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Letter

Chain-Length-Dependent Exciton Dynamics in Linear Oligothiophenes Probed Using Ensemble and Single-Molecule Spectroscopy Tae-Woo Kim, Woojae Kim, Kyu Hyung Park, Pyosang Kim, JaeWon Cho, Hideyuki Shimizu, Masahiko Iyoda, and Dongho Kim J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.5b02864 • Publication Date (Web): 14 Jan 2016 Downloaded from http://pubs.acs.org on January 17, 2016

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The Journal of Physical Chemistry Letters

Chain-Length-Dependent Exciton Dynamics in Linear Oligothiophenes Probed Using Ensemble and Single-Molecule Spectroscopy Tae-Woo Kim†, Woojae Kim†, Kyu Hyung Park†, Pyosang Kim†, Jae-Won Cho†, Hideyuki Shimizu‡, Masahiko Iyoda*,‡, and Dongho Kim*,† † Spectroscopy Laboratory for Functional π-electronic Systems and Department of Chemistry, Yonsei University, Seoul 03722, Korea ‡ Department of Chemistry, Graduate School of Science and Engineering, Tokyo Metropolitan University, Hachioji, Tokyo 192-0397, Japan

Corresponding Author *E-mail: [email protected] (D.K.). *E-mail: [email protected] (M.I.).

ACS Paragon Plus Environment

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ABSTRACT

Exciton dynamics in -conjugated molecular systems is highly susceptible to conformational disorder. Using time-resolved and single-molecule spectroscopic techniques, the effect of chain length on the exciton dynamics in a series of linear oligothiophenes, for which the conformational disorder increased with increasing chain length, was investigated. As a result, extraordinary features of the exciton dynamics in longer chain oligothiophene were revealed. Ultrafast fluorescence depolarization processes were observed due to exciton self-trapping in longer and bent chains. Increase in exciton delocalization during dynamic planarization processes was also observed in the linear oligothiophenes via time-resolved fluorescence spectra but was restricted in L-10T because of its considerable conformational disorder. Exciton delocalization was also unexpectedly observed in a bent chain using single-molecule fluorescence spectroscopy. Such delocalization modulates the fluorescence spectral shape by attenuating the 0-0 peak intensity. Collectively, these results provide significant insights into the exciton dynamics in conjugated polymers.

TOC GRAPHICS

Single-Molecule Study

Ensemble Study Ph Bu

Ph

S Bu Bu Bu

S

Bu Bu S Bu Bu S S Bu Bu S Bu Bu S Bu Bu Bu Bu

Straight

S

Bu Bu

Bu Bu S

Bu Bu

Excitonself-trapping

Bu Bu

S

Ph

S

S

Bu Bu

Bu Bu

Ph Bu Bu S

Bu Bu S S Bu Bu S Bu Bu S S Bu Bu Bu Bu

Bu Bu S

S

Bu Bu

Bent

Ph S

Bu Bu

Bu S Bu S

Dynamic planarization

Ph

Bu S

S Bu

Bu S

Bu Bu

Bu

Bu Bu S

S

Bu Bu

S

S

S

Bu S

S

Bu Bu

Bu

S

Bu Bu S

Bu Bu

Bu S

Bu Bu

S

Bu Bu S

Bu Bu

Ph S

S Bu

Bu

Ph

Ph Bu

Bu Bu

Bu Bu

Bu Bu S

S

Ph

Bu S Bu

Ph

S

S

Bu Bu

Ph

Bu Bu

Bu Bu S

S

Ph

S

S

Bu Bu

Bu S Bu

Bu Bu

S

Bu Bu S Bu

Bu

Bu Bu S

Bu Bu S

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S

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S Bu

Bu

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KEYWORDS

linear

oligothiophene;

exciton

dynamics;

fluorescence

up-conversion

spectroscopy; single-molecule fluorescence spectroscopy; conformational disorder, chain bending

