# Synthesis and Photophysical Properties of Tetra- and Octasubstituted Phosphorous Oxide Triazatetrabenzcorrole Photosensitizers

Authors: Edith M. Antunes; Tebello Nyokong
Journal: Metal-Based Drugs
 (2008)

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## Abstract

The synthesis of phosphorous oxide triazatetrabenzcorroles (TBC) tetra- (9, 11) or octa- (13)
substituted on the ring with halogenated functional groups is reported. The complexes are not aggregated
in dimethylsulfoxide (DMSO) and show solubility in solvents such as pyridine. The Q
band absorption spectra
of the complexes are red-shifted compared to unsubstituted PTBC. The latter complex shows a large triplet
lifetime (1.7 milliseconds), higher than for MPc derivatives. The chlorinated derivatives show good triplet
yields (ΦT∼ 0.46 and 0.36) and relatively long lifetimes (256 and 452 microseconds), respectively, for 11 and 13.


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## Body

## 1. Introduction

The
immense and diverse potential of phthalocyanines (Pc) in a variety of technical
(chemical sensors [1], liquid crystals [2], electrocatalysis [3], and nonlinear
optics [4]) and medicinal (primarily photodynamic therapy [5–8]) applications
has generated a great deal of interest in these macrocyclic compounds. This,
together with the extraordinary stability of these complexes, has resulted in
considerable research being carried out on the phthalocyanine complex upon
incorporation of nearly all metals in the periodic table into the Pc core.  Complexes
with metalloids and nonmetals of Groups IVA and VA are of particular interest
due to the two different valence/oxidation states available to the central
atom.Triazatetrabenzcorroles
(TBC) are phthalocyanine-like compounds which have lost one of the bridging
nitrogen atoms [9]. The
synthesis of Ge, Si, Ga, and Al TBC complexes have been reported [10, 11]. We
have recently reported on a microwave synthesis of a sulfonated SnTBC [12]. Gouterman
et al. were the first to report on the synthesis and unusual electronic spectra
of the PcPIII compared to PcPV [9, 13]. It became apparent
that upon complexation with the trivalent phosphorous, a bridging nitrogen was
lost to form the phosphorous oxide tetrabenzcorroles (PTBC).  Since then, a number of octa- and
tetrasubstituted PTBC derivatives have been synthesized [14–19]. Most complexes
synthesized contained alkyl chain ring substituents. High fluorescence quantum
yields [18] and singlet oxygen quantum yields [15] have been reported for the
complexes. The water-soluble tetrasulphonated PTBC showed good photodynamic
therapy towards HeLa cells [19]. For applications in PDT, high triplet
lifetimes and yields are desired; however there are no reports on the triplet-state
behavior of the PTBC complexes.  In this
work, we report on PTBC complexes tetra- or octasubstituted with chloride and
bromide ring substituents (see Schemes 1 and 2), since the halogens are expected to
enhance intersystem crossing, resulting in high triplet yields due to the heavy
atom effect.  A PTBC derivative
containing a mixture of butoxy and chloro ring substituents has been reported
[17], but no photophysical data has been reported for halogenated PTBCs.Scheme 1
Synthetic procedure for the tetrasubstituted PTBC complexes.Scheme 2
Synthesis of the octachlorosubstituted PTBC (13).


## 2. Experimental Procedures

Materials
Dimethylsulfoxide
(DMSO), methanol (MeOH), phosphorous tribromide (PBr3), deuterated
chloroform (CDCl3), deuterated dimethylsulfoxide (DMSO-d6), deuterated pyridine
(Pyr-d5), 4,5
dichlorophthalonitrile, 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU),
4-nitrophthalonitrile, copper (I) bromide, copper (I) chloride, hydrobromic
acid, hydrochloric acid, palladium on carbon (Pd/C), and dicyanobenzene were
purchased from Sigma-Aldrich (Miss, USA) and used as received. Pyridine and
1-pentanol were obtained from Sigma-Aldrich and dried prior to use. Column
chromatography was performed on silica gel 60 (0.04–0.063 mm).Equipment
Ground-state
electronic absorption spectra were recorded on a Varian Cary 500 UV-Vis-NIR
spectrophotometer.H1 and C13 NMR spectra were obtained using
a Bruker AMX 400 MHz and a Bruker Avance II+ 600 MHz NMR spectrometer.
Fluorescence emission and excitation spectra were recorded on a Varian Cary
Eclipse spectrofluorimeter, while FT-IR spectra (KBr pellets) were recorded on
a Perkin-Elmer spectrum 2000 FT-IR spectrometer. MS data was recorded on a Shimadzu
KRATOS Maldi MS instrument.
Triplet
absorption and decay kinetics were recorded on a laser flash photolysis system,
the excitation pulses were produced by an Nd: YAG laser (Quanta-Ray, 1.5 J/90 ns) pumping a dye laser (Lambda Physic FL 3002, Pyridin 1 in methanol). The
analyzing beam source was from a Thermo Oriel xenon arc lamp, and a
photomultiplier tube was used as detector. Signals were recorded with a
two-channel digital real-time oscilloscope (Tektronix TDS 360) where the
kinetic curves were averaged over 256 laser pulses. Triplet lifetimes were
determined by exponential fitting of the kinetic curves using OriginPro 7.5
software. Solutions for triplet yield and lifetime determinations were degassed
with argon before use.


### 2.1. Syntheses and Characterization

4,5-Dichlorophthalonitrile
and dicyanobenzene were commercially obtained.  Unsubstituted PTBC (7) was synthesized from unmetallated phthalocyanine (6) and PBr3 in a 16.4%
yield, and characterized as reported in the literature [9, 13, 19]. 4-Aminophthalonitrile
(2) and 4-bromophthalonitrile (4) were synthesised following literature
procedures [20, 21] with some minor modifications. 4-Chlorophthalonitrile (5) was synthesised by modifying the
procedure used for compound 4.


#### 2.1.1. Preparation of 4-Aminophthalonitrile (2)

4-Nitrophthalonitrile
(1) (1.00 g, 5.78 mmol) was placed
in a round bottom flask and 100 mL of ethanol added to obtain a suspension. The
catalyst Pd/C (55 mg) was added to the flask, the apparatus evacuated and then
filled with hydrogen and the mixture vigorously stirred at room temperature
until the absorption of hydrogen had completely stopped. The reaction mixture
was subsequently filtered over celite and the solution evaporated in vacuo. Yield:
98%. H1 NMR (400 MHz, DMSO-d6) δ ppm 7.39 (dd, J = 8.66, 1.34 Hz, 1H), 6.93 (d, J = 2.19 Hz, 1H), 6.81 (dd, J = 8.67, 2.18 Hz, 1H), 6.36 (br s, 2H).


#### 2.1.2. Preparation of 4-Bromophthalonitrile (4)

Compound2 (500 mg, 3.5 mmol) was taken up in
a mixture of water (4 mL) and hydrobromic acid (4 mL, 48%) and the solution
cooled to 0C° using an ice-salt bath. A solution of sodium nitrite
(276 mg, 4 mmol) in water (2 mL) was then added dropwise to the acid mixture to
form the diazonium salt 3a as an
intermediate. Copper (I) bromide (1.00 g, 6.97 mmol) was dissolved in HBr (4 mL, 48%) and cooled to 0C°. The cold diazonium salt solution was
then added dropwise to the CuBr reagent and the solution stirred for 1 hr at 0C°.
After an hour, the solution was left to stand at room temperature overnight. The
aqueous solution was then extracted with ethyl acetate (3 × 15 mL), the
combined organic extracts washed with brine, dried over magnesium sulphate and
concentrated to give 248 mg of 4.  Yield: 34%. H1 NMR (400 MHz, CDCl3) δ ppm 7.96 (d, J
= 1.78 Hz, 1H), 7.89 (dd, J = 8.37, 1.89 Hz, 1H), 7.68 (d, J = 8.37 Hz, 1H). C13 NMR (75 MHz, CDCl3)
δ ppm 136.6 (d), 136.3 (d), 134.4 (d), 128.1 (s), 117.3 (s), 114.7 (s), 114.5
(s), 114.0 (s).


