Photophysical and photochemical parameters of octakis (benzylthio) phthalocyaninato zinc, aluminium and tin: Red shift index concept in solvent effect on the ground state absorption of zinc phthalocyanine derivatives

Article abstract of DOI:10.1016/j.molstruc.2010.08.033

This paper addresses the synthesis of octa-substituted benzylthio metallophthalocyanines (OBTMPcs) that contain the central metal ions of Zn ²⁺, Al³⁺ and Sn⁴⁺. The ground state absorption of ZnPc(SR)₈ (OBTZnPc) along with the ZnPc derivatives, well documented in literature were used to study a new concept called the red shift index (R_sI). The concept is based on the empirical values of R _sI of the different complexes in solvent media. Unequivocally, parameters used in this paper show strong correlations that are consistent with the results obtained. For instance, R_sI of the complexes tend to increase as the refractive index, n_D, and solvent donor, DN, of solvent increases. Photodegradation (photobleaching) quantum yield, φ_d measurements of these compounds show that they are highly photostable, φ_d (0.03-0.33 × 10^-5). The triplet quantum yield, φ_T (0.40-0.53) and the triplet lifetime, τ_T (610-810 μs) are within the typical range for metallophthalocyanines in DMSO. The photosensitisation efficiency, S _Δ, is relatively high for all the molecules (0.74-0.90).

Full text of DOI:10.1016/j.molstruc.2010.08.033

Journal of Molecular Structure 984 (2010) 1–14
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Photophysical and photochemical parameters of octakis (benzylthio)
phthalocyaninato zinc, aluminium and tin: Red shift index concept in solvent
effect on the ground state absorption of zinc phthalocyanine derivatives
a,b,c,
⇑
b
c
a
Victor Akpe
, Hjalmar Brismar , Tebello Nyokong , P.O. Osadebe
a
Department of Pharmaceutical and Medicinal Chemistry, University of Nigeria, Nsukka, Nigeria
Cell Physics Division, Department of Applied Physics, Royal Institute of Technology, S-106 91, Stockholm, Sweden
Department of Chemistry, Rhodes University, P.O. Box 94, Grahamstown 6140, South Africa
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 2 May 2010
Received in revised form 13 August 2010
Accepted 16 August 2010
Available online 21 August 2010
This paper addresses the synthesis of octa-substituted benzylthio metallophthalocyanines (OBTMPcs)
2+
3+
4+
that contain the central metal ions of Zn , Al and Sn . The ground state absorption of ZnPc(SR)
OBTZnPc) along with the ZnPc derivatives, well documented in literature were used to study a new
concept called the red shift index (R I). The concept is based on the empirical values of R I of the different
complexes in solvent media. Unequivocally, parameters used in this paper show strong correlations that
are consistent with the results obtained. For instance, R I of the complexes tend to increase as the refrac-
tive index, n , and solvent donor, DN, of solvent increases. Photodegradation (photobleaching) quantum
yield, / measurements of these compounds show that they are highly photostable, /
(0.03–0.33 ꢀ
). The triplet quantum yield, / (0.40–0.53) and the triplet lifetime, (610–810 s) are within the
, is relatively high
8
(
s
s
s
Keywords:
Solvent effect
Polarisation red shift
Thioether phthalocyanine
Photosensitiser
D
d
d
ꢁ5
10
T
s
T
l
typical range for metallophthalocyanines in DMSO. The photosensitisation efficiency, S
D
Photophysics and photochemistry
for all the molecules (0.74–0.90).
Ó 2010 Elsevier B.V. All rights reserved.
1
. Introduction
ysis [3–5], photoactivation and photosensitisation [6–9], non-lin-
ear optics (NLO) [10,11], and gas sensing devices [12,13].
Metallophthalocyanines (MPcs) are among the class of organic
MPcs aggregate in solutions [14–17] and aggregation diminishes
their photosensitising abilities. In some cases, aggregation may re-
sult in low singlet oxygen production or reduced lifetime. Aggrega-
tion in MPc complexes is caused by electronic interactions between
neighbouring rings. The addition of electron donating groups to the
periphery of the macrocycle increases the distance between the pla-
dyes that show an intense blue–green colour, with extinction
5
ꢁ1
ꢁ1
coefficients greater than 10 M cm . They have two distinct
bands; the Q-band in the visible region (650–700 nm) and the B
(
Soret) band in the UV region (300–350 nm). These bands are
transitions that arise from the highest occupied molecular orbital
HOMO) and the lowest unoccupied molecular orbital (LUMO),
(
nar macrocycle rings carrying the p-electrons, thereby making the
where the Q and B bands depend largely on the nature of metallic
substitution and the type of group present at the periphery of the
phthalocyanine (Pc) compound. Structurally, MPcs have a well-or-
solvation process easier. Thioether substitution (SR) in the phthalo-
cyanine moiety is particularly suited for this purpose, as they are
highly electron rich. Recently, the aggregation effect in MPcs has
also been documented, showing some interesting optical uses.
Kenta Adachi et al. [18a–d] documented a series of reports of thioe-
ther MPc complexes in the aggregated state in mixed solvents. In
one of their reports, (OBTMgPcs) was shown as a complex resource
that can be tuned optically when induced by Pd -BINAP complex at
the liquid–liquid interface. Despite interesting findings by Kenta
Adachi et al. and other literature work on thioether MPcs in the last
decade [19–22], to our knowledge, little is known about the solvent
parameters that affect their ground state absorption, photophysics
and photochemistry.
dered macrocyclic backbone that contains 18
p-electron systems.
ꢂ
The
p–p
stacking interactions, coupled with the facile substitu-
tion at the periphery of the phthalocyanine ring, and the nature
of central metal ions introduced during cyclotetramerisation of
the phthalocyanine (Pc) can indeed have an overall effect on
the chemical and physical properties. Thus MPcs can be tailored
to a wide variety of applications. For example, in recent years
they have received extensive study in catalysis [1,2], photocatal-
2
+
This work investigates the photophysical and photochemical
properties of octa-substituted benzylthio metallophthalocyanines
(OBTMPcs) in DMSO. The ground state absorption of (OBTZnPcs)
⇑
Corresponding author at: Cell Physics Division, Department of Applied Physics,
Royal Institute of Technology, S-106 91, Stockholm, Sweden.
E-mail address: victorakpe@gmail.com (V. Akpe).
0
022-2860/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2010.08.033

