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Langmuir 1996, 12, 3980-3985
Adsorption Behavior and Orientation of Tetrakis(methylpyridiniumyl)porphyrin on Silica Martin A. Bos, Thekla M. Werkhoven, and J. Mieke Kleijn* Department of Physical and Colloid Chemistry, Wageningen Agricultural University, Dreijenplein 6, 6703 HB Wageningen, The Netherlands Received February 12, 1996. In Final Form: May 22, 1996X The adsorption of 5,10,15,20-tetrakis(4-N-methylpyridiniumyl)porphyrin (H2TMPyP) from aqueous solutions on silica has been studied by reflectometry, while order parameters of the orientation distribution of the adsorbed porphyrins have been obtained by total internal reflection fluorescence (TIRF). The initial adsorption rate of the positively charged porphyrins on the negative surface is transport-controlled. Subsequently, the adsorption continues at a lower rate, leading to significantly higher adsorbed amounts in the presence of electrolyte than from pure water. From a combination of reflectometer and TIRF results it is found that ordering in the adsorption layer is primarily determined by steric and electrostatic interactions between the adsorbed molecules and not by charge interactions between the surface and the porphyrin molecules. In the presence of buffer (screening lateral electrostatic interactions) the H2TMPyP molecules are more or less randomly oriented at low surface coverage, while, at higher surface coverage or in the absence of background electrolyte, a broad orientation distribution around an angle of ca. 45° between the porphyrin macrocycle and the surface is found. From TIRF measurements at various solution viscosities it is found that the orientation of the adsorbed porphyrins does not change on the time scale of fluorescence.
Introduction Porphyrins, which have the same macrocycle structure as the natural chlorophylls, are frequently used as photosensitizers in artificial solar energy systems. They have a very efficient light absorption in the visible part of the spectrum, where the solar energy spectrum has a maximum.1,2 For example, porphyrins, adsorbed on a semiconducting surface, are used to generate charge carriers in the semiconductor with photons of lower energy than the band gap energy of the semiconductor.3-6 It has been shown4,5,7 that the orientation of the dye molecules in the sensitizer film is important for the observed photoresponse. One of the methods to determine the orientation distribution of adsorbed (fluorescent) molecules like porphyrins is based on the technique of total internal reflection fluoresence (TIRF).8-12 Orientation distributions obtained with this method for tetrakis(methylpyridiniumyl)porphyrin adsorbed on glass11,12 and for bis-, tris-, and tetrakis(methylpyridiniumyl)porphyrin on quartz13 have been reported. In this paper TIRF orientation measurements on adsorbed tetrakis(methylpyridiniumyl)porphyrin (H2TMPyP) are combined with the adsorption behavior of this porphyrin on silica as studied by * Corresponding author. X Abstract published in Advance ACS Abstracts, July 1, 1996. (1) Boxer, S. G. Biochem. Biophys. Acta 1983, 762, 265. (2) Fendler, J. J. Phys. Chem. 1985, 89, 2730. (3) Suto, S.; Ikehara, T.; Koike, A.; Uchida, W.; Goto, T. Solid State Commun. 1990, 73, 331. (4) Yanagi, H.; Ashida, M.; Harima, Y.; Yamashita, K. Chem. Lett. 1990, 385. (5) Yanagi, H.; Douko, S.; Ueda, Y.; Ashida, M.; Wo¨hrle, D. J. Chem. Phys. 1992, 96, 1366. (6) Savenije, T. J.; Mare´e, C. H. M.; Habraken, F. H. P. M.; Koehorst, R. B. M.; Schaafsma, T. J. Thin Solid Films 1995, 265, 84. (7) Wo¨hrle, D.; Tennigkeit, B.; Elbe, J.; Kreienhoop, L.; Schnurpfeil, G. Liq. Cryst. 1993, 228, 221. (8) Thompson, N. L.; McConnell, H. M.; Burghardt, T. P. Biophys. J. 1984, 46, 739. (9) Thompson, N. L.; Burghardt, T. P. Biophys. Chem. 1986, 25, 91. (10) Fraaije, J. G. E. M.; Kleijn, J. M.; Van der Graaf, M.; Dijt, J. C. Biophys. J. 1990, 57, 965. (11) Bos, M. A.; Kleijn, J. M. Biophys. J. 1995, 68, 2566. (12) Bos, M. A.; Kleijn, J. M. Biophys. J. 1995, 68, 2573. (13) Wienke, J.; Kleima, F. J.; Koehorst, R. B. M.; Schaafsma, T. J. Thin Solid Films, in press.
