Emission Decay Pathways Sensitive to Circular Polarization of Excitation

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Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

Emission Decay Pathways Sensitive to Circular Polarization of Excitation Ashish Sharma,† Stavros Athanasopoulos,‡ Patrick C. Tapping,§ Randy P. Sabatini,† Olivia F. McRae,∥ Markus Müllner,∥ Tak W. Kee,§ and Girish Lakhwani*,†

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ARC Centre of Excellence in Exciton Science, School of Chemistry, and ∥Key Centre for Polymers and Colloids, School of Chemistry, The University of Sydney, Sydney, New South Wales 2006, Australia ‡ Departamento de Física, Universidad Carlos III de Madrid, Avenida Universidad 30, Leganés 28911, Madrid, Spain § School of Physical Science, The University of Adelaide, Adelaide, South Australia 5005, Australia S Supporting Information *

ABSTRACT: The photophysical properties of conjugated materials can be strongly affected by the nature of intermolecular interactions. Toward this aim, supramolecular self-assembly facilitates efficient packing of molecules into ordered architectures, which allows efficient intermolecular coupling. However, the resulting electronic overlap imparts additional sensitivity to the disorder, predominantly because of the “nonlocal” origins of the photophysical properties. Understanding the nature and origin of the disorder in conjugated systems is a prerequisite to exploit the benefits of efficient intermolecular coupling. In this report, we utilize chirality as a marker to sensitively probe the nature of the disorder in thermodynamically assembled helical nanoaggregates of a chiral conjugated polymer poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-(benzothiadiazole)] (PFBT). Surprisingly, we find that one-handed intermolecular coupling in helical PFBT aggregates leads to differences in the decay pathways of left- and right-handed excitations. We attribute the emergence of this sensitivity to the disorder in the excitonic coupling of flexible, nonplanar polymer chain conformations, likely predominant at the edges of the aggregate. Our findings shed insights into the effect of disorder on the photophysical properties, which open up new opportunities for sensitively exploring the links between intermolecular coupling and photophysical properties of conjugated systems.

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INTRODUCTION π-conjugated materials1 have a proven potential in the field of optoelectronics, as evidenced by the success of organic lightemitting diodes in display technology.2,3 The ability to finetune the optoelectronic response through subtle variations in a molecular structure has motivated an expanding interest in the photophysical properties of conjugated materials. The photophysical response of conjugated systems emanates from the interplay of local and long-range intermolecular interactions, intimately related to the nature of molecular packing in the device.4 Although efficient intermolecular coupling is desirable, as it translates to long-range energy and charge transport, high intermolecular electronic overlap also imparts additional sensitivity to disorder. Understanding the relationship between intermolecular interactions and the photophysical properties of conjugated systems is crucial in unmasking the origins and effects of disorder in organic electronic devices. In this regard, supramolecular self-assembly,5 which utilizes directional noncovalent interactions between conjugated molecules, has been extensively utilized to exercise control on the packing geometry of the molecules, enabling the fabrication of tailored molecular architectures. As such, supramolecularly self-assembled systems act as appropriate © XXXX American Chemical Society

test beds for investigating the correlation between intermolecular interactions and photophysical properties of conjugated systems. Notably, one-handed helical nanostructures assembled from chiral molecular building blocks interact preferentially with a specific handedness of circularly polarized light, which allows chirality to be employed as a reporter of the nature of intermolecular interactions.6,7 Specifically, the strength of the chiroptical response of a helically assembled nanostructure correlates with the degree of net helical order in the assembled structure.8−10 Furthermore, as the chiroptical response emanates from one-handed excitonic coupling of neighboring transition dipole moments, it is very sensitive to the nature and degree of intermolecular interactions.11 Thus, chiroptical techniques such as circular dichroism (CD) have come up as sensitive spectroscopic tools to investigate how changes in the intermolecular coupling affect the photophysical properties of a helically assembled system.12 Received: August 2, 2018 Revised: September 18, 2018 Published: September 24, 2018 A

