5986
J. Phys. Chem. B 2000, 104, 5986-5992
Structural and Photophysical Properties of a Water-Soluble Porphyrin Associated with Polycations in Solution and Electrostatically-Assembled Ultrathin Films P. Gregory Van Patten,† Andrew P. Shreve, and Robert J. Donohoe* Michelson Resource, MS J586, Bioscience DiVision, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 ReceiVed: February 22, 2000; In Final Form: April 18, 2000
A water-soluble porphyrin, meso-tetra(4-sulfonatophenyl)porphyrin (TSPP), has been associated with two different polycations, poly(diallyldimethylammonium chloride) (PDDA) and poly(ethyleneimine) (PEI), to investigate the effects of polymer binding upon the TSPP structure and excited-state dynamics both in solution and in ultrathin (∼10-30 Å) films deposited on glass slides by electrostatic assembly. Association of the porphyrin with PEI intrinsically quenches the singlet state dynamics of TSPP, both in solution and in films, while quenching is observed upon association with PDDA only for high concentrations of porphyrins or in films where TSPP aggregates are observed. For PDDA:TSPP films without significant aggregate content, the fluorescence decay time (τ1/e ∼ 5-6 ns) approaches that observed for monomeric or polymer-bound porphyrins in dilute solution (τ1/e ) 10.2 and 11.3 ns, respectively). However, rapid ( 1 ns. Control of Aggregation in TSPP-Impregnated Thin Films. The structural and photodynamic properties of aggregated TSPP in solution have been discussed in detail by several authors.13-17 As reported there (and shown in Figure 2b), solution TSPP aggregation entails reduction of the luminescent state lifetime.13,17b As discussed above, this effect is amplified in films, where rapid energy transfer to the aggregate sites leads to large scale quenching even in films containing primarily monomeric dyes. The presence of aggregates in the films (and in polymer-containing solutions) is principally related to the dye concentration, although other factors are relevant. Therefore, the ability to derive a long-lived fluorescent TSPP-impregnated film with useful absorption cross section depends on achieving a balance between optical density and aggregation. Without a full understanding of the thermodynamics and kinetics of dye association in polymer-modified substrates, the control of aggregation must proceed upon a largely phenomenological basis. We have investigated the means of controlling aggregation by regulating various parameters associated with the deposition process (structural modifications of the dye, not explored here, would almost certainly further this goal). Although the effects of several deposition parameters were examined, four were observed to have the most significant impact on the optical density and degree of aggregation in the films. These were pH, premixing with polyanions, deposition temperature, and annealing time. Of these, the first two entail the achievement of reduced aggregation via reduced TSPP content while the last two can yield reduced aggregation without a commensurate decrease in dye content. Results from all four approaches are now presented. As described above, the TSPP species generated in solution depend on pH and concentration. It is reasonable to expect that the nature of the TSPP dipping solution can have a strong impact upon the dye structure(s) obtained in a polymer film. Sample spectra from films prepared by dipping a PDDA film substrate into differing kinds of TSPP solutions are shown in Figure 6. The spectrum from a film deposited from free base TSPP (Figure 6a) is similar to that of the solution-phase polymer-bound free base, including a small PDDA-induced red shift. Likewise, a solution containing the aggregated diacid gives rise to a film whose spectrum resembles the solution-phase J-aggregate. Upon
Van Patten et al.
Figure 6. (a) Absorption spectra from PDDA films dipped into various TSPP solutions as indicated. (b) Spectral and relative emission intensity (PLQE) data for two of the films described in (a). The film with large amounts of aggregate exhibited very weak fluorescence.
