Chain Length and Substituent Effects on the Formation of Excimer

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Chain Length and Substituent Effects on the Formation of Excimer-Like States in Nanoaggregates of CN-PPV Model Oligomers Gizelle A. Sherwood, Ryan Cheng,† Kelly Chacon-Madrid, Timothy M. Smith,‡ and Linda A. Peteanu* Department of Chemistry, Carnegie Mellon UniVersity, 4400 Fifth AVe, Pittsburgh, PennsylVania 15213

Jurjen Wildeman Zernike Institute for AdVanced Materials, UniVersity of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands ReceiVed: January 19, 2010; ReVised Manuscript ReceiVed: May 23, 2010

The effects of aggregate formation on the photophysical properties of alkoxy and cyano-substituted polyphenylene phenylene vinylene oligomers (CN-PPVs) were studied in bulk solution to better understand the consequences of aggregation for the emission properties of the polymer. Nanoaggregates of oligomers from 5 to 13 repeat units in length were formed using a solvent reprecipitation method. The propensity for these aggregates to exhibit excimer-like emission in solution was found to be a strong function of oligomer chain length and the solvents used in the reprecipitation process. Short-chain oligomers produced nanoaggregates with absorption and fluorescence spectra and emission lifetimes essentially identical to those of the monomer. The aggregates of long-chain oligomers have broad and red-shifted emission spectra and relatively long emission lifetimes, both of which are characteristic of excimer states. However their absorption spectra are also perturbed suggesting that the oligomer chains in these aggregates interact strongly in their electronic ground states as well. For intermediate chain lengths, dual monomer-like (green) and excimer-like (red) emission is observed. Single aggregate dispersed emission spectra from aggregates deposited onto glass coverslips demonstrate that, in the absence of solvent, the predominant emitters are monomer-like rather than excimer-like. Moreover, the monomer-like emitters are found to be far more photostable than the analogous non-CN substituted aggregates, whereas the photostability of the excimer-like emitters is exceptionally poor under the illumination conditions used for microscopy. Comparisons between the properties of these nanoaggregates and the corresponding CN-substituted polymer are drawn. Introduction Ever since electroluminescence in a conjugated polymer was first reported by Burroughs et al.1 considerable effort has focused on optimizing both polymer structures and processing methods to enhance device performance. Cyano-substituted PPVs (e.g., CN-MEH-PPV, Chart 1) initially attracted a great deal of attention because their high electron affinities, relative to PPV itself, facilitate electron injection and permit electrodes with lower work functions and therefore higher stabilities to be used.2 Recently, efficient photodiodes and photovoltaic devices have been constructed in which CN-PPVs are used as the electrontransporting component of the heterojunction containing either noncyano-substituted PPV derivatives3,4 or polythiophene derivatives5 as the hole-transporting materials. One advantage to the CN-PPV-based systems in this application is that the excitations are believed to be predominantly inter-chain in nature6 meaning they are relatively long-lived and can therefore migrate efficiently between chains in the device. It had been suggested that these excitations can move rapidly to junctions * Corresponding author. Phone: 412-268-1327. Fax: 412-268-6897. E-mail: [email protected]. † Current address: Department of Chemistry, The University of Texas at Austin, 1 University Station A5300 Austin, TX 78712. ‡ Current address: Department of Biology, Chemistry and Environmental Studies (BCES), Christopher Newport University, 1 University Place, Newport News, VA 23606.

where charge separation is favored, enhancing the production of free charges and therefore the efficiency of the photovoltaic device.7,8 Despite their high fluorescence yields and desirable redox properties, one impediment to the use of CN-PPVs, particularly in light-emitting diodes (LEDs), has been that their emission spectra, and therefore the electronic character of their excited states, are more sensitive to processing conditions than those of non-CN-substituted PPVs. Specifically, the emission of CNPPV in the solid state or in a poor solvent exhibits a substantial Stokes shift (∼5000 cm-1) and diminished vibronic resolution as compared to the polymer in a good solvent such as toluene.9,10 Samuel and co-workers proposed that the broad red-shifted emission characteristic of CN-PPVs is due to an interchain excitation which is most probably excimer-like in nature.6,11 An excimer state is one which is bound in the excited state but unbound in the ground state. This is distinguished from an aggregate in which the chains also interact in the ground state, often resulting in a perturbed absorption spectrum. Moreover, it differs from a charge-transfer state or polaron pair in that these involve respectively a partial or complete separation of charge from one chain to an adjacent chain or chain segment. Excimers have relatively long radiative lifetimes of several nanoseconds, consistent with the forbidden nature of the transition from the bound excited state to the unbound ground state. As a result, nonradiative pathways compete effectively