DOI: 10.1021/acs.jpcc.8b07482 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C The chiral conjugated polymer poly[(9,9-di-n-octylfluorenyl2,7-diyl)-alt-(benzothiadiazole)]13 (PFBT) is well-known for its excellent optoelectronic properties (e.g. high quantum yield, high electron hole mobilities, and high band gap) and serves as an appropriate test bed. As a donor−acceptor-conjugated polymer, the lowest energy excited state of PFBT is charge transfer (CT) in character,14,15 which makes emissive properties of PFBT uniquely sensitive to the nature of intermolecular interactions. Investigations into the photophysical properties of PFBT films suggest that packing geometry of polymer chains determines the relative proportion of localized intrachain excitonic and interchain CT states,16−19 which in turn controls the nature of emission. For example, grazing angle photoluminescence (PL) measurements on PFBT films reveal that emissive states in the bulk of the film can be very different from those closer to the substrate.20 However, the inherent disorder in the molecular packing in films precludes direct insights into the intermolecular interactions responsible for the origin of these effects. We thus utilize chiral PFBT, where the chiral nature of its side chains promotes self-assembly of the polymer chains into one-handed helical nanofibers in solution. This allows chirality to serve as a spectroscopic marker of the extent of intermolecular coupling. Toward this aim, we utilize the fluorescence-detected CD21 (FDCD) measurements to investigate the sensitivity of the photophysical properties on the nature of intermolecular interactions in PFBT aggregates. Although the CD is the differential absorbance of left (L) and right (R) circularly polarized light measured in the transmission mode, the FDCD is the difference in the emission intensity of L and R circularly polarized excitations. Thus, although CD only depends on the net helical arrangement of ground-state conformations, FDCD is sensitive to factors such as excited-state relaxation, which affect the photophysics of the emissive states.21−23 Although FDCD has been studied extensively in the past,24−28 to our knowledge, this is the first report utilizing FDCD to explore the photophysical properties of supramolecular aggregates of organic semiconductors. In this report, we demonstrate that helical PFBT aggregates exhibit different decay pathways for the absorption of left and right circularly polarized light. The comparison between CD and FDCD reveals that the quantum yield of emission is sensitive to circular polarization of excitation. Further, we show that these sensitivities originate because of significant polarization decay at picosecond time scales. We attribute the origins of polarization decay to disorder in the excitonic coupling of flexible nonplanar geometries, which affects the CT character and subsequently the relaxation dynamics of excitedstate conformations at the picosecond time scale.

Figure 1. (A) CD and (B) absorption spectra collected at different temperatures while cooling a 8 μg/mL solution of PFBT in octanol at a rate of 0.5 °C/min. The molecular structure of the PFBT monomer is shown in the inset. (C) Dissymmetry parameter (gabs = ΔA/A) at different temperatures monitored at 498 nm. The inset shows an atomic force microscopy image of the aggregates in octanol, after evaporation onto a silica substrate.

octanol at this temperature (Figure 1A). However, after slowly cooling the molecularly dissolved PFBT solution in octanol at a controlled rate, the solubility of the polymer chains is lowered, and a CD feature develops, suggesting temperaturedependent aggregation of polymer chains in octanol. The optimal cooling rate was identified to be 0.5 °C/min; dynamic light scattering revealed fibers of approximately 160 nm in size (see Supporting Information). This cooling rate results in aggregates that are homogeneous, show high CD magnitudes and are very stable in octanol at room temperature (see Supporting Information). The absorption spectra of both molecularly dissolved PFBT and aggregates have the characteristic dual-band “camel-back” shape, with two peaks at ∼320 and ∼450 nm, attributed to π−π* and CT transitions, respectively 14,30 (Figure 1B). In comparison to their molecularly dissolved counterpart, the aggregates show an increase in the extinction coefficient concomitant with a slight red shift in the absorption spectrum, which implies a more planar backbone configuration enabling π-stacking of the polymer chains. The CD spectrum of the aggregates shows two bisignate features in the wavelength regions 300−380 and 380−510 nm, similar to what has been reported previously.31 The origin of these bands is attributed to excitonic coupling of transition dipoles on neighboring chromophores held in a lefthanded helical arrangement.32,33 The aggregation process can be followed conveniently by monitoring the dissymmetry parameter, gabs, as a function of temperature. gabs is defined as the ratio of differential absorbance (ΔA = AL − AR) to net absorbance at a given

