J . Phys. Chem. 1985, 89, 5225-5235 when compared with AHDO for H2SH+-SH2 (14.1 f 1.3 kcal mol-', see Table I). The difference is unexpected, since in both symmetric R20H+-OR2 and R3NH+.-NR3 dimers with APA = 0 the values of AHDO are constant, Le., 30 and 24 kcal mol-', respectively, regardless of the identity of R. The reason for the different behavior of the sulfur dimer is not obvious. 2. Mutual Effects of Weak and Strong Ligands; The Clusters CH3NH3+.CH3SH.nCH3CN. The examination of a large series of clusters of the type BH+.nA showed that the mutual effects of ligand molecules A weaken the bonding to each subsequent A molecule added to the cluster. For all of the hydrogen-bonded clusters, the relative weakening effect of consecutive ligands turned out to be unexpectedly constant. Thus, for a large variety of clusters, the attachment energy of the second A molecule to the cluster is smaller by a factor of 0.75 f 0.05 than the attachment energy of the first ligand; Le., the first ligand molecule decreases the attachment energy of the second ligand molecule to BH+ by 25%. In all cases examined experimentally, the first and second ligand molecules were identical. It is of interest to examine the mutual effects of ligands when the two molecules are different, especially when one is a highly polar, strongly bonding ligand and the other is a weakly polar, weakly bonding ligand. The weak attachment energies of sulfur ligands to NH+- groups affords such a test. For this purpose we examined the mutual effects of attaching CH3SH and CH3CN ligands to CH3NH3+.The results are shown in Table 11. The difference between the bonding of the ligands is illustrated by the difference between the first attachment energies, 13.4 vs. 26.2 kcal mol-'. The data for the second clustering steps can be used to examine the mutual effects of the ligands. For example, to find the effect of CH3SH on the attachment energy of CH3CN to the cluster, we compare AHo for reactions 4 and 5 . The results show that
+ CH$N
5225
-
C H ~ N H ~ + S C H ~ AHo C N = -26.2 (4) CH3NH3+*CH$H + CH3CN CH3NH3+.CH3SH*CH3CN AHo = -22.7 ( 5 ) CH3NH3'
-+
the addition of the weak ligand CH3SH decreases the attachment energy of CH3CN by a factor of 22.7/26.2 = 0.87, Le., AHo is lowered by 13%, while the prior addition of the strong ligand CH3CN lowers the attachment energy of CH3SH by a factor of 9.9/13.4 = 0.74, Le., by 26%. By comparison, the addition of one CH3CN molecule lowers the attachment energy of a second CH3CN molecule by a factor of 18.6/26.2 = 0.71, Le., by 29%. Thus, in the present cases the effect of the strong ligand on the attachment of a second ligand is proportionally similar whether the second ligand is weak or strong. However, if the first ligand is weak, its effect on attachment energy of a second, strong ligand is significantly smaller. This may be expected since the effect of a weak ligand on the charge distribution in CH3NH3+should be minimal. The differences between the effects of weak and strong ligands are seen to extend to higher clustering steps. Thus, -AHo for the addition of CH,CN to CH3NH3+CH3SHis larger by 4.1 kcal mol-' than the addition to CH3NH3+.CH3CN. By comparison, -AHo for the addition of CH3CN to CH3NH3+. CH3CN.CH3SH is still larger by 3.1 kcal mol-' than the addition to CH3NH3+.2CH3CN.Therefore, the bond-weakening effects of CH3CN and CH3SH in the mixed clusters are roughly additive. The present data seem to be the only available for protons solvated by more than two different ligands. Further study of mixed clusters is desirable, since many association ions occurring in nature may involve a variety of components. Acknowledgment. This research was supported by the Office of Basic Energy Sciences, United States Department of Energy.
Intramolecular Electron Transfer in Donor-Acceptor Systems. Porphyrins Bearing Trinitroaryl Acceptor Group G. Bhaskar Maiya and V. Krishnan* Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560 01 2, India (Received: February 20, 1985)
Porphyrins bearing picryl group in ortho, meta, and para positions of one of the mesoaryl groups of tetraphenylporphyrin (TPP) have been synthesized in the free-base form. The metal [Cu(II) and Zn(II)] derivativesof the free-base picrylporphyrins (PPc) have been prepared. The broadened Soret absorption, the decreased optical absorbance values of Q bands, and the reduced singlet emission quantum yields of PPc indicate the existence of intramolecular interaction. The extent of this interaction is found to be greater than those observed for the intermolecular systems and varies with the position at which the picryl moiety is attached to the porphyrin as ortho > meta > para. The energies of the redox states, E(CT) of P'Pc-, calculated from the electrochemical redox potentials depend on the nature of the metal ion as free-base PPc > CuPPc > ZnPPc. The nature of intramolecular interaction between the picryl moiety and porphyrin unit is essentially T-T (CT). Conclusive evidence for light-induced electron transfer in ZnPPc is presented from EPR studies. The decay profiles of the EPR signals vary with the position of the picryl moiety as ortho < para < meta. Computer simulation of structures substantiated by the 'H NMR results point out the restricted conformational freedom of the picryl moiety in ZnPPc. Arguments based on symmetry considerations of HOMO of the excited singlet state PPc and LUMO of picryl group indicate plane-to-plane orientation of the donor and acceptor in the picrylporphyrins.
