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Ground state charge transfer complex formation of some metalloporphyrins with aromatic solvents
Chemical Physics Letters 592 (2014) 149–154
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Ground state charge transfer complex formation of some metalloporphyrins with aromatic solvents Mihir Ghosh, Biswajit Roy, Abhimanyu Jha, Subrata Sinha ⇑ Integrated Science Education and Research Centre, Siksha-Bhavana, Visva-Bharati, Santiniketan 731 235, India
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Article history: Received 13 September 2013 In final form 18 December 2013 Available online 25 December 2013
a b s t r a c t Porphyrin derivatives are known to have absorption spectra in the visible region (400–700 nm). In the present Letter, a new (to the best of our knowledge) broad band is reported in the NIR region (700–900 nm) of the absorption spectra of zinc tetraphenylporphyrin, zinc octaethylporphyrin and magnesium octaethylporphyrin in aromatic solvents at high concentrations (10 4 mol/L). The broad absorption band in the NIR region is ascribed to be due to ground state charge transfer complex formation between the presently used metalloporphyrins and aromatic solvents. Crown Copyright Ó 2013 Published by Elsevier B.V. All rights reserved.
1. Introduction Porphyrin derivatives play very important roles in various fields of science viz., chemistry, physics and biology. Porphyrins generally have absorption bands in the visible region (400–700 nm). They exhibit a wide variety of redox properties. Furthermore, it is possible to control the photochemical and electrochemical properties of porphyrin derivatives by modification of the substituents and selection of the central metal atom. These unique properties of porphyrins have attracted great interest among various research groups for potential applications in solar energy harvesting and artificial photosynthesis [1–3], anticancer pharmaceutical drugs [4], photodynamic destruction of virus [5] and photodynamic therapy [6]. The electronic absorption spectra of a free base porphyrin consist of an intense band at about 400 nm, called Soret band (or B band) and four weaker bands in the visible region (about 500– 700 nm), called Q bands. It is well known that p, p⁄ transitions from the ground singlet state to the second excited singlet state (S0 ? S2) and from the ground singlet state to the first excited singlet state (S0 ? S1) are responsible for the B and Q bands, respectively. Metalation of the free base porphyrin leads to more symmetrical structure and consequently the four Q bands in the absorption spectra collapse into two. The origins of Q and B bands in the absorption spectra (visible region, 400–700 nm) of porphyrins and metalloporphyrins were first proposed by Martin Gouterman and co-workers [7–9] based on the four-orbital model in the 1960s. Here, we report the observation of a new, to the best of our knowledge, broad band in the NIR region (700–900 nm) of the steady state absorption spectra of some metalloporphyrins viz., zinc tetraphenylporphyrin, zinc octaethylporphyrin and ⇑ Corresponding author. Fax: +91 3463 261029. E-mail address: [email protected] (S. Sinha).
magnesium octaethylporphyrin in aromatic solvents at high concentrations (10 4 mol/L). The origin of this broad absorption band in the NIR region is suggested to be due to the formation of solute–solvent charge transfer (CT) complex of the presently used metalloporphyrins with aromatic solvents in the ground state.
2. Experimental 2.1. Chemicals The samples (purity P98%) free base tetraphenylporphyrin (5,10,15,20-tetraphenyl-21H,23H-porphine, TPhP), zinc tetraphenylporphyrin (5,10,15,20-tetraphenyl-21H,23H-porphine zinc, ZnTPhP), free base octaethylporphyrin (2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine, OEtP), zinc octaethylporphyrin (2,3,7,8, 12,13,17,18-octaethyl-21H,23H-porphine zinc, ZnOEtP) and magnesium octaethylporphyrin (2,3,7,8,12,13,17,18-octaethyl-21H, 23H-porphine magnesium, MgOEtP) were used as supplied by Sigma–Aldrich. The solvents chlorobenzene (CB), 1,2-dichlorobenzene (DCB), benzonitrile (BN), benzene, toluene, acetonitrile (ACN), acetone and tetrahydrofuran (THF) of spectroscopic grade were purchased from Sigma–Aldrich and were tested before use for the absence of any impurity emission in the wavelength region studied.
2.2. Spectroscopic apparatus The steady state electronic absorption spectra of all the samples were recorded by using 1 cm pathlength rectangular quartz cuvette by means of JASCO V-650 absorption spectrophotometer. The absorption spectra of the samples were measured at different temperatures by using a temperature controller water circulator
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(Spectralab, India, MCS-12–500) and a constant temperature rectangular cell holder (JASCO HMC-711). 3. Results and discussion Figure 1 shows the steady state absorption spectra of TPhP, ZnTPhP, OEtP, ZnOEtP and MgOEtP in chlorobenzene at low concentrations (10 6 mol/L). All the absorption spectra exhibit a highly intense Soret band in the blue side of the spectra and weak Q bands in the red side of the spectra (Table 1). The Q bands in the absorption spectra of free base porphyrins (TPhP and OEtP) consist of four peaks, while those of metalloporphyrins (ZnTPhP, ZnOEtP and MgOEtP) consist of only two peaks. All these features are in good accord with the literature data [3,7–12]. However, at high concentrations (10 4 mol/L), the metalloporphyrins (ZnTPhP, ZnOEtP and MgOEtP) show a new broad absorption band in the NIR region (700–900 nm) in chlorobenzene (Figure 2). The broad absorption band has a peak energy position at 812, 836 and 830 nm for ZnTPhP, ZnOEtP and MgOEtP, respectively (Figure 2, Table 1). To the best of our knowledge, no literature data are available on this broad absorption band. However, no emission in the NIR region (measured up to 1300 nm) is observed from ZnTPhP, ZnOEtP and MgOEtP at high concentrations in chlorobenzene at 298 K upon photoexcitation at the respective absorption peak energy position in the NIR region. Also, it is noteworthy that the free base porphyrins (TPhP and OEtP) do not show any absorption band in the NIR region at high concentrations in chlorobenzene (Figure 2). Earlier, formation of dimers and aggregates of porphyrins at high concentrations under certain specific conditions were
Figure 1. Steady state absorption spectra in chlorobenzene at 298 K of (a) TPhP (2.5 10 6 mol/L, solid line) and ZnTPhP (1.8 10 6 mol/L, dashed line); (b) OEtP (4.0 10 6 mol/L, solid line), ZnOEtP (1.5 10 6 mol/L, dashed line) and MgOEtP (2.9 10 6 mol/L, dotted line). Insets show the Q bands clearly.
