Article pubs.acs.org/JPCC
Interchain Charge-Transfer States Mediate Triplet Formation in Purified Conjugated Polymer Aggregates Alan K. Thomas, Hunter A. Brown, Benjamin D. Datko, Jose A. Garcia-Galvez, and John K. Grey* Department of Chemistry and Chemical Biology, University of New Mexico, Albuquerque, New Mexico 87131, United States S Supporting Information *
ABSTRACT: The pathways and dynamics of converting spinallowed (S = 0) singlet excitons to spin-forbidden (S = 1) triplets have significant implications in determining performance metrics of conjugated polymers in optoelectronic devices. We study the effect of structural ordering factors on triplet formation in self-assembled aggregate π-stacked chains of poly(3hexylthiophene) (P3HT) using single-molecule time-resolved intensity modulation and electric-field-dependent photoluminescence (PL) spectroscopy. Triplet generation is only efficient in P3HT aggregates of high purity, and formation yields are found to increase with nanofiber size. We propose that the high intrachain order in purified aggregates that extends exciton coherence lengths, leading to J-aggregate spectral signatures, is also important for populating interchain charge transfer (CT) states that, at longer times, recombine preferentially to triplets according to spin statistics. Electric-field-dependent PL decays of isolated P3HT aggregates show large modulation of a long-lived emitting state attributed to the delocalized intrachain exciton with substantial CT state admixture. Our results demonstrate the importance of dark CT states in mediating exciton relaxation and spin conversion processes that are usually obscured in conventional thin films by heterogeneity. We further demonstrate the utility of subtle structural factors for selecting photophysical outcomes by careful control of processing conditions.
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INTRODUCTION The formation of electrically neutral but spin-forbidden triplet excitons in conjugated organic polymer materials is generally regarded as a loss channel for various optoelectronic device applications, such as solar cells and light-emitting diodes.1−5 Ongoing efforts aimed at bypassing or harvesting these longlived states6 have experienced limited success mostly due to the lack of detailed, molecular-level structure−function relationship information governing triplet formation. Recent reports of singlet fission7 in conjugated organic small molecule and polymers alike have inspired new debate over the structural and electronic factors governing the efficacies of this and more common perturbative triplet generating mechanisms, such as intersystem crossing.8−10 However, heterogeneity arising from polydispersity or polymorphism presents serious obstacles for elucidating mechanistic details of triplet formation and the structural factors regulating their branching ratios.11−13 Here we use self-assembly approaches to effectively manage polydispersity-induced disorder effects in conjugated polymer aggregates, which reveals new views of exciton delocalization, coupling and spin conversion. Purified aggregate nanofibers of poly(3-hexylthiophene) (P3HT) are assembled via the whisker method in toluene solutions over long periods of time (>24 h).14 These nanofibers consist of single P3HT chains folded into highly anisotropic π-stacked conformations with stem lengths often exceeding 40 nm and interchain stacking distances of ∼3.7 Å.15,16 Importantly, only the higher molecular weight fraction forms aggregates,17 and individual folded chains subsequently associate into extended nanofiber structures, leading to morphologies distinct from more common partially aggregated P3HT fibrillar nanostructures.18 Fractionation, or © 2016 American Chemical Society
purification, toward higher molecular weight chains leads to high intrachain order (monomer coplanarity) within π-stacked stems.19 Consequently, singlet excitons are delocalized along the intrachain direction for their entire lifetime,20,21 owing to exceptionally large conjugation lengths (coherence lengths) and reduced aromaticity. These characteristics give rise to spectroscopic signatures resembling classical J-aggregates; that is, the strength of the electronic origin transition (0−0) is always larger than that of the vibronic sidebands (0 − n, n ≥ 1) in addition to relatively small (1 and relatively sharp vibronic line widths (fwhm ∼300 cm−1). By fitting and subtracting the nonaggregated P3HT absorption spectrum, the absorption spectrum of the fundamental J-aggregate chromophore becomes apparent (see Figure 1a, inset). On the basis of previous TEM imaging studies,16 X-ray diffraction,33 and time-resolved fluorescence polarization anisotropy20 measurements, P3HT segments within J-aggregate conformations are highly planarized and favor extended exciton and polaron delocalization, consistent with greater quinoidal character.15,16 The nearly instantaneous emergence of well-resolved Jaggregate signatures in Figure 1a is consistent with our previous model of single P3HT chains self-folded into highly ordered, anisotropic conformations that subsequently associate into hierarchical nanofibers. Vogelsang et al. demonstrated similar rod-like conformations that could be produced in noncrystalline, alkoxy-substituted poly(phenylene-vinylene) (PPV) derivatives by solvent vapor-annealing solid dispersions of isolated polymer chains.34 By carefully controlling the polymerization synthesis and processing conditions, Mazzio et al. also observed J-aggregate-like features in thin films of high molecular weight and regioregularity.33 More recently, Fauvell et al. demonstrated similar conformations in a push−pull type conjugated polymer where chains self-fold into anisotropic conformations, then associate into larger structures.35 These authors further note that the basic spectroscopic unit of aggregates can be as small as a single folded polymer chain.35 Assuming a nominal molecular weight of ∼65 kDa for the high-molecular-weight P3HT fraction, the expected contour length is ∼100 nm, implying that a single J-aggregate P3HT chain folds up to ∼3× onto itself. At longer times (>24 hr), nanofibers reach lengths of 1−10 μm and J-aggregate spectral
