Solvent effects for fluorescence and absorption of tetra (fluoroalkyl) metallophthalocyanines: Fluorocarbon solvent cage

Article abstract of DOI:10.1016/j.jphotochem.2010.06.012

The UV-vis absorption and fluorescence spectra of a series of tetra (fluoroalkyl) metallophthalocyanines were measured in CH₃OH, ethyl acetate, THF, CH₂Cl₂, C₆H₆, perfluorooctane. Their absorption spectra and fluorescence spectra were related not only to the peripheral substitutes, but also to the solvent environment. In the strong polar solvents, their maximum absorption and emission wavelengths were blue-shifted. However, in perfluorooctane, the blue shift was found for the absorption, and the red shift for the emission. The fluorocarbon solvent cage concept was put forward to explain this special phenomenon.

Full text of DOI:10.1016/j.jphotochem.2010.06.012

Journal of Photochemistry and Photobiology A: Chemistry 214 (2010) 86–91
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Solvent effects for fluorescence and absorption of tetra (fluoroalkyl)
metallophthalocyanines: Fluorocarbon solvent cage
Tao Qiua,b, Xiaoyong Xua, Xuhong Qiana,∗
a Shanghai Key Lab of Chemical Biology, School of Pharmacy, East China University of Science and Technology, 130 Meilong Road, P.O. Box 544,
Shanghai 200237, People’s Republic of China
b Institute of Fine Chemicals, Jiangsu Polytechnic University, Changzhou 213164, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
The UV–vis absorption and fluorescence spectra of a series of tetra (fluoroalkyl) metallophthalocyanines
were measured in CH3OH, ethyl acetate, THF, CH2Cl2, C6H6, perfluorooctane. Their absorption spectra
and fluorescence spectra were related not only to the peripheral substitutes, but also to the solvent
environment. In the strong polar solvents, their maximum absorption and emission wavelengths were
blue-shifted. However, in perfluorooctane, the blue shift was found for the absorption, and the red shift for
the emission. The fluorocarbon solvent cage concept was put forward to explain this special phenomenon.
Received 2 January 2010
Received in revised form 31 May 2010
Accepted 16 June 2010
Available online 23 June 2010
JEL classification:
31.70.Dk
33.50.Dq
© 2010 Elsevier B.V. All rights reserved.
33.20.−t
07.57.−c
82.80.−d
78.47.da
Keywords:
Fluoroalkyl metallophthalocyanines
UV–vis absorption spectra
Fluorescence spectra
Fluorocarbon solvent cage
Solvents effect
1. Introduction
Fluorocarbon solvents (perfluorocarbons) have been widely
used as green solvents in Fluorous Biphasic Catalysis (FBC), artificial
Recently, fluorinated metallophthalocyanines (MPcs) have
attracted more and more attention [1–3]. They have been proved
to be advantageous over non-fluorinated derivatives as photo-
sensitizers for the photodynamic therapy (PDT) treatment [4,5].
Generally, fluoroalkyl substituted compounds have high solubil-
ity in polar solvents due to the highest electronegativity of the
fluorine atom in all elements [6]. The spectral characteristics of
phthalocyanines and MPcs have been investigated in detail for
many years because of their wide applications [7–9]. At present,
more researchers focused on their study on the spectra of fluori-
nated MPcs. Sugimori reported the electronic absorption spectra of
the tetrakis-octafluoropentoxy nickel phthalocyanine complexes
[6]. Huang reported the characteristics of the UV–vis absorption
spectra of the tetrakis (3-trifluoromethylphenoxy) phthalocyanine
complexes with cobalt, nickel or zinc in THF [10].
blood substitutes [11], new phase-screen medium [12]. They hold
peculiar chemical and physical properties such as high-density,
colorlessness, non-toxicity, chemical inertness, thermal stability,
nonflammability, non-polarity, low intermolecular interaction, low
surface energy, wide-range boiling point [13]. In general, fluorocar-
bons are immiscible with water and can be used as the nonaqueous
phase [14–16]. Up to now, there is no report about the fluorocar-
bon solvents effects on metallophthalocyanines (MPcs) and even
fluorinated MPcs.
The properties of phthalocyanines are decided not only by the
nature of the substituent (electron-donating or withdrawing) of
the ligand but also by the metal cation in the core of the ligand.
