Photophysical characterization of dysprosium, erbium and lutetium phthalocyanines tetrasubstituted with phenoxy groups at non-peripheral positions

Polyhedron 30 (2011) 1612–1619

Contents lists available at ScienceDirect

Polyhedron journal homepage: www.elsevier.com/locate/poly

Photophysical characterization of dysprosium, erbium and lutetium phthalocyanines tetrasubstituted with phenoxy groups at non-peripheral positions Ruphino Zugle, Christian Litwinski, Tebello Nyokong ⇑ Department of Chemistry, Rhodes University, P.O. Box 94, Grahamstown 6140, South Africa

a r t i c l e

i n f o

Article history: Received 20 October 2010 Accepted 18 March 2011 Available online 24 March 2011 Keywords: Lanthanide ion Phenoxy-substituted phthalocyanine Photophysical and photochemical properties

a b s t r a c t Dysprosium bis-phthalocyanine and monomeric phthalocyanines of erbium and lutetium with nonperipheral phenoxy substituents have been synthesized using two different preparative routes. Photophysical studies on these phthalocyanines revealed that the triplet states of dysprosium and erbium are not populated while the monomeric phthalocyanine complex of lutetium is populated with a quantum yield of 0.83 and a lifetime of 25 ls in DMSO. It was further found that the phthalocyanine complex of lutetium was capable of photochemical generation of singlet state molecular oxygen with yield of 0.71 in THF, thus a promising photosensitizer. However, the three phthalocyanine molecules have very low fluorescence quantum yields of less than 0.01. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction In recent years, compounds of lanthanide ions (Ln3+) with organic ligands have found wide application in areas such as fluorescence materials [1,2], electroluminescence [3], and as fluorescence probes and labels in a variety of biological systems [4–6]. Phthalocyanines (Pcs) are remarkable macrocyclic compounds that possess interesting physical and chemical properties [7–9]. Thus synthesis and characterization of novel phthalocyanine metal complexes, including those of the lanthanides continue to attract the attention of many researchers. Interest in lanthanide phthalocyanine complexes in particular, has been as a result of possible coordination of two or more phthalocyanine macrocyclic units per metal atom forming LnPc2, or Ln2Pc3 [10–13]. These lanthanide phthalocyanine derivatives have high intrinsic conductivity and interesting electrochemical and electrochromic properties [14]. The lanthanide phthalocyanine complexes obtained by conventional chemical methods usually contain one or two macrocycles per metal atom [10]. The type (whether monomeric or oligomeric) and amounts of the particular derivative in a reaction mixture depend on the ratio of starting metal salt to phthalonitrile as well as other reaction conditions [11,12]. Phthalonitrile: metal salt ratio of 6:1 or larger results in oligomeric derivatives, whereas 4:1 or smaller results in monomeric complexes [11–13]. Thus oligomeric forms of phthalocyanine complexes of lanthanides can be obtained if the initial phthalonitrile content is high. Apart from the initial phthalonitrile:metal ratio in deciding the nature of the lanthanide phthalocyanine complex, it has also been established that the particular lanthanide metal also determines ⇑ Corresponding author. Tel.: +27 46 603 8260; fax: +27 46 622 5109. E-mail address: [email protected] (T. Nyokong). 0277-5387/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2011.03.030

the type of phthalocyanine complex formed, whether monomeric or oligomeric. Thus it has been reported [10,15] that the heaviest lanthanides form phthalocyanine complexes with fewer phthalocyanine units per lanthanide ion while the lighter lanthanides form complexes with higher number of phthalocyanine units with lanthanum and neodymium being predominant in this regard. There has been a lot of attention on the oligomeric lanthanide phthalocyanines, with only a few reports on the monomeric derivatives [16]. The current work is thus devoted to synthesis, photochemical and photophysical study of novel phenoxy-substituted phthalocyanine complexes of the heavier lanthanides (Dy, Er and Lu), by direct chemical reaction with initial phthalonitrile-metal ratio of 4:1 and reaction of the metal free phthalocyanine with the excess of the metal salt, with the view of forming monomeric phthalocyanines as major reaction products. The complexes are: 1(4), 8(11), 15(18), 22(25)-(tetraphenoxyphthalocyaninato) lutetium(III) acetate (4) and 1(4), 8(11), 15(18), 22(25)-(tetraphenoxyphthalocyaninato) erbium(III) chloride (6). Attempts to synthesize the monomeric dysprosium phthalocyanine resulted in bis-{1(4), 8(11), 15(18), 22(25)-(tetraphenoxyphthalocyaninato)} dysprosium(III) complex (5), Scheme 1. The synthesis of a lanthanide phthalocyanine containing phenoxy groups at the non-peripheral position is reported for the first time as well as the photophysical behavior of the complexes. 2. Experimental 2.1. Materials The following chemicals were purchased from SAARCHEM; 1-pentanol, n-hexane, tetrahydrofuran (THF), dimethyl sulfoxide