10.1021/jp100517n  2010 American Chemical Society Published on Web 06/29/2010

Nanoaggregates of CN-PPV Model Oligomers

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CHART 1: CN-PPV Oligomer and Polymer Structures

with the radiative decay, reducing the excimer fluorescence yield relative to that of the isolated chain. These features are often associated with other interchain excitations (aggregate, polaron pair, and charge-transfer state) as well. In the case of an aggregate, a forbidden state can also arise from out-of-phase combination of the transition moments of two adjacent molecules (H-aggregate). In this case, however, a blue shift of the absorption band will occur.6,10 Surprisingly, the emission yields of interchain excitations in CN-PPV are relatively high, nearly equaling those of the well-solvated polymer6 despite their long radiative lifetimes. Conwell and co-workers previously suggested that these excimers are highly emissive due to their very short interchain distances which favor radiative over nonradiative decay.12 Samuel et al. attributed this high yield to interchain excitations, such as excimers, being less mobile than intrachain excitations and therefore less likely to encounter a quench site before emitting.6 Though there is some indirect evidence that exciton migration is rapid in the aggregated polymer, this remains an open question as the dynamics of energy transfer in CN-PPVs has received less attention in the literature than that in other PPV-like molecules.7,13 Measurements on oligomers and oligomer aggregates are often very useful in unraveling the complex photophysics of the polymer system.14–19 Earlier work demonstrated that the molecular structure and the photophysics of CN-PPVs and their oligomers are very sensitive to the precise position of the CN substituent on the PPV backbone.14–19 In fact, the fluorescence yields and lifetimes of CN-PPV oligomers differ by over an order of magnitude depending on the degree of steric interaction between the CN and alkoxy substituents.15,16 In addition, many investigations of the intermolecular interactions in this class of molecules have focused primarily on excimer formation in the polymer and in well-defined crystals of 5-ring oligomers.16 These

studies were especially powerful in establishing the link between packing structure, as measured by X-ray diffraction (XRD), and the propensity for excimer formation. The current study focuses on how the spectral and lifetime perturbations induced by aggregation depend on oligomer chain length and substitution pattern in CN-PPV oligomers. Our ultimate goal is to understand the spectral and dynamical effects of interchain interactions on the polymer form, particularly as it aggregates during the casting of films from solution. The current work differs from previous studies in two important regards. First, we focus on the properties of oligomer aggregates formed in solution and deposited on solid substrates rather than on the properties of well-ordered crystals.16 Second, the oligomers studied here range in length from 5-13 repeat units while published work has thus far focused primarily on shorter (3-5 ring) systems.14–19 Both here and in our previous work on MEHPPV oligomer aggregates,20 it is clear that the spectral properties of the longer-chain oligomer aggregates differ in important ways from those of the corresponding shorter-chain systems. Moreover, aggregates formed by longer-chain oligomers best mimic the spectral properties and emission dynamics of the aggregated polymer. The aggregates studied here were formed by addition of a poor solvent to a solution of the monomeric oligomer in a good solvent and their size distributions were characterized via dynamic light scattering (DLS). Two solvent pairs were used: 2-methyltetrahydrofuran (MeTHF)/methanol (MeOH) and tetrahydrofuran (THF)/water. In general, THF/water tends to be more strongly aggregating than MeTHF/MeOH, probably because of the larger polarity difference between THF and water. This manifests itself in the emission properties of the aggregates formed and the strength of the chain-chain interactions within them as will be shown below. A variety of oligomer chain