In general, their solubility can be improved by the introduction
of different kinds of substituent, such as alkyl, alkoxyl, phenoxyl
and macrocyclic groups, into the peripheral of the Pc ring [17].
Nyokong et al. [18] reported the solvent effect for the absorption
spectra of the zinc octaestrone phthalocyanine, and the largest red
shift of the Q band observed in the aromatic solvents. Based on
our knowledge, the study on the spectral characteristics of fluo-
∗
Corresponding author. Tel.: +86 21 64253589; fax: +86 21 64252603.
E-mail address: xhqian@ecust.edu.cn (X. Qian).
1010-6030/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jphotochem.2010.06.012

T. Qiu et al. / Journal of Photochemistry and Photobiology A: Chemistry 214 (2010) 86–91
87
Fig. 1. Structures of tetra (fluoroalkyl) metallophthalocyanines: (i) CF3CF2CF2CF2I or CF3CF2CF2CF2CF2CF2I/Cu, DMF, 125 ◦C; (ii) H2NCONH2, MX, NH4Cl, (NH4)6Mo7O24·4H2O,
200 ◦C.
roalkyl metallophthalocyanines (FMPcs) in the fluorous solvent will
be significant.
that there was no aggregation in these systems. However, the spec-
trum of iron (compound 4a, 4b) was split into two distinct peaks in
THF, which indicated an obvious aggregation tendency for iron (4a,
4b) centered fluoroalkyl phthalocyanines. The same phenomenon
was observed in ethyl acetate. The excitation spectra of 3a was
recorded and compared with its absorption, two spectra were sim-
ilar which indicated the sample is pure. The spectra are shown in
Fig. 3.
In this paper, UV–vis absorption and fluorescence properties of
a series of tetra (fluoroalkyl) metallophthalocyanines (Fig. 1) in the
organic solvents and perfluorooctane, which were synthesized in
our previous research, were investigated [19]. The fluorocarbon sol-
vent cage effect was introduced to elucidate the unusual correlation
between their electronic spectra and the solvents.
2. Experimental
3.2. Fluorescence spectra in the organic solvent
2.1. Materials and methods
The fluorescence spectral data are shown in Table 1. Their flu-
orescence and quantum yields (˚F) were determined in different
solvents as shown in Table 1.
Perfluorooctane used was purchased from Aldrich Chemicals
Co., and the solvents were purified with standard methods and
dried as needed. All reagents are off-the-shelf commercial products.
Perfluoroalkyl phthalocyanine metal derivatives (1a–4b) (FMPcs)
were synthesized according to the method reported in the previ-
ous paper [19]. The desired compounds were prepared according to
the route shown in Fig. 1. Absorption spectra were determined on
PGENRAL TU-1901 UV-vis Spectrophotometer. Fluorescence spec-
tra were measured on a Perkin Elmer LS 50 spectrophotometer. The
relative fluorescence quantum yield was estimated using the 0.1 M
quinine sulfate with ˚F = 0.55 in 0.1 N sulfuric acid solution as a
standard sample [20,21]. Fluorescence lifetimes were measured on
an Edinburgh Lifespec-Ps spectrofluorometer (FL920).
The maximum emission wavelengths of all the synthesized
compounds were around 670–695 nm in THF as shown in Fig. 4,
and the compound 3b had higher ˚F value in THF and benzene,
compared with that of the reported fluoroalkoxy MPcs [25]. For
example, the ˚F value of tetra (trifluoroethoxy) zinc phthalocya-
nine was 0.058 (ethyl acetate, 25 ◦C), while the ˚F value of 3b was
up to 0.38 (ethyl acetate, 25 ◦C). However, the fluorescence inten-
sities of compound 2a and 2b were too weak to be detected, which
might be caused by the strong fluorescence-quenching effect due
to their core-metal cation.
3. Results and discussion
3.1. UV–vis absorption spectra in the organic solvent
The UV–vis spectral data of tetra (fluoroalkyl) metallophthalo-
cyanine derivatives (1a–4b) are shown in Table 1. The maximum
absorption wavelengths and values of log ε in different solvents
were listed. The intense Q absorption bands around 665 nm was
observed and shown in Fig. 2.
