Inhomogeneity in the Excited-State Torsional Disorder of a

Article pubs.acs.org/JPCB

Inhomogeneity in the Excited-State Torsional Disorder of a Conjugated Macrocycle Jaesung Yang,† Sujin Ham,† Tae-Woo Kim,† Kyu Hyung Park,† Kazumi Nakao,‡ Hideyuki Shimizu,‡ Masahiko Iyoda,*,‡ and Dongho Kim*,† †

Spectroscopy Laboratory for Functional π-Electronic Systems and Department of Chemistry, Yonsei University, Seoul 120-749, Korea ‡ Department of Chemistry, Graduate School of Science and Engineering, Tokyo Metropolitan University, Hachioji, Tokyo 192-0397, Japan S Supporting Information *

ABSTRACT: The photophysics of conjugated polymers has generally been explained based on the interactions between the component conjugated chromophores in a tangled chain. However, conjugated chromophores are entities with static and dynamic structural disorder, which directly affects the conjugated polymer photophysics. Here we demonstrate the impact of chain structure torsional disorder on the spectral characteristics for a macrocyclic oligothiophene 1, which is obscured in conventional linear conjugated chromophores by diverse structural disorders such as those in chromophore size and shape. We used simultaneous multiple fluorescence parameter measurement for a single molecule and quantum-mechanical calculations to show that within the fixed conjugation length across the entire ring an inhomogeneity from torsional disorder in the structure of 1 plays a crucial role in causing its energetic disorder, which affords the spectral broadening of ∼220 meV. The torsional disorder in 1 fluctuated on the time scale of hundreds of milliseconds, typically accompanied by spectral drifts on the order of ∼10 meV. The fluctuations could generate torsional defects and change the electronic structure of 1 associated with the ring symmetry. These findings disclose the fundamental nature of conjugated chromophore that is the most elementary spectroscopic unit in conjugated polymers and suggest the importance of engineering structural disorder to optimize polymer-based device photophysics. Additionally, we combined defocused wide-field fluorescence microscopy and linear dichroism obtained from the simultaneous measurements to show that 1 emits polarized light with a changing polarization direction based on the torsional disorder fluctuations.

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INTRODUCTION Conjugated polymers are important components in optoelectronic devices such as light-emitting diodes, field-effect transistors, and photovoltaic cells. The photophysics of conjugated polymers have generally been explained based on interactions between chromophoresconjugated segments segregated by structural or chemical defectsin the tangled chain.1−4 However, recent single chain spectroscopic studies have shown that the conjugated chromophores are entities with static and dynamic structural disorder and have a pronounced effect on the conjugated polymer photophysics. The physical shapestraight or bentof model oligomers of poly(phenylenevinylene) (PPV) was shown to affect the spectral position and line width.5−7 Aggarwal et al. showed that absorption and emission anisotropy of carbazole-based conjugated macrocycles is associated with the ring symmetry.8 In addition, this structural disorder in conjugated chromophores exhibits a dynamic behavior. Cryogenic temperature single chain spectroscopy of poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV),9−12 methyl-substituted ladder-type poly(p-phenylene) (MeLPPP),9,13−15 poly(3-hexylthiophene) (P3HT),16 and polyfluorenes (PF)17,18 © 2015 American Chemical Society

provided a straightforward framework to identify component single chromophores and revealed temporal fluctuations in their spectral position ranging from a few to a few tens of meV caused by subtle conformational changes. Such conformational changes of poly[9,9-bis(3,6-dioxaheptyl)fluorene-2,7-diyl] (BDOH-PF) at room temperature were shown to bring neighboring chromophores close enough to form groundstate aggregates and result in spectral jumps of a few hundreds of meV.19 Kobayashi et al. also showed that torsional flips in PPV oligomers cause spectral jumps on the order of hundreds of meV at room temperature.20 Based on the Frenkel exciton model, an exciton delocalizes along a conjugated chain via two mechanisms: Coulombinduced dipole and through-bond mechanisms. Exciton transfers via the through-bond interactions are described by J ∝ −t(ϕ)2/ΔE,21 where t(ϕ) is the HOMO and LUMO transfer integral and is proportional to the overlap of neighboring πorbitals: t(ϕ) ∝ cos ϕ. ϕ is the torsional angle between Received: December 11, 2014 Revised: February 4, 2015 Published: February 20, 2015 4116

DOI: 10.1021/jp5123689 J. Phys. Chem. B 2015, 119, 4116−4126

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The Journal of Physical Chemistry B

delocalized in the smallest macrocycle with 12 repeating units.27 Similarly, as will be described, the electronic structure of 1 is collectively contributed from all the component repeating units and thus dominated by the ring symmetry, which is equivalent to the fact that the π-conjugation in 1 is extended across the entire ring. This fixed π-conjugation length and the structural rigidity of 1 minimize the previously demonstrated implications of the varying conjugation length20 and chain bending6 on the spectral characteristics in nominally linear conjugated chromophores, thereby rendering the macrocycle 1 pertinent for exploring the role of chain structure torsional disorder. We present simultaneous multiple fluorescence parameter measurement for a single molecule and time-dependent density functional theory (TD-DFT) and electron density difference map (EDDM) calculations for 1. Because of the ring symmetry in the electronic structure of 1, its lowest-energy transition is directly associated with the ring structure torsional disorder. Based on this characteristic, correlation studies of fluorescence lifetimes and emission energies for single molecules of 1 demonstrate the inhomogeneities and fluctuations in torsional disorder for the ring structure in the lowest-energy excited state, which afford a spectral variability of approximately 220 meV within the fixed conjugation length. The torsional disorder fluctuations occasionally generate torsional defects in 1, which break the ring symmetry and cause a considerable spectral blueshift. Defocused wide-field fluorescence imaging for single molecules of 1 and linear dichroism from the simultaneous measurements provide a complementary explanation for the polarized emission of 1 and its susceptibility to fluctuations in the torsional disorder.