#### 2.1.3. Preparation of 4-Chlorophthalonitrile (5)

A
similar procedure to the one above was used to prepare compound5. Compound 2 (500 mg, 3.5 mmol) was dissolved in a water (4 mL)/hydrochloric
acid (5 mL, 32%) mixture, and the sodium nitrite (276 mg, 4 mmol) in water (2 mL) reagent was added to the amine hydrochloride solution to form the diazonium
salt (3b). Copper (I) chloride (1.00
g, 10.1 mmol) was dissolved in HCl (5 mL) and cooled to 0C°. The
diazonium salt was then added to the CuCl reagent and treated in the same
manner as reported above. Compound 5 was thus produced in a 58% yield. H1 NMR (400 MHz, CDCl3) δ ppm 7.80 (d, J
= 1.88 Hz, 1H), 7.75 (s, 1H), 7.73 (d, J = 1.87 Hz, 1H). C13
NMR (75 MHz, CDCl3)
δ ppm 140.2 (s), 134.5 (d), 133.7 (d), 133.5 (d), 117.4 (s), 114.6 (s), 114.2
(s), 114.1 (s).


#### 2.1.4. Preparation of Tetrabromo Phosphorous Oxide Triazatetrabenzcorrole (9, PTBrTBC)

Complex9 was synthesized from the
unmetallated derivative (8). The first step was the synthesis of 8,
following established methods [22] as follows: 4 (200 mg, 0.97 mmol) was
reacted with DBU (0.36 mL) in 1-pentanol under reflux for 12 hours under a
nitrogen atmosphere. The dark green mixture was cooled down to room
temperature, then methanol (5 mL) and water (2 mL) were added and the mixture
precipitated out and centrifuged. This green solid complex 8, confirmed to be a phthalocyanine
molecule by its UV spectrum, was not purified any further and was
employed as is for the synthesis of complex 9. Complex 9 was synthesized by heating (under
reflux) a mixture of complex 8 (100 mg, 0.12 mmol) and PBr3(0.34 mL, 3.60 mmol) in pyridine at 90 to
100C° for 2 hours. The reaction was then allowed to cool to room temperature
and poured carefully into water and allowed to stand overnight. The green
product obtained was then centrifuged and washed with copious amounts of water.
Upon drying, the precipitate was chromatographed on a silica gel column using
pyridine as the eluant. The complex was collected as a dark green band, while
the unmetallated Pc remained at the top of the column. Yield: 13.5%. UV/Vis (DMSO), λmax⁡/nm (log⁡ε)]: 662 (4.86), 630 (4.56), 602 (4.29), 578 (3.79), 448
(5.19), 439 (4.94), 418 (4.70).    [(KBr) vmax⁡/cm−1]: 3063, 1717, 1608, 1506, 1451, 1396,
1326, 1298 (P=O), 1271, 1151, 1109, 1077, 1043, 967, 956, 832, 813, 758, 723, 691,
578. H1 NMR (600 MHz, Pyr-d5)
δ ppm 10.15–9.92 (m, 4H), 9.83–9.57 (m, 4H), 8.59–8.41
(m, 4H). 31P NMR (162 MHz, Pyr-d5)
δ ppm –198.6. MALDI m/z: Calc. 860.8, found 861.1 {Br4(TBC)P(O)}.


#### 2.1.5. Preparation of Tetrachloro Phosphorous Oxide Triazatetrabenzcorrole (11, PTClTBC)

Complex11 was synthesized as
explained above for 9, except that complex 5 (200 mg, 1.23 mmol) was reacted with DBU (0.24 mL) in 1-pentanol instead of
compound 4 to obtain the 4-chloro H2Pc analogue (complex 10).
The presence of the metal-free Pc was again confirmed by its UV spectrum.
Complex 10 was then employed for the synthesis of complex 11 (100 mg,
0.15 mmol) by reacting it with PBr3(0.42 mL, 4.50 mmol) in hot
pyridine. Yield: 27.7%. UV/Vis (DMSO), λmax⁡/nm (log⁡ε)]: 661 (4.56), 630 (4.26), 603 (3.99), 447 (4.82), 438
(4.59), 417 (4.39).  [(KBr) vmax⁡/cm−1]: 2922, 1717, 1609, 1505, 1456, 1399,
1328, 1297 (P=O), 1143, 1068, 973, 910, 814, 769, 724, 690, 580. H1 NMR (600 MHz, Pyr-d5) δ ppm 9.97–9.66 (m, 8H), 8.45–8.26 (m, 4H). 31P NMR (162 MHz, Pyr-d5) δ ppm –198.5. MALDI m/z: Calc. 682.9, found 682.0 {Cl4(TBC)P(O)}.


#### 2.1.6. Preparation of Octachloro Phosphorous Oxide Triazatetrabenzcorrole (13, POClPBC)

The first step was the synthesis of metal-free complex12, using the same procedure as outlined above
for the synthesis of 8, except that 4,5-dichlorophthalonitrile (300 mg 1.53 mmol) was reacted with DBU (0.12 mL) instead of complex 4.
Confirmation of the formation of the H2Pc (12) was provided by the UV spectrum.  Complex 13 was then
synthesized as described for complexes 9 and 11 using 12 (100 mg, 0.128 mmol) and PBr3 (0.36 mL, 3.84 mmol). A paucity of proton
signals was expected and observed in the H1 NMR spectrum of this octasubstituted
complex. Yield: 8.6%. UV/Vis (DMSO), λmax⁡/nm  (log⁡ε)]: 667 (3.47), 635 (3.19), 606 (2.94), 452 (3.83), 442
(3.61), 422 (3.46), 411 (3.42).  [(KBr) vmax⁡/cm−1]: 2927, 1726, 1586, 1521, 1349,
1298 (P=O), 1248, 1197, 1138, 948, 908, 837, 743, 602. H1 NMR (600 MHz, Pyr-d5) δ ppm 8.07 (m, 8H). 31P NMR (162 MHz, Pyr-d5) δ ppm −197.4. MALDI m/z: not observed.


### 2.2. Photophysical and Photochemical  Parameters

#### 2.2.1. Fluorescence Quantum Yields and Lifetimes

The
comparative method was used to determine the fluorescence quantum yields(ΦF) according to the following
equation [23], utilizing unsubstituted ZnPc in DMSO as the standard (ΦF=0.18) [24]:
(1)ΦF=ΦF(Std)F⋅AStd⋅η2FStd⋅⁢⁢A⁢⋅ηStd2,
where F and FStd
are the areas under the fluorescence curve of the
sample and the standard, respectively. Similarly, A and AStd
are
the absorbance of the compound and the standard at the excitation wavelength, η
and ηStd are the refractive indices of solvents used for the sample
and the standard, respectively.Natural or radiative
lifetimes (τN)
were estimated using PhotochemCAD program which uses the Strickler-Berg
equation [25]. The fluorescence lifetimes (τF) were evaluated using the following equation:
(2)ΦF=τFτN.The rate constants for
intersystem crossing from the excited singlet state to the triplet state (kISC(S−T)) were estimated using the following equation [26]:
(3)kISC(S−T)=(1/τf)−(1/τf0),
where τf and  τf0 are the excited
singlet-state lifetimes for the halogenated derivatives and unsubstituted PTBC,
respectively.  Similarly, the rate
constants for intersystem crossing from the triplet state to the ground state
(kISC(T−S)) were estimated using the following equation:
(4)kISC(T−S)=(1/τT)−(1/τT0),
where τT and τT0 are the excited
triplet-state lifetime for the halogenated derivatives and unsubstituted PTBC,
respectively.


#### 2.2.2. Triplet Quantum Yields and Lifetimes

Solutions of the PTBC complexes were bubbled with argon in a 1 cm
pathlength spectrophotometric cell, irradiated at theQ band of the respective PTBC complexes, with the triplet quantum yields (ΦT)
determined by the triplet absorption method. The comparative method [27] was applied
as in the following equation, using ZnPc in DMSO as the standard:
(5)ΦT=ΦTstd⋅ΔAT⋅⁢εTStdΔATStd⋅εT.Changes
in the triplet-state absorbances of the PTBC derivative and the standard are
represented byΔAT and ΔATStd, respectively; while εT and εTstd  are the triplet-state
molar extinction coefficients for the PTBC derivative and the standard, respectively;
while ΦTstd is the triplet quantum
yield for the standard (ΦT=0.65 for ZnPc in DMSO) [28].  Triplet
lifetimes (τT) were determined by
exponential fitting of the kinetic curves using OriginPro 7.5 software.Quantum
yields of internal conversion(ΦIC) were obtained from the following
equation, which assumes that only three processes (fluorescence, intersystem
crossing, and internal conversion) jointly deactivate the excited singlet state
of PTBC derivatives:
(6)ΦIC=1−(ΦF+ΦT).


