J. Phys. Chem. B 2006, 110, 587-594
587
Sequential Deprotonation of meso-(p-Hydroxyphenyl)porphyrins in DMF: From Hyperporphyrins to Sodium Porphyrin Complexes Hongwei Guo,†,‡ Junguang Jiang,† Yingyan Shi,§ Yuling Wang,† Yizhe Wang,† and Shaojun Dong*,† State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin, 130022, P. R. China, Graduate School of the Chinese Academy of Sciences, Beijing, 100039, P. R. China, and Department of Chemistry, Jilin UniVersity, Changchun, Jilin, 130023, P. R. China ReceiVed: May 8, 2005; In Final Form: October 14, 2005
Sequential deprotonations of meso-(p-hydroxyphenyl)porphyrins (p-OHTPPH2) in DMF + H2O (V/V ) 1:1) mixture have been verified to result in the appearance of hyperporphyrin spectra. However, when the deprotonations of these p-OHTPPH2 are carried out in DMF, the spectral changes differ considerably from those in the mixture mentioned above. At low [OH-], the optical spectra in the visible region are still considered to have characteristics of hyperporphyrin spectra. Further deprotonation at much higher basicity makes the optical spectra form three-banded spectra similar to those in the acidic solution. To clarify the molecular origins of these changes, UV-vis, resonance Raman (RR), proton nuclear magnetic resonance (1H NMR) experiments are carried out. Our data give evidence that p-OHTPPH2 in DMF can be further deprotonated of pyrrolic-H by higher concentrated NaOH, due to an aprotic medium like DMF effectively weakening the basicity of the porphyrin relative to that of the NaOH, and coordinates with two sodium ions (except the sodium ions that interact with the peripherial phenoxide anions) to form the sodium complexes of p-OHTPPH2 (Na2P, to lay a strong emphasis on the sodium ions that coordinate with the central nitrogen atom), which can be regarded as the porphyrin anions being perturbed by the sodium cations due to their highly ionic character. The negative centers generated by deprotonation of pyrrolic-H and phenolic-H are not thoroughly delocalized between the substituents and the porphyrin ring. Thus the negative centers generated by deprotonation of pyrrolic-H only act as electron-donating groups on the porphyrin π system, and the negative charges of the phenoxide anion are also mainly localized on the peripheral substituents. As a result, the porphyrin π orbitals cross over the phenoxide anion π orbital and turn into HOMOs, which turns hyperporphyrin spectra of deprotonated phenolic-H of p-OHTPPH2 into three-banded spectra of regular metalloporphyrins.
1. Introduction meso-(p-Hydroxyphenyl)porphyrins (p-OHTPPH2) are a series of very important porphyrins because of their wide use as photosensitizers1 in the model system of photosynthesis. Furthermore, they have been found to be suitable candidates for use in the photodynamic therapy of cancer of internal organs.2-4 Thus their optical properties and photophysical behaviors continue to attract much interest. Spectral properties of pOHTPPH2 are observed to be strongly modified by pH change, because they have ionizable protons (H) at the two centers, the comparatively acidic phenolic-H in the peripheral region, and pyrrolic-H on the two N-H groups. At pH between 6 and 7, these porphyrins are electrically neutral and their optical absorption spectra include an intense Soret band around 420 nm and four weak bands (Q-bands) through the visible region. These are typically so-called “regular porphyrin spectra”, whose theory is long established.5-7 The excited states are based on the four-orbital model, involving two nearly degenerate HOMO orbitals b1 and b2 (symmetry in D4h, respectively, a2u(π) and * Corresponding author. Fax: +86-431-5689-711. E-mail:
[email protected]. † Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. ‡ Graduate School of the Chinese Academy of Sciences. § Jilin University.
Figure 1. The energy level diagram for four frontier orbitals of porphyrin adapted from ref 5. The broken line represents the shifted a2u orbital level upon electron-donating meso substitution.
