Article pubs.acs.org/JPCB
Aqueous Colloidal Acene Nanoparticles: A New Platform for Studying Singlet Fission Joseph N. Mastron, Sean T. Roberts, R. Eric McAnally, Mark E. Thompson, and Stephen E. Bradforth* Department of Chemistry and the Center for Energy Nanoscience, University of Southern California, Los Angeles, California 90089, United States S Supporting Information *
ABSTRACT: Singlet fission is a process that occurs in select molecular systems wherein a singlet excited state divides its energy to form two triplet excitations on neighboring chromophores. While singlet fission has been largely studied in molecular crystals, colloidal nanoparticles offer the ability to investigate fission using liquid suspensions, allowing questions regarding the importance of molecular arrangement and charge transfer to be assessed. Herein, we report the synthesis of aqueous colloidal nanoparticles of 5,12-diphenyltetracene (DPT), a material recently demonstrated to undergo singlet fission in disordered films. Upon synthesis, nanoparticles display absorption features that lie between those of monomeric DPT and disordered DPT films. These features evolve over a few days in a manner that suggests an increase in the degree of association between neighboring molecules within the nanoparticles. Transient absorption and time-resolved emission experiments indicate that photoexcited DPT nanoparticles undergo fission, but produce a lower triplet yield than disordered films.
I. INTRODUCTION Singlet fission (SF) refers to the ability of certain molecular materials to divide the energy of an excited singlet electronic state to form two lower energy triplet excitations on neighboring chromophores.1 More than a mere spectroscopic curiosity, SF provides a potential avenue toward the development of organic optoelectronics that benefit from exciton multiplication. Detailed balance models predict that SF can be used to boost the photocurrent of organic photovoltaics, leading to devices with limiting efficiencies above 45%.2,3 Such predictions are bolstered by the demonstration of pentacenebased photodetectors4 and photovoltaics5 that have demonstrated quantum efficiencies over 100% as well as recent twophoton photoemission experiments that have suggested that photoexcitation of a single electron in either pentacene6 or tetracene7 can lead to two-electron transfer to C60. However, for molecular devices to maximally benefit from SF, compounds are needed that display triplet yields near 200%. While the number of reported SF materials with triplet yields above 100% has steadily grown to encompass a number of acenes,8−10 carotenoids,11 and biradicaloids,12 the design space afforded by these materials remains small compared to the larger library of materials currently available for the design of organic optoelectronics. One factor that hinders the design of new SF materials is a lack of understanding regarding the key molecular parameters that control the fission process. While SF is a spin-allowed process since the formed triplet pair state, 1(TT), is spin correlated and of overall singlet character,1,13,14 the microscopic mechanism that leads to the formation of this state from the initial singlet excited state, S1, remains a topic of debate. One © 2013 American Chemical Society
potential scenario involves a pair of sequential electron transfers between an excited molecule and an unexcited neighbor, necessitating the involvement of an intermediate charge transfer (CT) state that can participate either as a real intermediate15,16 or as a virtual state through which the S1 and 1(TT) states are coupled.17−19 An alternate mechanism suggests that the S1 state can directly transition to the 1(TT) state without the need for intermediates due to a direct two-electron coupling between the S1 and 1(TT) states, which may be facilitated by motion along the S1 surface toward a conical intersection.20−22 Currently, many efforts to identify new SF materials focus solely on locating materials that fit the thermodynamic criterion that E(S1) ≥ 2E(T1).23,24 While this energy constraint is clearly central to the design of SF materials, understanding the mechanism by which SF proceeds can suggest additional criteria by which to screen candidate materials, such as the energy of CT states or degree of interchromophore coupling. Unfortunately, definitive experimental evidence showing the predominance of either mechanism remains elusive. Transient absorption experiments of polycrystalline pentacene films have shown that the time scales for the decay of singlet excitons and the rise of triplets are nearly identical, ∼80 fs,25 implying that singlets can directly evolve to a triplet pair. Likewise, twophoton photoemission experiments of pentacene6 and tetracene26 thin films have suggested that the S1 and 1(TT) states are directly coupled in acene films. However, the magnitude of the two-electron coupling strength needed to Special Issue: Michael D. Fayer Festschrift Received: June 12, 2013 Published: June 21, 2013 15519
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NPs’ UV absorption band broadens and changes in amplitude over a few days following synthesis, indicating a restructuring of the NPs that we associate with an increase in the interaction between neighboring DPT molecules. A combination of transient absorption and time-resolved emission experiments indicate that fully developed NPs indeed undergo SF. To the best of our knowledge, our work represents the first time that SF has been reported in acene NPs. Comparison of the SF kinetics between NP and thin film samples also provides some insight into the SF process. NPs are found to display a triplet yield (75%) that is just over half of that observed for thin films, suggesting that, due to enhanced disorder within the NPs, less DPT dimer pairs adopt configurations conducive to SF. Nevertheless, the forward SF rate displayed by NPs remains remarkably close to that of neat films, indicating that the aqueous environment that surrounds the NPs does little to speed the SF rate by stabilizing states of CT character and may call into question the role of such states as direct intermediates in SF between DPT molecules.