Many -conjugated polymers have drawn significant attention during the past few decades because of their semiconducting and optical properties and potential applications in solar cells, field-effect transistors, and organic light-emitting diodes.1-5 In such devices, the exciton dynamics in conjugated polymers is an important factor for determining device performance.6,7 As a consequence, many researchers have devoted considerable effort to unravel the exciton dynamics in conjugated polymers for technological applications and academic interest.8-12 Nevertheless, our understanding of the exciton dynamics in conjugated polymers remains limited because of the inherent existence of severe conformational disorder.8 To overcome this problem, oligomers have been proposed as promising model systems.13 Oligomers have similar electronic and optical properties to their corresponding polymers but with less polydispersity.14,15 Furthermore, it is possible to elucidate the structure/property relationships of well-defined oligomers with finely controlled size and shape. As a result, the exciton dynamics in various conjugated oligomers has been the subject of intensive research in recent years.16-28 Moreover, the results of these studies have enhanced intuitive understanding of the optical properties of conjugated polymers. In general, after photoexcitation (exciton formation), the exciton dynamics in conjugated oligomers can be summarized as follows: exciton self-trapping due to strong electron-phonon coupling,16 structural reorganization such as torsional relaxation or dynamic planarization,17 and


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exciton radiative recombination as fluorescence emission or intersystem crossing (ISC) to form triplet excitons. These processes in conjugated oligomers have been investigated with appropriate spectroscopic methods. Li et al.18 and Bednarz et al.19 reported size-dependent effects in the photophysical properties of oligothiophenes by observing steady-state optical spectra. Chang et al. reported the influence of nuclear geometric relaxation on the extent of exciton delocalization in conjugated zinc porphyrin oligomers.17 Thiessen et al. reported the extent of exciton localization and structural relaxation in carbazole-based linear and cyclic oligomers by combining transient fluorescence spectroscopy of ensembles with single-molecule polarization anisotropy analyses.20 Nevertheless, the effect of chain length on exciton dynamics in linear oligomers has still not been well understood. Here, we describe our investigation on the exciton dynamics in linear oligothiophenes with varying the chain length. As the chain length increases, conformational disorder is expected to increase, and therefore the longer chains tend to adopt bent geometries. Through a comparative analysis of model systems L-4T29, L-6T30, and L-10T24 (see Chart 1), the impact of chain length and conformational disorder on the exciton dynamics of linear conjugated oligomers can be revealed. In this regard, we have comprehensively investigated the exciton dynamics in the linear oligothiophenes as a function of chain length using time-resolved and single-molecule spectroscopic methods. Time-resolved fluorescence anisotropy measurements were used to investigate exciton self-trapping process, which induces ultrafast fluorescence depolarization in longer chains of linear oligothiophenes. Through time-resolved fluorescence spectra, chainlength-dependent increase in exciton delocalization due to dynamic planarization processes was observed. Single-molecule fluorescence spectroscopy was employed to investigate dynamic


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changes in exciton sizes and the effect of chain bending on fluorescence spectra of single oligothiophene chains. Basic photophysical properties of the linear oligothiophenes were obtained using steady-state spectroscopic methods (see Figure S1, and Table S1 in Supporting Information). As the chain length increased, the absorption and fluorescence spectra exhibited bathochromic shifts and the ratios of the 0-0 to 0-1 vibronic peak intensities in the fluorescence spectra increased. These results are attributed to an increase in the exciton delocalization length, in accordance with a previous work.18 Especially, the lack of mirror-image symmetry between the absorption and emission spectra was observed, which indicates excited-state relaxation in linear oligothiophenes as suggested in previous studies.24,31,32 To obtain information on reorientation of transition dipole moments during excited-state relaxation, we performed steady-state and time-resolved fluorescence anisotropy measurements (Figures 1a, and 1b).

Chart 1. Molecular Structures of the Linear Oligothiophenes.