a1u(π)) and two degenerate or nearly degenerate LUMO orbitals c1 and c2 (symmetry in D4h, respectively, egy(π*) and egx(π*)). The model is illustrated in Figure 1. It encompasses two pairs of degenerate orbital excitations, and the two resulting singlet levels interact strongly, producing the characteristic intense soret or B bands around 400 nm and the weak visible or Q-bands of the metalloporphyrins. In D4h symmetry (metalloporphyrins) each absorption band arises from two degenerate excited states of x and y polarization. In the free base porphyrins (D2h), the degeneracy of x and y polarization is strongly lifted by the central proton axis. As a result, the two-banded visible spectrum
10.1021/jp0523827 CCC: $33.50 © 2006 American Chemical Society Published on Web 12/08/2005
588 J. Phys. Chem. B, Vol. 110, No. 1, 2006 Q(0,0) and Q(1,0) (this latter is a vibronic band) splits into four bands Qx(0,0), Qx(1,0), Qy(0,0), and Qy(1,0). On titration of these p-OHTPPH2 by sodium hydroxide in DMF + H2O (1:1) mixture as solvent, deprotonation of the phenolic-H of these compounds has been shown to induce dramatic optical changes which are the appearance of the new red bands in the visible region and the splitting or broadening of the Soret band.8,9 Our previous RR and FTIR data9 support these compounds as hyperporphyrins (which have been defined as porphyrins that exhibit prominent extra absorption bands ( > 1000 M-1 cm-1) in the region λ > 320 nm which are not porphyrin π - π* transitions) under basic conditions, and the extra absorption bands are considered to be due to π (phenoxide anion)- π* (porphyrin) charge transfer (CT) transitions. In the present study, we observe that not only pH but also the nature of the solvent has a profound effect on the acidbase characteristics of these compounds and modification of their spectra. When the titrations of these p-OHTPPH2 by sodium hydroxide are carried out in DMF as solvent, the spectral changes differ considerably from those in DMF + H2O (1:1) mixture or methanol as solvent. At low [OH-], the optical spectra in the visible region are still red-shifted and intensified, and the new absorption bands appear. Further titration at much higher basicity makes the optical spectra form three-banded spectra similar to those in the acidic solution. This dramatic spectral change has been observed by Manna et al.8 in DMF as solvent, for which they suggested that the delocalization of these peripheral charges (only phenolic-H are ionized) in solution along the conjugative pathways into the core region causes accumulation of negative charges only on the pyridine-type N. Further redistribution of the negative charge density on these N can occur by tautomerism between pyrrole-type N and the pyridine-type N. Steric distortion primarily caused by charge accumulation in the center results in three-banded spectrum of the acid type. This is not the case, because the titration of TPP in DMF at much higher basicity also turns its regular porphyrin spectra into three-banded spectra. Thus, what factors lead to the electronic spectra to change? In this paper we report UV-vis, RR, and 1H NMR studies of these p-OHTPPH2 in DMF at higher concentrated sodium hydroxide in attempt to clarify molecular origins of these spectral changes. 2. Experimental Section Materials. The preparation and characterization of the porphyrins had been described in a previous paper.9 The labeling of specific carbon atoms on the macrocycle is given in Figure 2, and the nomenclature used is given in the figure caption. The p-hydroxy-substituted porphyrins studied have increasing numbers of hydroxy substituents, from one to four. The porphyrin with one p-hydroxy substituent is labeled (OH)1PH2. When two hydroxy groups are added to the molecule, they can be added to adjacent phenyl groups, resulting in the hydroxy groups being cis to each other or, on opposite phenyl groups, trans to each other. In this study only the cis compound labeled as (OH)2PH2 was used. The three-hydroxy-substituted compound is labeled (OH)3PH2 and the four-substituted compound, (OH)4PH2. N,N-dimethylformamide-d7 (99.5%, Cambridge Isotope Laboratories, Inc.) and spectroscopic methanol were used as received. Spectroscopic reagent N,N-dimethylformamide (Fluka) (used as received) was used as solvent for UV-vis and RR spectra. Doubly distilled water was used in all sample preparations. All other chemicals used were analytical grade and were used as received.
Guo et al.
Figure 2. Structure of p-hydroxy-substituted tetraphenylporphyrins and the labeling of the carbon atoms in the macrocycle. (I) TPP: A, B, C, and D ) H; (II) (OH)1PH2: A ) OH, B, C and D ) H; (III) (OH)2PH2: A and B ) OH, C and D ) H; (IV) (OH)3PH2: A, B and C ) OH, and D ) H; (V) (OH)4PH2: A, B, C, and D ) OH.
