Electron Paramagnetic Resonance Study of Dimerization Effects on

10 Electron Paramagnetic Resonance Study of Dimerization Effects on Porphyrins in the Photoexcited Triplet State 1

Hans van Willigen, T. K. Chandrashekar, U. Das, and Marie H. Ebersole Downloaded by IOWA STATE UNIV on February 18, 2015 | http://pubs.acs.org Publication Date: October 15, 1986 | doi: 10.1021/bk-1986-0321.ch010

Department of Chemistry, University of Massachusetts at Boston, Boston, MA 02125

An EPR study is made of the effect of dimerization on the photoexcited triplet state of a series of porphyrins. The systems of interest are tetra(benzo-15crown-5)porphyrin (TCP), tetra(4-sulfonatophenyl)porpliyrin (TPPS), tetra(4trimethyl-ammoniumphenyl)porphyrin (TTAP), and Zn-substituted derivatives of these porphyrins. Dimerization of TCP and ZnTCP can be induced by addition of cations. In aqueous solution TPPS and ZnTPPS are present in the form of monomers and dimers. Finally, admixture of TPPS (ZnTPPS) and TTAP (ZnTTAP) leads to dimer formation. The study of these systems is concerned with the question of whether or not dimerization effects on zero field splitting (zfs) and kinetic parameters of the photoexcited triplet state can be related to dimer geometry. In the case of ZnTCP dimerization effects can be interpreted in terms of fast triplet excitation energy transfer between dimer constituents. This means that there is a well-understood relationship between dimerization effects and dimer geometry. The data on the other systems best fit a model in which the excitation energy is localized on one of the porphyrin rings. Dimerization induced changes in zfs and kinetic parameters in this case must be attributed to a perturbation of porphyrin structure and do not give an insight into dimer geometry. Zero f i e l d s p l i t t i n g (zfs) values i n photoexcited t r i p l e t s of primary donor bacteriochlorophyll a i n photosynthetic bacteria are much lower than those found for i n v i t r o BChla t r i p l e t s . There i s a pronounced difference i n kinetics of population and depopulation of the t r i p l e t sublevels as well. The differences have been attributed to the effect of BChla dimerization and i t i s now generally accepted that the primary electron donor i n photosynthetic bacteria consists of a BChla dimer (special pair) (1-6^)· The data on the photoexcited t r i p l e t s are a potential source of structural information. S p e c i f i c a l l y , dimerization effects on zfs's and kinetics may be of a i d i n the characterization of special p a i r ' C u r r e n t address: M I 48824

Department o f C h e m i s t r y , M i c h i g a n State University, East L a n s i n g ,

0097-6156/86/0321-0140$06.00/ 0 © 1986 A m e r i c a n C h e m i c a l Society

In Porphyrins; Gouterman, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

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geometry and electronic structure (2-6). To obtain an insight into the r e l a t i o n between spectroscopic data and structure, studies are made of photoexcited t r i p l e t s of model dimer systems. The studies focus on covalently-1inked chlorophylls(7) and porphyrins(8) as well as dimers formed by spontaneous aggregation of tetrapyrroles (9-13). This paper i s concerned with r e s u l t s of studies of model systems belonging to the l a t t e r category. The molecules of interest are tetra(benzo-15-crown-5)porphyrin (TCP, I ) , tetra(4-sulfonatophenyl)porphyrin (TPPS, I I ) , and tetra(4trimethylammoniumphenyl)porphyrin (TTAP, III).(See Structure.) TCP and i t s metal-substituted derivatives can be made to dimerize quantitatively by the addition of cations (NH , K , Cs , Ba ) to solutions of these porphyrins (14). The stoichiometry of the dimerization reaction establishes that two porphyrins are linked by four crown-cation-crown bridges (14). This forces the porphyrins in a p a r a l l e l configuration. For s t e r i c reasons, one ring system must be rotated r e l a t i v e to the other. EPR (14) and ENDOR (15) studies of the ground state t r i p l e t [CuTCP] and [VOTCP] dimers f i t a model of two p a r a l l e l porphyrins with the t r a n s i t i o n metal ions positioned on a common axis perpendicular to the planes, with a center-to-center distance of 0.43 nm. UV-VIS spectroscopic studies (9-11,16) e s t a b l i s h that in aqueous solution TPPS (and i t s metal-substituted derivatives) i s in equilibrium with [TPPS]2» Dimerization can be promoted by increasing the ionic strength of the solution or by addition of cation-18crown6 (9-11) The EPR spectrum of CuTPPS i n frozen H 0-glycerol establishes the presence of a mixture of monomers and dimers. The EPR spectrum of [CuTPPS] shows that the Cu-Cu distance i s ~ 0.56 nm. The combination of c a t i o n i c (TTAP) and anionic (TPPS) water soluble porphyrins also leads to the formation of a stable 1:1 complex [TTAP/TPPS] (13,17). It is evident that dimerization i s promoted by e l e c t r o s t a t i c i n t e r a c t i o n between the oppositely charged substituents. It i s l i k e l y , therefore, that a face-to-face dimer is formed. EPR spectra of the [CuTTAP/CuTPPS] dimer indicate that the Cu-Cu distance i s larger than 0.6 nm. The study of these face-to-face dimers i s concerned with the question of whether or not dimerization effects on zfs and k i n e t i c parameters of the photoexcited t r i p l e t state can be related to dimer structure. +

