Fluorescence properties of phenol-modified zinc phthalocyanine that tuned by photoinduced intra-molecular electron transfer and pH values

Article abstract of DOI:10.1007/s10895-011-0844-0

Tetra[α-(4-hydroxyphenoxy)] zinc phthalocyanine, ZnPc(α-OPhOH) ₄, was synthesized and its photophysics was found to be sharply pH dependent. Dual fluorescence emission around 700 nm was observed when it is dissolved in basic solution. The fluor

Full text of DOI:10.1007/s10895-011-0844-0

J Fluoresc (2011) 21:1559–1564
DOI 10.1007/s10895-011-0844-0
ORIGINAL PAPER
Fluorescence Properties of Phenol-Modified Zinc
Phthalocyanine that Tuned by Photoinduced Intra-Molecular
Electron Transfer and pH Values
Xian-Fu Zhang
Received: 11 November 2010 /Accepted: 10 January 2011 /Published online: 25 January 2011
#
Springer Science+Business Media, LLC 2011
Abstract Tetra[α-(4-hydroxyphenoxy)] zinc phthalocya-
nine, ZnPc(α-OPhOH)4, was synthesized and its photo-
physics was found to be sharply pH dependent. Dual
fluorescence emission around 700 nm was observed when
it is dissolved in basic solution. The fluorescence of the
phthalocyanine can be sharply switched off at pH 9.1 due to
the intramolecular photoinduced electron transfer (PET) in
ZnPc(α-OPhONa)4, formed by the deprotonation of ZnPc(α-
OPhOH)4. The photophysics of both ZnPc(α-OPhOH)4 and
ZnPc(α-OPhONa)4 were studied in detail by UV-vis
absorption, steady state and time-resolved fluorescence and
transient absorption (TA) to reveal the fluorescence quench-
ing mechanism. Intra-molecular PET in ZnPc(α-OPhONa)4
from the donor, PhONa subunits, to the acceptor, ZnPc
moiety, was characterized by the much smaller fluorescence
quantum yield (0.003) and lifetime (<0.20 ns). PET was
further evidenced by the occurrence of charge separation
state (CSS) in TA spectra, i.e. the bands due to anion radical
of ZnPc and phenol radical. The lifetime of the charge
separation state is ca. 3 ns, the efficiency of PET is ca. 99%
and the rate constant of PET is 2.3×1010 s−1.
areas [1–4], such as chemical sensors to hydrogen peroxide
and NO2 etc [5–8], due to the excellent physical and
chemical properties, as well as their thermal and photo
stabilities. Pcs have the potential as PET (photoinduced
electron transfer) based fluorescent probes owning to the
following advantages: 1) their fluorescence emission is in
the red region which is remarkably longer than the auto
fluorescence that interferes with the measurement, 2) their
adaptability is well known since the wavelength absorption
and other properties can be tuned by easy structural
modification or through substitution of metal centers, 3)
they are biocompatible which have been shown by many
tests during the study of photodynamic therapy of tumor, 4)
Pcs are good electron acceptors or donors for PET as we
have shown in previous reports [9–14]. Fluorescein and its
derivatives, the most popular probes [15–17], however, are
almost exclusively sensitive to acidic or near neutral pH
values.
Phenolate is a proton acceptor due to the equilibrium:
PhOH ⇆ PhO- + H+, the pKa of this reaction is close to 9.5
and therefore suitable for the measurement of basic pH
values. In this study, we synthesize the phenol modified
ZnPc, measure the dependence of fluorescence on the pH
values, and elucidate the mechanism of the dependency,
which is revealed to be PET from phenolate to ZnPc.
.
.
Keywords Phthalocyanine Fluorescence Fluorescent
.
probe Photoinduced electron transfer
Introduction
Materials and Methods
Phthalocyanines (Pcs), with structures as shown in
Scheme 1, have gained widespread applications in diverse
Reagents and Apparatus
X.-F. Zhang (*)
Chemistry Department, Hebei Normal University of Science
and Technology,
Qinghuangdao, Hebei Province, China 066004
e-mail: zhangxianfu@tsinghua.org.cn
All reagents for synthesis were analytical grade and used as
received. H NMR spectra were recorded at room temper-
ature on a Bruker dmx 300 MHz NMR spectrometer. MS
spectra were recorded either on a Bruker APEX II or
1

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(Ar-OH), 3029 (Ar-H), 1636 (Ar), 1506 (Ar), 1477 (Ar), 1450
(Ar), 1331, 1233, 1194, 1125, 1091, 744. 1H NMR (DMSO-
d6, ppm): δ 9.322 (s, 1H), 8.863–8.796 (d, 1H), 8.089–8.017
(t, 1H), 7.605–7.502 (d, 1H), 7.408–7.292 (d, 2H), 6.968–
6.718 (d, 2H). MS, m/z: 1010.7 [M+H]+.