## 2.1. Syntheses and Characterization

4,5-Dichlorophthalonitrile
and dicyanobenzene were commercially obtained.  Unsubstituted PTBC (7) was synthesized from unmetallated phthalocyanine (6) and PBr3 in a 16.4%
yield, and characterized as reported in the literature [9, 13, 19]. 4-Aminophthalonitrile
(2) and 4-bromophthalonitrile (4) were synthesised following literature
procedures [20, 21] with some minor modifications. 4-Chlorophthalonitrile (5) was synthesised by modifying the
procedure used for compound 4.


### 2.1.1. Preparation of 4-Aminophthalonitrile (2)

4-Nitrophthalonitrile
(1) (1.00 g, 5.78 mmol) was placed
in a round bottom flask and 100 mL of ethanol added to obtain a suspension. The
catalyst Pd/C (55 mg) was added to the flask, the apparatus evacuated and then
filled with hydrogen and the mixture vigorously stirred at room temperature
until the absorption of hydrogen had completely stopped. The reaction mixture
was subsequently filtered over celite and the solution evaporated in vacuo. Yield:
98%. H1 NMR (400 MHz, DMSO-d6) δ ppm 7.39 (dd, J = 8.66, 1.34 Hz, 1H), 6.93 (d, J = 2.19 Hz, 1H), 6.81 (dd, J = 8.67, 2.18 Hz, 1H), 6.36 (br s, 2H).


### 2.1.2. Preparation of 4-Bromophthalonitrile (4)

Compound2 (500 mg, 3.5 mmol) was taken up in
a mixture of water (4 mL) and hydrobromic acid (4 mL, 48%) and the solution
cooled to 0C° using an ice-salt bath. A solution of sodium nitrite
(276 mg, 4 mmol) in water (2 mL) was then added dropwise to the acid mixture to
form the diazonium salt 3a as an
intermediate. Copper (I) bromide (1.00 g, 6.97 mmol) was dissolved in HBr (4 mL, 48%) and cooled to 0C°. The cold diazonium salt solution was
then added dropwise to the CuBr reagent and the solution stirred for 1 hr at 0C°.
After an hour, the solution was left to stand at room temperature overnight. The
aqueous solution was then extracted with ethyl acetate (3 × 15 mL), the
combined organic extracts washed with brine, dried over magnesium sulphate and
concentrated to give 248 mg of 4.  Yield: 34%. H1 NMR (400 MHz, CDCl3) δ ppm 7.96 (d, J
= 1.78 Hz, 1H), 7.89 (dd, J = 8.37, 1.89 Hz, 1H), 7.68 (d, J = 8.37 Hz, 1H). C13 NMR (75 MHz, CDCl3)
δ ppm 136.6 (d), 136.3 (d), 134.4 (d), 128.1 (s), 117.3 (s), 114.7 (s), 114.5
(s), 114.0 (s).


### 2.1.3. Preparation of 4-Chlorophthalonitrile (5)

A
similar procedure to the one above was used to prepare compound5. Compound 2 (500 mg, 3.5 mmol) was dissolved in a water (4 mL)/hydrochloric
acid (5 mL, 32%) mixture, and the sodium nitrite (276 mg, 4 mmol) in water (2 mL) reagent was added to the amine hydrochloride solution to form the diazonium
salt (3b). Copper (I) chloride (1.00
g, 10.1 mmol) was dissolved in HCl (5 mL) and cooled to 0C°. The
diazonium salt was then added to the CuCl reagent and treated in the same
manner as reported above. Compound 5 was thus produced in a 58% yield. H1 NMR (400 MHz, CDCl3) δ ppm 7.80 (d, J
= 1.88 Hz, 1H), 7.75 (s, 1H), 7.73 (d, J = 1.87 Hz, 1H). C13
NMR (75 MHz, CDCl3)
δ ppm 140.2 (s), 134.5 (d), 133.7 (d), 133.5 (d), 117.4 (s), 114.6 (s), 114.2
(s), 114.1 (s).


### 2.1.4. Preparation of Tetrabromo Phosphorous Oxide Triazatetrabenzcorrole (9, PTBrTBC)

Complex9 was synthesized from the
unmetallated derivative (8). The first step was the synthesis of 8,
following established methods [22] as follows: 4 (200 mg, 0.97 mmol) was
reacted with DBU (0.36 mL) in 1-pentanol under reflux for 12 hours under a
nitrogen atmosphere. The dark green mixture was cooled down to room
temperature, then methanol (5 mL) and water (2 mL) were added and the mixture
precipitated out and centrifuged. This green solid complex 8, confirmed to be a phthalocyanine
molecule by its UV spectrum, was not purified any further and was
employed as is for the synthesis of complex 9. Complex 9 was synthesized by heating (under
reflux) a mixture of complex 8 (100 mg, 0.12 mmol) and PBr3(0.34 mL, 3.60 mmol) in pyridine at 90 to
100C° for 2 hours. The reaction was then allowed to cool to room temperature
and poured carefully into water and allowed to stand overnight. The green
product obtained was then centrifuged and washed with copious amounts of water.
Upon drying, the precipitate was chromatographed on a silica gel column using
pyridine as the eluant. The complex was collected as a dark green band, while
the unmetallated Pc remained at the top of the column. Yield: 13.5%. UV/Vis (DMSO), λmax⁡/nm (log⁡ε)]: 662 (4.86), 630 (4.56), 602 (4.29), 578 (3.79), 448
(5.19), 439 (4.94), 418 (4.70).    [(KBr) vmax⁡/cm−1]: 3063, 1717, 1608, 1506, 1451, 1396,
1326, 1298 (P=O), 1271, 1151, 1109, 1077, 1043, 967, 956, 832, 813, 758, 723, 691,
578. H1 NMR (600 MHz, Pyr-d5)
δ ppm 10.15–9.92 (m, 4H), 9.83–9.57 (m, 4H), 8.59–8.41
(m, 4H). 31P NMR (162 MHz, Pyr-d5)
δ ppm –198.6. MALDI m/z: Calc. 860.8, found 861.1 {Br4(TBC)P(O)}.


### 2.1.5. Preparation of Tetrachloro Phosphorous Oxide Triazatetrabenzcorrole (11, PTClTBC)

Complex11 was synthesized as
explained above for 9, except that complex 5 (200 mg, 1.23 mmol) was reacted with DBU (0.24 mL) in 1-pentanol instead of
compound 4 to obtain the 4-chloro H2Pc analogue (complex 10).
The presence of the metal-free Pc was again confirmed by its UV spectrum.
Complex 10 was then employed for the synthesis of complex 11 (100 mg,
0.15 mmol) by reacting it with PBr3(0.42 mL, 4.50 mmol) in hot
pyridine. Yield: 27.7%. UV/Vis (DMSO), λmax⁡/nm (log⁡ε)]: 661 (4.56), 630 (4.26), 603 (3.99), 447 (4.82), 438
(4.59), 417 (4.39).  [(KBr) vmax⁡/cm−1]: 2922, 1717, 1609, 1505, 1456, 1399,
1328, 1297 (P=O), 1143, 1068, 973, 910, 814, 769, 724, 690, 580. H1 NMR (600 MHz, Pyr-d5) δ ppm 9.97–9.66 (m, 8H), 8.45–8.26 (m, 4H). 31P NMR (162 MHz, Pyr-d5) δ ppm –198.5. MALDI m/z: Calc. 682.9, found 682.0 {Cl4(TBC)P(O)}.