Aggregation is usually depicted as a coplanar association of rings
progressing from monomer to dimer and higher order complexes
[22,23]. Different solvents or various central metals also greatly
influence the aggregation [24]. The aggregation behaviors of the
FMPcs (1a–4b) with different central metal cations were investi-
gated in ethyl acetate and THF (tetrahydrofuran), and it was found
that a sharper and narrower absorption peak was observed at about
665 nm for cobalt (compound 1a, 1b), copper (compound 2a, 2b)
and zinc (compound 3a, 3b) coordinated fluoroalkyl phthalocya-
nines, respectively, in THF at the concentration 10−5 M. This means
Fig. 2. UV–vis absorption spectra of tetra (fluoroalkyl) metallophthalocyanines
(1a–4b) in THF. Concentration: 1.0 × 10−5 mol L−1
.

88
T. Qiu et al. / Journal of Photochemistry and Photobiology A: Chemistry 214 (2010) 86–91
Table 1
The spectral properties of fluoroalkyl metallophthalocyanines in different solvents.
Solvent
Solvent polarity
Methanol
6.60
Ethyl acetate
4.30
THF
4.20
Dichloromethane
3.50
Benzene
3.00
Perfluorooctane
1a
ꢀmax/nm (log ε)
ꢀem/nm (˚F)
(ꢁabs − ꢁem)/cm−1
2a
646 (3.84)
–
662 (4.47)
690 (0.024)
613
653 (3.78)
694 (0.032)
904
662 (3.16)
693 (0.042)
676
663 (3.86)
697 (0.016)
736
610 (2.40)
698 (0.0073)
2067
a
ꢀmax/nm (log ε)
ꢀem/nm (˚F)
3a
607(3.17)
–
663 (3.31)
–
665 (5.22)
–
668 (3.02)
–
673 (3.60)
–
592 (3.20)
–
ꢀmax/nm (log ε)
ꢀem/nm (˚F)
(ꢁabs − ꢁem)/cm−1
4a
661 (3.90)
673 (0.19)
270
667 (4.28)
676 (0.032)
200
666 (4.75)
677 (0.024)
244
672 (3.94)
678 (0.077)
132
673 (3.88)
688 (0.079)
324
617 (3.77)
700 (0.0037)
1922
ꢀmax/nm (log ε)
ꢀem/nm (˚F)
(ꢁabs − ꢁem)/cm−1
1b
–
–
684 (3.24)
692 (0.17)
169
687 (3.70)
693 (0.074)
126
692 (3.21)
694 (0.16)
42
692 (2.28)
696 (0.063)
83
598 (3.12)
701 (0.07)
2457
ꢀmax/nm (log ε)
ꢀem/nm (˚F)
(ꢁabs − ꢁem)/cm−1
2b
645 (3.41)
–
660 (4.26)
–
656 (4.70)
676 (0.01)
451
–
–
668 (2.69)
678 (0.008)
221
598 (3.17)
701 (0.065)
2457
ꢀmax/nm (log ε)
ꢀem/nm
609(3.10)
–
665 (3.23)
–
666 (4.27)
–
671 (2.84)
–
673 (3.40)
–
588 (4.31)
–
3b
ꢀmax/nm (log ε)
ꢀem/nm (˚F)
(ꢁabs − ꢁem)/cm−1
4b
657 (3.38)
673 (0.12)
362
666 (4.12)
674 (0.38)
178
668 (3.29)
674 (0.68)
133
670 (3.46)
677 (0.09)
154
672 (3.40)
679 (0.875)
153
598 (3.36)
700 (0.04)
2437
ꢀmax/nm (log ε)
ꢀem/nm (˚F)
(ꢁabs − ꢁem)/cm−1
–
–
685 (3.13)
691 (0.09)
126
663 (4.36)
674 (0.014)
178
670 (3.47)
694 (0.014)
516
693 (2.81)
697 (0.018)
83
598 (3.35)
700 (0.02)
2437
˚F: fluorescence quantum yield (in general the standard used for determination of ˚F is 0.1 M quinine sulfate in sulfuric acid (˚F = 0.55).
a
Spectrum of the compound in these solvents were not detected.
It is well known that the absorption and emission wavelengths
are sensitive to the environmental factors [26,27]. As indicated in
Table 1, the absorption of FMPc was very sensitive to the solvent.
For example, with the increase of solvent polarity, the maximum
absorption wavelength of the compound 2a was blue-shifted about
65 nm (from 673 nm in benzene to 607 nm in methanol).