Spectroscopy. UV-visible absorption spectra were taken on a Cary 500 Scan UV-vis-NIR spectrophotometer (Varian, Inc.). FT-IR spectra were obtained with a Bruker IFS-66v/S (Germany) FT-IR spectrometer. RR spectra were recorded with a Renishaw system 2000 Raman spectrophotometer equipped with an integral microscopy (Leica DMLM). Radiation of 514.5 nm was obtained from a Ar+ laser. Baseline corrections were carried out using Labspec (J-Y) software (ver 3.03T). The proton NMR spectra were run on a Bruker AV600 (Germany) spectrometer operating at 600 MHz at ambient temperature. UV-vis Spectrophotometric Titration. Solutions of porphyrins in DMF were titrated directly in 1-cm absorption cells by successive additions of aqueous solution of sodium hydroxide while monitoring the spectra. Increasingly concentrated NaOH (0.01-2 M) solution was added in 0.2 to 5 µL aliquots using a microliter syringe. The original porphyrin solution was ∼4 µM in a total volume of 3.0 mL. The total volume change during the titration was negligible. After addition of each aliquot, the cell was capped and was mixed by inversion, and the spectrum was retaken. In all cases, the reversibility of the deprotonation was demonstrated by addition of 1.0 M HCl. 3. Results 3.1 UV-vis Spectroscopy. Figures 3-5 show spectrophotometric titrations for (OH)1PH2, (OH)3PH2, and (OH)4PH2 in DMF as solvent (the titration procedures are completely reversible on addition of acid). Comparing the spectral changes of these three species in DMF, we find that these deprotonations occur in two well-separated stages. The first deprotonation step gives a reduced and blue-shifted Soret peak, the disappearance of the original four-banded spectra (Q-band), and two new bands, a very broad, flat absorption around 485 nm in (OH)1PH2 (which turns into the low-energy component of sharply split Soret structure, 467 nm in (OH)3PH2 and 459 nm in (OH)4PH2, respectively.) and a strong, broad band at about 660 nm, extending almost to 800 nm. At this stage, these optical spectra are still considered to have characteristics of hyperporphyrin spectra, although they differ from those in basic DMF + H2O (V/V ) 1:1) mixture (we surmise that the difference may be relevant to the nature of the solvent). So we attribute these spectral changes to deprotonation of comparatively acidic phenolic-H in the peripheral region and forming hyperporphyrins. The second step at much higher concentrated NaOH (0.04 M) shifts the Soret band peak toward the red, sharpens it dramatically, and restores a more “normal” two-banded structure in the visible, as in the acidic solution. We presume that the formation of three-banded spectra might be connected with
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J. Phys. Chem. B, Vol. 110, No. 1, 2006 589
Figure 3. Spectrophotometric titration of (OH)1PH2 with NaOH in DMF. A is the deprotonation of the first stage. The ×4 is the scale factor used for Q-bands. B is the deprotonation of the second stage. The ×3 is the scale factor used for Q-bands.
Figure 4. Spectrophotometric titration of (OH)3PH2 with NaOH in DMF. A is the deprotonation of the first stage. The ×2 is the scale factor used for Q-bands. B is the deprotonation of the second stage.
deprotonation of pyrrolic-H on the two N-H groups. To verify this inference, the spectra of successive NaOH-titration products of TPP are given in Figure 6. Deprotonation occurs in one stage marked by a good set of isosbestics and is completely reversible on addition of acid. No spectral red shift is observed. The original regular porphyrin spectrum of TPP disappears into a three-banded spectrum. This experiment demonstrates that deprotonation of pyrrole-H is responsible for the formation of three-banded spectra. Herein we note that the three-banded spectrum of TPP is in good accord with that of alkali metal porphyrin complex Na2(TMPP)10 which was obtained following treatment of dimethoxyethane (DME) solutions of meso-tetrakis(3,4,5-tri-methoxyphenyl) porphyrin (TMPP) with methanolic solution of NaOH. These alkali metal porphyrin complexes (M2(por)) are thought to be chemically labile and can quickly change in the presence of water to the free base porphyrins (H2P), which have been prepared from a variety of nonaqueous solvents, such as