(

)n

L-4T : n = 1 L-6T : n = 3 L-10T : n = 7


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a

b

L-4T

0.24

0.40

L-4T

0.16 0.08

L-6T

0.16 0.08

Anisotropy

Absorbance (Norm.)

L-6T 0.35

0.00 0.24

Anisotropy

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0.30

0.00 0.24

L-10T

L-10T 0.25

0.16 0.08

300

400

0.00 600

500

0.20 0

500

Wavelength (nm)

1000

1500

2000

Time (fs)

Figure 1. (a) Steady-state fluorescence excitation anisotropy spectra (dashed lines) with absorption spectra (solid lines) for the linear oligothiophenes (L-4T, L-6T, and L-10T). Fluorescence excitation anisotropy spectra were probed near the emission maximum for each oligomer. (b) Time-resolved fluorescence anisotropy decay profiles for L-4T (circles), L-6T (squares), and L-10T (triangles). Fluorescence anisotropy decay profiles were probed near the emission maximum for each oligomer after photoexcitation at 450 nm. The steady-state fluorescence excitation anisotropy spectra for the linear oligothiophenes are shown in Figure 1a (dashed lines). As the chain length increased from L-4T to L-10T, the anisotropy value at the maximum of each absorption spectrum decreased, indicating that the relative orientation of the absorption and emission dipoles changed. Previously reported


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theoretical simulations have suggested that kink or worm-like structures may exist in the ground state of conjugated thiophene oligomers, leading to different directions of transition dipole moments for the higher and lowest excited states.33 Accordingly, the conformational disorder of the linear oligothiophenes appeared to increase as the chain length increased, resulting in the fluorescence depolarization during excited-state relaxation processes, such as internal conversion, exciton self-trapping and torsional relaxation.10,17,28,33,34 Time-resolved fluorescence anisotropy measurements were employed to investigate the fluorescence depolarization, particularly on exciton self-trapping on the subpicosecond time scale. We measured fluorescence anisotropy decay profiles for the linear oligothiophenes after photoexcitation at 450 nm which is corresponding to S1 state,24 in order to exclude internal conversion as a factor for inducing fluorescence depolarization (Figure 1b). The initial fluorescence anisotropy values clearly decreased going from L-4T to L-10T, which is implicative of ultrafast fluorescence depolarization within the instrumental response of ~100 fs due to exciton self-trapping.33-35 It has been proposed that in -conjugated oligomers and polymers, geometrical relaxation toward the minimum for the C=C stretching coordinate (13001500 cm-1) leads to exciton self-trapping to the spectroscopic segment in the excited state. In particular, for a bent geometry such as a worm-like structure, this process can induce reorientation of the transition dipole moment with the C=C stretching vibration period (T ~ 25 fs).35 Therefore, the longer chains exhibited a smaller initial anisotropy value due to ultrafast depolarization through exciton self-trapping because long chains tend to adopt bent geometries. Interestingly, although L-4T did not clearly undergo anisotropy decay in the ~2 ps time window with rinf = 0.38, L-6T and L-10T exhibited an additional fluorescence depolarization with time constants of ~200 and ~400 fs, respectively. In this case, it is thought that torsional motions