Introduction Intramolecular systems comprising of an electron donor and an acceptor are important in the study of excited-state energytransfer and electron-transfer mechanisms. Investigations of systems containing porphyrins and metalloporphyrins as electron donors bear a direct relevance to photosynthetic research because of their close structural resemblance to the plant pigments. A 0022-3654/85/2089-5225$01.50/0
study of the model compounds comprising a donor porphyrin and an acceptor at prefixed distances and orientations is paramount to elucidate the influence of these parameters in the intramolecular photochemical electron transfer. Several interesting studies to mimic the reaction-center complex through the synthesis of covalently linked porphyrin-quinone moieties are reported in literature.' In all these systems, evidence has been presented for 0 1985 American Chemical Society
Maiya and Krishnan
5226 The Journal of Physical Chemistry, Vol. 89, No. 24, 1985
SCHEME I : Synthetic Route for the Preparation of Picrylporphyrins ..
H
/
d
OH
I
1 P-A
Figure 1. Schematic representation of the possible events of photoexcited porphyrin covalently linked to an acceptor (P-A).
intramolecular interaction, and in a few cases light-induced charge separation has been demonstrated. Recent studies of McIntosh et Siemiarczuk et and Wasielewski and Niemczyk4 on a series of porphyrin-quinone systems are noteworthy in this context. These investigations deal with the influence of donoracceptor separation and orientation of the acceptor on the kinetics of light-induced charge separation. Further, Moore et aL5 have elegantly assembled a quinone and a carotenoid to a porphyrin and demonstrated an ultrafast excited-state electron transfer accompanied by a slow charge recombination reaction. Thus far, the studies to mimic reaction-center complexes involve the use of quinone as an acceptor. The reasons for this choice stem from the observation that in early stages of photosynthesis in photosystem I of both green plant and bacterial systems, quinones are involved as primary acceptors.6 Besides quinones, methylviologen (MV2+) has been used as an acceptor in the model compounds involving MV2+covalently linked to porphyrins.' Time-resolved optical studies of these compounds reveal the existence of excited-state charge transfer in these systems. The energetics of light-induced charge separation is schematically represented in Figure 1. The energetics of radical pair P+wA- (where represents a covalent linkage) formation is
-
~~~
(1)(a) Tabushi, I.; Koga, N.; Yanagita, M. Tetrahedron Lett. 1979, 257-260. (b) Dalton, J.; Milgrom, L. R. J. Chem. SOC.,Chem. Commun. 1979,609-610.(c) Kong, J. L. Y.;Loach, P. A. J. Heterocycl. Chem. 1980, 17,737-744. (d) Ho, T.-F.; McIntosh, A. R.; Bolton, J. R. Nature (London) 1980,286, 254-256. (e) Harriman, A,; Hoise, R. J. J. Photochem. 1981, 15, 163-167. (f) Netzel, T. L.; Bergkamp, M. A.; Chang, C. K.; Dalton, J. J. Photochem. 1981, 15,451-460. (g) Nishitani, S.; Kurata, N.; Sakata, Y.; Misumi, S.; Migita, N.; Okada, T.; Mataga, N. Tetrahedron Lett. 1981, 2099-2102. (h) Migita, M.; Okada, T.; Mataga, N.; Nishitani, S.; Kurata, N.; Sakata, Y.; Misumi, S. Chem. Phys. Lett. 1981,84,263-266. (i) Kong, J. L. Y.; Spears, K. G.; Loach, P. A. Photochem. Photobiol. 1982, 35, 545-553. 6 ) Ganesh, K.N.; Sanders, J. K. M. J. Chem. Soc., Perkin Trans. 1 1982,1617-1624. (k) Bergkamp, M. A.; Dalton, J.; Netzel, T. L. J. Am. Chem. SOC.1982,104,253-254. (I) Lindsey, J. S.; Mauzerall, D. C. J. Am. Chem. SOC.1982, 104,4498-4500. (m) Lindsey, J. S.; Mauzerall, D. C. J. Am. Chem. SOC.1983,105,6528-6529.(n) Nishitani, S.; Kurata, N.; Sakata, Y.; Misumi, S.; Karen, A.; Okada, T.; Mataga, N. J. Am. Chem. SOC.1983, 105,7771-7772. (0)Weiser, J.; Staas, H. A. Angew. Chem., (In?.Ed. Engl.) 1984,23,623625.(p) Leighton, P.; Sanders, J. K. M. J. Chem. Soc., Chem. Commun. 