2
V. Akpe et al. / Journal of Molecular Structure 984 (2010) 1–14
along with the ZnPc derivatives, which is well documented in liter-
ature are also investigated in different solvent media. The red shift
added in portions over a period of 2 h while maintaining the inert
atmosphere and with constant stirring to obtain a homogenous
mixture. The reaction mixture was stirred under nitrogen at ambi-
ent temperature for additional 12 h. The reaction mixture was
index (R
interactions as a macroscopic entity, thus enabling good empirical
deductions. The R I values may provide a useful empirical guide in
s
I), an entirely a new concept, explains the solute–solvent
3
s
quenched with water (ꢃ110 cm ) and the aqueous phase extracted
the selection of compounds intended for use in optical chemical
sensing materials, and may also find use as solvent indicators.
Overall, this work shows three key ways in which this class of
dyes may be employed, namely: as photocatalysts due to their
low photodegradation yields and high triplet lifetime; as photosen-
sitisers due to the calculated values of their photosensitisation effi-
ciency and the photon yields per molecule they generate; and
lastly as advanced optical chemical sensing materials based on
the excellent spectroscopic trends of OBTZnPc in both single and
mixed solvents.
with chloroform. The chloroform portion was then treated with 5%
sodium carbonate solution to remove the excess phenyl metha-
nethiol, followed by additional treatment with water to remove
unreacted potassium carbonate and potassium chloride. The
resulting product was then dried over anhydrous sodium sulphate
to yield an oily product. The oily product was dried by evaporating
the inherent chloroform. The final product (compound 2) was crys-
tallised from absolute ethanol and dried in a dessicator (silica gel)
to yield golden yellow needle-like crystals: yield: 10.1 g (80%) m.p.
m H
/cm : 2228 (C–Nstr) prominent peak, 1
ꢁ
1
190–191 ꢀC . IR (KBr)
NMR (DMSO-d )/ppm: 8.20 (s, 2 H, benzyl), 7.30 (m, 10 H, phenyl),
.30 (s, 4 H, methylene).
6
4
2
. Experimental procedures
2
.3.2. Synthesis of octakis(benzylthio)phthalocyaninato lithium,
OBTLiPc (3)
Lithium metal (12 mg, 0.0081 mmol) was suspended in 8 mL
2
.1. Materials
4
,5-dichloro-1,2-dicyanobenzene, zinc acetate, anhydrous
n-pentanol under nitrogen and was refluxed at 90 °C, while being
stirred, for 2 h until a homogeneous mixture containing lithium
pentanoalate was formed. The mixture was allowed cooled to
room temperature followed by addition of 30 mg (0.081 mmol)
of compound 2 (1,2-(benzylthio)-4,5-dicyanobenzene) which was
previously suspended in n-pentanol. The temperature was steadily
raised to 135 °C while stirring. The reaction mixture was stirred for
an additional 2 h and monitored at intervals using a UV–vis. spec-
trophotometer for the formation of the Q-band for the lithium
complex, compound 3. The complex was not isolated but employed
in the formation of compounds 4–6, as explained in the following
sections.
tin(IV)chloride, aluminium monoacetate ((HO) AlC ), dimeth-
2
2 3 2
H O
ylsulfoxide (DMSO), 1,3-diphenylisobenzofuran (DPBF), and alu-
minium oxide (Type WN-3, neutral) were purchased from Sigma–
0
Aldrich. N,N dimethylformamide (DMF), ammonium hydroxide
(
25% and 35% solutions), lithium metal, and glacial acetic acid were
purchased from SAARChem. Potassium carbonate (anhydrous), so-
dium sulphate (anhydrous) and diazabicyclo[2.2.2]octan (DABCO)
were purchased from Merck. Phenylmethanethiol and 1-pentanol
were purchased from Fluka. DMSO was dried over alumina, and
DMF was freshly distilled before use.
2.2. Methods
2
.3.3. Synthesis of octakis(benzylthio)phthalocyaninato zinc,
UV–vis. spectra were recorded on a Varian 500 UV/Visible/NIR
OBTZnPc (4)
spectrophotometer. Nitrogen gas was purchased from Afrox. Fluo-
rescence excitation and emission spectra were recorded on a Var-
ian Eclipse spectrofluorimeter. Photo-irradiation was done using a
General Electric Quartzline lamp (300 W). A 600 nm glass cut-off
filter (Schott) and a water filter were used to filter off ultraviolet
and infrared radiations respectively. A 700 nm interference filter,
with a bandwidth of 40 nm, was also placed in the light path before
the sample. Light intensities were measured with a PowerMax
To the mixture containing OBTLiPc, compound 3, a solution of
ꢁ4
anhydrous zinc acetate (64 mg, 3.488 ꢀ 10 mol) in dry DMSO
was added and refluxed for 45 min until a deep green colour of
OBTZnPc, compound 4 was observed. The formation of compound
4
was monitored with a UV–vis. spectrophotometer, and the reac-
tion was deemed complete when the Q-band position of com-
pound disappeared and new Q-band position due to
3
a
compound 4 was formed. The reaction mixture was cooled to room
temperature. The Pentanol was removed under reduced pressure
and hexane (1 mL) was added to the mixture to precipitate the
compound. The product was collected by vacuum filtration and
5
100 (Molelectron Detector Inc.) power meter. Triplet absorption
and decay kinetics were recorded on a laser flash photolysis sys-
tem, and the excitation pulses were produced by Nd: YAG laser
(
Quanta-Ray, 1.5 J/90 ns) pumping a tunable dye laser (Lambda
washed with hot EtOH (2 ꢀ 5 mL) and then dried in under vacuum.
Physic FL 3002, pyridine 1 dye in methanol). The analysing beam
source was from a Thermo Oriel xenon arc lamp, and a photomul-
tiplier tube was used as a detector. Signals were recorded with a
two-channel digital real-time oscilloscope (Tektronix TDS 360);
the kinetic curves were averaged over 256 laser pulses.
ꢁ1
Compound 4: Yield: 65%. IR (KBr)
m
/cm : 2916 (C–Cstr), 1589
(
(
C@Cstr), 695 (C–Sstr). UV–vis. (DMSO) kmax (nm) (log
e
): 373
1
6
4.87), 634 (4.48), 710 (5.33). H NMR (DMSO-d )/ppm: 8.29 (s, 8
H, Pc), 7.99 (m, 40 H, phenyl), 4.48 (m, 16 H, methylene). MALDI-
+
TOF (m/z): Molecular mass of 4: 1555.42 g/mol, found: {{M + 1} ,
1
556.42 g/mol, (100).
2
2
.3. Synthesis and characterisation
2
.3.4. Synthesis of octakis(benzylthio)phthalocyaninato aluminium,
.3.1. Synthesis of 1,2-(benzylthio)-4,5-dicyanobenzene (2)
OBTAlPcOH (5)
Compound 1, 4,5-dichloro-1,2-dicyanobenzene was synthesised
A solution of aluminium acetate (25 mg, 0.0162 mmol) in n-
pentanol was added to the mixture containing compound 3, and
the solution was refluxed for 1 hour until a deep blue colour was
observed. The complete formation of the OBTAlPc was confirmed
by monitoring the disappearance of the split Q-bands related to
the metal free Pc with the simultaneous formation of a well de-
fined Q-band corresponding to OBTAlPcOH. The mixture was
cooled to room temperature. The solid product was washed in
hot EtOH (2 ꢀ 3 mL) collected by centrifuge, and dried in a desicca-
according to literature methods [22]. Compound 2 was a modifica-
tion of the previously procedure reported in the literature [23].
Phenyl methanethiol (8420 mg, 68 mmol) was dissolved in dry
3
DMSO (35 cm ) under nitrogen, and compound 1 (5 910 mg,
3
0 mmol) was added while gently stirring the mixture under inert
nitrogen condition. The mixture was then stirred for additional
0 min after compound 1 was completely added. Finely ground
anhydrous potassium carbonate (15,000 mg, 109 mmol) was
3