DMSO, acetone, THF, and DME.10,11 However, we consider that it is possible to form M2(por), which are stable enough to be measured in the presence of a small amount of water. Thus the absorption maxima of TPP, (OH)1PH2, (OH)2PH2, (OH)3PH2, and (OH)4PH2 in DMF treated by aqueous and methanolic solution of sodium hydroxide, respectively, are shown in Table 1. The spectra obtained in the presence of a small amount of water (VDMF/VH2O ) 50:1) agree to within 2% of the corresponding spectra of sodium complexes of p-OHTPPH2 obtained from solutions prepared as above in DMF-methanol mixtures (VDMF/VH2O ) 50:1). We conclude that the appearance of three-
banded spectra is due to the formation of sodium complexes Na2P, which can be regarded as the porphyrin anions (with D4h symmetry) being perturbed by the sodium cations due to their highly ionic character.12-13 3.2. Resonance Raman Spectroscopy. To further ensure our assignment, the high-frequency region (900-1600 cm-1) RR spectra of (OH)4PH2 in basic DMF + H2O (V/V ) 1:1), DMF + H2O (V/V ) 50:1), and DMF + methanol (V/V ) 50:1) mixtures using 514.5 nm excitation are shown in Figure 7 ([OH-] ) 0.04 M). Comparing Figure 7b with 7c, the RR spectrum of (OH)4PH2 in basic DMF + H2O (V/V ) 50:1) mixture is similar to that of its sodium salt obtained from basic DMF + methanol (V/V ) 50:1) mixture. This further demonstrates that in the presence of a small amount of water (VDMF/ VH2O ) 50:1) at much higher concentrated NaOH we can still obtain alkali metal porphyrin complexes Na2P, which are stable enough to be measured by treatment of DMF solutions of p-OHTPPH2 with aqueous solution of sodium hydroxide. RR spectrum of p-OHTPPH2 in basic DMF + H2O (V/V ) 1:1) mixture is assigned in analogy with the previous results.9 To our knowledge, there is no experimental report on the vibrational spectra of these alkali metal porphyrin complexes yet. The assignment of RR of Na2P is based on a recent density functional theory (DFT) calculation11 on the vibrational frequencies of Li2P, Na2P, K2P, and Zn2P, and the results of Li et al.14 The atom labeling is defined in the structural diagram (Figure 2). We have
590 J. Phys. Chem. B, Vol. 110, No. 1, 2006
Figure 5. Spectrophotometric titration of (OH)4PH2 with NaOH in DMF. A is the deprotonation of the first stage. The ×2 is the scale factor used for Q-bands. B is the deprotonation of the second stage.
Figure 6. Spectrophotometric titration of TPP with NaOH in DMF. The ×10 is the scale factor used for Q-bands.
retained the D4h symmetry notation characteristic of the porphyrin core, and the results are presented in Table 2. In resonance with the Q-band (514.5 nm excitation), the RR spectrum is dominated by depolarized (dp) and anomalously polarized (ap) bands. They arise from B1g or B2g (dp) and A2g (ap) modes which are vibronically active in Q-B mixing and are enhanced via the B (vibronic) term. The RR spectrum of (OH)4PH2 (Figure 7a) in basic DMF + H2O (V/V ) 1:1) mixture shows strong enhancement of nontotally symmetric
Guo et al. modes at 1551 (ν19), 1489 (ν11), and 1239 cm-1 (ν26) (only the phenolic-H of (OH)4PH2 are ionized, forming hyperporphyrin in basic DMF + H2O (V/V ) 1:1) mixture and whose macrocyclic skeleton vibrations are nearly unaffected compared to those in neutral solution9). Upon further deprotonation of pyrrolic-H and formation of Na2P, the porphyrin macrocyclic skeleton vibrations are greatly perturbed. In particular, the ν19, ν11, and ν9 modes shift down by 42, 17, and 18 cm-1, respectively. These vibrational modes are structurally sensitive and are known as the structural markers in RR studies. The ν19, ν11, and ν9 modes involve mainly the CR-Cm and Cβ-Cβ bond stretching, respectively, thus the downshift of ν19, ν11, and ν9 modes reflects that Na2P molecules have prolonged CR-Cm and Cβ-Cβ bonds compared with p-OHTPPH2, which agrees with DFT calculated results. The discrepancies between observed and calculated frequencies (calculation is based on planar porphyrin geometry) in Na2P reflect the additional CR-Cm expansion, attributable to distortion of the skeleton from the planar porphyrin geometry.15-18 3.3. 