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Films of polyvinylpyrrolidone containing zinc tetraphenylporphyrin: evidence for aggregation of porphyrins in the presence of pyridine
thin
ELSEVIER
o
Thin Solid Films 303 (1997) 295-30i
Films of polyvinylpyrrolidone containing zinc tetraphenylporphyrin: evidence for aggregation of porphyrins in the presence of pyridine. I. Leray, M.C. Verni~res, R. Pansu, C. Bied-Charreton ~, J. Faure Laboratoire de Photophysique et Photochimie Supramoldculaires et Macromoldculaires, (URA ]906 du CNRS), Ecole Not~nale Supdrieure de Cachan, 61 Acenue du Prdsidetlt Wilson. 94235 Cachan Cedex. Fra~zce
Received 27 June I996: accepted30 September1996
Abstract Films of zinc porphyrins in polyvinylpyrrolidone have been formed and studied by spectrophotometric methods. Diffusion of gaseous pyridine through a film of polyvinylpyrrolidone containing zinc tetraphenylporphyfin leads to a rearrangement of the porphyrin dye with the formation of aggregates. Absorption and fluorescence spectra are complemented by observations under an inverted Nikkon microscope equipped for epifluorescence, using a Titane-Sapphire laser as the excitation source. Fluorescence is collected through an optical fiber. The technique allows for the study of well-defined zones in the film. © 1997 Elsevier Science S.A. © 1997 Elsevier Science S.A. Kev,~'ords: Diffusion; Fluorescence; Optical spectroscopy;Polymers
1. Introduction We are interested by complexes formed between polymers and porphyrins because of their relevance to structures involved in energy transfer processes. In natural processes, the reaction center where porphyrin initiation takes place, is very complex. In photosynthesis, for example, the chromophore is linked to a protein and encaged in a hydrophobic environment which is entirely suspended in an aqueous medium [1]. Furthermore, recent investigations on cancer phototherapy using porphyrin derivatives have demonstrated the importance of the porphyrin-membrane interaction [2,3]. However little is known about the migration of porphyrin-like chromophores under the influence of a change in polarity. Through the use of block-copolymers (polystyrene-block-polyvinyl pyridine), we were able to localize the chromophore either in a hydrophobic or in a hydrophilic environment depending on the structure of the dye [4]. In the course of our work, we observed that a change in complexation of zinc porphyrins immobilized in a film of polyvinylpyrrolidone leads to a reorganization of the dye within the polymer. Spectroscopic evidence points to the comptexation of the porphyrin by the hydrophilic polymer, followed by decomplexation after exposure to gaseous pyridine. Such behaviour is emphasized at the
" Correspondingauthor. 0040-6090/97/$17.00 © I997 Elsevier Science S.A. A]i rights reserved. PII S0040-6090(9 6)09404-7
surface of the polymer where we observe spectacular auto-assemblies of the dyes in presence of pyridine vapours. By using a confocal microscope adapted to a titane-sapphire picosecond laser we were able to explore the lifetime of the dye at the surface of the polymer with a spatial resolution limited by the diffraction and especially the spectacular dendrites growing up when the polymer is exposed to the pyridine vapours.
2. Experimental 2.J. Reagents
All standard reagents and solvents were purchased and used without further purification. Zinc tetraphenylporphyrin (ZnTPP) (Fig. 1) was synthesized according to Adler et al. [5] and metallated with zinc as described by the same authors [6]. Tetramethylpyridiniumporphyrin ( H J M P y P ) (Fig. 1) was synthesized according to Pasterhack and coworkers and metallated with zinc as described by the same authors [7,8]. Polyvinylpyrrolidone (PVP) is a commercial polymer (Aldrich M w = 17 000). 2.2. Preparation o f f i h n s
Films were prepared by spin coating a mixture of PVP and ZnTPP in CHC13 of varying relative concentrations.
296
L Leray et aL /Thin Solid Films 303 (1997) 295-301
R
The time resolution of detection was 50 ps. Acquisition of the decays was achieved within 10 min.
3. Results and discussion
3.I. Visible spectra of Zn porphyrins in the presence of poA,vinytpyrrolidon e
R
ZnTPP: R= ZnTMPyP : R =
Two kinds of experiments have been performed on Zn porphyrins: one series using porphyrins in solution at low concentration preventing aggregation of the dyes, the other in films at higher dye concentrations.
--@ N
I"
3.1.]. Visible spectra of Zn porphyrins in solution Visible spectra of ZnTPP in CH2C1," and ZnTMPyP in water have been recorded in the presence of increasing concentrations of polyvinylpyrrolidone. The resulting spectra are reported as Fig. 2(a) and 2(b). For ZnTPP in CI-{2C12 (Fig. 2(a)), the Soret absorption is shifted from 421.6 to 428 nm and the absorbance increases with the addition of PVP. The half-width of the band stays almost constant at 12 +_ 1 nm. The presence of an isobestic point indicates that in each case two species are present and we assume that they are in equilibrium, This is consistent with the results obtained by Manna et al. [9] concerning the axial complexation of zinc by the carbonyl of a pyrrolidine group. Complexation of ZnTPP by carbonyl groups was already observed in a previous work [ 10]. The following complexation equation can be assumed. ZnP + L ~ ZnPL where ZnP is the zinc porphyrin molecule, L is the ligand pyrrolidine and ZnPL is the complex formed by their association. The equilibrium constant can be related to absorbances using the relationship proposed by Leggett et al. [11] A -A 0 log At. _ A log K + log[PVP]
Fig. 1. Structures of the zinc porphyrins.
