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Ultrafast Exciton Delocalization, Localization and Excimer Formation Dynamics in a Highly Defined Perylene Bisimide Quadruple #-Stack Christina Kaufmann, Woojae Kim, Agnieszka Nowak-Król, Yongseok Hong, Dongho Kim, and Frank Würthner J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b11571 • Publication Date (Web): 05 Mar 2018 Downloaded from http://pubs.acs.org on March 5, 2018
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Journal of the American Chemical Society
Christina Kaufmann,†,§ Woojae Kim,‡,§ Agnieszka Nowak-Król,† Yongseok Hong,‡ Dongho Kim,‡,* Frank Würthner†,* † Universität Würzburg, Institut für Organische Chemie & Center for Nanosystems Chemistry, Am Hubland, 97074 Würzburg, Germany, ‡ Department of Chemistry and Spectroscopy Laboratory for Functional -Electronic Systems, Yonsei University, Seoul 03722, Korea KEYWORDS Dye aggregate, excimer, perylene bisimide, pi-stack, ultrafast spectroscopy
ABSTRACT: An adequately designed, bay-tethered perylene bisimide (PBI) dimer Bis-PBI was synthesized by Pd/Cu-catalyzed Glaser-type oxidative homo-coupling of the respective PBI building block. This newly synthesized PBI dimer self-assembles exclusively and with high binding constants of up to 10 6 M−1 into a discrete -stack of four chromophores. Steady-state absorption and emission spectra show the signatures of H-type excitonic coupling among the dye units. Broadband fluorescence upconversion spectroscopy (FLUPS) reveals an ultrafast dynamics in the optically excited state. An initially coherent Frenkel exciton state that is delocalized over the whole quadruple stack rapidly (τ = ~ 200 fs) loses its coherence and relaxes into an excimer state. Comparison with Frenkel exciton dynamics in PBI dimeric and oligomeric H-aggregates demonstrates that in the quadruple stack coherent exciton propagation is absent due to its short length of aggregates, thereby it has only one relaxation pathway to the excimer state. Furthermore, the absence of pump-power dependence in transient absorption experiments suggests that multi-exciton cannot be generated in the quadruple stack, which is in line with time-resolved fluorescence measurements.
Unravelling the electronic coupling in photoexcited molecular aggregates1,2 has developed into a research field of high interest due to prospects for applications in photonics, 3 photovoltaics,4 and sensing5 as well as its relevance for the understanding of light harvesting (LH) processes6,7 in photosynthetic plants and bacteria. Unfortunately, fundamental questions of exciton coupling and delocalization or electronic coherences so far can only be studied for larger chromophore ensembles isolated from natural light harvesting complexes,8 rather undefined extended aggregates where size and exact packing are typically not well characterized9 or small dimer aggregates.10 For the latter, arranged as -stacked H-coupled dimers, during the last years a lot of fundamental insights were gained that include the characterization of ultrafast relaxation of initial Frenkel exciton states into excimers,11 symmetry breaking charge separation (SBCS),12 or triplet state population either via SBCS or singlet fission.13 Due to their ideal properties (strong stacking interactions, high fluorescence quantum yield, excellent photostability) perylene bisimide (PBI) dyes evolved as the most utilized chromophore for such studies. 14 Accordingly, in our endeavor to advance the understanding of excitons in molecular aggregates of highly defined size and geometry, we envisioned stacks of four closely stacked dyes which should become accessible in a zipper-type approach15 by the self-assembly of properly designed covalent dimers (Figure 1). Thus, we have designed the bay-tethered PBI dye Bis-PBI by
molecular modeling, in which two PBI chromophores are covalently linked by a diacetylene spacer in the bay position (Scheme 1). The spacer unit ensures a distance of approximately 7 Å between the -scaffolds which is exactly twice the --distance, that is known to enable the intercalation of an additional chromophore.16 Different from previously reported examples of tethered bis-PBIs,17,18,19 in which the PBI subunits are connected at the imide positions, we wanted to utilize these positions to introduce long branched alkyl chains to ensure adequate solubility in nonpolar solvents which is required for these studies. Thus, for the first time, we have used an ether functionality in bay area to anchor the tether unit. By this approach, an exclusive PBI quadruple -stack, which is a dimerized Bis-PBI at high concentration, has been successfully synthesized and characterized. Furthermore, with the aid of broadband fluorescence upconversion spectroscopy (FLUPS),27 ensuring broadband detection range and ultrafast time resolution at once, we have investigated ultrafast Frenkel exciton relaxation and excimer formation dynamics in the PBI quadruple -stack within 1 ps. By analyzing the vibronic line shape in the early-time transient fluorescence spectra in detail, we demonstrate that the Frenkel exciton immediately after photoexcitation is entirely delocalized along the quadruple stack and immediately loses its coherence followed by the formation of the excimer state.