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RESULTS AND DISCUSSION In this article, we utilize CD as a spectroscopic marker of supramolecular ordering, attributed to the aggregation of polymer chains. Samples with no net CD suggest a lack of aggregates. We term these samples as “molecularly dissolved” in the subsequent discussion, consistent with previous literature on supramolecular self-assembly of chiral polyfluorenes.29 Thermodynamically stable aggregates of PFBT were obtained by controlled cooling of an 8 μg/mL PFBT solution in octanol, with lower cooling rates leading to larger aggregates. At 80 °C, the PFBT solution in octanol showed no CD, suggesting that PFBT was molecularly dissolved in B


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The Journal of Physical Chemistry C

difference in the absorbance of parallel and perpendicular polarizations of linearly polarized light. The low LD values (∼10−4) of PFBT aggregates confirm that there is no preferential macroscopic alignment of the aggregates in octanol (see Supporting Information). Further, to rule out any artifacts arising from the photoselection effects, the FDCD measurements were performed using an ellipsoidal device to collect the emission in almost 360°.38,39 The photoselected FDCD is marked by a high degree of linear polarization in the emission, which necessitates the use of appropriate slit masks in the ellipsoidal device;38 the FDCD spectra of PFBT aggregates do not change on varying the width of the slit masks, which confirms that the photoselection effects do not interfere with the FDCD detection. Further, the shape of the emission spectrum and average PL lifetime of the aggregates is independent of the excitation wavelength as well as concentration, indicating that only one kind of aggregate species is present in the solution (see Supporting Information for more information). Instead, to understand the observed differences between CD and FDCD, we look to the underlying assumption in correlating CD and FDCD measurements: that the emission quantum yields of L and R excitations are the same. This may not always be true40differences in emission from L and R excitations can be argued if the electronic couplings are very sensitive to the conformation of the excited state. In fact, if we hold ourselves to this assumption, we cannot use the measured CD spectrum to calculate the FDCD spectrum accurately (Figure 2B). However, by accounting for differences in the quantum yield of L and R excitations (ΔΦ ≈ 0.1%), we can satisfactorily reproduce the FDCD spectrum from the measured CD spectrum (see Supporting Information for detailed theoretical formalism of FDCD). Thus, comparison between CD and FDCD reveals that the decay of the emissive state in PFBT aggregates is sensitive to circular polarization of the excitation. Sensitivity of the quantum yield to the circular polarization of excitation suggests a crucial role of the excitedstate conformation on the radiative and nonradiative decay pathways in PFBT aggregates. The differences in PL quantum yield (PLQY) for L and R excitations motivated us to probe the transient dynamics of the aggregates. For comparison, we examined the PL decay of both molecularly dissolved PFBT and PFBT aggregates. We chose to probe independently the red edge (575 nm) and blue edge (530 nm) of the emission peak (Figure 3A,B) to investigate the role of processes such as excitation energy transfer and excited-state relaxation on the overall emissive properties of the aggregate. All kinetic features show a PL lifetime of ∼2 ns. In general, the 575 nm kinetics for molecularly dissolved PFBT and aggregates exhibit a similar behavior, including an initial rise time (∼15 ps) before the slower decay. These features have been previously ascribed to relaxation of the emissive intramolecular CT state, which evolves into a more stable relaxed geometry through solvent-assisted conformational changes.41,42 However, early time kinetics at 530 nm is markedly different for the dissolved and aggregated sample. Although both exhibit a short ( 350 nm, where similar quantum yields allow direct comparison between CD and FDCD. Surprisingly, for PFBT aggregates, ΔA measured from CD and FDCD are significantly different, including a difference in sign within the region 375−425 nm (Figure 2A). We have ruled out the effects from the photoselection of ground-state conformations,36 self-absorption, or macroscopic alignment37 of the aggregates. Toward this end, we employed the linear dichroism (LD) measurements, which probe the C


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Figure 3. (A) PL decay and fits of molecularly dissolved PFBT in tetrahydrofuran (THF) monitored at 530 nm (red) and 575 nm (blue) (λex = 440 nm) at room temperature. (B) PL decay and fits of PFBT aggregates in octanol monitored at 530 nm (red) and 575 nm (blue) at room temperature (λex = 440 nm). Contour plot showing the evolution of the SE spectra of (C) molecularly dissolved PFBT in THF and (D) PFBT aggregates in octanol at room temperature. The white line in the contours denotes a break in the vertical axis from linear to log scale.

