J. Phys. Chem. C 2007, 111, 4425-4430
4425
Photomodulated PPV Emission in a Photochromic Polymer Film Steven M. Lewis and Elizabeth J. Harbron* Department of Chemistry, The College of William and Mary, Williamsburg, Virginia 23187-8795 ReceiVed: October 24, 2006; In Final Form: January 4, 2007
We present studies of fluorescence intensity modulation in neat films of a photochromic poly(pphenylenevinylene) (PPV) derivative. Poly(2-hexyloxy-5-((10-(4-(phenylazo)phenoxy)decyl)oxy)-1,4-phenylenevinylene) (HPA-10-PPV), a PPV derivative functionalized with photoaddressable azobenzene side chains, is known to undergo modulated changes in fluorescence intensity upon azobenzene isomerization in solution. These changes are caused by nonradiative transfer of excitation energy from the PPV backbone to the azobenzene moiety. The efficiency of energy transfer is higher for the cis azobenzene isomer than for trans, a difference that enables fluorescence intensity modulation. Upon film formation, the fluorescence spectra of neat HPA-10-PPV films undergo a red shift of ca. 30 nm compared to those measured in dilute solution. Application of ultraviolet irradiation to induce trans f cis azobenzene isomerization yields a reduction in fluorescence intensity in HPA-10-PPV films that is reversed upon azobenzene cis f trans back isomerization. These reversible intensity changes can be cycled many times in the film with a modulation efficiency that is virtually identical to that measured in dilute solution. The isomeric state of the azobenzene side chains at the time of film casting has a negligible impact on film fluorescence properties and subsequent isomerization behavior in the film. This result implies that the azobenzene has ample free volume for isomerization in the film and that its isomeric state does not substantially impact film morphology in a way that affects the fluorescence. Spatial control of fluorescence intensity in HPA-10-PPV films is demonstrated by inscribing an image with increased fluorescence intensity on a lower intensity background. The successful photomodulation of fluorescence intensity in polymer films further demonstrates the potential for photochromic conjugated polymers as a class to find applications in optical data storage.
Introduction Luminescent poly(p-phenylenevinylene) (PPV) derivatives continue to be developed for a variety of applications,1 including polymer light-emitting diodes.2 While PPV itself is insoluble in organic solvents, its most common derivatives feature solubilizing alkoxy side chains that facilitate the formation of polymer films through spin coating from solution. Multilayer devices are routinely constructed by sequential coating and deposition of soluble PPVs and other components. In addition to enhancing solubility and processability, PPV side chains can be used to confer or enhance functionality. Alkoxy substituents on the PPV main chain phenyl group have been altered or replaced to enable sensor applications,3-7 alter the emission color,8,9 and improve emission properties, including fluorescence quantum yield,7,10-14 photostability,10 and electroluminescence efficiency in devices.11,15-17 PPVs have also been functionalized with pendent azobenzenes for investigations of anisotropy18,19 and nonlinear optical properties.20,21 Our work focuses on PPV derivatives with pendent photochromic side chains that enable photoswitching of PPV fluorescence properties.22-24 Here we report fluorescence intensity modulation in neat films of an azobenzene-functionalized PPV derivative. Photochromic molecules switch between two forms with different absorption spectra in response to a light signal.25 This unique behavior can be exploited to create a wide variety of systems with photoswitchable properties. Numerous types of polymers have been functionalized with pendent photochromic * To whom correspondence should be addressed. E-mail: ejharb@ wm.edu.