V. Akpe et al. / Journal of Molecular Structure 984 (2010) 1–14
3
ꢁ
tor. Compound 5: yield: 69%. IR (KBr)
m
/cm 1: 3461 (O–Hstr), 2959
2complexes were characterised by several methods including
UV–vis., IR, and H NMR spectroscopies and MALDI-TOF mass spec-
1
(
(
C–Cstr), 1543 (C@Cstr), 669 (C–Sstr). UV–vis. (DMSO) kmax (nm)
1
log
e
): 379 (4.807), 627 (4.31), 699 (5.14). H NMR (DMSO-d
ppm: 9.18 (s, 8 H, Pc), 7.70 (m, 40 H, Phenyl), 4.76 (m, 16 H, meth-
ylene). MALDI-TOF (m/z): Molecular mass of 4: 1551.06 g/mol,
6
)/
troscopy. As shown in the experimental section, the mass spectral
data (Maldi-TOF) for all the MPcs showed prominent molecular ion
peaks.
The UV–vis. spectra of OBTMPcs: 4, 5 and 6 are shown in Fig. 1.
The Q-bands occurred at 710 nm (for compound 4, OBTZnPc),
+
found: {{M + 2} , 1553.06 g/mol.
6
99 nm (for compound 5, OBTAlPcOH) and 703 nm (for compound
2
.3.5. Synthesis of octakis(benzylthio)phthalocyaninato tin,
6, OBTSnPcCl ). The compounds obeyed Beer’s law at concentra-
2
OBTSnPcCl (6)
2
ꢁ5
ꢁ3
tions less than 1 ꢀ 10 mol dm
Disappearance of the characteristic CN stretch at 2228 cm
from compound 2, Fig. 2) for all the OBTMPcs indicates complete
.
A solution of tin (II) chloride (105 mg, 0.0647 mmol) in n-pent-
anol was added to the mixture containing compound 3 and re-
fluxed for one hour until a deep green colour was observed. The
reaction was monitored with a UV–vis. spectrophotometer, as for
compound 4 above. The mixture was cooled to room temperature.
Hexane (1 mL) was added and the solid product precipitated out.
The resultant product (Compound 6) was collected by vacuum fil-
tration, washed with hot methanol (1 ꢀ 4 mLs) and immediately
ꢁ
1
(
cyclotetramerisation of the Pc ring (Fig. 3).
1
6
The H NMR spectra of OBTMPcs (4–6) in deuterated DMSO-d
exhibit patterns consistent with metallophthalocyanines. The 1
NMR peaks of compounds: 4, OBTZnPc; 5, OBTAlPcOH and 6,
OBTSnPcCl of the aromatic protons on the phenyl groups ap-
peared at 8.29, 9.18, and 9.18 ppm, and methylene protons ap-
peared at 4.48, 4.76 and 4.76 ppm, respectively. The large upfield
shift in the proton is caused by the influence of the diamagnetic
ring currents of OBTMPc, in the shielding core. Consequently, sig-
nals are shifted upfield and the magnitude depends on the distance
between a given proton and the Pc moiety. This effect is particu-
larly pronounced in OBTZnPc because of the relative net contribu-
H
2
ꢁ
1
dried in a dessicator. Compound 6: Yield: 73%. IR (KBr)
2
m/cm :
915 (C–Cstr), 1562 (C@Cstr), 687 (C–Sstr), 327 (Sn–Clstr). UV–vis.
1
(
DMSO) kmax (nm) (log
e
): 377 (4.56), 6304 (3.97), 703 (4.86).
H
NMR (DMSO-d
4
4
6
)/ppm: 9.18 (s, 8 H, Pc), 7.76 (m, 40 H, phenyl),
.76 (m, 16 H, methylene). MALDI-TOF (m/z): Molecular mass of
+
: 1657.5 g/mol, found: {{M + 2} , 1659.5 g/mol.
tion of electronegativity changes and Pp–dp back donation
between the Pc ring and the central atom (Zn). The Pp–dp back
3
. Results and discussion
donation is explained in reference [24].
3.1. Synthesis and characterisation
The synthesis of the octakis (benzylthio) phthalocyanato zinc,
3.2. Solvent effects of the UV–vis absorption spectra of OBTZnPc
aluminium and tin were carried out using established procedure
in the literature [23] illustrated in Scheme 1.
Compound 2 was synthesised according to the procedure
described in literature [23] involving the conversion of 4,5-di-
chloro-1,2-dicyanobenzene (1) with an 80% yield. The MPc
The electronic absorption spectrum of an organic dye is primar-
ily dependent on the type of chromophore, the nature of transition
ꢂ
ꢂ
(in MPc:
p ! p (Q-band), n ! p (Soret or B band) and charge
transfer absorptions. Solvent effects on electronic absorption spec-
Cl
Cl
CN
CN
RS
RS
CN
CN
Li
HSCH Ph
2
Li Pc
DMSO
K CO
2
Pentanol
2
3
(
1)
(2)
(3)
RS
SR
N
N
N
SR
RS
RS
Li Pc
2
N
M
N
Metal Salt
(
3)
SR
N
NH
N
RS
SR
4.M = Zn
5
.M = Al(OH)
.M = Sn(Cl2)
R = CH
2
6
2
Scheme 1. Synthesis of complexes 4: OBTZnPc; 5: OBTAlOH; 6: OBTSnPcCl .