1H NMR Spectroscopy. Figure 8 shows the aromatic region of the NMR spectra of TPP, (OH)3PH2, (OH)4PH2 in neutral and basic ([OH-] ) 0.04 M) DMF-d7. The chemical shift data for phenolic-H, β-pyrrole H (Hβ), ortho, meta, and para phenyl H, and N-H for these compounds are summarized in Table 3. In a comparison of the pyrrole-H patterns and chemical shifts observed for the three compounds in neutral and basic DMF-d7 of Figure 8 and the chemical shift data for phenolic-H, phenyl H, and N-H presented in Table 3, we can draw several conclusions.19-23 Considering first the phenolic-H and N-H, we can see that at higher concentration of NaOH the signals of both phenolicHs and pyrrolic-H of the compounds disappear. Thus NMR gives direct evidence for forming porphyrin anions (Na2P, in brief, can be thought to be porphyrin anions). We call attention to the aromatic protons. Also included in Figure 8 are symmetry labels for the types of pyrrolic-H. There are four possible types of pyrrole protons, based upon the identity of their nearest and next-nearest neighbors: Ha has a p-hydroxyphenyl group as nearest and a phenyl group as nextnearest meso neighbors, while Hb has the reverse. Hc has phenyl groups as both nearest and next-nearest meso neighbors, while Hd has p-hydroxyphenyl groups as both types of meso neighbors, Further subclassification of d can be made if the third-nearest meso neighbor is considered: we define type d as that which has a p-hydroxyphenyl group as third-nearest neighbor and d′ as that which has a phenyl group in that position.19 First, introduction of negative charges into the porphyrin macrocycle can be expected to cause upfield shift of Hβ signal,22 which is evident that forming porphyrin anions move the Hβ signals upfield 0.162-0.290 ppm in these compounds (Figure 8AI to 8AII, Figure 8BI to 8BII, and Figure 8CI to 8CII). The effect of forming a porphyrin anion on ring current is contrary to the phenomena previously observed of conversion of porphine to its dication20,24 (introduction of positive charges into the porphyrin macrocycle) or replacement of NH groups of tetraphenylporphyrin by the heteroatoms S, Se, and Te22 (the core interactions between heteroatoms act as an electron drain on the π system), which moves the Hβ signal downfield. Second, p-hydroxyphenyl substituents which are electron-donating in symmetrical tetra-substituted TPP ((OH)4PH2) cause a slight downfield pyrrole-H shift with respect to TPP in neutral (Figure 8CI to 8AI) and basic (Figure 8CII to 8AII) DMF-d7, because the former have larger ring current and thereby larger deshielding. Third, the unsymmetrical p-hydroxyphenyl substituents in
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TABLE 1: Absorption Maxima and Extinction Coefficientsa of TPP, (OH)1PH2, (OH)2PH2, (OH)3PH2, and (OH)4PH2 in Basic DMF + H2O (V/V ) 50:1), and DMF + Methanol (V/V ) 50:1) Mixturesb compd
Soret band
TPP TPP (OH)1PH2 (OH)1PH2 (OH)2PH2 (OH)2PH2 (OH)3PH2 (OH)3PH2 (OH)4PH2 (OH)4PH2 Na2(DME)n(TMPP)d
435 (42.34) 435 (40.62) 437 (12.42) 441 (14.61) 456 (11.06) 453 (13.00) 458 (14.34) 455 (15.97) 458 (22.85) 455 (23.54) 434 (39.81)
538 (0.39) 540 (0.47)
557 (0.41)
I band
II band
II/I
solvent
580 (1.43) 579 (1.53) 590 (0.77) 588 (0.84) 602 (0.59) 598 (0.64) 610 (0.63)c 605 (0.65)c 612 (0.67)c 607 (0.65)c 576 (1.26)
623 (1.54) 623 (1.69) 641 (1.73) 637 (1.72) 656 (1.86) 651 (1.77) 666 (2.20) 659 (2.10) 674 (3.14) 668 (2.88) 618 (1.26)
1.08 1.10 2.25 2.05 3.15 2.77 3.49 3.23 4.69 4.43 1.00
DMF + H2O DMF + methanol DMF + H2O DMF + methanol DMF + H2O DMF + methanol DMF + H2O DMF + methanol DMF + H2O DMF + methanol DME
a λmax in nm; (in parentheses) in 104 mol L -1 cm -1. b The concentration of NaOH in DMF + H2O (V:V ) 50/1), and DMF + methanol (V/V ) 50:1) mixture is 0.04 M. c Shoulder bands. d Data from ref 10.
TABLE 2: Raman Frequencies (cm-1) of (OH)4PH2 in Basic DMF + H2O (V/V ) 1:1) (A), DMF + H2O (V/V ) 50:1) (B), and DMF + Methanol (V/V ) 50:1) (C) Mixtures Using 514.5 nm Excitation (OH)4PH2 (OH)4PH2 (OH)4PH2 (A) (B) (C) 1551 1489 1328 1280 1239 1079 1003
1509 1472 1327 1274
1509 1477 1328 1273
1061 1003
1064 1005
assignment ν(CRCm)+δ(CRCmCPh) ν(CβCβ)+δ(CβH) ν(CRCβ)+δ(CβH) ν(Cmph)+ν(NCR)+δ(CH)ph ν(NCR)+ν(CRCβ)+δ(CRCβCβ)+δ(CRCm) δ(CβH)+ν(CβCβ) ν(CRCβ)+ν(NCR)+ ν(CC)ph
a The concentration of (OH)4PH2 in basic DMF + H2O (V/V ) 1:1), DMF + H2O (V/V ) 50:1), and DMF + methanol (V/V ) 50:1) mixtures is 5 × 10-4 M.