model the experimental data is roughly an order of magnitude larger than that obtained from theoretical treatments.18,19,27 Rather, it has been suggested that large couplings that lead to SF in linear acenes can be obtained via a superexchange mechanism involving the coupling of S1 and 1(TT) to CT states that transfers population from S1 to 1(TT) directly without populating a CT state itself.18,19 Other experiments provide some evidence for CT state participation, such as lowtemperature time-resolved emission experiments of polycrystalline tetracene thin films that display kinetics consistent with the population of an intermediate state along the SF reaction path.28 More direct evidence comes from measurements of covalently tethered dimers of 1,3-diphenylisobenzofuran,29 which observed the formation of a CT state in addition to triplet formation in polar solvents, but the measured triplet yield was sufficiently low (9%) that triplet formation via intersystem crossing could not be ruled out. Moreover, additional questions arise when considering how the couplings that lead to SF, through either direct or CT mediated pathways, vary with molecular organization. A theoretical treatment of perylenediimide dimers found that the couplings involved in both potential pathways varied substantially with stacking geometry, leading to predicted triplet yields that varied from 132% to essentially zero,30 while measurements of 1,3-diphenylisobenzofuran thin films identified two polymorphs of which only one readily underwent SF.31 To date, efforts to produce covalently tethered dimers that undergo SF have been unsuccessful32 in part due to the use of linking groups that force the tethered chromophores into geometries that limit coupling to the 1(TT) state.18 Experiments on amorphous rubrene films33 as well as our group’s measurements of disordered 5,12-diphenyltetracene (DPT) films10 have provided evidence that SF occurs at select dimer sites that adopt couplings conducive to SF, conclusions that have recently been supported by nonadiabatic quantum molecular dynamics simulations of SF in amorphous DPT.34 To better understand the role that CT states and molecular geometry play in SF, we have synthesized colloidal DPT nanoparticles (NPs) that can be easily formed as an aqueous suspension through reprecipitation,35−38 a method that has been previously used to prepare carotenoid aggregates capable of SF.11 The small size of our DPT NPs, ∼11 nm in diameter, ensures that ∼50% (see below) of the DPT molecules that comprise individual particles will be placed in close contact with the surrounding high dielectric aqueous environment. For many of the acenes including tetracene and pentacene, the lowest energy CT state is higher in energy than the S1 state by 250−350 meV.15,21,39 The aqueous environment that surrounds the NPs is expected to stabilize the CT state, lowering its energy, and increasing the 1(TT)−CT coupling which is inversely proportional to their energy difference. Thus, if DPT’s lowest CT state facilitates SF, the rate of formation and yield of triplets observed for DPT NPs is expected to differ from that measured for disordered DPT films. In addition, it has been demonstrated that by altering the reaction conditions used to prepare organic NPs, control can be exhibited over the resulting NP size distribution and crystallinity,40,41 suggesting that DPT NPs can serve as a tunable model system for exploring connections between SF rate and molecular organization. In this report, we present a spectroscopic characterization of aqueous DPT NP suspensions. These NPs display optical properties and excited state dynamics similar to disordered DPT films, but differ in a few key aspects. In particular, the
II. EXPERIMENTAL METHODS Sample Preparation and Steady-State Characterization. DPT NPs were synthesized using the reprecipitation method35−37 wherein a dilute solution of DPT in a good solvent is injected into a poor solvent, nucleating NP growth. DPT was synthesized according to literature methods42,43 and purified twice by vacuum thermal gradient sublimation prior to use. DPT NPs were prepared by injecting a 1.0 mM solution of DPT in tetrahydrofuran (THF) into a large volume of vigorously stirred ultrapure water (18.2 MΩ resistivity) in a volume ratio of 5:100 THF solution to water. Due to THF’s miscibility with water and DPT’s low aqueous solubility, DPT molecules spontaneously coalesce to form NPs, yielding a suspension pale yellow-green in color. These suspensions have been found to be remarkably stable, remaining dispersed in