Photophysics
The absorption and fluorescence spectra, fluorescence
quantum yields and excited singlet-state lifetimes, as well
as triplet properties were investigated at room temperature.
Steady-state fluorescence spectra were acquired on a FLS 920
with 1 nm slit width for both excitation and emission
monochromators. All spectra were corrected for the sensitivity
of the photo-multiplier tube. The fluorescence quantum yield
Scheme 1 Chemical structures of adopted phthalocyanines
Autoflex III Maldi-TOF spectrometer. IR spectra were
recorded at room temperature on a Shimadzu FTIR-8900
spectrometer. UV-visible spectra were recorded on a
Shimadzu 4500 spectrophotometer using 1 cm matched
quartz cuvettes.
(Φf) was calculated by Φf ¼ FsA0 Φ0 =ðF0AsÞ, in which F
f
is the integrated fluorescence intensity, A is the absorbance at
excitation wavelength, the subscript 0 stands for a reference
compound and s represents samples. Zinc phthalocyanine
0
Synthesis of 3-(4-hydroxyphenoxy)phthalonitrile
was used as the reference ð Φf ¼ 0:30Þ [18]. Excitation
wavelengths of 615 nm corresponding to S0 to S1 transitions
were employed. The sample and reference solutions were
prepared with the same absorbance (Ai) at the excitation
wavelength (near 0.09 per cm). All solutions were air
saturated.
5.19 g (0.03 mol) 3-nitrophthalonitrile and 16.5 g
(0.15 mol) hydroquinone were added to 30 ml DMF, the
resulted solution was heated at 80°C under nitrogen for 6 h,
during which 20.7 g (0.15 mol) anhydrous potassium
carbonate was added. After cooling down, the product
solution was added to 500 ml ice-water to precipitate and
filter, the obtained solid was washed with dilute acetic acid to
pH 6.0 and dried under vacuum. 3.3 g slight yellow solid was
obtained after recrystallization with absolute ethanol. Yield
47%. m.p. 178–179°C. IR(KBr),ν(cm−1): 3402 (Ar-OH),
2236 (CN), 3029 (Ar-H), 1585 (Ar), 1508 (Ar), 1467 (Ar),
1445 (Ar), 1310, 1255, 1197, 1096, 1075, 982, 809, 791.
1H NMR (DMSO-d6, ppm): δ9.615 (s, 1H), 7.761–7.718
(t, 2H), 7.138–7.071 (q, 1H), 7.071–7.009 (d, 2H), 6.858–
6.785 (d, 2H). MS, m/z: 237.3 [M+H]+.
Fluorescence lifetime of S1 was measured by time-
correlated single photon counting method (Edinburgh
FLS920 spectrophotometer) with excitation at 672 nm
diode laser (50 ps FWHM) and emission was monitored
at 710 nm.
Transient Absorption spectra were recorded in degassed
methanol (prepared by bubbling with Argon for 20 min)
with an Edinburgh LP920 laser flash photolysis system. A
Nd:YAG laser (Brio, 355 nm and 5 ns FWHM) was used as
excitation source. The analyzing light was from a xenon
lamp. The laser and analyzing light beams perpendicularly
passed through a quartz cell with an optical path length of
1 cm. The signal was displayed and recorded on a
Tektronix TDS 3012B oscilloscope and an Edinburgh
LP920-R928B detector. The laser energy incident at the
sample was attenuated to a few mJ per pulse. Time profiles
at a series of wavelengths from which point by-point
spectra were assembled were recorded with the aid of a Pc
controlled kinetic absorption spectrometer. The concentra-
tions of the target compounds were typically 10 μM
providing A355=0.25 in a 10 mm cuvette.