Films of PVP containing zinc tetramethylpyridiniumporphyrin (ZnTMPyP) were obtained by spin coating a mixture of polymer and dye dissolved in ethanol/water (4/1). The solutions were spin coated on a square glass plate (2.5 cm X 2.5 cm), thicknesses of ca. 1 bLm measured using a DEKTAK 3ST were obtained.
2.3. Apparatus Visible spectra were obtained using a Philips PU8720 UV-Vis spectrophotometer using films or cuvets of 1 cm optical length. CH2C12 or H 2 0 were the solvents. Static fluorescence measurements were performed using a SLM8000 spectrofluorimeter. The experimental device used for fluorescence decay measurements was a picosecond Tsunami titane-sapphire laser characterized by a 1 ps excitation pulse. The microscope was an inverted Nikon equipped for epifluorescence measurements for which a titane-sapphire laser was used as the excitation source. The excitation wavelengths were 417, 430 and 450 nm. Fluorescence was collected through a 400 ~ m diameter optical fiber and detected at 655 nm by a single photon counting set-up, based on a Hamamatsu multichannel plate photomultiplier.
where A is the absorbance at a given wavelength A, A 0 is
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1.3
I
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0.4
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400 450 Wavelength (nm)
0 500
350
400
450
500
Wavelength (nm)
Fig. 2. Evolution of the visible spectra of zinc tetraphenylporphyrin during the comptexation by polyvinylpyrrolidone in (a) CH,C1, and (b) H20.
297
L Leray et aI. / Thin Solid Films 303 (1997) 295-301
2.2
the absorbance of the porphyrin at A before the addition of polymer, At. is the absorbance when all of the porphyrins are complexed by the polymer, K is the equilibrium constant and [PVP] represents the concentration of pyrrolidine units L. Plots of A-Ao/AL-A versus log[PVP] give the complexation constant: K = 3.8 for ZnTPP in CH2C1 z with a slope of ca. 1 indicating that only one ligand is attached to a zinc.
1 1.65
1.1
o
0.55
3.1.2. Visible spectra of Zn porphyrins in fihns of polyL'inylpyrrolidone By spin coating, we prepared polyvinylpyrrolidone films, each of them containing different concentrations of ZnTPP from 1% to 10% (mass/polymer). The porphyrin absorption is slightly red-shifted from 427 nm in the solution containing PVP, to 432 nm in the film, a change characteristic for porphyrins in polymer films [4]. In addition the band is significantly broadened (half-height width, 28 rim). By plotting the absorbances versus thicknesses for three different mass ratios of porphyrin/polymer we obtained straight lines as expected. At a given film thickness, we could derive the absorbance versus the concentration of porphyrins in the films. Fig. 3 shows the relationship between concentration of porphyrin and absorbance at 432 nm for films having the same thickness. This graph shows that the films do not obey the Beer-Lambert law but the curves could be fitted using the equation proposed by Pasternack et al. [7]. Thus in the film before exposure to pyridine, the presence of dimers a n d / o r small aggregates can be assumed. From the equation of Pasternack et al. a dimerisation constant of ca. 150 tool- 1 1 can be evaluated. Fig. 4 represents the spectra of a film of polyvinylpyrrolidone containing 10 wt.% ZnTPP, recorded every 5 rain during exposure to pyridine vapors. Initially the Soret absorption at 432 nm decreases, broadens and moves towards the blue (430 nm) due to complexation with pyridine then two new bands at 408 and 450 nm arised on each side accompanied by two isosbestic points
ul
0
350
420 490 560 630 Wavelength (run)
700
Fig. 4. Modification of the absorption spectrum of the film of zinc tetraphenylporphyrin in polyvinylpyrrolidone during I h of exposure to gazeous pyridine. (1) after 5 min, (2) after 10 min, (3) after 15 min, (4) after 20 rain, (5) after 25 min.
0.55
o
/ 0.t 350
I 450 550 650 Wavelength (rim)
Fig. 5. Modification of the absorption spectrum of the film after i h of exposure to gazeous pyridine. (1) Film containing 1% of porphyrin/polymer; (2) film containing 5% of porphyrin/polymer.
0.5
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2
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% porphyrin/polymer Fig. 3. Absorbance of zinc tetraphenylporphyrin as a function of the relative concentrations of porphyrin and polymer. (1) O, 2000 i : (2) C),
4000 7~,
350
I 400
F 450
q 500
f 550
F 600
I 650
700
Wavelength (run) Fig. 6. Modification of the absorption spectrum of the fLIm of zinc tetramethylpyridiniumporphyrin in polyvinylpyrrolidone: (1) before exposure to pyridine; (2) after 1 h of exposure to gaseous pyridine.
I. Leray
298
er
a L / Thin Solid Fihns 303 (1997) 295-301
at 412 and 447 nm. The Q(I,0) band and the Q(0,0) band are displaced to lower energy and the intensity of the Q(0,0) band is increased. This effect depends on the concentration of porphyrin in the film as shown on Fig. 5. A possible explanation for these changes is the formation of aggregates: aggregates in solution [12,13] and in films [14] are characterized by new absorption bands on each side of the initial Sorer absorption. Comparable modifications of the spectrum are observed for crystalline zinc porphyrins which produce organized assemblies [15]. When ZnTPP is complexed by Py, it becomes more hydrophobic
and to minimize interactions with the hydrophilic polymer, PyZnTPP migrates in the polymer to form aggregates. We have successfully produced a film of PVP containing ZnTMPyP which is hydrophilic. In this film, the Sorer absorption is again red-shifted from 442 to 446 nm with a half-width significantly increased compared to the solution (36 rim). When this film is exposed to pyridine vapors there is no splitting of the Sorer band (Fig. 6). The Soret band becomes larger and narrower and is displaced towards higher energy from 446 to 443 nm, the half-width being 26 nm. The Q bands are also displaced towards
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Fig. 7. Microscopy of the ~ of zinc tetrapheny]po~hyrin in potyviny]pyrro]idone: (a) ffdm before exposure (eniargment 100 X ); (b) film after exposure (enlaIgment tO0 X ); (c) film after exposure (entargment lO00 X ); (d) observation by atomic force microscopy.