S0743-7463(96)00124-2 CCC: $12.00
reflectometry. Furthermore, by variation of the viscosity of the solution it is evaluated if the time scale of rotational movements of the adsorbed molecules is of the same order as their fluorescence lifetime. In TIRF, fluorescent molecules are excited by the evanescent field originating from total internal reflection of a laser beam at the interface between an optically transparent medium and a solution. It has been shown11 that, by measuring the fluorescence intensity and its polarization at different polarization angles of the incident light beam, two order parameters of the orientation distribution of the fluorophores, 〈P2〉 and 〈P4〉, are obtained, i.e.,
1 〈P2〉 ) (3〈cos2 θ〉 - 1) 2
(1a)
1 〈P4〉 ) (35〈cos4 θ〉 - 30〈cos2 θ〉 + 3) 8
(1b)
In the case of porphyrins θ represents the angle between the porphyrin macrocycle and the sorbent surface. The brackets 〈 〉 denote an average over all abundant orientations. From the restricted knowledge of these two order parameters it is not possible to retrieve the orientation distribution N(θ) of the adsorbed porphyrins exactly. However, by assuming a model distribution (e.g., Gaussian) or by applying the so-called maximum-entropy method,11,14 one can obtain an estimate for N(θ). The latter method has been applied here; i.e., no a priori assumptions are made concerning the shape of the distribution function. One of the assumptions made in the underlying theory is that the orientation of the adsorbed molecules does not change on the time scale of fluorescence. It can be shown11 that, in the case that the rotational mobility of the adsorbed fluorophores is much faster than the fluoresence lifetime, polarization of the fluoresence will be completely lost. In the case that the correlation time of any significant changes in the orientation of the molecules is of the same order as their fluorescence lifetime, the order parameters calculated from the experimental data will vary systematically (14) Bevensee, R. M. Maximum Entropy Solutions to Scientific Problems; Prentice-Hall Inc.: Englewood Cliffs, NJ 1983.
© 1996 American Chemical Society
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Figure 1. Viscosities (relative to that of water) of sucrose (b) and glycerol (O) solutions in water.
with the viscosity of the solvent. The fluorescence lifetime (τf) of H2TMPyP in aqueous solutions is ca. 5 ns;15-17 Vergeldt et al.17 have established that for the adsorbed state (on quartz) τf is somewhat lower, i.e., ca 3 ns. Materials and Methods All chemicals used were analytical reagent grade. Water was purified using a millipore purification system, involving reversed osmosis and subsequent percolation through charcoal and a mixed-bed ion exchange resin. 5,10,15,20-Tetrakis(4-N-methylpyridiniumyl)porphyrin (H2TMPyP) solutions were prepared from H2TMPyPl4 obtained from Strem Chemicals Inc. The H2TMPyP molecule is disk-shaped, carries four positive charges, and has a molecular weight of 678 g/mol. The porphyrin macrocycle has a flat structure. From energy minimization studies and molecular-dynamics simulations18 it has been found that the energetically most favorable tilt of the four methylpyridyl side groups is 80° relative to the plane of the porphyrin ring, but the flexibility of the