solution for upward of 8 months. However, as described below, we find evidence that the NPs within the suspension evolve over the first few days following preparation. Sample suspensions were stored in the dark when not in use to avoid photo-oxidation. For steady-state optical studies, NP suspensions were degassed with N2 and placed in either a 1 mm or 1 cm path length quartz cell for absorption or emission experiments, respectively. CHCl3 solutions were not degassed prior to use. Thin film samples were vapor deposited on a 1/16″ thick quartz substrate, and encapsulated under N2 by covering with an additional quartz window and sealing with epoxy. Absorption spectra were obtained with a Cary 50 UV−vis spectrometer in transmission geometry using NP suspensions with an optical density of 0.05 at the peak of DPT’s 1La ← 1A absorption transition (∼500 nm). Such a solution corresponds to a total DPT concentration of 37 μM and a NP concentration of 36 nM assuming an average NP diameter of 11 nm (see below). NP absorption spectra displayed in Figure 2A were corrected for scatter by fitting the spectral baseline to a 1/λ function in nonabsorbing areas.44 Steady-state emission spectra were measured with a Horiba FluoroMax-3 in a right angle collection geometry, exciting at 435 nm. The spectral bandshapes were found to be identical regardless of where DPT was excited within the 1La band. Atomic Force Microscopy (AFM). DPT NP films were prepared by drop-casting aqueous suspensions onto a glass microscope slide and allowed to dry overnight prior to use. 15520
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III. STEADY-STATE CHARACTERIZATION OF DPT NANOPARTICLES The size distribution of the synthesized NPs was characterized using a combination of AFM and dynamic light scattering. For AFM measurements, NPs that had been allowed to mature for 1 month (see below) were drop-cast on a glass slide with rootmean-square (RMS) roughness of 1.7 nm and allowed to dry overnight. A representative AFM image of the resulting film is plotted in Figure 1A, and highlights that the NP size
AFM experiments were carried out in tapping mode using a Digital Instruments Dimension 3100 atomic force microscope. Time Resolved Emission. Data was recorded using a TCSPC system (Becker and Hickl SPC 630) operating in tandem with a 250 kHz Ti-sapphire regenerative amplifier (Coherent RegA 9050). Excitation pulses centered near 500 nm were produced using a 400 nm pumped type I optical parametric amplifier (Coherent OPA 9450). The resulting fluorescence emission was collected at a right angle to the sample, passed through a 0.125 m double monochromator (Digikröm CM112) set to transmit 535 nm light, and detected using a Hamamatsu R3809U-50 photomultiplier tube with a 20 ps instrument response time. N2 degassed NP samples housed in a 1 mm path length quartz cell and thin film samples were oriented at ∼30° with respect to the excitation laser. CHCl3 solutions were not degassed and held in a 2 × 10 mm quartz cell held normal to the excitation source. Excitation pulses were centered at 505 nm for CHCl3 measurements and the resulting emission was passed through a polarizer set to the magic angle with respect to the pump prior to detection. For thin film and NP measurements, excitation pulses were instead centered at 501 nm and no polarization discrimination was applied to the collected probe light. Femtosecond Transient Absorption (TA). NP solutions used for TA experiments were prepared 1 week prior to TA measurement and were housed in the dark prior to use. These solutions were held in a 1 cm quartz cell, bubbled with N2 prior to use, and had an optical density of ∼0.4 at 500 nm. Measurements were carried out using a Coherent Legend Ti:sapphire amplifier operating at a 1 kHz repetition rate that pumps a type II Spectra Physics OPA-800C to produce excitation pulses centered at 500 nm with ∼9 nm of bandwidth. White light supercontinuum probe pulses (320−950 nm) were generated by focusing a small portion of the amplifier output into a 2 mm thick CaF2 window that was continuously rotated to ensure probe stability. The probe was collimated and focused into the sample using a pair of off-axis aluminum parabolic mirrors, while a CaF2 lens focused the pump. After the sample, a spectrograph (Oriel MS1271) dispersed the probe onto a 256 pixel silicon array (Hamamatsu S3901−256Q). The experimental time resolution for thin film measurements was determined to be ∼200 fs across the probe spectrum from a cross-correlation of the pump and probe in a quartz window. The time