I. Leray et a l . / T h i n Solid Films 303 (1997) 295-301
higher energies. This result can be explained by the fact that the pyridine complex of ZnTMPyP is still hydrophilic (it has a charge of + 4) and therefore remains compatible with the polymer, consistent with our interpretation.
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3.2. Observation under optical microscopy
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Observations of the film have been made using a microscope with an enlargement of 100 × - 1 0 0 0 X . Films before and after 1 h exposure to pyridine are shown in Fig. 7. Before exposure (Fig. 7(a)) the film is uniform. After exposure (Fig. 7(b)), an important change is observed even at an enlargement of 100 × . A higher enlargement (Fig. 7(c)) of 1000 × shows the presence of small rods indicating that microdomains containing PyZnTPP are formed. From the enlargement by the microscope, we estimated a size of 1 b m for the rods with a comb-like structure. Observation with the epifluorescence microscope shows that these rods are fluorescent. Observations using AFM confirm this structure (Fig. 7(d)). The absorption spectroscopy has provided evidence for the presence of aggregates on a molecular level and microscopy shows that the aggregates extend over microns.
3.3. Fluorescence spectra polyvinylpyrrolidone
of fihns
of ZnTPP
in
3.3.1. Concentration effects on the static fluorescence of films of ZnTPP in polyvinylpyrrolidone before exposure to pyridine Films containing increasing relative concentrations of porphyrin and polymer have been prepared. The thicknesses have been set to obtain comparable absorbances for all films. The static fluorescence spectra were recorded using an excitation wavelength of 433 nm. The fluorescence yields were calculated using the fluorescence yield of ZnTPP in toluene (3.3%) as a reference [16]. The results reported in Fig. 8 show that the fluorescence yield decreases while the concentration of porphyrins dimers in the
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0 0.5 t 1.5 2 2.5 3 3.5 4 [ porphyrin dimer] Fig. 8. Quantum yield of fluorescence as a function of the concentrations of porph~ria dimer. 2t~xc = 430 nm.
0.40
0.00 560
600
640
680
Wavelength (nm) Fig. 9. Stationary fluorescence emission at 650 nm of the film of zinc tetraphenylporphyrin in polyvinylpyrrolidone: (i) excitation at 430 nm before exposure, (2) excitation at 430 nm after exposure, (3) excitation at 450 nm after exposure, (4) excitation at 417 nm after exposure.
film increases. This is consistent with a quenching of fluorescence by porphyrin dimers [17]. Absorption spectroscopy has shown that the B e e r - L a m b e r t law is not obeyed in that region of concentrations, thus a part of the porphyrins population in the film before exposure is in the form of dimers or small aggregates. The fluorescence yield decrease could be explained by the existence of energy transfer between porphyrins by FSrster mechanism [4]. Thus dimers and small aggregates efficiently capture the energy of the surrounding porphyrins leading to efficient quenching.
3.3.2. Fluorescence of fihns of ZnTPP in polyvinylpyrroIidone after exposure to pyridine After exposure of the film to pyridine, fluorescence emission spectra were recorded at several different excitation wavelengths corresponding to the Soret absorption maxima observed in the absorption spectrum of PyZnTPP in polyvinylpyrrolidone (Fig. 4). It is worth noting that the absorbance of the three bands used as the excitation wavelength have quite the same intensity. Different fluorescence spectra are observed depending on the excitation wavelength as reported in Fig. 9. From these results it can be concluded that the maxima correspond to different species present in the film with different emission spectra. Excitation at 408 nm leads to lower quantum yield of fluorescence than excitation at 450 nm. The observation of two different emission spectra for excitation at 408 (Fig. 9, curve 4) and 450 nm (Fig. 9, curve 3) suggest the existence of at least two distinct entities. This is consistent with the theory which predicts that fluorescence quenching for face-to-face aggregates (H-aggregates) is more efficient than that for edge-to-edge aggregates (J-aggregates). At 430 nm the spectrum of the exposed film (Fig. 9, curve 2) has a fluorescence enhancement compared to the starting film (Fig. 9, curve 1). We assume that at this wavelength the excited molecules are the non-rearranged molecules PyZnTPP. However the molecules are now less concen-
l. Leray et al./Thin Solid Films 303 (1997) 295-301
300 (a)
(b) J
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6
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t (ns)
t(ns) Fig. t0. (a) Randomized fluorescence decays of the film at 650 nm of zinc tetraphenyl porphyrin in polyvinylpyrrolidone: (l) excitation at 430nm before exposure, (2) excitation at 430 nm after exposure, (3) excitation at 450 nm after exposure, (4) excitation at 417 nm after exposure. (b) Space-resolved fluorescence decays of the film at 650 nm: excitation at 417 nm after exposure: (1) between rods, (2) randomized, (3) on rods.
trated and so the quenching by another porphyrin or a small aggregate is less probable than in the non-exposed film, consistent with what was observed in the non-exposed film (Section 3.4.1). The quantum yield of PyZnTPP was found to be 3% [18].