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Synthesis and Characterization of the Self-assembled Quadruple -stack. The target compound Bis-PBI was synthesized by palladium/copper-catalyzed Glaser-type oxidative homo-coupling of acetylene-functionalized precursor Ref-PBI, the latter was obtained by nucleophilic substitution of the respective bromo-substituted PBI with propagyl alcohol (Scheme 1). Detailed synthetic procedures and characterizing data of literature unknown compounds are reported in the Supporting Information.
Figure 2. Concentration-dependent UV/Vis absorption spectra (colored lines) of Bis-PBI (c0 = 3.51 x 104 M – 3.75 x 107 M) in toluene at 298 K. Also shown are the calculated spectra obtained by a global fit analysis for supramolecular dimerization (black dashed lines). Arrows indicate the changes in intensity of the 00 and the 01 band with decreasing concentration. Furthermore depicted are steady state fluorescence spectra of Bis-PBI monomers in CHCl3 (red dashed line, c0 = 107 M) and dimers in toluene (blue dashed line, c0 = 103 M) at 298 K.
Scheme 1. Synthetic route to diacetylene-tethered Bis-PBI.
Moreover, the spectral shift of the aggregate absorption band in comparison to the monomer absorption band can allow a classification to H- or J-type exciton coupling between the interacting PBI chromophores within the Kasha’s molecular exciton theory and its extended model by Spano.22,26 Indeed, the data could be properly fitted by using the monomer-dimer model also in a global fit approach. In Figure 2 it is nicely shown that the global dimer fit, which is depicted as black dashed lines, is covering the complete spectra formed by the experimental data (colored lines),24 providing a dimerization constant of KD = 1.4 x 105 M1 in pure toluene at 298 K. Also the observation of three well-defined isosbestic points at 561 nm, 489 nm and 477 nm, respectively, indicates the presence of a thermodynamic equilibrium between two species that can be clearly attributed to the respective monomer and dimer. Similar results were obtained in THF with an even higher dimerization constant of KD = 1.3 x 106 M1 (Figure S2), whereas the concentration-dependent UV/Vis spectra of the monochromophoric reference compound Ref-PBI do not show any prominent changes upon dilution in the same concentration range (Figure S8). Furthermore, temperature-dependent UV/Vis absorption studies of Bis-PBI in toluene (Figure S9) also revealed spectral changes from monomer to dimer species upon decreasing temperature. With increasing temperature, a reversal of the band intensities of the 00 and 01 absorption bands occurs, which confirms the observations of the concentrationdependent UV/Vis data. By performing concentrationdependent experiments at different temperatures in both pure toluene (Figure 3) and THF (Figures S2S4, Table S1) it was possible to calculate the thermodynamic parameters by a van’t Hoff plot. From these studies we can derive conditions where
The self-assembly behavior of Bis-PBI was explored by concentration-dependent UV/Vis absorption studies in various solvents. In toluene, the transition from almost pure monomeric to almost fully aggregated species of Bis-PBI could be analyzed for the accessible concentration range of UV/Vis experiments (for further details of related studies in other solvents see Supporting Information Figure S2–S6). The corresponding spectra in toluene are shown in Figure 2 where significant spectral changes are induced by continuous concentration variations. At low concentrations (c0 =3.75 x 107 M), Bis-PBI displays a typical monomer absorption band for PBI chromophores with a maximum at 545 nm similar to the one of monochromophoric reference dye Ref-PBI (Figure S7 and S8). By increasing the concentration a reversal of the intensities of the 00 and 01 absorption bands can be observed, which is characteristic of a co-facial -stacking of these dyes (Figure 2).20,21 This arises from the interplay of exciton and vibrational couplings22,23 and can be taken as a clear indication for the formation of H-type aggregates. By performing concentration-dependent UV/Vis studies the aggregation mechanism for supramolecular self-assembly processes can be clarified by fitting the experimental data with different mathematical models.24,25
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Figure 3. Plot of the extinction coefficient against the concentration from concentration-dependent UV/Vis absorption data of BisPBI (c0 = 2.91 x 104 M – 2.48 x 107 M) in toluene at different temperatures from 60 °C – 10 °C. Also shown are the calculated thermodynamic parameter obtained from the van’t Hoff plot.