Additionally, it was observed that the fluorescence of FMPc also
depended on the “polarity” of the environment, which was con-
sistent with their absorption spectra. As shown in Table 1, the
maximum emission wavelength of compound 3b was blue-shifted
about 6 nm (from 679 nm in benzene to 673 nm in methanol). Sim-
ilar blue shifts were also observed in other compounds.
It showed that the maximum absorption (ꢀmax) and emission
wavelengths (ꢀem) of FMPcs were blue-shifted with the increase of
the solvent polarity in Table 1. This indicated the energy gap (ꢂE)
between the ground state and the excited state gradually became
large with the increase of the solvent polarity. Such increase of
energy level indicated its instability in the excited state in strongly
polar solvent.
The polarity of FMPcs molecule after the excitation in strongly
polar solvents became weak as compared with its polarity in the
ground state. This led to the rising of the energy level in the excited
state in polar solvents and the decrease of the energy level of the
ground state. Accordingly, the gap of the energy became larger
and the maximum absorption wavelength was blue-shifted. This
phenomenon was consistent with the result in our previous report
[25].
are at 590–610 nm, even much shorter than those in the strongly
polar organic solvents (as shown in Fig. 5), leading to the blue shift.
Accordingly, perfluorooctane has the typical characteristics of the
strongly polar solvents. However, the fluorescence maximum emis-
sion wavelength (ꢀem) of FMPcs is near 700 nm, even longer than
that in the weakly polar organic solvents, leading to the red shift.
Perfluorooctane also has typical characteristics of the non-polar
solvents.
In order to investigate the effect of perfluorooctane, the fluo-
rescence lifetime of one of compound 3b was obtained by time
resolved fluorescence measurement. The fluorescence lifetime of
3b is 1.24 ␮s (43.75%) and 9.60 ␮s (56.25%), fitted with double
exponential decaying with ꢃ2 = 1.271 in THF at the concentration
10−5 M, while in perfluorooctane, the value changes to 1.15 ␮s
(49.02%) and 9.89 ␮s (50.98%) with ꢃ2 = 1.272. In addition, the range
of Stokes shift of 3b is 130–360 cm−1 in common solvents, how-
ever, the Stokes shift of 3b is 2437 cm−1 in perfluorooctane solvent.
These values mean that the spectral characteristics of FMPcs in per-
fluorooctane are quite different from that in the normal organic
solvents. Therefore, in this paper we introduce the fluorocarbon
solvent cage concept to explain this special phenomenon.
According to the Franck–Condon Principle [28] and the solvent
cage molecular spectroscopy [29,30], both intermolecular inter-
action (solute–solute interactions, solute–solvent interactions)
and the multiplexed excited states interactions of the composite
molecule have to be taken into consideration when investigating
the spectral characteristics of the molecular system. The target
molecular system should be regarded as the molecule of an aggre-
gation one. The study should not be focusing just on the electronic
configuration of a single molecule in excited states, and it is also
important to study the influence of the interaction between the
host molecule and the surrounding solvent molecules in the excited
state.
3.3. The solvent effects of the perfluorooctane
The UV–vis absorption spectra and fluorescence emission spec-
tra of eight FMPcs in perfluorooctane were measured and listed in
Table 1. The maximum absorption (ꢀmax) wavelengths of FMPcs

T. Qiu et al. / Journal of Photochemistry and Photobiology A: Chemistry 214 (2010) 86–91
89
Fig. 5. UV–vis absorption spectrum of 3a in THF and perfluorooctane. Concentra-
tion: 1.0 × 10−5 mol L−1
.
of solute molecule of the FMPcs, leading to the special change of the
spectral characteristics. In the ground state S00, a directional fluo-
rocarbon solvent cage was induced around the FMPcs molecule,
having a static stability effect in the solute’s ground state and
accordingly, a lower energy at the ground state S00 in the perfluo-
rooctane than that in the common organic solvents. When the FMPc
molecule was excited by light, its electronic configuration instantly
reached to a higher energy level S1, an unstable excited state S1n
,
whereas the molecular conformation could not catch up with the
change immediately. From the spectral characteristics of FMPcs
in polar and non-polar organic solvents described above, it was
known that the polarity of the FMPc in the unstable excited state
usually became weak after the excitation. Meanwhile, the perflu-
orooctane could not immediately re-orient through conformation
changes during the light-irradiation and still kept the original flu-
orocarbon solvent cage and showed an induced lagging effect, that
is, the fluorocarbon solvent cage corresponding to S1n still stayed
as the solvent cage to S00 at the ground state.