3.4. Time-resoh,ed fluorescence of fihns of ZnTPP in pol},t'in),Ip3'rrolidone
long lifetime of 3 ns is for isolated molecules of PyZnTPP, and that aggregates absorbing at 417 nm are present in the microcrystals. For 449 nm excitation, the correlation is the same. The emission is also associated with excitation of aggregated species but to a lesser extent.
4. Conclusion
3.4.1. Space-averaged data Before exposure of the film to pyridine, fluorescence decays at 650 nm are not dependent on the excitation wavelength. Fluorescence decays of the film after exposure to pyridine are reported in Fig. 10. tn a first series of measurements (Fig. 10(a)), we have registered decays at 650 nm while moving the film in a random manner so that the decays measured are randomized. Upon excitation at 417 nm, a very steep decay is observed corresponding to the lifetime of the laser pulse. At 430 nm the fluorescence decay is longer than that observed before exposure. These results agree with the yields measurements. Samples with a high yield correspond to samples with a long lifetime. After exposure, all decays approach the same long lifetime of 3 ns. At 449 nm the faster component is longer than that of the one at 417 nm. If 417 and 449 nm aggregates where part of the same crystal a common decay would be observed. The question is: which of the 417 nm and 449 nm bands is that of the crystal? One band could be that of isolated dimers whereas a crystal would force a different structure.
3.4.2. Space-resolved data In Fig. 10(b), decays obtained after focussing on aggregates or between ag~egates are gathered. The excitation wavelength is 417 nm. The long lifetime component contributes more when films are excited between aggregates as expected for isolated monomers. This confirms that the
These results demonstrate that PyZnTPP migrates in the polymer relatively quickly to rearrange into microdomains clearly identifiable through microscopic investigations. It is a good example of the interest of the method of spaceresolved fluorescence decay measurements using an epifluorescence microscope coupled with a titane-sapphire laser. From the spectroscopic characteristics of the species, it is concluded that J-aggregates and H-aggregates are formed. The presence of broad bands, especially for the J-aggregates, can be explained by the fact that the phenomenon is observed in a polymer matrix in contrast to what is observed for solids which are pure porphyrins, spin coated on a surface [14,15].
Acknowledgements We thank CNRS (Centre National de la Recherche Scientifique) and Thomson-CSF Company for the grant supporting Isabelte Leray and ADEME (Agence de FEnvironnement et la Maitrise de l'Energie) for financial support. We are grateful to Guilhem Alibert for "kindly recording the optical microscopy pictures shown in Fig. 8 and for helping us with the films thicknesses measurements and to Professeur R.F. Pasternack (Swarthmore College, PA) for fruitful discussions.
I. Leray et al. / Thin Solid Films 303 (1997) 295-301
References [1] J. Deisenhofer, O. Epp, K. Mild, R. Huber and H. Michel, J. Mol. Biol., 180 (1984) 385. [2] D. Dolphin, Can. J. Chem., 72 (1994) i005. [3] G.G. Meng, B.R. James, K.A. Skov and M. Korbelik, Can. J. Chem.., 72 (1994) 2447. [4] S. Salhi, C. Bied-Charreton, R. Pansu and J. Faure, J. Photochem. Photobiol. B., 23 (1994) 253. [5] A.D. Adler, F.R. Longo, J.D. Finarelli, J.D. Goldmaeher and J. Assour, J. Org. Chem.., 32 (1967) 475. [6] A.D. Adler, F.R. Longo, F. Kampas and J. Kim, J. Inorg. Nucl. Chem., 32 (1970) 2443. [7] R.F. Pastemack, P.R. Huber, P. Boyd, G. Engasser, L. Francesconi, E. Gibbs, P. Fasella, G.C. Venturo and L de C. Hinds, J. Am. Chem. Soc., 94 (1972) 4511. [8] R.F. Pasternack, L. Francesconi, D. Raft and E. Spiro, Inorg. Chem., 12 (1973) 2606. [9] B.K. Manna, D. Sen, S.C. Bera and K.K. Rohatgi-Mukherjee, Spectroehim. Acta., 11/12 (1992) 1657.
301
[10] C. Bied-Charreton, C. Merienne and A. Gaudemer, New J. Chem., 11 (1987) 633. [11] D.G. Leggett, S.L. Kelly, L.R. Shiue, Y.T. Wu, D. Chang and K.M. Kadish, Talanta., 30 (1983) 579. [12] J.M. Ribo, J. Crusats, J.A. Fan-era and M.L. Valero. J. Chem. Soc. Chem. Commun., (I994) 681. [13] D.L. Aldns, H.R. Zhu and C. Guo, J. Phys. Chem., 100 (i996) 5420. [14] J.M. Kroon, P.S. Schenkels, M. Van Dijk and E.J.R. Sudh~5Iter, J. Mater. Chem., 5 (1995) 1309. [15] B.A. Gregg, M.A. Fox and A.J. Bard, J. Phys. Chem., 93 (1989) 4227. [16] M. Gouterman, in D. Dolphin (ed.), The Porphyrins, VoI. III, Academic Press, New York, I978, p. 1. [i7] D.P. Millar, R.J. Robbins and A.M. Zewail, J. Chem. Phys., 75 (1981) 3649. [I8] S. Salhi, M.C. Verni~res, R. Pansu, C. Bied-Charreton and J. Faure, React. Funct. Pol., 26 (1995) 209.