>95% of Bis-PBI are self-assembled in quadruple -stacks. Thus, at 298 K (313 K) in THF concentrations c0 > 1.5 x 103 M (6.0 x 103 M) and in toluene concentrations c0 > 7.5 x 103 M (5.0 x 102 M) are needed. To further confirm the presence of Bis-PBI dimer aggregates under these conditions two-dimensional diffusion ordered spectroscopy (DOSY) NMR studies (Figure 4) were performed in toluene (c = 2 x 103 M, 313 K) providing a diffusion coefficient of D = 3.95 x 1010 m2 s1. By solving the Stokes-Einstein equation a hydrodynamic radius of r = 12.1 Å can be calculated, which is in good agreement with the results gained from DFT calculations taking into account that the long alkyl chains were not included in the calculations (B97D3/def2-SVP, Figure 4), where the geometry optimized structure of Bis-PBI dimers shows a perfect intermolecular distance of 3.23.3 Å for -stacking between the PBI chromophores (Figure 1).
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Figure 4. (A) Illustration of the energy minimized dimer structure of Bis-PBI obtained from DFT calculations (B97D3/def2-SVP) and hydrodynamic radius as obtained from (B) the StokesEinstein equation. (C) 2D plot of DOSY NMR (600 MHz, 313 K) spectrum of Bis-PBI in toluene-d8 (c0 = 2 x 103 M).
Further evidence for the exclusive presence of discrete BisPBI dimers even at higher concentration was obtained by ESI mass spectrometry (Figure S10) and atomic force microscopy (AFM) of spin-coated solution of Bis-PBI in methylcyclohexane on a HOPG substrate (Figure S11) that both clearly confirm the absence of larger dye stacks. Steady-state Optical Characteristics. With these structural features, Bis-PBI dimers are ideally suited to unravel the electronic interactions among PBI chromophores in their quadruple H-aggregated state by fluorescence spectroscopy (Figure 2). In chloroform at low concentrations (c0 = 107 M), where only monomeric species are present, the UV/Vis and fluorescence spectra of Bis-PBI show very characteristic wellresolved vibronic structures with mirror-image relationship (Figure 2), a small Stokes shift of 525 cm1 and a high fluorescence quantum yield of about 76%. On the contrary, for BisPBI dimers at high concentrations (c0 = 103 M) in toluene, a prominent hypsochromic shift of the absorption maximum from 545 nm to 510 nm can be observed. In this case the Stokes shift is much higher with 4622 cm1 and the fluorescence quantum yield is decreased to only 9%. Additionally, below 600 nm a small shoulder corresponding to remaining monomer fluorescence can be recognized (for corresponding excitation spectra of Bis-PBI see Supporting Information Figure S12). The change of the emission properties can be attributed to the formation of an excimer state in the aggregate, which is characteristic for -stacking PBI chromophores.20,21
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Journal of the American Chemical Society Broadband Fluorescence Upconversion Spectroscopy. In order to get deeper insights into the initial Frenkel exciton state and concomitant excimer formation dynamics in H-type PBI quadruple stacks, we have measured transient fluorescence spectra with the aid of femtosecond broadband fluorescence upconversion spectroscopy (FLUPS).27 We have chosen THF as a solvent for the time-resolved fluorescence measurements, because it allows already at quite low concentrations (c0 = ~ 1 x 10−3 M) and at room temperature the almost exclusive presence of quadruple PBI stacks owing to a larger dimerization constant (Table S1). As shown in Figure 5A, the early-time transient fluorescence spectra of Bis-PBI not only deviate from the steady-state fluorescence spectrum but also show drastic spectral changes within 1 ps.