### 2.1.6. Preparation of Octachloro Phosphorous Oxide Triazatetrabenzcorrole (13, POClPBC)

The first step was the synthesis of metal-free complex12, using the same procedure as outlined above
for the synthesis of 8, except that 4,5-dichlorophthalonitrile (300 mg 1.53 mmol) was reacted with DBU (0.12 mL) instead of complex 4.
Confirmation of the formation of the H2Pc (12) was provided by the UV spectrum.  Complex 13 was then
synthesized as described for complexes 9 and 11 using 12 (100 mg, 0.128 mmol) and PBr3 (0.36 mL, 3.84 mmol). A paucity of proton
signals was expected and observed in the H1 NMR spectrum of this octasubstituted
complex. Yield: 8.6%. UV/Vis (DMSO), λmax⁡/nm  (log⁡ε)]: 667 (3.47), 635 (3.19), 606 (2.94), 452 (3.83), 442
(3.61), 422 (3.46), 411 (3.42).  [(KBr) vmax⁡/cm−1]: 2927, 1726, 1586, 1521, 1349,
1298 (P=O), 1248, 1197, 1138, 948, 908, 837, 743, 602. H1 NMR (600 MHz, Pyr-d5) δ ppm 8.07 (m, 8H). 31P NMR (162 MHz, Pyr-d5) δ ppm −197.4. MALDI m/z: not observed.


## 2.1.1. Preparation of 4-Aminophthalonitrile (2)

4-Nitrophthalonitrile
(1) (1.00 g, 5.78 mmol) was placed
in a round bottom flask and 100 mL of ethanol added to obtain a suspension. The
catalyst Pd/C (55 mg) was added to the flask, the apparatus evacuated and then
filled with hydrogen and the mixture vigorously stirred at room temperature
until the absorption of hydrogen had completely stopped. The reaction mixture
was subsequently filtered over celite and the solution evaporated in vacuo. Yield:
98%. H1 NMR (400 MHz, DMSO-d6) δ ppm 7.39 (dd, J = 8.66, 1.34 Hz, 1H), 6.93 (d, J = 2.19 Hz, 1H), 6.81 (dd, J = 8.67, 2.18 Hz, 1H), 6.36 (br s, 2H).


## 2.1.2. Preparation of 4-Bromophthalonitrile (4)

Compound2 (500 mg, 3.5 mmol) was taken up in
a mixture of water (4 mL) and hydrobromic acid (4 mL, 48%) and the solution
cooled to 0C° using an ice-salt bath. A solution of sodium nitrite
(276 mg, 4 mmol) in water (2 mL) was then added dropwise to the acid mixture to
form the diazonium salt 3a as an
intermediate. Copper (I) bromide (1.00 g, 6.97 mmol) was dissolved in HBr (4 mL, 48%) and cooled to 0C°. The cold diazonium salt solution was
then added dropwise to the CuBr reagent and the solution stirred for 1 hr at 0C°.
After an hour, the solution was left to stand at room temperature overnight. The
aqueous solution was then extracted with ethyl acetate (3 × 15 mL), the
combined organic extracts washed with brine, dried over magnesium sulphate and
concentrated to give 248 mg of 4.  Yield: 34%. H1 NMR (400 MHz, CDCl3) δ ppm 7.96 (d, J
= 1.78 Hz, 1H), 7.89 (dd, J = 8.37, 1.89 Hz, 1H), 7.68 (d, J = 8.37 Hz, 1H). C13 NMR (75 MHz, CDCl3)
δ ppm 136.6 (d), 136.3 (d), 134.4 (d), 128.1 (s), 117.3 (s), 114.7 (s), 114.5
(s), 114.0 (s).


## 2.1.3. Preparation of 4-Chlorophthalonitrile (5)

A
similar procedure to the one above was used to prepare compound5. Compound 2 (500 mg, 3.5 mmol) was dissolved in a water (4 mL)/hydrochloric
acid (5 mL, 32%) mixture, and the sodium nitrite (276 mg, 4 mmol) in water (2 mL) reagent was added to the amine hydrochloride solution to form the diazonium
salt (3b). Copper (I) chloride (1.00
g, 10.1 mmol) was dissolved in HCl (5 mL) and cooled to 0C°. The
diazonium salt was then added to the CuCl reagent and treated in the same
manner as reported above. Compound 5 was thus produced in a 58% yield. H1 NMR (400 MHz, CDCl3) δ ppm 7.80 (d, J
= 1.88 Hz, 1H), 7.75 (s, 1H), 7.73 (d, J = 1.87 Hz, 1H). C13
NMR (75 MHz, CDCl3)
δ ppm 140.2 (s), 134.5 (d), 133.7 (d), 133.5 (d), 117.4 (s), 114.6 (s), 114.2
(s), 114.1 (s).


## 2.1.4. Preparation of Tetrabromo Phosphorous Oxide Triazatetrabenzcorrole (9, PTBrTBC)

Complex9 was synthesized from the
unmetallated derivative (8). The first step was the synthesis of 8,
following established methods [22] as follows: 4 (200 mg, 0.97 mmol) was
reacted with DBU (0.36 mL) in 1-pentanol under reflux for 12 hours under a
nitrogen atmosphere. The dark green mixture was cooled down to room
temperature, then methanol (5 mL) and water (2 mL) were added and the mixture
precipitated out and centrifuged. This green solid complex 8, confirmed to be a phthalocyanine
molecule by its UV spectrum, was not purified any further and was
employed as is for the synthesis of complex 9. Complex 9 was synthesized by heating (under
reflux) a mixture of complex 8 (100 mg, 0.12 mmol) and PBr3(0.34 mL, 3.60 mmol) in pyridine at 90 to
100C° for 2 hours. The reaction was then allowed to cool to room temperature
and poured carefully into water and allowed to stand overnight. The green
product obtained was then centrifuged and washed with copious amounts of water.
Upon drying, the precipitate was chromatographed on a silica gel column using
pyridine as the eluant. The complex was collected as a dark green band, while
the unmetallated Pc remained at the top of the column. Yield: 13.5%. UV/Vis (DMSO), λmax⁡/nm (log⁡ε)]: 662 (4.86), 630 (4.56), 602 (4.29), 578 (3.79), 448
(5.19), 439 (4.94), 418 (4.70).    [(KBr) vmax⁡/cm−1]: 3063, 1717, 1608, 1506, 1451, 1396,
1326, 1298 (P=O), 1271, 1151, 1109, 1077, 1043, 967, 956, 832, 813, 758, 723, 691,
578. H1 NMR (600 MHz, Pyr-d5)
δ ppm 10.15–9.92 (m, 4H), 9.83–9.57 (m, 4H), 8.59–8.41
(m, 4H). 31P NMR (162 MHz, Pyr-d5)
δ ppm –198.6. MALDI m/z: Calc. 860.8, found 861.1 {Br4(TBC)P(O)}.


## 2.1.5. Preparation of Tetrachloro Phosphorous Oxide Triazatetrabenzcorrole (11, PTClTBC)

Complex11 was synthesized as
explained above for 9, except that complex 5 (200 mg, 1.23 mmol) was reacted with DBU (0.24 mL) in 1-pentanol instead of
compound 4 to obtain the 4-chloro H2Pc analogue (complex 10).
The presence of the metal-free Pc was again confirmed by its UV spectrum.
Complex 10 was then employed for the synthesis of complex 11 (100 mg,
0.15 mmol) by reacting it with PBr3(0.42 mL, 4.50 mmol) in hot
pyridine. Yield: 27.7%. UV/Vis (DMSO), λmax⁡/nm (log⁡ε)]: 661 (4.56), 630 (4.26), 603 (3.99), 447 (4.82), 438
(4.59), 417 (4.39).  [(KBr) vmax⁡/cm−1]: 2922, 1717, 1609, 1505, 1456, 1399,
1328, 1297 (P=O), 1143, 1068, 973, 910, 814, 769, 724, 690, 580. H1 NMR (600 MHz, Pyr-d5) δ ppm 9.97–9.66 (m, 8H), 8.45–8.26 (m, 4H). 31P NMR (162 MHz, Pyr-d5) δ ppm –198.5. MALDI m/z: Calc. 682.9, found 682.0 {Cl4(TBC)P(O)}.


