Excited state dynamics of zinc and aluminum phthalocyanine carboxylates

Spectrochimica Acta Part A 68 (2007) 995–999

Short communication

Excited state dynamics of zinc and aluminum phthalocyanine carboxylates Mopelola Idowu a , Abimbola Ogunsipe a,b , Tebello Nyokong a,∗ a

Department of Chemistry, Rhodes University, Grahamstown 6140, South Africa b Department of Chemistry, University of Lagos, Lagos, Nigeria

Received 21 November 2006; received in revised form 22 January 2007; accepted 24 January 2007

Abstract Photophysical parameters for zinc and aluminium tetracarboxylphthalocyanines (ZnTCPc and AlTCPc, respectively) and their octacarboxy substituted counterparts (ZnOCPc and AlOCPc) were studied. Data for the fluorescence quenching of the complexes by benzoquinone (BQ) were treated using the Stern–Volmer analysis, and the quenching was found to follow a diffusion-controlled (dynamic) bimolecular mechanism. Theoretical values of bimolecular rate constant for complex-BQ interactions were determined using the Stokes–Einstein–Smoluchowski model; and the values, together with the Stern–Volmer quenching constants were used in calculating the fluorescence lifetimes of the complexes. The thermodynamics of the MPc-BQ interaction, in terms of solvent reorientation energy is also discussed. © 2007 Elsevier B.V. All rights reserved. Keywords: Carboxyphthalocyanine; Benzoquinone; Stern–Volmer; Fluorescence lifetime; Quenching

1. Introduction Metallophthalocyanines (MPcs) have been a focus of attention because of their exclusive properties. Their intense absorption in the red and near infra-red regions of the solar spectrum, ease of identification coupled with their non-toxicity, has made them attractive as chromophores for light-driven processes. MPcs have found applications in nonlinear optics [1], catalysis [2], liquid crystals [3] and as chemical sensors [4]. Worth mentioning is the complexes’ well-documented application in oncology [5] and in photovoltaic cells [6–8]. MPc complexes may also be used as photosynthetic mimickers. An essential requirement for a good photosynthetic mimicker is the ability to undergo excited state charge transfer with ease, and MPc-quinone systems have proved to be favoured candidates in this respect [9,10]. In this work, we present a study of the light harvesting and energy transducing tendencies of mixtures of some non-transition metal phthalocyanine carboxylates and 1,4-benzoquinone (BQ). Photophysicochemical studies on some phthalocyanine carboxylates has been documented [11]. An important prerequisite for light harvesters

∗

Corresponding author. Tel.: +27 46 6038260; fax: +27 46 6225109. E-mail address: [email protected] (T. Nyokong).

1386-1425/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2007.01.025

in a photovoltaic cell is appreciable photoactivity; hence an investigation of their photophysical characteristics is desirable. The photophysics of the MPc derivatives used in this work is studied. 2. Experimental and method 2.1. Materials Zinc and aluminium tetracarboxyphthalocyanines (ZnTCPc and AlTCPc, respectively) and their corresponding octacarboxy substituted counterparts (ZnOCPc and AlOCPc) were synthesized, purified and characterized according to literature methods (Fig. 1) [12,13], respectively. ZnPc was synthesized and characterized according to literature [14]. 1,4-Benzoquinone (BQ) was purchased from Aldrich and dimethylsulfoxide (DMSO, SAARCHEM) was dried before use. 2.2. Equipment Ground state electronic absorption spectra were recorded on a Varian 500 UV–vis/NIR spectrophotometer, fluorescence excitation and emission spectra recorded on a Varian Eclipse spectrofluorimeter. Laser flash photolysis experiments were performed with light pulses produced by a Quanta-Ray Nd:YAG

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Fig. 1. Chemical structures of (a) octacarboxyl-substituted and (b) tetracarboxyl-substituted zinc and aluminium phthalocyanine.