moieties to enable photocontrol over a range of polymer properties.26-30 Most of these polymers have been nonfluorescent, but fluorescent polymeric and small molecule systems in which a photochromic unit is incorporated to facilitate photocontrol of fluorescence are becoming increasingly common.31-34 This approach is exemplified by bichromophoric systems in which a fluorescent donor is covalently linked to a photochromic energy transfer acceptor for the purpose of fluorescence modulation.35-53 Because of their different absorption properties, the two forms of a photochromic molecule typically have very different efficiencies of nonradiative energy transfer from a given fluorescent donor. Hence, the fluorescence intensity of a donor can be modulated by using light to switch between photochromic forms of the acceptor, reversibly activating and deactivating its ability to act as an energy transfer acceptor. Such photocontrol of fluorescence intensity potentially has applications in optical data storage.53 We have employed this photochromic photoswitch strategy to PPV derivatives in which the polymer backbone acts as a donor and covalently linked azobenzenes function as photochromic energy transfer acceptors.22,24 Azobenzene shows photochromism through isomerization: the thermally stable trans form isomerizes to cis in response to ultraviolet light, and the cis form returns to trans thermally or upon irradiation with visible light (Scheme 1).54 The two isomeric forms have overlapping but unique absorption spectra as well as different dipole moments, shapes, and molecular volumes. Nonradiative energy transfer to one or both forms of azobenzene with different efficiencies has been observed previously in small molecule photoswitches.35,37
10.1021/jp0669759 CCC: $37.00 © 2007 American Chemical Society Published on Web 02/27/2007
4426 J. Phys. Chem. C, Vol. 111, No. 11, 2007
Lewis and Harbron
SCHEME 1
We have studied energy transfer in a family of azobenzenefunctionalized PPVs based on poly(2-methoxy-5-((10-(4-(phenylazo)phenoxy)decyl)oxy-1,4-phenylenevinylene) (MPA-10PPV), shown in Scheme 1.22,24 In dilute solutions of THF, toluene, or other good solvents, the emission of the PPV backbone in thermally equilibrated MPA-10-PPV is quenched relative to a control polymer lacking the azobenzene side chains. The fluorescence is quenched even further when the thermally stable trans azobenzenes undergo photoisomerization to the photostationary state concentration of cis isomers upon application of ultraviolet irradiation. We previously determined that this isomer-dependent fluorescence quenching is caused by nonradiative energy transfer from the PPV backbone to the azobenzene side chains.22,24 Energy transfer occurs to both azobenzene isomers but with a higher efficiency to the cis form. Dynamic quenching by both trans and cis azobenzene is consistent with fluorescence resonance energy transfer, and differences in energy transfer efficiency reflect the difference in Fo¨rster radii for the PPV-trans and PPV-cis donor-acceptor pairs.24 This isomeric difference in energy transfer efficiencies enables photomodulation of the fluorescence intensity: the intensity of PPV emission can be tuned by controlling the isomeric state of the side chain with light. Initial energy transfer studies of MPA-10-PPV were performed entirely in dilute solution. Given that most applications for PPVs involve polymer films rather than solutions, it is critically important to determine whether and to what extent this fluorescence modulation phenomenon translates to polymer films. Here we present studies of fluorescence modulation in polymer films of HPA-10-PPV, a derivative of the previously studied MPA-10-PPV with a longer alkoxy side chain for enhanced processability (Scheme 1). The fluorescence spectra of neat films of HPA-10-PPV show a red shift from dilute solution that is typical of PPV derivatives. The emission intensity of HPA-10-PPV films decreases upon trans f cis azobenzene isomerization and recovers upon cis f trans back-isomerization. Thus, the fluorescence modulation initially observed in solution remains unchanged, demonstrating that this phenomenon is not limited to solution-phase studies. The fluorescence photomodulation also enables spatial control of emission intensity, which we demonstrate by inscribing an image with increased fluorescence intensity on a lower intensity background. The successful photomodulation of fluorescence intensity in polymer films further demonstrates the potential for photochromic conjugated polymers as a class to find applications in optical data storage.
Figure 1. Absorbance (a) and fluorescence (b) spectra of HPA-10PPV in dilute THF solution (solid lines) and neat film cast from THF solution (dotted lines).
(THF) by spin coating at 1000 RPM (Cookson Electronics, G3-8 Spin-Coater). Samples were then capped by spin coating a solution of 0.6% w/w poly(vinyl alcohol) (Acros, average molecular weight ) 16 000) in water (Millipore Milli-Q) to protect the film from atmospheric oxygen.55,56 Fisher cover slips (12-544-10) were used both as substrate for the film and as a secondary cap sandwiching the sample to protect it. The absorption of the slides in the UV region limited study of polymer films to wavelengths of ca. 320 nm and above. Exposure to ambient light was minimized for all polymer solutions and films. Ultraviolet irradiation was provided by a UV pencil lamp (Spectronics Corporation, 36-380). The 488 nm line from a tunable argon ion laser (Melles Griot), passed through a liquid light guide (PTI) to diffuse the beam, was used for visible irradiations. Irradiation times of 15 s (UV) and 35 s (488 nm) were determined to be sufficient to yield the photostationary states in films. Absorbance spectra were recorded on a Varian Cary50 Bio equipped with a slide holder. Fluorescence studies were performed on a Perkin-Elmer LS 55 with a solid sample holder. Emission spectra were obtained at an excitation wavelength of 482 nm (films) or 488 nm (solutions). The fluorimeter excitation source is capable of inducing azobenzene isomerization during collection of fluorescence spectra. The fluorimeter’s relatively low lamp intensity, combined with manipulation of integration times and monochromator slit widths, mitigates this problem. Fluorescence spectra for peak intensity measurements were recorded over a narrow wavelength range (30 nm) to reduce the duration of exposure to the fluorimeter lamp. Fluorescence microscopy images were collected on an inverted microscope (Carl Zeiss Axiovert 200). Excitation (488 nm) from an argon ion laser (Melles Griot) is introduced to the microscope via an optical fiber. A dichroic beamsplitter (Chroma Technology) directs the laser beam to the sample via an oil immersion objective (Zeiss Fluar, 100X, N.A. ) 1.3). The sample is raster scanned over the laser beam by a nanopositioning stage (NanoBio2, Mad City Labs). Fluorescence is collected by the microscope objective, passed back through the dichroic beamsplitter and a 488 nm holographic notch filter (Kaiser Optical), and directed to an avalanche photodiode detector (SPCM-AQR-14, Perkin-Elmer Optoelectronics). Nanopositioning stage movement and data collection are controlled by Labview software (National Instruments).