resolution for NP experiments is primarily limited by a solvent response that has largely decayed by Δt = 300 fs. This response was carefully measured using a 1 cm quartz cell filled with water and subtracted from the data presented in Figure 4. Transient spectra were recorded with pump and probe pulses oriented at the magic angle with respect to one another. During measurements, samples were slowly translated perpendicular to the path of the pump and probe by a linear stage to prevent photodamage. Spectra reported in Figure 4 were measured using a pump fluence of 15 μJ/cm2. Measurements of disordered DPT films have indicated that TA spectra can be influenced by singlet−singlet annihilation,10 but we believe these effects to be minor in the data presented in Figure 4. Section VIS of the Supporting Information provides a brief discussion of how the kinetics measured in TA experiments of NP suspensions varies with excitation density.
Figure 1. (A) AFM image of a dropcast DPT NP film. Peaks are broadened along the x and y axes due to convolution with the AFM tip size and scanning of the tip. (B) NP size distribution determined from AFM images.
distribution is polydisperse, containing NPs that range in size from a few nanometers to just upward of 30 nm. However, despite the large breadth of this distribution, it displays a welldefined peak near 11 nm (Figure 1B) that agrees well with the results of dynamic light scattering experiments (Figure S1, Supporting Information). Assuming that the NPs adopt a density similar to that of a neat DPT film (0.94 g/cm3), a spherical NP with an 11 nm diameter will contain roughly 1000 DPT molecules, ∼50% of which are expected to be located at the NP’s surface and thus in contact with the surrounding aqueous medium (Section IIIS, Supporting Information). Figure 2A compares absorption spectra of aqueous DPT NPs to that of monomeric DPT dissolved in CHCl3 and a neat vapor deposited DPT film. Given the similarity of DPT’s solution phase spectra to that of solution phase tetracene,10 we have labeled DPT’s spectroscopic states in accordance with the perimeter free electron orbital (PFEO) model for unsubstituted acenes.45,46 From 400 to 500 nm, a vibronically broadened absorption band appears due to a transition to DPT’s lowest excited singlet state (1La)47 from which SF proceeds (Figure 2A inset). Unlike spectra of polycrystalline tetracene films, which display a strong Davydov splitting of the lowest exciton band48−50 that reflects the delocalization of tetracene’s 1La state over multiple molecules,51 the absorption spectrum of DPT’s S1 state is only slightly red-shifted upon moving from CHCl3 to a 15521
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the spectra in Figure 2A reveals that the transition to DPT’s 1Bb state46,47 is sharply peaked at 287 nm in CHCl3 solution, but broadens and red-shifts when moving to a thin film environment. This spectral broadening suggests that the 1Bb state is more delocalized than the 1La state in disordered DPT films. Dipolar interactions that result from the close arrangement of neighboring DPT molecules are expected to more effectively delocalize the 1Bb state over the 1La since the transition to the 1Bb state from the 1A ground state carries ∼11× the oscillator strength of the 1La ← 1A transition. Interestingly, we find that spectra measured for the 1Bb state of DPT NPs slowly change with time, which we ascribe to an evolution of the NPs’ structure. Absorption spectra were taken of DPT NPs at set intervals during the weeks following their preparation (Figure 2A). Immediately following synthesis, the 1 Bb ← 1A transition appears slightly broader and red-shifted compared to that of DPT in CHCl3, indicating some degree of interaction between neighboring DPT molecules. Over the course of 2 days, this transition red-shifts and broadens, suggesting that with time the NPs undergo some degree of restructuring that results in stronger interactions between neighboring DPT molecules. Such rearrangements may involve the expelling of excess THF solvent that was kinetically trapped within the NPs as they nucleate. NPs continue to evolve over the course of a few weeks, but at a slower rate than that seen during the initial 48 h following their preparation (Figure 2B). During this period, the 1La ← 1A transition appears unchanged, indicating that it is not sensitive to the morphological rearrangements occurring in the NPs. The NP 1Bb line shape asymptotes toward a line width that is even broader than that measured for a disordered thin film, which may be