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contributed to exciton localization. Using theoretical simulations, Tretiak et al. found that a slow nuclear mode referred to as torsional mode also induce exciton self-trapping in conjugated systems.36 Furthermore, the vibrational period of the torsional mode in a quinquethiophene was previously observed using coherent vibrational spectroscopy to have a time constant of 240 fs. 37 These results suggest that the exciton self-trapping along the torsional modes may result in the additional depolarization in the longer chains. Time-resolved fluorescence spectra (TRFS) obtained using femtosecond broadband upconversion technique38-40 were then used to investigate the effect of dynamic planarization processes on exciton delocalization in the linear oligothiophenes after the subpicosecond time scale. The changes in the vibronic peak ratios, the spectral red shifts, and the increases in fluorescence intensities observed in the time-resolved fluorescence spectra of conjugated polyand oligomers are known to reflect the significant role that torsional relaxation plays in exciton delocalization.10,17,41,42 Accordingly, TRFS was used to analyze the linear oligothiophenes in order to gain insight into their exciton delocalization processes. Because the fluorescence dynamics in linear oligothiophenes is nonexponential and depends on the fluorescence wavelength (energy), the decay profiles at various probe wavelengths (energies) were each analyzed via global fitting with the summation of three exponential functions, one of which had a decay time (3) fixed as the lifetime of the lowest excited state (see Table S2, and Figure S3 in Supporting Information). The first two time constants for L-4T, L-6T, and L-10T were 1 = 8.2, 8.7, and 9.1 ps and 2 = 24, 35, and 45 ps, respectively. These time constants are in the range of picoseconds or tens of picoseconds and reflect the torsional relaxation involved in molecular conformational changes from distorted to more planar


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geometries in the excited states of the oligothiophpenes, as previously reported.10,12,22,42,43 The slightly longer time constants for the longer linear oligothiophenes are attributed to greater conformational disorder in the torsional angles because of the innate structural flexibility of the longer chains, which leads to an increase in the overall time for torsional relaxation following photoexcitation. a 150

Time (ps)

0.10

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1.1 L-4T L-6T L-10T

1.0 0.9 0.8 0.7 0.6

0

50

100

150

0

Time (ps)

50

100

150

Time (ps)

Figure 2. (a) Time-resolved fluorescence spectra for the linear oligothiophenes (L-4T, L-6T, and L-10T) after photoexcitation at 400 nm. (b) Temporal shifts of the main emission peak energies of the linear oligothiophenes for the relative energy change ∆E(t) = E(t) – E(0). (c) Total emission intensities for the linear oligothiophenes calculated from the spectrally integrated emission intensities of the 0-0 and 0-1 vibronic peaks.


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As can be seen in Figure 2a, all of the TRFS for the linear oligothiophenes exhibited spectral red shifts and increases in the fluorescence intensity, indicative of increase in exciton delocalization through dynamic planarization processes. In addition, decrease in the vibronic peak intensity ratio (I0-0/I0-1), which is an evidence of dynamic planarization (see Figure S4 in Supporting Information), was also observed. To perform a more detailed analysis of the relationship between the spectral evolution and dynamic planarization processes, TRFS for the linear oligothiophenes at various time delays after photoexcitation were each fitted with the sum of two to three Gaussian functions using the vibronic transition energies (0-0, 0-1 and 0-2) observed in the fluorescence spectra.24,25 Based on the fitting results, the 0-0 transition energy (E0-0), and the total emission intensity, or the sum of the spectrally integrated intensities for the 0-0 and 0-1 vibronic peaks (I0-0+I0-1) were calculated and plotted as a function of time in Figures 3b and 3c, respectively. As can be seen in these figures, all of the linear oligothiophenes exhibited transient red shifts of their emission peaks after photoexcitation and increases in their total emission intensities during early emission dynamics. The evolution of the changes in the 0-0 transition energies and total emission intensities for the linear oligothiophenes were found to be well-matched with the time scales observed in the plots of the decay profiles analyzed via global fitting. Therefore, it can be concluded that these phenomena resulted from dynamic planarization processes. It has been previously reported that this type of process is accompanied by an increase in excitonic delocalization leading to growth of the oscillator strength with a spectral red shift.17 The extents of the red shifts of the transition energies were 15 meV for L-4T, 26 meV for L-6T, and 19 meV for L-10T in the ~150 ps time window. Likewise, the amplitudes of the increases in the total emission intensities of the linear oligothiophenes increased in the order of L-4T < L-10T < L-6T.