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Figure 5. (A) Two-dimensional transient fluorescence contour map (top) and transient fluorescence spectra (bottom) of Bis-PBI (c0 = ~ 1 x 103 M) in THF. The excitation wavelength was tuned to be resonant with the electronic transition of the symmetric upper Frenkel excitonic state (λpump = 510 nm). The pump power at the sample was approximately 300 μW (~ 0.25 W/cm2). The transient spectra below 570 nm correspond to the sum-frequency signals of the pump and gate pulses. (B) Kinetic traces of λ2I00(t*)/I01(t*) (red circle) and I0v’(t*)/I0-1(t*) (blue square) extracted from transient fluorescence spectra.
At initial time delay, emission from the excimer state is negligible but that from the Frenkel state is rather prominent. The ultrafast (< 50 fs) population of the latter is evident from the apparent vibronic features of the fluorescence spectra, 27 especially the vibronic peak ratio between 0−0 electronic and 0−1 vibronic side bands. However, within less than 1 ps these spectral signatures disappear with the appearance of broad excimer bands at longer wavelength region. Meanwhile, Ref-
Here, the Huang-Rhys factor (λ2 = 0.714, Ref-PBI in THF) can be simply estimated from the linear absorption spectrum of the monomeric units.31 In the case of J-aggregates, Ncoh accords with the vibronic peak ratio throughout the entire time window. On the other hand, the vibronic peak ratio traces in H-aggregates do not totally follow the evolution of coherence length over time due to gradual population from the symmetric upper- to the antisymmetric lower Frenkel excitonic states, giving rise to an additional fluorescence quenching from the 0−0 electronic transition.26,30 Therefore, especially for Haggregates, λ2I00(t*)/I01(t*) values at later time delays should underestimate the size of excitation. As shown in Figure 5B, the experimental λ2I00(t*)/I01(t*) kinetic traces indicate that the initial Ncoh value of PBI quadruple stacks is approximately 3. Nevertheless, considering the limitations for the timeresolution of the FLUPS setup, this result reflects that the nascent Frenkel exciton within a highly defined H-aggregate is almost fully-delocalized over the whole quadruple stack. This value rapidly decays to 1 with a time constant of ~ 200 fs, which can be attributed to decoherence of the delocalized Frenkel exciton, i.e. exciton localization process. With the same time constant, we could observe the accompanying rise dynamics of the I0v’(t*)/I01(t*) peak ratio (v’ > 1, the assumption is that the excimer emission rather than the higher vibronic progression from the Frenkel state mainly contributes to the I0v’(t*) band) which we previously11 suggested as an indicator of excimer formation (Figure 5B). These results emphasize two important aspects of Frenkel exciton relaxation and excimer formation dynamics in PBI H-aggregates: (i) the excimer formation process suppresses the coherence of the delocalized Frenkel exciton,10 (ii) the mixing between Frenkel exciton and charge-transfer states and/or the existence of conical intersection can trigger the ultrafast (< 1 ps) exciton localization and excimer formation process.32-34
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Figure 6. Schematic illustration of Frenkel exciton relaxation and excimer formation dynamics in PBI dimer, tetramer, and extended oligomeric H-aggregates. These dynamics occur within 1 ps, which can be inferred by ultrafast time-resolved fluorescence measurements.