Synthesis of tetra[α-(4-hydroxyphenoxy)] Zinc
Phthalocyanine, ZnPc(α-OPhOH)4
In 5 ml n-pentanol, 30 mg (4.25 mmol) lithium was added
under N2 and heated for several minutes until it completely
dissolved. 47 mg (0.21 mmol) zinc acetate, and 200 mg
(0.85 mmol) 3-(p-hydroxylphenoxy)phthalonitrile were
then added and heated at 135°C for 1 h. after cooling
down, the resulted solution was poured into 200 ml
methanol containing 1 M HCl. Methanol was removed by
rotavapor under reduced pressure and the remaining
solution was treated with chloroform to obtain green solid
product. After filtration and washing with chloroform,
acetonitrile and water, the solid was purified by column
chromatography with THF-ethanol as eluent, 64 mg dried
product was obtained, yield 30%. IR (KBr), ν (cm−1): 3419
Results and Discussion
Figure 1 shows the dependence of the fluorescence of ZnPc
(α-OPhOH)4 on pH values. The spectra were measured by
excitation at 615 nm and the concentration of ZnPc(α-OPh-

J Fluoresc (2011) 21:1559–1564
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fluorescence of ZnPc moiety. The negative charged
phenolate is a much better electron donor than its conjugate
acid, which facilitates intramolecular PET. The isosbestic
points were observed in the absorption spectra (Fig. 2),
suggesting that the dissociation equilibrium holds for pH
range from 6.4 to 8.9. At pH 9.4, however, the spectrum
deviated from the isosbestic points, indicating that only
phenolate exists under the condition. The absorption
maxima were also gradually shifted to longer wavelengths
with the increase of pH, matching the observations in the
emission spectra. This red shift is apparently due to the
stronger electron-donating effect of PhO- than that of PhOH
on ZnPc π-system.
One additional emission band around 685 nm showed up
(Fig. 1 and its inset) when pH reached the values such that
the solution is sufficiently basic and PhO− was formed, this
suggests that a new emitting species was generated in these
cases and it exists only in basic solution, so the deproto-
nated ZnPc−O−PhO−Na+ is the fluorophore responsible for
the weak emission. The dual emissive nature of the
fluorescence permits that the pH measurement can be
carried out based on the intensity ratio of two emission
bands. This single excitation-dual emission is highly
desirable for an ideal probe since this overcomes the
shortcomings in a single emissive fluorescent probe, since
parameters such as optical path length, local probe
concentration, photobleaching, and leakage from the cells
are irrelevant because both signals come from the probe in
exactly the same environment.
Fig. 1 The fluorescence spectra of ZnPc(α-OPhOH)4 at different pH
values, which were measured by excitation at 615 nm in 4.0 mM
NaH2PO4-Na2HPO4 water-DMSO (1:1.5 v/v) buffer. The concentra-
tion of ZnPc(α-OPhOH)4 was kept constant at 10 μM. The pH value
was adjusted by changing the ratio of NaH2PO4 to Na2HPO4. The
inset top is the plot of fluorescence integrated intensity against pH
values. The inset bottom is the amplified view of the emission
spectrum at pH 9.4
OH)4 at each pH was kept constant. The corresponding UV-
vis absorption spectra were displayed in Fig. 2. Due to the
easy formation of H-aggregate in pure water buffer, Water-
DMSO (1:1.5 v/v) buffer was employed as the solvent.
The decrease of emission intensity occurred upon the
increase of pH, accompanied by the red shift of fluores-
cence maximum. In addition, an abrupt drop was observed
at pH ~9.2 (inset top), close to the pKa of phenol. At pH 9.4
more than 98% fluorescence was quenched. At this pH,
phenol moiety is almost completely dissociated to pheno-
late, which is responsible for the efficient quenching of the
To firmly reveal PET in the super molecule and the
mechanism of dual emission, we next check its photo-
physical process in more detail.
Intramolecular PET in ZnPc(α-OPhONa)4
Comparison of the UV-vis absorption maximum shows
that the value is increased slightly in the order: 670 nm
for ZnPc < 690 nm for ZnPc(α-OPh)4 < 696 nm for ZnPc
(α-OPhOH)4 < 704 nm for ZnPc(α-OPhONa)4, the rank
matches the electron donating ability of the substituents. It
is the bridging oxygen atoms rather than phenyls,
however, that make the major contribution to the red shift,
since the absorption maximum of 692 nm for ZnPc(α-
OCH3)4 is greater than that of ZnPc(α-OPhOH)4 [11].