laser providing 400 mJ, 90 ns pulses of laser light at 10 Hz, pumping a Lambda-Physik FL3002 dye (Pyridine 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 a photomultiplier tube was used as detector. Signals were recorded with a two-channel digital real-time oscilloscope (Tektronix TDS 360) and kinetic curves were averaged over 256 laser pulses. 2.3. Photophysical studies Fluorescence (ΦF ) and triplet (ΦT ) quantum yields were determined by comparative methods [9,15] using ZnPc in DMSO as a standard (ΦF = 0.18 [16], ΦStd T = 0.65 [17]). Triplet lifetimes were determined by exponential fitting of the kinetic curves using the program OriginPro 7.5. Solutions for triplet yield and lifetime experiments were purged of oxygen (by bubbling nitrogen) before laser irradiation. Quantum yields of internal conversion (ΦIC ) were obtained from Eq. (1), which assumes that only three processes (fluorescence, intersystem crossing and internal conversion), jointly de-activate the excited singlet states of the complexes. ΦIC = 1 − (ΦF + ΦT )

(1)

2.4. Fluorescence quenching by benzoquinone Fluorescence quenching experiments on the various MPc complexes were carried out by the addition of varying concentrations of BQ to a fixed concentration of the respective MPc complexes, and the concentrations of BQ in the resulting mixtures were 0, 0.0028, 0.0056, 0.0084, 0.0112 and 0.0140 M. The fluorescence spectra of MPc complexes at each BQ concentration were recorded, and the changes in fluorescence intensity related to BQ concentration by the Stern–Volmer (Eq. (2)) [18]: I0 = 1 + KSV [BQ] (2) I where I0 and I are the fluorescence intensities of fluorophore in the absence and presence of quencher, respectively; [BQ] the concentration of the quencher, and KSV is the Stern–Volmer

constant, which is the product of the bimolecular quenching constant (kQ ) and the fluorescence lifetime τ F , i.e. KSV = kQ · τF

(3)

The theoretical bimolecular rate constant for diffusioncontrolled reactions (kD ) is related to the bimolecular quenching constant (kQ ) by Eq. (4) [19]: kQ = fkD

(4)

where f is the collision efficiency. The bimolecular rate constant (kD ) can be obtained from the Einsten–Smoluchowski relationship: kD = 4πNA (Df + Dq )(rf + rq )

(5)

where NA is the Avogadro’s number; Df and Dq the diffusion coefficients of the fluorophore and quencher, respectively; and rf and rq are the radii of fluorophore and quencher, respectively.The diffusion coefficient D is given by the Stokes’ equation (Eq. (6) [19]. D=

kT 6πηr

(6)

where k is the Boltzman constant; T the absolute temperature; η the solvent’s viscosity and r is the fluorophore (or quencher) radius. The radii were determined using ACD/ChemSketch program. 2.5. Thermodynamics of quenching 2.5.1. Solvent reorganization energy Solvent reorientation energies (λs ) were calculated using the Marcus dielectric continuum formula [20] (Eq. (7)),    1 1 e2 1 1 1 λs = (7) − + − 4πε0 n2 εs 2RA 2RD RAD where e is the electronic charge, ε0 the permittivity of vacuum; n and εs the solvent’s refractive index, and dielectric constant, respectively; RA and RD the molecular radii of the electron acceptor (BQ) and donor (MPc), respectively, while RAD is the sum of RA and RD .

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Fig. 2. Ground state electronic absorption spectra of ZnOCPc and AlOCPc (9.6 × 10−6 M) in DMSO.

3. Results and discussion 3.1. Ground state electronic absorption and fluorescence spectra Fig. 2 shows the ground state electronic absorption spectra of AlOCPc and ZnOCPc in DMSO. These spectra are typical of MPc complexes in organic solvents. AlTCPc and ZnTCPc also exhibit sharp absorptions in their Q band regions, as observed for AlOCPc and ZnOCPc in Fig. 2. It is believed that the band around 630 nm could have a partial contribution from n → ␲* [21]. It can be noted in Table 1 that the molar extinction coefficients of the Zn(II) complexes are slightly smaller than those for the corresponding Al(III) complexes; due to the heavy atom effect (which introduces some triplet character to the excited singlet state of the Zn(II) derivatives), the S0 → S1 transition probability is reduced, hence the molar extinction coefficients are expected to reduce. Fluorescence excitation and emission spectra of the complexes (Fig. 3) are normal for MPc complexes; the excitation spectra are identical to the absorption spectra, and both are mirror images of the emission spectra.

