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Chinese Science Bulletin © 2008
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Solvent effects on photophysical properties of copper and zinc porphyrins LI Ye† The photophysics of Zn(tetraphenylporphyrin,TPP), Zn(tetra-2,4,6-trimethylphenyl porphyrin, TMP), Zn (tetra-(o-dichlorophenyl) porphyrin, TPPCl8), Cu(tetraphenylporphyrin,TPP), Cu(tetra-2,4,6-trimethylphenyl porphyrin,TMP), and Cu(tetra-(o-dichlorophenyl) porphyrin, TPPCl8, TPPCl8) in several solvents have been investigated on steady state and time-resolved spectroscopy. The Cu(TPPCl8 ) is normal and shows no evidence of CT transition in the visible or near UV regions in nonpolar solvent. However, Cu(TPPCl8)shows a blue shift in the absorption spectrum and intramolecular CT bands at absorption spectra in polar solvent, which shows a fluorescence maximum emission at 650 nm and 8.4 ns lifetime. The reason can be attributed to two points. Firstly, the increase of solvent polarity can enlarge outer reorganisational energy, which is favorable to reduce the activation free energy of charger-transfer transition based on Marcus theory of electron transfer. Moreover, the internal heavy-atom effect on Cu(TPPCl8) is encouraging to stabilize the 2T1 state also, which increases the possibility of population to CT band from 2T1 state. This result is in accord with an earlier estimate of a 10 ns lifetime and CT absorption at 640 nm bands for the CT state of Cu (II) octethylporphyrins. Other possible reasons arousing unusual fluorescence like H-bonding, axial ligands, molecular aggregation are excluded. metalloporphyrins, fluorescence, charge transfer, polarity
The photophysical properties of metalloporphyrins have been extensively studied over recent years, primarily due to their importance in biological systems[1–2]. Their photophysical properties exhibit a huge difference in incorporating different metals into the center of the ring or grafting various substituents at their peripheral positions. The lowest energy-excited state of paramagnetic metal Zn porphyrins and Mg porphyrins can be described as normal S1 state (π, π*) with a very strong fluorescence[3]. But other remaining first-row transition metal porphyrins, charge-transfer-excited state and tripmultiplet-excited state have to be considered. In some metalloporphyrins like Fe (III), Cr (III) and Mn(II), the interaction between metal and porphyrin gives rise to charge transfer (CT) absorption bands[4]. However, copper porphyrins, and the absorption spectra are normal and show no evidence of CT transition in the visible or near UV regions. www.scichina.com | csb.scichina.com | www.springerlink.com
Moreover, it is well known that the energy level of the CT transition is particularly sensitive to molecular geometry, the binding of axial ligand, and solvent polarity [5]. In this work, the photophysics of Zn(TPP), Zn(TMP), Zn(TPPCl 8 ), Cu(TPP), Cu(TMP), and Cu(TPPCl8) in several solvent have been investigated on steady state and time-resolved spectroscopy. Effects of polarity of solvents on photophysics of those metalloporphyrin are discussed. The Cu(TPPCl8) is proved to have no aggregates in protic solvent. The CT bands of Cu(TPPCl8) porphyrin are also observed in absorption spectra at 620 nm with lifetime of 8.4 ns. For other Cu(II) metalloporphyrins, fluorescence emission is undetectable, Received April 11, 2008; accepted October 5, 2008 doi: 10.1007/s11434-008-0498-8 † Corresponding author (email:
[email protected]) Supported by National Natural Science Fundation of China (Grant No. 20873005), Beijing Natural Science Fundation (Grant No. 2083028), Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry and 422 Funds from University of Science and Technology Beijing
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PHYSICAL CHEMISTRY
Department of Chemistry, School of Applied Science, Beijing University of Science and Technology, Beijing 100083, China
which is attributed to difference in their molecular geometry.
1 Materials and methods Zinc tetraphenylporphyrin (99.5%, Alfa Aesar), mesotetra (o-dichlorophenyl) porphine, meso-tetra (2,4,6trimethylphenyl) porphine, meso-tetraphenylporphine (low chlorin, Frontier Scientific Inc.), and Sephadex LH-20 were bought from Fluka BioChemika. Dichloromethane (analytic grade) methanol (HPLC grade), copper acetate, and zinc acetate were all purchased from Aldrich Chemical Company. Zinc (II) and Copper (II) metalloporphyrins were prepared according to a reported method of Adler et al[6]. The chlorine impurities in synthetic metaloporphyrins were oxidized by 2,3-dichloro-5, 6-dicyano-p-benzoquinone (DDQ). A dry-column chromatography procedure was used to remove DDQ and impurities using dichloromethane as the mobile phase[7]. Absorption spectra were measured on a Cary 500 ultra-vis spectrophotometer. Fluorescence emission spectra were measured on a F4500 spectrofluorimeter at room temperature. The sample was diluted with absorbance from 0.20 to 0.25 in experiments to ensure that the spectra were undistorted by reabsorption. All the ex-
periments were performed in deoxygenated solution obtained via a sequence of freeze-pump-thaw cycles. The quantum yields of porphyrins were estimated from the emission and absorption spectra by a comparative method using the equation with a correction for the difference in solvent index of refraction[8]. Fluorescence lifetimes were measured by time-correlated single photon counting using laser excitation and photon detection methods [9]. The experiment did not measure the fluorescence lifetimes of the species with quantum yield lower than 10−5 due to sensitivity limitations at the wavelengths, where the porphyrins emit and lifetimes shorter than 25 ps are beyond the time resolution of our apparatus.