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These results suggest that the extent of increase in exciton delocalization also increased in the order of L-4T < L-10T < L-6T. The excitons in L-4T were already delocalized through most of the molecule, even after exciton self-localization, because of its rigid conformation and the straight geometry of the chain. Conversely, the excitons in L-6T, which had already experienced self-localization arising from conformational disorder, further expanded along the conjugated backbone during dynamic planarization to increase the pi overlap. In contrast, in L-10T, the increase in exciton delocalization appeared to be restricted by the considerable conformational disorder, including kink and/or worm-like structures, in its long, distorted chain. To gain further insight into the effects of molecular chain length on the exciton properties, static and dynamic heterogeneities in exciton state of linear oligothiophenes were directly investigated by employing single-molecule fluorescence spectroscopy without obfuscation by an ensemble averaging effect.44-51 For this purpose, the fluorescence intensity trajectories (FITs) and fluorescence spectra corresponding to each discrete step were observed by recording the fluorescence signal from a single molecule of each oligomer embedded in an inert polymer matrix. Each fluorescence spectrum was fitted with the sum of two to three Gaussian functions that exhibited the vibronic transition energies (0-0, 0-1 and 0-2) observed in the fluorescence spectrum. The exciton dynamics was then studied by analyzing the fluorescence spectral peak positions and peak ratios (I0-0/I0-1) for the single chain of each linear oligothiophene.


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b

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0

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Time (s)

20

30

400 500 600 400 500 600 400 500 600 Wavelength (nm)

Figure 3. Representative fluorescence intensity trajectories (FITs) and corresponding fluorescence spectra for (a) L-4T, (b) L-6T, and (c),(d) L-10T. Representative data for single L-4T, L-6T, and L-10T molecules are shown in Figure 3. All of the FITs exhibited stepwise intensity jumps and concurrent changes in corresponding fluorescence spectra in each trajectory. As can be seen in Figure 3a, the fluorescence emission spectrum for L-4T exhibited a hypsochromic shift (from 484 to 464 nm) and a decrease in the spectral peak ratio (from 1.50 to 1.35) as the fluorescence intensity jumped from the first step to the second step. A similar tendency was also observed in the fluorescence emission spectral


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changes for L-6T and L-10T: hypsochromic shifts with a decrease in peak ratio (Figures 3b and 3c, respectively). For a conjugated polymer or oligomer, a change in the conjugation length, or exciton size, associated with a conformational change is accompanied by changes in the fluorescence intensity, lifetime, and spectrum, as previously reported.47,49,52-56 Accordingly, the segmental dynamic behavior observed for the linear oligothiophenes evaluated in the present study can be attributed to changes in the exciton sizes of the samples due to conformational changes (mostly in the torsional angles because of the restriction in the polymer matrix) in the single chain of each compound. The decrease in exciton delocalization subsequently resulted in shifts in the fluorescence emission spectra to higher energies and decreases in the peak ratios because of attenuation of the electronic coupling between the constituent thiophene units. Interestingly, the representative fluorescence emission spectrum for a single L-10T molecule displayed an unusual tendency that is contrary to the explanation described above. As can be seen in Figure 3d, when the fluorescence intensity jumped to the next step, the fluorescence emission spectrum exhibited a bathochromic shift (528 → 561 → 579 nm), but the spectral peak ratio decreased gradually (1.90 → 1.62 → 1.49). To elucidate the reason for this unusual segmental dynamic behavior in the single chain of this linear oligothiophene, a statistical analysis of the fluorescence parameters obtained using single-molecule fluorescence spectroscopy, including the fluorescence energy shift (∆E) and change in the spectral peak ratio (∆I0-0/I0-1), was performed for all three compounds.