1984,24-25. (9)Mataga, N.; Karen, A,; Okada, T.; Nishitani, S.; Kurata, N.; Sakata, Y.;Misumi, S. J. Phys. Chem. 1984,88, 5138-5141. (2)McIntosh, A. R.;Ho, T.-F.; Stillman, M. J.; Roach, K. J.; Weedon, A. C. J. Am. Chem. SOC.1983,105,7215-7223. (3)Siemiarczuk, A.; McIntosh, A. R.; Ho, T.-F.; Stillman, M. J.; Roach, K. J.; Weedon, A. C.; Bolton, J. R.; Connolly, J. S. J. Am. Chem. SOC.1983, 105, 7224-7230. (4)Wasielewski, M. R.; Niemczyk, M. P. J. Am. Chem. SOC.1984,106, 5043-5045. (5) Moore, T. A,; Gust, D.; Mathis, P.; Mialocq, J.-C.; Chachaty, C.; Bensasson, R. V.; Land, E. J.; Doizi, D.; Liddell, P. A.; Lehman, W. R.; Nemeth, G. A.; Moore, A. L. Nature (London) 1984, 307, 630-632. (6)(a) Blankenship, R. E. Ace. Chem. Res. 1981, 14, 163-170. (b) Vermeglio, A. In 'Function of Quinones in Energy Conserving Systems"; Trumpower, B., Ed.; Academic Press: New York, 1982;pp 169-180. (c) Blankenship, R. E; Parson, W. W. In "Photosynthesis in Relation to Model Systems"; Barber, E., Ed.; Elsevier: Amsterdam, 1979;pp 71-1 14. (7)Harriman, A,; Porter, G.; Wilowska, A. J . Chem. Soc., Faraday Trans. 2 1984,80, 191-204.
M = ZH,
Zn ( I l l , C u ( l l )
dictated by both the oxidation potential of P and the reduction potential of A. For the substituted 1,Cquinones as acceptors the reduction potentials are poised favorably to have exergonic values of AGet for electron-transfer process.s The magnitudes of reduction potentials of the quinones that are covalently linked to porphyrins suggest that in all the cases light-induced radical-pair generation should result. The usage of quinones as acceptors, however, restricts the energy that can be stored in the photoinduced radical pair and the mechanisms through which radical-pair generation takes place.9 In order to study a wide range of mechanisms, it is essential to have acceptors other than quinones whose reduction potentials can systematically be varied such that the energy of P+-A- falls between the energies of excited singlet, triplet state and beyond. Another criterion that needs fulfillment is the ability of these molecules to function as a-acceptors since electron donation essentially arises from the a-electronic manifold of the donor. In this paper we report the synthesis of porphyrins bearing a trinitroaryl (picryl) group covalently linked to one of the mesoaryl groups of porphyrins a t ortho, meta, and para (0, m, and p) positions. We describe here the optical, magnetic resonance, and electrochemical properties of these porphyrin systems. A report concerning the porphyrins bearing a cyclodextrin cavity in which nitroaromatic acceptors are hosted has appeared recently.I0 (8) The free energy change accompanying the excited-state electron transfer (A&) can be expressed as AGET = E(D+./D) - E(A/A--) - E& where E(D+-/D) and E(A/A--) refer to one-electron reduction potentials of the oxidized donor and the acceptor, respectively. E&, refers to the first excited singlet state energy of the donor. (Weller, A.; Zachariasse In "Molecular Lumnescence"; Lim, E., Ed.; W. A. Benjamin: New York, 1969; p 685). This expression neglects Coulomb interactions and solvation effects. It is e x w t e d that when -AGET > 150 meV, the electron transfer occurs and the rate increases with increasing exergonicity. (Sutin, N. Annu. Rev. Nucl. Sei. 1962,12, 285. Jortner, J. J. Am. Chem. SOC.1980,102, 6676). (9)Connolly (Connolly, J. S. In "Photochemical Conversion and Storage of Solar Energy-1982";Rabani, J., Ed.; The Weizmann Science Press of Israel: Jerusalem, 1982;Part A, pp 175-204)suggests an alternative to the quinones and recommends the usage of nitroaromatics with varying reduction potentials, so as to obtain various mechanistic informations concerning the excited-state electron transfer. (10)Gonazalez, M. C.; McIntosh, A. R.; Bolton, J. R.; Weedon, A. C. J . Chem. Soc., Chem. Commun. 1984, 1139-1141.