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of the hitherto spin-forbidden transition and reducing that for spin-allowed fluorescence. Triplet quantum yield values (ΦT , Table 1) show an opposite trend to that observed for ΦF . ΦT values are higher for the tetrasubstituted derivatives than for the octasubstituted derivatives, implying that most of the non-fluorescing molecules in the tetrasubstituted derivatives (AlTCPc, ZnTCPc) undergo intersystem crossing to the excited triplet (T1 ) state. Comparing species with the same number of ring substituents, ΦT values are higher for the zinc-complexed species than for the aluminiumcomplexed species, which is attributed to stronger spin–orbit coupling induction by Zn(II) ion compared to that by Al(III) ion, as discussed above. Triplet lifetimes (τ T ) are shorter for the zinc-complexed species than for the aluminium-complexed species. The reason that the S1 → T1 intersystem crossing is more promoted in the zinc-complexed species should also suffice in explaining the shorter triplet lifetimes of these complexes. Quantum yields of internal conversion (ΦIC , Table 1) are higher for the octasubstituted derivatives than for the corresponding tetrasubstituted derivatives. The eight substituent groups are expected to be more effective in presenting pathways through which excitation energy can “leak out” non-radiatively (via internal conversion). Rate constants for the intrinsic processes deactivating the excited singlet state (kF , kISC and kIC ) are shown in Table 2. These values follow the same trends as their respective quantum yield values. 3.3. Fluorescence quenching by benzoquinone (BQ) Fig. 4 shows the spectral changes accompanying the fluorescence quenching of AlTCPc by BQ in DMSO, as well as

3.2. Photophysical parameters The photophysical parameters of AlTCPc, ZnTCPc, AlOCPc and ZnOCPc are shown in Table 1. Fluorescence quantum yields (ΦF ) are lower for the tetrasubstituted derivatives AlTCPc, ZnTCPc than for the corresponding octasubstituted ones—AlOCPc and ZnOCPc. As expected on the basis of spin–orbit coupling, ΦF values are lower for Zn(II) complexes than for the Al(III) complexes. Considering the relative masses of the central metal ions, Zn(II) will induce a stronger spin orbit coupling than Al(III), thereby enhancing the likelihood

Fig. 3. Normalized fluorescence excitation and emission spectra of ZnOCPc in DMSO.

Table 1 Spectral and photophysical parameters for MPc complexes in DMSO

AlOCPc ZnOCPc AlTCPc ZnTCPc

λmax Q band (nm)

log ε

λmax Ems (nm)

ΦF

ΦT

ΦIC

τ T (␮s)

707 700 690 688

5.20 5.18 5.21 5.00

714 708 698 696

0.44 0.24 0.33 0.16

0.34 0.62 0.56 0.82

0.22 0.14 0.11 0.02

1270 480 340 240

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Scheme 1. Mechanism of charge transfer in MPc-quinone mixture.

the corresponding Stern–Volmer plot for the course of quenching. The Stern–Volmer plots for all four complexes gave straight lines, depicting diffusion-controlled quenching mechanisms. In addition, there were no changes in the absorption spectra of the complexes in the presence of BQ; hence the quenching mechanism is dynamic and not via the formation of non-fluorescent ground state complexes (static quenching). Again, energy transfer from the excited MPc to BQ is not plausible, as the energy of the lowest excited state for quinones is greater than the energy of the excited singlet state of MPc complexes [22]. These facts point to the conclusion that MPc fluorescence quenching by BQ

Fig. 4. Spectral effects of AlTCPc (1.24 × 10−5 mol dm−3 ) quenching by benzoquinone (BQ). Excitation wavelength = 630 nm.