4
V. Akpe et al. / Journal of Molecular Structure 984 (2010) 1–14
mixture where OBTZnPc exhibits the highest monomeric behaviour,
the following criteria discussed in reference [28] were partly used:
1) the solvent component of the higher refractive index should be a
(
very good solvent of the given polymer (OBTZnPc), while the lower
refractive index should be an effective participant in the polymer;
(
2) the solvent with the higher refractive index should exhibit a bet-
ter affinity for the polymer; (3) linear relationships exist between
the refractive index measurements of single solvent and mixed sol-
vents at 20 °C [29]; (4) hydrogen-bonding interaction between sol-
vent and polymer is assumed to be absent.
Bayliss et al. [25] qualitatively described three factors that could
possibly affect the ground state electronic absorption spectra.
These are: dipole, polarisation and hydrogen-bonding forces,
which they summarised as the momentary transition dipole during
the optical absorption process, the difference between the perma-
nent dipole moment(between the ground and the excited states of
the solute) and the effect of the Franck–Condon principle. The lat-
ter two factors are assumed to be non-operative in the ground
state electronic absorption spectra of the complex investigated,
or are negligible factors that can be ignored. The only operative
force is assumed to be the momentary polarisation that is induced
in the solvent by the transition dipole of OBTZnPc, which can be
interpreted as the red shift and is a function of the solvent refrac-
tive index. The polarisation red shift of ZnPc has been reported [26]
but for the purpose of this study, the absorption spectra at the Q-
band of ZnPc were determined in nine selected solvents using a
UV–vis spectrophotometer.
Fig. 1. Electronic absorption spectra in DMSO: 4 = OBTZnPc (kmax = 710 nm);
5
2
= OBTAlPcOH (kmax = 699 nm); 6 = OBTSnPcCl (kmax = 703 nm). Concentration =
ꢁ
6
ꢁ3
ꢃ1 ꢀ 10 mol dm
.
tra of donor-substituted thioether derivatives of OBTZnPc were
investigated in eight selected solvents. The data (Table 1) were
evaluated following literature methods [25–27]. The refractive
indices of the solvent mixture were calculated using the equation
below:
n
1
V
V
1
þ n
2
V
2
þ ꢄ ꢄ ꢄ n
n n
V
mixn
¼
ð1Þ
D
1
þ V
2
þ ꢄ ꢄ ꢄ V
n
where mix
n
is the refractive index of the solvent mixture and n
and
and
D
1
2
n are the refractive indices of solvents 1 and 2 respectively. V
1
V
2
are the volumes of solvents 1 and 2 respectively. OBTZnPc
Table 1 shows the polarisation red shift behaviour of OBTZnPc
in different solvents. The polarisation red shift of a substituted
MPc can be calculated, provided the Q-band (kmax) of the unsubsti-
tuted MPc is known in the vapour phase. In addition, the Q-band of
the unsubstituted MPc in solution for any given solvent must be
spectra in single and mixed solvents showed monomer peaks at
ꢁ6
ꢁ3
the Q-bands with concentrations as low as <1 ꢀ 10 mol dm , ex-
cept in a water/pyridine mixture, where aggregation behaviour was
noticed (not shown as a figure legend). In the search for a solvent
(
a)
(b)
2
Fig. 2. (a) Polarisation red shift of OBTZnPc in single solvents. Solvents: 1: pyridine; 2: DMSO; 3: DMF; 4: THF. F = 2n
ꢁ1
, n is the refractive index of solvent and (b) polarisation
n
2
þ1
red shift of OBTZnPc in mixed solvents. Solvents: a: benzene/pyridine; b: toluene/pyridine; c: chloroform/pyridine; d: petroleum ether/pyridine.
(
a)
(b)
Fig. 3. (a) Q-band dependence on the refractive index of single solvents (OBTZnPc in single solvents) and (b) Q-band dependence on the refractive index of mixed solvents. a:
petroleum ether/pyridine; b: chloroform/pyridine; c: toluene/pyridine; d: benzene/pyridine (OBTZnPc in mixed solvents).