pyridylporphyrin bond is large. Porphyrin concentrations were determined by measuring the extinction at 424 nm in a Hamamatzu spectrophotometer (extinction coefficient 220 × 103 M-1 cm-1 (refs 6 and 19)). The pH values of 10 mM acetate- and phosphate-buffered solutions were 4.0 and 7.0, respectively; unbuffered solutions had pH values between 5.5 and 6.0. The viscosity of the H2TMPyP solutions was varied by adding sucrose or glycerol (Baker Chemicals). An Ostwald viscosity meter was used to measure the viscosities (results are given in Figure 1), and the refractive indices of the solutions were determined with an Abbe refractometer. Depletion Measurements. Adsorption experiments were carried out in 10 cm3 polycarbonate tubes in which porphyrin solutions of various concentrations were added to a dispersion of SiO2 (borosilicate from Solvirel, Levallais, France, specific surface area 0.6 m2/g) in the same buffer. The tubes were gently rotated for about 16 h. After centrifugation, the porphyrin concentrations in the clear supernatant were determined. Reflectometry Measurements. At a jet flow of 2.0 × 10-8 m3 s-1 a porphyrin solution impinges perpendicularly onto the sorbent surface, an SiO2 film on a silicon wafer. The flux Jp of porphyrin molecules to the surface may be derived from20
Jp ) 0.283v-1/3Φ2/3R-5/3D2/3cp
(2)
where v is the kinematic viscosity of the solution, Φ the volume flux, R the radius of the nozzle of the flow channel, D the diffusion coefficient of the porphyrin in solution, and cp its concentration (15) Kalyanasundaram, K. Inorg. Chem. 1984, 23, 2453. (16) Kano, K.; Nakajima, T. J. Chem. Soc. Jpn. 1987, 60, 1281. (17) Vergeldt, F. J.; Koehorst, R. B. M.; Van Hoek, A.; Schaafsma, T. J. J. Phys. Chem. 1995, 99, 4397. (18) Schrijvers, R.; Van Dijk, M.; Sanders, G. M.; Sudho¨lter, E. J. R. Recl. Trav. Chim. Pays-Bas. 1994, 113, 351. (19) Kalyanasundaram, K.; Neumann-Spallart, M. J. Phys. Chem. 1982, 86, 5163. (20) Dabros, T.; Van de Ven, T. G. M. Colloid Polym. Sci. 1983, 26, 694.
Figure 2. (a) Adsorption isotherms of H2TMPyP in water obtained with reflectometry and depletion measurements. (b) Adsorption isotherms of H2TMPyP in water, 0.1 M KNO3 and 10 mM phosphate buffer pH 7 obtained with reflectometry. in solution. The amount of porphyrin adsorbed was determined in situ by reflectometry. The equipment and the procedure to calculate the adsorbed amount have been described by Dijt et al.21,22 TIRF Measurements. The TIRF instrument used has been described in detail by Bos and Kleijn.12 Adsorption of H2TMPyP onto a glass plate took place in a laminar flow cell at a flow rate of 2.0 × 10-8 m3 s-1. Solutions containing sucrose or glycerol had lower fluxes: solutions of the highest viscosities (concentration of sucrose higher than 40 vol %, concentration of glycerol higher than 60 vol%) were introduced into the cell by hand using a syringe. Excitation of the adsorbed molecules took place at a wavelength of 514 nm, i.e. in the Qy(1,0) absorption band of the porphyrin. For determination of the orientation of the porphyrin macrocycle, the fluorescence spectrum was integrated between 600 and 800 nm. This was done at various polarization angles of the incident light beam. All measurements have been performed at ambient room temperature (20 °C).