reported by many research groups [13–39]. These reports suggest that the dimer/aggregate formations of porphyrins generally give rise to slightly red shifted absorption spectra (compared to the monomer), which lie in the visible region of the spectra. This seemingly indicates that dimer/aggregate formation may not be responsible for the presently observed broad absorption spectra in the NIR region for the metalloporphyrins (ZnTPhP, ZnOEtP and MgOEtP) in chlorobenzene at high concentrations (Figure 2). Also, it is well known that the concentration of the dimer, if any, is directly proportional to the square of the monomer concentration as per the law of mass action [40]. Hence, the absorbance of a dimeric band at a particular wavelength should be directly proportional to the square of the monomer concentration. On the contrary, the absorbance at the peak energy position for ZnTPhP in chlorobenzene is found to follow a linear relation (Beer–Lambert law) with the concentration of the fluorophore (Figure 3, at high concentrations till the solubility limit is reached). Similar trend is observed for ZnOEtP and MgOEtP in chlorobenzene. Thus, dimer/ aggregate formation may safely be ruled out for the presently observed broad absorption band in the NIR region for the metalloporphyrins (ZnTPhP, ZnOEtP and MgOEtP) in chlorobenzene at high concentrations. The fact that the broad absorption band in the NIR region is observed for some metalloprophyrins (ZnTPhP, ZnOEtP and MgOEtP), but not for their free base analogs (TPhP and OEtP), strongly indicates that the presence of the metal atom (Zn or Mg) at the centre of the metalloporphyrins might play a role in originating the broad absorption band in the NIR region. Clearly, incorporation of the metal atom (Zn or Mg) at the centre of the metalloporphyrins (ZnTPhP, ZnOEtP and MgOEtP) makes these molecules more electron donating in nature compared to their free base analogs (TPhP and OEtP). This is supported by the reported lower values of the one-electron half-wave oxidation potentials of the metalloporphyrins (+0.66, +0.63 and +0.54 V for ZnTPhP, ZnOEtP and MgOEtP, respectively) [41,42] compared to those of the free base porphyrins (+1.04 and +0.81 V for TPhP and OEtP, respectively) [41,43]. Again, in chlorobenzene, the electronegative chlorine atom makes the p-bonded benzene ring slightly electron deficient. Consequently, partial charge transfer may occur from the presently used metalloporphyrins to the p-bonded benzene ring of chlorobenzene. This results in some sort of solute–solvent CT complex in the singlet ground state of the presently used metalloporphyrins with chlorobenzene. This type of CT transition was first reported by Benesi and Hildebrand in 1949 [44]. They observed a new absorption band in the UV region for a solution of iodine and benzene in n-heptane, which is not present in the absorption spectra of either iodine or benzene. Mulliken [45] proposed a theory of CT complexes to explain the observation of Benesi and Hildebrand. According to the modified theory, partial charge transfer occurs from the HOMO (highest occupied molecular orbital) of a donor (D) molecule to the LUMO (lowest unoccupied molecular orbital) of an acceptor (A) molecule, when the two orbitals overlap with each other. The CT complex so formed is postulated to be a resonance between a non-bonded structure (D-A) and an ionically bound structure (D+-A-). The new absorption band results from a transition from the ground state molecular orbital to the excited state molecular orbital of this CT complex upon photoexcitation. Earlier, Evans et al. [46] reported solute–solvent CT complex formations between iron tetraphenylporphyrin cation and p-arenes, though the benzene ring acts as electron donor in their investigations for obvious reason. Our observations seemingly indicate a kind of inverted solute–solvent CT interactions (benzene ring acts as electron acceptor) between the presently used metalloporphyrins and chlorobenzene. Nevertheless, the aforementioned picture regarding the nature of the ground state solute–solvent CT complex of the
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M. Ghosh et al. / Chemical Physics Letters 592 (2014) 149–154 Table 1 Data from the steady state absorption spectra of TPhP, ZnTPhP, OEtP, ZnOEtP and MgOEtP in chlorobenzene at 298 K. Sample
Absorption peak (nm) Soret band (concentration 10
TPhP ZnTPhP OEtP ZnOEtP MgOEtP
6
mol/L)
420 423 401 404 411
Q band (concentration 10 515, 550, 498, 532, 545,
550, 592, 649 589 531, 568, 622 570 581
6
mol/L)
NIR region (concentration 10
4
mol/L)
– 812 – 836 830
presently used metalloporphyrins (ZnTPhP, ZnOEtP and MgOEtP) in chlorobenzene is further corroborated by the following facts: (i) No broad absorption band in the NIR region is observed for the metalloporphyrins (ZnTPhP, ZnOEtP and MgOEtP) at high concentrations in non-aromatic solvents viz., ACN, acetone and THF. This is expected as non-aromatic solvents cannot form the aforementioned CT complex with the metalloporphyrins. (ii) Figure 4a shows the temperature dependence of the broad absorption band in the NIR region for ZnTPhP at high concentration in chlorobenzene. Apparently, the peak energy position in the broad absorption band shifts from 812 nm at 298 K to 816 nm at 328 K, thereby suffering a red shift of about 4 nm as the temperature is increased from 298 to 328 K. It is well known that the polarity of a liquid medium decreases with increasing temperature [47]. However, the dielectric constant of chlorobenzne decreases from 5.7 to 4.1 only as the temperature is increased from 293 to
Figure 2. Steady state absorption spectra (in the NIR region) in chlorobenzene at 298 K of (a) TPhP (8.9 10 5 mol/L, solid line) and ZnTPhP (2.6 10 4 mol/L, dashed line); (b) OEtP (1.2 10 4 mol/L, solid line), ZnOEtP (1.6 10 4 mol/L, dashed line) and MgOEtP (3.7 10 4 mol/L, dotted line).