V. Akpe et al. / Journal of Molecular Structure 984 (2010) 1–14
5
Table 1
Solvent effects on the electronic absorption spectra of OBTZnPc in various solvents. kmax (at the Q-band) for the unsubstituted ZnPc in vapour = 660 nm [30].
2
Solvent
Refractive
index, nD20
Solvent donor
number (DN)
Dipole
moment,
Q-band kmax
(nm) ZnPc
Q-band kmax (nm)
OBTZnPc
D
ZnPc
k
a
(nm)
D
k
b
(nm)
F = 2
n
ꢁ1
þ1
n
2
l
OBTZnPc
DMSO
DMF
THF
1.479
1.430
1.406
29.8
26.6
20.0
33.1
3.96
3.79
1.69
2.21
(0.00, 2.21)
(1.90, 2.21)
672
670
667
674
672
673
710
705
704
713
711
710
12
10
7
14
12
13
50
45
44
53
51
50
0.2209
0.2053
0.1972
0.2299
0.2285
0.2157
Pyridine
Benzene/pyridine (2:1) 1.504
Chloroform/pyridine
1.509
a
e
b
–
a
e
c
1.462
–
(
2:1)
Toluene/pyridine (2:1) 1.501a
–
e
(0.38, 2.21)
(– , 2.21)
671d
666
711
708
11
6
51
47
0.2276
0.1996
a
e
e
Petroleum ether/
pyridine (2:1)
O/pyridine (2:1)
1.413
–
H
2
1.3915a
Agg
(1.87, 2.21)
Agg
Agg
–e
–e
–e
ZnPcsolution
ZnPcvapour
D
D
k
k
a
b
polarisation red shift of ZnPc. Values calculated using;
polarisation red shift of OBTZnPc. Values calculated using;
D
k
a
= k
ꢁ kmax
.
ZnPcvapour
max
OBTZsolution
D
k
b
= k
ꢁ kmax
.
max
Agg = aggregation.
a
Refractive index of solvent mixture calculated using Eq. (1).
b
c
k
k
k
max for ZnPc measured in benzene.
max for ZnPc measured in chloroform.
max for ZnPc measured in toluene.
d
e
Not found.
greater than the Q-band in the vapour phase. The values obtained
are approximate. Ideally the polarisation red shift is the difference
between the Q-band in solution and the Q-band in the vapour
phase. The equations below show an empirical method for calcu-
lating the polarisation red shift of any given substituted MPc.
refractive index was also observed in both single and mixed sol-
vents, indicating a Q-band dependence on the solvent’s refractive
index, as shown in Eqs. (6a) and (6b) (Fig. 3a and b).
2
k
max ¼ 90:421n þ 576:35 with R ¼ 0:9863
ðOBTZnPcÞðin single solventsÞ
ð6aÞ
ð6bÞ
MPcðsubstitutedÞsolution
MPcðsubstitutedÞsolution
MPcðunsubstitutedÞvapour
MPcðunsubstitutedÞvapour
2
D
k
¼ k
k
ꢁ kmax
ꢁ kmax
ð2Þ
k ð3Þ
ð4Þ
max
k
max ¼ 43:462n þ 645:86 with R ¼ 0:9541
ꢀ
ꢁ
MPcðsubstitutedÞsolution
MPcðunsubstitutedÞsolution
0
k
¼
þ
D
ðOBTZnPcÞðin mixed solventsÞ
max
max
D
k0 ¼ k
MPcðsubstitutedÞsolution
ꢁ k
MPcðunsubstitutedÞsolution
Linear expressions are also shown for the following parameters
in Eqs. (7a) and (7b) (Figs. 4a and b):
max
max
0
where
D
k is the increased value brought about by the interaction of
2
D
k ¼ 0:6973DN þ 28:912 with R ¼ 0:8432
the solvent and the substituted electron donating group in the MPc
moiety. Substituting Eq. (4) for Eq. (3) gives Eq. (2). The polarisation
red shift of OBTZnPc is therefore the difference between the Q-band
of OBTZnPc in solution and the Q-band of unsubstituted ZnPc in the
vapour phase.
ðOBTZnPcÞðin single solventsÞ
ð7aÞ
ð7bÞ
2
k
max ¼ 0:6973DN þ 688:91 with R ¼ 0:8432
ðOBTZnPcÞðin single solventsÞ
but there was no direct correlation between the polarisation red
shift (or the Q-band) and the solvent’s dipole moment.
2
D
D
k ¼ ꢁ12:807F þ 108:25 with R ¼ 0:972
ðOBTZnPcÞðin single solventsÞ
ð5aÞ
ð5bÞ
2
3.3. Interpretation of the photophysical and photochemical data
k ¼ ꢁ6:3289F þ 78:89 with R ¼ 0:9745
ðOBTZnPcÞðin mixed solventsÞ
The photophysical and photochemical parameters of photo-
The two plots (Fig. 2a and b) suggest that the polarisation red
sensitising dyes can be measured in terms of the light harvesting
property of the dye (i.e. molar extinction coefficient ( ), fluores-
cence quantum yield (/ ), fluorescence lifetime ( ), triplet quan-
tum yield (/ ), triplet lifetime ( ), singlet oxygen quantum yield
(/ ), phosphorescence quantum yield(/ ), photosensitising
shifts are largely due to solvation, and are expressed by the equa-
tions above ((5a) and (5b)). R is the correlation coefficient and
F(OBTZnPc) is a constant for a particular solvent or solvent mix
at a given volume ratio. A linear variation of the Q-band with
e
F
s
F
T
s
T
D
p
(a)
(b)
Fig. 4. (a) Variation of red shift at the Q-band with solvent’s coordination no. (DN) (OBTZnPc) and (b) variation of the Q-band with solvent’s coordination no. (DN) (OBTZnPc).

6
V. Akpe et al. / Journal of Molecular Structure 984 (2010) 1–14
efficiency (S
D
). All three complexes were measured in dimethyl
were further filtered before transferring into a standard spectro-
photometer cell.
sulphoxide (DMSO) at the room temperature and the solutions
(
a)
(b)
s s
Fig. 5. (a) Polarisation red shift vs. red shift index (R I) (OBTZnPc) and (b) Q-band vs. red shift index (R I) (OBTZnPc).
(a)
(b)
(c)
Fig. 6. (a) Scale control chart. DMSO represents the universal solvent for the five complexes and it also the highest polarity index of solvent (7.2). ZnPc(SR)
polarity index chart of single and mixed solvents (2:1 volume ratio). PET: petroleum ether; Py: pyridine; Tol: toluene; Bz: benzene; THF: tetrahydrofuran; Chl: chloroform;
DMF: dimethyl formamide; DMSO: dimethyl sulphoxide. Red bars represent solvents employed in OBTZnPc only, (c) relative red shift index (R I) in DMF:
ZnOEPc > OBTZnPc > ZnOPPc > ZnPc > (py)ZnPc, (d) relative red shift index (R I) in THF: OBTZnPc > ZnOEPc > ZnPc, (e) relative red shift index (R I) in pyridine:
ZnPc > ZnOEPc > ZnOPPc > OBTZnPc, (f) relative red shift index (R I) in Bz (or Bz/Py): ZnOEPc > OBTZnPc > ZnPc = (py)ZnPc, (g) relative red shift index (R I) in Chl(or Chl/
Py): ZnPc > OBTZnPc, (h) relative red shift index (R I) in Tol (or Tol/Py): ZnOEPc > OBTZnPc > ZnPc = (py) ZnPc and (i) relative red shift index (R I) in PET/Py: OBTZnPc > ZnPc.
For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
8
is OBTZnPc, (b)
s
s
s
s
s
s
s
(

V. Akpe et al. / Journal of Molecular Structure 984 (2010) 1–14
7
(
(
d)
e)
(
f)
Fig. 6 (continued)
std
3
.3.1. Fluorescence quantum yields
Fluorescence quantum yield (/
D
D
T
A :e
T
AT
Sample
Std
/T
¼ /T
ð9Þ
Std
F
) was determined by the com-
:e
T
parative method used in literature [31].
Std
where
D
A
T
and
D
AT are the changes in the triplet state absorbance
Std
s
2
F ꢄ AStd ꢄ n
D
of the OBTMPc and standard respectively; eT and
state extinction coefficients for OBTMPc and standard respectively.
The triplet quantum yield for standard, zinc phthalocyanine (ZnPc)
in DMSO (/T = 0.65 cited in Ref. [33]).
e
are the triplet
/F ¼ / ðStdÞ F
ð8Þ
F
2
Std ꢄ A ꢄ n
DStd
Std
F represents the areas under the fluorescence curves of the
2
+
3+
4+
Quantum yields of internal conversion (/ ) were obtained
OBTMPc (M = Zn , Al , Sn ), and FStd is area under the fluores-
cence curve of the standard. A and AStd are the respective absor-
bances of sample, OBTMPc and the standard at the excitation
wavelengths, and n and nDStd are the refractive indices of solvents
D
used in the sample and standard respectively. Fluorescence quan-
IC
using Eq. (10). The assumption is based on the fact that there are
only three processes that are solely responsible for the joint deac-
tivation processes of OBTMPc, namely fluorescence, intersystem
crossing and internal conversion.
Std
tum yield for standard, ZnPc in DMSO (/F = 0.18) [32a,b]. Both the
sample and standard were excited at the same wavelength.
/IC ¼ 1 ꢁ ð/ þ / Þ
ð10Þ
F
T
Triplet lifetimes were obtained by the exponential fitting of the
kinetic curves using OriginPro 7.5 software.
3.3.2. Triplet quantum yields and lifetimes
De-aerated solutions of the respective OBTMPc were introduced
into a 2 mm ꢀ 10 mm spectrophotometer cell and irradiated at the
3.3.3. Singlet oxygen and photodegradation quantum yields
Q-band maxima with the laser system described above. Triplet
D
d
Singlet oxygen (/ ) and photodegradation (/ ) quantum yield
quantum yields (/ ) were determined by the comparative method
based on triplet decay in Eq. (9).
determinations were carried out using the experimental set-up de-
scribed in detail in this paper [34]. Typically, a 2 mL portion of the
T