Figure 7. Resonance Raman spectra of (A) (OH)4PH2 in basic DMF + H2O (V/V ) 1:1); (B) (OH)4PH2 in basic DMF + H2O (V/V ) 50:1); (C) (OH)4PH2 in basic DMF + methanol (V/V ) 50:1) mixture with 514.5 nm excitation in the 900-1600 cm-1. The small negative peaks are due to solvent subtraction or noise. * ) solvent band.
(OH)3PH2 make the Hβ signal split. The chemical shifts of Ha, Hd, Hd′ protons appear as one broad peak at lower field than that of Hb (Figure BI). Upon forming porphyrin anion, pphenoxide anion groups produce larger splitting than that of the p-hydroxyphenyl substituents (p-phenoxide anion is a stronger electron-donating substituent). Pyrrole rings in which one proton is near a p-hydroxyphenyl (Ha) and the other is near a phenyl (Hb) have their proton resonances split into an AB quartet. The two doublets are centered “outside” the resonance
positions of the split signals of Hd,d′, with Ha being centered 0.059 ppm downfield from Hd,d′. Thus Ha feels an effect (0.059 ppm) from the electron-donating effect of their next-nearest neighbor. The splitting of Hd and Hd′ is also observed at 600 MHz, and therefore the electronic effect of the third-nearest meso neighbor is measurable at 600 MHz. Thus Ha feels a larger ring current than it should, according to the behavior of TPP and (OH)4PH2. While Hb has been “robbed”. Note, however, that Ha, which is adjacent to Hb in the same pyrrole ring, still has a slightly greater downfield (ring current) shift than Hd and Hd′, thus indicating that the two meso positions adjacent to Ha are able to “rob” that meso position adjacent to Hb slightly better than the meso position opposite the phenyl group. As a result, the pyrrole-H NMR patterns of Figure 8 suggest that the electronic effects of substituents on the meso phenyl rings on the pyrrole proton environments of the porphyrin are not thoroughly delocalized throughout the π system.19 In addition, conversion of TPP to its dianion moves the oH and mH signals upfield about 0.106 and 0.123 ppm, respectively, which are quite small compared to that of pyrrole-H (0.290 ppm). The perturbation of porphyrin macrocycles decreases as the group becomes more distant from the negative centers. This further suggests the negative centers are not thoroughly delocalized between substituents and the porphyrin ring. Upon forming porphyrin anion, the oOH and mOH signals of (OH)4PH2 move upfield about 0.332 and 0.504 ppm. The oH, oOH, mH, and mOH signals of (OH)3PH2 only have a little change, according to the behavior of TPP and (OH)4PH2. Thus the negative centers generated by deprotonation of pyrrolic-H only act as electron-donating groups on the porphyrin π system and the negative charges of the phenoxide anion also mainly localize on the peripheral substituents.
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Figure 8. 600 MHz NMR spectra of the aromatic regions of the TPP, (OH)3PH2, (OH)4PH2 in neutral (AI, BI, and CI) and basic DMF-d7 (AII, BII, and CII) vs Me4Si. Symmetry labels a, b, c, d, and d′ are discussed in the text. * ) solvent band.
TABLE 3: Chemical Shifts (ppm) of TPP, (OH)3PH2, and (OH)4PH2 Protons in Neutral and Basic DMF-d7a compd TPP TPP2-d (OH)3PH2 (OH)3P2-d (OH)4PH2 (OH)4P2-d
O-H
10.211 -b 10.112 -b
Ha
Hd′d
8.985c 8.823
8.764 8.969 8.733
Hb
Hc
o-HH
8.926 8.636
8.321 8.215 8.309 8.209
8.885 8.494
o-HOH
8.120 7.786 8.110 7.778
m-HH 7.888 7.765 7.881 7.730
m-HOH
7.338 6.849 7.337 6.833
N-H -2.755 -b -2.699 -b -2.691 -b
a Measured in N,N-dimethylformamide-d7, with Me4Si as reference. Chemical shifts are in ppm. b The disappearance of O-H and N-H signals of TPP, (OH)3PH2, and (OH)4PH2 in basic DMF-d7. c The Ha and Hd′d signals in (OH)3PH2 in neutral DMF-d7 appear as one peak. d Indicate the sodium salts of TPP, (OH)3PH2, and (OH)4PH2.