neighboring repeating units. This equation illustrates that the through-bond excitonic interactions in the conjugated chain and the resultant photophysics are highly dependent on a distribution of the torsional angles between repeating units in the chaintermed as a chain structure torsional disorder hereafter. Comparing glassy- and β-phase PFs provides the most obvious experimental example, where the latter were shown to have a nearly coplanar structure and act as a single quantum system despite their average length of ∼500 repeating units.17,18,22 Feist et al. showed that the polychromism in MEHPPVthe presence of distinct blue and red emitting species can be accounted for by the difference in chromophore conjugation length because the chain-packing effects existing in regions in a polymer chain with a high local chromophore density result in the high degree of chain ordering.12 Schmidtke et al. showed that the photoluminescence spectrum for a dilute solid-state solution of poly(9,9-di-n-octylfluorene-alt-benzothiadiazole) (F8BT) red-shifts at high pressure because of the improved chain ordering.23 However, such increase in the degree of chain ordering in conventional conjugated polymers includes not only variations in both the chain structure torsional disorder and the chain bending within a given conjugation length but also a possible increase in chromophore conjugation length resulted from both effects. To exclusively investigate the impact of chain structure torsional disorder in conjugated chromophores on their spectral characteristics, here we introduce a macrocyclic oligothiophene decamer 1 composed of 2,5-thienylene, ethynylene, and vinylene moieties (Figure 1). Previous X-ray crystal structure

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RESULTS AND DISCUSSION Single-Molecule Fluorescence Spectroscopy. The simultaneous multiple fluorescence parameter measurement maximizes the information obtained from each single molecule. Specifically, investigating correlated time-dependent changes in different variables on a molecule-by-molecule basis can uncover dynamic processes obfuscated by ensemble-averaged measurements.6,11,16,22,28−37 On the basis of these benefits, we simultaneously measured the fluorescence intensity, lifetime, emission spectrum, and linear dichroism of single molecules of 1. Picosecond pulsed excitation light at 470 nm with an irradiation power at the sample of 260 W/cm2 was used. A schematic illustration of the experimental setup is shown in Figure S1 of the Supporting Information. Single molecules of 1 predominantly exhibited segmental dynamic behaviors with time. A typical example is shown in Figure 2: this single-molecule switched fluorescence intensity among four different levels, I−IV (Figure 2b). Figure 2d shows the fluorescence decay profiles for I−IV, which all decayed exponentially with time constants of 1.15, 0.91, 1.03, and 0.65 ns. Corresponding emission spectra are shown in Figure 2e. Each spectrum adequately decomposed into three Gaussian curves (0−0, 0−1, and 0−2 vibronic bands from the highestenergy curve). We used the 0−1 vibronic band for further statistical analysis because 8% of the 258 investigated single molecules of 1 included intensity levels in which the blue spectral region of their corresponding emission spectra was truncated by the 488 nm long-pass filter used to remove light scattering from the 470 nm excitation source. In Figure 2e, the 0−1 vibronic peaks for I−IV appear at 2.09, 2.15, 2.09, and 2.21 eV. Comparing the determined fluorescence lifetimes (τF) with

Figure 1. Molecular structure of 1.

analysis revealed that 1 has a rigid two-dimensional cyclic structure with all repeating units in s-cisoid form.24 Optimum size estimation based on the bond angles showed that the angle between the two adjacent ethyne−thiophene bonds is slightly smaller than the inner angle of reported 2,5-diethynylthiophene, which suggested that accommodating ten repeating units into the cyclic structure imparts the structural rigidity to 1.24 Similar s-cisoid conformation was also observed for the Xray crystal structure of directly linked macrocyclic oligothiophene decamer.25 Yet slightly larger macrocyclic oligothiophene dodecamer composed of 2,5-thienylene, ethynylene, and vinylene moieties was shown to exhibit the mixed s-cisoid and s-transoid conformations in the X-ray crystal structure most likely because of the increased conformational flexibility.26 On the other hand, the electronic structures of macrocyclic oligothiophenes were shown to vary with ring size: Donehue et al. used ultrafast time-resolved solution spectroscopy to show that whereas an exciton localizes on a part of the rings in the larger macrocycles with 18−30 repeating units, it is fully 4117