Figure 3. Plot of absorbance (at the peak energy position) vs. concentration for ZnTPhP in chlorobenzene at 298 K.
Figure 4. (a) Steady state absorption spectra (in the NIR region) of ZnTPhP (2.6 10 4 mol/L) in chlorobenzene at different temperatures; (b) Peak fitting of the steady state absorption spectra (in the NIR region) of ZnTPhP (2.6 10 4 mol/L) in chlorobenzene at 298 K (dashed curves: individual peaks) and 328 K (dash dotted curves: individual peaks). Solid black lines represent the fitted curves.
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383 K. Therefore, it is expected that the peak energy position of the broad absorption spectra in the NIR region for ZnTPhP in chlorobenzene will practically remain unaffected (may suffer very little blue shift) with the increase in temperature from 298 to 328 K, contrary to the observed red shift of 4 nm. This apparent contradiction may be due to the presence of a weak shoulder in the shorter wavelength side of the absorption spectra. A simple fit of the observed broad absorption spectra by two GAUSSIAN peaks at 298 and 328 K clearly shows that the main peak energy position in the longer wavelength side of the absorption spectra remains practically unaltered (at 819 nm) with increasing temperature (Figure 4b), thereby removing the aforementioned apparent contradiction. Again, the absorbance (taking the area of the broad absorption spectra in the NIR region) decreases by about 5% as the temperature is increased from 298 to 328 K. Obviously, the effective concentration of ZnTPhP decreases with increase in temperature due to thermal expansion of the volume of the solvent. Taking a value of 1 10 3 K 1 for the volumetric coefficient of expansion for the solvent chlorobenzene [48], it is found that the effective concentration of ZnTPhP decreases by only 2% (approximately) with increase in temperature from 298 to 328 K. Therefore, the overall intensity of the broad absorption band in the NIR region for ZnTPhP in chlorobenzene definitely decreases with increasing temperature, which cannot be fully accounted by mere alteration in the effective concentration of ZnTPhP due to thermal expansion of the solvent. This unaccounted decrease in the absorbance can be explained on the basis of the fact that the CT complexation would be less favorable at higher temperatures. Consequently, the overall intensity of the broad absorption band in the NIR region should decrease with increase in temperature as observed in Figure 4a. Similar trends are observed for the temperature dependence of the broad absorption spectra in the NIR region for ZnOEtP and MgOEtP in chlorobenzene. Moreover, our observations negate the possibility of any impurities present in the metalloporphyrins (ZnTPhP, ZnOEtP and MgOEtP) being responsible for the broad absorption spectra in the NIR region in chlorobenzene. This is because of the fact that if the broad absorption band in the NIR region is due to impurities, the overall intensity of the broad absorption band should either be independent of temperature or increase at higher temperatures due to better solubility of the impurities. (iii) Figure 5a shows that the broad absorption band in the NIR region for ZnTPhP becomes gradually weaker as the aromatic solvent is changed in the following order: benzonitrile, 1,2-dichlorobenzene, chlorobenzene, benzene, toluene (keeping the high concentration of ZnTPhP same in all the solvents). This feature may be explained on the basis of the formation of ground state solute–solvent CT complex of ZnTPhP via the p-bonded benzene rings of the aromatic solvents. To support our conjecture, we calculated the HOMO and LUMO energies for ZnTPhP and the five aromatic solvent molecules (toluene, benzene, CB, DCB and BN) using the quantum chemical HF/6-31++G(d,p)//6-31G(d,p) method. Our calculated HOMO, LUMO energies are listed in Table 2. HOMO, LUMO energies are known to be related to the ionization energy (IE, negative of HOMO energy) and electron affinity (EA, negative of LUMO energy), respectively, by Koopman’s theorem. The HOMO energy values in Table 2 indicate that IE value for ZnTPhP (6.16 eV) is much lower than that for the solvents (BN: 9.83 eV, DCB: 9.43 eV, CB: 9.25 eV, benzene: 9.18 eV, toluene: 8.84 eV). On the other hand, the difference of EA values between ZnTPhP and
Figure 5. (a) Steady state absorption spectra (in the NIR region) of ZnTPhP (2.6 10 4 mol/L) in different aromatic solvents at 298 K; (b) Peak fitting of the steady state absorption spectra (in the NIR region) of ZnTPhP (2.6 10 4 mol/L) in benzene (dashed curves: individual peaks) and 1,2-dichlorobenzene (dash dotted curves: individual peaks) at 298 K. Solid black lines represent the fitted curves.