Experimental Methods The syntheses of poly(2-hexyloxy-5-((10-(4-(phenylazo)phenoxy)decyl)oxy)-1,4-phenylenevinylene) (HPA-10-PPV) and control polymer poly(5-decyloxy-2-hexyloxy-1,4-phenylenevinylene) (DH-10-PPV) were described previously.23 The synthesis of model compound decyloxyazobenzene was also described previously.22 Polymer films were prepared from saturated polymer solutions in spectral grade tetrahydrofuran
Results and Discussion HPA-10-PPV is a PPV derivative with tethered azobenzene side chains that photoisomerize in response to light, as depicted in Scheme 1.23 In dilute THF solution prepared in the dark, the azobenzene side chains are predominantly in the trans form, which shows a strong π-π* absorption at 350 nm (Figure 1a, solid line). This peak broadens slightly but remains otherwise
PPV Emission in a Photochromic Polymer Film
J. Phys. Chem. C, Vol. 111, No. 11, 2007 4427 SCHEME 2
Figure 2. Modulation behavior of HPA-10-PPV in dilute THF solution (a traces) and in a neat film cast from THF solution (b traces): fluorescence spectra of initial, thermally equilibrated samples with essentially all trans side chains (solid lines); at the photostationary state concentration of cis azobenzenes (dotted lines); and upon return to the trans state (dashed lines). Each set of three spectra was normalized to the peak intensity of the spectrum of the thermally equilibrated solution (a) or film (b) but retains the authentic intensity variations induced by irradiation. The photostationary state concentration of cis azobenzenes was induced by UV irradiation (45 s solution, 15 s film), while the return to trans was induced by 488 nm irradiation (4 min solution, 35 s film). Irradiation times were determined by absorbance measurements to be sufficient to generate the photostationary state.
unchanged in an HPA-10-PPV film prepared by spin coating from THF solution (Figure 1a, dotted line). Both the solution and film absorption spectra show an additional broader peak at 475-500 nm due to the absorbance of the PPV backbone, which obscures the weak absorbance of cis azobenzene in the same region when it is present. The PPV absorbance in the film is somewhat red-shifted from solution. This phenomenon is also reflected in the emission spectra shown in Figure 1b and is attributed to an increase in conjugation length and formation of aggregated interchain species in the film.57,58 Relative to the solution-phase spectrum, the λmax of the film emission is shifted 33 nm to the red, a magnitude typical of PPV derivatives.59 The solution and film emission spectra are otherwise similar in shape and width although the film spectrum has greater vibronic character. The emission is entirely from the PPV backbone as the azobenzene side chains do not emit at detectable levels. Fluorescence intensity modulation can be generated in both solutions and films of HPA-10-PPV, as shown in Figure 2. Here, the initial spectrum (solid line) represents polymer with azobenzene side chains in thermal equilibrium, which is virtually all in the trans isomeric state. Ultraviolet (UV) irradiation isomerizes some of the azobenzene side chains to cis, driving the system to a photostationary state (pss) with lower emission intensity in both solution and film (dotted line). Absorbance studies demonstrate that the decrease in fluorescence intensity tracks with the trans f cis azobenzene isomerization.22 We previously determined that the increased efficiency of energy transfer to the cis isomers is responsible for the observed decrease in emission intensity relative to the predominantly trans form of the polymer.24 Application of visible light drives the system back toward a photostationary state with predominantly trans side chains (dashed line); this state closely resembles the thermally equilibrated state. Visible irradiation induces the cis f trans back-isomerization via two mechanisms: primarily by sensitized isomerization due to energy transfer from the excited polymer with a smaller contribution from direct excitation of the cis isomer. Figure 2 demonstrates that the fluorescence modulation is virtually identical in magnitude in solution and film. This result is not necessarily expected and is discussed further below. Fluorescence studies of DH-10-PPV, a control polymer lacking the terminal azobenzene in the side chain (Scheme 2), were conducted to determine the extent to which HPA-10-PPV’s