## 2.1.6. Preparation of Octachloro Phosphorous Oxide Triazatetrabenzcorrole (13, POClPBC)

The first step was the synthesis of metal-free complex12, using the same procedure as outlined above
for the synthesis of 8, except that 4,5-dichlorophthalonitrile (300 mg 1.53 mmol) was reacted with DBU (0.12 mL) instead of complex 4.
Confirmation of the formation of the H2Pc (12) was provided by the UV spectrum.  Complex 13 was then
synthesized as described for complexes 9 and 11 using 12 (100 mg, 0.128 mmol) and PBr3 (0.36 mL, 3.84 mmol). A paucity of proton
signals was expected and observed in the H1 NMR spectrum of this octasubstituted
complex. Yield: 8.6%. UV/Vis (DMSO), λmax⁡/nm  (log⁡ε)]: 667 (3.47), 635 (3.19), 606 (2.94), 452 (3.83), 442
(3.61), 422 (3.46), 411 (3.42).  [(KBr) vmax⁡/cm−1]: 2927, 1726, 1586, 1521, 1349,
1298 (P=O), 1248, 1197, 1138, 948, 908, 837, 743, 602. H1 NMR (600 MHz, Pyr-d5) δ ppm 8.07 (m, 8H). 31P NMR (162 MHz, Pyr-d5) δ ppm −197.4. MALDI m/z: not observed.


## 2.2. Photophysical and Photochemical  Parameters

### 2.2.1. Fluorescence Quantum Yields and Lifetimes

The
comparative method was used to determine the fluorescence quantum yields(ΦF) according to the following
equation [23], utilizing unsubstituted ZnPc in DMSO as the standard (ΦF=0.18) [24]:
(1)ΦF=ΦF(Std)F⋅AStd⋅η2FStd⋅⁢⁢A⁢⋅ηStd2,
where F and FStd
are the areas under the fluorescence curve of the
sample and the standard, respectively. Similarly, A and AStd
are
the absorbance of the compound and the standard at the excitation wavelength, η
and ηStd are the refractive indices of solvents used for the sample
and the standard, respectively.Natural or radiative
lifetimes (τN)
were estimated using PhotochemCAD program which uses the Strickler-Berg
equation [25]. The fluorescence lifetimes (τF) were evaluated using the following equation:
(2)ΦF=τFτN.The rate constants for
intersystem crossing from the excited singlet state to the triplet state (kISC(S−T)) were estimated using the following equation [26]:
(3)kISC(S−T)=(1/τf)−(1/τf0),
where τf and  τf0 are the excited
singlet-state lifetimes for the halogenated derivatives and unsubstituted PTBC,
respectively.  Similarly, the rate
constants for intersystem crossing from the triplet state to the ground state
(kISC(T−S)) were estimated using the following equation:
(4)kISC(T−S)=(1/τT)−(1/τT0),
where τT and τT0 are the excited
triplet-state lifetime for the halogenated derivatives and unsubstituted PTBC,
respectively.


### 2.2.2. Triplet Quantum Yields and Lifetimes

Solutions of the PTBC complexes were bubbled with argon in a 1 cm
pathlength spectrophotometric cell, irradiated at theQ band of the respective PTBC complexes, with the triplet quantum yields (ΦT)
determined by the triplet absorption method. The comparative method [27] was applied
as in the following equation, using ZnPc in DMSO as the standard:
(5)ΦT=ΦTstd⋅ΔAT⋅⁢εTStdΔATStd⋅εT.Changes
in the triplet-state absorbances of the PTBC derivative and the standard are
represented byΔAT and ΔATStd, respectively; while εT and εTstd  are the triplet-state
molar extinction coefficients for the PTBC derivative and the standard, respectively;
while ΦTstd is the triplet quantum
yield for the standard (ΦT=0.65 for ZnPc in DMSO) [28].  Triplet
lifetimes (τT) were determined by
exponential fitting of the kinetic curves using OriginPro 7.5 software.Quantum
yields of internal conversion(ΦIC) were obtained from the following
equation, which assumes that only three processes (fluorescence, intersystem
crossing, and internal conversion) jointly deactivate the excited singlet state
of PTBC derivatives:
(6)ΦIC=1−(ΦF+ΦT).


## 2.2.1. Fluorescence Quantum Yields and Lifetimes

The
comparative method was used to determine the fluorescence quantum yields(ΦF) according to the following
equation [23], utilizing unsubstituted ZnPc in DMSO as the standard (ΦF=0.18) [24]:
(1)ΦF=ΦF(Std)F⋅AStd⋅η2FStd⋅⁢⁢A⁢⋅ηStd2,
where F and FStd
are the areas under the fluorescence curve of the
sample and the standard, respectively. Similarly, A and AStd
are
the absorbance of the compound and the standard at the excitation wavelength, η
and ηStd are the refractive indices of solvents used for the sample
and the standard, respectively.Natural or radiative
lifetimes (τN)
were estimated using PhotochemCAD program which uses the Strickler-Berg
equation [25]. The fluorescence lifetimes (τF) were evaluated using the following equation:
(2)ΦF=τFτN.The rate constants for
intersystem crossing from the excited singlet state to the triplet state (kISC(S−T)) were estimated using the following equation [26]:
(3)kISC(S−T)=(1/τf)−(1/τf0),
where τf and  τf0 are the excited
singlet-state lifetimes for the halogenated derivatives and unsubstituted PTBC,
respectively.  Similarly, the rate
constants for intersystem crossing from the triplet state to the ground state
(kISC(T−S)) were estimated using the following equation:
(4)kISC(T−S)=(1/τT)−(1/τT0),
where τT and τT0 are the excited
triplet-state lifetime for the halogenated derivatives and unsubstituted PTBC,
respectively.


## 2.2.2. Triplet Quantum Yields and Lifetimes

Solutions of the PTBC complexes were bubbled with argon in a 1 cm
pathlength spectrophotometric cell, irradiated at theQ band of the respective PTBC complexes, with the triplet quantum yields (ΦT)
determined by the triplet absorption method. The comparative method [27] was applied
as in the following equation, using ZnPc in DMSO as the standard:
(5)ΦT=ΦTstd⋅ΔAT⋅⁢εTStdΔATStd⋅εT.Changes
in the triplet-state absorbances of the PTBC derivative and the standard are
represented byΔAT and ΔATStd, respectively; while εT and εTstd  are the triplet-state
molar extinction coefficients for the PTBC derivative and the standard, respectively;
while ΦTstd is the triplet quantum
yield for the standard (ΦT=0.65 for ZnPc in DMSO) [28].  Triplet
lifetimes (τT) were determined by
exponential fitting of the kinetic curves using OriginPro 7.5 software.Quantum
yields of internal conversion(ΦIC) were obtained from the following
equation, which assumes that only three processes (fluorescence, intersystem
crossing, and internal conversion) jointly deactivate the excited singlet state
of PTBC derivatives:
(6)ΦIC=1−(ΦF+ΦT).