ELSEVIER
o
Thin Solid Films 303 (1997) 295-30i
Films of polyvinylpyrrolidone containing zinc tetraphenylporphyrin: evidence for aggregation of porphyrins in the presence of pyridine. I. Leray, M.C. Verni~res, R. Pansu, C. Bied-Charreton ~, J. Faure Laboratoire de Photophysique et Photochimie Supramoldculaires et Macromoldculaires, (URA ]906 du CNRS), Ecole Not~nale Supdrieure de Cachan, 61 Acenue du Prdsidetlt Wilson. 94235 Cachan Cedex. Fra~zce
Received 27 June I996: accepted30 September1996
Abstract Films of zinc porphyrins in polyvinylpyrrolidone have been formed and studied by spectrophotometric methods. Diffusion of gaseous pyridine through a film of polyvinylpyrrolidone containing zinc tetraphenylporphyfin leads to a rearrangement of the porphyrin dye with the formation of aggregates. Absorption and fluorescence spectra are complemented by observations under an inverted Nikkon microscope equipped for epifluorescence, using a Titane-Sapphire laser as the excitation source. Fluorescence is collected through an optical fiber. The technique allows for the study of well-defined zones in the film. © 1997 Elsevier Science S.A. © 1997 Elsevier Science S.A. Kev,~'ords: Diffusion; Fluorescence; Optical spectroscopy;Polymers
1. Introduction We are interested by complexes formed between polymers and porphyrins because of their relevance to structures involved in energy transfer processes. In natural processes, the reaction center where porphyrin initiation takes place, is very complex. In photosynthesis, for example, the chromophore is linked to a protein and encaged in a hydrophobic environment which is entirely suspended in an aqueous medium [1]. Furthermore, recent investigations on cancer phototherapy using porphyrin derivatives have demonstrated the importance of the porphyrin-membrane interaction [2,3]. However little is known about the migration of porphyrin-like chromophores under the influence of a change in polarity. Through the use of block-copolymers (polystyrene-block-polyvinyl pyridine), we were able to localize the chromophore either in a hydrophobic or in a hydrophilic environment depending on the structure of the dye [4]. In the course of our work, we observed that a change in complexation of zinc porphyrins immobilized in a film of polyvinylpyrrolidone leads to a reorganization of the dye within the polymer. Spectroscopic evidence points to the comptexation of the porphyrin by the hydrophilic polymer, followed by decomplexation after exposure to gaseous pyridine. Such behaviour is emphasized at the
" Correspondingauthor. 0040-6090/97/$17.00 © I997 Elsevier Science S.A. A]i rights reserved. PII S0040-6090(9 6)09404-7
surface of the polymer where we observe spectacular auto-assemblies of the dyes in presence of pyridine vapours. By using a confocal microscope adapted to a titane-sapphire picosecond laser we were able to explore the lifetime of the dye at the surface of the polymer with a spatial resolution limited by the diffraction and especially the spectacular dendrites growing up when the polymer is exposed to the pyridine vapours.
2. Experimental 2.J. Reagents
All standard reagents and solvents were purchased and used without further purification. Zinc tetraphenylporphyrin (ZnTPP) (Fig. 1) was synthesized according to Adler et al. [5] and metallated with zinc as described by the same authors [6]. Tetramethylpyridiniumporphyrin ( H J M P y P ) (Fig. 1) was synthesized according to Pasterhack and coworkers and metallated with zinc as described by the same authors [7,8]. Polyvinylpyrrolidone (PVP) is a commercial polymer (Aldrich M w = 17 000). 2.2. Preparation o f f i h n s
Films were prepared by spin coating a mixture of PVP and ZnTPP in CHC13 of varying relative concentrations.
296
L Leray et aL /Thin Solid Films 303 (1997) 295-301
R
The time resolution of detection was 50 ps. Acquisition of the decays was achieved within 10 min.
3. Results and discussion
3.I. Visible spectra of Zn porphyrins in the presence of poA,vinytpyrrolidon e
R
ZnTPP: R= ZnTMPyP : R =
Two kinds of experiments have been performed on Zn porphyrins: one series using porphyrins in solution at low concentration preventing aggregation of the dyes, the other in films at higher dye concentrations.
--@ N
I"
3.1.]. Visible spectra of Zn porphyrins in solution Visible spectra of ZnTPP in CH2C1," and ZnTMPyP in water have been recorded in the presence of increasing concentrations of polyvinylpyrrolidone. The resulting spectra are reported as Fig. 2(a) and 2(b). For ZnTPP in CI-{2C12 (Fig. 2(a)), the Soret absorption is shifted from 421.6 to 428 nm and the absorbance increases with the addition of PVP. The half-width of the band stays almost constant at 12 +_ 1 nm. The presence of an isobestic point indicates that in each case two species are present and we assume that they are in equilibrium, This is consistent with the results obtained by Manna et al. [9] concerning the axial complexation of zinc by the carbonyl of a pyrrolidine group. Complexation of ZnTPP by carbonyl groups was already observed in a previous work [ 10]. The following complexation equation can be assumed. ZnP + L ~ ZnPL where ZnP is the zinc porphyrin molecule, L is the ligand pyrrolidine and ZnPL is the complex formed by their association. The equilibrium constant can be related to absorbances using the relationship proposed by Leggett et al. [11] A -A 0 log At. _ A log K + log[PVP]
Fig. 1. Structures of the zinc porphyrins.
Films of PVP containing zinc tetramethylpyridiniumporphyrin (ZnTMPyP) were obtained by spin coating a mixture of polymer and dye dissolved in ethanol/water (4/1). The solutions were spin coated on a square glass plate (2.5 cm X 2.5 cm), thicknesses of ca. 1 bLm measured using a DEKTAK 3ST were obtained.