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lengths and substitution patterns were studied (Chart 1) to correlate photophysics and molecular geometry. Both timeresolved and steady-state fluorescence measurements were used to assign the emitting state as an intra- versus interchain exciton and the degree of heterogeneity among aggregates was probed via measurements of the dispersed emission spectra of single aggregates using fluorescence microscopy. As we have seen previously for non-CN oligomer PPVs (OPPVs),20 the spectral perturbations due to aggregation depend strongly on oligomer chain length. Moreover, the details of the substitution pattern are critical to determining whether or not interchain emission is seen, presumably due to the sensitivity of this process to interchain distances and chromophore packing. Interestingly, all of the longer-chain aggregates that exhibit red-shifted emission, similar to that assigned to an excimer state in the CN-PPV polymer, also exhibit perturbed absorption spectra. Therefore, the species formed appear not to be classical excimers in that this term implies that the chains interact strongly only in the excited state, not in the ground state. This finding forces a reconsideration of the earlier assignment of the red-shifted emission in these molecules as being due to the formation of an excimer state.6,12 Experimental Section The oligomers were synthesized as described in ref 21. The aggregates were prepared via a reprecipitation method in which volumes between 20 and 1000 µL of a 10-6 M stock solution of each oligomer in the good solvent (MeTHF or THF) were added to 1 mL of the nonsolvent (MeOH or water, respectively). Their size distributions were determined by DLS (Malvern Zetasizer Nanoseries ZS). For shorter-chain oligomers (∼5 rings), aggregates with a mean size of ∼200 nm were formed independent of the starting volume of the good solvent used. However, with the longer-chain oligomers (7-13 rings), the mean aggregate size can be varied from ∼200-1000 nm depending on the initial volume of the stock solution used. Specifically, increasing the volume of the stock solution increases the mean aggregate size in a nearly linear fashion. For the longer-chain oligomers, the aggregate sizes formed and hence their electronic properties depend both on the initial concentration of the stock solution and the volume used. Therefore, both parameters are specified when describing these spectra. Prior to and following the addition of the nonsolvent, the absorption and emission spectra of the samples were measured. The absorption spectra were obtained (Cary 50, Varian) using both conventional transmitted light detection and a second detector assembly that minimizes the distance between the cuvette and the detector so as to decrease the losses due to scattered light. This was done due to a concern that scattering could distort the spectral shape and intensity of the absorption spectrum of the aggregate suspensions. As the two methods were found to yield nearly identical results, only the spectra measured using the conventional optics for transmitted light are reported here. The monomer and aggregate emission spectra were obtained at the absorption maximum of the monomer. To facilitate comparison of the emission intensities of the monomer and the aggregate, the emission intensity of the aggregate suspensions were scaled to account for any differences observed in the extinction of the aggregate versus the monomer at this wavelength. Because of the methodology used to form these aggregates, it is possible that residual unaggregated monomer also contributes to the spectrum of the bulk suspension. To help identify

Sherwood et al. the spectral contribution due specifically to aggregates, each suspension was filtered with a 200 nm pore size syringe filter, and the absorption and spectra of the eluant was compared to those obtained prior to filtration. The emission spectrum of the eluant is invariably monomer-like and its intensity, relative to that of the initial suspension, gives a measure of the strength of chain-chain interactions in the aggregate. This process is referred to in the Results section below as the filtration experiments. We have previously shown that this process provides a qualitative gauge of the strength of the intermolecular interactions in oligomer aggregates. This is because the filtration process not only removes aggregates that are larger than the pore size from the initially formed suspension but, in some cases, appears to resolubilize weakly bound aggregates or otherwise shift the equilibrium between their monomeric and aggregated state in the eluant.20 In addition, the spectrum of the aggregates which do not pass through the filter pores may be inferred by the difference between the spectra of the initial suspension and that of the eluant. For fluorescence microscopy, the monomer solutions and aggregate suspensions were deposited onto a glass coverslip which was previously treated by briefly passing it through the flame of a butane torch to remove surface impurities. The morphology of the samples proved highly sensitive to the deposition method used. Spin-casting both the monomer and aggregate solutions (3000-10 000 rpm) yielded a uniform film with no evidence of aggregates. However, as drop casting the aggregate solutions onto the slides preserved identifiable aggregate structures, this sample preparation method was used to obtain the spectra and images shown here. This is contrary to what was previously observed regarding aggregates of non-CNsubstituted PPV oligomers which formed readily identifiable structures when either method was used.20 The casting methods used here were not successful with the THF/water suspensions due to the low rate of evaporation of the water. Therefore, all of the single-aggregate data shown here was obtained from aggregates made from MeTHF/MeOH mixtures. To obtain steady-state fluorescence images, samples were excited using the 458 nm output of an argon ion laser (BeamLok 2060, Spectra-Physics) with a power of 25 µW at the back of the objective. They were imaged onto a CCD detector (Cool Snap HQ) using an inverted IX-71 Olympus microscope in through-objective total internal reflection (TIRF) mode (Olympus PlanApo 60X oil immersion objective (N.A. ) 1.45)). A 465 nm long pass filter is used to diminish background due to scattered light. For some oligomers, both red and green emissive aggregates were formed on the coverslip that could be examined separately using appropriate filters. Fluorescence lifetimes of individual aggregates were obtained by directing the output of a pulsed diode laser at 437 nm (LDH-PC-440, PicoQuant GmbH) into the back port of the microscope, using an iris to restrict the field of view to a single aggregate, and detecting the emission through band-pass filters (Chroma) via an actively quenched photodiode (SPD-5-CTC, Micro Photon Devices) and time correlated single photon counting electronics (Picoharp 300, Picoquant) in place of the CCD detector. These components were reconfigured to obtain bulk solution-phase lifetimes of the monomeric and aggregated species as well. Dispersed fluorescence spectra of the aggregates were obtained by selecting wellseparated aggregates to image using an iris to restrict the TIRF field in the microscope and directing the emission from the same output port of the microscope used for imaging through a transmission grating (300 g/mm, Edmunds Scientific) and onto the CCD detector. When the CCD is in position for imaging,