is via excited state electron transfer, with the MPc being the electron donor, and BQ, the electron acceptor. Scheme 1 explains the mode of interaction of BQ with an excited MPc complex. kQ is the diffusion-controlled bimolecular quenching constant, kCT , the charge transfer rate constant, and knr , the rate constant for the non-radiative decay of the non-fluorescent ion–radical pair, which also involves back electron transfer. Stern–Volmer constants (KSV ), Table 2, for fluorescence quenching by BQ are within the same range for all four MPc complexes in this work; this may be attributed to the similarity in structure and chemical nature of the MPc complexes. One expects the complexes to exhibit somewhat similar reduction potentials, substituent parameters (Hammett and Taft) and diffusion coefficients. The diffusion coefficients (Eq. (6), Table 2) of the complexes range between 2.34 and 2.65 × 1010 m2 s−1 . Bimolecular quenching constants (kQ ), calculated from Eqs. (3)–(5) (assuming that collision efficiency, f, is unity) for the MPc complexes range between 5.58 and 5.75 × 109 M−1 s−1 ; these values are very close to 1010 M−1 s−1 , which is in agreement with the Einstein–Smoluchowski approximation at room temperature for diffusion-controlled bimolecular interactions [23]. Fluorescence lifetimes, τ F (Eq. (3), Table 2), values are within the expected range for MPc derivatives [22,23]. As this treatment is a relatively new dimension to the determination of τ F [9], it is necessary to compare the results with those obtained from familiar procedures. The values obtained from our treatment compare well with those obtained using the Strickler–Berg equation-adapted software by Lindsey group (PhotochemCAD

Table 2 Fluorescence quenching and kinetic data for MPc complexes in DMSO

AlOCPc ZnOCPc AlTCPc ZnTCPc a b

KSV (M−1 )

˚ RMPc a (A)

10−10 DMPc (m2 s−1 )

10−9 kQ (M−1 s−1 )

τ F(MPc) b (ns)

10−8 kF (s−1 )

10−8 kISC (s−1 )

10−7 kIC (s−1 )

λS (eV)

13.25 18.68 16.22 13.04

10.87 10.90 9.65 10.30

2.35 2.34 2.65 2.48

5.74 5.75 5.58 5.66

2.3 (1.8) 3.3 (3.5) 2.9 (3.1) 2.3 (2.3)

1.90 0.74 1.13 0.70

1.47 1.91 1.92 3.57

9.52 4.31 3.78 0.87

0.6564 0.6577 0.6548 0.6556

RBQ = 3.9 × 10−10 [25]. RMPc were calculated using ACD/ChemSketch program. Values in brackets were obtained using PhotochemCAD software [24].

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[24]), as given in Table 2. The fluorescence lifetimes range between 1.8 and 3.5 ns, and do not follow a well-defined trend among the MPc complexes studied. 3.4. Solvent reorganization energy Marcus [20] suggested that a reorganization of the solvent molecules around the electron donor and acceptor occurs prior to the electron transfer process. The clustering of solvent molecules around the interacting species inhibits electron transfer; hence a solvent reorganization is essential for the electron transfer process. The reorganization would involve a partial disengagement of solvent molecules attached to the donor and acceptor, leading to an increase in entropy. This entropy increase provides a driving force for the spontaneity of the reaction via a negative free energy value. Solvent reorganization energies were calculated according to Eq. (7), and the values are essentially the same for all the complexes (∼0.66 eV). This is not unexpected, because the same solvent was employed in all cases. Again, the interacting species were of similar physical and chemical nature. 4. Conclusion In conclusion, this work provides evidence that metallophthalocyanines carboxylates of aluminium and zinc are wellsuited for catalytic applications in light-driven processes. Their high yields of metastable excited state particularly should make them suitable as photosensitizers in PDT and other applications. The effective quenching of the complexes’ fluorescence by benzoquinone suggests that systems that are composite of MPc carboxylates and quinones could well serve as good light harvesters and energy transducers. We are at present looking at the possibility of architecting MPc-quinone conjugates, which should be more efficient as a mimicker of the natural photosynthesis. Acknowledgements This work was supported by Rhodes University and by the National Research Foundation (NRF, GUN 2053657) of South Africa.

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