Photophysical study of Zn phthalocyanine in binary solvent mixtures - Staicu, Pascu, Pascu, Enescu.pdf

7/27/2019 Photophysical study of Zn phthalocyanine in binary solvent mixtures - Staicu, Pascu, Pascu, Enescu.pdf

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Photophysical study of Zn phthalocyanine in binary

solvent mixtures

A. Staicua, A. Pascua, M.Bonia, M.L.Pascua, M.Enescub

aLaser Department, Atomistilor 409, 077125 Magurele, Bucharest, National Institute for

Lasers, Plasma, and Radiation Physics, RomaniabUFR-ST Laboratoire Chrono-Environnement UMR CNRS 6249, 16 Route de Gray,

25030 Besancon Cedex, Universite de Franche-Comte, France

Abstract

Photophysical properties of phthalocyanines are important in photodynamic

therapy, where these compounds are used as photosensitizing agents. We

report here some significant solvent effects on the photophysical properties

of Zn phthalocyanine (ZnPc) observed in binary solvent mixture dimethyl

sulfoxide/water at several rates of cosolvents. The absorbance of ZnPc at the

maximum of Q band has a sharp drop in intensity for a water mass fraction

in the solvent mixture larger than 40%. The same characteristic shows also

the quantum yield of fluorescence. A particular result is the increase of

singlet oxygen lifetime for water percentage raise up to 20% in the solvent

mixture. The effects are discussed in connection with the particular solvent

microenvironment, involving DMSO/water clusters formation and the strong

interaction between the solute and the solvent.

Keywords: Zn phthalocyanine, photophysics, DMSO/water solvent

Author for correspondence. Tel.: +40-21-4575739; Fax: +40-21-4575739, E-

mail address: [email protected]

Preprint submitted to Journal of Molecular Structure October 8, 2012


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1. Introduction

Phthalocyanines constitute a class of organic compounds with wide application

in different fields as dyes and pigments, solar cells [1], optical materials [2], sensors,

electronic devices [3]. One of their main applications is as photosensitizers in

photodynamic therapy (PDT). The photophysical properties of phthalocyanines

are important to be determined in order to establish their efficiency in PDT.

It is known that unsubstituted phthalocyanines show in aqueous solution a high

aggregation tendency [3]. Sulphonated phthalocyanine photosensitizers for PDT

were widely synthesized because of the advantage of their water solubility [4, 5, 6].

New methods of drug delivery developed in the last years as liposomal formulation,

encapsulation or conjugation with nanoparticles [7, 8, 9] were employed also for

phthalocyanines in order to overcome the water solubility problem. These allow a

reconsideration of the potential role of basic metallophthalocyanines as photosen-

sitizers, especially taking into account their higher efficiency in generating active

species.

Among metallated phtahlocyanines, Zn phthalocyanine (ZnPc) has a high

quantum yield of singlet oxygen generation and it was used in different drug de-

livery vehicles as liposomes and nanoparticles [7, 8, 9].

On the other hand, DMSO is a solvent that is often used as drug delivery

support due to its efficient penetration in the tissues [10]. DMSO/water is one of

the most interesting solvent mixtures, showing a strong non-ideal behavior. Shin

et al observed [11, 12] by mass spectrometry that the cluster formation between

water and DMSO molecules is nonlinearly dependent on the solvent composition,

at certain ratios of mixing drastic changes in the microscopic solvent structure

take place. It was also demonstrated that in such a solvent mixture the solvation

depends on the microscopic solvent structures, implying that solute species interact

with the solvent clusters, rather than with individual solvent molecules.

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Reports were also made regarding theoretical studies on the binary mixture

DMSO / water in order to explain the nonlinear and non-aditive behavior of cluster

formation [13, 14].

The studies about photosensitive dyes in binary solvent mixtures and their

photophysical behavior are quite scarce. Absorption spectra of a benzoporphyrin

derivative in DMSO/water mixture showed, starting from about 40% upwards wa-

ter in solvent, the dimmer formation [15]. In the case of diphenylhexatriene in

solvent mixtures containing water and DMSO, the absorption of the compound

versus water content of the solvent showed a steep drop at about 46% water in sol-

vent assigned to aggregates formation [16]. Also, studies on hypericin aggregation

in DMSO/water mixture were recently reported [17].