Results Adsorption Kinetics and Adsorbed Amounts. Figure 2a shows adsorption isotherms of H2TMPyP on SiO2 from pure water obtained with reflectometry and depletion measurements. The results of both types of experiments are in good agreement up to a concentration of 200 mg/L. Above this concentration reflectometry cannot be used due to strong light absorption by the porphyrin solution in the reflectometer cell. In Figure 2b the adsorption is shown of H2TMPyP from water, 0.1 M KNO3, and 10 mM phosphate buffer as obtained with reflectometry. The adsorbed amounts from 0.1 M KNO3 and 10 mM phosphate buffer solutions are remarkably higher than the adsorbed amounts from water. This must be the result of the screening effect of the salt ions, resulting in a lower (21) Dijt, J. D.; Cohen Stuart, M. A.; Hofman, J. E.; Fleer, G. J. Colloids Surf. 1990, 51, 141. (22) Dijt, J. D.; Cohen Stuart, M. A.; Fleer, G. J. Adv. Colloid Interface Sci. 1994, 50, 79.
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Figure 4. Emission spectrum of H2TMPyP in the adsorbed state, as obtained with TIRF. Excitation wavelength 514 nm.
Figure 3. (a) Amounts of H2TMPyP adsorbed from different solutions as a function of time. Porphyrin concentration in solution 10 mg/L. (b) Initial parts of the curves given in part a. The flux Jp of porphyrin molecules to the surface is indicated.
repulsion between the positively charged porphyrin molecules in the adsorption layer. Although the ionic strength in the phosphate buffer is approximately 10 times lower, the effect on the adsorbed amounts is even higher than the effect of the presence of 0.1 M KNO3. This might be caused by specific adsorption of the phosphate ions, so that the adsorption layer is in fact composed of both positively charged porphyrin molecules and negatively charged phosphate ions. The maximum adsorbed amounts of 2-2.5 mg/m2 in buffer and 0.1 M KNO3 correspond to a surface area of 0.8-1.0 nm2 per molecule. Considering that the TMPyP molecule is disk-shaped with a diameter of ca 1.6 nm, the molecules cannot be adsorbed in a flat orientation (this would take an area of at least 2 nm2/ molecule) but make a tilt with the surface or are adsorbed in more than one layer. For a compact monolayer in which the molecules are adsorbed with the porphyrin macrocycle perpendicular to the surface, the area per molecule would be about 0.8 nm2.23 The adsorption of H2TMPyP from water, 0.1 M KNO3, and 10 mM phosphate buffer is given as a function of time in Figure 3. In the first few seconds adsorption takes place very quickly, followed by a much slower adsorption which continues for a long time, especially in the case of adsorption from the buffer solution. Similar adsorption behavior of H2TMPyP on glass has been reported by Savenije et al.6 The initial adsorption rate (dΓ/dt)tf0 can be compared with the flux of the porphyrin molecules to the surface. This flux, shown in Figure 3b, has been calculated according to eq 2 and using the following data: v ) 10-6 m2 s-1, Φ ) 2.0 × 10-8 m3 s-1, R ) 9.5 × 10-4 m, D ) 4.3 × 10-10 m2 s-1 and cp ) 10 mg/L. Irrespective of the electrolyte composition of the solvent, (dΓ/dt)tf0 is equal to Jp. Apparently, on the bare silica surface every molecule that arrives is attached. The desorbability of the porphyrin adsorbed from water and phosphate buffer has been determined by flushing (23) De´sormeaux, A.; Rinquet, M.; Leblanc, R. M. J. Colloid Interface Sci. 1991, 147, 57.