2.3. Apparatus Visible spectra were obtained using a Philips PU8720 UV-Vis spectrophotometer using films or cuvets of 1 cm optical length. CH2C12 or H 2 0 were the solvents. Static fluorescence measurements were performed using a SLM8000 spectrofluorimeter. The experimental device used for fluorescence decay measurements was a picosecond Tsunami titane-sapphire laser characterized by a 1 ps excitation pulse. The microscope was an inverted Nikon equipped for epifluorescence measurements for which a titane-sapphire laser was used as the excitation source. The excitation wavelengths were 417, 430 and 450 nm. Fluorescence was collected through a 400 ~ m diameter optical fiber and detected at 655 nm by a single photon counting set-up, based on a Hamamatsu multichannel plate photomultiplier.
where A is the absorbance at a given wavelength A, A 0 is
1.8
1.3
I
8 0.8 U3
/
0.4
-0.i
;' .ZZ
I
350
400 450 Wavelength (nm)
0 500
350
400
450
500
Wavelength (nm)
Fig. 2. Evolution of the visible spectra of zinc tetraphenylporphyrin during the comptexation by polyvinylpyrrolidone in (a) CH,C1, and (b) H20.
297
L Leray et aI. / Thin Solid Films 303 (1997) 295-301
2.2
the absorbance of the porphyrin at A before the addition of polymer, At. is the absorbance when all of the porphyrins are complexed by the polymer, K is the equilibrium constant and [PVP] represents the concentration of pyrrolidine units L. Plots of A-Ao/AL-A versus log[PVP] give the complexation constant: K = 3.8 for ZnTPP in CH2C1 z with a slope of ca. 1 indicating that only one ligand is attached to a zinc.
1 1.65
1.1
o
0.55
3.1.2. Visible spectra of Zn porphyrins in fihns of polyL'inylpyrrolidone By spin coating, we prepared polyvinylpyrrolidone films, each of them containing different concentrations of ZnTPP from 1% to 10% (mass/polymer). The porphyrin absorption is slightly red-shifted from 427 nm in the solution containing PVP, to 432 nm in the film, a change characteristic for porphyrins in polymer films [4]. In addition the band is significantly broadened (half-height width, 28 rim). By plotting the absorbances versus thicknesses for three different mass ratios of porphyrin/polymer we obtained straight lines as expected. At a given film thickness, we could derive the absorbance versus the concentration of porphyrins in the films. Fig. 3 shows the relationship between concentration of porphyrin and absorbance at 432 nm for films having the same thickness. This graph shows that the films do not obey the Beer-Lambert law but the curves could be fitted using the equation proposed by Pasternack et al. [7]. Thus in the film before exposure to pyridine, the presence of dimers a n d / o r small aggregates can be assumed. From the equation of Pasternack et al. a dimerisation constant of ca. 150 tool- 1 1 can be evaluated. Fig. 4 represents the spectra of a film of polyvinylpyrrolidone containing 10 wt.% ZnTPP, recorded every 5 rain during exposure to pyridine vapors. Initially the Soret absorption at 432 nm decreases, broadens and moves towards the blue (430 nm) due to complexation with pyridine then two new bands at 408 and 450 nm arised on each side accompanied by two isosbestic points
ul
0
350
420 490 560 630 Wavelength (run)
700
Fig. 4. Modification of the absorption spectrum of the film of zinc tetraphenylporphyrin in polyvinylpyrrolidone during I h of exposure to gazeous pyridine. (1) after 5 min, (2) after 10 min, (3) after 15 min, (4) after 20 rain, (5) after 25 min.
0.55
o
/ 0.t 350
I 450 550 650 Wavelength (rim)
Fig. 5. Modification of the absorption spectrum of the film after i h of exposure to gazeous pyridine. (1) Film containing 1% of porphyrin/polymer; (2) film containing 5% of porphyrin/polymer.
0.5
0.4
12_ ,
r 0.3
2
1.6-
2
1 0.2
0.8
0.1 0.4 0~ 0
T 2
I 4
I 6
I 8
l I0
0 12
% porphyrin/polymer Fig. 3. Absorbance of zinc tetraphenylporphyrin as a function of the relative concentrations of porphyrin and polymer. (1) O, 2000 i : (2) C),
4000 7~,
350
I 400
F 450
q 500
f 550
F 600
I 650
700
Wavelength (run) Fig. 6. Modification of the absorption spectrum of the fLIm of zinc tetramethylpyridiniumporphyrin in polyvinylpyrrolidone: (1) before exposure to pyridine; (2) after 1 h of exposure to gaseous pyridine.
I. Leray
298
er
a L / Thin Solid Fihns 303 (1997) 295-301
at 412 and 447 nm. The Q(I,0) band and the Q(0,0) band are displaced to lower energy and the intensity of the Q(0,0) band is increased. This effect depends on the concentration of porphyrin in the film as shown on Fig. 5. A possible explanation for these changes is the formation of aggregates: aggregates in solution [12,13] and in films [14] are characterized by new absorption bands on each side of the initial Sorer absorption. Comparable modifications of the spectrum are observed for crystalline zinc porphyrins which produce organized assemblies [15]. When ZnTPP is complexed by Py, it becomes more hydrophobic
and to minimize interactions with the hydrophilic polymer, PyZnTPP migrates in the polymer to form aggregates. We have successfully produced a film of PVP containing ZnTMPyP which is hydrophilic. In this film, the Sorer absorption is again red-shifted from 442 to 446 nm with a half-width significantly increased compared to the solution (36 rim). When this film is exposed to pyridine vapors there is no splitting of the Sorer band (Fig. 6). The Soret band becomes larger and narrower and is displaced towards higher energy from 446 to 443 nm, the half-width being 26 nm. The Q bands are also displaced towards
[
Nanone~r~ 35.0
70.82
_o
28.3
1'7,5
!\ \
J
,
, 8,8
!