1613

R. Zugle et al. / Polyhedron 30 (2011) 1612–1619

OAC CN CN

K2CO3,

25oC,

48 hrs

NO2

N

RO

CN

ROH, DMF

N

LuOAc3, pentanol

N

DBU, Reflux, 7 hrs

CN

N Lu

N

OR

1

N N

N

2

OR

OR

4

DyOAc3, pentanol DBU, Reflux, 7 hrs

OR

N

RO

N

N

N

N

N

N OR

Where R

Dy

OR

N

N

N

N

N

OR

Reflux, 7 hrs

N

N OR

ErCl3, DBU

N H N

N

OR

2

N H

N

OR

Cl

N

RO

N

5

RO

CN

OR

N

=

DBU, pentanol

N OR

N

CN

RO

N

RO

Reflux, 2 hrs

N N

N Er

N

OR

N N

N

OR

3

RO

OR

6

Scheme 1. Synthetic route of compound 4, 5 and 6.

(DMSO), N,N-dimethylformamide (DMF), ethanol and dichloromethane (DCM). Zinc phthalocyanine (ZnPc), 1,3-diphenylisobenzofuran (DPBF), 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), potassium carbonate, phenol, lutetium(III) acetate, dysprosium(III) acetate and erbium(III) chloride were from Sigma– Aldrich. 3-Phenoxyphthalonitrile was prepared through base catalyzed nucleophilic aromatic displacement reaction as described in literature [17]. Column chromatography was performed on silica gel 60 (0.04–0.063 mm) and preparative thin layer chromatography was performed on silica gel 60 P F254. 2.2. Equipment Fluorescence excitation and emission spectra were recorded on a Varian Cary Eclipse spectrofluorimeter. Infrared (IR) spectra were recorded on a Perkin–Elmer Fourier transform-IR (FT-IR) Spectrum 2000 spectrometer using potassium bromide (KBr) disks. UV–Vis absorption (UV–Vis) spectra were recorded on a Varian-Cary 500 UV/Vis/NIR spectrophotometer. 1H NMR spectra were recorded on a Bruker AMX600 MHz in deuterated DMSO. Microanalyses

were performed using a Vario-Elementar Microcube ELIII. Mass spectral data were recorded on ABI voyager De-STR Maldi TOF instrument at University of Stellenbosch, South Africa using 2,5dihydroxy benzoic acid as a matrix. Laser flash photolysis experiments were performed with light pulses produced by a Quanta-Ray Nd:YAG laser providing 400 mJ, 9 ns pulses of laser light at 10 Hz, pumping a Lambda-Physik FL3002 dye laser (Pyridin 1 dye in methanol). Single pulse energy ranged from 2 to 7 mJ. The analyzing beam source was from a Thermo Oriel Xenon arc lamp, and photomultiplier tube was used as a detector. Signals were recorded with a two-channel 300 MHz digital real time oscilloscope (Tektronix TDS 3032C); the kinetic curves were averaged over 256 laser pulses. Fluorescence lifetimes were measured using a time correlated single photon counting setup (TCSPC) (Fluo Time 200, Picoquant GmbH) with a diode laser as excitation source (LDH-P-670 driven by PDL 800-B, 670 nm, 20 MHz repetition rate, Picoquant GmbH). Fluorescence was detected under the magic angle with a peltier cooled photomultiplier tube (PMT) (PMA-C 192-N-M, Picoquant GmbH) and integrated electronics (PicoHarp 300E, Picoquant

1614

R. Zugle et al. / Polyhedron 30 (2011) 1612–1619

GmbH). A monochromator with a spectral width of about 4 nm was used to select the required emission wavelength. The response function of the system, which was measured with a scattering Ludox solution (DuPont), had a full width at half-maximum (FWHM) of about 300 ns. The ratio of stop to start pulses was kept low (below 0.05) to ensure good statistics. All luminescence decay curves were measured at the maximum of emission peak. The data were analyzed with the program FluoFit (Picoquant GmbH). The support plane approach [18] was used to estimate the errors of the decay times. Singlet oxygen quantum yields (UD) were determined by detecting the phosphorescence of 1O2 at 1270 nm. An ultra sensitive germanium detector (Edinburgh Instruments, EI-P) combined with a 1000 nm long pass filter (Omega, 3RD 1000 CP) and a 1270 nm band-pass filter (Omega, C1275, BP50) was used to detect 1 O2 phosphorescence under the excitation using a dye laser system described above, with a pulse period of 9 ns and repetition rate of 10 Hz. The near-infrared phosphorescence of the sample was focused to the germanium detector by a lens (Edmund, NT 48-157) with detection direction perpendicular to the excitation laser beam. The detected signals were averaged with a digital oscilloscope (Tektronics, TDS 3032C) to show the dynamic phosphorescence decay of 1O2. The data obtained was analyzed using Microsoft Excel and ORIGIN PRO 8 software. 2.3. Photophysical parameters 2.3.1. Triplet state yields and lifetimes Triplet quantum yields were determined in DMSO using a comparative method [19], Eq. (1) with ZnPc as the standard.