4. Discussions The p-OHTPPH2 have two kinds of ionizable protons (H), the comparatively acidic phenolic-H in the peripheral region and pyrrolic-H on the two N-H groups. The N-H groups are very weakly acidic9 (pK > 15). Thus, as has been experimentally demonstrated that on titration with NaOH in DMF + H2O (V/V ) 1:1) mixture or methanol, only the phenolic-H of p-OHTPPH2 are expected to ionize, while pyrrolic-Hs do not. Note, however, this acid-base reaction is in equilibrium between a soft (porphyrin) and a hard (NaOH) base and therefore solvent effects can be used to alter the relative strength of the bases.25 Hard bases are greatly stabilized by H-bonding solvents, whereas soft bases are not very affected by H-bonding.26-31 Changing the solvent from water to a polar, aprotic medium such as DMSO or DMF will increase the base strength of the NaOH but will hardly affect the porphyrin. Thus, a solvent like DMF effectively weakens the basicity of the porphyrin relative to that of the NaOH. As a result, when the titrations of p-OHTPPH2 are carried out in DMF as solvent, the pyrrolic-H at higher concentrated NaOH are expected to ionize. As shown in Figures 3-5, for all three macrocycles in DMF, the lower concentrated NaOH results in the markedly broaded, red-shifted, strong new bands in the visible region, and the splitting of the Soret band. At this stage, these optical spectra are still considered to be hyperporphyrin spectra, which, based on different HOMOs or LUMOs due to charge localization, are generally considered to originate from charge-transfer transitions. In this case, as had been suggested that the negative
charges on the phenoxide anion groups increase all the orbital energies. However, the deprotonation increases the energy of π orbital localized on the phenoxide anion substituents proportionally more than that of the macrocycle π orbitals. Hence the phenoxide anion π orbital crosses over the porphyrin π orbital, creating a different HOMO and as a result a charge-transfer transition (π (phenoxide anion) f π* (porphyrin)) at lower energy and in higher oscillator strength. This results in redshifted, enhanced new bands in the visible region.9 On increasing the concentration of NaOH in DMF, the deprotonation of pyrrolic-H on the two N-H groups changes hyperporphyrin spectra into three-banded spectra of acid type. No experimental data about orbital energy before and after forming porphyrin anion is obtained. We consider these reverse phenomena. As has been demonstrated by NMR data that protonation of porphine or exchange NH groups in the case of TPP by the heteroatoms S, Se, and Te has contrary effect on ring current to that for forming porphyrin anion. The effect of substituting two NH groups in TPP by two S atoms on the first oxidation and reduction potentials is a shift of both to more positive values.32 But the effect on the reduction process is larger. By subsequent substitution of S by Se and Te, which can be stronger interaction and be thereby more effective drain on the π electron, the first reduction potential continues to increase, while the first oxidation potential stays more or less constant or even decreases somewhat. To a first approximation,33 the redox span of the first oxidation to the first reduction potential corresponds to the HOMO-LUMO energy gap. Thus
Sequential Deprotonation of p-OHTPPH2 we expect that forming porphyrin anion shifts both the first oxidation and the first reduction potentials to more negative values. That is, forming porphyrin anion increases the energy of porphyrin HOMO and LUMO. The inference are supported that the energy level of conversion of porphine to its dication is estimated to be decreased about 0.2 eV.34 The conclusion is also in accord with several previous DFT (B3LYP/6-31G(d)) calculations that the HOMO - 1, HOMO, LUMO, and LUMO + 1 of porphine had been calculated to be -5.542, -5.407, -2.485, and -2.472 eV, respectively,35 while the corresponding orbital energies of its disodium salts were -4.90, -4.74, -1.84, and -1.84 eV.11 Thus conversion of porphine to its disodium salts had been calculated to increase the energy levels of LUMO and HOMO by about 0.65 ev. However, this increase is mainly applied to the porphyrin macrocycle π orbitals but not the peripheral phenyl orbitals, because our NMR data have demonstrated that the negative centers generated by deprotonation of pyrrolic-H only act as electron-donating groups on the porphyrin π system and the negative charges of the phenoxide anions also mainly localize on the peripheral substituents. As a result, the deprotonation of pyrrolic-H increases the energy of the macrocycle π orbitals