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state A, the longer-lifetime/lower-energy state (τF,A = 0.85 ns, E0−1,A = 2.14 eV), and state B, the shorter-lifetime/higherenergy state (τF,B = 0.63 ns, E0−1,B = 2.21 eV). The population ratio for these two states, PA/PB, was 1.75. Note that the same distribution shown in the contour plot is color-coded for these two states in Figure S3 of the Supporting Information. In state A, the τF and E0−1 exhibited a negative linear correlation with a ρ(τF,A,E0−1,A) value of −0.58, which reproduces the τF−E0−1 distribution for the initial intensity level. In contrast to state A, no apparent linear correlation between τF and E0−1 was observed for state B: whereas the E0−1 values spread over approximately 200 meV, the τF values remained within a narrow range from 0.4−0.6 ns. State B only emerged after continuous illumination: 24% of the investigated single molecules of 1 exhibited transitions from states A to B. Among these molecules, 11 demonstrated that these transitions are reversible. An example of this reversible transition is provided in Figure S2a of the Supporting Information. It is worth noting that no obvious correlation between the fluorescence intensity and lifetime was found for the investigated single molecules of 1. For example, in Figure 2, the fluorescence intensity decreased by ∼71% as it switched from the first to second level, whereas the decrease in τF was only ∼21%. This incompatibility between the two fluorescence parameters has also been observed for other conjugated polymers such as P3HT and polyfluorenebis(vinylphenylene) (PFBV), and its origin is still under debate.38−41 One of the possible reasons suggested by Scheblykin and co-workers is static or ultrafast (faster than the time resolution of the TCSPC measurements) quenching.38−41 The mechanism of this quenching process includes energy transfer, electron transfer, and a reduced conformational freedom imposed by surrounding host polymers. Because 1 is single chromophoric system, it is rather unlikely that the observed discrepancy of the fluorescence intensity and lifetime for 1 arises from either energy or electron transfer. Electron transfer would be more related to fluorescence blinking behavior based on the report by Zhang et al. that oxidation of similar macrocyclic oligothiophene decamer greatly changes its absorption properties.25 At present, we conjecture that little correlation between the fluorescence intensity and lifetime for 1 is likely associated with changes in the degree of freedom for conformations of 1 to relax, which result from fluctuations in chain structure torsional disorder in 1. Quantum-Mechanical Calculations. To interpret the relationship between the τF and E0−1 for states A and B, we examined the electronic structure of 1 using ground-state quantum-mechanical calculations. We performed a geometry optimization based on the X-ray crystal structure of 124 using DFT with the Coulomb-attenuating method (CAM)-B3LYP42 employing a basis set consisting of 6-31G(d,p). We then performed TD-DFT calculations with the same functional and basis set to simulate the ground-state absorption spectrum. Except for the geometry optimization, the TD-DFT and EDDM calculations (vide inf ra) were performed upon replacing the peripheral n-butyl substituents with hydrogen atoms to reduce computational costs. Figure 4a shows the energies and oscillator strengths for the five lowest-energy vertical transitions of 1. Whereas the k = ±1 transitions are strongly allowed and close in energy, the k = 0 transition is fundamentally forbidden, as evidenced by its significantly decreased oscillator strength. In agreement with these results, the absorption spectrum for a dilute chloroform solution of 1 adequately decomposed into

Figure 2. Example of simultaneous multiple fluorescence parameter measurements from a single molecule of 1. (a) Fluorescence linear dichroism (LD) trajectory. (b) Fluorescence intensity trajectory with its horizontally (H) and vertically (V) polarized components. (c) Fluorescence lifetime trajectory (circle) was drawn by successive exponential fitting of fluorescence decays constructed for every 4000 photons. Fitting errors were determined based on ref 87. The full width at half-maximum of the instrument response function obtained for Erythrosin B in chloroform was typically ∼400 ps in our TCSPC system. 0−1 vibronic emission peak energy trajectory (square) was drawn by successive Gaussian fitting of emission spectra for every 1 s. According to segmental dynamics, the trajectories were classified into four parts, I−IV. Fluorescence decay profiles with exponential fits shown in semilogarithmic scale (d) and fluorescence emission spectra with decomposed three Gaussian curves (e) for I−IV.

the 0−1 vibronic emission peak energies (E0−1) verifies that these two fluorescence parameters are inversely correlated, which is more clearly visible in the τF and E0−1 trajectories shown in Figure 2c. Further examples of the inverse correlation between τF and E0−1 for single molecules of 1 are provided in Figure S2 of the Supporting Information. Figure 3a shows the two-dimensional statistical distribution for τF and E0−1 for 1 based on the initial intensity level in the 258 single-molecule fluorescence intensity trajectories. Obviously, the distribution elongated diagonally and fit a single Gaussian distribution with average τF and E0−1 values of 0.81 ns and 2.14 eV, respectively. Pearson’s correlation coefficient, ρ(τF,E0−1), for the distribution was determined to be −0.68, indicating a negative linear correlation between τF and E0−1 of 1. Shown in Figure 3b is the τF−E0−1 distribution for every constant intensity level in the fluorescence intensity trajectories. Distinct from that for the initial intensity level, Gaussian mixture model identified two domains for this distribution: 4118


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Figure 3. Two-dimensional statistical distributions for τF and E0−1 for (a) initial and (b) all intensity levels in fluorescence intensity trajectories of single molecules of 1. 258 and 577 intensity levels were used, respectively. Contour plots in each part are fit to the distributions based on Gaussian mixture model. (c) E0−1 histograms for initial (blue) and all (gray) intensity levels. A single Gaussian fit to the former histogram is also shown (solid line). (d) Schematic representation illustrating the relationship between the ring structure torsional disorder and electronic structure for 1.