Figure 1. Monomer (red) and aggregate (blue) absorption and emission spectra of (a) O2P5V4RCN2 and (b) O2P5V4βCN2. Aggregates are a bulk suspension in a MeTHF/MeOH solvent mixture. Emission spectra are scaled to the peak absorbance.

TABLE 1: Photo-Physical Parameters of Monomeric CN-OPPVs and CN-MEH-PPV and of Their Aggregates in MeTHF/ MeOH Solution molecule O2P5V4RCN2 O2P5V4βCN2 O4P5V4CN2 O4P7V6CN2 O12P13V12CN2 CN-MEH-PPV

monomer extinction coefficienta

monomer quantum yield, φ

99 000 ( 9000 50 000 ( 2000 60 000 ( 4000 109 000 ( 5000 408 000 ( 10 000

0.03 ( 0.01 0.70 ( 0.10c 0.017 ( 0.008 0.135 ( 0.008 0.13 ( 0.01 c

monomer lifetime, τb 0.02 1.6c 0.18 0.5 0.9 0.8

c

aggregate lifetimes, τb 0.12/0.66 (80/20)d 1.3 0.22 0.27 (green)e; 1.0/2.7 (80/20) (red)f 4.5 0.6/2.9 (80/20)

In LM-1 cm-1. b In ns. c Values are consistent with those in ref 30. d Percentages refer to amplitudes of each lifetime component. Emission obtained using 540/50 nm band-pass filter (see text and Figure 7b). f Emission obtained using 680/30 nm band-pass filter (see text and Figure 7d). a

e

the zeroth order emission hits the detector such that an image showing the spatial distribution of aggregates can be recorded. Displacement of the CCD laterally by a small amount allows the first order emission, which appears as a single horizontal line for each aggregate imaged, to be recorded. The dispersion of the grating is calibrated by imaging the emission of two different diameter quantum dots having distinct and well separated emission maxima (515 and 605 nm). This step is repeated each time the experiment is performed. The dispersion is then used to convert the abscissa of the line spectrum from pixel number to wavelength. The results were verified by comparison to spectra obtained on a commercial scanning confocal instrument (Leica TCS SP5 100× objective 1.4 NA in XY Lambda scan mode with photomultiplier detection). The spectral results from both instruments were similar for photochemically stable aggregates though the TIRF system proved superior for nonphotostable species because of the speed with which spectra can be obtained. For aggregates of any given oligomer type and size distribution, the spectra obtained from each individual aggregates showed remarkably little variation in peak wavelength and overall shape. Therefore only the averaged spectra are presented here. Results This section is divided into three parts. First, the effects of R-CN versus β-CN substitution of a 5-ring oligomer on the emission properties of the corresponding aggregates are described. Next, results on a series of 5-, 7-, and 13-ring oligomer aggregates are presented to explore trends in aggregate photophysics with chain length and aggregate size. The effects of altering the aggregate solvent mixture from MeTHF/MeOH to the more strongly aggregating mixture THF/water are also