## 3. Results and Discussion

### 3.1. Synthesis and Characterization

Substituted phthalocyanines are generally prepared by
cyclotetramerization of substituted phthalonitriles. 2(3), 9(10), 16(17), 23(24)-Tetrasubstituted
phthalocyanines can be synthesized from 4-substituted phthalonitriles [22],
while octasubstituted phthalocyanines can be synthesized from
4,5-dichlorophthalonitrile [29]. In the case of tetrasubstituted derivatives, a
mixture of four possible structural isomers are obtained, which can be designated
by their molecular symmetry as C4h, C2υ, Cs, and
D2h.
In
this study, synthesized tetrasubstituted phthalocyanine compounds are obtained
as isomer mixtures as expected. No attempt was made to separate the isomers of 9 and 11.A variety of halogenated phosphorous
oxide triazatetrabenzcorroles (complexes9, 11, and 13) were
prepared by treatment of unmetallated phthalocyanines with PBr3 in
pyridine according to literature procedures [19]. The products thus obtained
were then subjected to silica gel column chromatography using pyridine as an
eluant.Generally,
phthalocyanine complexes are insoluble in most organic solvents; however
introduction of substituents to the ring increases the solubility. The
halogenated complexes (particularly9 and 11) exhibited excellent solubility in organic solvents
such as pyridine and DMSO. For comparative purposes, the unsubstituted PTBC was
synthesized and found to be soluble in DMSO, but only sparingly in pyridine.The new
compounds were characterized by UV-vis, IR, mass, and NMR spectroscopies
(including31P NMR) and the analyses were consistent with the
predicted structures as shown in Section 2.
However, mass spectral data proved to be difficult to obtain for complex 13. The P=O vibrations were observed at ~1295 cm-1(in accordance
with [14]) in the IR spectra, confirming the presence of O coordinated to the phosphorous
atom. This was corroborated by the 31P NMR shifts obtained, that is,
~ −198 ppm, which is typical of a P=O bond [14]. H1 NMR
investigations of 9 and 11 gave the characteristic
chemical shifts, with three proton signals integrating for a total of 12 for each
complex. For complex 13,
a multiplet due to the nonperipheral protons, was observed in the H1
NMR spectrum.TBC complexes have
distinct UV-Vis spectra with a sharp peak at ~450 nm [16–19], which can be
employed in their characterization.  The
formation of the TBC complexes occurs when the MPc molecules no longer retains
a Pc moiety as they no longer have the fourth azomethine nitrogen (see Schemes 1 and 2).
It is believed [11] that in the presence of excess metal halide, the bridge
nitrogen of the Pc is eliminated, forming TBC.  In this study, the unmetallated Pc derivatives
(8,10 and 12) were formed first,
which, upon reaction with PBr3, resulted in the formation of PTBC
derivatives. This was judged spectroscopically by the collapse of the sharp Q
band in the visible region of unmetallated Pcs to three bands (in the Q band
region), together with the formation of the sharp Soret band at 440 nm (see Figure 1).   The spectra of unmetallated Pcs 6, 8, 10, and 12 in DMSO and pyridine
showed a single Q band (see Figure 1) which is uncharacteristic of unmetallated
Pcs.
Typically, unmetallated Pcs show a split Q band due to lack of
symmetry.  Solvation in polar aprotic
solvents (such as DMSO and pyridine) occurs through their unshared
electrons.  Thus, in DMSO (see Figure 1)
and pyridine, the spectra of 8, 10, and 12 did not show the normal splitting of the Q
band that is typical of free-base phthalocyanines, showing instead a single
sharp Q band.   The same applies to H2Pc in
pyridine. The nonsplit Q band is a
result of the basicity of the
solvents.  It has been documented that in
strongly basic solvents, the inner pyrrole hydrogens are acidic enough to
dissociate resulting in a charged system (Pc-2) which becomes
symmetric and thus possesses an unsplit Q band [30].Figure 1
Spectrum of metal-free Pc (10) and transformation upon formation
of PTClTBC (11) in DMSO. Concentration = 3.59×10−5 mol dm-3.Figure2 compares the
spectra of the complexes synthesized in this work, while Table 1 lists the Q
and B band maxima. It is clear in Table 1 and Figure 2 that the presence of
bromines and chlorines shifts the spectra to the red region. The
red shift of spectra on halogenation has been observed before [26] for ZnPc
derivatives.Table 1
UV-Vis and fluorescence spectral data for the PTBC derivatives.
CompoundλQband  (abs)ε(mol-1⋅ L⋅cm-1)λQband (abs)λQband (Em)λQband  (Exc)Stokes shift (nm)PTBC (7)6551.50×1044426606555PTBrTBC (9)6627.28×1044486706618PTClTBC (11)6624.56×1044476686596POClTBC (13)6672.93×1034526756688ZnPc6722.38×105[29]3526816729Figure 2
Comparison of the
electronic spectra of various PTBC complexes synthesized in DMSO. Concentrations
=8.06×10−5 mol dm-3(7),
1.95×10−5 mol dm-3(9),
9.27×10−5 mol dm-3(13).Aggregation in
phthalocyanines and related complexes is usually depicted as a coplanar
association of rings progressing from monomer to dimer and higher-order
complexes. It is dependent upon the concentration of the complex, the nature of
the solvent, as well as the nature of the substituents and the complexed metal
ions. In this study, the aggregation behavior of the TBC complexes (7, 9, 11, and 13) was investigated in DMSO (see Figure 3). The complexes did not
show aggregation at concentrations less than 8×10−5 mol dm-3.
A linear plot of absorbance versus concentration was obtained in this
concentration range.Figure 3
Absorption spectra of (11) in DMSO at different decreasing concentrations.
Concentrations from 8×10−5 to 5×10−6 M. Inset: plot
of absorbance versus concentration.The shapes of the excitation spectra for the TBC
complexes were similar to the absorption spectra (see Figure4). However, these
spectra were not mirror images of the fluorescence spectra for the PTBC
derivatives, in that the emission spectra showed only a single band, while the
Q band of the absorption spectra has a split Q band. The observation of a single-emission
band in the Q band region is typical of unsymmetric phthalocyanine complexes such as
unmetallated derivatives [31–33]. Metal-free Pcs are known to fluoresce with
only one main peak in
non-aqueous media which has been assigned as the 0–0 transition of the
fluorescence [31].Figure 4
Comparison of the absorbance (a), excitation
(b), and emission (c) spectra of complex9. Excitation wavelength = 380 nm.The emission spectra were slightly
red-shifted with Stokes shifts ranging from 5 to 9 nm, suggesting no change in
nuclear configurations following excitation.  The largest shifts were observed for the
POClTBC (13) and PTBrTBC (9), while the smallest shift was
observed for the PTBC derivative (7).


### 3.2. Photophysical and Photochemical Studies

The fluorescence quantum yields(ΦF) of the PTBC
derivatives are given in Table 2. These values are much lower than reported for
MPc complexes, except for the unsubstituted PTBC complex, which gives ΦF values in the range for
MPc complexes [34]. The low values obtained for the halogenated derivatives are
most likely due to the heavy atom effect of the halide functional group, which
encourages intersystem crossing to the triplet state. Halogenation of ZnPc [26]
has been reported to give a remarkable decrease in fluorescence quantum yields
and lifetimes, since incorporation of a halogen into the photosensitizer
increases the level spin-orbit coupling.Table 2
Photophysical and photochemical parameters of the
PTBC derivatives in DMSO. References given in square brackets.
CompoundaτT(μs)ΦTτF(ns)ΦFΦICPTBC (7)17400.272.60.120.61PTBrTBC (9)6570.050.30.030.92PTClTBC (11)4520.360.90.070.57POClTBC(13)2360.461.10.060.48ZnPca standard:  τT=353(μs); ΦT=0.65 [28];
ΦF=0.18 [24].Fluorescence lifetimes (τF, Table 3) were calculated using
the Strickler-Berg equation.  Using this
equation, a good correlation has been [35] found between
experimentally and theoretically determined lifetimes for the unaggregated
molecules as found in this work. Thus, we believe that the values obtained
using this equation are a good measure of fluorescence lifetimes. Halogenation
is expected to decrease fluorescence quantum yields and lifetimes, increase
triplet-state formation, and shorten triplet lifetime. Thus, as expected, the τF values of the halogenated
derivatives were lower
than for unsubstituted PTBC and lower than generally observed for MPc complexes [35]. Octasubstitution with
chlorines increased the τF values compared to tetrasubstituted derivatives when comparing complexes 11 and 13. It is also interesting to note that a decrease in fluorescence
lifetime was observed upon contraction of the ring in tin tetrasulphonated α,β,γ-tetrabenzcorroles
compared to tin  tetrasulphonated phthalocyanines
[12].Table 3
Rate contacts for the photophysical processes
occurring in PTBC derivatives in DMSO.
Compound1kF(s-1) (×107)(kISC(S−T))(× 109)(kISC(S−T))(s-1)2KIC(s-1)
(×108)PTBC (7)2.9——2.3PTBrTBC (9)10329.5× 10231PTClTBC (11)7.89.21.6× 1036.3POClTBC(13)5.56.73.6× 1034.41kF is the rate constant for fluorescence. Values
calculated using kF=ΦF/τF.2kIC is the rate constant for internal conversion.
Values calculated using kIC=ΦIC/τF.The acquisition ofτF values allowed us to determine the
rate constants for various processes. The rate constant for fluorescence (kF),
Table 3, was highest for complex 9,
which also had the lowest triplet quantum yield as will be discussed below. Likewise,
the rate constants for intersystem crossing from the singlet state to the
triplet state (kISC(S−T)) was the highest for complex 9, Table 3.The transient absorption
spectra were recorded in argon-degassed solutions by exciting the
photosensitizer (in DMSO) in theQ band region and recording the transient absorption
spectra point by point from 400 to 750 nm (see Figure 5). A representative
decay profile is shown in Figure 6.  The
Q and the Soret bands showed a negative absorption (bleaching) and the
transient spectra showed a broad positive absorption ~500 nm (see Figure 5).Figure 5
Transient differential spectrum of
complex unsubstituted PTBC (7)
in DMSO. Excitation wavelength = 655 nm. The inset shows the weak
transient absorption of the triplet state.Figure 6
Triplet decay curve of unsubstituted PTBC (7) in
DMSO at 490 nm. Excitation wavelength = 655 nm.The triplet lifetimes for
the PTBC derivatives, ranging from 256 to 1740 microseconds, are listed in Table2.   The latter value
was observed for the unsubstituted PTBC (Table 2) and it is an unusually high
triplet lifetime.  Such high values are
rare for MPc complexes [34]. Values in the millisecond range have been reported
for AlPc derivatives, however they are still not as high as the value observed
here for unsubstituted PTBC.  The
presence of the halogens was expected to lower the triplet lifetimes (when
compared to unhalogenated PTBC (7)), as
observed in Table 2; with the octasubstituted complex, 13, giving the lowest triplet lifetime.
Contrary to the heavy atom effect, which results in the decrease in triplet lifetimes
with an increase in the size of the halogen [26], this work shows an increase
in lifetime on going from the chlorinated (11)
to brominated (9) PTBC derivatives, with
the octachlorinated (13) complex showing
the lowest triplet lifetime.  The latter
(13) could have a lower triplet
lifetime than the tetrachlorinated derivative (11)
due to the plurality of chlorine atoms and the heavy atom effect.We have recently [12]
shown that upon contraction of the ring in tin tetrasulphonated α,β,γ-tetrabenzcorrole compared to tin  tetrasulphonated
phthalocyanine, there was a decrease in ΦT values and the triplet lifetimes.
However, the lifetimes reported here for PTBC derivatives are high compared to
MPc complexes in general.In general, there is an
increase inΦT values upon halogenation with a striking exception of the brominated complex  (9). The octachlorinated derivatives (13) gave the highest triplet quantum
yield (ΦT),
while surprisingly the bromo substituted
complex 9 gave the lowest value.  The ΦT value for 9 is almost ten-fold lower
than for 11.  Again, the larger value of ΦT for 13 could be due to the plurality of chlorines and the heavy atom
effect.  The low ΦT value for the brominated
derivative compared to the chlorinated one contradicts the heavy atom effect. The
increased ΦT value for the chlorinated derivatives 11 and 13 compared to 9 will result in shorter triplet
lifetime for the former, and this is the case in Table 2.Quantum yields of
internal conversion(ΦIC)
were calculated using (6)  and are high
due to low ΦT values. Using the triplet lifetimes, the rate constants for intersystem
crossing from the triplet state to the ground state (kISC(T−S)) were
determined and are shown in Table 3. The lowest (kISC(T−S)) value obtained
is for complex 9  (Table 3) yet this complex has the
lowest triplet quantum yield.