Table 2 HOMO, LUMO energies of ZnTPhP and different aromatic solvents, obtained from HF/ 6-31++G(d,p)//6-31G(d,p) calculations. System ZnTPhP BN DCB CB Benzene Toluene
HOMO (a.u.) 0.22644 0.36144 0.34666 0.33993 0.33736 0.32475
LUMO (a.u.)
IE(ZnTPhP) (a.u.)
0.01641 0.03265 0.03680 0.03929 0.04167 0.04283
– 0.25909 0.26324 0.26573 0.26811 0.26927
EA(solvent)
solvent molecules range from 0.44 (BN) to 0.72 eV (toluene) only. This clearly indicates that charge transfer is possible from ZnTPhP to all the five aromatic solvents. The value of [IE(ZnTPhP) EA(solvent)] determines the feasibility, ease and direction of charge transfer. The lower is the value, the greater is the expected charge transfer. Therefore, the amount of charge transfer should be in the order BN > DCB > CB > benzene > toluene as evident from the values of [IE(ZnTPhP) EA(solvent)] (Table 2). Consequently, the broad absorption band in the NIR region for ZnTPhP should gradually decrease in the overall intensity in the following order of the aromatic solvents: BN > DCB > CB > benzene > toluene, as observed in Figure 5a. Again, the polarity of the aromatic solvents increases as the solvent is changed from benzene to toluene, CB, DCB and BN (dielectric constants are 2.27, 2.38, 5.62, 9.93 and 26, respectively). Therefore, the broad absorption band in the NIR region should suffer gradual red shift (to some extent) as the aromatic solvent is changed from benzene to toluene, CB, DCB and BN, contrary to the observed slight blue shift of the
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4. Conclusions
Figure 6. Steady state absorption spectra (in the NIR region) in different aromatic solvents at 298 K of (a) ZnOEtP (1.6 10 4 mol/L) and (b) MgOEtP (3.7 10 4 mol/L).
A new broad absorption band in the NIR region (700–900 nm) is observed for some metalloporphyrins (ZnTPhP, ZnOEtP and MgOEtP), but not for their free base analogs, in aromatic solvents at high concentrations (10 4 mol/L). No such band is observed in non-aromatic solvents. As the temperature is increased, the broad absorption band in the NIR region for ZnTPhP, ZnOEtP and MgOEtP goes down in intensity, but the peak absorption energy position remains unaltered (obtained by peak-fitting). Also, the broad absorption band in the NIR region for the presently used metalloporphyrins becomes gradually weaker as the aromatic solvent is changed in the order: benzonitrile, 1,2-dichlorobenzene, chlorobenzene, benzene, toluene (keeping the high concentration of the metalloporphyrin same in all the solvents). The peak absorption energy position suffers red shift (obtained by peak-fitting) for ZnTPhP with the increase in solvent polarity. However, no such shift is observed for ZnOEtP and MgOEtP. All these findings seemingly indicate the formation of ground state solute–solvent CT complex of the presently used metalloporphyrins in aromatic solvents at high concentrations. However, no emission in the NIR region (measured up to 1300 nm) is observed from ZnTPhP or ZnOEtP or MgOEtP at high concentrations in aromatic solvents at 298 K upon photoexcitation at the maximum of the respective broad absorption spectra in the NIR region. High-level quantum chemical calculations are presently under progress in our laboratory to find the exact nature and extent of CT interactions in the proposed ground state solute–solvent CT complexes of the metalloporphyrins (ZnTPhP, ZnOEtP and MgOEtP) in aromatic solvents at high concentrations and will be reported later. Acknowledgments
overall absorption peak energy position. Again, this apparent contradiction may be due to the presence of a weak shoulder in the shorter wavelength side of the broad absorption spectra. A simple fit of the observed broad absorption spectra by two GAUSSIAN peaks reveals that the main peak energy position in the longer wavelength side of the spectra indeed suffers a gradual red shift as the aromatic solvent is changed from benzene (818 nm) to CB (819 nm), DCB (821 nm) and BN (824 nm) (Figure 5b). It is to be noted that the absorption band in the NIR region for ZnTPhP in toluene is too broad and weak to get good fitting by two GAUSSIAN peaks.
Subrata Sinha acknowledges the Department of Atomic Energy (DAE) – Board of Research in Nuclear Sciences (BRNS), India (Project No.: 2010/37P/12/BRNS) for providing financial assistances in the form of grants and fellowship. Also, S.S. thanks Asit K. Chandra (Department of Chemistry, North Eastern Hill University, Shillong 793 022, India) for performing the quantum chemical calculations and Sukhendu Nath (Radiation and Photochemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India) for useful discussions. Finally, S.S. thanks the reviewers for their extremely valuable comments and suggestions. References
The overall intensity of the broad absorption band in the NIR region for ZnOEtP and MgOEtP in different aromatic solvents (Figure 6) is found to change with the aromatic solvents in a way similar to that observed for ZnTPhP (Figure 5a) and the explanation seems to follow the same arguments presented for ZnTPhP (vide supra). However, Figure 6 shows that the overall peak energy position of the broad absorption band in the NIR region remains nearly the same for either ZnOEtP (836 nm) or MgOEtP (830 nm) in all the five aromatic solvents (simple fitting of the broad absorption band with two GAUSSIAN peaks gives the same trend regarding the main absorption peak energy position in the longer wavelength side of the absorption spectra). The exact reason for the absence of the expected red shift (to some extent) in the broad absorption spectra in the NIR region for ZnOEtP and MgOEtP with the increase in solvent polarity is not clear at the present state of investigations. However, it may be due to some specific orientation of the excited state dipole moment of the solute–solvent CT complex for ZnOEtP and MgOEtP so that the reaction field (solvent relaxation) remains more-or-less unaffected with the change in solvent polarity [49].