Figure 5. Calculated electron and hole natural transition orbitals (NTOs) for the first excited-state transition in coupled FBT (monomer unit of the polymer PFBT) dimers for LE (nonplanar) and DE (planar) geometries.

time scale: ∼200 ps. In addition to the slow shift of the SE peak, the SE spectra are quenched at a much faster rate for the aggregates resulting in about 50% decay of the total intensity in the first 200 ps (Figure 3D). Thus, both time-resolved PL and TAS point to the emergence of an intermediate decay component in the aggregate sample. We attribute the emergence of this intermediate decay component at short emission wavelengths to slow conformational relaxation of nonplanar excited-state geometries in the aggregates. The torsional relaxation dynamics of the excited state in polyfluorene derivatives43 is indeed intimately related to the planarization of the polymer backbones and can vary from 10 to 300 ps. This sensitivity to the polymer backbone suggests that the packing of polymer chains in the aggregate facilitates regions with rigidly packed planar conformations, as well as flexible nonplanar conformations, the latter we speculate to be predominant at the edges of the aggregate. This is especially the case of PFBT films, where rigidly packed planar conformations in the bulk of the film account for lower-energy (redder) emission in comparison to the high-energy emission

Figure 4. PL decay of PFBT aggregates monitored at 530 nm for excitation wavelengths of 440 nm (red) and 400 nm (blue) for (A) right-handed excitation and (B) left-handed excitation.

To probe further the changes in CT dynamics, we performed ultrafast transient absorption spectroscopy (TAS). For molecularly dissolved PFBT, the stimulated emission (SE) red-shifts during the first 15 ps (Figure 3C), concomitant with the observed fast time component in the 575 nm PL decay (Figure 3A). After this initial CT relaxation, the SE peak remains at 560 nm, decaying with an approximate 2 ns lifetime. The SE of the aggregates, however, red-shifts for a much longer D


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Figure 6. (A) Test parameters (δx, φ) for identifying the role of perturbations on the calculated S0→S1 excited state energies (TD-DFT) of planar and ground-state optimized (nonplanar) conformations of the dimers. (B) Heat maps representing the effect of the test parameters on the calculated energies. The effect of varying the chiral angle (φ) on the calculated rotational strengths of (C) planar and (D) nonplanar conformations.

from flexible polymer conformations closer to the film substrate.20 As discussed earlier, the FDCD measurements reveal that the PLQY of PFBT aggregates is sensitive to the circular polarization of excitation; to probe further, we investigate the dynamics of excited-state relaxation for left- and right-handed excitations. We probed the PL decay at two different excitation regions, one where the aggregates predominantly absorb L excitation (λ = 400 nm) and the other where the aggregates predominantly absorb R (λ = 440). Expectedly, the PL decay of right-handed excitations was both independent of the excitation wavelength and very similar to that observed using unpolarized excitation (Figure 4A). Surprisingly, the PL decay of left-handed states depends on the excitation wavelength. The L excitation at 440 nm results in a fast 400 fs decay, twice as fast as the decay for excitation at 400 nm (Figure 4B). These differences persist for a few hundred picoseconds. This dissimilarity is also corroborated in the “SE” spectra when the excitation wavelengths of 400 and 440 nm are compared for L excitation. The time scales for which the differences persist suggest that the differences in PLQY for L and R excitations emerge because of the sensitivity of torsional relaxation dynamics to the handedness of excitation. Interestingly, the differences only emerge for the PL decay monitored at short emission wavelengths, indicating that the sensitivities to circular polarization of excitation arise because of the disorder in the relaxation of less stabilized nonplanar geometries, predominantly likely at the edge of the aggregate (see Supporting Information for more information). No significant difference in the decay dynamics was observed for parallel and perpendicular excitations; this confirms that the observed sensitivity toward circularly polarized excitation does not originate because of residual