the forbidden character of this transition. We performed EDDM calculations for the lowest-energy k = 0 transition using the optimized structure of 1. EDDM visualizes the difference in electron densities between the occupied and unoccupied molecular orbitals that collectively contribute to a given electronic transition.43 As shown in Figure 4b, the EDDM of the optimized structure reveals that the entire π-system along the ring annulus in 1 contributes to the lowest-energy transition, which indicates that the transition is forbidden because of the cancellation of component transition dipoles under ring symmetry. The electronic transition characteristics for 1 resemble those for not only other conjugated macrocycles26,27,44−46 but also the B850 ring in the light-harvesting complex LH2,47,48 porphyrin-based macrocycles,49−51 and cylindrical molecular aggregates:52,53 assuming zero disorder, the k = ±1 transitions are degenerate transitions and carry the majority of the oscillator strength, whereas the k = 0 transition is forbidden because component transition dipoles cancel out. The forbiddenness of the lowest-energy transition was shown to be directly affected by the degree of ring symmetry. For example, Sprafke et al.49 showed that π-conjugated butadiynelinked hexameric porphyrin rings retain higher ring symmetry with hexadentate templates than without them because the template binding to the component porphyrin moieties decreases porphyrin−porphyrin torsional angles. Consequently, the lowest-energy transition for the ring−template complex systems was more strictly forbidden, accompanied by a significant decrease in the near-infrared absorption shoulder intensity and the fluorescence quantum yield. In Figure 4d, 1 shows a nonplanar optimized structure whose component repeating units exhibit a distribution of torsional angles relative to the ring plane. The ring plane is the average plane between the sulfur and carbon atoms in the conjugation backbone. This chain structure torsional disorder in 1 is the primary factor to reduce the ring symmetry and decrease the forbiddenness of the lowest-energy transition, which accounts for a clear manifestation of the low-energy absorption shoulder (Figure 4a) and a moderate fluorescence quantum yield (17%) of 1 compared to the cyclic systems with higher ring symmetry.46,49,51 To elaborate the relationship between the chain structure torsional disorder and the lowest-energy transition of 1, we first

Figure 4. (a) Steady-state absorption spectrum for a chloroform solution of 1 (solid line) and its decomposition into three Gaussian curves (dashed lines). Vertical solid lines represent the five lowestenergy transitions for the optimized structure of 1. Inset shows the schematic representation for torsional angle of a component thiophene ring with respect to the ring plane (α) and torsional angle between neighboring thiophene rings (β). EDDMs for the lowest-energy transition for the optimized structure (b) and the structure with an αavg of 18.1° (c). (d) Optimized structure of 1 depicted upon replacing the peripheral n-butyl substituents with hydrogen atoms for clarity.

three Gaussian curves with peaks at 2.41, 2.73, and 2.94 eV, which correspond to the k = 0, k = ±1 transitions, respectively (Figure 4a). The smaller contribution from the k = 0 transition to the absorption relative to the k = ±1 transitions manifested as a low-energy absorption shoulder, which reasonably matches 4119


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values, which indicate that structures with an αavg ≥ 18.1° are thermodynamically unstable at room temperature. We calculated the EDDMs for the lowest-energy transitions in structures of 1 with differing αavg values. The EDDM for the structure with an αavg of 18.1° is shown in Figure 4c as a representative example. The EDDMs for all of these structures are provided in Figure S5 of the Supporting Information. In contrast to the EDDM for the optimized structure (αavg = 15.0°), the EDDM for the structure with an αavg of 18.1° shows that the component thiophene rings, 7, 8, and 9, have a negligible contribution to the lowest-energy transition. This low contribution occurs because the molecular orbitals that contribute the most to this transition, i.e., HOMO−LUMO (∼49%), HOMO−1−LUMO+1 (∼23%), and HOMO−2− LUMO+2 (∼13%), have little electron density on these thiophene rings (Figure S6 in Supporting Information). Thus, for the given structure of 1, thiophene rings 7, 8, and 9 act as torsional defects that prevent the π-conjugation from extending over the entire ring, which breaks the ring symmetry in the lowest-energy transition. The sites with disrupted π-conjugation exhibit the β values of 57.1° (β6−7) and −56.6° (β9−10; see the Figure S4 in Supporting Information). These π-conjugation cutoff angles agree reasonably with report from De Leener et al. that suggested the cutoff angle of approximately 70° from the INDO/SCI calculations for a model oligomer of MEH-PPV.55 Inhomogeneity in Excited-State Torsional Disorder. Because the photon absorption of a conjugated chromophore is followed by structural relaxation processes such as bond length alternation,58−60 C−C bond stretching,60−63 and torsional relaxation,60,64−67 the quantum-mechanical calculations for the lowest-energy transition of 1 may not be immediately related to the spectral characteristics. Nevertheless, the relationship between the Eex and 1/μ2 as a function of the αavg values qualitatively reproduces that of τF and E0−1 for 1 from the single-molecule fluorescence measurements. On the basis of the relationship among the Eex, 1/μ2, and the EDDMs, we therefore propose that the electronic structure of 1 differs for states A and B. As schematically illustrated in Figure 3d, the π-conjugation is extended over the entire ring for 1 in state A, and thus the electronic structure is dominated by the ring symmetry. However, the presence of torsional defects disrupts the π-conjugation and breaks down the ring symmetry, which accounts for state B. Most importantly, the negative linear correlation between the τF and E0−1 for state A demonstrates that within the fixed conjugation length across the entire ring, the energetic disorder for the lowest-energy excited state of 1 is highly associated with inhomogeneities from torsional disorder in the ring structure. This energetic disorder led to the spectral breadth of approximately 220 meV (see Figure 3a,c). Single chromophores in conventional conjugated polymers were shown to generally have broader spectral breadths based on the cryogenic temperature fluorescence emission spectrum measurements. Feist et al. showed that the emission peak energies for single chromophores in MEH-PPV (Mw ∼ 200 kDa) exhibit a bimodal distribution with its breadth of approximately 0.5 eV (at 1.2 K).12,68 Similar results were observed by Barbara group for the emission peak energies of the MEH-PPV samples with a higher Mw of ∼1000 kDa (at 20 K).2,69,70 Thiessen et al. recently showed that the distribution of the emission peak energies for single chromophores in P3HT (Mw ∼ 65.2 kDa) is trimodal and spans almost 0.8 eV (at 4 K).16 Additionally, the room-temperature measurements by Kobayashi et al. showed that the model oligomers of PPV