emission properties depend on the presence of the azobenzene and/or its covalent attachment to the polymer backbone. The control polymer does not show intensity photomodulation in either solution or film, indicating that the presence of the azobenzene is required for this phenomenon to occur. DH-10PPV was doped with decyloxyazobenzene, an azobenzene model compound (Scheme 2), to facilitate comparison of doped and covalently attached azobenzene energy transfer acceptors. The quality of the doped films prepared from solutions of DH-10PPV and decyloxyazobenzene was extremely low as the two components experienced phase separation upon casting. Alternatively, films were cast from solutions of DH-10-PPV, optically transparent poly(methyl methacrylate), and decyloxyazobenzene. These films did not exhibit intensity modulation. However, their components were also visibly separated, reducing the impact of this result. Efforts to obtain a meaningful comparison between systems with doped and covalently attached azobenzene are ongoing. Intensity measurements of neat DH-10-PPV films were also performed to determine whether thermally equilibrated HPA10-PPV is substantially quenched relative to its control polymer, as is the case in solution. These preliminary measurements indicate that thermally equilibrated HPA-10-PPV is only slightly, if at all, quenched relative to control polymer DH-10-PPV. The quantum yields for films of PPV derivatives are typically much lower than solution due to chain-chain interactions.60 The presence of the bulky azobenzene side chain on each subunit in HPA-10-PPV may inhibit the chain-chain interactions that likely reduce the emission intensity of the control polymer. Many studies have shown an increase in film quantum efficiency with the addition of bulky side chain groups that inhibit close chain contacts.13,14,61-63 In this case, reduced chain-chain interactions in HPA-10-PPV may fortuitously compensate for intensity quenching due to energy transfer to the trans azobenzene side chain. Ideally, a polymer designed for intensity modulation would undergo a large change in fluorescence intensity between the two photostationary states. We have previously defined the modulation efficiency (Emod) as a means of quantifying the magnitude of fluorescence modulation according to eq 1:24
Emod ) 1 -
Icis Itrans
(1)
where Icis and Itrans are the peak emission intensities measured at the pss concentration of cis azobenzene side chains obtained upon UV irradiation and at the pss concentration of trans azobenzene isomers obtained upon visible irradiation, respectively. The polymer intensity observed after cis f trans backisomerization is used instead of the initial intensity to minimize the contribution of non-modulating intensity loss due to irreversible photobleaching. Hence, Emod as defined here is the fraction of maximal intensity that can be repeatedly lost and recovered. We find that HPA-10-PPV in film and solution exhibit essentially the same Emod value of 0.3 for optimal samples. Film samples with photobleaching problems, described
4428 J. Phys. Chem. C, Vol. 111, No. 11, 2007
Figure 3. (A) Fluorescence intensity of HPA-10-PPV at λmax over five cycles of isomerization induced by UV and visible irradiation in dilute THF solution (dashed line, open circles) and in a film cast from THF (solid line, closed squares). The points represent peak emission intensities at the photostationary state concentrations of cis or trans azobenzene, and the lines are merely guides for the eye. (B) Peak fluorescence (squares) and absorbance (triangles) of films cast from THF solutions with thermally equilibrated, trans azobenzenes (solid symbols and lines) and from films cast from THF solutions at the photostationary state concentration of cis azobenzenes (open symbols and dashed lines) over cycles of UV and visible irradiation. The solution from which the “initially cis” film was cast was UV-irradiated for 2 min immediately prior to spin coating. All data are normalized to the recovered intensity following the first visible irradiation period. The daggers mark points for which fluorescence spectra are shown in C and D. (C) Normalized partial fluorescence spectra from the initially trans (solid) and initially cis (dashed) films measured at the pss concentration of cis azobenzenes from the point marked with a dagger (†) in Figure 3B. (D) Spectra analogous to those in C but measured at the pss concentration of trans azobenzenes after visible irradiation from the point marked with a double dagger (‡) in Figure 3B. The spectra in C and D were renormalized to permit comparison of spectral shapes.
