J. P h p . Chem. 1982, 86, 5163-5169
5163
Photophysical and Redox Properties of Water-Soluble Porphyrins in Aqueous Media K. Kalyanasundaram and M. Neumann-Spallart Instirut de Chimie Physique, €cole Polytechnique F&6rale de Lausanne, CH- 1015 Lausanne, Switzerland (Received May 11. 1982; I n Final Form: July 30, 1982)
Various photophysical (spectra,lifetimes, and quantum yields for fluorescence, phosphorescence,T-T absorption spectra, and quantum yields for triplet formation) and redox (potentials for one- and two-electron oxidation and reduction) properties of several water-soluble porphyrins (H,TPPS, H,TMPyP, ZnTPPS, ZnTMPyP, ZnTPPC, PdTPPS, PdTMPyP, and PdTPPC where TPPS = tetrakis(sulfonatophenyl)porphyrin, TMPyP = tetrakis(N-methylpyridyl)porphyrin,and TPPC = tetrakis(carboxypheny1)porphyrin)in aqueous solution have been studied. The tetrakis(N-methylpyridy1)porphyrinsare characterized by reduced fluorescence yields, higher triplet yields, and long lifetimes with intense T-T absorptions extending up to 1200 nm. The redox potentials of these cationic porphyrins are also shifted to more positive potentials by 350-400 mV with respect to those of anionic TPPS systems.
Introduction Photochemical studies of metalloporphyrins have been of increasing interest in fields extending from chemistry to bio1ogy.l The ability of the porphyrin excited states to undergo electron-transfer reactions is being pursued vigorously partly toward an understanding of the primary photosynthetic process2 and partly toward development of systems capable of photochemical conversion and storage of solar en erg^.^^^ Water-soluble porphyrins are of special interest for the latter. It has been shown recently that these porphyrins can sensitize H2evolution from water with rather high effi~iency.~ Development of photosensitizers requires knowledge of the photochemical and redox properties. While extensive studies exist for several synthetic or naturally occurring porphyrins in nonaqueous media:" similar studies on water-soluble porphyrins in aqueous media are sparse. Herein we report on our extensive studies on the photophysical and redox properties of several porphyrins (free bases as well as Zn, Pd, Mn, and Co derivatives of tetrakis(N-methylpyridy1)porphyrin (TMPyP), tetrakis(sulfonatopheny1)porphyrin (TPPS), and tetrakis(carboxypheny1)porphyrin (TPPC)). A comprehensive analysis is presented on the influence of the porphyrin ring substituents as well as metal incorporation on the photo and redox properties, as applicable to water-soluble porphyrins in aqueous media. Earlier we reported on some aspects of the Zn porphyrin derivat i v e ~ , ~In* ~recent ~ years, several other publications have (1)(a) D. Dolphin, Ed., "The Porphyrins", Vols. I-VII, Academic Press, New York, 1978; (b) K. M. Smith, Ed., "Porphyrins and Metalloporphyrins", Elsevier, Amsterdam, 1975. . (2)(a) K. Sauer, Acc. Chem. Res., 13,249 (1980);(b)M. Calvin, ibid., 11, 869 (1978); (c) G. R. Seely, Photochem. Photobiol., 27,639 (1978). (3)(a) D. G. Whitten, Acc. Chem. Res., 13,83(1980);(b)M. Gritzel, ibid., 14,376 (1981). (4)J. R. Darwent, P. Doughlas, A. Harriman, G. Porter and M.-C. Richoux, Coord. Chem. Rev., 44, 83 (1982). (5)(a) K. Kalyanasundaram and M. Gratzel, Helu. Chim. Acta, 63,478 (1980);(b) G. McLendon and D. S. Miller, J. Chem. SOC.,Chem. Commun., 533 (1980); (c) A. Harriman, G. Porter, and M.-C. Richoux, J. Chem. SOC.,Faraday Trans. 2,77,833(1981); (d) A. Harriman and M.-C. Richoux, J. Photochem., 15,335(1981);(e) I. Okura and N. Kim-Thuan, J. Mol. Catal., 5,311 (1979). (6)(a) F. R. Hopf and D. G. Whitten, in ref la, Vol. IIB, Chapter 6; (b)D. G. Whitten, Reu. Chem. Intermed., 2,107(1978);(c) D. Mauzerall in ref la, Vol. VC, p 29; (d) F. R. Hopf and D. G. Whitten in ref lb,p 667; (e) M. Gouterman in ref la, Vol. 111, p 1. (7)(a)R. H. Felton in ref la, Vol. VC, p 53; (b) D. G. Davis, ibid., p 127; (c) J.-H. Fuhrhop, Struct. Bonding (Berlin),18, 1 (1974). 0022-3654/82/2086-5 163$01.2510
appeared dealing with either photo or redox properties of water-soluble porphyrin^.^