UT ¼ UStd T

DAT eStd T DAStd T T

e

ð1Þ

where DAT and DAStd are the changes in the triplet state absorT bances of 4 and the standard, respectively. eT and eStd T are the triplet state molar extinction coefficients for 4 and the standard, respectively. UStd is the triplet quantum yield for the standard, ZnPc T (UStd T = 0.65 in DMSO) [20]. The triplet lifetimes were determined by exponential fitting of the kinetic curves using the program ORIGINPRO 8. 2.3.2. Fluorescence quantum yields Fluorescence quantum yields (UF) were determined in DMSO by the comparative method, Eq. (2) [21],

UF ¼ UStd F

F AStd n2 FStd An2Std

ð2Þ

where F and FStd are the areas under the fluorescence emission curves of complexes 4–6, and the standard, respectively. A and AStd are the respective absorbances of the samples and standard at the excitation wavelengths. The same solvent was used for samples and standard. Unsubstituted ZnPc in DMSO (UStd F = 0.20) [22] was employed as the standard. Both samples and standard were excited at the same wavelength of 628 nm and emission spectra were recorded from 650 to 800 nm. The absorbances of the solutions at the excitation wavelength were about 0.05 to avoid any inner filter effects. 2.3.3. Singlet oxygen quantum yield UD Singlet oxygen (1O2) quantum yield (UD ) values were determined in air by direct detection of the (1O2) phosphorescence (1270 nm). Determinations were made in the absence and presence of sodium azide (NaN3), a physical quencher of singlet oxygen and compared to a standard, ZnPc [23].

The dynamic course of 1O2 phosphorescence can be clearly recorded, following Eq. (3) as theoretically described in literature [24].

IðtÞ ¼ B

sD ½et=sT  et=sD  sT  sD

ð3Þ

where, I(t) is the phosphorescence intensity of 1O2 at time t, sD is the lifetime of 1O2 phosphorescence decay, sT is the MPc derivative triplet state lifetime in the presence of oxygen and B is a coefficient involved in sensitizer concentration and 1O2 quantum yield. 1 O2 quantum yields (UD of MPc derivatives) were then determined using Eq. (4)

UD ¼ UStd D 

B  AStd BStd  A

ð4Þ

where UStd is the singlet oxygen quantum yield for the standard D Std ZnPc (UStd refer to coefficient involved D = 0.53 in THF) [23]. B and B 1 in sensitizer concentration and O2 quantum yield for the sample and standard respectively and, A and AStd to the absorbances of the sample and standard respectively at the excitation wavelength. 2.4. Synthesis Two synthetic pathways were used; the well known template cyclotetramerisation of substituted phthalonitriles (for complexes 4 and 5) with metal salts and reaction of the metal free phthalocyanine with a metal salt (for complex 6) as shown in Scheme 1. Synthesis of complex 3 has been reported before [25]. 2.4.1. Synthesis of 1(4), 8(11), 15(18), 22(25)-(tetraphenoxy phthalocyaninato) lutetium(III) acetate (4) A mixture of anhydrous lutetium(III) acetate (134 mg, 0.38 mmol), 3-phenoxyphthalonitrile (2) (339 mg, 1.54 mmol) in 1-pentanol (2 mL) was refluxed for 7 h under nitrogen atmosphere with DBU as catalyst. After cooling, the crude product was precipitated with n-hexane, filtered and washed with excess n-hexane and then dried in air. Column chromatography (silica gel) was employed using THF:methanol (10:1) as the eluting solvent mixture. Compound (4) was obtained as the major product, which was recrystallized from hexane. Yield: 23%. IR [KBr, m, cm1] 748, 802, 862, 880, 969, 1024 (Pc skeleton), 1248, 1324, 1482 (C–O–C), 1727, 1771 (C@O, acetate), 2955 (C–H, aromatic), 3635 (CH3). UV–Vis (THF): kmax nm (log e) 321 (4.55) 430 (4.30) 627 (4.31), 690 (5.12). Anal. Calc. for C58H35N8O6Lu: C, 62.48; H, 3.16; N, 10.05. Found: C, 61.50; H, 4.86; N, 9.37%. 1H NMR (DMSO-d6): d, ppm 7.76–7.86 (12-H, m, Pc-H), 6.58–6.87 (20-H, m, Phenyl-H), 2.09 (3-H, s, acetate-CH3); MS (MALDI-TOF): (m/z): Calc. 1113; Found: 1119 [M+2H+]. 2.4.2. Synthesis of bis-{1(4), 8(11), 15(18), 22(25)-(tetraphenoxy phthalocyaninato)} dysprosium(III) complex (5) A mixture of anhydrous dysprosium(III) acetate (129 mg, 0.38 mmol), 3-phenoxyphthalonitrile (2) (339 mg, 1.54 mmol) in 1-pentanol (2 mL) was refluxed for 7 h under nitrogen atmosphere with DBU as catalyst. After cooling, the crude product was precipitated with n-hexane, filtered and washed with excess n-hexane and then dried in air. The crude product was purified as explained above for 4 to give 5 which was recrystallized from hexane. Yield: 40%. IR [KBr, m, cm1], 751, 893 (Pc skeleton), 1090, 1220, 1246, 1360, 1433 (C–O–C), 2954 (C–H, aromatic). UV–Vis (THF): kmax nm (log e), 430 (4.34), 625 (4.72), 690 (5.11). Anal. Calc. for C112H64N16O8Dy: C, 70.50; H, 3.91; N, 5.57. Found: C, 69.47; H, 4.86; N, 6.64%. MS (MALDI-TOF): (m/z): Calc. 1924; Found: 1926 [M+2H+].