We report here a photophysical study of ZnPc in binary DMSO/water solvent

mixture. Different spectroscopic techniques were used: absorption spectroscopy for

monitoring the UV-VIS spectral changes, laser induced fluorescence spectroscopy

to assess intensity and quantum yield of the ZnPc fluorescence, time-resolved phos-

phorescence of singlet oxygen spectroscopy to check the quantity and kinetics of

photosensitized singlet oxygen species generated by ZnPc.

2. Materials and Methods

Dimethyl sulfoxide for synthesis (purity 99%) was purchased from Merck while

zinc phthalocyanine (ZnPc) was supplied by Fluka. Distilled water was home-made

by Merit Water Still W4000 equipment. Sample preparation was straightforward:

stock solutions of ZnPc were prepared in DMSO. Adequate quantities of binarysolvent solution were prepared at different water rates in order to prepare a sample.

After parts mixing, the solutions were stirred and relaxed to the room temperature

(21C) in order to obtain homogenous samples.

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For absorption spectroscopy, steady-state measurements were carried out with

a Perkin Elmer UV/Vis/NIR Spectrometer, model Lambda 950.

The laser radiation used as excitation in phosphorescence kinetics measure-

ments and laser induced fluorescence spectroscopy was provided by the third

harmonic generation (THG) of a Nd:YAG (Continuum, Excel Technology) laser,

model Surelite II with 10 Hz frequency, 6 ns pulse duration, and 120 mJ maxi-

mum energy at 355 nm. The laser beam was properly conveyed to and tailored

according to the requested conditions of each experimental setup.

Laser pulse energy was monitored with an energy meter Quanta QE 25 from

Gentec by properly splitting of the laser beam. The laser pulse energies used in the

experiments were smaller than 5 mJ to avoid a saturation effect of the processes

and the ZnPc photodegradation.

For laser induced fluorescence measurements, the experimental arrangement

employs the third harmonic of the Nd:YAG laser beam as the excitation source,

interacting with the liquid sample contained in a fluorescence-photometer type

cuvette (1 cm diameter). The fluorescence was collected in a right angle geometry

by an optical fiber and sent to the entrance slit of a spectrograph (Acton Research,

model SpectraPro 2750) equipped with a 150 tr/mm diffraction grating blazed at

500 nm. The detection was made by an electronic cooled iCCD from Princeton

Instruments, model PIMAX 1024 RB. The camera was triggered by a TTL laser-

generated synchronizing pulse. Stored data were subsequently computer processed

using the software of the spectrograph/camera system.

Fluorescence quantum yields of the dye in different binary solvent mixtures

were determined by a relative method, comparing the wavelength-integrated in-

tensity of the measured fluorescence spectra of the dye with that of a standard

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with a known quantum yield:

Q = QrefI

Iref

Aref

A

n2

n2ref(1)

where Q is the quantum yield, I is the integrated fluorescence intensity, n is the

solvent refractive index, and A is the sample absorbance at 355 nm; ref index

corresponds to the the standard sample. In our case, this was chosen as ZnPc in

DMSO with 0.19 fluorescence quantum yield [18].

Singlet oxygen was generated by photoexcitation of the ZnPc molecules in their

Soret band with laser pulses at = 355 nm followed by the energy transfer between

the triplet state of ZnPc and the ground state of the solvated O2 molecules. The

singlet oxygen phosphorescence at = 1270 nm was used to measure its lifetime

as well as the generation quantum yield. All samples were air equilibrated.

Time-resolved phosphorescence of singlet oxygen was measured by using a

cooled NIR photomultiplier (Hamamatsu H-10330) whose output was fed to a

digital scope (Tektronix DPO 7254). The sample was placed in a 1 cm cuvette

crossed by the laser beam. The phosphorescence was collected in right-angle ge-ometry and heavily filtered against wavelengths other than 1270 nm by an optical

arrangement (lenses, apertures and filters) placed in front of the NIR photomul-

tiplier. Triggering of the oscilloscope run was ensured by a TTL signal from the

laser source.

It is worth noting that the possibility to have parasitic radiation originating

from the phosphorescence of ZnPc itself at the same wavelength with that of the

singlet oxygen phosphorescence (1270 nm) may be neglected, given the working

temperature and the liquid solvent matrices. Even at 77 K, according to [19], the

faintest phosphorescence of metallated phthalocyanines terminates before 1250 nm.