This results clearly indicate that the phenyl moieties are
not π-conjugated with the Pc unit. The shape of the
absorption spectra, on the other hand, remains the same as
that of ZnPc, suggesting that –PhOH or PhONa only
weakly interacts with ZnPc moiety in their ground states.
The molecular mechanics calculation suggests that the
phenyl moiety is almost perpendicular to the Pc ring.
Based on above results, we conclude that the electron
cloud of phenyl units is actually independent from that of
Fig. 2 The UV-vis spectra of ZnPc(α-OPhOH)4 at different pH
values in 4.0 mM NaH2PO4-Na2HPO4 water-DMSO (1:1.5 v/v)
buffer. The concentration of ZnPc(α-OPhOH)4 was kept constant at
6 μM

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Pc moiety in their ground states, which satisfy the
prerequisite for intra-molecular PET.
The fluorescence quantum yield in methanol is 0.25,
0.18 and 0.0030 for ZnPc(α-OPh)4, ZnPc(α-OPhOH)4, and
ZnPc(α-OPhONa)4, respectively, which shows that PhONa
is a much more effective quencher than PhOH.
The measurement of fluorescence lifetime, as shown
in Fig. 3, gives a further support of intra-molecular PET.
Both ZnPc(α-OPh)4 and ZnPc(α-OPhOH)4 exhibit the
typical mono-exponential decay, ZnPc(α-OPh)4 has a
slightly longer fluorescence lifetime (τf) of 3.02 than
2.92 ns of ZnPc(α-OPhOH)4. ZnPc(α-OPhONa)4, howev-
er, shows bi-exponential behavior. The short-lived com-
ponent features a τf of less than 0.20 ns (detection limit)
and is responsible for 86% photons emitted, the long one
has a τf of 3.22 ns and assume 14% of emission. The τf of
major emitting species is significantly shortened to less
than 6.8% of the value for ZnPc(α-OPhOH)4, due to
Fig. 4 Transient triplet–triplet absorption of ZnPc(α-OPhOH)4 in
argon purged methanol, 10 μM, excitation wavelength 355 nm. Inset
shows the decay at 570 nm
efficient PET from PhONa moieties to the excited ZnPc
sub-unit.
Thermodynamic analysis provides us with more firmed
evidence that PET can occur while thermal (i.e. ground
state) electron transfer is forbidden.
Thermodynamics of PET
The oxidation potential of phenol is known to be
ꢀþ
E
¼ 0:60V (vs SCE) and the value of pheno-
ðPhOH=PhOH
Þ
late is decreased to 0.40 V [19]. ZnPc(α-OPh)4 exhibits
ꢀꢁ
reduction potential at E
¼ ꢁ1:0V [20].
ðZnPc =ZnPcÞ
The electron transfer in their ground states is not
thermodynamically allowed, due to the large positive value
of free energy change (ΔG) calculated by ΔGET ¼ Eoxꢁ
Ered ꢁ C ¼ 0:40 ꢁ ðꢁ1:0Þ ꢁ 0:06 ¼ 1:34eV, in which Eox
is the oxidation potential of a donor, Ered represents the
Fig. 3 Time profile of fluorescence decay and fitting curves in
methanol with excitation at 672 nm diode laser (50ps), the emission
was monitored at 710 nm, the concentration of dyes is ca. 5.0 μM.
Top: ZnPc(α-OPhOH)4, bottom: ZnPc(α-OPhONa)4. Fitting residues
were also included under each figure. Chi squared value is 1.07 and
1.12 for top and bottom case. ZnPc(α-OPhONa)4 was obtained from
ZnPc(α-OPhOH)4 in methanol by adding 1.0 mM NaOH
Fig. 5 Transient absorption of ΖnPc(α-OPhONa)4 after 10 ns delay of
excitation in argon purged methanol containing 1 mM NaOH, 10 μM,
excitation wavelength 355 nm. Inset shows the formation and decay at
595 nm

J Fluoresc (2011) 21:1559–1564
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reduction potential of an acceptor, and C is a small constant
associated with solvent.
Fig. 4 is obvious. The 630, 595 and 480 nm positive
absorptions bands are assigned to the anion radical of the
phthalocyanine [20], the fitting of decay at 595 nm gives a
lifetime of 5.6 ns. The positive 520 nm band is due to
phenoxy radical [19, 23]. These positive bands are
accompanied by the current occurrence of negative bands
due to the bleaching of ground state of ZnPc moiety,
indicating that the positive bands of charge separation state
is indeed originated from the light excitation of ground
state.