2 Results and discussion 2.1 Absorption spectra and emission spectra The six different zinc (II) or copper (II) metalloporphyrins were employed in this work. The structures of those metalloporphyrins are shown in Figure 1. In Table 1, zinc porphyrins, and the photophysical data of Zn(TMP) are almost the same as those of Zn (TPP). It implies that methylation of the phenyl-substituted group has no effect on its excited state properties.
Figure 1 The structures of zinc (II) or copper (II) metalloporphyrins. (a) Metal meso-tetraphenylporphine, M = Zn2+ (ZnTPP) or Cu 2+(CuTPP); (b) metal meso-tetra (2,4,6-trimethylphenyl) porphine, M = Zn2+ (ZnTMP) or Cu 2+(CuTMP); (c) metal meso-tetra(o-dichlorophenyl)porphine, M = Zn2+ (ZnTPPCl8) or Cu 2+ (CuTPPCl8). Table 1 Spectroscopic and photophysical data for metalloporphyrins in dichloromethane at room temperature Absorption Spectra of Q bands Absorption Spectra of Soret band Luminescent wavelength Compound λmax (nm) λ max (nm), Logε (M−1 cm−1) λ max (nm), Logε (M−1 cm−1) Zn (TPP) 420(5.74) 552 (4.60) 587 603 Zn (TMP) 423(5.78) 547 (4.81) 585 605 Zn (TPPCl88AA) 421(5.61) 549 (4.32) 587 647 Cu (TPP) 414(5.66) 538 (4.28) 574 805b) Cu (TMP) 415(5.63) 540 (4.36) 575 775b) Cu (TPPCl8) 415(5.90) 541 (4.26) 576 756b) a) fluorescence emission lifetime; b) phosphorescences emission lifetime. 3616
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Lifetime (ns) 1.78a) 2.22a) 0.33a) 40b) 32b) 300b)
Figure 3 The fluorescence emission spectra of Cu(TPPCl8) in either dichloromethane (solid line) or methanol (dotted line).
Figure 2 Absorption spectra of Cu(TPPCl8) in dichloromethane (solid line) and methanol (dotted line). The small figure represents an enlarged Q band of Cu(TPPCl8) in dichloromethane (solid line) and methanol (dotted line).
The photophysical properties of Cu(TPPCl8) in other different solvents are summarized in Table 2. It is noticed that Cu(TPPCl8) also shows a measurable fluorescence emission in CH3CN and C2H5OH with almost the same lifetimes. The absorption spectra of Cu(TPPCl8) in CH3CN and C2H5OH also shows a blue shift of soret band and obvious CT bands around 620 nm, which is the same as those described in methanol. It is understandable because all of them are strong polar solvents. The charger-transfer transitions are usually observed in polar solvent based on the Marcus theory of electron transfer, since increase of solvent polarity can enlarge outer reorganisational energy, which is favorable to decreasing the decrease activation free energy of charger-transfer
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which can be attributed to CT-excited band. It is well known that the charger-transfer transitions involve considerable redistribution of electron density within the metalloporphyrins. It can be categorized as CT between porphyrins and metal. Moreover, the CT-excited state can be defined as a (d,π*) CT-excited state or a (π, d) CT-excited state. The former, a (d,π*) metal-to-ring CT-excited state, arises from promotion of the dX2-Y2 electron to a ring eg(π*) LUMO. The latter, a (π,d) ring-to-metal CT-excited state, originates from promotion of one charge of the ring-filled HOMOs to dX2-Y2. The fluorescence emission spectra of Cu(TPPCl8) in either dichloromethane or methanol are shown in Figure 3. Although fluorescence emission of Cu(TPPCl8) in dichloromethane (dotted line) is undetectable, fluorescence emission of Cu(TPPCl8) methanol (solid line) is noticeable with a maximal emission at 620 nm.