The singlet oxygen lifetime was determined by recording the phosphorescence

decay curves over 1000 averaged laser shots. Resulted data were processed by

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fitting the decaying part of the phosphorescence transients with mono-exponential

functions, the time constant of the fitting curves giving the lifetime of the singlet

oxygen population.

Singlet oxygen quantum yields were determined relative to the standard ZnPc

in DMSO (0.67 conform to [20]). The samples were measured under the same

experimental conditions, in air equilibrated solutions. Using time-resolved mea-

surements for the sample and standard, and estimating the phosphorescence inten-

sities of singlet oxygen extrapolated to the zero-time of laser pulse excitation, the

quantum yields for the examined solution was found according to the formula [21]:

= refI

Iref

Aref

A

n2

n2ref

ref(2)

where stands for the quantum yield of singlet oxygen, I for the phospho-

rescence intensity of singlet oxygen extrapolated at t = 0, A for the samples

absorbance at 355 nm and and for the singlet oxygen lifetime. Subscripts ref

correspond to the standard. The phosphorescence intensity at t = 0 is obtained by

extrapolating to t = 0 the mono-exponential fitting curve of the phosphorescence

kinetics.

3. Results and discussion

3.1. Absorption spectroscopy studies

ZnPc displays absorption spectra with two main electronic bands, the Q band

with a maximum around 672 nm and the Soret band with a maximum at 350 nm.

The Q band presents a vibronic structure with vibration bands at 608 and 644 nm.

The spectra of the ZnPc at 5 106M concentration in solution of DMSO/water

for different water mass fractions are shown in Figure 1. The shape of the spectra

suggests their grouping in two packs: one corresponding to solutions with water

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mass fraction less than 35% and the second containing more than 40% water. One

notes a faint shift of 3 nm to a longer wavelength for the Q band peak when

water fraction rises from 0% to 35% (ZnPc is a non-polar molecule). Also in this

range of water percentage, there is a small decrease (about 20%) of the maximum

absorbance of this band upon the mass fraction increase. A sort of bleaching

takes place when the water mass fraction increases beyond 35%, the Q band has

a suddenly decrease in intensity and two wide low-intensity bands with flat peaks

at 620 nm and 750 nm appear in the spectra.

By plotting the maximum absorbance of Q band versus water content, the data

shown in Figure 2 are obtained, showing the same steep changes occuring at 35%

water fraction.

We have considered in the first instance two potential explanations for such

behavior: a change in the protonation sate of ZnPc due to the water fraction

variation and a dimerisation of the phthalocyanine.

Because the pH dependence of some photophysical paramaters of phthalo-

cyanines have shown a steep variation (e.g. the fluorescence quantum yield of

disulphonated phthalocyanine, as in [4]), we checked the evolution of the DMSO/water

mixture as function of water mass fraction. The results are consistent with the

data from [22], indicating that the increase of water mass fraction causes a smooth

increase of the pH in the range 7.5 - 10. We examined the effect of the pH on the

absorbance of the dye in a DMSO/water solution with the cosolvents proportion

70:30. The pH of the solution was varied by adding suitable quantities of NaOH

or HCl. The spectra (not shown) revealed that within the limits of the pH from

6 to 12, apart from an increasing of the background, there are no considerable

spectral changes of the dye absorption bands intensity. The absorption bands did

not exhibit peaks wavelengths shift, splitting of the bands or new bands. Conse-

quently, the pH modification of the solution produced by water rate increase can

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not explain the steep variation of the absorbance as seen in Figure 2.

We have also directly tested the effect of the ZnPc protonation on the absorp-

tion spectra by adding sulphuric acid to a ZnPc DMSO solution. The spectral

modifications thus observed are in agreement with those reported by Ogunsipe et

al. [6]. They show no similarity with the presently reported spectral modifications

occurring in DMSO/water mixture.

According to the literature [23], an absorption band of ZnPc derivatives with

a maximum at around 620 nm could be attributed to the dimeric form of the

compounds. We tested this hypothesis by measuring absorption spectra for ZnPc

concentrations ranging from 5 107M to 1 104M. For all solutions the water

mass fraction was 40%. The results are given in Figures 3 and 4. One notes

that the shape of the absorbtion spectrum is independent with respect to the dye

concentration (Figure 3). Moreover, the intensity of the absorption bands at 633

and 673 nm shows a good linearity as a function of ZnPc concentration (Figure 4).