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Downloaded by IOWA STATE UNIV on February 18, 2015 | http://pubs.acs.org Publication Date: October 15, 1986 | doi: 10.1021/bk-1986-0321.ch010

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Experimental TPPS (sodium salt) was obtained from Strem Chemicals and used as received. TTAP and some of i t s metal-substituted derivatives were kindly by Dr. E. Ojadi of Brandeis u n i v e r s i t y . The other porphyrins used in this study were synthesized following published procedures (14,18). EPR spectra were recorded with a Varian E9 X-band spectrometer using f i e l d (100 kHz) and l i g h t (13 or 83 Hz) modulation with phasesensitive detection at the modulation frequencies (19). Typically, the f i e l d modulation amplitude employed ranged from 20 to 40 gauss, the microwave power from 0.1 to 0.5 mW. Measurements were performed on frozen solutions of the porphyrins at about 100 Κ using the standard Varian variable temperature accessory or at about 10 Κ with an Oxford Instruments helium gas cryostat. Light sources used for photoexcitation were a 1000 W Xe arc source powered by a Photochemical Research Associates Supply with electronic modulation


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DYNAMICS

c a p a b i l i t y and Spectra Physics argon (514.5 nm, 0.5 W) or krypton (647.1 nm, 0.5 W) lasers. Optical absorption spectra were recorded with a Cary 14 and fluorescence spectra with a Perkin-Elmer 650-40 spectrometer. A Nicolet 1180E computer interfaced to the Varian and Cary spectrometers was used for data acquisition and analysis. The k i n e t i c s of formation and decay of t r i p l e t ESR signals was recorded by exciting with square-wave-modulated l i g h t . The computer, used in the time-averaging mode, was triggered at the light-on edge, and the EPR signal amplitude vs. time p r o f i l e was stored in a IK memory block.


Results

Optical spectra Dimerization effects on absorption and emission spectra of TCP are shown in Figures 1 and 2. Similar effects are observed upon dimerization of ZnTCP (12), formation of [TPPS/TTAP] and [ZnTPPS/ZnTTAP] (Γ7). It is found that dimerization i s accompanied by (a) a blue s h i f t of the Soret band, (b) red s h i f t s of the Q bands, (c) red s h i f t of the fluorescence maximum, and (d) strong quenching of fluorescence. Optical spectra of face-to-face covalently-1inked diporphyrins show the same trends (20,21)· However, in one instance formation of a covalently-1inked diporphyrin does not lead to a Soret band s h i f t (22). Of the systems considered here, [TPPS] and [ZnTPPS] also f a i l to show a Soret s h i f t even though the v i s i b l e absorption bands and fluorescence show clear evidence of dimer formation (10). A summary of observed dimerization induced absorption band s h i f t s i s given in Table I. 2

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EPR

spectra

The EPR spectrum of the photoexcited t r i p l e t of TCP randomly oriented in frozen solution, recorded with 83-Hz l i g h t modulation at 10 K, i s shown in Figure 3a. The e f f e c t of addition of Na or K to the TCP solution is i l l u s t r a t e d in Figure 3, b and c, respectively. In general, EPR spectra of randomly oriented t r i p l e t s w i l l show three pairs of peaks (23) (labeled x, y, ζ in Figure 3). One p a i r for each of the three orientations for which the magnetic f i e l d is p a r a l l e l to a p r i n c i p a l axis (x, y, z) of the zfs tensor. The separations between the pairs of lines are given by 2D and D + 3E, where D and Ε are the zfs parameters (23). The spectra in Figure 3 show absorption as well as emission peaks as a result of the spin s e l e c t i v i t y of the intersysten: crossing and the decay from the t r i p l e t sublevels (19,24). The observed pattern of absorption (A) and emission (E) peaks (marked in the figure) corresponds to that found for other tetraphenylporphyrin (TPP) t r i p l e t s (9,10,25). It can be attributed to p r e f e r e n t i a l population of the z e r o - f i e l d T sublevel (24) (following convention the order of the energy levels i s given as Τ > Τ > T ) . It i s evident that the introduction of sodium ions (which does not cause dimerization (14)) does not affect the values of the zfs parameters. It merely causes some enhancement in signal amplitude which may be the result of a s l i g h t reduction in l i n e width. On the other hand, upon addition of ET ( i . e . dimerization +

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Figure 1. Optical absorption spectra of TCP (Soret, 3 χ 10~ M; Q bands, 6 χ 10 M) i n CH^Cl-CE^OH (1:1): (a) before and (b) after addition of KC1 (~ 10 M). The v e r t i c a l scale i s based on nonomer concentration. 5

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Figure 2. Fluorescence spectra of TCP ( 1 0 ~ ° M) in CH3CI-CH3OH (1:1) (a) before addition of K C 1 , excitation wavelength λ = 426 nm, and (b) after addition of E C 1 (~ 1

Electron Paramagnetic Resonance Study of Dimerization Effects on

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