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Ground state charge transfer complex formation of some metalloporphyrins with aromatic solvents Mihir Ghosh, Biswajit Roy, Abhimanyu Jha, Subrata Sinha ⇑ Integrated Science Education and Research Centre, Siksha-Bhavana, Visva-Bharati, Santiniketan 731 235, India
a r t i c l e
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Article history: Received 13 September 2013 In final form 18 December 2013 Available online 25 December 2013
a b s t r a c t Porphyrin derivatives are known to have absorption spectra in the visible region (400–700 nm). In the present Letter, a new (to the best of our knowledge) broad band is reported in the NIR region (700–900 nm) of the absorption spectra of zinc tetraphenylporphyrin, zinc octaethylporphyrin and magnesium octaethylporphyrin in aromatic solvents at high concentrations (10 4 mol/L). The broad absorption band in the NIR region is ascribed to be due to ground state charge transfer complex formation between the presently used metalloporphyrins and aromatic solvents. Crown Copyright Ó 2013 Published by Elsevier B.V. All rights reserved.
1. Introduction Porphyrin derivatives play very important roles in various fields of science viz., chemistry, physics and biology. Porphyrins generally have absorption bands in the visible region (400–700 nm). They exhibit a wide variety of redox properties. Furthermore, it is possible to control the photochemical and electrochemical properties of porphyrin derivatives by modification of the substituents and selection of the central metal atom. These unique properties of porphyrins have attracted great interest among various research groups for potential applications in solar energy harvesting and artificial photosynthesis [1–3], anticancer pharmaceutical drugs [4], photodynamic destruction of virus [5] and photodynamic therapy [6]. The electronic absorption spectra of a free base porphyrin consist of an intense band at about 400 nm, called Soret band (or B band) and four weaker bands in the visible region (about 500– 700 nm), called Q bands. It is well known that p, p⁄ transitions from the ground singlet state to the second excited singlet state (S0 ? S2) and from the ground singlet state to the first excited singlet state (S0 ? S1) are responsible for the B and Q bands, respectively. Metalation of the free base porphyrin leads to more symmetrical structure and consequently the four Q bands in the absorption spectra collapse into two. The origins of Q and B bands in the absorption spectra (visible region, 400–700 nm) of porphyrins and metalloporphyrins were first proposed by Martin Gouterman and co-workers [7–9] based on the four-orbital model in the 1960s. Here, we report the observation of a new, to the best of our knowledge, broad band in the NIR region (700–900 nm) of the steady state absorption spectra of some metalloporphyrins viz., zinc tetraphenylporphyrin, zinc octaethylporphyrin and ⇑ Corresponding author. Fax: +91 3463 261029. E-mail address: [email protected] (S. Sinha).
magnesium octaethylporphyrin in aromatic solvents at high concentrations (10 4 mol/L). The origin of this broad absorption band in the NIR region is suggested to be due to the formation of solute–solvent charge transfer (CT) complex of the presently used metalloporphyrins with aromatic solvents in the ground state.
2. Experimental 2.1. Chemicals The samples (purity P98%) free base tetraphenylporphyrin (5,10,15,20-tetraphenyl-21H,23H-porphine, TPhP), zinc tetraphenylporphyrin (5,10,15,20-tetraphenyl-21H,23H-porphine zinc, ZnTPhP), free base octaethylporphyrin (2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine, OEtP), zinc octaethylporphyrin (2,3,7,8, 12,13,17,18-octaethyl-21H,23H-porphine zinc, ZnOEtP) and magnesium octaethylporphyrin (2,3,7,8,12,13,17,18-octaethyl-21H, 23H-porphine magnesium, MgOEtP) were used as supplied by Sigma–Aldrich. The solvents chlorobenzene (CB), 1,2-dichlorobenzene (DCB), benzonitrile (BN), benzene, toluene, acetonitrile (ACN), acetone and tetrahydrofuran (THF) of spectroscopic grade were purchased from Sigma–Aldrich and were tested before use for the absence of any impurity emission in the wavelength region studied.
2.2. Spectroscopic apparatus The steady state electronic absorption spectra of all the samples were recorded by using 1 cm pathlength rectangular quartz cuvette by means of JASCO V-650 absorption spectrophotometer. The absorption spectra of the samples were measured at different temperatures by using a temperature controller water circulator
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(Spectralab, India, MCS-12–500) and a constant temperature rectangular cell holder (JASCO HMC-711). 3. Results and discussion Figure 1 shows the steady state absorption spectra of TPhP, ZnTPhP, OEtP, ZnOEtP and MgOEtP in chlorobenzene at low concentrations (10 6 mol/L). All the absorption spectra exhibit a highly intense Soret band in the blue side of the spectra and weak Q bands in the red side of the spectra (Table 1). The Q bands in the absorption spectra of free base porphyrins (TPhP and OEtP) consist of four peaks, while those of metalloporphyrins (ZnTPhP, ZnOEtP and MgOEtP) consist of only two peaks. All these features are in good accord with the literature data [3,7–12]. However, at high concentrations (10 4 mol/L), the metalloporphyrins (ZnTPhP, ZnOEtP and MgOEtP) show a new broad absorption band in the NIR region (700–900 nm) in chlorobenzene (Figure 2). The broad absorption band has a peak energy position at 812, 836 and 830 nm for ZnTPhP, ZnOEtP and MgOEtP, respectively (Figure 2, Table 1). To the best of our knowledge, no literature data are available on this broad absorption band. However, no emission in the NIR region (measured up to 1300 nm) is observed from ZnTPhP, ZnOEtP and MgOEtP at high concentrations in chlorobenzene at 298 K upon photoexcitation at the respective absorption peak energy position in the NIR region. Also, it is noteworthy that the free base porphyrins (TPhP and OEtP) do not show any absorption band in the NIR region at high concentrations in chlorobenzene (Figure 2). Earlier, formation of dimers and aggregates of porphyrins at high concentrations under certain specific conditions were
Figure 1. Steady state absorption spectra in chlorobenzene at 298 K of (a) TPhP (2.5 10 6 mol/L, solid line) and ZnTPhP (1.8 10 6 mol/L, dashed line); (b) OEtP (4.0 10 6 mol/L, solid line), ZnOEtP (1.5 10 6 mol/L, dashed line) and MgOEtP (2.9 10 6 mol/L, dotted line). Insets show the Q bands clearly.