Experimental Section Materials. Free-base porphyrins tetrapyridylporphyrin (TPyP), tetrakis(sulfonatopheny1)porphyrin(TPPS),and tetrakis(carboxypheny1)porphyrin (TPPC) were obtained from Strem Chemicals. Free-base porphyrin H2TMPyP (tetrakis(N-methylpyridy1)porphyrin)was prepared by methylation of HzTPyP with methyl p-toluenesulfonate in DMF. All metal insertions (Zn, Pd, Fe, Co, and Mn) were carried out by refluxing the free-base porphyrins with the corresponding metal(I1) chloride or sulfate (as hydrate) overnight in DMF according to the procedures of Adler et and subsequently purified on Dowex or Sephadex columns. Methods. Photochemical. Flash photolysis studies were carried out by using the 20-ns 530-nm light pulses from a Q-switched Nd laser flash photolysis setup. Transient absorption studies in the red, near-IR region employed a fast ITT photodiode as the photodetector. Steady-state emission properties (fluorescence and phosphorescence) were recorded on a Hitachi Perkin-Elmer spectrofluorimeter. Fluorescence lifetimes were measured with a PRA time-correlated single-photon counting system. Triplet excited-state extinction coefficients were determined by the saturation technique described ear1ier.l' The method consists of measuring absorbance changes (AA),at the end of the laser pulse, of a dilute porphyrin solution of known concentration at the T-T absorption maxima, as a function of laser intensity. At saturating laser light intensities (correspondsto complete conversion to the triplets) EA
= (AAA"
+ AA"))/C
(1)
(8)M. Neumann-Spallart and K. Kalyanasundaram, 2.Naturforsch. B, 36,596 (1981). (9)(a) G. S.Nahor, J. Rabani, and F. Grieser, J. Phys. Chem., 85,697 (1981);(b)R. Bonnett, R. J. Ridge, E. J. Land, R. S. Sinclair, D. Tait, and T. G. Truscott, J. Chem. Faraday Trans. 1, 78,127 (1982);(c) P. Neta, J. Phys. Chem., 85, 3678 (1981); (d) R. H. Schmehl and D. G.
Whitten, ibid., 85,3473(1981);(e) K. Hatano, K. Usui, and M. Ischida, Bull. Chem. SOC. Jpn. 54,413 (1981). (10)(a) A. D. Adler, F. R. Longo, F. Kampas, and J. Kim, J . Inorg. Nucl. Chem.,32,2443(1970);(b) A. D. Adler, F. R. Longo, and V. Varadi, Inorg. Synth., 16,213 (1976). (11)U. Lachish, P.P. Infelta, and M. Gratzel, Chem. Phys. Lett., 62, 317 (1979).
0 1982 American Chemical Society
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The Journal of Physical Chemistry, Vol. 86, No. 26, 1982
Kalyanasundaram and Neumann-Spallart
TABLE I : Ground-StateAbsorption Data for Various Porphyrins in Watera
H,TMPyP ZnTMP yP PdTMP yP M n"ITMPyP
c0"ITMPYP
H,TPPS ZnTPPS PdTPPS Mn'IITPPS Co"'TPPS H,TPPC ( D M F ) ZnTPPC PdTPPC (9O%DMF) Mn'I'TPPC a
424 (226) 4 3 6 (180) 419 (146) 463 ( 9 2 ) 434 ( 1 6 8 ) 412 (530) 421 (683) 412 (127) 469 (95) 424 ( 2 7 0 ) 419 (339) 420 (160) 415 468 (93)
520 562 522 560 550 515 555 520 562 538 514 556 520 564
(14.5) (16.0) (12.5) (11.0) (8.0) (16.5) (22.1) (10.7) (11.7) (13.0) (16.0) (11.5)
5 5 8 (9.2) 602 5 6 6 (5.1)
(11.2)
598 (9.0)
584
638
15.6 11.3 11.7 8.4 20.1 32.1 30.9 11.9 8.2 20.8 21.2 13.9
553 (6.8) 5 9 4 (9.6) 5 9 6 (7.5) 548 (8.6) 5 9 4 (6.4)
588 (5.9)
8.3
Unless stated otherwise all data are for aqueous solutions at pH 5.0.
where MAmaX and Ah@refer to maximum transient absorbance changes and ground-state absorbance at the monitoring wavelength X and C refers to the total concentration of the porphyrin. Quantum yields for the lowest triplet excited state were determined by the comparison technique using known standardd2 ZnTPP ($triplet = 0.88) and Ru(bpy),2+($CT = 1.0). The principle of the method is to compare the concentration of triplets formed on excitation of the porphyrin (AA/Ae) with the concentration of triplets formed by the same number of photons from a solution of a standard (MStd/Ae,td) with a known triplet spectrum (estd vs. A) and intersystem-crossover yield (&): AAunk/
= hnkAeunk/(&dAestd)
-@
___
100%
DMF
@
20% DMF
@
2 % DMF
t
(2)
ElectrochemicaL Cyclic voltammetric studies were carried out with the aid of a Tacussell potentiostat as described earlier.13 Aqueous saturated calomel or AgAgCl electrodes were used as reference electrodes. A glassy carbon electrode or a hanging drop mercury electrode was used as the working electrode for oxidations and reductions, respectively.