These results clearly indicate that ZnPc dimerization is negligible in the present

concentration range.

Therefore, we attribute the peculiar behavior of ZnPc absorbance in DMSO/water

to the strong dye interaction with the solvent. As already mentioned in the intro-

ductory section, the DMSO/water mixture has a highly non-ideal behavior due to

its associative character, which depends on the ratio of the two compounds. Im-

portant deviations that occur at certain values of the water fraction in the solvent

mixture are associated to the formation of hydrogen-bond molecular clusters which

consist of a DMSO molecule and two water molecules or two molecules of DMSO

and one molecule of water [24]. Shin et al. observed [11, 12] by mass spectrome-

try that the cluster formation between water and DMSO molecules is nonlinearly

dependent on the solvent composition and at certain mixing ratios drastic changes

in the microscopic solvent structure taking place. It was also demonstrated that

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in such a solvent mixture the solvation depends on the microscopic solvent struc-

tures, implying that solute species interact with the solvent clusters, rather than

with individual solvent molecules. At water mole fraction over 0.93 (water mass

fraction 75%), the solute is preferentially solvated by water clusters, whereas below

0.93, it is preferentially solvated by DMSO clusters.

Zakharov et al. defined [25] four behavioral levels of the bulk, depending

on the DMSO molar fraction: the domain with the XDMSO > 0.56 (85% DMSO

mass fraction) where DMSO molecules form clusters as large as in pure solvent, the

domain 0.56 > XDMSO > 0.28 (63% DMSO mass fraction) where DMSO molecules

are associated in large and small clusters, the domain 0.28 > XDMSO > 0.1 (34%

DMSO mass fraction) where DMSO-water clusters appear, and the domain with

XDMSO < 0.1 where the water clusters are dominant.

According to the present analysis, the step variation of the ZnPc absorption

spectra with respect to the water mass fraction can be attributed to a step change

in the microscopic structure of the solvent. This solvent structural change induces

a significant modification in the solvent-solute interaction by favoring the ZnPc-

water contact.

3.2. Laser induced fluorescence studies

The dispersed fluorescence spectra induced by laser radiation at 355 nm for

ZnPc at the concentration of 5 106M in mixtures of DMSO/water at different

water mass fractions are shown in Figure 5.

The spectra point out that the samples having water proportion up to 40%

generate quite intense fluorescence radiation while for the samples with water

mass fraction over 40%, the dye experiences a dramatic fluorescence quenching.

The variation of the intensity of the fluorescence peak as a function of solvent

composition is shown in Figure 6. The intensity of the fluorescence peak shows an

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increase until a water level of 20%, then a decrease with a dramatic fall at about

40% water mass fraction. Similar dropping behavior of the fluorescence quantum

yield was reported for hypericin in DMSO/water mixtures [17] and assigned to

hypericin aggregates formation.

The fluorescence quantum yields calculated according to the relative mentioned

procedure ( Eq. (1)), are shown in Figure 7. We took into account as standard,

the fluorescence quantum yield of ZnPc in DMSO (0.19 [18]). The fluorescence

quantum yields dependence with respect to the solvent mixture is similar to that

observed for the fluorescence intensity: it increases when the water mass fraction

increases up to 20% and decreases dramatically for water mass fraction greater

than 40%.

It is obvious that the changes in the fluorescence properties are directly re-

lated to that observed for the absorption spectra. Together they indicate that the

changes in the ratio of the two cosolvents induces a major changes in the ZnPc

photophysics by activating a very efficient radiationless dezexcitation process. On

the other hand, the moderate increase of the fluorescence quantum yield occurring

in the domain of low water fractions can be attributed to the increase of the solvent

viscosity. Indeed, several reports indicate the non-ideal behavior of the viscosity

in DMSO/water mixtures [26, 27, 17]. The viscosity of the two solvents at 20C

are 1.0 cP for water and 1.99 cP for DMSO. Adding water to DMSO induces a

viscosity increase to a maximum of 3.72 cP that is reached at about 30% water

mass fraction [26].The increase in fluorescence quantum yield in a more viscous

solvent is due to a decrease in the rate of nonradiative decay [28].