1615

R. Zugle et al. / Polyhedron 30 (2011) 1612–1619

1.6 1.4

4

1.2

Absorbance

2.4.3. Synthesis of 1(4), 8(11), 15(18), 22(25)-(tetraphenoxy phthalocyaninato) erbium(III) chloride (6) A mixture of tetraphenoxyphthalocyanine (3, synthesis as reported in Ref. [25]) (500 mg, 0.56 mmol) and erbium chloride (164 mg, 0.6 mmol) was heated in 1-pentanol (2 mL) under reflux for 2 h in the presence of DBU and under nitrogen gas. The product was precipitated with hexane filtered and air dried. The crude product was then purified on a column chromatography (silica gel) with THF:methanol (10:1) as eluting solvent mixture. Finally the product was recrystallized from hexanes. Yield: 17%. IR [KBr, m, cm1] 784, 861, 886, 926, 1024 (Pc skeleton), 1120, 1201, 1249, 1363, 1474 (C–O–C), 2871, 2956 (aromatic C–H). UV–Vis (THF): kmax nm (log e) 322 (4.64), 623 (4.38), 690 (5.03). Anal. Calc. for C56H32N8O4ClEr: C, 62.07; H, 2.98; N, 10.34. Found: C, 62.04; H, 3.21; N, 10.10%. MS (MALDI-TOF): (m/z): Calc. 1084; Found: 1088 [M+4H+].

1.0 0.8 0.6 0.4 0.2 0.0 400

600

800

1000

1200

1400

1600

Wavelength/nm

1.2

5

The MALDI-TOF method was employed for mass spectrometry study, using 2,5-dihydroxybenzoic acid as the matrix. Compound 4 showed a molecular ion peak corresponding to a protonated species [M+4H]+ at 1119 amu which is consistent with the calculated values of 1115 amu. For compound 6, the most intense ion peak at 1088 amu corresponding to a protonated species [M+4H]+ is comparable to the calculated mass of 1084 amu. In the case of compound (5), a dysprosium bis-phthalocyanine, the molecular ion base peak occurred at 1926 amu, which compares well with the calculated value of 1924 amu and is consistent with the protonated species [M+2H]+, where M is (Pc(2)DyIIIPc(1) in this case, as will be proved by the near infra red spectra below. The matrix employed in this work is 2,5-dihydroxybenzoic acid, which is known [26] to intensify the fragmentation process hence the observed mass spectral data. 1H NMR data was consistent with the structure for 4. All the protons were observed in their respective regions. The Pc ring protons integrated for 12 and phenyl for 20 as expected. In addition, protons due to the acetate axial ligand were observed. The isomeric nature of the molecules only results in the broadening of the NMR spectra. Complexes 5 and 6 are paramagnetic hence no NMR data was obtained. Elemental analysis results were in agreement with the proposed structure. A mixture of four possible structural isomers is expected for the complexes. In this study, synthesized phthalocyanine compounds are obtained as isomeric mixtures as expected and no attempt was made to separate them. The fact that for Er and Lu, the main product was the MPc species while for Dy the main product was the DyPc2 is consistent with the tendency of the lighter lanthanides to form complexes with higher coordination number than the heavier ones. The three phthalocyanines are all soluble in most polar organic solvents such as DMF, DMSO, THF and DCM, this is typical for phthalocyanines with alkyl and alkoxy substituent groups in either or both peripheral and non-peripheral positions of the phthalocyanine framework [27]. The UV–Vis spectra of the phthalocyanine complexes of the three lanthanides in THF and DMSO are shown in Figs. 1 and 2, respectively and the Q band maxima are listed in Table 1. The spectra are typical of metallated phthalocyanines, with a similar Q-band absorption maximum of 690 nm in THF. The introduction of the phenoxy substituents and lanthanide central metals into the phthalocyanine macrocycle has led to appreciable red shift in the visible region compared to unsubstituted ZnPc (Q band maximum at 670 nm) [28]. Such a red shift in phenoxy substituted phthalocyanines has also been reported by Lukyanets et al. [29]. In DMSO the peaks are more red-shifted than