indicative of a broader distribution of local environments within the NPs due to the possibility for individual DPT molecules to reside at either the NP surface or interior. To verify that these line shape changes are not due to chemical degradation of the NPs, we intentionally photooxidized an NP sample by leaving it in an open cuvette under ambient room illumination. Figure 2C plots spectra of this sample alongside spectra measured for NPs kept under a N2 environment. Oxidation of the NPs leads to a disappearance of the 1La and 1Bb lineshapes and growth of a new transition peaked near 250 nm. This band does not appear in spectra of NPs housed under N2, indicating that the spectral changes we observe as the NPs age do not result from photo-oxidation. Although prior work on tetracene NPs has shown that acene NPs can be more robust with regards to photodegradation than corresponding dilute solutions,36 we do not find this to be the case for the DPT NPs suspensions described here. DPT NPs were found to have a half-life of only 24 min under exposure to ambient conditions (Figure S3, Supporting Information) while a corresponding solution of DPT dissolved in CHCl3 displayed a half-life of 130 min despite the higher solubility of oxygen in CHCl3 over water.52,53 Steady-state emission spectra measured following excitation of DPT’s 1La state appear in Figure 3A. As reported previously,10 spectra measured for monomeric DPT and vapor deposited DPT films are nearly identical, reflecting the localized nature of DPT’s 1La state in disordered films. The spacing between the vibronic features that appear in both film and solution spectra can be fit well with a vibronic progression with a frequency spacing of 1360 cm−1 associated with ring stretching vibrations of DPT’s acene core.48,54,55 By contrast, the emission spectrum of freshly prepared NPs is substantially
Figure 2. (A) Absorption spectra of dilute DPT dissolved in CHCl3 (blue dashed), a vapor deposited DPT film (red dotted), and aqueous DPT NPs measured at different times following their preparation (solid). Each plotted trace is normalized to the peak of the 1La band (∼500 nm). NP absorption spectra show a time-dependent shift of the 1 Bb band (∼290 nm) that indicates structural evolution (black arrow). (A, inset) Expanded view of the 1La absorption band that is largely unchanged moving from solution to disordered film or NP environments. (B) Peak position (red) and variance (blue) of the NP 1Bb absorption band following their preparation. Corresponding values for DPT dissolved in CHCl3 and a vapor deposited DPT film appear as dashed and dotted lines, respectively. (C) Absorption spectrum of an intentionally photo-oxidized DPT NP suspension (magenta) compared with spectra measured for DPT NP suspensions kept under N2.
thin film environment. During thin film growth, DPT’s bulky phenyl groups frustrate crystal formation, creating disordered films with no long-range order.10 Within such films, the interaction between neighboring DPT molecules is sufficiently weak that the 1La state remains localized on a single molecule. Examining the absorption spectra of freshly formed DPT NPs, we see that the transition to the 1La state displays a spectrum identical in shape to that of monomeric DPT, but slightly redshifted, suggesting that the NP 1La state remains localized on individual molecules. In contrast, distinct differences in the spectra of DPT solutions, films, and NPs are observed for absorption transitions to higher energy singlet states. The UV portion of 15522
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rapid decay component that accelerates and grows in amplitude as the NPs age. After 2 weeks following their preparation, DPT NPs display a rapid decay nearly identical to that observed in disordered films. Although rapid emission quenching is not direct evidence of SF, the similarity of the decay kinetics observed for NPs and disordered DPT films suggests that the NPs undergo SF. TA measurements definitively confirm that DPT NPs indeed undergo SF. Figure 4 compares TA spectra of vapor deposited
Figure 3. (A) Emission spectra of DPT dissolved in CHCl3 (blue dashed), a vapor deposited DPT film (red dotted), and DPT NP suspensions taken on the day of their preparation and 6 days afterward (solid). (B) TCSPC decay traces detected at 535 nm for DPT in CHCl3 (blue), a vapor deposited DPT film (red), and DPT NPs following excitation of the 0 ← 0 transition of DPT’s 1La (S1) state. As the NPs age, the decay profile of the S1 state moves to resemble that of disordered films.