described. Finally, comparisons to monomeric and aggregated CN-MEH-PPV are drawn. Here we use the adjective “monomerlike” to describe all emission similar in wavelength and lifetime to that of the isolated monomer and “excimer-like” to describe all emission that is red-shifted, has essentially no vibronic structure, and has a fluorescence lifetime longer than that of the monomer. However, as will be shown below, our results suggest that such emission arises instead from aggregate states in which there are significant chain-chain interactions in the ground state as well. Figure 1 contains both the monomer and aggregate absorption and emission spectra for R-CN- and β-CN-substituted OPPV5, respectively (O2P5V4RCN2 and O2P5V4βCN2) precipitated from a MeTHF/MeOH solvent mixture. The absorption spectrum of the R-CN-substituted OPPV5 monomer (λmax ∼ 355 nm) is blueshifted relative to that of the β-CN-substituted form (λmax ∼ 445 nm) and shows far less vibronic structure, consistent to what has been previously reported by van Hutten et al.16 These spectral effects have been attributed to steric hindrance between the R-CN groups and the alkoxy groups causing a reduction in the planarity of the R-CN form relative to the β-CN form and a consequent increase in spectral congestion. In addition, the R-CN form shows a significantly faster fluorescence decay and a smaller quantum yield (Table 1) than does the β-CN form. Both effects are consistent with a more efficient nonradiative decay process to the ground state.15,16 Changing the solvent mixture to THF/water does not noticeably change the aggregate emission characteristics of O2P5V4RCN2 (data not shown). Though the emission spectrum of O2P5V4RCN2 appears unchanged on aggregation, the fluorescence dynamics are clearly altered (Table 1). Whereas the monomer exhibits a singleexponential fluorescence decay with a very short lifetime (∼0.1

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Figure 2. (a) Averaged single aggregate emission spectrum (black) of O2P5V4RCN2 deposited onto a glass coverslip from a MeTHF/MeOH suspension. The bulk emission of monomer (red) and aggregate (blue) in MeTHF/MeOH are shown for comparison. (b) Evolution of the single aggregate emission spectrum with time as indicated by the arrow. The half-life (t1/2) is ∼100 s.

ns), the aggregate shows two lifetime components (0.12 and 0.66 ns). The dominant component (80%) is indistinguishable from that of the monomer given our time resolution while the smaller component is significantly longer. A suggestion as to the origin of the long-lived component comes from published studies of O2P5V4RCN2 in a solid matrix by Oelkrug and coworkers.15 These authors find that restricting the molecular motion of this compound leads to a substantial increase in the fluorescence lifetime and reduction in the nonradiative decay rate. By analogy, the longer-lived component in the fluorescence decay of the aggregates may arise from either subpopulation of aggregates in which molecular torsions are frozen out due to stacking interactions or regions within all the aggregates in which this is the case. Likewise, the shorter-lived component would correspond to chains that are less closely packed and therefore more monomer-like. Note that, for these aggregates, an increase in fluorescence lifetime is observed with no concomitant red shift or broadening of the emission spectrum. Therefore, we presume that the longer lifetime in these aggregates is not due to excimers or other interchain aggregate states but rather to increased planarization of the molecule in the aggregate. The results of filtration experiments on the O2P5V4RCN2 aggregate suspensions are consistent with the existence of two types of emissive aggregates having different interchain interactions. Following filtration, only ∼60% of the emission intensity is recovered in the eluant suggesting that the remaining ∼40% can be ascribed to emissive aggregates excluded by the filtration process. As argued earlier, aggregates that are trapped by filtration presumably contain relatively tightly packed chains that are not readily resolublized whereas the emission in the eluant is attributable to small aggregates, free monomer chains, and aggregates that have been disrupted in the filtering step. The single-aggregate emission spectra of O2P5V4RCN2 cast from MeTHF/MeOH shows only a green species with an emission spectrum centered at ∼550 nm (Figure 2a). The ∼30 nm red-shift in the maximum of this spectrum relative to the bulk is consistent with that seen by van Hutten et al. in going from solution phase to single crystals of O2P5V4RCN2.16 Time lapse images (Figure 2b) of the single aggregate emission spectrum of O2P5V4RCN2 show that these species are remarkably photostable (half-life (t1/2) ∼ 60s). Like the R-CN-substituted isomer, O2P5V4βCN2 aggregated in MeTHF/MeOH forms particles (∼200 nm) having absorption and fluorescence spectra nearly identical to the corresponding

Figure 3. Monomer (0%, red line) and aggregate emission spectra of O2P5V4βCN2 in a bulk suspension of THF/water as a function of increasing the percentage of water.