Laser-induced fluorescence measurements of ZnPc in DMSO/water solutions

has confirmed as in the case of absorption that the solvent behavior have a strong

impact on the dye photophysics.

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3.3. Singlet oxygen generation

The lifetime of the singlet oxygen generated by exciting ZnPc at a concentration

of 5106M in DMSO/water solution was measured for different cosolvents mass

fractions. Its dependence on the water content is represented in Figure 8. The

results indicate that the water addition up to 20% mass fraction induces an increase

of the measured singlet oxygen lifetime, followed by a decrease for larger water

content. Beyond 60% water mass fraction the phosphorescence signal was to weak

for evaluating the lifetime.

It is well known that lifetime of singlet oxygen in water is 3 - 4 s. Regarding

DMSO, literature gives scarce and spread data: 19 s according to [29], 1.8 s

according to [30] and 30 s according to [31]. We determined for the lifetime of

1O2 in DMSO a value of 5.6 s.

To the best of our knowledge, data on singlet oxygen lifetime in binary DMSO/water

solvent mixtures were not yet reported. On the other hand, the photophysical

properties of singlet oxygen (lifetimes, radiative rate constants) in solvent mix-

tures containing water, D2O, dioxane, acetonitrile, propylene carbonate and ethy-lene carbonate, photophysical properties of singlet oxygen (lifetimes, radiative rate

constants) were found to be correlated with the solvent polarizability [32]. More-

over, for solvent mixtures containing acetonitrile and D2O, data suggest additional

interactions more solvent specific than collisions [32]. In water and dioxane mix-

tures there is a linear correlation between singlet oxygen lifetime and the fraction

of dioxane in water, while in water/acetonitrile mixtures the dependence is not

linear [33]. In some cases specific interactions between singlet oxygen and solvent

molecules cannot be described by solvent scales [34].

It is to be noticed that the singlet oxygen lifetime has an increase at 20% water

mass fraction to a value (10 s), more than the lifetime either in water,or DMSO.

20% water perecntage in the solvent DMSO/water corresponds to a mole fraction

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of 0.5. At this value, the calculation of Brussel et. al [13] gave maximum of the

concentration for mixed clusters formed by 2 molecule of water and 1 molecule of

DMSO.

The quantum yield of singlet oxygen generation was determined using the

Eq. 2. The results for different cosolvents mass fractions are given in Figure 9.

This shows the same kind of the quantum yield variation versus the water mass

fraction as in the case of the lifetime of singlet oxygen. Moreover, the curve is

very similar to that obtained for the fluorescence quantum yield 7. This result

strongly suggests that only the fluorescent S1 state of the dye is involved in the

singlet oxygen generation while the ZnPc solvated in a solvent with predominant

water mass fraction has a negligible contribution to this process, if any.

For the case of ZnPc in DMSO/water mixtures, the similar behavior depen-

dence on solvent composition observed for singlet oxygen lifetime and quantum

yield generation, on the one hand, and the fluorescence intensity and quantum

yield, on the other hand, suggests that the main factor involved in this depen-

dence is the peculiar variation of the microscopic structure of the solvent. This

structure strongly affect the dye photophysics, thus the singlet oxygen generation.

Moreover, the evolution of the singlet oxygen lifetime as a function of water mass

fraction correlates very well with the variation of the solvent viscosity. These con-

clusions were checked by measuring the lifetime of the singlet oxygen generate with

a different dye used as photosensitizer, verteporfin. As expected, the same solvent

dependence was obtained.

4. Conclusions

The photophysical properties of ZnPc in DMSO/water presently reported high-

light the highly non-ideal behavior of this binary solvent mixture. These properties

correlate very well with the reported molecular clustering patterns of the solvent.

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Absorption spectroscopy reveals a dramatic variation in the intensity of the Q

absorption bands of ZnPc occurring for a water mass fraction greater than 35%

- 40%. This behavior was assigned to the strong variation in the solute-solvent

interaction due to changes in the solvent molecules clustering. In the solvent con-

figuration characteristic for water mass fractions greater than 35%, the S1 state

of the dye is no longer fluorescent. Moreover, the quantum yields for singlet oxy-

gen generation becomes negligible small. This result suggest that, in this solvent

configuration, a new channel for the decay of the S1 state, more efficient than

the fluorescence emission and the intersystem crossing, is activated. On the other

hand, the rise in the fluorescence intensity, singlet oxygen lifetime and quantum

yields, observed for water mass fractions up to 20%, can be explained by the rise in

the viscosity of the solvent mixture. As a practical conclusion, it appears that the

optimum composition of the DMSO/water mixture for singlet oxygen generation

is that obtained for a 20% water mass fraction.