reported by many research groups [13–39]. These reports suggest that the dimer/aggregate formations of porphyrins generally give rise to slightly red shifted absorption spectra (compared to the monomer), which lie in the visible region of the spectra. This seemingly indicates that dimer/aggregate formation may not be responsible for the presently observed broad absorption spectra in the NIR region for the metalloporphyrins (ZnTPhP, ZnOEtP and MgOEtP) in chlorobenzene at high concentrations (Figure 2). Also, it is well known that the concentration of the dimer, if any, is directly proportional to the square of the monomer concentration as per the law of mass action [40]. Hence, the absorbance of a dimeric band at a particular wavelength should be directly proportional to the square of the monomer concentration. On the contrary, the absorbance at the peak energy position for ZnTPhP in chlorobenzene is found to follow a linear relation (Beer–Lambert law) with the concentration of the fluorophore (Figure 3, at high concentrations till the solubility limit is reached). Similar trend is observed for ZnOEtP and MgOEtP in chlorobenzene. Thus, dimer/ aggregate formation may safely be ruled out for the presently observed broad absorption band in the NIR region for the metalloporphyrins (ZnTPhP, ZnOEtP and MgOEtP) in chlorobenzene at high concentrations. The fact that the broad absorption band in the NIR region is observed for some metalloprophyrins (ZnTPhP, ZnOEtP and MgOEtP), but not for their free base analogs (TPhP and OEtP), strongly indicates that the presence of the metal atom (Zn or Mg) at the centre of the metalloporphyrins might play a role in originating the broad absorption band in the NIR region. Clearly, incorporation of the metal atom (Zn or Mg) at the centre of the metalloporphyrins (ZnTPhP, ZnOEtP and MgOEtP) makes these molecules more electron donating in nature compared to their free base analogs (TPhP and OEtP). This is supported by the reported lower values of the one-electron half-wave oxidation potentials of the metalloporphyrins (+0.66, +0.63 and +0.54 V for ZnTPhP, ZnOEtP and MgOEtP, respectively) [41,42] compared to those of the free base porphyrins (+1.04 and +0.81 V for TPhP and OEtP, respectively) [41,43]. Again, in chlorobenzene, the electronegative chlorine atom makes the p-bonded benzene ring slightly electron deficient. Consequently, partial charge transfer may occur from the presently used metalloporphyrins to the p-bonded benzene ring of chlorobenzene. This results in some sort of solute–solvent CT complex in the singlet ground state of the presently used metalloporphyrins with chlorobenzene. This type of CT transition was first reported by Benesi and Hildebrand in 1949 [44]. They observed a new absorption band in the UV region for a solution of iodine and benzene in n-heptane, which is not present in the absorption spectra of either iodine or benzene. Mulliken [45] proposed a theory of CT complexes to explain the observation of Benesi and Hildebrand. According to the modified theory, partial charge transfer occurs from the HOMO (highest occupied molecular orbital) of a donor (D) molecule to the LUMO (lowest unoccupied molecular orbital) of an acceptor (A) molecule, when the two orbitals overlap with each other. The CT complex so formed is postulated to be a resonance between a non-bonded structure (D-A) and an ionically bound structure (D+-A-). The new absorption band results from a transition from the ground state molecular orbital to the excited state molecular orbital of this CT complex upon photoexcitation. Earlier, Evans et al. [46] reported solute–solvent CT complex formations between iron tetraphenylporphyrin cation and p-arenes, though the benzene ring acts as electron donor in their investigations for obvious reason. Our observations seemingly indicate a kind of inverted solute–solvent CT interactions (benzene ring acts as electron acceptor) between the presently used metalloporphyrins and chlorobenzene. Nevertheless, the aforementioned picture regarding the nature of the ground state solute–solvent CT complex of the
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M. Ghosh et al. / Chemical Physics Letters 592 (2014) 149–154 Table 1 Data from the steady state absorption spectra of TPhP, ZnTPhP, OEtP, ZnOEtP and MgOEtP in chlorobenzene at 298 K. Sample
Absorption peak (nm) Soret band (concentration 10
TPhP ZnTPhP OEtP ZnOEtP MgOEtP
6
mol/L)
420 423 401 404 411
Q band (concentration 10 515, 550, 498, 532, 545,
550, 592, 649 589 531, 568, 622 570 581
6
mol/L)
NIR region (concentration 10
4
mol/L)
– 812 – 836 830
presently used metalloporphyrins (ZnTPhP, ZnOEtP and MgOEtP) in chlorobenzene is further corroborated by the following facts: (i) No broad absorption band in the NIR region is observed for the metalloporphyrins (ZnTPhP, ZnOEtP and MgOEtP) at high concentrations in non-aromatic solvents viz., ACN, acetone and THF. This is expected as non-aromatic solvents cannot form the aforementioned CT complex with the metalloporphyrins. (ii) Figure 4a shows the temperature dependence of the broad absorption band in the NIR region for ZnTPhP at high concentration in chlorobenzene. Apparently, the peak energy position in the broad absorption band shifts from 812 nm at 298 K to 816 nm at 328 K, thereby suffering a red shift of about 4 nm as the temperature is increased from 298 to 328 K. It is well known that the polarity of a liquid medium decreases with increasing temperature [47]. However, the dielectric constant of chlorobenzne decreases from 5.7 to 4.1 only as the temperature is increased from 293 to
Figure 2. Steady state absorption spectra (in the NIR region) in chlorobenzene at 298 K of (a) TPhP (8.9 10 5 mol/L, solid line) and ZnTPhP (2.6 10 4 mol/L, dashed line); (b) OEtP (1.2 10 4 mol/L, solid line), ZnOEtP (1.6 10 4 mol/L, dashed line) and MgOEtP (3.7 10 4 mol/L, dotted line).