monomer though with ∼50% loss in emission intensity (Figure 1b). Despite this diminished yield, the fluorescence lifetime of O2P5V4βCN2 aggregates in solution is essentially identical to that of the monomer. This suggests that a substantial number of nonemissive species are formed within the aggregate suspension but that the emissive aggregates have quantum yields similar to that of the monomer. Filtration of the suspension with a 200 nm filter (see the Experimental Section) shows that over 95% of the emission intensity of the aggregates is recovered in the eluant (Supporting Information). Therefore the emissive species are aggregates smaller than the pore size and/or those easily dissociated by the filtration process. From this we conclude that the emissive aggregates in O2P5V4βCN2 consist of primarily weakly interacting chains. Weak chain-chain interactions in the aggregates would also be consistent with the fact that their spectra and fluorescence lifetimes are identical to those of the monomer itself. In contrast, any aggregates that are excluded by the filter are presumably much more weakly emissive and less susceptible to dissociation, suggesting they consist of more strongly interacting chains. Figure 3 shows the effect on the emission spectrum of aggregating O2P5V4βCN2 using a THF/water mixture. When the amount of water in the mixture is less than ∼30% the spectrum is again monomer-like. There is a notable shift in the absorption and emission spectra for higher percentages of water in the mixture. The spectrum of the aggregates formed at higher water to THF ratios appears intermediate between that of the monomer and what would be expected from an excimer-like state. That



Figure 4. (a) Averaged single aggregate emission spectrum (black) of O2P5V4βCN2 deposited onto a glass coverslip from a MeTHF/MeOH suspension. The bulk emission of monomer (red) and aggregate (blue) in MeTHF/MeOH are shown for comparison. (b) Evolution of the single aggregate emission spectrum with time as indicated by the arrow. The t1/2 is over 2 s. (c) Evolution of the spectrum obtained by scaling the long time (1.8 s) spectrum in (b) to the peak height of each of the earlier time spectra in (b) and taking the difference. This shows the rapid photobleaching of an excimer-like band with a t1/2 of ∼100 ms.

Figure 5. Monomer (red) and aggregate (blue) absorption and emission spectra of O4P5V4CN2. Aggregates were formed in (a) MeTHF/MeOH and (b) THF/water. .Aggregates are in bulk suspension in a MeTHF/MeOH solvent mixture. Emission spectra are scaled to the peak absorbance.

is, the aggregate spectrum is broader than that of the monomer and there is a loss of intensity at the 0-0 band and an enhancement of the red edge. The emission spectrum of single O2P5V4βCN2 aggregates cast from MeTHF/MeOH suspensions onto glass coverslips is similar to that in bulk suspension (λmax ∼ 540 nm) though with substantially reduced vibronic structure (Figure 4a). Similar spectral broadening is seen in all of the CN-substituted oligomer aggregates studied here though its cause remains unclear. The similarity of the wavelength maximum of this spectrum to that of the solution-phase monomer suggests that predominantly intrachain excitons are formed in O2P5V4βCN2 aggregates, consistent with a structure containing mostly weakly packed chains.22 In contrast to what is seen in bulk suspension, the timelapse single-aggregate emission spectrum (Figure 4b,c) actually contains two distinct spectral components. The emission maximum shifts to higher energies with time (Figure 4b) and difference spectra, generated by subtracting the final spectrum (t ) 1900 ms) from each of the other traces (Figure 4c) show a broad, weak, and red-shifted emission component (λmax ∼ 610 nm) not seen in the aggregate spectra obtained in bulk suspension. These properties are consistent with assignment to an excimer-like state that is significantly less photostable than the predominant green-emitting aggregate. In fact, this spectrum is very similar in its lack of vibronic structure and its emission maximum to that reported by van Hutten et al. for O2P5V4βCN2 in crystals and annealed films.16 These authors assign the emission they observe to an excimer state by virtue of its long fluorescence lifetime (8 ns) and hypothesized that it is favored