Fig. 3. (a) UV–vis absorption spectrum of 3a in THF. Concentration:
2.0 × 10−5 mol L−1. (b) Fluorescence excitation spectrum of 3a in THF. Concentration:
2.0 × 10−5 mol L−1
.
It was found that FMPcs dissolved easily in the perfluorooctane,
which suggested that perfluorooctane might form a directional
particular solvent cage around the FMPcs molecule, that is, fluo-
rocarbon solvent cage. The fluorocarbon solvent cage would affect
the energy variation between the ground state and the excited state
Fig. 4. Fluorescence emission spectrum of tetra (fluoroalkyl) metallophthalocya-
nines (1a–4b) in THF.
Fig. 6. The explanation of fluoro solvent cage on electronic transition of fluoroalkyl
metallophthalocyanines in perfluorooctane.

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T. Qiu et al. / Journal of Photochemistry and Photobiology A: Chemistry 214 (2010) 86–91
Fig. 7. The explanation of electronic transition of fluoroalkyl metallophthalocyanines in perfluorooctane and common solvent.
In this case, the weak polarity of the FMPc molecule in the unsta-
As compared in the common organic solvents, the blue shift
ble excited state S1n was not compatible to the strong polarity of
the original solvent cage, leading to the instability of the FMPc and
the energy increase of the unstable excited state, subsequently, the
energy gap ꢂE between S1n and S00 increased and blue shift of ꢀmax
occurred, as shown in Figs. 6 and 7.
value of the maximum absorption wavelength was larger in
perfluorooctane. In the fluorocarbon solvent perfluorooctane,
FMPcs solute molecules were enclosed by perfluorooctane solvent
molecules that formed the fluorocarbon solvent cage. The fluoro-
carbon solvent cage and its lagging effect influenced the energy
variation between the ground state and the excited state of the
solute molecule of FMPcs and further resulted in the special change
of the spectral characteristics. The measurement of fluorescence
lifetime also confirmed the solvent effect of fluorocarbon solvent
perfluorooctane. Using the fluorocarbon solvent cage concept, this
particular change of absorption and emission spectra in perfluo-
rooctane was easily and clearly explained.
After FMPc reached to its unstable excited state (S1n), its energy
was quickly released by the heat loss and the conformation slowly
fell to the equilibrium excited state S10, which induced the original
solvent cage to form the second compatible fluorocarbon solvent
cage with weak polarity. In this process, the FMPc’s conformation
changedand thefluorocarbonsolvent cagearound the FMPcwas re-
oriented, causing a static stability of the equilibrium excited state.
The energy of FMPc at state S10 was very low. Furthermore, the
FMPc returned from the equilibrium excited state to the unstable
ground state S0n and led to an emission of fluorescence. Similarly,
due to the induced lagging effect of the fluorocarbon solvent cage,
the solvent cage corresponding to S0n remained as the second fluo-
rocarbon solvent cage with weak polarity. The mismatch between
the FMPc at unstable ground state with strong polarity and the
second fluorocarbon solvent cage with low polarity caused the
unstable FMPc and its energy increase. Thus, the energy gap ꢂE
between S10 and S0n decreased and the red shift of ꢀem occurred,
as shown in Figs. 6 and 7.
Acknowledgements
This project was financially supported by the National Basic
Research Program of China (973 Program, 2010CB126104),
National High Technology Research and Development Program of
China (2010AA10A204). The authors also appreciate the support
from Shanghai Leading Academic Discipline Project (No. B507), 111
Project (No. B07023), Shanghai Education Committee and Shanghai
Foundation of Science and Technology.
Although the spectral characteristics of the fluorobutyl metal-
lophthalocyanine (FBMPc) were slightly different from that of the
fluorohexyl metallophthalocyanine (FHMPc) in the perfluorooc-
tane owing to the chain length difference of the substituent, the
blue shift of ꢀmax of the FHMPc was larger than that of the FBMPc
and the red shift of ꢀem of FHMPc was larger than that of FBMPc in
perfluorooctane. The special phenomenon was investigated under-
way.
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