with the solvent. The fraction of the adsorbed amount which can be desorbed varies between 10 and 30% for buffer and between 20 and 60% for water and 0.1 M KNO3, depending on the porphyrin concentration in solution during adsorption. The porphyrin molecules are bound more strongly to the surface in presence of phosphate buffer, supporting the idea of coadsorption of phosphate ions. Orientation Measurements and Mobility of Adsorbed Molecules. In Figure 4 the emission spectrum of H2TMPyP in the adsorbed stte as obtained with TIRF is given. Under all adsorption conditions applied the fluoresence spectrum of the adsorbed molecules was essentially the same. The two peaks in the spectrum, i.e. the Q(0,0) and Q(0,1) bands, correspond to two vibronic transitions from the first excited state (S1) to the ground state (S0) of the molecule. In aqueous solution the fluorescence spectrum shows one broad peak, which has been interpreted by Kano and Nakajima16 as evidence for the formation of (loose) dimers. However, Vergeldt et al.17 have shown that the molecule is monomeric in water below 10-3 M (ca. 7 g/L). In the fluorescence spectrum of the molecule in aqueous solution resolution of the Q(0,0) and Q(0,1) bands is lost due to resonance interaction between the porphyrin macrocycle and the pyridinium side groups.17 This interaction is determined by the degree of coplanarity of the porphyrin and the pyridinium π-systems and the polarity of the environment, which are different for the molecule in the adsorbed and in the dissolved state. The absorption spectrum of H2TMPyP adsorbed on glass is virtually the same as in solution and is given in ref 6. By measuring the fluorescence intensity and polarization at different polarization angles of the incident light beam, the order parameters 〈P2〉 and 〈P4〉 of the orientation distribution of adsorbed H2TMPyP molecules have been determined. Results for adsorption from different solutions are given in Figures 5 and 6. These solutions vary in pH (and ionic composition), viscosity of the solvent, and porphyrin concentration. All (〈P2〉; 〈P4〉) combinations derived were found to be within their physical boundaries, which follow from the definitions given in eq 1 and are indicated in Figure 5. The fluorescence intensities obtained in 80 vol % glycerol solutions were remarkably higher than those in water. It is known that the fluorescence quantum yield can vary with the solvent.17 Probably, the fluorescence quantum yield of H2TMPyP in glycerol is higher than that in water. From Figures 5 and 6 it can be seen that there is some scatter in the obtained order parameters, especially in 〈P4〉, which is more sensitive to experimental errors than 〈P2〉. This can be understood considering the definition of the order parameters given in eq 1 and the fact that 〈cos2 θ〉 and 〈cos4 θ〉 are the experimentally accessible parameters.11,12 No correlation was found between the order
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Langmuir, Vol. 12, No. 16, 1996 3983
Figure 5. Combinations of order parameters for H2TMPyP adsorbed from various solutions (of which only the porphyrin concentrations are indicated in this figure). The physical limits of 〈P2〉 and 〈P4〉 are given by drawn lines.
parameters and the nature of the solvent (pH, ionic strength, viscosity). On the other hand, a certain trend in the orientation distribution with porphyrin concentration in solution can be distinguished. The values for 〈P2〉 and 〈P4〉 for adsorption from solutions with a porphyrin concentration of 1 mg/L are around 0.2 and 0.0, respectively. In the case of concentrations of 10 and 100 mg/L, 〈P2〉 and 〈P4〉 are around 0.2 and -0.2, respectively. Orientation distribution functions corresponding with the average values for 〈P2〉 and 〈P4〉 and calculated using the maximum-entropy method for adsorption from 1, 10, and 100 mg/L porphyrin solutions are shown in Figure 7. At 1 mg/L the orientation distribution is almost random (there seems to be a slight preference of the molecules to lie flat on the surface). At concentrations of 10 and 100 mg/L the adsorbed molecules show clearly a preferential orientation angle θ of 45-46°. Figure 8 shows the fluorescence as a function of time of adsorption from H2TMPyP solutions of various concentrations in 10 mM phosphate buffer pH 7. For adsorption from 10 mM acetate buffer pH 4 comparable fluorescence curves were obtained. At concentrations of 0.1 and 1.0 mg/L H2TMPyP the curves show a gradual increase of the fluorescence, reflecting the adsorption process. The time scale of this process is different from that in