by π-stacking of the oligomer chains.16 Unfortunately, we were unable to obtain lifetime data on the red-emitting species observed here due to its poor photostability under imaging conditions. Interestingly, though excimer emission is predominant in bulk crystals and annealed films of O2P5V4βCN2,16 it is a minor component of the emission spectrum from the aggregates formed here, demonstrating that aggregates formed by solution reprecipitation favor different packing arrangements than those in the crystal or annealed films. Though the spectral and dynamical properties of the monomers and their aggregates are clearly sensitive to large differences in steric interactions such as those between R- and β-CN substituted oligomers, even more subtle changes in the substitution patterns produce noticeable effects. An example of this is O4P5V4CN2 which differs from O2P5V4βCN2 in the substitution pattern of the alkoxy groups relative to the central ring (Chart 1). This molecule has similar bulk absorption and fluorescence properties as O2P5V4βCN2 (Figure 5a) though it has a considerably smaller fluorescence yield (0.02 versus 0.7) and a much shorter lifetime (0.18 versus 1.4 ns, Table 1). These differences indicate that the nonradiative rates are highly sensitive to substitution patterns even in molecules for which the CN group has the same configuration (β-substituted in this case) relative to the alkoxy group. The origin of this trend is not known though it likely reflects a difference in the degree of planarity between the two oligomers. The effects of aggregation on O4P5V4CN2 in MeTHF/MeOH as measured in bulk suspension are very similar to those described earlier for O2P5V4βCN2. Specifically, the emission lifetime is unchanged on aggregation (Table 1)

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Figure 6. (a) False color TIRF image of red- and green-emitting O4P5V4CN2 aggregates obtained by merging images obtained through a 680/30 nm bandpass and a 540/50 nm bandpass filter, respectively. (b) Averaged single aggregate emission spectrum (black) compared spectra of monomer (red) and aggregate (blue) in bulk MeTHF/MeOH suspension. (c) Time evolution of single aggregate emission spectrum. The arrow shows the increase in intensity with time (see text).

though the yield drops ∼50% which suggests that two subpopulations of aggregates are formed. Again, we expect that the emissive aggregates consist of monomer-like or weakly interacting chains while the remaining subpopulation is essentially nonfluorescent. The results of filtration experiments, which were essentially the same as those obtained for O2P5V4βCN2, also suggest that the emissive aggregates are weakly bound (Supporting Information). Changing the solvent mixture to THF/water has a dramatic effect on the emission properties of the O4P5V4CN2 aggregates formed. Specifically, their emission is now entirely excimerlike (Figure 5b) and fairly intense. Moreover the absorption spectrum develops a tail at lower energies. We hypothesize that the excimer-like emission arises because stronger chain-chain interactions are formed in the THF/water aggregates than in the MeTHF/MeOH aggregates. Consistent with this is the fact that the eluant obtained by filtering the THF/water suspension is nonemissive meaning that essentially all of the oligomers are trapped by the filtration process. Therefore filtration does not appear to dissociate the aggregate structures made from this solvent pair (Supporting Information). Upon casting O4P5V4CN2 aggregates from MeTHF/MeOH onto glass coverslips for fluorescence microscopy, two species are observed with distinctly different emission wavelengths. Though the image shows regions of bright red aggregates (Figure 6a) (λmax ∼ 675 nm) the emission spectrum of the predominant form (λmax ∼ 540 nm) is essentially identical to that of the aggregate in fluid suspension but with reduced vibronic structure (Figure 6b). The spectral properties of the red aggregates suggest they are an excimer-like species and fluorescence lifetime measurements on the single aggregates confirm this interpretation (data not shown). Both aggregates exhibit dual exponential lifetimes but the green aggregates show somewhat faster decay components (0.75 and 1.96 ns) than do the red aggregates (1.1 and 3.9 ns; Table 1). Unlike O2P5V4βCN2, both the red and green aggregates of O4P5V4CN2 are highly photostable (Figure 6c), and remarkably, the emission intensity initially appears to increase at early times, saturating in intensity after ∼6 s. A similar phenomenon was previously reported by Kuzuoka et al. in various CN-substituted PPV polymers and attributed to the photobleaching-induced loss of a site that serves as an acceptor or “trap” of the aggregate emission.23 However, as the red and green emissions increase simultaneously, the trap would have to be yet another species not directly observed in our experiments. In summary, not only are the photophysical properties of these short CN-substituted PPVs very sensitive to the precise position