proportionally more than that of π orbitals localized on the phenoxide anion substituents. Thus the porphyrin π orbitals cross over the phenoxide anion π orbital and turn into HOMO. Thereby a charge-transfer transition (π (phenoxide anion) f π* (porphyrin)) disappears. The spectra of Na2Ps restore regular porphyrin spectra. Due to deprotonation of two pyrrolic-H turning p-OHTPPH2 from D2h to D4h symmetry, the higher concentrated NaOH in DMF turns hyperporphyrin spectra of p-OHTPPH2 into three-banded spectra of metalloporphyrin. In addition, from Table 1 and Figures 3-6, we observe that both the absorption maxima of all bands and the extinction coefficients () ratios of Q(0,0) (II)/Q(1,0) (I) of Na2Ps increase markedly with increasing number of phenoxide anion substituents. Such changes can be explained according to four orbital model (Figure 1). The electron density associated with the a1u orbital is nonvanishing only on the pyrrolic R- and β-carbons (R > β), while that associated with the a2u orbital is concentrated mostly on the meso carbons and the nitrogen centers, with a small component from the pyrrolic R- and β-carbons. Orbital representations are shown in Figure 9. The substituents located in the meso position are known to mainly affect the energy of the a2u HOMO. It is expected that electron-donating substituents in the para position will raise the energy of this orbital by increasing its electron density. This will lead to a decrease in the energy of the transition (as shown in Figure 1). As a result, all bands of the Na2Ps are red-shifted, as the number of the electron-donating substituents increases. The four-orbital model predicts the Q-bands are due to (-) combinations of one-electron promotions between the two highest occupied π orbitals a2u and a1u and the two lowest unoccupied π* orbitals eg (Q ) (1/ x2) (a1ueg-a2ueg)). If the a1u and a2u orbitals are degenerate in energy, the two promotions contribute equally to Q-bands, and effectively cancel each other. When the electron donating substituents in the para position of phenyl ring raise the energy of a2u orbital, the degeneracy of the a1u and a2u orbitals is removed. It induces the intense Q-bands due to the enhanced transition probability of the forbidden Q-bands absorption. In all cases, intensities of the forbidden 0-0 transitions are affected much more significantly than those of the 0-1 bands, where perturbation by vibronic coupling has already removed the forbiddenness. This results in the increased ratios of Q(0,0)/Q(1,0). Increasing number
J. Phys. Chem. B, Vol. 110, No. 1, 2006 593
Figure 9. Schematic representation of the two highest filled π-orbitals (a1u, a2u symmetry) and lowest unoccupied π-orbitals (eg) in the D4h porphyrin ring adapted, from ref 36. (Size of circles proportional to atomic orbital coefficients with filled circles representing negative coefficients.)
of phenoxide anion substituents increases the energy of a2u orbital. Therefore, the more phenoxide anions substituents, the more all bands in the optical spectra of the sodium salts of p-OHTPPH2 are red-shifted and the larger the extinction coefficient ratios of Q(0,0)/Q(1,0) increases. 5. Summary The alkali metal porphyrin complexes M2(Por) (M ) Li, Na, K) were obtained as long ago as 1948,37and have been found to be useful reagents in the syntheses of early transition metal complexes of porphyrins38 and porphyrin-sandwich compounds.39 However, isolation and structural study of these alkali metal complexes are difficult since they are chemically labile and can quickly change to the free base porphyrin (H2P) in the presence of water. However, we observe that in the presence of a small amount of water, p-OHTPPH2 in DMF as solvent can be deprotonated of pyrrolic-H by higher concentrated NaOH and coordinate with sodium cations to form Na2P, which has been demonstrated by our UV-vis, RR, and NMR data. Herein we first report RR and NMR spectra of sodium salts of p-OHTPPH2, which demonstrates that Na2P molecules have prolonged CR-Cm and Cβ-Cβ bonds compared with H2P, and that the formation of Na2P only perturbs the porphyrin skeleton vibrations, leaving phenyl rings almost unaffected. The negative centers generated by deprotonation of pyrrolic-H act only as electron-donating groups on the porphyrin π system, and the negative charges of the phenoxide anions also mainly localize on the peripheral substituents. Hence the porphyrin π orbitals cross over the phenoxide anion π orbital and turn into HOMOs, which transforms the hyperporphyrin spectra of p-OHTPPH2 into three-banded spectra of regular metalloporphyrin.
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