below, have Emod values closer to 0.2. In previous work we have demonstrated the ability to improve Emod in solution via chemical changes to the polymer structure,24 and we will continue to investigate this possibility in polymer films. In addition to a high modulation efficiency, the reproducibility of the modulation over many cycles is an important feature. Figure 3A shows the peak intensity of HPA-10-PPV emission over repeated cycles of UV and visible irradiation. The robustness of intensity photomodulation in the film (closed squares) approaches that in degassed solutions (open circles), with minimal loss of emission intensity. Like most PPV derivatives, HPA-10-PPV is extremely oxygen sensitive and can undergo rapid photobleaching, particularly during UV
Lewis and Harbron irradiation. For example, fluorescence intensity of a film shown in Figure 3B (closed squares) recovers to only 83% of its initial value and continues to lose intensity in each cycle, in contrast to the more stable film shown in Figure 3A. Comparison of fluorescence and absorbance data demonstrates that the intensity loss is due to polymer photobleaching rather than to a decline of azobenzene photoswitching capability. The azobenzene photoisomerization is extremely stable over many cycles, as shown by the peak absorbance of the trans azobenzene isomer during UV-visible cycling (Figure 3B, closed triangles). With improved oxygen exclusion techniques to protect the polymer backbone, emission intensity losses over time would be minimized. The emission of PPV films has been shown to be extremely sensitive to film morphology due to the strong influence of interchain species on emission properties.60 The nature and number of interchain contacts are affected by film preparation conditions including casting solvent and solution concentration and by thermal annealing of the film.60 Our standard procedure is to cast HPA-10-PPV films from thermally equilibrated solutions, which have essentially all trans side chains. Cis azobenzene side chains, however, have a significantly different size and shape, with the end-to-end distance between para carbon atoms decreasing from 9.0 Å in the trans isomer to 5.5 Å in the cis.26 Given this difference between trans and cis azobenzene side chains, it is possible that the isomeric state of the side chain during spin coating could influence film formation and, hence, emission properties. To probe this possibility, we conducted parallel absorption and fluorescence experiments on two sets of film samples that differed in their azobenzene isomeric state at the time of spin coating. One set of samples was cast normally from a solution of thermally equilibrated polymer with mostly trans azobenzenes and the other from a solution at the photostationary state concentration of cis azobenzenes, generated by UV irradiation immediately prior to spin coating. Figure 3B shows the absorbance and fluorescence behavior of these samples over several cycles of UV and visible irradiation. The fluorescence modulation of the films is identical: both films show the same Emod and photobleaching described above. Additionally, the spectra from which the intensities in Figure 3B were obtained do not reveal any quantifiable differences between the initially trans and initially cis films. Figure 3C,D shows normalized partial fluorescence spectra from the initially trans and initially cis films that correspond to the points marked with daggers in Figure 3B. The spectra show identical λmax values although the spectra from the initially cis film are slightly broader on both the blue and red edges than those from the initially trans film in both cases. The insensitivity of the fluorescence spectra and modulation behavior to initial side chain isomeric state implies that the shape of the azobenzene does not play an important role in film morphology. Alternatively, the concentration of cis azobenzenes at the UV-induced photostationary state could simply be too low to have a measurable influence on film morphology. Figure 3B also shows the independence of the azobenzene photoisomerization from the azobenzene isomeric state during film casting. As was the case with fluorescence modulation, the azobenzene isomerization process appears to be identical in the initially trans and initially cis films. The isomeric composition in the films can be estimated from the absorbance at the trans azobenzene λmax as follows: the absorbance of the thermally equilibrated (trans) and pss (cis) forms are measured, the spectra are corrected for the absorbance of the polymer backbone in the UV region, and the ratio of the normalized
PPV Emission in a Photochromic Polymer Film absorbances of the pss and trans forms at the trans λmax is calculated. As the absorbance of the cis isomer in this region is very small, this method provides a reasonable estimate of isomeric composition.64 According to this method, the UVinduced photostationary state in the films shown in Figure 3B is 19% cis. In contrast, the UV-induced photostationary state of HPA-10-PPV in THF solution is 76% cis (data not shown). It is conceivable that some azobenzene side chains are prevented from isomerizing in the film as the azobenzene isomerization requires free volume estimated to be in the 0.12-0.28 nm3 range.65 However, the identical azobenzene behavior observed in the absorbance of initially trans and initially cis films in Figure 3B negates this idea. An alternate explanation for the