Electron Transfer in Donor-Acceptor Systems The choice of trinitroaryl group as an acceptor is dictated from our earlier studies" on intermolecular complexes formed between tetraphenylporphyrin (TPP) and its metallo-derivatives with sym-trinitrobenzene (TNB). These studies point out that in the 1:l complexes, the donor and acceptor have plane-to-plane orientation. Further, it is shown that the singlet quenching of the porphyrins by nitroaromatics essentially proceeds through exciplex formation. It is anticipated that similar effects would prevail in the covalently linked porphyrins. Employment of the ether group as a source of covalent linkage between the porphyrin and the acceptor has the distinct advantage of being photochemically and hydrolytically inert and permits a range of conformational mobility.I2 For brevity, we denote the picrylporphyrins synthesized in this study as oPPc, mPPc, and pPPc where 0,m, and p prefixes stand for ortho, meta, and para positions of the mesoaryl group at which the picryl group is attached. The divalent metal derivatives of these picrylporphyrins are represented with prefixing the metal to the respective porphyrins. While the optical absorption and emission studies of these porphyrins furnish a good deal of information concerning the intramolecular complexing ability of these systems, we are able to demonstrate the existence of light-induced radical-pair formation through EPR measurements. The electrochemical redox potentials of these porphyrins have been useful in arriving at the energies of charge-transfer (CT) states responsible for radical-pair generation. The existence of different conformers as revealed by N M R experiments is helpful in highlighting the influence of orientation of the acceptor relative to the donor on the photoreactions.
Experimental Section The synthetic strategy adopted for the preparation of picrylporphyrins is essentially the same as that reported for the podand porphyrins.13 The synthetic route begins with the normal porphyrin functionalized with a hydroxyl group at 0, m, and p positions of one of the meso aryl groups and followed by condensation with picryl chloride (Scheme I). Picryl chloride was prepared by reacting together PC15 and picric acid. (5Hydroxyphenyl)-10,-1 5,-20-triphenylporphyrin(I) was prepared in 5% yield by condensing together 1.34 g (1 1 mM) of either 0-,m-, or p-hydroxybenzaldehyde and 3.18 g (30 mM) of benzaldehyde with 2.68 g of pyrrole (40 mM) in 250 mL of refluxing propanoic acid. It was purified by a reported p r 0 ~ e d u r e . l ~ Picrylporphyrins: o-Picrylporphyrin. A 1.26-g (2.0 mM) sample of I and 0.52 g (2.1 mM) of picryl chloride were reacted together in presence of K2C03(1 .O g) in 50 mL of D M F for 36 h under vigorous stirring at room temperature. The residue obtained after the removal of D M F under reduced pressure was washed with water and taken in CHCl,. The solution was dried over anhydrous MgS04 and chromatographed on basic alumina column (activity 1). A solvent mixture, CHC1,:THF (100:5 v/v), was employed as eluent, and the fast moving fraction was shown to contain the desired picrylporphyrin. A similar procedure was adopted for the preparation of m- and p-picrylporphyrins starting from the m- and p-hydroxybenzaldehydes. In all the cases, the yields of the picrylporphyrins were found to be -50% based on the (hydroxypheny1)triphenylporphyrin. The metal [Cu(II) and Zn(II)] derivatives of these picrylporphyrins have been prepared by using a procedureI5 analogous to that of metal derivatives of tetraphenylporphyrin (TPP). Methods. The spectrometers employed in this study are the same as described in our earlier work.16 Spectroscopically pure
-
(11) (a) Chandrashekar, T. K.; Krishnan, V. Inorg. Chem. 1981, 20, 2782-2786. (b) Chandrashekar, T. K.; Krishnan, V. Bull. Sor. Chim. Fr. 1984, 1-42-48. (12) Jeyakumar, D.; Krishnan, V. N o w . J . Chem. 1983, 697-698. (13) Bhaskar Maiya, G.; Krishnan, V. Inorg. Chim. Acfa 1983, 77, L1315. (14) Little, R. G.; Anton, J . A,; Loach, P. A,; Iber, J. A. J . Hererocycl. Chem. 1975, 23, 243-348. (15) Rothemund, P.; Mennotti, A. R. J. Am. Chem. SOC.1948, 70, 1808-1 8 12.