## 3.1. Synthesis and Characterization

Substituted phthalocyanines are generally prepared by
cyclotetramerization of substituted phthalonitriles. 2(3), 9(10), 16(17), 23(24)-Tetrasubstituted
phthalocyanines can be synthesized from 4-substituted phthalonitriles [22],
while octasubstituted phthalocyanines can be synthesized from
4,5-dichlorophthalonitrile [29]. In the case of tetrasubstituted derivatives, a
mixture of four possible structural isomers are obtained, which can be designated
by their molecular symmetry as C4h, C2υ, Cs, and
D2h.
In
this study, synthesized tetrasubstituted phthalocyanine compounds are obtained
as isomer mixtures as expected. No attempt was made to separate the isomers of 9 and 11.A variety of halogenated phosphorous
oxide triazatetrabenzcorroles (complexes9, 11, and 13) were
prepared by treatment of unmetallated phthalocyanines with PBr3 in
pyridine according to literature procedures [19]. The products thus obtained
were then subjected to silica gel column chromatography using pyridine as an
eluant.Generally,
phthalocyanine complexes are insoluble in most organic solvents; however
introduction of substituents to the ring increases the solubility. The
halogenated complexes (particularly9 and 11) exhibited excellent solubility in organic solvents
such as pyridine and DMSO. For comparative purposes, the unsubstituted PTBC was
synthesized and found to be soluble in DMSO, but only sparingly in pyridine.The new
compounds were characterized by UV-vis, IR, mass, and NMR spectroscopies
(including31P NMR) and the analyses were consistent with the
predicted structures as shown in Section 2.
However, mass spectral data proved to be difficult to obtain for complex 13. The P=O vibrations were observed at ~1295 cm-1(in accordance
with [14]) in the IR spectra, confirming the presence of O coordinated to the phosphorous
atom. This was corroborated by the 31P NMR shifts obtained, that is,
~ −198 ppm, which is typical of a P=O bond [14]. H1 NMR
investigations of 9 and 11 gave the characteristic
chemical shifts, with three proton signals integrating for a total of 12 for each
complex. For complex 13,
a multiplet due to the nonperipheral protons, was observed in the H1
NMR spectrum.TBC complexes have
distinct UV-Vis spectra with a sharp peak at ~450 nm [16–19], which can be
employed in their characterization.  The
formation of the TBC complexes occurs when the MPc molecules no longer retains
a Pc moiety as they no longer have the fourth azomethine nitrogen (see Schemes 1 and 2).
It is believed [11] that in the presence of excess metal halide, the bridge
nitrogen of the Pc is eliminated, forming TBC.  In this study, the unmetallated Pc derivatives
(8,10 and 12) were formed first,
which, upon reaction with PBr3, resulted in the formation of PTBC
derivatives. This was judged spectroscopically by the collapse of the sharp Q
band in the visible region of unmetallated Pcs to three bands (in the Q band
region), together with the formation of the sharp Soret band at 440 nm (see Figure 1).   The spectra of unmetallated Pcs 6, 8, 10, and 12 in DMSO and pyridine
showed a single Q band (see Figure 1) which is uncharacteristic of unmetallated
Pcs.
Typically, unmetallated Pcs show a split Q band due to lack of
symmetry.  Solvation in polar aprotic
solvents (such as DMSO and pyridine) occurs through their unshared
electrons.  Thus, in DMSO (see Figure 1)
and pyridine, the spectra of 8, 10, and 12 did not show the normal splitting of the Q
band that is typical of free-base phthalocyanines, showing instead a single
sharp Q band.   The same applies to H2Pc in
pyridine. The nonsplit Q band is a
result of the basicity of the
solvents.  It has been documented that in
strongly basic solvents, the inner pyrrole hydrogens are acidic enough to
dissociate resulting in a charged system (Pc-2) which becomes
symmetric and thus possesses an unsplit Q band [30].Figure 1
Spectrum of metal-free Pc (10) and transformation upon formation
of PTClTBC (11) in DMSO. Concentration = 3.59×10−5 mol dm-3.Figure2 compares the
spectra of the complexes synthesized in this work, while Table 1 lists the Q
and B band maxima. It is clear in Table 1 and Figure 2 that the presence of
bromines and chlorines shifts the spectra to the red region. The
red shift of spectra on halogenation has been observed before [26] for ZnPc
derivatives.Table 1
UV-Vis and fluorescence spectral data for the PTBC derivatives.
CompoundλQband  (abs)ε(mol-1⋅ L⋅cm-1)λQband (abs)λQband (Em)λQband  (Exc)Stokes shift (nm)PTBC (7)6551.50×1044426606555PTBrTBC (9)6627.28×1044486706618PTClTBC (11)6624.56×1044476686596POClTBC (13)6672.93×1034526756688ZnPc6722.38×105[29]3526816729Figure 2
Comparison of the
electronic spectra of various PTBC complexes synthesized in DMSO. Concentrations
=8.06×10−5 mol dm-3(7),
1.95×10−5 mol dm-3(9),
9.27×10−5 mol dm-3(13).Aggregation in
phthalocyanines and related complexes is usually depicted as a coplanar
association of rings progressing from monomer to dimer and higher-order
complexes. It is dependent upon the concentration of the complex, the nature of
the solvent, as well as the nature of the substituents and the complexed metal
ions. In this study, the aggregation behavior of the TBC complexes (7, 9, 11, and 13) was investigated in DMSO (see Figure 3). The complexes did not
show aggregation at concentrations less than 8×10−5 mol dm-3.
A linear plot of absorbance versus concentration was obtained in this
concentration range.Figure 3
Absorption spectra of (11) in DMSO at different decreasing concentrations.
Concentrations from 8×10−5 to 5×10−6 M. Inset: plot
of absorbance versus concentration.The shapes of the excitation spectra for the TBC
complexes were similar to the absorption spectra (see Figure4). However, these
spectra were not mirror images of the fluorescence spectra for the PTBC
derivatives, in that the emission spectra showed only a single band, while the
Q band of the absorption spectra has a split Q band. The observation of a single-emission
band in the Q band region is typical of unsymmetric phthalocyanine complexes such as
unmetallated derivatives [31–33]. Metal-free Pcs are known to fluoresce with
only one main peak in
non-aqueous media which has been assigned as the 0–0 transition of the
fluorescence [31].Figure 4
Comparison of the absorbance (a), excitation
(b), and emission (c) spectra of complex9. Excitation wavelength = 380 nm.The emission spectra were slightly
red-shifted with Stokes shifts ranging from 5 to 9 nm, suggesting no change in
nuclear configurations following excitation.  The largest shifts were observed for the
POClTBC (13) and PTBrTBC (9), while the smallest shift was
observed for the PTBC derivative (7).