%, i,
? _ _ ~
i
I
0
1
0
8.8
I
175
I
28.3
I
35.0
Fig. 7. Microscopy of the ~ of zinc tetrapheny]po~hyrin in potyviny]pyrro]idone: (a) ffdm before exposure (eniargment 100 X ); (b) film after exposure (enlaIgment tO0 X ); (c) film after exposure (entargment lO00 X ); (d) observation by atomic force microscopy.
I. Leray et a l . / T h i n Solid Films 303 (1997) 295-301
higher energies. This result can be explained by the fact that the pyridine complex of ZnTMPyP is still hydrophilic (it has a charge of + 4) and therefore remains compatible with the polymer, consistent with our interpretation.
2.00
299
I
I
I
I
I
I
1.60
1.20
3.2. Observation under optical microscopy
'~ 0180
Observations of the film have been made using a microscope with an enlargement of 100 × - 1 0 0 0 X . Films before and after 1 h exposure to pyridine are shown in Fig. 7. Before exposure (Fig. 7(a)) the film is uniform. After exposure (Fig. 7(b)), an important change is observed even at an enlargement of 100 × . A higher enlargement (Fig. 7(c)) of 1000 × shows the presence of small rods indicating that microdomains containing PyZnTPP are formed. From the enlargement by the microscope, we estimated a size of 1 b m for the rods with a comb-like structure. Observation with the epifluorescence microscope shows that these rods are fluorescent. Observations using AFM confirm this structure (Fig. 7(d)). The absorption spectroscopy has provided evidence for the presence of aggregates on a molecular level and microscopy shows that the aggregates extend over microns.
3.3. Fluorescence spectra polyvinylpyrrolidone
of fihns
of ZnTPP
in
3.3.1. Concentration effects on the static fluorescence of films of ZnTPP in polyvinylpyrrolidone before exposure to pyridine Films containing increasing relative concentrations of porphyrin and polymer have been prepared. The thicknesses have been set to obtain comparable absorbances for all films. The static fluorescence spectra were recorded using an excitation wavelength of 433 nm. The fluorescence yields were calculated using the fluorescence yield of ZnTPP in toluene (3.3%) as a reference [16]. The results reported in Fig. 8 show that the fluorescence yield decreases while the concentration of porphyrins dimers in the
1.4
.O ./
1.2
/
/ /-
/
/ / /
0.8 II
/ /
0.6
,z /
0.4 0.2 0
/ / /
I
t
I
I
l
I
I
0 0.5 t 1.5 2 2.5 3 3.5 4 [ porphyrin dimer] Fig. 8. Quantum yield of fluorescence as a function of the concentrations of porph~ria dimer. 2t~xc = 430 nm.
0.40
0.00 560
600
640
680
Wavelength (nm) Fig. 9. Stationary fluorescence emission at 650 nm of the film of zinc tetraphenylporphyrin in polyvinylpyrrolidone: (i) excitation at 430 nm before exposure, (2) excitation at 430 nm after exposure, (3) excitation at 450 nm after exposure, (4) excitation at 417 nm after exposure.
film increases. This is consistent with a quenching of fluorescence by porphyrin dimers [17]. Absorption spectroscopy has shown that the B e e r - L a m b e r t law is not obeyed in that region of concentrations, thus a part of the porphyrins population in the film before exposure is in the form of dimers or small aggregates. The fluorescence yield decrease could be explained by the existence of energy transfer between porphyrins by FSrster mechanism [4]. Thus dimers and small aggregates efficiently capture the energy of the surrounding porphyrins leading to efficient quenching.
3.3.2. Fluorescence of fihns of ZnTPP in polyvinylpyrroIidone after exposure to pyridine After exposure of the film to pyridine, fluorescence emission spectra were recorded at several different excitation wavelengths corresponding to the Soret absorption maxima observed in the absorption spectrum of PyZnTPP in polyvinylpyrrolidone (Fig. 4). It is worth noting that the absorbance of the three bands used as the excitation wavelength have quite the same intensity. Different fluorescence spectra are observed depending on the excitation wavelength as reported in Fig. 9. From these results it can be concluded that the maxima correspond to different species present in the film with different emission spectra. Excitation at 408 nm leads to lower quantum yield of fluorescence than excitation at 450 nm. The observation of two different emission spectra for excitation at 408 (Fig. 9, curve 4) and 450 nm (Fig. 9, curve 3) suggest the existence of at least two distinct entities. This is consistent with the theory which predicts that fluorescence quenching for face-to-face aggregates (H-aggregates) is more efficient than that for edge-to-edge aggregates (J-aggregates). At 430 nm the spectrum of the exposed film (Fig. 9, curve 2) has a fluorescence enhancement compared to the starting film (Fig. 9, curve 1). We assume that at this wavelength the excited molecules are the non-rearranged molecules PyZnTPP. However the molecules are now less concen-
l. Leray et al./Thin Solid Films 303 (1997) 295-301
300 (a)
(b) J
lOCO,
1000
<~
2
lC0.
3
100
d
' t
10
111
j
I
I
0
2
1 4
I
t
I
I
6
8
10
12ns
I
1 2
I .l
I 6
[ 8
t 10
I t2ns
t (ns)
t(ns) Fig. t0. (a) Randomized fluorescence decays of the film at 650 nm of zinc tetraphenyl porphyrin in polyvinylpyrrolidone: (l) excitation at 430nm before exposure, (2) excitation at 430 nm after exposure, (3) excitation at 450 nm after exposure, (4) excitation at 417 nm after exposure. (b) Space-resolved fluorescence decays of the film at 650 nm: excitation at 417 nm after exposure: (1) between rods, (2) randomized, (3) on rods.
trated and so the quenching by another porphyrin or a small aggregate is less probable than in the non-exposed film, consistent with what was observed in the non-exposed film (Section 3.4.1). The quantum yield of PyZnTPP was found to be 3% [18].