quantified the degree of torsional disorder in the optimized structure by measuring the torsional angles for the repeating units with respect to the ring plane (α1, α2, ..., α10; see the inset of Figure 4a). The αavg for the optimized structure was determined to be 15.0°. We then modified the optimized structure by adjusting the torsional angles between neighboring repeating units (β1−2, β2−3, ..., β9−10; see the inset of Figure 4a) using steps of 10% for the βn−n+1 values from the optimized structure, which yielded a set of structures with different αavg values. The αn and βn−n+1 value distributions are provided in Figure S4 of the Supporting Information. A similar approach has proven to be fairly informative in previous studies of model systems of conjugated polymers.23,54,55 For each structure, we calculated the excitation energies (Eex) and dipole strengths (μ) for their lowest-energy transitions and the ground-state energies. Figure 5a shows the Eex and 1/μ2 as a function of

Figure 5. (a) Eex (circle) and 1/μ2 (square) for the lowest-energy transition and (b) ground-state energies for structures of 1 with different αavg values: αavg = 10.8°, 11.3°, 12.1°, 12.8°, 14.0°, 15.0°, 16.2°, 17.1°, 18.1°, 19.1°, 20.6°, and 23.1°. In (b), all of the groundstate energies are offset by the optimized structural energy.

the αavg values, where 1/μ2 is inversely proportional to the radiative rate of a quantum system based on Fermi’s golden rule.56,57 For structures of 1 with αavg < 18.1°, the Eex and 1/μ2 show a linear increase and decrease, respectively, with increasing αavg. This inverse correlation between Eex and 1/μ2 agrees with the experimentally observed negative linear correlation between τF and E0−1 for 1 in state A. On the other hand, for structures of 1 with αavg ≥ 18.1°, the Eex continues to increase linearly with the αavg, whereas the 1/μ2 shows asymptotic behavior. This trend is reminiscent of state B, where the E0−1 values spread over approximately 200 meV, and the τF values remained between 0.4 and 0.6 ns. Figure 5b shows the ground-state energies for structures of 1 with differing αavg 4120


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The Journal of Physical Chemistry B exhibit the emission peak energies with a trimodal distribution spread over approximately 0.5 eV.20 These broad spectral breadths for the nominally linear conjugated chromophores originate from not only variations in the degree of chain ordering within a given conjugation lengtha distribution of the torsional angles between repeating units and the degree of chain bendingbut also the varying conjugation length. We stress that the spectral breadth of 220 meV for 1 in state A, on the other hand, primarily originated from the inhomogeneous chain structure torsional disorder, given that the rigid closed ring structure and the fixed π-conjugation length of 1. It is noteworthy that fairly rigid conjugated polymers MeLPPP (Mw ∼ 25 kDa)9,13 and their undecameric model oligomers13 exhibited a much narrower monomodal emission peak energy distribution at 5 K (approximately 50 meV). This narrow spectral breadth, at least to some extent, might result from the limited torsional motion around the conjugated chain backbone. It was theoretically suggested that the slight structural rearrangements of a small number of atoms in the conjugated chain backbone28,71 and either the torsions or vibrations in the conjugated chain side groups15 can result in a transition frequency variation in conjugated chromophores. Such subtle conformational changes may not necessarily cause the correlated changes in τF and E0−1 for single molecules of 1. In addition, recent reports on isolated polythiophenes in solid films have shown that photochemistry such as photooxidation, radical pair or long-living charge separated species formation, and singlet−singlet or singlet−triplet annihilation processes can cause changes in fluorescence properties of single molecules.39,41 For this reason, the degree of linear correlation between the τF and E0−1 for 1 in the state A was restrained to the ρ(τF,E0−1) value of −0.68. Having torsional defect is most likely thermodynamically unviable for 1 at room temperature based on the EDDM and ground-state energy calculations (Figures 4c and 5b), which explains why none of the investigated single molecules of 1 exhibited state B in the initial intensity levels of the fluorescence intensity trajectories. The emergence of state B is discussed below. It should be mentioned that the structures of 1 shown in Figure 4c and Figure S5 of the Supporting Information are not the only possible candidates for state B. Either a larger or a smaller number of consecutive repeating units may constitute torsional defect sites in the ring annulus of 1. The presence of multiple distinct torsional defects is also conceivable. Because torsional defects necessarily shorten the effective conjugation length in 1, we conjecture that only the structures that exhibited a moderate absorption cross section at the excitation wavelength of 470 nm contributed to populate state B. Torsional Disorder Fluctuations. To examine the segmental dynamic behavior in single molecules of 1, we assessed the jump in fluorescence lifetime (ΔτF) and shift in the 0−1 vibronic emission peak energy (ΔE 0−1 ) between subsequent intensity levels in the fluorescence intensity trajectories. Figure 6a shows the two-dimensional statistical distribution for ΔτF and ΔE0−1. Excepting permanent photobleaching, 304 transition events were used to draw the distribution, in which each transition was color-coded by its starting and ending states: A → B, A → A, B → B, and B → A. At first glance, the distribution is shaped similarly to that of τF and E0−1 for 1. The A → A transitions, which preserve the ring symmetry in the electronic structure, exhibited ΔτF and ΔE0−1