0.8

0.4

0.0 400

600

800

1000 1200 1400 1600

Wavelength/nm 1.2 6

1.0 Absorbance

3.1. Spectral characterization of the complexes

Absorbance

3. Results and discussion

0.8 0.6 0.4 0.2 0.0 400

600

800

1000 1200 1400 1600

Wavelength/nm Fig. 1. UV–Vis spectra of compound (a) 4 (1.7  105 M), (b) 5 (1.5  105 M) and (c) 6 (8.2  106 M in THF.

in THF, Table 1. This is attributed to the fact that DMSO is more polar than THF. A similar observation and explanation have been put forward for the changes in band positions of Ti(IV)Pc complexes with changes in polarity of the solvent [30]. In order to further ascertain whether the phthalocyanine complexes are monomeric complexes or bis-phthalocyanines, the ground state electronic absorptions of the compounds were extended to 1600 nm. It has been reported that a broad near-IR band near 900 nm is highly characteristic of lanthanide double deckers which contain a hole in one of the ligands [31]. As shown in

1616

R. Zugle et al. / Polyhedron 30 (2011) 1612–1619

1.35  106 M to 1.04  105 M. All the complexes showed no aggregation in DMSO and THF at concentrations less than 1  105 M.

1

(a)

0.9

Absorbance

0.8

(b) (c)

0.7 0.6

3.2. Photophysical properties

0.5 0.4 0.3 0.2 0.1 0 300

400

500

600

700

800

Wavelength/nm Fig. 2. UV–Vis spectra of (a) 4 (5.7  106 M), (b) 5 (7.0  106 M), (c) 6 (6.1  106 M) in DMSO.

Table 1 Molar extinction coefficients (log e) of 4, 5, 6 in THF and DMSO at the Q-band region and the corresponding emission maxima. Complex

Solvent

Q-band absorption kmax (nm) (log e)

Fluorescence maxima kmax (nm)

4

THF DMSO THF DMSO THF DMSO

690 695 690 694 690 696

699 705 719 708 710 706

5 6

(5.12) (5.16) (4.87) (5.04) (5.03) (5.11)

Fig. 1, the spectrum of compound 5 (circle) has a weak near IR band near 920 nm, which is a diagnosis for the presence of an oxidized phthalocyanine ring in lanthanide bis-phthalocyanines. Thus complex 5 is the green neutral Pc(2)DyPc(1). Chemical reduction of this complex resulted in the change of the colour from green to blue and the disappearance of the band at 920 nm, as is typical for reduction of Pc(2)DyPc(1) to [Pc(2)DyPc(2)]. The spectrum also confirms that the complex is a double decker phthalocyanine complex as indicated by its mass spectral data. On the other hand, the spectra of compounds 4 and 6 do not contain such near IR absorption bands, Fig. 1, which also confirms their monomeric nature of the complexes. Fig. 3 shows the dependence of the absorbance of compound 4 (as a representative of the other complexes) on concentration and the insert is a linear calibration plot at 690 nm, showing that the Beer–Lambert law is obeyed for concentrations ranging from