PET from phenolate to S1 of ZnPc moiety (by the photo
excitation of the Pc moiety), on the other hand, is
thermodynamically allowed, because its ΔG is a negative
value obtained by: ΔGPET ¼ ΔGET ꢁ E00 (S1 excitation
energy of ZnPc)=1.34–1.75 = − 0.41 eV, which is more
favorable than the case of phenol: ΔGPET=−0.21 eV. PET
from Phenolate to T1 of ZnPc moiety is forbidden, since the
energy of the T1 state is 0.99 eV [21], which is fairly low
and suggesting a ΔG of +0.34 eV.
Efficiency and Kinetics of PET
Conclusion
The rate constant of PET (ket) in ZnPc(α-OPhONa)4 can
be calculated from the fluorescence quantum yield and
lifetime data using Eq. 1, in which Φ0f and tf0 is the
fluorescence quantum yield and lifetime of the model
compound, ZnPc(α-OPh)4, while Φf and τf are the values
for ZnPc(α-OPhONa)4. ket is thus obtained as 2.29×1010 s−1
in ZnPc(α-OPhONa)4 and 1.25×108 s−1 for (α-OPhOH)4.
The fluorescence of ΖnPc(α-OPhOH)4, shows profound
dependency on pH values. Very efficient intra-molecular
PET occurs in the phenol-substituted zinc phthalocyanine,
ΖnPc(α-OPhONa)4, upon photoexcitation which quenches
its fluorescence. Its conjugate acid, ΖnPc(α-OPhOH)4, on
the other hand, shows fairly weak PET and therefore highly
fluorescent. Upon pH increase in solution containing ΖnPc
(α-OPhOH)4, the fluorescence is switched off at pH 9.1. Dual
emission bands were observed when pH>7, one is attributed
to the ZnPc moiety, while the other is from the anion of ZnPc
generated by PET. The results suggest that ΖnPc(α-OPhOH)4
may be used as a fluorescent pH indicator.
ð Φ0f = Φf Þ ꢁ 1
ket ¼ tꢁf 1 ꢁ ðt0f Þꢁ1
¼
ð1Þ
tf0
Efficiency for PET (Φet) was calculated to be 99% and
14% , respectively, by using the Φf value
Acknowledgement We thank the Natural Science Foundation of
Hebei Province (Contract B2010001518) and HBUST for financial
support.
À
À
0
Φet ¼ 1 ꢁ Φf = Φf ÞÞ, which indicates that PET is very
efficient in ZnPc(α-OPhONa)4. kf (the rate constant of
fluorescence emission) is 0.83×108 s−1 for ZnPc (computed
by kf ¼ Φf =tf ), which is much slower than the rate of PET.
References
k
isc (the rate constant of inter system crossing or ISC) of ZnPc
is 0.20×109 s−1 (Φisc/τf) [11], smaller than 1% of ket. This
suggests that triplet generation by ISC in ZnPc(α-OPhONa)4
would be negligible.
1. Zagal JH, Griveau S, Silva JF, Nyokong T, Bedioui F (2010)
Metallophthalocyanine-based molecular materials as catalysts for
electrochemical reactions. Coord Chem Rev 254:2755–2791
2. Smith KM, Kadish KM, Guilard R (2003) The porphyrin
handbook. Elsevier
3. de la Torre G, Bottari G, Hahn U, Torres T. Functional
phthalocyanines: synthesis, nanostructuration, and electro-optical
applications. In: Jiang J ed, Functional Phthalocyanine Molecular
Materials. 1 ed. Heidelberg: Springer, pp 1–44
4. Claessens CG, Hahn U, Torres T (2008) Phthalocyanines: from
outstanding electronic properties to emerging applications. Chem
Rec 8:75–97
5. Alzeer J, Luedtke NW (2010) pH-mediated fluorescence and
G-quadruplex binding of amido phthalocyanines. Biochemistry
49:4339–4348
6. Bohrer FI, Colesniuc CN, Park J, Schuller IK, Kummel AC, Trogler
WC (2008) Selective detection of vapor phase hydrogen peroxide with
phthalocyanine chemiresistors. J Am Chem Soc 130:3712–3713
7. Mashazia P, Togoc C, Limsonc J, Nyokong T (2010) Applications of
polymerized metal tetra-amino phthalocya nines towards hydrogen
peroxide detection. J Porphyrins Phthalocyanines 14:252–263
8. Paoletti AM, Pennesi G, Rossi G, Generosi A, Paci B, Albertini
VR (2009) Titanium and ruthenium phthalocyanines for NO2
sensors: a mini-review. Sensors 9:5277–5297
Transient Absorption Studies
Transient absorption spectra were recorded in argon
saturated methanol solutions upon laser excitation of
355 nm (5 ns pulse). Figure 4 displays the μs-scale spectra
for ZnPc(α-OPhOH)4. It exhibited the typical triplet-triplet
(T1-Tn) transient absorptions as reported [22] with abs.