Figure 3. Plot of absorbance (at the peak energy position) vs. concentration for ZnTPhP in chlorobenzene at 298 K.
Figure 4. (a) Steady state absorption spectra (in the NIR region) of ZnTPhP (2.6 10 4 mol/L) in chlorobenzene at different temperatures; (b) Peak fitting of the steady state absorption spectra (in the NIR region) of ZnTPhP (2.6 10 4 mol/L) in chlorobenzene at 298 K (dashed curves: individual peaks) and 328 K (dash dotted curves: individual peaks). Solid black lines represent the fitted curves.
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383 K. Therefore, it is expected that the peak energy position of the broad absorption spectra in the NIR region for ZnTPhP in chlorobenzene will practically remain unaffected (may suffer very little blue shift) with the increase in temperature from 298 to 328 K, contrary to the observed red shift of 4 nm. This apparent contradiction may be due to the presence of a weak shoulder in the shorter wavelength side of the absorption spectra. A simple fit of the observed broad absorption spectra by two GAUSSIAN peaks at 298 and 328 K clearly shows that the main peak energy position in the longer wavelength side of the absorption spectra remains practically unaltered (at 819 nm) with increasing temperature (Figure 4b), thereby removing the aforementioned apparent contradiction. Again, the absorbance (taking the area of the broad absorption spectra in the NIR region) decreases by about 5% as the temperature is increased from 298 to 328 K. Obviously, the effective concentration of ZnTPhP decreases with increase in temperature due to thermal expansion of the volume of the solvent. Taking a value of 1 10 3 K 1 for the volumetric coefficient of expansion for the solvent chlorobenzene [48], it is found that the effective concentration of ZnTPhP decreases by only 2% (approximately) with increase in temperature from 298 to 328 K. Therefore, the overall intensity of the broad absorption band in the NIR region for ZnTPhP in chlorobenzene definitely decreases with increasing temperature, which cannot be fully accounted by mere alteration in the effective concentration of ZnTPhP due to thermal expansion of the solvent. This unaccounted decrease in the absorbance can be explained on the basis of the fact that the CT complexation would be less favorable at higher temperatures. Consequently, the overall intensity of the broad absorption band in the NIR region should decrease with increase in temperature as observed in Figure 4a. Similar trends are observed for the temperature dependence of the broad absorption spectra in the NIR region for ZnOEtP and MgOEtP in chlorobenzene. Moreover, our observations negate the possibility of any impurities present in the metalloporphyrins (ZnTPhP, ZnOEtP and MgOEtP) being responsible for the broad absorption spectra in the NIR region in chlorobenzene. This is because of the fact that if the broad absorption band in the NIR region is due to impurities, the overall intensity of the broad absorption band should either be independent of temperature or increase at higher temperatures due to better solubility of the impurities. (iii) Figure 5a shows that the broad absorption band in the NIR region for ZnTPhP becomes gradually weaker as the aromatic solvent is changed in the following order: benzonitrile, 1,2-dichlorobenzene, chlorobenzene, benzene, toluene (keeping the high concentration of ZnTPhP same in all the solvents). This feature may be explained on the basis of the formation of ground state solute–solvent CT complex of ZnTPhP via the p-bonded benzene rings of the aromatic solvents. To support our conjecture, we calculated the HOMO and LUMO energies for ZnTPhP and the five aromatic solvent molecules (toluene, benzene, CB, DCB and BN) using the quantum chemical HF/6-31++G(d,p)//6-31G(d,p) method. Our calculated HOMO, LUMO energies are listed in Table 2. HOMO, LUMO energies are known to be related to the ionization energy (IE, negative of HOMO energy) and electron affinity (EA, negative of LUMO energy), respectively, by Koopman’s theorem. The HOMO energy values in Table 2 indicate that IE value for ZnTPhP (6.16 eV) is much lower than that for the solvents (BN: 9.83 eV, DCB: 9.43 eV, CB: 9.25 eV, benzene: 9.18 eV, toluene: 8.84 eV). On the other hand, the difference of EA values between ZnTPhP and
Figure 5. (a) Steady state absorption spectra (in the NIR region) of ZnTPhP (2.6 10 4 mol/L) in different aromatic solvents at 298 K; (b) Peak fitting of the steady state absorption spectra (in the NIR region) of ZnTPhP (2.6 10 4 mol/L) in benzene (dashed curves: individual peaks) and 1,2-dichlorobenzene (dash dotted curves: individual peaks) at 298 K. Solid black lines represent the fitted curves.
