in Scheme 1). The cyclisation of (1) and (2), in 1-pentanol, at 150ºC, using DBU
as catalyst, under N2 atmosphere, resulted in the phthalocyanine–
naphthalocyanine hybrids compounds (4) and (5).
Scheme 1. Synthetic route of the compounds.
The phthalocyanine–naphthalocyanine hybrids were isolated by careful
column chromatography using toluene and THF. The co-eluent THF breaks up
9

the molecular aggregation and increases the mobility of these compounds on
silica gel columns[14]. In these conditions, the cis and trans hybrids could not be
isolated each other, and their properties are not presented here. All compounds
1
were characterized by spectroscopic methods including UV-vis and H NMR.
The results are consistent with the proposed structures.
In the IR absorption spectra, the formation of phthalocyanine was
confirmed by disappearance of phthalonitrile C≡N band at 2247cm-1. The
MALDI-TOF mass spectra of compounds (4) and (5) displayed the molecular
ion peaks at m/z = 1016.29 [M+] and 856.16 [M+] respectively, confirming the
1
proposed structures (Fig 1). The H NMR spectra of (4) and (5) exhibited
aromatic ring protons with δ ranging from 7.66 to 9.38 ppm and aliphatic protons
with δ ranging from 1.52 to 4.75 ppm (Fig 2).
10

4
x10
856.163
6
858.179
857.174
860.190
4
2
0
859.188
861.200
862.208
863.217
864.261
864
865.231
854
856
858
860
862
866
m/z
Fig. 1. MALDI-TOF mass spectra of compound (5).
9.5
9.0
8.5
8.0
7.5
7.0
6.5
6.0
5.5
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
Fig. 2. 1H NMR spectrum of compound (5) in DMSO-d6.
11

Phthalocyanines exhibit two strong absorption bands in the UV-vis
region. The Soret band (UV), due to electronic transitions from deeper π-HOMO
to n*-LUMO energy levels, and Q-band (visible region), because of electronic
transitions from π-HOMO to π*-LUMO energy levels[23].
The UV-vis spectra of macrocyclic compounds were recorded in DMSO
(Fig 3). The complexes exhibited a very intense Q(0, 0) band in the 600–800
nm region and a Soret band between 300 – 400 nm. Table 1 summarizes the
wavelengths and associated extinction coefficients in Q-band and Soret band.
Due to the extend conjugation, the Q-band was red-shifted in the order (3) < (4)
< (5) < ZnNc as much as new aromatic π-system is added to the macrocyclic
structure.
(3) (4) (5) ZnNc
300
400
500
600
700
800
Wavelength (nm)
Fig. 3. Normalized absorption spectra of (3), (4), (5) and ZnNc in DMSO.
12

The compounds (4) and (5) exhibited a Q-band split into two dominant Q-
band components. This is attributed to the lower complex symmetry arising from
the introduction of different substituents. The broadening in spectra of hybrid
compounds may suggest that the steric effects of the substituents could also
decrease the symmetry. The twisting of this moiety around the macrocycle bond
can distorts the molecule and promote a slight loss of symmetry[24,25]
.
Phthalocyanines and naphthalocyanines derivatives have high tendency
to aggregate in solution due to the π-π interaction. It affects their solubility,
photophysical, photochemical and electrochemical properties. The hybrids
synthesized herein showed monomeric behavior in DMSO in lower
concentration (from 1 to 15 M), as is possible to see in UV-vis studies (Fig. 4
and 5).
1,2
1,0
0,8
0,6
0,4
0,2
0,0
300
400
500
600
700
800
Wavelength nm)
13

Fig. 4. Absorption spectra of (5) in DMSO from 1.3 x 10-6 to 1.4 x 10-5 mol.L-1
1,2
(3)
(4)
(5)
1,0
ZnNc
0,8
0,6
0,4
0,2
0,0
0
5
10
15
Concentration (M)
Fig. 5. Intensity at maximum absorption wavelength versus concentration for all studied
compounds, in DMSO.
3.2. Fluorescence spectra, quantum yields and lifetimes
Fluorescence measurements of the phthalocyanine–naphthalocyanine
hybrids were recorded in DMSO (Table 1). The excitation spectra were similar
to absorption spectra and both were mirror images of the emission spectra. The
proximity of the wavelength of Q band maxima of the absorption and excitation
spectra for all compounds suggests that the nuclear configurations of the
ground and excited states are similar and not affected by excitation. Stokes
shifts is in agreement with the values found for other Pcs[26]. Due to the extend
14

conjugation, the Q-band maxima of the excitation spectra and emission was
red-shifted in the same order of absorption spectra.
Table 1 presents the maximum wavelength of absorption, excitation and
emission for all compounds in DMSO.
Table 1. Absorption, excitation and emission data for phthalocyanine–
naphthalocyanine hybrids in DMSO.
Q band
Excitation
Emission
Stokes Shift
Compound
̅
ZnPc
(3)
675
5.3
5.1
4.8
4.9
5.0
607
681
6
11
7
680
713
738
764
682
715
745
766
693
722
755
768
(4)
(5)
10
2
ZnNc
The fluorescence quantum yields ( ) of the compounds were
determined by comparative method and the values are given in Table 2. The
values for ZnPc were higher than phthalocyanine–naphthalocyanine hybrids
and for ZnNc. The fluorescence lifetime ( ) of the phthalocyanine–
naphthalocyanine hybrids are supplied in Table 2. Again, the values were
lower for ZnNc than ZnPc.
The fluorescence quantum yields and fluorescence lifetime values
decreases in the sequence from compound (3) to (4) and (5). These results
suggest that the addition of benzene rings to the periphery of phthalocyanine
macrocycle increases the non-radiative losses from excited states and
decreases the efficiency of fluorescence process.
15

Table 2. Photophysical data of phthalocyanine–naphthalocyanine hybrids in
DMSO.
Compound
3.30
3.22
2.62
2.23
2.46
ZnPc
(3)
0.20
0.18
0.10
0.06
0.07
0.67
0.24
0.18
0.15
0.03
(4)
(5)
ZnNc
3.3. Singlet oxygen quantum yields ( )
The were determined in DMSO applying a chemical quencher method
as DPBF. The reaction was monitored using UV-vis spectrophotometer (Fig. 6).
The values of ZnPc are higher than for other compounds (Table 2). Once
more, higher extended π-conjugation in phthalocyanine–naphthalocyanine
hybrids results in lower values for singlet oxygen generation.
16

0,8
0,6
0,4
0,2
0,0
0,8
0,6
0,4
0,2
0,0
0
20
40
60
80
100
120
140
160
180
Time(s)
300
400
500
600
700
800
900
Wavelength (nm)
Fig. 6. Changes in the absorbance intensity at 415 nm during the determination
of singlet oxygen yield of (5), in DMSO. (Inset: plots of 415 absorbance intensity
versus time.)
4. Conclusions
In our study we describe the synthesis and characterization of new
phthalocyanines and naphthalocyanines hybrids. The photophysical and
photochemical properties were investigated in DMSO to understand the effects
of extended π-conjugation on compounds. The presence of naphthalo groups in
the structure causes a bathochromic shift in UV-vis, emission and excitation
spectra. While UV-vis spectra did not showed aggregation behaviors in lower
concentration in DMSO, the quantum yield of singlet oxygen generation
decreases at the same range of concentration, indicating the presence of
aggregated species.
17

Acknowledgements
This work was supported by UFABC, CNPq (402289/2013-7), CAPES
and FAPESP (2014/18527-8).
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