Results and Discussion Ground-State Absorption Properties and Aggregation Phenomena. For a comprehensive understanding of the photochemical and redox properties, it is essential to have information on the ground-state absorption and aggregation properties. In this study we have focused our attention on the free bases and various metalloporphyrins derived from three porphyrin systems: H,TPPS, HZTPPC, and H2TMPyP. Except for the cases elaborated below, most of the metalloporphyrins and the free bases are readily soluble in water over a wide range of pH and concentrations. As monitored by their characteristic ground-state absorption and observance of Beer's law, they are present as monomers over a wide concentration range. These include H,TPPS, H,TMPyP, and their Zn2+,Pd2+, Co3+, and Mn3+ salts. Data on the molar extinction coefficients in water for the various free bases and metalloporphyrins used in this study are collected in Table I. Wherever information is available, our measured values are in good agreement with the literature values (free (12) (a) J. T.Richards and J. K. Thomas, Trans. Faraday Soc., 66,621 (1970); (b) B.h a n d and R. B e m a " , Chem. Phys. Lett., 34,44 (1975). (13) (a) M.Neumann-Spallart and K. Kalyanasundaram,Ber. Bunsenges. Phys. Chem., 85, 704 (1981); (b) M.Neumann-Spallart, K. Kalyanasundaram,C. Gratzel, and M. Gratzel, Helu. Chim. Acta, 63, 1111 (1980).
0
. i/nm
Flgure 1. Absorption spectral changes due to addiin of water to DMF solutions of PdTPPC: (1) 100% DMF, (2) 20% DMF, (3) 2% DMF.
bases,14 Zn porphyrin^,^^^^^ Mn porphyrin^,'^ PdTPPS,gd CoTMPyP14g)within 5%. There have been several studies on the aggregation and on the kinetics of incorporation of various metals on these porphyrin systems, especially by Pasternack, Hambright, and their co-workers.14 Fe3+salts of TMPyP and of TPPS undergo pH-dependent dimerization, as described earlier in the 1 i t e r a t ~ r e . lIt~ ~should ~ ~ be noted that the direct sulfonation of TPP yields a mixture of sulfonated TPP derivatives with one to four S032-groups attached to the porphyrin phenyl groups. Among the tri- and tetrasulfonated TPP derivatives (often denoted as TPPS3and (14) (a) W.I. White in ref la, Vol. VC, p 303; (b) R. F. Pasternack, Ann. N . Y. Acad. Sci., 206, 614 (1973); (c) P. Hambright and E. B. Fleischer, Inorg. Chem., 9, 1757 (1970); (d) R. F. Pasternack, L. Francesconi,D. F&ff, and E. Spiro, ibid., 12,2606 (1973); (e) R. F. Pasternack, H. Lee, P. Malek,and C. Spencer, J.Inorg. Nucl. Chem., 39,1865 (1977); (0 E.B. Fleischer, J. M. Palmer, T. S. Srivataava, and A. Chatterjee,J. Am. Chem. SOC.,93,3162 (1971); (g) R. F. Pasternack, E. G. Spiro, and M. Teach, J. Inorg. Nucl. Chem., 36,599 (1974). (15) A. Harriman and G. Porter, J. Chem. Soc., Faraday Trans. 2,75, 1532, 1543 (1979).
The Journal of Physical Chemistty, Vol. 86, No. 26, 1982 5165
Water-Soluble Porphyrins in Aqueous Media
TABLE 11: Emission Properties of Various Porphyrin Derivatives in Aqueous Solutions at R o o m Temperaturei fluorescence ( 2 9 8 K ) porphyrin H,TMPyP ZnTMPyP PdTMPyP H,TPPS ZnTPPS PdTPPS ZnTPPC PdTPPCh H,TPPg ZnTPPg PdTPPg
hm,/nm 675, 706 626, 666 570 645, 700 (644)d 606, 656 560, 606 610,658
rbo/ns 5.3 1 . 3 (1.4)b 1 0 . 4 (14.3)d 1.7
1.8 (1.6)b 13.6 2.7 0.02
653 602 561
phosphorescence ( 2 9 8 K ) @fill0
0.011 0.020 ( 0 . 0 2 5 ) b
< 10-4
0.080 0.043