Table 2 HOMO, LUMO energies of ZnTPhP and different aromatic solvents, obtained from HF/ 6-31++G(d,p)//6-31G(d,p) calculations. System ZnTPhP BN DCB CB Benzene Toluene
HOMO (a.u.) 0.22644 0.36144 0.34666 0.33993 0.33736 0.32475
LUMO (a.u.)
IE(ZnTPhP) (a.u.)
0.01641 0.03265 0.03680 0.03929 0.04167 0.04283
– 0.25909 0.26324 0.26573 0.26811 0.26927
EA(solvent)
solvent molecules range from 0.44 (BN) to 0.72 eV (toluene) only. This clearly indicates that charge transfer is possible from ZnTPhP to all the five aromatic solvents. The value of [IE(ZnTPhP) EA(solvent)] determines the feasibility, ease and direction of charge transfer. The lower is the value, the greater is the expected charge transfer. Therefore, the amount of charge transfer should be in the order BN > DCB > CB > benzene > toluene as evident from the values of [IE(ZnTPhP) EA(solvent)] (Table 2). Consequently, the broad absorption band in the NIR region for ZnTPhP should gradually decrease in the overall intensity in the following order of the aromatic solvents: BN > DCB > CB > benzene > toluene, as observed in Figure 5a. Again, the polarity of the aromatic solvents increases as the solvent is changed from benzene to toluene, CB, DCB and BN (dielectric constants are 2.27, 2.38, 5.62, 9.93 and 26, respectively). Therefore, the broad absorption band in the NIR region should suffer gradual red shift (to some extent) as the aromatic solvent is changed from benzene to toluene, CB, DCB and BN, contrary to the observed slight blue shift of the
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4. Conclusions
Figure 6. Steady state absorption spectra (in the NIR region) in different aromatic solvents at 298 K of (a) ZnOEtP (1.6 10 4 mol/L) and (b) MgOEtP (3.7 10 4 mol/L).
A new broad absorption band in the NIR region (700–900 nm) is observed for some metalloporphyrins (ZnTPhP, ZnOEtP and MgOEtP), but not for their free base analogs, in aromatic solvents at high concentrations (10 4 mol/L). No such band is observed in non-aromatic solvents. As the temperature is increased, the broad absorption band in the NIR region for ZnTPhP, ZnOEtP and MgOEtP goes down in intensity, but the peak absorption energy position remains unaltered (obtained by peak-fitting). Also, the broad absorption band in the NIR region for the presently used metalloporphyrins becomes gradually weaker as the aromatic solvent is changed in the order: benzonitrile, 1,2-dichlorobenzene, chlorobenzene, benzene, toluene (keeping the high concentration of the metalloporphyrin same in all the solvents). The peak absorption energy position suffers red shift (obtained by peak-fitting) for ZnTPhP with the increase in solvent polarity. However, no such shift is observed for ZnOEtP and MgOEtP. All these findings seemingly indicate the formation of ground state solute–solvent CT complex of the presently used metalloporphyrins in aromatic solvents at high concentrations. However, no emission in the NIR region (measured up to 1300 nm) is observed from ZnTPhP or ZnOEtP or MgOEtP at high concentrations in aromatic solvents at 298 K upon photoexcitation at the maximum of the respective broad absorption spectra in the NIR region. High-level quantum chemical calculations are presently under progress in our laboratory to find the exact nature and extent of CT interactions in the proposed ground state solute–solvent CT complexes of the metalloporphyrins (ZnTPhP, ZnOEtP and MgOEtP) in aromatic solvents at high concentrations and will be reported later. Acknowledgments
overall absorption peak energy position. Again, this apparent contradiction may be due to the presence of a weak shoulder in the shorter wavelength side of the broad absorption spectra. A simple fit of the observed broad absorption spectra by two GAUSSIAN peaks reveals that the main peak energy position in the longer wavelength side of the spectra indeed suffers a gradual red shift as the aromatic solvent is changed from benzene (818 nm) to CB (819 nm), DCB (821 nm) and BN (824 nm) (Figure 5b). It is to be noted that the absorption band in the NIR region for ZnTPhP in toluene is too broad and weak to get good fitting by two GAUSSIAN peaks.
Subrata Sinha acknowledges the Department of Atomic Energy (DAE) – Board of Research in Nuclear Sciences (BRNS), India (Project No.: 2010/37P/12/BRNS) for providing financial assistances in the form of grants and fellowship. Also, S.S. thanks Asit K. Chandra (Department of Chemistry, North Eastern Hill University, Shillong 793 022, India) for performing the quantum chemical calculations and Sukhendu Nath (Radiation and Photochemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India) for useful discussions. Finally, S.S. thanks the reviewers for their extremely valuable comments and suggestions. References
The overall intensity of the broad absorption band in the NIR region for ZnOEtP and MgOEtP in different aromatic solvents (Figure 6) is found to change with the aromatic solvents in a way similar to that observed for ZnTPhP (Figure 5a) and the explanation seems to follow the same arguments presented for ZnTPhP (vide supra). However, Figure 6 shows that the overall peak energy position of the broad absorption band in the NIR region remains nearly the same for either ZnOEtP (836 nm) or MgOEtP (830 nm) in all the five aromatic solvents (simple fitting of the broad absorption band with two GAUSSIAN peaks gives the same trend regarding the main absorption peak energy position in the longer wavelength side of the absorption spectra). The exact reason for the absence of the expected red shift (to some extent) in the broad absorption spectra in the NIR region for ZnOEtP and MgOEtP with the increase in solvent polarity is not clear at the present state of investigations. However, it may be due to some specific orientation of the excited state dipole moment of the solute–solvent CT complex for ZnOEtP and MgOEtP so that the reaction field (solvent relaxation) remains more-or-less unaffected with the change in solvent polarity [49].
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