PHYSICAL CHEMISTRY
However, Zn(TPPCl8) demonstrates an obvious decrease in both fluorescence intensity and quantum yield. The lifetime is changed from 1.78 ns to 0.33 ns after incorporating chloro atoms because of internal heavy-atom effect[10]. The copper porphyrins, and the unpaired d-electrons on central metal copper ion interact in the porphyrins (π, π*) triplet state, resulting in formation of tripmultiplet state[11]. Intersystem-crossing from the singlet 2 S1 state results in population of tripmultiple 2T1 state which can undergo spin-conversion to the lower energy 4T1 state. Both state can undergo radioactive and non-radiative decay to ground state. The 4T1 → 2S0 transition is spinningly forbidden and the accompanying luminescence can be described as phosphorescence. The fluorescence emission (2T1 → 2S0) is undetected at room temperature. The Cu (TPPCl8) shows a sharp increase in phosphoresces lifetimes, compared to those of Cu(TPP) and Cu(TMP). It can be explained that tetraphenylporphyrins with halogens in ortho positions are particularly efficient in promoting 2S1 → 2T1 radiative transit (internal heavy-atom effect), leading to increase of phosphoresce lifetime [11]. More interestingly, Cu(TPPCl8) shows excited state relaxation with dependency on solvent effect. Typical absorption spectra of Cu(TPPCl8) in dichloromethane and methanol are given in Figure 2. In Figure 2, both Soret band and Q bands of Cu(TPPCl8) show a 5 nm blue shift in methanol compared to that in dichloromethane. Moreover, a new band is observed at 622 nm,
transition[12]. The charger transfer is generally reported in polar solvent and is unobservable in nonpolar solvent. Although Zn(TTP) was reported to have aggregates at moderate concentrations in freshly prepared dry acetonitrile solution at room temperature, the Zn(TTP) aggregate is metastable and inclined to disaggregate in dilute solution[13]. In our experiment, it was impossible for all metalloporphyrins to congregate because all spectroscopic measurements were finished in highly dilute solution. The decay processes are summarized in Figure 4. When Cu(TPPCl8) molecules were excited to 2S1, it can result in a rapid intersystem crossing to 2T1, and then populate to CT-excited state. CT-excited state can not populate directly from 2S1. Although Yan and Holten[14] once reported a 10 ns lifetime and a CT absorption band at 640 nm for Cu (II) octethylporphyrins (Cu OEP) in methylcyclohexane, in our experiment, the 8.4 ns lifetimes component coming from CT-excited state is only observed in polar solvent. Moreover, the internal heavyatom effect on Cu(TPPCl8) is helpful to stabilize the 2T1 state also because it increases the possibility of population to CT band from 2T1 state.
Figure 4 Schematic energy diagram for Cu(TPPCl8) and origin of CT band emission.
The excited state lifetime of Cu(TPPCl8) in methanol measured on time-correlated single photon counting is shown in Figure 5. It was well fitting with one exponential. Likewise, the Cu(TPPCl8) in other solvents is listed in Table 2 in order to explore the reasons of solvents to cause a big difference of its photophysical properties. The Cu(TPPCl8) in neither no-coordinating solvents like chloroform, toluene and benzene nor weak or strong coordinating solvents, DMF was found to have fluores3618
Figure 5 The excited state lifetime of Cu(TPPCl8) in methanol measured with time-correlated single photon counting method fitting with one exponential.
cent emissions or a CT bands in spectroscopic measurement. Those results accord with the above-mentioned explanation that charger-transfer transitions are usually observed in polar solvents. Besides, we do not assume the 8.4 ns lifetimes component comes from the remaining freebase H2TPPCL8 because it was measured with only 220 ps lifetime. It was reported that H-banding affected the properties of energy level in the excited state, which led to a change in red shift of fluorescence spectra and aroused the increase of the fluorescence yield after hydrogen bonding[15]. In order to analyze the influence of solvent polarity and hydrogen bonding on excited state energy and fluorescence properties of CuCl8(TPP), the steady fluorescence emission spectra of CuTPPCl8 were carried out in deuterated methanol. However, it was found that the fluorescence intensity of Cu(TPPCl8) is almost the same in deuterated methanol with certain concentration and in H-bonding methanol solution. Other Cu (II) metalloporphyrins, fluorescence emission of Cu(TPP) and Cu(TMP) are undetectable, which are attributed to difference in their molecular geometry and lack of CT state in molecules.
3 Conclusion The photophysical properties of several Zn (II) and Cu (II) metalloporphyrins are examined in various solvents. The results point out that the tetraphenylporphyrins with halogens in ortho positions of phenyl are particularly efficient in promoting 2S1 → 2T1 intersystem crossing because of internal heavy-atom effect, leading to in-
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Table 2 Photophysical data of copper and zinc porphyrins measured in different solvents at room temperature Zn (TPP)
Cu (TPP) Zn (TMP) Cu (TMP) Zn (Cl8TPP) Cu (Cl8TPP)
Solvent
Static dielectric constant 32.70
3.41 × 10−2
1.8
CH3CN
37.5
2.82 × 10−2
1.3
Methanol
32.70
< 1 × 10−5
Dichloromethane
8.93
< 1 × 10−5
Methanol
32.70
3.51 × 10−2
2.2
Dichloromethane
8.93
3.61 × 10−2
2.3
Methanol
32.70
< 1.0 × 10−5
−
Dichloromethane
8.93
< 1.0 × 10−5
−
Methanol
32.70
3.34 × 10−3
0.31
Dichloromethane
8.93
3.35 × 10−3
0.32
Methanol
32.70
4.70 × 10−3
8.4
EtOH
24.58
1.49 × 10−3
8.6
CH3CN
37.5
2.67 × 10−3
8.3
< 1.0 × 10−5
−
4 5
6 7
8
9
3.30 × 10
−