Figure 6. (a) Two-dimensional statistical distribution for ΔτF and ΔE0−1 for single molecules of 1. Excepting permanent photobleaching, 304 transition events were used to draw the distribution. Each transition is color-coded by its starting and ending states: A → B, A → A, B → B, and B → A. Inset shows ΔτF−ΔE0−1 distribution for A → A transitions with its linear fit. (b) ΔE0−1 histogram shown in semilogarithmic scale. Solid azure lines represent a bimodal Gaussian distribution fit to the histogram.

values with a negative linear correlation with a ρ(ΔτF,A→A, ΔE0−1,A→A) of −0.68 (the inset of Figure 6a). This linearity agrees with the negative linear correlation between τF and E0−1 observed for state A. No such linear correlation was found for the A → B, B → B, and B → A transitions. Figure 6b shows the corresponding ΔE0−1 histogram, which adequately fit a bimodal Gaussian distribution with a mean and standard deviation of ΔE0−1,1 = 12 ± 35 meV (82.0%) and ΔE0−1,2 = 118 ± 52 meV (18.0%). Comparing Figures 6a and 6b directly reveals that the ΔE0−1,1 component corresponds to either the A → A or B → B transitions, whereas the ΔE0−1,2 component predominantly corresponds to the A → B transitions generating torsional defects in 1. These characteristics in the bimodal ΔE0−1 distribution and the linearity of ΔτF − ΔE0−1 for the A → A transition correlate with the relationship between the τF and E0−1 for 1 demonstrated in a previous section. Thus, we corroborate that the segmental dynamic behaviors of single molecules of 1 were caused primarily by fluctuations in the torsional disorder within the ring structure. It is important to note that the torsional disorder fluctuations are associated with the spectral drifts spanning a few tens of meV (ΔE0−1,1 = 12 ± 35 meV), which suggests that such spectral drifts observed for single chromophores in MEH-PPV,9−12 MeLPPP,9,13−15 and P3HT16 can be accounted for by temporal fluctuations in the chain structure torsional disorder. Transition paths specified further by the varying energies a− h and their probabilities and rates for single molecules of 1 are presented in Table 1. We calculated transition energy barriers for each path using the Arrhenius relationship, k = Aexp(−ET/ kBT). For the A → B transition, a transition rate kA→B of 0.38 s−1 and an approximate pre-exponential frequency factor A20,72 of 1 × 1012 s−1 yielded a transition energy barrier of 16.7 kcal mol−1. Similarly, the energy barriers for all transition paths were calculated to be in a range of 16.4−17.4 kcal mol−1 (Table 1), which agree reasonably with the rotational barriers for the phenylene units in poly(ethylene terephthalate) and poly(butylene terephthalate) revealed by the solid-state NMR measurements and the ab initio calculations.72−75 The 4121


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As exemplified in Figure 2, the inversely correlated changes in the τF and E0−1 for single molecules of 1 predominantly accompanied changes in the linear dichroism, which indicates that the torsional disorder fluctuations affect the fluorescence polarization of 1. To have more precise understanding, it is necessary to address the anisotropic character of these emissions. As 1 is a single chromophoric system, the simplest situation would be that its transition dipole moment orientation remains static for a structure with a given torsional disorder and 1 emits polarized light. Because of the symmetric twodimensional cyclic structure of 1, it is conceivable that the transition dipole moment orientation undergoes rapid random fluctuations, decreasing the fluorescence anisotropy of 1. It is also conceivable that single molecules of 1 exhibit alternating behaviors between the two invoked situations depending on their conformation, local environment, or time. Although the linear dichroism from the simultaneous measurements was useful to reveal the presence of correlation between the torsional disorder fluctuations and fluorescence polarization of 1, the combinatorial character of its value makes it difficult to determine the exact mechanism from the three possible situations suggested above. Defocused wide-field fluorescence microscopy provides a straightforward way to evaluate the three-dimensional orientation of molecule’s emission dipole moment.80−86 This technique exploits characteristic emission patterns of defocused fluorescence images and resolves both in-plane (φ) and out-ofplane (θ) transition dipole moment orientations (see the inset in Figure 7a). A library of calculated images for various orientations is used together to determine these angles. To examine the anisotropic character of emissions from 1, we performed defocused wide-field fluorescence imaging measurements for single molecules of 1. For excitation, 445 nm light from a continuous wave laser with an irradiation power at the