## 3.2. Photophysical and Photochemical Studies

The fluorescence quantum yields(ΦF) of the PTBC
derivatives are given in Table 2. These values are much lower than reported for
MPc complexes, except for the unsubstituted PTBC complex, which gives ΦF values in the range for
MPc complexes [34]. The low values obtained for the halogenated derivatives are
most likely due to the heavy atom effect of the halide functional group, which
encourages intersystem crossing to the triplet state. Halogenation of ZnPc [26]
has been reported to give a remarkable decrease in fluorescence quantum yields
and lifetimes, since incorporation of a halogen into the photosensitizer
increases the level spin-orbit coupling.Table 2
Photophysical and photochemical parameters of the
PTBC derivatives in DMSO. References given in square brackets.
CompoundaτT(μs)ΦTτF(ns)ΦFΦICPTBC (7)17400.272.60.120.61PTBrTBC (9)6570.050.30.030.92PTClTBC (11)4520.360.90.070.57POClTBC(13)2360.461.10.060.48ZnPca standard:  τT=353(μs); ΦT=0.65 [28];
ΦF=0.18 [24].Fluorescence lifetimes (τF, Table 3) were calculated using
the Strickler-Berg equation.  Using this
equation, a good correlation has been [35] found between
experimentally and theoretically determined lifetimes for the unaggregated
molecules as found in this work. Thus, we believe that the values obtained
using this equation are a good measure of fluorescence lifetimes. Halogenation
is expected to decrease fluorescence quantum yields and lifetimes, increase
triplet-state formation, and shorten triplet lifetime. Thus, as expected, the τF values of the halogenated
derivatives were lower
than for unsubstituted PTBC and lower than generally observed for MPc complexes [35]. Octasubstitution with
chlorines increased the τF values compared to tetrasubstituted derivatives when comparing complexes 11 and 13. It is also interesting to note that a decrease in fluorescence
lifetime was observed upon contraction of the ring in tin tetrasulphonated α,β,γ-tetrabenzcorroles
compared to tin  tetrasulphonated phthalocyanines
[12].Table 3
Rate contacts for the photophysical processes
occurring in PTBC derivatives in DMSO.
Compound1kF(s-1) (×107)(kISC(S−T))(× 109)(kISC(S−T))(s-1)2KIC(s-1)
(×108)PTBC (7)2.9——2.3PTBrTBC (9)10329.5× 10231PTClTBC (11)7.89.21.6× 1036.3POClTBC(13)5.56.73.6× 1034.41kF is the rate constant for fluorescence. Values
calculated using kF=ΦF/τF.2kIC is the rate constant for internal conversion.
Values calculated using kIC=ΦIC/τF.The acquisition ofτF values allowed us to determine the
rate constants for various processes. The rate constant for fluorescence (kF),
Table 3, was highest for complex 9,
which also had the lowest triplet quantum yield as will be discussed below. Likewise,
the rate constants for intersystem crossing from the singlet state to the
triplet state (kISC(S−T)) was the highest for complex 9, Table 3.The transient absorption
spectra were recorded in argon-degassed solutions by exciting the
photosensitizer (in DMSO) in theQ band region and recording the transient absorption
spectra point by point from 400 to 750 nm (see Figure 5). A representative
decay profile is shown in Figure 6.  The
Q and the Soret bands showed a negative absorption (bleaching) and the
transient spectra showed a broad positive absorption ~500 nm (see Figure 5).Figure 5
Transient differential spectrum of
complex unsubstituted PTBC (7)
in DMSO. Excitation wavelength = 655 nm. The inset shows the weak
transient absorption of the triplet state.Figure 6
Triplet decay curve of unsubstituted PTBC (7) in
DMSO at 490 nm. Excitation wavelength = 655 nm.The triplet lifetimes for
the PTBC derivatives, ranging from 256 to 1740 microseconds, are listed in Table2.   The latter value
was observed for the unsubstituted PTBC (Table 2) and it is an unusually high
triplet lifetime.  Such high values are
rare for MPc complexes [34]. Values in the millisecond range have been reported
for AlPc derivatives, however they are still not as high as the value observed
here for unsubstituted PTBC.  The
presence of the halogens was expected to lower the triplet lifetimes (when
compared to unhalogenated PTBC (7)), as
observed in Table 2; with the octasubstituted complex, 13, giving the lowest triplet lifetime.
Contrary to the heavy atom effect, which results in the decrease in triplet lifetimes
with an increase in the size of the halogen [26], this work shows an increase
in lifetime on going from the chlorinated (11)
to brominated (9) PTBC derivatives, with
the octachlorinated (13) complex showing
the lowest triplet lifetime.  The latter
(13) could have a lower triplet
lifetime than the tetrachlorinated derivative (11)
due to the plurality of chlorine atoms and the heavy atom effect.We have recently [12]
shown that upon contraction of the ring in tin tetrasulphonated α,β,γ-tetrabenzcorrole compared to tin  tetrasulphonated
phthalocyanine, there was a decrease in ΦT values and the triplet lifetimes.
However, the lifetimes reported here for PTBC derivatives are high compared to
MPc complexes in general.In general, there is an
increase inΦT values upon halogenation with a striking exception of the brominated complex  (9). The octachlorinated derivatives (13) gave the highest triplet quantum
yield (ΦT),
while surprisingly the bromo substituted
complex 9 gave the lowest value.  The ΦT value for 9 is almost ten-fold lower
than for 11.  Again, the larger value of ΦT for 13 could be due to the plurality of chlorines and the heavy atom
effect.  The low ΦT value for the brominated
derivative compared to the chlorinated one contradicts the heavy atom effect. The
increased ΦT value for the chlorinated derivatives 11 and 13 compared to 9 will result in shorter triplet
lifetime for the former, and this is the case in Table 2.Quantum yields of
internal conversion(ΦIC)
were calculated using (6)  and are high
due to low ΦT values. Using the triplet lifetimes, the rate constants for intersystem
crossing from the triplet state to the ground state (kISC(T−S)) were
determined and are shown in Table 3. The lowest (kISC(T−S)) value obtained
is for complex 9  (Table 3) yet this complex has the
lowest triplet quantum yield.


## 4. Conclusions

In conclusion, we have
synthesized halogenated PTBC derivatives (complexes9, 11, and 13) and
compared their photophysical data with that of the unsubstituted PTBC complex.
The latter complex shows a very high triplet lifetime value, higher than that for
MPc complexes. However, this complex also has a correspondent low ΦT value. Complexes 11 and 13 show reasonably
high triplet lifetimes and yields, making them possible candidates for PDT. Complex 9 having bromine substituents showed
a behavior different from the other halogenated complexes (11 and 13) in that it
gave a very low triplet quantum yield.



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*Source: 498916-2008-03-03.xml*