The Journal of Physical Chemistry, Vol. 89, No. 24, 1985 5227 solvents were used for all measurements. The solution preparation area was covered with a blanket of Nz and darkly lit. Precautions were taken not to expose the solutions prior to spectral measurements to bright light and the solutions are purged with N z free from oxygen before the measurements. The purity of the samples for spectral measurements were determined by TLC, 'H NMR, and optical spectral methods. The concentrations of picrylporphyrins employed for optical absorbance measurements in the red region (Q bands) range from 0.1 to 0.3 mM while a decadic dilution of this solution was used for the absorbance measurements in the Soret region. The fluorescence spectra were measured by using right angle detection method. The concentration of the solutions employed for these measurements had an optical absorbance less than 0.2 for Soret absorption. The excitation wavelength is the Soret band (A = 420 nm). The excitation band-pass slit width used is 2 nm and that of emission is 5 nm. The absorbance and emission intensities reported here are integrated over the absorption/emission envelope. The uncertainty involved is f 10%. The EPR measurements of Cu(I1) derivatives of picrylporphyrins (1 mM) were made in toluene at 100 K. DPPH was used as g marker. Light source for the study of photochemical properties observed through EPR was 150-W Varian E-MAC xenon lamp coupled to a regulated power supply. The UV irradiance was cut off by using glass filters so that only h > 350 nm falls on the sample. Light beam was channelled and focused into the EPR cavity through a focusing light guide. The data represent irradiance at broad band visible light. The concentration of solutions employed for irradiation experiments was approximately 0.1 mM. The solutions were taken in 0.4-cm-0.d. quartz tubes which were then introduced into the insert Dewar of the temperature controller in the EPR cavity. The 'H N M R measurements of the picrylporphyrins and their metal derivatives were carried out in CDCl, at ambient temperature using Me4Si as an internal standard. Cyclic voltammetric experiments were performed using a three-electrode potentiostatic circuit and a MPI Model M P 1042 voltammetry controller. A Beckmann Pt-button electrode (area = 0.25 cm2), a Pt wire counter electrode, and saturated calomel electrode (SCE) constitute the three electrode assembly. Tetrabutylammonium perchlorate (TBAP) twice crystallized from ethanol/ethyl acetate was used as the supporting electrolyte. Half-wave potentials were measured as the average of cathodic and anodic peak potentials for all reversible peaks. Liquid junction potentials were eliminated by using a salt bridge consisting of the same concentration of base electrolyte as that of the bulk solution and it was positioned between the bulk test solution and the reference electrode. All potentials are referenced with respect to SCE unless otherwise stated.
Results The synthetic procedure adopted in the present study leads to the preparation of spectrally pure picrylporphyrins in good yields. The presence of oxidized contaminants in solutions of these porphyrins was checked from time to time and it is found that the solutions are stable provided they are not unduely exposed to bright light. 'H N M R spectral features of these compounds have been used for the complete characterization. Optical Spectra. The optical absorption spectra of the free-base OPPCand its Zn(I1) derivative in CHCI, are shown in Figure 2. The spectra of mPPc and pPPc free-base porphyrins exhibit features similar to that of absorption of oPPc. The band maxima, their log t values and fwhm values of Soret bands of the porphyrins are given in Table I. It is of interest to note that the c values of the bands are diminished relative to the unsubstituted porphyrin (TPP) and the log c values follow the order TPP > pPPc > mPPc > oPPc. The decrease in absorbance values closely parallel the absorbance changes observed for TPP/ZnTPP on addition of TNB." Moreover, the absorption bands of picrylporphyrins are broadened relative to TPP or ZnTPP and this effect is pronounced ~~
(16) Thanabal, V.; Krishnan, V. J . Am. Chem. SOC.1982,104,3644-3650.
5228 The Journal of Physical Chemistry, Vol. 89, No. 24, 1985
Maiya and Krishnan
TABLE I: Optical Absorption Data on Free-Base Picrylporphyrins and Their Metal Derivatives in CHCl3 at 293 K"
compd TPP pPPc mPPc OPPC ZnTPP ZnpPPc ZnmPPc ZnoPPc CuTPP CupPPc CumPPc CUOPPC
4 2
Q1
650 648 648 648
(3.50) (3.46) (3.49) (3.46)
592 592 592 591
Q?
(3.72) (3.73) (3.71) (3.72)
550 552 552 552 549 551 551 552 540 541 542 543
fwhm for B band, nm
B
Q.4
(3.88) (3.80) (3.80) (3.82) (4.37) (4.24) (4.24) (4.24) (4.29) (4.20) (4.1 1) (4.05)
5 15 (4.26) 514 (4.26) 5 14 (4.23) 515 (4.21)
419 420 421 421 421 421 422 422 417 416 418 418
(5.60) (5.42) (5.41) (5.33) (5.70) (5.34) (5.32) (5.01) (5.62) (5.60) (5.60) (5.51)
15 17 17 19 11 13 12 13 13 16 16 16
aValues in the parantheses refer to log c values; absorption band positions in nm. TABLE 11: Fluorescence Maxima (nm) and Relative Quantum Yieldsa of the Free-Base and Zn(I1) Derivatives of Picrvlporphvrins*
CHCl3 compd pPPc mPPc OPPC ZnpPPc ZnmPPc ZnoPPc
CH,CI,
CH30H
CH3CN
Amax
@i
Amax
@i
Amax
9i
Amax
@f
650 649 649 650 652 653
0.92 0.84 0.76 0.50 0.46 0.43
654 654 652 657 658 658
0.95 0.70 0.64 0.77 0.71 0.51
650 652 652 656 657 657
0.85 0.64 0.59 0.49 0.36 0.17
652 650 652 658 658 658
0.3 1 0.28 0.19 0.45 0.35 0.21
"These are relative fluorescence quantum yields taking @[ for TPP and ZnTPP to be 1.O. bQuantum yields at other bands of PPc (-710 nm) and ZnPPc (-610 nm) were found to be not different from the corresponding values quoted above.