8
V. Akpe et al. / Journal of Molecular Structure 984 (2010) 1–14
(
(
g)
h)
(i)
Fig. 6 (continued)
Table 2
Red shift index table of ZnPc derivatives. Ref. [26] indicates the literature values used to calculate the red shift indices of the different complexes. ZnOPPc: zinc
octaphenoxyphthalocyanine; ZnOEPc: zinc octaestronephthalocyanine; (py) ZnPc: (pyridino) zinc phthalocyanine.
Solvent
Solvent
Red shift index,
Red shift index,
Red shift index, ZnOPPc Red shift Index, ZnOEPc Red shift index, (py)ZnPc
polarity index,
PI
ZnPc, Ndye = 560
OBTZnPc, Ndye = 142
[27], Ndye = 357
[27], Ndye = 324
[27], Ndye = 560
DMSO
7.2
10
10
10
10
10
DMF
THF
Pyridine
Benzene/pyridine
6.4
4.0
5.3
(2.7, 5.3)
8.358
5.877
11.632
9.064
8.875
10.555
10.186
8.965
10.500, 15.523
11.009
9.529
8.136
11.368
7.534
d
–
–
d
a
a
a
a
10
10.500, 15.523
10.452
10
(
2:1)
Chloroform/
pyridine (2:1)
Toluene/pyridine
(4.1, 5.3)
(2.4, 5.3)
(0.1, 5.3)
10.817b
9.180c
5.045
10
–d
–d
–d
10.186
9.627
10.500, 15.523c
–d
9.991c
–d
10c
–d
(
2:1)
Petroleum ether/
pyridine (2:1)
a
b
c
k
k
k
max measured in benzene.
max measured in chloroform.
max measured in toluene.
d
Not determined.

V. Akpe et al. / Journal of Molecular Structure 984 (2010) 1–14
9
ꢁ
1
ꢁ2
respective OBTMPc solution (absorbance ꢃ0.5) containing the sin-
glet oxygen quencher, 1,3-diphenylisobenzofuran(DPBF), was irra-
diated in the Q-band region using the photo-irradiation set-up
s
cm . Photodegradation quantum yields were determined using
Eq. (12),
ðC
0
ꢁ C
I ꢄ S ꢄ t
and C are OBTMPc concentrations before and after irradia-
tion respectively, V is the reaction volume, N the Avogadro’s con-
t
Þ ꢄ V ꢄ N
A
described above. Singlet oxygen quantum yields (/
D
) for the com-
/ ¼
ð12Þ
d
plexes were calculated according to Eq. (11):
C
0
t
Std
A
R ꢄ I
Std
abs
/D ¼ /D
ð11Þ
stant, S the irradiated cell area and t the irradiation time. I is the
overlap integral between the radiation source and OBTMPc absorp-
Std
R
ꢄ Iabs
1
6
ꢁ1
ꢁ2
tion. The light intensity of 5.02 ꢀ 10 photons s cm was em-
Std
/
(
is the singlet oxygen quantum yield for the standard ZnPc
ployed for the photodegradation studies of the complexes.
D
Std
= 0.67 in DMSO (cited in Ref. [32])); R and RStd are the DPBF
/
D
photobleaching rates in the presence of OBTMPc and standard
3.4. The concept of the red shift index (R
unsubstituted ZnPc in different solvents
s
I). Case study: OBTZnPc and
Std
respectively; Iabs and I
are the rates of light absorption by the
abs
OBTMPc and ZnPc, standard respectively. DPBF concentrations were
calculated using log = 4.36 at 417 nm in DMSO. The light intensity
used for /D determinations was found to be 1.14 ꢀ 10 photons
e
The red (bathochromic) shift of an organic dye is the shift of the
absorption spectrum to longer wavelengths due to a substitution
16
(
a) 60
5
5
4
5
0
5
4
3
0
5
30
5
4.8
2
5
4.6
4.4
20
DN
4.2
F
15
4
(
b) 70
6
5
4
0
0
0
3
1
0
5
35
30
10
25
RsI
20
DN
5
15
Fig. 7. (a) 3D correlation multiparameter plot: polarisation red shift (
k), red shift index (R I) and solvent donor number (DN), (c) 3D correlation multiparameter plot: Q-band (kmax of OBTZnPc, red shift index, R
linear dependence string plot function of the solvent parameters: k(F), kmax(n) and kmax(R I).
D
k), solvent donor (DN) and F(OBTZnPc), (b) 3D correlation multiparameter plot: polarisation red shift
(D
s
s
I and refractive index, n and (d)
D
s