reduced cis concentration at the photostationary state in the film, relative to solution, is the energy transfer process itself. As observed for MPA-10-PPV22 and other bichromophoric systems36,66 in solution, the energy transfer sensitizes cis f trans azobenzene isomerization but not the trans f cis process. If the sensitization process is more efficient in the film, then the cis concentration would be limited compared to solution. It is intriguing to note that the same modulation efficiency (Emod ) 0.3) is observed in both solution and film in spite of the large difference in cis azobenzene concentrations. It has previously been demonstrated that a very small number of quenchers can efficiently quench an entire conjugated polymer chain due to exciton migration processes that can funnel the exciton to a quencher.3,67,68 In the case of HPA-10-PPV, a small number of cis azobenzene quenchers could have the same effect on emission intensity as a larger number. The fluorescence modulation behavior demonstrated above can be used to exert spatial control over emission intensity in a polymer film, which could have potential applications in optical data storage. Writing to and reading from a film can be accomplished with two light sources, one UV and one visible. Figure 4A shows a 30 × 30 µm image of a neat HPA-10-PPV film following UV irradiation to generate the photostationary state concentration of cis azobenzenes, i.e., the dim state. Select lines were irradiated with 488 nm light to induce cis f trans back-isomerization to the brighter state. As shown in Figure 4B, this effort yields a bright-on-dim image of the letter “H” in the film. The image is then erased by irradiating the entire image area with 488 nm light, producing an all-bright image (Figure 4C). Alternatively, it could be erased by UV irradiation, which would return the film to the state seen in Figure 4A. As seen in Figure 4A-C, the film is somewhat heterogeneous, with some regions consistently brighter than others. Figure 4D shows a background-subtracted image of the inscribed character that eliminates the film heterogeneity, thus enhancing the contrast of the character. While the diffraction limit of light prevents a photochromic polymer such as HPA-10-PPV from attaining extremely high data densities with conventional fluorescence microscopy techniques, the system has other useful qualities. First, it is inherently eraseable and rewriteable, allowing it to be used as live memory instead of write-once-read-many memory. Second, unlike most current data storage techniques, it does not necessarily have to store binary data. Photochromic polymers could be used to store analog signal directly by using spots of continuously varying intensity. In the case of HPA-10-PPV films, excitation of the polymer to read stored data also weakly induces erasure, which limits its usefulness in such applications. Nevertheless, these results serve as demonstration of the potential for photochromic conjugated polymers as a class to be used in data storage applications.
J. Phys. Chem. C, Vol. 111, No. 11, 2007 4429
Figure 4. Images of an HPA-10-PPV film (30 × 30 µm). (A) After 30 s UV irradiation to produce the dim cis azobenzene state. (B) After writing the character “H” by irradiating two vertical lines and one horizontal line for 30 s each with 488 nm light. (C) After additional whole-image 488 nm irradiation to erase the image. A watermark in the upper left corner can be used as a reference point in images A-C. (D) Background-corrected image produced by subtracting image A from image B. For images A-C, each pixel represents photons collected over 3 ms, and the scale bar correlates the color of each pixel to the number of fluorescent photons collected.
Conclusions We have demonstrated fluorescence intensity modulation in neat films of an azobenzene-functionalized PPV derivative, HPA-10-PPV. Polymer film emission is brighter when the azobenzene side chains are predominantly trans and dimmer when they are at the photostationary state concentration of cis isomer. On the basis of previous solution phase work, we attribute this phenomenon to a nonradiative energy transfer process that is more efficient to the cis isomer than to the trans. Thus, the azobenzene isomerization can be used as a photoswitch to control PPV emission intensity. The efficiency of intensity modulation in the film is comparable to that observed in solution in spite of the fact that far fewer cis azobenzenes are present at that photostationary state in the film than in solution. The low concentration of cis azobenzenes is not due to restricted free volume in the film but may be a result of efficient sensitization of the cis f trans back-isomerization by the energy transfer process. We have demonstrated spatial control of fluorescence intensity in this film by inscribing an image formed by intensity differences. These results further demonstrate potential for photochromic luminescent polymers as a class to find application in optical data storage and related areas. Acknowledgment. We gratefully acknowledge partial support of this work by the Camille and Henry Dreyfus Foundation through a Faculty Start-Up Grant (E.J.H.). S.M.L. thanks The College of William and Mary for a Student Research Grant and the Department of Chemistry for the Debra L. Allison Summer Fellowship. We thank Ryan D. Clayton and Stacy S. Tse for experimental assistance.
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