I
-
II
0.8 -
c
u
Y
5
0.6-
d SI
2
-
n
-
N
$ 0.4-
a
P I
I ! II 0.2
-
500
550
€00
650
700
WAVELENGTH ( n m )
Figure 2. Optical absorption spectra of the free-base oPPc(-) Zn(I1) derivative (- - -) in CHCI, at 293 K.
and its
in the Soret region as seen from the fwhm values. The normalized Soret band profiles of the free-base picrylporphyrins are shown in Figure 3. This broadening implies the interaction of the picryl moiety with the porphyrin unit. The fluorescence spectra of oPPc and its Zn(I1) derivative are shown in Figure 4. The spectra of picrylporphyrins were measured in four different solvents, CHC13,CH2C12,CH30H,and CH3CN. The fluorescence yields were calculated by making use of the expression given by Austin and Gouterman." The fluorescence quantum yields of ZnTPP and TPP are chosen to be unity for the sake of comparison. The emission bands and their relative quantum yields are given in Table 11. The fluorescence maxima of both the free-base picrylporphyrins and their Zn(I1) derivatives (17) Austin, E.; Gouterman, M. Bioinorg. Chem. 1978, 9,281-298
0 I
1
1
I
380
400
420
44 0
The Journal of Physical Chemistry, Vol. 89, No. 24, 1985 5229
Electron Transfer in Donor-Acceptor Systems
TABLE 111: EPR Parameters of Cu(I1) Derivatives of Picrylporphyrins in Toluene at 100 K compd
Rll
RI
A? x 104 cm-1
A? x 104 cm-'
A: x io4 cm-'
A? x io4 cm-'
A&
CuTPP CupPPc CumPPc CUOPPC
2.185 2.156 2.166 2.160
2.047 2.039 2.030 2.017
208.0 201.0 201 .o 200.0
31.5 32.3 32.3 32.1
14.8 14.6 14.6 15.0
16.0 16.0 16.1 16.0
0.052 0.047 0.064
TABLE IV: g and AH,, Values for the Signals Generated on Irradiation of Zinc(I1) Picrylporphyrins at 300 and 140 K 300 K compd ZnpPPc ZnmPPc ZnoPPc
y 720 I
I
I
680
I
I"\,
1
640
600 WAVELENGTH l n m I
I
I
560
Figure 4. Fluorescence spectra of oPPc (-- -) and ZnoPPc(-) in CH2CI2 at 293 K. The excitation wavelengths used were 421 and 422 nm for the free-base and Zn(I1) derivative, respectively.
and CH3CN). However, for the free-base porphyrins a dramatic decrease in &value is observed in CH3CN though the dielectric constants of C H 3 0 H and CH3CN are not very different. The results obtained herein indicate the existence of interaction between picryl group and porphyrin and these are governed by the polarity of the solvents. 'HNMR Spectra. The 'H N M R spectra of picrylporphyrins are highly characteristic and provide evidence for structural integrity in these compounds. IH N M R spectra of free-base picrylporphyrins are shown in Figure 5. The @-pyrroleprotons of picrylporphyrins resonate as a complex multiplet (8.94-8.70 ppm) due to the nonequivalence of @-pyrroleprotons as a consequence of mesoaryl substitution.Is It is of interest to note that the complexity of the multiplet depends on the nature of substitution and increases from para-substituted porphyrin to an ortho-substituted one. This indicates the presence of the picryl group influences the @-pyrrole proton resonance. The resonances arising from 0,m, and protons of the meso aryl groups occur at higher fields relative to @-pyrroleprotons. The o protons appear as a multiplet scanning the region 8.30-8.10 ppm whereas m and p protons resonate at 7.85-7.61 ppm. The resonances of the substituted mesoaryl protons possibly are merged with the resonances of other mesoaryl protons and it is found difficult to locate them in an unambiguous manner. The proton resonance of picryl group occur ca. 8.90 ppm in the picrylporphyrins, the region in which the protons of trinitroanisole r e ~ o n a t e . ' ~It is noteworthy that the position of the picryl resonance in the porphyrins varies with the nature of the substitution and occur at higher fields relative to the trinitroanisole resonance. Thus in the 0-,m- and p-picrylporphyrins, the proton resonance of the trinitroaryl group occur at 8.50, 8.91, and 8.95 ppm, respectively. This observation closely parallels the upfield shifts of T N B proton resonance in the intermolecular complexes of TPP and ZnTPP with T N B in CDCl,." The imino protons of the free-base picrylporphyrins resonate as a complex multiplet in the range -2.60 to -3.10 ppm (Figure 5) while only a singlet resonance is observed for TPP at -2.78 ppm.20 The complexity of the multiplet and the upfield shift of (18) Walker, A. N . ; Blake, V. L.; McDermott, G. A. Inorg. Chem. 1982, 21, 3342-3348. (19) "The Sadtler Hand Book of NMR Spectra"; Simon, W. W., Ed.;
Sadtler Research Lab Inc., Pennsylvania; p 682.