Table 1. Transition Paths, Probabilities (P), Rates (k), and Energy Barriers (ET) for Single Molecules of 1a a b c d e f g h

transition path

Pb (%)

kc (s−1)

ET (kcal mol−1)

photobleaching from A A → A with decreasing energy A → A with increasing energy A→B B→A B → B with decreasing energy B → B with increasing energy photobleaching from B

22.0 10.6 18.2 12.2 1.7 6.2 10.5 18.6

0.11 0.26 0.33 0.38 0.43 0.37 0.53 0.30

17.4 16.9 16.7 16.7 16.6 16.7 16.4 16.8

a

A and B represent the states A and B, respectively. b512 transition events extracted from the 258 fluorescence intensity trajectories of single molecules of 1 were used. cThe transition rates were calculated by dividing the number of occurrences of a given transition by the sum of the dwell times at the intensity levels from which the transition initiated.

calculated transition energy barriers indicate that fluctuations in the chain structure torsional disorder in 1, including those generating torsional defects in the ring annulus (A → B transition), are unviable with thermal energy at room temperature, which therefore suggests that the fluctuations most likely result from photoinduced processes under continuous illumination during single-molecule measurements. These results are incompatible with the calculated ground-state energies shown in Figure 5b, which predict that the torsional disorder fluctuations occurring within the state A can be caused by thermal energy at room temperature. This discrepancy may arise from the fact that the peripheral n-butyl substituents of 1 were replaced by hydrogen atoms in the quantum-mechanical calculations because of the computational reason. Realistically, complex conformational states of the n-butyl substituents between neighboring thiophene rings and/or interactions between the n-butyl substituents and their surrounding host polymer molecules could cause the emergence of local potential energy minima, which limit thermally driven fluctuations in chain structure torsional disorder in 1. In Table 1, the transitions from state B (paths e, f, g, and h) had 1.6 times the rate of those initiated from state A (paths a, b, c, and d): the probability-weighted average rates for transitions from states A and B are 0.27 and 0.39 s−1, respectively. Furthermore, half of the transitions from state B resulted in permanent photobleaching (path h). Both of these traits indicate that the structure of 1 is less photostable under continuous illumination with torsional defects on its annulus than without these defects. Fluorescence Emission Polarization. The linear dichroism allows the single-molecule fluorescence polarization characteristics to be examined and is calculated by (IV − GIH)/(IV + GIH).1,8,76−79 IV and IH represent the intensities of vertically and horizontally polarized components of fluorescence emission, respectively. G represents a correction factor that accounts for the difference in sensitivity in the two detection channels. The linear dichroism generally provides combinatorial information: experimentally determined linear dichroism values can represent either the degree of fluorescence anisotropy for multiple emitting dipoles coexisting in a diffraction limited volume or a linear dipole angle projected on the plane perpendicular to the optical axis. For example, a linear dichroism of 0 results from both unpolarized emission and emissions with an approximately 45° dipole relative to the two detectors.

Figure 7. Example of defocused wide-field fluorescence imaging measurements from a single molecule of 1. (a) Fluorescence intensity trajectory reconstructed from consecutive defocused images obtained for a single molecule. Image integration time was 1 s. The trajectory was classified into three intensity levels I−III. Inset illustrates polar (φ, θ) coordinate system for the three-dimensional orientation of a transition dipole moment. (b) Experimentally observed defocused images (upper) and corresponding calculated images (lower) for I−III. The determined transition dipole moment orientations are drawn in polar plot in (c). (d) Histogram for angle difference between transition dipole moments from subsequent intensity levels in the fluorescence intensity trajectories for single molecules of 1. 4122