Comparison of Frenkel Exciton Relaxation and Excimer Formation Dynamics in PBI Dimeric, Tetrameric and Oligomeric H-Aggregates. Combining the time-resolved fluorescence results of the PBI quadruple stacks with our previous work on PBI dimeric and oligomeric aggregates,11 we can recapitulate the Frenkel exciton relaxation and excimer formation dynamics depending on the size of PBI aggregates (Figure 6). This comparative analysis should be valid because these systems have nearly the same intermolecular packing geometry with similar exciton coupling strengths appertaining to the intermediate coupling regime.11,35 According to the previous observation,11 extended PBI oligomeric H-aggregates showed the initial value of about 3 and a double exponential decay with the time constants of < 80 and 250 fs in kinetic profiles of Ncoh (λ2I00(t*)/I01(t*)). The former decay component was assigned to the exciton localization process caused by coherent exciton propagation along the long onedimensional aggregate, which was also evident from the nearly constant I0v’(t*)/I01(t*) peak ratio within the first 110 fs. This is due to the result that the energy fluctuation during exciton propagation in a coherent fashion leads to a reduction in the size of excited-state wave function along the molecular aggregate,11,36,37 giving rise to an ultrafast decay of Ncoh to the value of about 2. Actually, this process and its time scale are quite similar to coherent excitation energy transfer (EET) observed in conjugated polymers in the intermediate coupling regime where the delocalization is perturbed by nuclear vibrational modes.38-40 In this case, a partially delocalized exciton undergoes quantum-assisted transport within long polymer chains on the ~100 fs time scale, thereby the exciton is finally relaxed to a more localized site. On the other hand,
this kind of coherent transport process was not revealed in PBI quadruple stacks, despite of the similar initial Ncoh value of about 3. Likewise, PBI dimeric aggregates did not show any fast decay component before relaxing to the excimer state with the initial Ncoh value of about 2. The instantaneous rise dynamics in kinetic profiles of excimer formation processes (I0v’(t*)/I01(t*)) without a plateau also substantiates that the excimer state of PBI dimer and quadruple stacks is directly formed alongside the localization of the initial fullydelocalized Frenkel exciton. The disparity of the initial Frenkel exciton relaxation and excimer formation dynamics between highly defined dimer/tetramer stacks and extended oligomeric aggregates can be attributed to the size-limit of molecular aggregates. Since the PBI quadruple stacks are composed of only ‘four’ monomeric units, the initial fullydelocalized exciton is immobile within a highly defined aggregate and thereby it only has a relaxation pathway to the excimer state with ultrafast exciton localization. This situation can be applied in the case of PBI dimer aggregates in the same manner. Interestingly, another size-effect on exciton dynamics in the aggregates was revealed by femtosecond transient absorption spectroscopy (Figure S22). Contrary to the oligomeric aggregates,41 there are no excitation power dependent behaviors in the quadruple stacks, like in the case of dimeric aggregates,42,43 implying that multiple excitons cannot be generated. Namely, only ‘one’ delocalized exciton, which is also immobile, can be formed in a stack of four PBIs, further supporting the time-resolved fluorescence results.
In this work, a PBI quadruple dye stack formed by selfassembly of bay-tethered Bis-PBI dyes provided for the first time insights into the exciton dynamics within a highly de-
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fined synthetic dye aggregate beyond dimers. The unambiguous detection of the initial fully-delocalized Frenkel exciton state and its localization, i.e. exciton decoherence, occurring within 1 ps are important outcomes of our study. With the now available spectroscopic insights for the series of structurally defined dimer, tetramer and extended -stacks a sound understanding of exciton migration versus exciton relaxation in Haggregates has been achieved. Unraveling such processes in artificial dye aggregates is of relevance for their comparison to natural dye arrays as given in the light-harvesting systems of photosynthesis6,7 as well as for the design of materials for organic electronics4,44 and photonics.1-3
The Supporting Information is available free of charge on the ACS Publications website at DOI: Detailed experimental methods (PDF)
*
[email protected] *
[email protected] ORCID Dongho Kim: 0000-0001-8668-2644 Frank Würthner: 0000-0001-7245-0471 These authors declare no competing financial interests. § C.
Kaufmann and W. Kim contributed equally to this work.
The research at the Universität Würzburg was supported by the German Science Foundation (DFG) within the research unit FOR 1809. The research at Yonsei University was supported by the Global Research Laboratory Program (2013K1A1A2A02050183) funded by the Ministry of Science, ICT & Future, Korea.
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