maximum at 565 nm. The fitting of transient decay by
mono-exponential function gave a lifetime (τT) of 38 μs in
the absence of oxygen, but it is shortened to 0.11 μs in the
air saturated solution, a typical feature of triplet signal.
On the same μs time scale, however, no signal could be
detected for ZnPc(α-OPhONa)4, this is due to the efficient
quenching of S1 by PET, so that no remarkable T1 state was
formed. TA absorption on ns scale, on the other hand, could
be recorded and is shown in Fig. 5. The difference from

1564
9. Zhang XF, Cui X, Liu Q, Zhang F (2008) Multiple-charge
J Fluoresc (2011) 21:1559–1564
16. APd S, Moody TS, Wright GD (2009) Fluorescent PET
(Photoinduced Electron Transfer) sensors as potent analytical
tools. Analyst 134:2385–2393
separation in nanoscale artificial photosynthetic models. Chem-
physchem 9:1514–1518
10. Zhang XF, Cui X, Liu Q, Zhang F (2009) Photoinduced multi-
electron transfer in the Dn–A system consisting of multi-
phthalocyanines linked to one carbon nanotube. PhysChem Chem
Phys 11:3566–3572
11. Zhang XF, Di Y, Zhang F (2009) Photoinduced single-and double-
electron transfer in a photosynthetic model consisting of one-
acceptor with four equally linked donors (D4-A). J Photochem
Photobiol, A 203:216–221
17. Han J, Burgess K (2010) Fluorescent indicators for intracellular
pH. Chem Rev 110:2709–2728
18. Montalti M, Credi A, Prodi L, Gandolfi MT (2006) Photophysical
properties of organic compounds. In: Handbook of photochemis-
try, 3rd edn. CRC, pp 255
19. Steenken S, Neta P (1982) One-electron redox potentials of
phenols. Hydroxy-and aminophenols and related compounds of
biological interest. J Phys Chem 86:3661–3667
12. Zhang XF, Li X, Niu L, Sun L, Liu L (2009) Charge transfer
photophysics of tetra (α-amino) zinc phthalocyanine. J Fluoresc
19:947–954
13. Zhang XF, Zheng H (2010) Tetra (β-Phenothiazinyl) zinc
phthalocyanine: an easily prepared D4-A system for efficient
photo-induced electron transfer. Inorg Chim Acta 363:2259–
2263
14. Zhang XF, Zheng H, Jin S, Wang R (2010) Photoinduced electron
transfer (PET) within D4-A and D-A photosynthetic systems:
enhanced intramolecular PET achieved by increasing the number
of donors. Dyes Pigm 87:139–141
15. Lavis LD, Raines RT (2008) Bright ideas for chemical biology.
ACS Chem Biol 3:142–155
20. Ou Z, Jiang Z, Chen N, Huang J, Shen J, Kadish KM (2008)
Electrochemistry and spectroelectrochemistry of tetra-α-substituted
metallophthalocyanines. J Porphyrins Phthalocyanines 12:1123–1133
21. Carmichael I, Hug GL (1986) Tripet-triplet absorption spectra of
organic molecules in condensed phase. J Phys Chem Ref Data
15:1–250
22. Ishii K, Kobayashi N (2003) The photophysical properties of
phthalocyanines and related compounds. In: Kadish KM, Smith
RM, Guilard R (eds) The porphyrin handbook. Academic, San
Diego, pp 1–42
23. Abdullah J, Ahmad M, Heng LY, Karuppiah N, Sidek H (2007) An
optical biosensor based on immobilization of laccase and MBTH in
stacked films for the detection of catechol. Sensors 7:2238–2250