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The Journal of Physical Chemistry B sample of 287 W/cm2 was used. For the analysis, fluorescence intensity trajectories were reconstructed from consecutive defocused fluorescence images obtained for each single molecule, after which these defocused images were stacked according to distinct intensity levels. We assumed that the torsional disorder in the structure of 1 remains unchanged at constant intensity levels, based on the fact that the changes in τF and E0−1 of single molecules of 1 were marginal when the fluorescence intensity remained constant from the simultaneous measurements. Note that states A and B cannot be distinguished in the defocused fluorescence imaging measurements. A typical example is shown in Figure 7a−c: this single molecule of 1 exhibited three distinct fluorescence intensity levels I−III before permanent photobleaching. Corresponding defocused fluorescence images showed characteristic bilobal emission patterns with slightly different long axis orientations and each image was matched with one of the calculated images. The transition dipole moments for the intensity levels I−III were determined to have angles (φ, θ) of (290°, 75°), (310°, 75°), and (270°, 75°), respectively. Remarkably, all defocused fluorescence images for the 304 intensity levels extracted from the 173 fluorescence intensity trajectories for single molecules of 1 adequately defined corresponding transition dipole moments with single orientations. The resultant transition dipole moment orientations represented as dots in the polar plot are provided in Figure S7 of the Supporting Information with an associated discussion. These results indicate that the transition dipole moment orientation for 1 is essentially static when the torsional disorder in the ring structure remains unchanged, which results in the polarized emission. If the transition dipole moment orientation fluctuated under either a constant intensity or an image integration time (1 s), the defocused image would appear as the overlap of several bilobal emission patterns that cannot be assigned to any calculated images, similar to reports on perylenediimide molecular assemblies.33,82,85,86 The polarized emission of 1 led us to conclude that the correlated changes among the τF, E0−1, and the linear dichroism of single molecules of 1 observed from the simultaneous measurements indicate changes in a direction of the polarized emission of 1 caused by the torsional disorder fluctuations. To evaluate the magnitude of the changes in fluorescence polarization direction, we calculated the angle difference between transition dipole moments from subsequent intensity levels in the fluorescence intensity trajectories. For example, the single molecule of 1 shown in Figures 7a and 7b changed its transition dipole moment orientation by 19.3° and 38.6°, respectively, as the fluorescence intensity switched in order from I to III (Figure 7c). Figure 7d shows the angle difference distribution for single molecules of 1 that exhibited multiple fluorescence intensity levels (60/173). The distribution is centered at approximately 20° and extended up to 85°. We conjecture that the greater changes in the transition dipole moment orientation presumably resulted from the emergence of state B due to the generation of torsional defects in 1, given that the interstate A → B and B → A transitions caused greater changes in the linear dichroism values than transitions within a single state (Figure S8 in Supporting Information). Comparing the fluorescence polarization characteristics of 1 with the carbazole-based conjugated macrocycle recently reported by Aggarwal et al.8 is informative. The latter macrocycle is composed of six N-phenylcarbazole units linked

each other by phenylene−ethynylene−butadiynylene moieties. The single carbazole-based rings were shown to exhibit fluctuations in the fluorescence emission polarization on two time scales: for the first 100 ms of the fluorescence linear dichroism measurements, the rings predominantly emitted unpolarized light with a linear dichroism of approximately 0 because of rapid emission switching between equally weighted component chromophores. Thereafter, the rings typically exhibited a nonzero linear dichroism with discrete jumps between values on the time scale from milliseconds to seconds, which were explained by random exciton localization and its light-driven fluctuation. On the other hand, we showed that the emission of 1 is polarized in a direction associated with variations in torsional disorder within the ring structure. These different polarization characteristics for the two macrocycles result from the fundamental difference in their electronic structures: whereas the effective size of the emissive exciton, i.e., chromophore size, for the carbazole-based macrocycle was defined as a dimeric carbazole unit, 1 is a single chromophoric system in which excitonic interactions between the component repeating units collectively generate a forbidden lowest-energy excited state. This comparison reveals a close relationship between electronic structure and fluorescence polarization in cyclic molecular systems.

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CONCLUSIONS In this study, we presented the correlation studies of singlemolecule fluorescence intensity, lifetime, emission energy, and linear dichroism, and the TD-DFT and EDDM calculations for 1. This macrocycle provided a straightforward framework to explore the relationship between chain structure torsional disorder and spectral characteristics in conjugated chromophores for the following reasons: first, the rigid closed ring structure and the fixed π-conjugation length of 1 minimize the effects of the chromophore size and shape on its spectral characteristics, which generally coexist with the torsional disorder effect in linear conjugated chromophores. Second, the lowest-energy transition of 1 is directly associated with the degree of torsional disorder within the ring structure because of the ring symmetry. On the basis of these characteristics, we showed that inhomogeneities from chain structure torsional disorder in 1 play a crucial role in causing the energetic disorder for the lowest-energy excited state within the fixed conjugation length, which affords the spectral variability that spans approximately 220 meV. The torsional disorder fluctuations in 1 typically led to the spectral drifts on the order of approximately 10 meV and could generate torsional defects in the ring annulus that cause a considerable spectral blue-shift. We believe that our findings are general for chromophores in a broad class of conjugated chain materials and disclose the importance of engineering structural disorder to optimize polymer-based device photophysics.

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ASSOCIATED CONTENT

* Supporting Information S

Experimental methods; additional examples from the simultaneous multiple fluorescence parameter measurements on single molecules of 1; two-dimensional statistical distribution for τF and E0−1 color-coded by states A and B; both αn and βn−n+1 distributions and EDDMs for the lowest-energy transition for structures of 1 with different αavg values; frontier molecular orbitals for the structure of 1 with an αavg of 18.1°; polar plot of the transition dipole moment orientations for single molecules 4123


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of 1 and an associated discussion; histograms for the changing linear dichroism for the intra- and interstate transitions of single molecules of 1. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (D.K.). *E-mail [email protected] (M.I.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was financially supported by the Midcareer Researcher Program (NRF-2005-0093839) of the MEST of Korea (D.K.). The quantum calculations were performed using supercomputing resources of the KISTI. The work at Tokyo Metropolitan University was supported by a Grant-in-Aid for Scientific Research from the JSPS and by the JST of Japan (M.I.).

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