the reflectometer experiments (see Figure 3) because of different flowing conditions (laminar flow versus impinging jet). For adsorption from 10 and 100 mg/L porphyrin solutions the fluorescence increases sharply and then gradually decreases. In the reflectometer experiments a comparable decrease has not been found, so it is not the result of desorption. A possible explanation could be that during adsorption reorientation of the adsorbed molecules takes place in such a way that the absorption efficiency (and therefore the fluorescence intensity) would decrease. However, by following the fluorescence intensity and polarization at various polarization angles of the incident light, it was found that during the adsorption processsat least on a time scale of minutes and largersno reorientation occurs at any porphyrin concentration studied. Probably, as the surface concentration of porphyrin molecules increases, a shift in the absorption spectrum takes place as a result of mutual interactions.16,17 Adsorption from 10 and 100 mg/L H2TMPyP solutions containing 10 mM phosphate buffer leads to relatively high surface concentrations, possibly even multilayer formation, giving rise to shifts in the Q-absorption bands, which in turn results in less efficient excitation of the molecules and hence a lower fluorescence
Figure 6. Order parameters for H2TMPyP adsorbed from various solutions as a function of viscosity (relative to that of water). (a and b) Concentration of H2TMPyP in solution ) 1 mg/L; (4) 10 mM acetate buffer pH 4; (O) 10 mM phosphate buffer pH 7. (c and d) Concentration of H2TMPyP in solution ) 10 mg/L; (4) unbuffered; (O) 10 mM phosphate buffer pH 7. The squares (0) indicate results for adsorption from 100 mg/L H2TMPyP in 10 mM phosphate buffer pH 7.
intensity. We did not observe significant changes in the emission spectrum as a function of surface concentration. Discussion The difference between the orientation distributions at solution concentrations of 1 and 10-100 mg/L H2TMPyP can be related to differences in surface coverage and electrostatic interactions between the adsorbed molecules. For adsorption from 1 mg/L H2TMPyP in 10 mM phosphate
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Figure 7. Orientation distribution functions of adsorbed H2TMPyP, corresponding to the average values for 〈P2〉 and 〈P4〉 for adsorption from solutions with porphyrin concentrations of 1, 10, and 100 mg/L and calculated following the maximumentropy method.
Figure 8. Fluorescence intensity as a function of adsorption time for solutions with varying H2TMPyP concentration (10 mM phosphate buffer pH 7).
or acetate buffer the orientation of the adsorbed porphyrins turned out to be almost random. Figure 2b shows that the adsorbed amount from 1 mg/L porphyrin in 10 mM phosphate buffer is approximately 0.8 mg/m2, corresponding to an area per molecule of 2.5 nm2. In this case there is enough room left at the surface for the molecules to adopt any orientation angle θ without being sterically hindered by neighboring molecules; the presence of phosphate ions in the adsorption layer diminishes electrostatic interactions between the adsorbed porphyrins. For adsorption from 10 mg/L porphyrin in the same buffer the adsorbed amount is about 1.5 mg/m2, corresponding to only 1.3 nm2/molecule, so that the orientation of each molecule is affected by that of its neighbors. From the observation that for low surface coverage ordering in the adsorption layer is much less than that for higher surface coverage, it is concluded that adsorption does not occur in clusters. Otherwise, also at low surface coverages porphyrin-porphyrin interactions in the clusters would lead to a preferential tilt angle of the molecules. If adsorption from 10 mM buffer solution does not occur in clusters, it is very unlikely that this would be the case when the molecules adsorb from water without background electrolyte. Therefore, the finding that for adsorption from 10 mg/L H2TMPyP in water there is clearly a preferential tilt angle in the adsorption layer although the surface coverage is relatively low must be attributed to long-range electrostatic repulsion between the adsorbed porphyrins. From the finding that the order parameters of the orientation distribution for adsorption from 10 mM phosphate buffer pH 7 are the same as those for adsorption from 10 mM acetate buffer pH 4 or unbuffered solutions (pH 5.5-6) it is concluded that in this pH range the charge of the silica surface is of minor importance for the organization of the adsorption layer. At pH 4 and pH 7
Bos et al.