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EXPERIMENTAL SECTION Sample and Device Preparation. P3HT (>99% regioregularity, Mn ≈ 65 kDa) nanofibers were self-assembled in solution using procedures previously described in detail.14 Nanofibers were further purified by several centrifugation and washing steps, then dispersed in polystyrene hosts (∼1−3% w/ w). Electric-field-only (capacitor)-type devices were fabricated by spin coating nanofiber−polystyrene dispersions onto glass coverslips coated with indium tin oxide (ITO) and silicon dioxide (SiO2, 50 nm). The additional layer of SiO2 ensures unwanted complications from charge injection, movement, and contact effects. Similar measures were employed at the metal contact by depositing a polystyrene spacer layer prior to thermal evaporation of a metal contact. Nanofiber−polystyrene film dispersion final thicknesses were ∼600 nm, and the device was completed by depositing a metal overcoating by thermal evaporation at a base pressure of ∼10−7 Torr. Capacitor devices were checked for leakage prior to use and monitored during operation using a digital oscilloscope. Instrumentation. P3HT nanofibers were characterized using UV−vis absorption, Raman, and PL spectroscopy. Single-molecule PL spectroscopy and imaging of isolated nanofibers was carried out using a scanning confocal microscope spectrometer previously described in detail. Laser excitation light was provided by a Ti:sapphire oscillator (77 MHz repetition rate) and white-light-continuum-generating crystal fiber. Appropriate bandpass and long-pass filters were used to select wavelengths on resonance with the P3HT aggregate transition maximum (∼570 nm). Intensity modu23231
DOI: 10.1021/acs.jpcc.6b06526 J. Phys. Chem. C 2016, 120, 23230−23238
Article
The Journal of Physical Chemistry C
of these self-folded P3HT chains in aged J-aggregate nanofiber structures lead to increased likelihood of registry mismatch between neighboring π-stacks. When delocalized J-type excitons encounter a chain−chain contact, one carrier can undergo interchain transfer, leading to an interchain CT state. According to Spano and coworkers, positive wave function overlap can result, leading to spectroscopic signatures of Haggregates.31 Unlike traditional Coulombic dipole−dipole coupling descriptions of aggregate excitons that are mediated by intrachain order, the evolution of positive-wave function overlap must proceed via interchain CT states in purified aggregates.31 This phenomenon can be observed in Figure 1b in absorption spectra of aged J-aggregates. Here nanofibers are much longer and therefore contain more self-folded chains and more chain−chain contacts with possible registry mismatch. Indirect evidence of chain−chain contacts as interchain CT sites can be observed from sonication or dilution of aged Jaggregate nanofibers in solvent or solid dispersions, respectively.14 In both cases, pristine J-aggregate character is recovered in absorption and emission spectra due to breaking up the nanofiber into constituent self-folded, J-aggregate type P3HT chains (see the Supporting Information). Figure 1c shows a representative PL spectrum of an isolated J-aggregate P3HT chain from an aged sample diluted into a polystyrene host. The distributions of PL 0−0 maxima and 0− 0/0−1 ratios are shown as a histogram for ∼50 particles. The latter reflects the extent of intrachain delocalization with some particles appearing as ideal J-aggregates (0−0/0−1 > 4), with others more closely resembling H-aggregates (0−0/0−1−1) with much lower signal-to-noise ratios (see the Supporting Information). Comparisons with time-dependent nanofiber absorption spectra in Figure 1a reveal consistent trends where larger nanofibers possess greater interchain H-like character. We now show that the combination of delocalized intrachain excitons and chain−chain contacts in J-aggregate nanofibers expedites the formation of triplet excitons on longer (>1 ns) time scales. In a previous study, we demonstrated efficient quenching of emissive singlet excitons by triplets in isolated P3HT Jaggregates using time-resolved, excitation intensity-dependent PL modulation spectroscopy.36 The full multistate photodynamic model describing time-dependent triplet populations and PL quenching is outlined in the Supporting Information, which is similar to that used in previous studies of conjugated polymer single molecules.37 This multistate model can be reduced to an effective two-level system (see the Supporting Information), and the time-dependent probability of forming a triplet on a chromophore site is given by
Figure 1. (a) Time-dependent optical absorption spectra of P3HT Jaggregate nanofibers during self-assembly. Inset: Spectrum of isolated single chain aggregate. (b) Comparison of fresh and aged P3HT Jaggregates with H-aggregate nanofibers. (c) Representative PL spectrum of a single J-aggregate and distributions of PL maxima (λmax). Inset: Histogram of vibronic 0−0/0−1 ratios.
signatures are diminished (i.e., 0−0/0−1 ratios