3.2.1. Triplet state quantum yields and lifetimes Complexes 5 and 6 showed no triplet state decay curves. Dysprosium(III) and erbium(III) are open shell, paramagnetic metal ions which cause a short-lived excited triplet state [32,33]. The lifetime of the excited triplet state seems to be faster than the resolution of the flash photolysis system used. Also for DyPc2 (5), the strong intramolecular p–p interactions increase relaxation routes and therefore decreases the triple-state lifetime [34]. In the case of complex 4, a triplet state quantum yield of 0.83 was obtained after three replicate measurements in DMSO. The triplet state lifetime of 25 ls was obtained. Lutetium(III) is a closed shell diamagnetic ion and that could explain the observed triplet state behavior which is completely different from the two other lanthanides, dysprosium and erbium. The corresponding triplet state decay profile which follows first order kinetics is shown in Fig. 4. The triplet–triplet absorption peak was observed at 478 nm for complex 4. 3.2.2. Fluorescence spectra, quantum yields and lifetimes The fluorescence properties of the three investigated complexes (4–6) were studied in DMSO degassed with argon. The fluorescence spectra were mirror images of the excitation spectra for 4–6, Fig. 5. The proximity of the Q band maxima of the absorption and excitation spectra for all complexes suggests that the nuclear configurations of the ground and excited states are similar and not affected by excitation in DMSO. Fluorescence band maxima are listed in Table 1. Stokes shifts were typical for MPc complexes [34]. The fluorescence quantum yields (UF) values were determined by the comparative method and the values were all lower than 0.01. These low UF values could be attributed to the heavy atom effect of the central metals, which encourages intersystem crossing to the triplet state. A similar argument has also been suggested for hafnium phthalocyanines which do not fluoresce at all [35]. In the case of compounds 5 and 6 the central metals are of paramagnetic nature. Paramagnetic metal ions enhance the yield of the triplet state but inevitably shortens the lifetime of the excited state [33]. The triplet lifetime of phthalocyanines containing a central paramagnetic transition metal have been found to be very short (nanosecond scale), hence the lack of triplet decay curves

1.6

Change in absorbance(A)

1.2 Absorbance

Absorbance

(h)

0.8

1.6 1.2 0.8 0.4 0 0

0.4

5

10

15

Concentration/x10 -6Moldm-3

(a)

0 300

400

500

600

700

800

Wavelength/nm

0.2

0.1

0.0 0

50

100

150

200

250

Time/µs Fig. 3. UV–Vis spectra of compound 4 showing variation of absorbance with concentration (a) minimum concentration: 1.35  106 M and (h) maximum concentration 1.04  105 M.

Fig. 4. Triplet state decay curve of compound 4 in DMSO at 478 nm. Excitation wavelength = 695 nm.

1617

R. Zugle et al. / Polyhedron 30 (2011) 1612–1619

1.2

1

measurement fit

0.8

counts

8000

Em

0.6

4000

Abs Ex

0.4

6000

2000

Residuals

Intensity (Normalised)

10000

0.2

0

600

650

700

750

800

0 6 4 2 0 -2 -4 -6

5

Wavelength/nm

Intensity (Normalised)

15

20

25

30

time [ns] Fig. 6. Fluorescence decay profile for compound 4 in DMSO.

1.2

1

Table 2 Fluorescence lifetimes and amplitudes of compounds 4, 5 and 6 in DMSO.

Em

MPc

0.8

Abs 4 5 6

0.6

Ex

s1F /ns (relative

s2F /ns (relative

amplitude)

amplitude)

s3F /ns (relative amplitude)

0.09 ± 0.01 (0.96) 0.10 ± 0.05 (0.53) 0.08 ± 0.02 (0.96)

1.70 ± 0.60 (0.01) 2.20 ± 0.20 (0.17) 2.40 ± 0.50 (0.02)

4.70 ± 0.20 (0.03) 5.00 ± 0.10 (0.30) 4.60 ± 0.10 (0.02)

0.4

0.2

0 600

650

700

750

800

Wavelength/nm 1.2

Intensity (Normalised)

10

1

Em

0.8

Abs

0.6

Ex 0.4 0.2 0 600

650

700

750

800

Wavelength/nm Fig. 5. Fluorescence (Em), absorption (Abs), excitation (Ex) spectra in DMSO for 4 (a), 6 (b), 5 (c).

for complexes 5 and 6 as discussed above. The low triplet lifetimes impose a severe limitation on the use of phthalocyanines containing paramagnetic central metals as photosensitizers. The fluorescence decay profiles Fig. 6, for compound 4, of the phthalocyanines are characterized by three exponential decay processes. The lifetimes and amplitudes of all compounds are listed in Table 2. Biexponential fluorescence decay profiles of phthalocyanines are attributed to the formation of ground-state dimers which

can quench the monomer fluorescence, leading to a quenched and unquenched lifetime [36]. The longer (unquenched) lifetime is attributed to the monomer and the shorter lifetime is attributed to quenched lifetime of the dimer [36]. In this work three lifetimes were observed. The short-lived component (s1F ) of the decay profile could then be attributed to the set of molecules in which the coordinating phthalocyanine ligand and the lanthanide ion behave as coordinating entities resulting in the observed fluorescence. This is a general characteristic feature in fluorescence of lanthanides Pcs which due to their big mass, have shorter fluorescence lifetimes than the metal free Pcs or porphyrins [37,38]. The long lifetime (s3F ), Table 2, could then be attributed to some small amount of Pc molecules in which lanthanide ions seem to have been lost and the observed fluorescence attributed to the coordinating phthalocyanine without the central metal. The values for s3F are in the range observed for monomeric phthalocyanines [36]. The intermediate lifetime (s2F ) might be due to metal free Pc dimers or oligomers which are formed in the solution. The value of s2F lies in the range of the quenched lifetimes in MPc complexes [36]. The estimated amplitudes of all three compounds confirm the above assignments. In compounds 4 and 6 which are supposed to be monomeric the amplitude of s1F is about two to three orders of magnitude greater than the amplitudes of s2F and s3F . Here only very small amounts of species without lanthanide central metal and dimers/oligomers are in the solution. In the case of compound 5 which is supposed to be dysprosium bis-phthalocyanine the amplitudes of s1F is only about two times bigger than the amplitude of the other two lifetimes. The reason for this is the lower stability of compound 5 which results in more Pcs which can lose their central metal (s3F ) and stay monomeric or form dimers/oligomers (s2F ). 3.2.3. Singlet oxygen quantum yield UD Singlet oxygen quantum yield (UD) values are expected to depend on the corresponding triplet quantum yield (UT) values of