modified, displaying peaks at long wavelengths (>600 nm) that are more intense and broader than those measured for either DPT dissolved in CHCl3 or a vapor-deposited DPT film. Analogous red-shifted emission has been observed in rubrene crystals with high defect densities and assigned to the emission of singlet excitons generated at these defects from the fusion of triplet pairs. 56 Time-correlated single photon counting (TCSPC) experiments that measure emission from the NPs at 650 nm show that this emission band displays a delayed rise of 370 ps that is not observed for higher energy peaks in the NP emission spectra, consistent with the notion that this band is indicative of low energy sites that are diffusively populated (Figure S4, Supporting Information). While the precise molecular origin of the long wavelength emission seen from DPT NPs is not yet fully clear, we find that this feature is suppressed as the NPs age.
Figure 4. (A) TA spectra of DPT NPs compared to that for a disordered DPT film, showing the decay of the S1 population and creation of triplets. Decay traces are normalized to the maximum of the S1-induced absorption band and traces for subsequent time delays are offset by 0.3 units. The inset shows TA spectra measured at a time delay of 900 ps focusing on the region of Tn ← T1 absorption. (B) Decay of the singlet induced absorption band (410−430 nm) and growth of triplet absorption (490 −510 nm) for DPT NPs and disordered films. The early portion (Δt < 0.2 ps) of the NP transients is omitted due to overlapping solvent response. Reported spectra were measured using an excitation fluence of 15 μJ/cm2 for both samples. Dashed lines represent a triexponential function that is included as a guide to the eye. (B, inset) Early portion of the S1 induced absorption decay showing that the fastest decay component is nearly identical for NP and thin film samples.
IV. DPT NANOPARTICLE EXCITED STATE KINETICS To characterize the excited state kinetics of DPT NPs and assess their ability to undergo SF, time-resolved emission and TA experiments were performed on NP suspensions at various periods following their preparation. Figure 3B compares TCSPC traces measured for DPT dissolved in CHCl3 and a vapor deposited DPT film with that of NP suspensions. While DPT dissolved in CHCl3 displays a monoexponential decay with a time constant of 11 ns, neat DPT films show a rapid loss of S1 attributed to SF followed by a slowly decaying tail that results from the recombination of triplet excitons following SF (Figure 3B).10 Within 24 h of their preparation, DPT NPs display an emission profile that substantially differs from the monoexponential decay seen for DPT in CHCl3, possessing a
DPT thin films and a sample containing NPs that were aged for one week. Rapidly following photoexcitation at 500 nm, thin film spectra display a strong Sn ← S1 transition peaked near 420 nm (Figure 4, blue traces) that decays over two time regimes of roughly 800 fs and a few hundred picoseconds as DPT molecules undergo SF,10 yielding an induced absorption spectrum that resembles that previously reported for DPT triplet excitons (Figure 4A inset).57,58 SF in vapor deposited DPT films occurs over two time scales due to the film’s disordered nature, wherein some dimer pairs adopt appropriately coupled geometries capable of rapid SF, while excitations created elsewhere within the film first need to diffuse to these 15523
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regions to undergo SF.10,34 TA spectra measured for DPT NPs are qualitatively similar to those of DPT films (Figure 4, red traces). These spectra display a strong Sn ← S1 induced absorption band at short delays that decays by 900 ps to a line shape similar to that observed for triplets in neat DPT films. Given that the intrinsic intersystem crossing rate of isolated DPT is 89 ns,57 the rapid production of triplets on a subnanosecond time scale in DPT NPs strongly suggests that they are produced via SF. Figure 4B plots a comparison of the decay of the S1 induced absorption band and the corresponding rise of T1 induced absorption for DPT NPs and thin films. The fastest component of the S1 decay is related to the intrinsic SF rate that occurs at DPT dimer pairs that directly undergo SF following photoexcitation. Examination of the TA data at early times shows that this component is largely unchanged between NP suspensions and thin films (Figure 4B inset). Since we expect a significant fraction of the DPT molecules that comprise the NPs to be in close contact with the surrounding aqueous medium (Section IIIS, Supporting Information), the lack of influence on the SF rate exhibited by the surrounding liquid environment is surprising. If a CT state was populated as an intermediate along the SF reaction path, the polar solvent would stabilize the intermediate and pull its energy closer to that of the 1(TT) state, improving the forward SF rate. One means of interpreting this result is that rather than