140 K
g
AH,.., mT
R
2.0032 2.0039 2.0031
0.53 0.70 0.68
2.0032 2.0039 2.0031
AH,., mT 0.50 0.53 0.53
the center of this multiplet resonance relative to TPP in the synthesized picrylporphyrins follow the order pPPc < mPPc < oPPc. The appearance of a multiplet for N H protons resonance can either arise from the existence of N H tautomers and/or interaction of N H with NO2 group of the picryl moiety. In view of these possibilities, we have carried out a few N M R experiments the results of which are summarized as follows. (i) The proton resonance spectra of free-base picrylporphyrins obtained by irradiating at the resonance peaks of P-pyrrole protons did not show any effect on the resonance position of N H protons and picryl protons. This indicates the absence of any specific interaction between the protons of @-pyrrole, trinitroaryl, and the imino groups. (ii) The ' H N M R spectra of the free-base picrylporphyrins obtained at higher temperature (325 K) exhibit reduced complexity of the multiplet for N H protons resonance relative to the observation at ambient temperature (303 K). (iii) The change of solvent from CDC1, to CD3CN:CDC13(90:10), v/v) results in the collapse of the multiplet structure of N H proton resonance of picrylporphyrins followed by a marginal upfield shift. (iv) The IH N M R spectra of a 1:l mixture of mPPc and CF3CO O H in CDC13 in the temperature range 300-213 K exhibits a coalescence point at 227 K featuring the -NH tautomerization equilibrium analogous to that reported for TPPH22+.21 These observations are suggestive of the involvement of N H protons in H bonding.22 EPR Spectra. The EPR spectra of Cu(I1) derivatives of picrylporphyrins are well resolved displaying characteristic features both in parallel and perpendicular regions. The EPR spectrum of CumPPc is compared with the spectrum obtained for CuTPP in the same solvent (toluene) at 140 K and this is shown in Figure 6. The EPR parameters lgl and IAl are calculated from the spectral features assuming a spin Hamiltonian for axial symmetry23 (Table 111). It is possible to estimate the bonding parameter ( a 2 ) which is a measure of covalency of the in-plane bonding of the Cu-N bonds in the copper(I1) picrylporphyrin~.~~ The values thus calculated are given in Table 111. An examination of Table I11 reveals that in all Cu(I1) derivatives there is a marginal decrease in g and A T values and this is accompanied by an increase in A; and AT values. However, there is no regular change in the magnitude of these parameters in the Cu(I1) derivatives of differently substituted picrylporphyrins. It may be pointed out that the marginal changes observed in the EPR parameters relative to CuTPP are in accordance with intermolecular complexes of CuTPP and TNB." The change in the a2values ( A a 2 )of the CuPPc suggests that the M-N bond becomes more covalent relative to CuTPP owing to the interaction of trinitroaryl group with the porphyrin. (20) Storm, C. B.; Teklu, Y.; Sokoloski, E. A. Ann. N.Y. Acad. Sci. 1973, 206, 63. (21) Abraham, R. J.; Hawkes, G.E.; Smith, K. M. Tetrahedron Lett. 1984, 7 1-74. (22) Laszlo, P. In "Progress in NMR Spectroscopy";Emsely, J., Feeney, J., Sutchlifte, L. H., Eds.; Pergamon Press: Oxford, 1967; Vol. 111, Chapter 6, pp 231-402. (23) Assour, J . M. J . Chem. Phys. 1965, 43, 2477-2489. (24) Kievelson, D.; Lee, S. K. J . Chem. Phys. 1964, 41, 1896-1909.
5230
The Journal of Physical Chemistry, Vol. 89, No. 24, 1985
Maiya and Krishnan
41
_i
I
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"-2 p p m 16)
I
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Figure 6. EPR spectra of (a) CuTPP and (b) CumPPc in toluene at 100
K.
Figure 5. 'HNMR spectra of the free-base picrylporphyrins in CDC13 at 293 K: (I) pPPc, (11) mPPc, and (111) oPPc. (a) represents both vertical and horizontal expansion of the NH proton resonances.