dipping such a film into a solution of 0.1 N NaOH, the dye deprotonates, and a new spectrum, also shown in Figure 6a, results. This behavior was first reported by Ariga et al.;7b however, two important differences, not discussed in their report, can be discerned between the spectra of films prepared by direct deposition of the free base compared with those prepared by deposition of the diacid followed by deprotonation. The first difference is that film preparation from the diacid aggregate solution results in the deposition of much more dye than dipping into a similarly concentrated monomeric free base solution. Comparison of the integrated Soret band absorbance indicates that TSPP is deposited at least 10 times more efficiently from diacid-containing solutions than from free-base-containing solutions. This is possibly due to the fact that extended aggregates, rather than individual molecules are adsorbed from the aggregated solution. A second difference between the films prepared from free base TSPP and the aggregated diacid plus NaOH dipping is the observation that the latter exhibits approximately equal intensity for both the blue- and red-shifted Soret peaks, while the free-base-dipped film contains primarily the latter. Further work, in which the two species have been more completely isolated, has shown that the short wavelength peak appears at ∼404 nm and the long wavelength peak is at 420 nm. It is not surprising that the diacid aggregates in solution give rise to aggregates in the film; however, the blue-shifted aggregate peak in the diacid-derived film spectrum is indicative of an H-type (cofacial, or “head-to-head”) aggregate, while the solution spectrum indicates a J-type (“head-to-tail”) aggregate, as mentioned above. Thus, deprotonation of the diacid in the films is accompanied by significant perturbation of the aggregate
Porphyrin Dye on Polycationic Films
Figure 7. Dependence of film spectral characteristics on: (a) TSPP dipping solution temperature and (b) age following dipping in TSPP solution (at room temperature).
geometry but not by complete dissociation: excitonic coupling remains evident. Figure 6b shows a comparison of the PLQE data for the films described above and in Figure 6a. Note that even though the peak absorbance is increased roughly 4-fold by depositing from the aggregated diacid followed by deprotonation, the peak emission intensity is increased only by a factor of 2. As was the case for the low ratio PDDA/TSPP solutions, the presence of aggregates causes a distinct red shift in the emission and a concomitant decrease in PLQE. The emission spectra from such films (not shown) are also much more excitation wavelength dependent, demonstrating greater diversity of structures in such films. Interestingly, we have observed that even the use of monomeric diacid dipping solutions also increases both the amount of material deposited and the extent of aggregation in the films compared with deposition from TSPP solutions. Premixing the TSPP with a polyanion such as PSS was observed to have some impact on aggregate content of the films. Of course, the reduction of aggregate content is accompanied by a commensurate reduction in optical density: the PSScontaining film exhibits an optical density that is smaller by a factor of about 2 compared with a film prepared from a similarly concentrated TSPP-alone solution. The deposition temperature affects the extent of aggregation in the films. Figure 7a shows spectra of single PDDA/TSPP bilayers deposited at different TSPP solution temperatures. There is a significant improvement in the optical density and a slight improvement in the monomeric quality of the film deposited at 45 °C. Thus, thermal regulation of aggregation does not require a tradeoff of optical density for film quality. However, the gains offered by increased temperature are limited: at 70 °C PDDA is desorbed from the substrate and dye deposition is thwarted.
J. Phys. Chem. B, Vol. 104, No. 25, 2000 5991 [Desorption was verified by allowing the substrate to cool and attempting TSPP deposition at room temperature. Failure to observe deposition of TSPP as detected by UV-vis was taken as an indication of the absence of PDDA on the substrate.] It is worth noting that current theoretical models suggest that polymer deposition on a substrate is entropically driven by virtue of having the increased disorder associated with the displacement of surface-bound monocations to solution outweigh the increased order associated with surface binding of a polycation.22,23 We have initiated some van’t Hoff experiments to test the equilibrium constants of this process, although an improved understanding of the kinetics of deposition, which includes long-time components7b and annealing is also required. Allowing the films to anneal in air and in the dark over a period of a couple of days to several months has yielded one of the most dramatic effects on