1618

R. Zugle et al. / Polyhedron 30 (2011) 1612–1619

the photosensitiser. That is if the triplet state of a photosensitiser is populated, it can then interact with ground state triplet molecular oxygen exciting it to its singlet excited state. Compounds 5 and 6 have no observed triplet state population as stated above, possibly due to the open-shell, paramagnetic nature of Dy3+ and Er3+. In the case of compound 4 which gave a very high triplet quantum yield, because of its closed-shell diamagnetic nature of Lu3+, there was evidence of its ability to generate singlet oxygen with quantum yield of 0.71, which was detected by the phosphorescence decay of singlet oxygen at 1270 nm in THF. Fig. 7 shows the singlet oxygen decay curve profile for complex 4 in THF. This solvent was found to give better results for singlet oxygen detection compared to DMSO. The latter was however employed for triplet state studies since it gave the best results. However, even though direct comparison between UT and UD is not appropriate due to the use of different solvents, the obtained UD is reasonable and suggests that it is a potential photosensitizing agent. To further demonstrate the ability of compound 4 to generate singlet oxygen, 1,3-diphenylisobenzofuran (DPBF) was used as a singlet oxygen quencher and its conversion by the singlet oxygen was monitored by UV–Vis spectroscopy, Fig. 8. There were no changes in the Q-band intensities during the process suggesting that the phthalocyanine was not degraded, but DPBF degraded

due to singlet oxygen production by complex 4. For complexes 5 and 6 there were no changes in the DPBF spectra even after long period of light exposure in the presence of these complexes. That is the DPBF was not degraded as evidence of the presence of singlet state oxygen, due to lack of singlet oxygen production by these complexes. 4. Conclusion In this work we have found that the type of phthalocyanine complex (whether monomeric or oligomeric) with lanthanide ions depend more on the particular metal rather than the phthalonitrile/metal ratio. The open-shell dysprosium bis-phthalocyanine complex and the monomeric complex of the open-shell erbium are neither fluorescent nor showed the ability to generate singlet state molecular oxygen. On the other hand, the monomeric phthalocyanine complex of the close shell lutetium is a promising photosensitizer. Acknowledgements This work was supported by the Department of Science and Technology (DST) and National Research Foundation (NRF), South Africa through DST/NRF South African Research Chairs Initiative for Professor of Medicinal Chemistry and Nanotechnology as well as Rhodes University.

0.25

References

Intensity/a.u

0.20

0.15

0.10

0.05

0.00

0

20

40

60

80

100

Time/µ s Fig. 7. Singlet oxygen phosphorescence decay profile for compound 4 in THF.

1.4

DPBF

Absorbance

1.2 1 0.8

4

0.6 0.4 0.2 0 300

400

500

600

700

800

Wavelenght/nm Fig. 8. UV–Vis monitored conversion of DPBF by excited state singlet oxygen. Complex 4.