SF being mediated by pathways involving CT state participation, SF between DPT molecules occurs through a direct two-electron pathway.20,21 However, this interpretation presupposes that DPT molecules located at the NPs surface are arranged in such a manner that they can still undergo SF. It may be the case that the fast decay component of the S1 state is solely related to SF that occurs between molecules within the interior of the NPs that are insulated from the surrounding aqueous environment, and that, due to a disordering of the NP surface, few molecules located at the surface adopt pair configurations conducive to SF. While we cannot rule out this latter possibility entirely, we note that the fast S1 decay component possesses similar amplitude for both thin film and NP suspensions, which suggests that the concentration of fission sites within the NPs is likely similar to that of disordered thin films. Since ∼50% of the DPT molecules that comprise the NPs are expected to reside near their surface, it is likely that molecules located at the NP surface can still readily undergo SF. Although our results argue against the direct population of a CT state during SF in DPT, they do not rule out the possibility of CT states facilitating SF through a virtual superexchange pathway.17,18 Such a scenario eliminates the ability of the solvent to lower the CT state energy through dipolar solvation, but allows the polarizability of the surrounding medium to alter the SF rate. How the influence of local polarizability varies for DPT NPs with respect to disordered films is at present unclear. While the intrinsic SF rate is found to be similar for NPs and thin films, further examination of the TA spectra shows that the NP Tn ← T1-induced absorption band that grows following photoexcitation (Figure 4A inset) reaches an amplitude by 900 ps that is only about half that observed for a neat DPT film. This reduced T1 amplitude corresponds with a slowed decay of the NP Sn ← S1 absorption transition over time scales longer than 1 ps (Figure 4B) during which the diffusion of singlets created far from SF sites begins to occur.10 The slower decay of the NP S1 state during this later time regime suggests a slight reduction of SF dimer sites and a slowing of diffusive Förster
transfer within the NP that could reflect a lower density of DPT molecules within the NPs compared to neat films. While we have not yet applied a full kinetic model to the NP data that accounts for competing radiative and nonradiative decay pathways and exciton annihilation, fitting of the data presented in Figure 4 to extinction spectra of DPT’s S1 and T1 states10,57,58 indicates that the T1 population rises to 75% of the initial S1 population within 0.9 ns after excitation. This value corresponds to just over half the triplet population generated in vapor deposited DPT films over a similar time window.10
V. CONCLUSIONS In conclusion, we have demonstrated that acene NPs are a viable platform for fundamental studies of SF. DPT NPs formed through reprecipitation display absorption and emission spectra that indicate they are initially formed in a thermodynamically unstable structure that slowly evolves over a few days into one with a greater degree of association between neighboring molecules. The slow structural evolution of the NPs as well as the potential to tune their structure by altering preparation conditions offers the possibility of testing how the relative arrangement of neighboring molecules and the local dielectric environment each influence SF. Time-resolved experiments reveal that these particles readily undergo SF, albeit with a lower overall yield than neat DPT films. Surprisingly, we find that the high dielectric environment that surrounds the NPs does little to alter their intrinsic SF rate, which may question the degree to which CT states directly participate in SF for DPT.
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ASSOCIATED CONTENT
S Supporting Information *
Dynamic light scattering measurements of DPT NP suspensions, additional NP film AFM images, a description of the methodology used to estimate the number of DPT molecules per NP, photooxidation kinetics of DPT NPs and dilute solutions, and a brief discussion of the excitation fluence dependence of NP TA measurements and fitting procedure used to extract the SF yield from TA data are provided. This material is available free of charge via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The research presented here is based upon work supported through the Center for Energy Nanoscience, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences (DESC0001013). J.N.M. thanks the Rose Hills Foundation for additional support through a Rose Hills Summer Undergraduate Research Fellowship. We also thank Dr. Chongwu Zhou for use of his AFM, Jia Liu for AFM training, and Dr. Nickolas Chelyapov of the USC Nanobiophysics Core Facility for DLS training. 15524
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