of the CN groups on the PPV backbone structure but also the same appears to be true of their aggregates, particularly in the solid state. More work is clearly needed to understand the link between theses photophysical effects and presumed differences in the packing structures of these aggregates. As the chain length of the oligomers increases, their propensity to form excimer-like aggregates also increases. Aggregates formed in MeTHF/MeOH from the O4P7V6CN2 oligomers show emission that is intermediate between monomerlike and excimer-like depending on the aggregate size (Figure 7). Smaller aggregates (200 nm) have broad red-shifted emission spectra and an absorption spectrum that contains a new low-energy transition at ∼575 nm not seen in the monomer. The red emission shows well-resolved vibronic bands spaced at ∼1270 cm-1 which is consistent with a progression in the CdC double bond stretching mode. Because the emission spectrum of the small aggregates is essentially identical to that of the monomer, we suggest that these contain predominantly weakly interacting chains. This is supported by filtration experiments which show that over 90% of the emission intensity is recovered in the eluant (Supporting Information). However, the emission properties of this molecule are not entirely unperturbed by aggregation. The fluorescence lifetime of these aggregates (0.27 ns) is nearly a factor of 2 shorter than that of the monomer (Table 1), indicating the opening of a new nonradiative pathway. Interestingly, this is the only species studied here in which the lifetime decreases upon aggregation. When the suspension containing predominantly large redemitting O4P7V6CN2 aggregates is filtered, essentially all of the emission is lost from the eluant (Supporting Information). This is again consistent with the larger average size of these aggregates and also indicates that they are strongly bound. Strong chain-chain interactions are also indicated by significant perturbation seen to their emission spectra and fluorescence decay properties. The fluorescence decay of the larger aggregates in bulk suspension contains two components, one (80% amplitude) with a lifetime of 1 ns and a minor component with a 3 ns lifetime. Both components are significantly longer than the emission decay of the monomer (0.5 ns) but shorter than typical excimer lifetimes in CN-PPVs.6,7,9,10,24 As before, more classically excimer-like aggregates are formed when O4P7V6CN2 is aggregated in THF/water (Figure 8). The lifetime of this species is biexponential (1.6 and 5 ns) with the contribution of the longer lifetime component increasing



Figure 7. O4P7V6CN2 in MeTHF/MeOH. Small aggregate preparation: (a) DLS and (b) aggregate absorption and emission spectra (blue) compared to the monomer (red). Large aggregate preparation: (c) DLS and (d) aggregate absorption and emission spectra (blue) compared to the monomer (red).

Figure 8. Absorption and emission spectra of excimer-like aggregates of O4P7V6CN2 formed in THF/water solvent mixtures (blue). The corresponding monomer spectra are shown for comparison (red).

as the proportion of THF in the solvent mixture increases (data not shown). The emission spectrum of the eluant obtained by filtering these aggregates is again nonfluorescent indicative of strong chain-chain interactions in this species. When aggregates of O4P7V6CN2 are cast from MeTHF/MeOH into the solid state (Figure 9), the emission spectra of the small and large aggregates are essentially identical and intermediate in wavelength maximum between the monomer-like and excimer-like bands observed in bulk suspension. In contrast to what has been seen thus far, the aggregates of the longest-chain oligomer, O12P13V12CN2, show exclusively excimer-like emission in bulk suspension in both solvent

Figure 9. Averaged single aggregate spectrum of O4P7V6CN2 (black) compared to the monomer (red) and aggregate (blue) spectra in bulk suspension. Though a large aggregate preparation is shown, the single aggregate spectra of the small and large aggregates are essentially identical (see text).

mixtures (Figure 10). In MeTHF/MeOH a reduction in the emission yield and a relatively long fluorescence lifetime (4.5 ns; Table 1) is observed. In THF/water solvent mixtures, the emission yield is higher and the lifetime is biexponential exhibiting a shorter (1.6 ns) and longer component (5 ns) with roughly equal amplitudes (data not shown). As was seen for O4P7V6CN2, both aggregates show a broadened absorption spectrum along with a weak tail in the red (Figure 10). Not surprisingly, filtration experiments are consistent with the picture that this excimer-like species contains closely packed chains

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Figure 10. Aggregates of O12P13V12CN2 formed from (a) MeTHF/MeOH and (b) THF/water solvent mixtures. Aggregate spectra are shown in blue and monomer spectra are shown in red.

Figure 11. (a) False color TIRF image of red- and green-emitting O12P13V12CN2 aggregates obtained by merging images obtained through 700/75 nm and 540/50 nm bandpass filters, respectively. (b) Averaged single aggregate emission spectrum (black) compared to spectra of the monomer (red) and aggregates (blue) in MeTHF/MeOH suspension.

(Supporting Information) in that the eluant contains

Chain Length and Substituent Effects on the Formation of Excimer

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