[1] G.E. Buono-Core, H. Li, B. Marciniak, Coord. Chem. Rev. 90 (1990) 55. [2] W.-N. Wu, N. Tang, L.J. Yan, Fluoresc 18 (2008) 101. [3] A. Edward, T.Y. Chu, C. Claude, I. Sokolik, Y. Okamoto, R. Dorisinville, Synth. Met. 84 (1997) 433. [4] C.F. Meares, T.G. Wensel, Acc. Chem. Res. 17 (1984) 202. [5] A. Kukhta, E. Kolesnik, I.J. Grabchev, Fluoresc 16 (2006) 375. [6] F.B. Wu, C. Zhang, Anal. Biochem. 311 (2002) 57. [7] N.A. Kuznetsova, N.S. Gretsova, V.M. Derkacheva, S.A. Mikhalenko, L.I. Solov’eva, O.A. Yuzhakova, O.L. Kaliya, E.A. Luk’yanets, Russ. J. Gen. Chem. 72 (2002) 300. [8] C.C. Leznoff, A.B.P. Lever (Eds.), Phthalocyanines: Properties and Applications, VCH Publishers, New York, 1996. vols. 1–4. [9] K. Kadish, K.M. Smith, R. Guilard (Eds.), The Porphyrin Handbook, Academic Press, Boston, 2003. vols. 15–20. [10] A.L. Thomas, Phthalocyanine Research and Application. CRC Press, 1990. [11] F.H. Moser, A.L Thomas (Eds.). Phthalocyanine Properties 1, CRC Press, 1983. [12] T.N. Sokolova, T.N. Lomova, V.V. Morozov, B.D. Berezin, Koord. Khim. 20 (1994) 637. [13] I.P. Kalashnikova, S.E. Nefedov, L.G. Tomilova, N.S. Zefirov, Russ. Chem. Bull. Int. Ed. 56 (2007) 2426. [14] R. Weiss, J. Fischer, in: K.M. Kadish, K.M. Smith, R. Guilard (Eds.), The Porphyrin Handbook, vol. 16, Academic Press, New York, 2003, p. 171. [15] M. M’Sadac, J. Ronclay, F.J. Garrier, Chim. Phys. Phys.-Chim. Biol. 83 (1986) 211. [16] S. Bo, D. Tang, X. Liu, Z. Zhen, Dyes Pigm. 76 (2008) 35. [17] N.B. McKeown, J. Painter, J. Mater. Chem. 4 (1994) 1153. [18] J.R. Lakowicz, Principles of Fluorescence Spectroscopy, second ed., Kluwer Academic/Plenum Publishers, New York, 1999, p. 35. [19] P. Kubàt, J. Mosinger, J. Photochem. Photobiol. A 96 (1996) 93. [20] T.H. Tran-Thi, C. Desforge, C. Thiec, J. Phys. Chem. 93 (1989) 1226. [21] S. Fery-Forgues, D. Lavabre, J. Chem. Ed. 76 (1999) 1260. [22] A. Ogunsipe, J.-Y. Chen, T. Nyokong, New J. Chem. 28 (2004) 822. [23] L. Kaestner, M. Cesson, K. Kassab, T. Christensen, P.D. Edminson, M.J. Cook, I. Chambrier, G. Jori, Photochem. Photobiol. Sci. 2 (2003) 660. [24] M.S. Patterson, S.J. Madsen, R.J. Wilson, Photochem. Photobiol. B. Biol. 5 (1990) 69. [25] M. Idowu, T. Nyokong, J. Photochem. Photobiol. A 199 (2008) 282. [26] A.Y. Tolbin, V.E. Pushkarev, G.F. Nikitin, L.G. Tomilova, Tetrahedron Lett. 69 (2009) 4848. [27] A. Beck, K.M. Mangold, M. Hanack, Chem. Ber. 124 (1991) 2321. [28] M.J. Stillman, T. Nyokong, in: C.C. Leznoff, A.B.P. Lever (Eds.), Phthalocyanines: Properties and Applications, vol. 1, VCH, New York, 1989 (Chapter 3). [29] E.A. Luk’yanets, V.M. Derkacheva, J. Gen. Chem. USSR 50 (1980) 1874. [30] P.D. Hale, W.J. Pietro, M.A. Ratner, D.E. Ellis, T.J. Marks, J. Am. Chem. Soc. 109 (1987) 5943. [31] Y. Brien, R. Wang, D. Wang, P. Zhu, R. Li, J. Dou, Helv. Chim. Acta 87 (2004) 2581. [32] T. Nyokong, Coord. Chem. Rev. 251 (2007) 1707.

R. Zugle et al. / Polyhedron 30 (2011) 1612–1619 [33] J.R. Darwent, P. Douglas, A. Harriman, G. Poter, M.C. Richoux, Coord. Chem. Rev. 44 (1982) 83. [34] M.O. Liu, C.-H. Tai, A.T. Hu, T.-H. Wei, J. Organomet. Chem. 689 (2004) 2138. [35] L.A. Tomachynski, I.N. Tretyakova, V.Ya. Chernii, S.V. Volkov, M. Kowalska, J. Legendziewicz, Y.S. Gerasymchuk, S. Radzki, Inorg. Chim. Acta 361 (2008) 2569.

1619

[36] J.A. Lacey, D. Phillips, Photochem. Photobiol. Sci. 1 (2002) 378. [37] M.P. Tsvirko, G.F. Stelmakh, V.E. Pyatosin, K.N. Solovyov, T.F. Kachura, A.S. Piskarskas, R.A. Gadonas, Chem. Phys. 106 (1986) 467. [38] M. Durmus, S. Yesilot, B. Cosut, A. Guel Guerek, A. Kilic, V. Ahsen, Synth. Met. 160 (2010) 436.