polarization artifacts. Furthermore, the PL decay of molecularly dissolved PFBT does not show any difference for different handedness of circularly polarized excitation, confirming that the observed differences emerge because of the helical coupling of chromophores in the aggregates (see Supporting Information). In comparison to the R excitations, the decay of L excitation is slightly faster resulting in a small ΔΦ value of 0.5%, which agrees with the small differences in the quantum yield of L and R excitations as revealed by FDCD. The observed sensitivity of the excited-state relaxation dynamics on the circular polarization of excitation motivated us to probe the nature of intermolecular coupling in the excited states of PFBT aggregates. Performing time-dependent density functional theory (TD-DFT) on two interacting dimers (i.e. four co-monomer units of FBT), we calculated the hole and electron NTOs for the planar and ground-state optimized polymer backbone conformations. Of note, this reveals a difference in the extent of delocalization for planar versus nonplanar geometries, with a slight red-shifted absorption of the planar geometries compared to that of the nonplanar conformations (see Supporting Information). Although planar conformations support lower energy (redder) delocalized interchain CT states, predominantly leading to delocalized excited-state (DE) geometries, nonplanar geometries display localized intramolecular CT character resulting in localized excited states [locally excited (LE)] (Figure 5). We then investigated how differences in intermolecular coupling of planar and nonplanar geometries can result in the observed sensitivity of excited-state relaxation on circular polarization of excitation. We probed the relative energies of LE and DE states for polymer conformations upon varying two test parameters: (i) sliding one dimer with respect to the other (δx) and (ii) introducing a chiral angle between the aligned E



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dimers (φ) (Figure 6A). We find that, in contrast to the planar conformations, the energies of nonplanar conformations are very sensitive to perturbations (Figure 6B). Furthermore, slight changes in the test parameters switch the lowest energy transition between LE to delocalized (DE). Further, the calculated CD spectra reveal that slight perturbations in the chiral angle cause minimal change in the planar conformations, whereas the flexible, nonplanar geometries exhibit large changes, including CD signal of the opposite sign (Figure 6C,D). These results show that disorder in the relaxation dynamics of nonplanar (LE) geometries can induce a sensitivity of PL decay to circular polarization of excitation, which could lead to significant decay of L polarization.

ACKNOWLEDGMENTS This work was supported by the Australian Research Council Centre of Excellence in Exciton Science (funding grant number CE170100026). The authors thank Dr. Robert Abbel for the generous gift of the polymer material. S.A. acknowledges funding from the Universidad Carlos III de Madrid, the European Union’s Seventh Framework Programme for research, technological development, and demonstration under Grant Agreement No. 600371, el Ministerio de Economı ́a, Industria y Competitividad (COFUND201451509), el Ministerio de Educación, cultura y Deporte (CEI15-17), and Banco Santander.

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CONCLUSIONS In conclusion, the comparison between steady-state CD and FDCD measurements reveal that PLQY of PFBT aggregates is sensitive to circular polarization of excitation, suggesting a crucial role of the excited-state conformation on the radiative and nonradiative decay pathways in PFBT aggregates. Utilizing time-resolved PL and TAS measurements, it was identified that packing of polymer chains in the aggregate facilitates regions with rigidly packed planar conformations, as well as flexible nonplanar conformations, likely predominant at the edges of the aggregate. Although planar conformations support delocalized interchain CT states, nonplanar geometries display localized intramolecular CT character resulting in localized excited states. The dynamics of excited-state relaxation in flexible, nonplanar conformations is prone to disorder, as revealed by significant polarization decay of left-handed polarization at ultrafast time scales. These results provide insights into the origins of disorder in molecular assemblies and its impact on the overall photophysical properties, crucial in identifying the design principles of next-generation optoelectronic devices.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b07482. Materials, methods, and instrumentation; theoretical framework for analyzing FDCD; CD, FDCD, and absorbance spectra for aggregates obtained by cooling dissolved PFBT at different cooling rates; time-resolved PL and TAS measurements; and autocorrelation analysis of TAS data and analysis of computational data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Stavros Athanasopoulos: 0000-0003-0753-2643 Patrick C. Tapping: 0000-0002-2359-1304 Randy P. Sabatini: 0000-0002-5975-4347 Markus Müllner: 0000-0002-0298-554X Tak W. Kee: 0000-0002-4907-4663 Girish Lakhwani: 0000-0003-1070-5859 Notes

The authors declare no competing financial interest. F


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