1
0
V. Akpe et al. / Journal of Molecular Structure 984 (2010) 1–14
sample
solution
reference
or solvent effect. We defined the red shift of a substituted MPc in
Eq. (2) to be the difference between the Q-band in solution and
(
Dk
in Eq. (2)), k
of the unsubstituted dye, and the sol-
maxðvapourÞ
vent choice that has the highest solvent polarity index value (PI).
The red shift index can be applied to any type of organic dye,
provided the Q-band shows a monomer peak in the absorption
spectrum. It is a useful parameter for estimating, in empirical
terms, the coordinating power of a solvent in the presence of a
dye, irrespective of the state of the dye (charged or neutral). The
linear plots of the polarisation red shift and the Q-band (kmax) vs.
the red shift index of OBTZnPc and ZnPc (Fig. 5a and b) are given
below:
the Q-band in the vapour phase. The red shift index, R
fore be defined as the fraction of the red shift of the substituted dye
OBTZnPc) to the Q-band (kmax) of the unsubstituted dye [ZnPc] in
s
I, may there-
(
the vapour phase multiplied by a scaling factor, Ndye
.
solution
D
ksample
ꢀ Ndye
ðR
s
IÞ ¼
ð13aÞ
ðvapourÞreference
k
max
ꢀ
ꢁ
sample
maxðsolutionÞ
reference
1
0 k
ꢁ k
2
maxðvapourÞ
D
D
k ¼ 5:4295R
k ¼ 1:2139R I ꢁ 0:13943 with R ¼ 1 ðZnPcÞ
max ¼ 5:3517R
kmax ¼ 1:2139R I þ 659:87 with R ¼ 1 ðZnPcÞ
s
I ꢁ 4:3971 with R ¼ 0:9875 ðOBTZnPcÞ
ð14aÞ
ð14bÞ
ð14cÞ
ð14dÞ
N
dye
¼
¼
reference
k
2
maxðvapourÞ
s
!
sample
2
k
maxðsolutionÞ
k
s
I þ 656:49 with R ¼ 1 ðOBTZnPcÞ
10
ꢁ 1
ð13bÞ
reference
maxðvapourÞ
2
k
s
reference
maxðvapourÞ
k
is the unsubstituted dye in the vapour phase. The red
Fig. 6a–i shows the chart variation of the data in Table 2. In
DMF, except for (py)ZnPc, the four other complexes represented
show red shift indices that are almost the same. In THF, only ZnPc
shows a low red shift index compared to the other two complexes
represented. Surprisingly in pyridine, the red shift index of ZnPc
shift index is a modified formula from Freed et al. [35]. Ndye is the
scaling factor of the dye when dissolved in the solvent with the
highest polarity index value. The scaling factor is measured from
three parameters, namely: the red shift of the substituted dye
(c)
8
7
7
6
6
00
50
00
50
00
15
2.5
2
10
1.5
1
RsI
0.5
n
5
0
(d)
58
5
5
5
5
4
4
4
4
6
4
2
0
8
6
4
2
6
00
7
30
6
50
720
7
10
700
λ
(n) (nm)
700
max
750
λ
(RsI) (nm)
max
690
Fig. 7 (continued)

V. Akpe et al. / Journal of Molecular Structure 984 (2010) 1–14
11
represents the highest value. Also good red shift values are repre-
sented for the other complexes in the same solvent. The red shift
indices of the various complexes in pyridine may be due to the
high coordinating activity of pyridine, which is a function of its sol-
vent donor, DN (33.1). This may also explain why the red shift indi-
ces for OBTZnPc in the mixed solvents remain exceptionally high
except for Fig. 6i, where the red shift index of ZnPc in PET/Py is
low (which may be due to low coordinating activity in the mix-
ture). The red shift index shows a strong correlation with the fol-
lowing solvent parameters represented in Table 1: the refractive
(R
s
I) and the refractive index, n(n
D
). The linear dependence func-
tions of these parameters were further confirmed using the string
plot shown in Fig. 7d.
3.5. Explanation of the photophysical and photochemical parameters
The excitation spectra of the three complexes OBTMPc repre-
sent mirror images of their emission spectra. Fig. 8a and b also
shows that the complexes absorbing light in the ground state are
the same complexes that are being excited. This confirms that
the absorbing species is also the fluorescing species. In general,
all the parameters in Table 3 show strong correlations that can
be summarised using the Jablonski energy level diagram (not
shown as a figure legend). Following the absorption of light at
the excitation wavelength of these complexes, electrons are pro-
moted from the ground state to the singlet state by the absorption
index, n
D
and the solvent donor, DN. The red shift index increases
and DN). The red shift indices
as these two parameters increase (n
D
of the complexes represented show good empirical scale, since the
index values are in agreement with the refractive indices of single
and mixed solvents. There is no clear correlation between the red
shift index with the following parameters: the polarity index of
1
ꢂ
solvent, PI and the dipole moment,
l
in Table 1. Fig. 7a–c shows
of photon Iab. The excited state complexes ( M ) fluoresce by emit-
the solvation pattern of OBTZnPc. The pattern illustrates six para-
ting photons, with decreasing / efficiency as follows: OBTAlPcO-
F
metric values used in the study of solvent effects of OBTZnPc,
H > OBTSnPcCl ꢅ OBTZnPc. Fluorescence quantum yield values
2
namely: the Q-band (kmax), the polarisation red shift
(
D
k),
are usually expected to be higher for MPcs with lighter atoms
F(OBTZnPc), the solvent donor number (DN), the red shift index
and lower for MPcs with heavier atoms, where intersystem
(
a) 1,2
1
0
0
0
0
,8
,6
,4
,2
0
6
60
680
700
720
740
760
780
800
Wavelength (nm)
(
b) 1,2
1
0
0
0
0
,8
,6
,4
,2
0
6
00
620
640
660
680
700
720
740
760
780
800
Wavelength (nm)
Fig. 8. (a) Normalized fluorescence excitation (blue curve) and emission spectra (red curve) of OBTZnPc in DMSO. Excitation wavelength at 645 nm and (b) normalized
ground state absorption (black curve) and excitation (blue curve) spectra of ZnPc(SR)
is referred to the web version of this article.)
8
in DMSO. (For interpretation of the references to colour in this figure legend, the reader