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0.8

ρ = -0.67

L-4T

ρ = -0.61

L-6T

ρ = -0.56

L-10T

0.4  I0-0 / I0-1

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0.0 -0.4 -0.8

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-0.3

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-0.1

0.1

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 Energy (eV)

Figure 4. Two-dimensional statistical distributions of ∆E and ∆I0-0/I0-1 for single molecules of the linear oligothiophenes. Figure 4 shows two-dimensional plots for the statistical distributions of ∆E (x-axis) and ∆I00/I0-1

(y-axis) for L-4T, L-6T, and L-10T. The distributions of all three of the linear

oligothiophenes were elongated diagonally and exhibited negative linear correlations. This negative linear correlation between ∆E and ∆I0-0/I0-1 for the single chains indicates that the fluorescence spectra exhibited bathochromic (hypsochromic) shifts with increases (decreases) in the spectral peak ratios due to the expansion (shrinkage) of the exciton sizes, which is in accordance with the results presented in Figures 3a, 3b, and 3c, respectively. The Pearson’s correlation coefficients, ρ(∆E,∆I0-0/I0-1), for the distributions were determined to be -0.67 for L4T, -0.61 for L-6T, and -0.56 for L-10T and decreased as the chain length increased. The reduced linear correlation for the longer chain clearly appears to result from exciton extension over a bent molecular chain. A theoretical study on the impact of chain bending on photoluminescence (PL) spectral line shapes and radiative decay rates by Hestand et al. revealed the attenuation of the 0-0/0-1 PL ratio and radiative decay rate due to the misalignment of monomeric transition dipole moments with chain bending.57 Accordingly, the expansion of an exciton over a bent chain results in the fluorescence spectrum with a bathochromic shift but a


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decrease in the spectral peak ratio that reflects the decrease in the 0-0 intensity. Therefore, ρ(∆E,∆I0-0/I0-1) decreases as the chain length increases, because longer chains are more likely to adopt bent geometries. In summary, we have investigated the effect of chain length on the exciton dynamics in a series of -conjugated linear oligothiophenes using time-resolved and single-molecule spectroscopic analyses. Although the oligomers were all relatively short, the conformational disorder distinctly increased as the chain length increased, as indicated by the results obtained using time-resolved fluorescence anisotropy decay, time-resolved fluorescence spectra, and single-molecule fluorescence spectroscopy. Ultrafast fluorescence depolarization was observed because the longer chains tended to adopt bent geometries, resulting in ultrafast reorientation of their transition dipole moments during exciton self-trapping. Analysis of time-resolved fluorescence spectra revealed that the linear oligothiophenes experienced the increase in exciton delocalization induced by dynamic planarization processes but to different extents. The increase in exciton delocalization of L-10T was restricted due to the considerable conformational disorder in its long chain. Changes in the exciton sizes in single oligothiophene chains were observed using single-molecule spectroscopy, as was the effect of chain bending on the fluorescence spectra of the linear oligothiophenes; the 0-0 peak intensity was attenuated because of expansion of excitons over a bent chain. Consequently, we suggest that the chain length and conformational disorder of conjugated oligomers play crucial roles in determining the optical properties and exciton dynamics of these materials. Furthermore, we believe that these results will provide intuitive knowledge for enhancing future investigations of the exciton dynamics in conjugated polymers.


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ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. Experimental methods, photophysical parameters obtained using steady-state measurements, fluorescence decay profiles obtained using TCSPC measurements, time-resolved fluorescence decay profiles at various wavelengths obtained using femtosecond broadband fluorescence upconversion measurements, temporal evolution of the vibronic peak ratios from the TRFS results, and statistical distributions of fluorescence parameters obtained using the singlemolecule fluorescence spectroscopy. (PDF) (This information can be found on the internet at http://pubs.acs.org.)

AUTHOR INFORMATION Notes

The authors declare no competing financial interests. ACKNOWLEDGMENT The work was supported by the Global Frontier R&D Program on Center for Multiscale Energy System funded by the National Research Foundation under the Ministry of Science, ICT & Future, Korea (NRF-2014M3A6A7060583) (D.K.), and the work at Tokyo Metropolitan University was supported by a Grant-in-Aid for Scientific Research from JSPS (22245024 and 26288040) (M.I.). The authors thank J. Sung for experimental support.


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Chain-Length-Dependent Exciton Dynamics in ... - ACS Publications

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