and an electrolyte concentration of 10-2 M, the surface charge densities of SiO2 are approximately -0.01 and -0.1 C m-2, respectively.24 However, the tenfold more negative charge density of the surface at pH 7 does not result in a lower preferential orientation angle θ between the porphyrin macrocycle and the sorbent surface, as one might expect assuming that the molecules are predominantly bound through their positively charged methylpyridinium groups. In this respect it should be noted that the adsorbed amount of 0.8 mg/m2 from 1 mg/L H2 TMPyP in 10 mM phosphate buffer corresponds already to a charge of ca. 0.3 C/m2, which means that the surface charge density is too small to induce a four-point binding of the porphyrin molecules. This overcompensation of surface charge again points to coadsorption of phosphate ions. Wienke et al.13 have reported that at pH 10 H2TMPyP, adsorbed on quartz from 20 mM borate buffer solutions, is preferentially parallel oriented with respect to the sorbent surface. In the range of porphyrin concentrations studied (approximately 1-100 mg/L) the amount of adsorbed porphyrin was found to be independent of the solution concentration and corresponding to an area of 2.2 nm2 per molecule. Apparently, at this high pH value the surface charge density of the quartz substrate is so large that it gives rise to formation of a complete monolayer of flat-lying porphyrin moleucles at any solution concentration. In all orientation measurements it was found that the fluorescence signal was polarized. From this it can be concluded that any significant changes in the orientation of the adsorbed porphyrin molecules on a time scale much faster than that of the fluorescence can be ruled out, since they would lead to a complete loss of polarization. If fluctuations in the orientation of the molecules in the adsorption layer would occur at the time scale of fluorescence, a systematic effect of the viscosity of the solvent on the obtained order parameters is expected. Admittedly, it is not known how the fluoresence lifetime depends on the composition of the solvent, in this case the concentration of sucrose or glycerol. For water/methanol mixtures Vergeldt et al.17 have found that for the adsorbed state τf hardly depends on the composition of the solvent (it varies from 2.7 ns for pure water to 3.3 ns for pure methanol). If in our case τf would vary with the viscosity of the solvent (i.e., the solvent composition), we do not expect that this would be in the same way as the correlation time of orientational changes of the adsorbed porphyrins. Therefore, since the data given in Figure 6 show no correlation at all between the viscosity and the order parameters of the orientation distribution, it is concluded that the orientation of the adsorbed molecules does not change on the time scale of fluorescence. This conclusion is further supported by considering the lower limit for the rotation correlation times of the porphyrin molecules in the various solutions, estimated according to the equation τrot ) 1/(6Drot) for spherical molecules, in which Drot ) kT/(6ηV).25,26 Here Drot is the rotation diffusion coefficient, k is Boltzmann’s constant, T is the absolute temperature, η is the absolute viscosity of the solution, and V is the volume of the molecule. It is assumed that the lowest rotation correlation time of the H2TMPyP molecule is comparable to or larger than that of a spherical molecule with a radius of 0.8 nm. In this way it is found that for the porphyrin in water (relative viscosity of 1.00) τrot is at least 0.5 ns, while for a relative viscosity of 45 this value (24) Tadros, Th.F.; Lyklema, J. J. Electroanal. Chem. 1968, 17, 267. (25) Axelrod, D. Biophys. J. 1979, 26, 557. (26) Holde van, K. E. Physical Biochemistry, 2nd ed.; Prentice-Hall Inc.: Englewood Cliffs, 1985; p 159.
Tetrakis(methylpyridiniumyl)porphyrin on Silica
is about 24 ns. These correlation times for the dissolved state are already of the same order as the fluorescence lifetime. For the adsorbed state we expect much higher values for the rotation correlation time. In conclusion, the positively charged H2TMPyP molecule adsorbs readily on the negatively charged silica surface. However, in the pH range studied (pH 4-7) plateau values of adsorption and ordering in the adsorption layer are
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primarily determined by interactions between neighboring molecules and not by porphyrin-surface interactions. The assumption made in the theory underlying the TIRF orientation measurements that the orientation of the adsorbed porphyrins does not change on the time scale of fluorescence is justified. LA960124I