1
2
V. Akpe et al. / Journal of Molecular Structure 984 (2010) 1–14
Table 3
Photophysical and photochemical parameters obtained in DMSO.
a
b
ISCc ratio,
5
d
e
Complex
k
Q
(nm)
k
F
(nm)
D
stokes(nm)
s
T
(
ls)
/
F±(0.03)
/
T
/
IC
v
/
d
ꢀ 10
/
D
n
p
S
D
5
5
4
OBTZnPc
OBTAlPcOH
OBTSnPcCl
710
699
703
719
707
717
9
8
14
630
810
610
0.13
0.31
0.15
0.53
0.40
0.47
0.34
0.29
0.38
4.08
1.29
3.13
0.03
0.04
0.33
0.43
0.36
0.35
4 ꢀ 10
8 ꢀ 10
5 ꢀ 10
0.81
0.90
0.74
2
The photophysical and photochemical properties were all measured in DMSO at the average LT (laboratory temperature = 20 °C).
a
D
/
stokes is the stokes shift and values calculated using
IC is the internal conversion. Values calculated using Eq. (10), /IC = 1 ꢁ (/
ISC ratio = / //
is the photon yields per molecule. Values calculated using a rough estimation n
D D T
is the fraction of triplet state quenched by triplet state oxygen. Values calculated using S = / // .
D
stokes = k
F
ꢁ k
Q
.
b
c
F T
+ / ).
v
T
F
.
d
e
n
p
p F d
= / // .
S
D
0
0
0
0
0
0
0
,7
,6
,5
,4
,3
,2
,1
0
compete with phosphorescence [37], with OBTAlPcOH, exhibiting
the highest lifetime. Fig. 9 shows the triplet decay curve of OBTAlP-
cOH in DMSO. Also from Table 3, there is a strong correlation be-
T D
tween the following parameters: s , S and np (photon yields per
molecule). The three parameters indicate that the higher the pho-
ton yields per molecule of the complex, the higher the energy
transfer efficiency and the higher the triplet lifetime. Singlet oxy-
gen is most commonly produced by photosensitisation from the
triplet state of molecules. Consequently, the singlet oxygen quan-
tum yield (/
from Table 3, the presence of axial ligands in OBTAlPcOH and
OBTSnPcCl may have also contributed to their low singlet oxygen
D
) should depend on the triplet quantum yield. Also
2
yields. This has also been reported in this review [37].
0
0,005
0,01
0,015
time (s)
0,02
0,025
0,03
The degradation of the molecules under irradiation is used to
study the photostability of the molecules, which is especially
important for those molecules intended for use as photocatalysts.
Fig. 9. Triplet decay curve of OBTAlPcOH in DMSO. Exponential fit was obtained
with OriginPro 7.5 software.
d
Table 3 shows the photodegradation yields (/ ) of complexes
investigated. These values are generally lower than reported for
most MPcs, which further shows that thioether substituents im-
part a reasonable degree of photostability to the OBTMPc com-
plexes. Fig. 10a shows the kinetic studies of OBTAlPcOH in air,
oxygen, DABCO (a singlet oxygen scavenger), deuterated DMSO
and nitrogen. A similar study was carried out with OBTZnPc and
crossing (ISC) of heavier atoms is expected to cause a decrease in
fluorescence quantum yield [36].
The complexes can lose energy in three different ways: (i)
through internal conversion to move to the ground state (/IC), with
decreasing yields as follows: OBTSnPcCl
2
> OBTZnPc > OBTAlPcOH;
2
OBTSnPcCl . A faster rate is observed in oxygen than in nitrogen,
a direct correlation with the Stokes shift values are also compared.
From Table 3, the larger the Stokes shift, the greater the internal
conversion yield; meaning that more of the electronic energy of
the excited singlet state is lost through internal conversion to the
air or deuterated DMSO, meaning that oxygen may be involved
in the photodegradation, but not just as singlet oxygen, since the
complex is highly photostable in deuterated DMSO. However when
DABCO was employed, the spectral changes shown in Fig. 10b were
observed, which might be attributed to the phototransformation of
OBTAlPcOH, accompanied by a shift in band positions (Q and B
bands) to a highly photostable complex. Similar studies on
OBTZnPc and OBTAlPcOH indicate the lowering of the Q-band posi-
tions without the accompanied phototransformation observed for
OBTAlPcOH in DABCO. The observed trends of these complexes in
DABCO suggest that radical mechanisms may play a major role in
ground state in OBTSnPcCl
TAlPcOH and OBTZnPc). (ii) Through intersystem crossing, ISC
moving to the triplet state, with decreasing yields of / as follows:
OBTZnPc > OBTSnPcCl > OBTAlPcOH, which is the direct inverse of
. These trends observed in quantum yields of fluorescence and
2
when compared to the two other (OB-
T
2
/
F
triplet are due to the heavy atom effect. The effect is caused by
the spin–orbit coupling of atoms of higher atomic number, which
causes a reduction in fluorescence quantum yield and increases
the probability of such compounds undergoing successful intersys-
tem crossing from the singlet state to the triplet state. The pres-
ence of axial OH in complex 5 and axial Cl in complex 6 could in
addition facilitate radiationless decay to the ground state, and
hence a subsequent drop in the fluorescence quantum yield as
shown for the corresponding values obtained in OBTAlPcOH and
2
the degradation of OBTZnPc and OBTSnPcCl , and that this mecha-
nism play a less important role in OBTAlPcOH.
4. Conclusion
Solvent effects on the ground state absorption and physico-
2+
OBTSnPcCl
2
in Table 3. Again from Table 3, the intersystem cross-
chemical parameters of OBTMPc with central metal ions of Zn ,
3
+
4+
ing ratio is directly proportional to the triplet quantum yield and
inversely proportional to the fluorescence quantum yield.
Another way in which these complexes may lose energy is (iii)
through intermolecular processes via excited state quenching. In
Al and Sn are not well understood. The inclusion of a thioether
derivative (SR, where R is CH Ph) in the phthalocyanine moiety re-
2
sulted in OBTZnPc exhibiting excellent spectral properties, as
monomeric peaks were observed in single and mixed solvents.
These complexes also showed a high photon yield per molecule
D
Table 3, S represents one such physical parameter, and this is a
fraction of the triplet state quenched by triplet state oxygen, with
decreasing energy transfer efficiency as follows: OBTAlPcO-
(n
P
), very low photodegradation yield (/
d
), and high photosensiti-
sation-efficiency (S
D
), which suggests they should be further inves-
H > OBTZnPc > OBTSnPcCl
Table 3 are long enough for a variety of quenching processes to
2
. The triplet lifetime values listed in
tigated for light-driven applications such as photosensitisers,
photocatalysts and as advanced optical materials.

V. Akpe et al. / Journal of Molecular Structure 984 (2010) 1–14
13
(
a) 1,2
1
D6
Air
0
0
0
0
,8
,6
,4
,2
0
N
2
O2
DABCO
0
200
400
600
800
1000
time (s)
(
b) 1,2
1
0
0
0
0
,8
,6
,4
,2
0
300
350
400
450
500
550
600
650
700
750
800
-0,2
Wavelength (nm)
Fig. 10. (a) Kinetic studies of OBTAlPcOH in DMSO in the presence of (i) oxygen, (ii) nitrogen, (iii) DABCO and (iv) AlPc(SR)
8
OH in deuterated DMSO. Excitation wavelength
ꢁ
6
ꢁ3
ꢃ699 nm. (b) Electronic absorption spectral changes observed during the photolysis of AlPc(SR)
Excitation wavelength ꢃ699 nm.
8
OH in DMSO containing DABCO. AlPc(SR)
8
OH ꢃ1.3 ꢀ 10 mol dm
.
[
[
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Acknowledgements
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The work supported by Rhodes University, South Africa and
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