3.4. Time-resoh,ed fluorescence of fihns of ZnTPP in pol},t'in),Ip3'rrolidone
long lifetime of 3 ns is for isolated molecules of PyZnTPP, and that aggregates absorbing at 417 nm are present in the microcrystals. For 449 nm excitation, the correlation is the same. The emission is also associated with excitation of aggregated species but to a lesser extent.
4. Conclusion
3.4.1. Space-averaged data Before exposure of the film to pyridine, fluorescence decays at 650 nm are not dependent on the excitation wavelength. Fluorescence decays of the film after exposure to pyridine are reported in Fig. 10. tn a first series of measurements (Fig. 10(a)), we have registered decays at 650 nm while moving the film in a random manner so that the decays measured are randomized. Upon excitation at 417 nm, a very steep decay is observed corresponding to the lifetime of the laser pulse. At 430 nm the fluorescence decay is longer than that observed before exposure. These results agree with the yields measurements. Samples with a high yield correspond to samples with a long lifetime. After exposure, all decays approach the same long lifetime of 3 ns. At 449 nm the faster component is longer than that of the one at 417 nm. If 417 and 449 nm aggregates where part of the same crystal a common decay would be observed. The question is: which of the 417 nm and 449 nm bands is that of the crystal? One band could be that of isolated dimers whereas a crystal would force a different structure.
3.4.2. Space-resolved data In Fig. 10(b), decays obtained after focussing on aggregates or between ag~egates are gathered. The excitation wavelength is 417 nm. The long lifetime component contributes more when films are excited between aggregates as expected for isolated monomers. This confirms that the
These results demonstrate that PyZnTPP migrates in the polymer relatively quickly to rearrange into microdomains clearly identifiable through microscopic investigations. It is a good example of the interest of the method of spaceresolved fluorescence decay measurements using an epifluorescence microscope coupled with a titane-sapphire laser. From the spectroscopic characteristics of the species, it is concluded that J-aggregates and H-aggregates are formed. The presence of broad bands, especially for the J-aggregates, can be explained by the fact that the phenomenon is observed in a polymer matrix in contrast to what is observed for solids which are pure porphyrins, spin coated on a surface [14,15].
Acknowledgements We thank CNRS (Centre National de la Recherche Scientifique) and Thomson-CSF Company for the grant supporting Isabelte Leray and ADEME (Agence de FEnvironnement et la Maitrise de l'Energie) for financial support. We are grateful to Guilhem Alibert for "kindly recording the optical microscopy pictures shown in Fig. 8 and for helping us with the films thicknesses measurements and to Professeur R.F. Pasternack (Swarthmore College, PA) for fruitful discussions.
I. Leray et al. / Thin Solid Films 303 (1997) 295-301
References [1] J. Deisenhofer, O. Epp, K. Mild, R. Huber and H. Michel, J. Mol. Biol., 180 (1984) 385. [2] D. Dolphin, Can. J. Chem., 72 (1994) i005. [3] G.G. Meng, B.R. James, K.A. Skov and M. Korbelik, Can. J. Chem.., 72 (1994) 2447. [4] S. Salhi, C. Bied-Charreton, R. Pansu and J. Faure, J. Photochem. Photobiol. B., 23 (1994) 253. [5] A.D. Adler, F.R. Longo, J.D. Finarelli, J.D. Goldmaeher and J. Assour, J. Org. Chem.., 32 (1967) 475. [6] A.D. Adler, F.R. Longo, F. Kampas and J. Kim, J. Inorg. Nucl. Chem., 32 (1970) 2443. [7] R.F. Pastemack, P.R. Huber, P. Boyd, G. Engasser, L. Francesconi, E. Gibbs, P. Fasella, G.C. Venturo and L de C. Hinds, J. Am. Chem. Soc., 94 (1972) 4511. [8] R.F. Pasternack, L. Francesconi, D. Raft and E. Spiro, Inorg. Chem., 12 (1973) 2606. [9] B.K. Manna, D. Sen, S.C. Bera and K.K. Rohatgi-Mukherjee, Spectroehim. Acta., 11/12 (1992) 1657.
301
[10] C. Bied-Charreton, C. Merienne and A. Gaudemer, New J. Chem., 11 (1987) 633. [11] D.G. Leggett, S.L. Kelly, L.R. Shiue, Y.T. Wu, D. Chang and K.M. Kadish, Talanta., 30 (1983) 579. [12] J.M. Ribo, J. Crusats, J.A. Fan-era and M.L. Valero. J. Chem. Soc. Chem. Commun., (I994) 681. [13] D.L. Aldns, H.R. Zhu and C. Guo, J. Phys. Chem., 100 (i996) 5420. [14] J.M. Kroon, P.S. Schenkels, M. Van Dijk and E.J.R. Sudh~5Iter, J. Mater. Chem., 5 (1995) 1309. [15] B.A. Gregg, M.A. Fox and A.J. Bard, J. Phys. Chem., 93 (1989) 4227. [16] M. Gouterman, in D. Dolphin (ed.), The Porphyrins, VoI. III, Academic Press, New York, I978, p. 1. [i7] D.P. Millar, R.J. Robbins and A.M. Zewail, J. Chem. Phys., 75 (1981) 3649. [I8] S. Salhi, M.C. Verni~res, R. Pansu, C. Bied-Charreton and J. Faure, React. Funct. Pol., 26 (1995) 209.