We shall examine now the EPR featrues of the photoinduced radical of picrylporphyrins. It is found that only Zn(I1) derivatives of picrylporphyrins generate radical pair with the appearance of an EPR signal in CH2C12under the conditions described in the Experimental Section. Two temperatures (300 and 140 K) are chosen for this study. Signals generated on irradiation of ZnoPPc for different time periods are shown in Figure 7A. Irradiation of a -0.1 mM solution of ZnPPc at a modulation amplitude of 0.4 mT results in a signal with fine structures at the wings of the main signal. The signal thus generated at the end of 15 min for ZnmPPc is displayed in Figure 7B. It is found that the features of the EPR signals observed in C H Q , remain unaltered when CH3CN is used as a solvent. The shape of the signal is Gaussian and remains so for the signals observed for Zn(I1) derivatives of other substituted picrylporphyrins both at ambient and low temof these EPR peratures. The g factors and the line widths signals are given in Table IV. It may be pointed aut that a change of concentration of zinc(I1) picrylporphyrins from 0.1 to 1.0 mM and a change in the microwave power of EPR settings from 1 to 10 mW does not have any effect on the line widths and g factors of the photoinduced signals. The assertion that these signals are the consequence of charge-separated radical-pair generation stems from the optical absorbance measurements of the solutions before and after irrad i a t i ~ n .The ~ ~ optical data indicate the absence of any irreversible photochemical reactions. Radical generations can also arise from (25) The optical absorption spectra, typically of ZnoPPc in CH2CI,, at the cessation of irradiation reveals bands at 552 nm (log c = 4.24) and 422 nm (log e = 5.01). This agrees well with the spectrum obtained prior to irradiation. In fact, the same sample could be used 2-3 times for irradiation studies.
Figure 7. (A) EPR spectra of light-generated radicals in CH2C12observed for ZnoPPc at 140 K at different time intervals: (a) 2 min, (b) 4 min, (c) 8 min, (d) 15 min, and (e) 18 min. EPR settings: time constant, 0.064 s; scan time, 4 min; modulation frequency, 100 kHz; modulation amplitude, 0.2 mT; microwave power, 2 mW; microwave frequency, 9.046 GHz. (B) EPR signal observed for ZnmPPc in CH,CI, at 140 K after 15 min of irradiation using the same field setting as in A. The modulation amplitude employed, however, is 0.4 mT.
fast reversible photochemical reactions occurring in the course of continuous radiation and/or individual photochemical reactivities of the constituent partners in the EPR tube. These possibilities are discarded by the following control experiments. (i) Irradiation of the solutions in CHzClz containing a mixture
Electron Transfer in Donor-Acceptor Systems
Figure 8. (a) E P R spectra of photolyzed samples containing equimolar (-1 m M ) or 1:20 molar mixture of ZnTPP and T N B (ii) and 1:250 molar mixture of ZnTPP and T N B (i). (b) E P R spectra of ZnTPP+. (- 1) m M ) produced electrolytically in CH2C12 containing TBAP (ii) and TNB-- (-1 m M ) produced photolytically in THF (i). (c) E P R spectrum of equimolar, (- 1 m M ) mixture of ZnTPP'. and TNB-e in a (1:l) mixture of CH2CI2 and T H F . (d) The digital addition of the spectra of ZnTPP+. and TNB-. from (b). All the EPR spectra are at 140 K. The E P R settings are time constant, 0.064 s; scan time, 4 min; modulation frequency, 100 kHz; modulation amplitude, 0.4 mT; microwave power, 2 mW; microwave frequency, 9.055 GHz.
of ZnTPP and T N B at the same concentration as that of ZnPPc or even at 1:20 mole proportions in EPR cavity does not result in the generation of EPR signal under the conditions employed for Zn(I1) derivatives of picrylporphyrins [Figure 8a]. However, on prolonged irradiation (ca. 30 min.) of an intermolecular mixture of ZnTPP and TNB in 1:250 mole proportions in CHzC12,an EPR signal is observed [Figure Sa] whose quantum yield is found to be very low.26 (ii) ZnTPP/TPP in CHzCIzdoes not produce any EPR signal on irradiation under the present experimental conditions. (iii) Irradiation of other constituents like TNB, picric acid, trinitrotoluene in CHZCl2,or the solvent CH2CI2alone does not produce any EPR signals2' (iv) The EPR spectra of ZnTPP. (produced electrochemically) in CHZCl2and TNB-. (produced photolytically) in T H F are shown in Figure 8b. A comparison of the EPR spectrum obtained on addition of equimolar ZnTPP+and TNB-a [Figure 8c] with that of the digital addition [Figure 8d] of individual EPR spectrum of ZnTPP+. and TNB-. reveals very close similarity. However, it is important to recognize that the EPR features of the signals generated upon photolysis of ZnPPc (Figure 7) are different from those of the control experiments (Figure 8). These findings suggest that for radical-pair generation, a covalent linkage is necessary to initiate donor-acceptor interaction (vide infra). It is found that the photoinduced EPR signals of Zn(I1) derivatives of picrylporphyrins under aerobic conditions develop after a lapse of few minutes of irradiation, attaining a saturation at the end of about 20 min. At the cessation of irradiation, the signal (26) The quantum yields for the generation of the radical pair were estimated by comparing the intensities of the light-induced EPR signals of Zn(I1) derivatives of picrylporphyrins and of a solution containing ZnTPPTNB (1:250). (27) The EPR signals of the TNB anion radical generated upon light impingement in different donor solvents have been reported. (Lagererantz, C.; Yhland, M. Acra Chem. Scand. 1962, 16, 1043-1044). It is to be noted that the irradiation was performed by using a Hg lamp or a high-pressure Xe lamp and the wavelength used was