monomer/aggregate proportions in these films. Figure 7b shows two spectra from the same film, taken immediately after preparing the film and after one month aging in a dark environment. The spectrum of the aged film exhibits a very sharp, red-shifted absorbance band, indicating the predominance of monomeric dye. There are several factors that could lead to the annealing of the films. Migration of the dyes within the films is one very probable contributor. It has been shown elsewhere that this migration occurs in the shorter term for a similar case.24 In addition, the polymer structure may be slowly stabilizing itself in a manner that could include disruption of aggregated dyes. As expected, elevated temperatures (∼100 °C) accelerate the annealing process; however, heating at such temperatures can also cause rapid removal of the films as evidenced by absorbance measurements. Although the various samples we have prepared appear to be robust under ambient conditions, exposure to a water-free nitrogen atmosphere for several hours was also observed to lead to film desorption. Thus, water content appears to be critical to the stability of these films. Migration of the charged porphyrins within the films has thwarted attempts to develop stratified materials with isolated, well-defined dye-impregnated layers. As an example, deposition of ZnTSPP in one PDDA layer followed by deposition of a PSS spacer layer and then deposition of free base TSPP in a second PDDA layer does not yield isolated, well defined energydonor- (ZnTSPP) and acceptor- (TSPP) impregnated layers. Instead, deposition of the charged porphyrins using such a multiple dipping process leads to fairly indiscriminate association with the various polycation layers. This was demonstrated by a number of observations, including an experiment in which samples with differing numbers of PDDA/PSS bilayers were ultimately dipped once into a TSPP solution. The uptake of TSPP was significantly dependent upon the number of bilayers, with the thicker samples exhibiting higher optical density. The use of toluenesulfonate as a coadsorbate with the dyes did not reduce the observed interlayer scrambling. Conclusions The association of ionic dyes with water-soluble polymers offers a simple means to develop photonically active thin films. Intrinsic quenching of dye excited states can occur with some (imine) polymers but the alteration of lifetimes is otherwise found to depend primarily upon energy transfer to trapping sites. The development of optimized (long-lived) films will require modeling of the energy transfer processes and control of the trapping site density. When the trapping sites consist of dye aggregates, optimization will require control of the balance of optical density, energy transfer rates, and aggregate density. The
5992 J. Phys. Chem. B, Vol. 104, No. 25, 2000 structural properties of thin films evolve over extended periods of time, including the migration of dyes within the films. Acknowledgment. This research was supported by the Laboratory Directed Research and Development program at Los Alamos National Laboratory. We thank Drs. Hsing-Lin Wang and Duncan McBranch for helpful discussions. References and Notes (1) (a) Hong, H. P.; Davidov, D.; Avny, Y.; Chayet, H.; Faraggi, E. Z.; Neumann, R. AdV. Mater. 1995, 7, 846-847. (b) Hong, H.; Tarabia, M.; Chayet, H.; Davidov, S.; Faraggi, E. Z.; Avny, Y.; Neumann, R.; Kirstein, S. J. Appl. Phys. 1996, 79, 3082-3088. (c) Hong, H.; Davidov, D.; Chayet, H.; Faraggi, E. Z.; Tarabia, M.; Avny, Y.; Neumann, R.; Kirstein, S. Supramol. Sci. 1997, 4, 67-73. (2) (a) Onitsuka, O.; Fou, A. C.; Ferreira, M.; Hsieh, B. R.; Rubner, M. F. J. Appl. Phys. 1996, 80, 4067-4071. (b) Fou, A. C.; Onitsuka, O.; Ferreira, M.; Rubner, M. F.; Hsieh, B. R. J. Appl. Phys. 1996, 79, 75017509. (c) Fou, A. C.; Onitsuka, O.; Ferreira, M.; Rubner, M. F.; Hsieh, B. R. Mater. Res. Soc. Symp. Proc. 1995, 369, 575. (d) Ferreira, M.; Rubner, M. F.; Hsieh, B. R. Mater. Res. Soc. Symp. Proc. 1994, 328, 119. (3) Li, D. Q.; Bishop, A.; Gim, Y.; Shi, X. B.; Fitzsimmons, M. R.; Jia, Q. X. Appl. Phys. Lett. 1998, 73, 2645-2647. (4) Sun, Y. P.; Zhang, X.; Sun, C. Q.; Wang, B.; Shen, J. C. Macromol. Chem. Phys. 1996, 197, 147-153. (5) (a) Onda, M.; Lvov, Y.; Ariga, K.; Kunitake, T. Biotechnol. Bioeng. 1996, 51, 163-167. (b) Onda, M.; Lvov, Y.; Ariga, K.; Kunitake, T. J. Ferment. Bioeng. 1996, 82, 502-506. (6) (a) Keller, S. W.; Johnson, S. A.; Brigham, E. S.; Yonemoto, E. H.; Mallouk, T. E. J. Am. Chem. Soc. 1995, 117, 12879-12880. (b) Kaschak, D. M.; Mallouk, T. E. J. Am. Chem. Soc. 1996, 118, 4222-4223. (c) Kaschak, D. M.; Lean, J. T.; Waraksa, C. C.; Saupe, G. B.; Usami, H.; Mallouk, T. E. J. Am. Chem. Soc. 1999, 121, 3435-3445. (7) (a) Lvov, Y.; Ariga, K.; Ichinose, I.; Kunitake, T. J. Am. Chem. Soc. 1995, 117, 6117-6123. (b) Ariga, K.; Lvov, Y.; Kunitake, T. J. Am.
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