Acknowledgements

The support of the Romanian National Authority for Scientific Research, CNCS-

UEFISCDI by project number PN-II-ID-PCE-2011-3-0922 is fully acknowledged.

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3 0 0 3 5 0 4 0 0 4 5 0 5 0 0 5 5 0 6 0 0 6 5 0 7 0 0 7 5 0 8 0 0

A

b

s

o

r

b

a

n

c

e

w a v e l e n g t h ( n m )

2 0 %

2 5 %

3 0 %

3 5 %

4 0 %

5 0 %

6 0 %

8 0 %

Figure 1: Absorption spectra of ZnPc, 5 106M in DMSO/water solvent, water mass

fraction as a parameter.

0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0

A

b

s

o

r

b

a

n

c

e

a

t

m

a

x

m

u

m

o

f

Q

b

a

n

d

w a t e r p e r c e n t a g e ( % )

Figure 2: Absorbance of ZnPc (c = 5 106M) at 672 nm (maximum of the Q band)

versus water percentage in the solvent mixture.

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3 0 0 3 5 0 4 0 0 4 5 0 5 0 0 5 5 0 6 0 0 6 5 0 7 0 0 7 5 0 8 0 0 8 5 0

5 x 1 0

7 . 5 x 1 0

1 x 1 0

3 x 1 0

5 x 1 0

7 . 5 x 1 0

1 x 1 0

3 x 1 0

5 x 1 0

A

b

s

o

r

b

a

n

c

e


Figure 3: Absorption spectra of ZnPc at concentration between 5 107and5 105M

in DMSO/water at 40% water mass fraction.

2 x 1 0 4 x 1 0 6 x 1 0 8 x 1 0 1 x 1 0

p e a k @ 6 3 3 n m

p e a k @ 6 7 2 n m

A

b

s

o

r

b

a

n

c

e

c o n c e n t r a t i o n ( M )

Figure 4: Absorbances at 633 nm and 672 nm peaks versus ZnPc concentration in

DMSO/water at 40% water mass fraction.

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6 0 0 6 5 0 7 0 0 7 5 0 8 0 0

1 x 1 0

2 x 1 0

3 x 1 0

4 x 1 0

5 x 1 0

6 x 1 0

7 x 1 0

F

u

o

r

e

s

c

e

n

c

e

n

t

e

n

s

t

y

(

r

e

.

u

.

)


2 0 %

2 5 %

3 0 %

3 5 %

4 0 %

5 0 %

6 0 %

8 0 %

Figure 5: Disperse fluorescence spectra of ZnPc (c = 5 106M) in binary solvent

DMSO/water, water mass fraction as a parameter.]

0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0

1 x 1 0

2 x 1 0

3 x 1 0

4 x 1 0

5 x 1 0

6 x 1 0

F

u

o

r

e

s

c

e

n

c

e

p

e

a

k

n

t

e

n

s

t

y

(

r

e

.

u

.

)


Figure 6: The intensity of the fluorescence maximum of ZnPc in DMSO/water solvent as

a function of water percentage.

19


20/21

0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0

0 . 0 0

0 . 0 5

0 . 1 0

0 . 1 5

0 . 2 0

0 . 2 5

F

u

o

r

e

s

c

e

n

c

e

q

u

a

n

t

u

m

y

e

d


Figure 7: Fluorescence quantum yields of ZnPc dye in DMSO/water solvent versus water

mass fraction.

- 1 0 0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0

(

s

)


Figure 8: Lifetime of ZnPc photosensitized singlet oxygen in DMSO/water binary solvent.

20


21/21

0 1 0 2 0 3 0 4 0 5 0 6 0

Q

u

a

n

t

u

m

y

e

d

o

f

1

O

2

g

e

n

e

r

a

t

o

n


Figure 9: Quantum yield of singlet oxygen versus water percentage in DMSO/water binary

solvent.

21

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Photophysical study of Zn phthalocyanine in binary solvent mixtures - Staicu, Pascu, Pascu, Enescu.pdf