Intramolecular Interactions of Highly Ï-Conjugated Perylenediimide

Letter pubs.acs.org/JPCL

Intramolecular Interactions of Highly π‑Conjugated Perylenediimide Oligomers Probed by Single-Molecule Spectroscopy Jae-Won Cho,† Hyejin Yoo,† Ji-Eun Lee,† Qifan Yan,‡ Dahui Zhao,*,‡ and Dongho Kim*,† †

Spectroscopy Laboratory for Functional π-Electronic Systems and Department of Chemistry, Yonsei University, Seoul 120-749, Korea ‡ Beijing National Laboratory for Molecular Sciences, the Key Laboratory of Polymer Chemistry and Physics of the Ministry of Education, College of Chemistry, Peking University, Beijing 100871, China S Supporting Information *

ABSTRACT: Highly π-conjugated perylenediimide (PDI) oligomers are promising low band gap organic materials for various applications in optoelectronics. In this work, individual fluorescence dynamics of ethynylene- and butadiynylene-bridged dimeric and trimeric PDIs (PEP, PBP, and PEPEP) were monitored and analyzed by single-molecule fluorescence spectroscopy to gain information on the degree of extension of πconjugation through the acetylene bridge in PDI multichromophores. The simultaneous measurements of fluorescence intensity, lifetime, and spectrum indicate a sequential decrease in π-conjugation upon photobleaching of PDI monomer units. Furthermore, Huang−Rhys (HR) factors, S, are obtained to evaluate the degree of electronic coupling in view of π-conjugation and overall rigidity between the PDI units in PDI oligomers at the single-molecule level. In addition, butadiynylene-bridged dimeric PDI (PBP) reveals conformational heterogeneity due to the long butadiynylene linker. These results suggest a new way to control the photophysical properties of the PDI multichromophoric system by expansion of π-conjugation and modification with different linker groups. SECTION: Spectroscopy, Photochemistry, and Excited States

P

coupling can be tuned by varying the number of acetylene bridge units, such as ethynylene and butadiynylene linkers, between adjacent PDI units. Yan et al.26 demonstrated that the ethynylene and butadiynylene linkers lower the LUMO levels of the molecules, which is demonstrated by the UV−visible absorption spectra and cyclic voltammetry that show the extension of π-conjugation length in the electronic structure. In addition, Jumper et al.32 reported the dipole coupling model in butadiynylene-bridged PDI dimer. However, the spectral shift solely by dipole coupling is somewhat too small to explain a large red shift observed in the fluorescence spectra and singlemolecule spectroscopic data in both PEP and PBP (see the “Description of dipole coupling model” section and Figure S1 in the Supporting Information). These results show that the electronic communication of the PDI dimers are primarily determined by the expansion of the π-conjugation. However, although ensemble studies in solution develop the basic photophysical properties of the molecules, they often conceal the unique contribution of each molecular entity in multichromophores. Single-molecule fluorescence spectroscopy (SMFS) is a powerful technique to observe dynamic processes in individual molecules by recording the fluorescence signal as a function of

erylenediimide (PDI) is a promising molecular unit for optoelectronic applications because of its high photochemical stability, electron affinity, and high tinctorial strength over the entire visible spectrum.1−6 As a result of these properties, PDI and its derivatives have been utilized for various applications such as organic light emitting devices (OLEDs),7−9 organic field effect transistors (OFETs),10−13 and organic solar cells.14−16 Apart from their diverse applications, structural modifications of PDI have been made to understand the photophysical properties of PDI.17−22 In particular, the bay substitution of PDI is an easy approach to fine-tune the properties of PDI by modifying its frontier orbital energy levels compared to the modification at the imide nitrogen group of PDI.23−26 By controlling the band gap of the molecule, a low band gap material can be prepared, which is promising for various applications such as OLEDs, OFETs, and organic photovoltaics (OPVs).7−16,27−30 It has been reported to achieve PDI derivatives with low band gaps by modifying the bay substituents of PDI toward efficient NIR absorber.31 However, few detailed discussions of their associated excited state photochemistry have been investigated. In this regard, highly π-conjugated PDI oligomers were prepared to construct electronically coupled ladder-type molecules that would manifest relatively strong electronic intramolecular interactions and desirable photophysical properties as a result of being connected through sp-hybridized acetylene bridges.26 In this case, the degree of electronic © XXXX American Chemical Society

Received: August 21, 2014 Accepted: October 21, 2014

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dx.doi.org/10.1021/jz501765x | J. Phys. Chem. Lett. 2014, 5, 3895−3901

The Journal of Physical Chemistry Letters

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Chart 1. Molecular Structures of PEP, PBP, and PEPEP

time without ensemble averaging effect. SMFS can also provide the information on the heterogeneous processes based on distributions of spectroscopic parameters such as fluorescence lifetimes.20 In this work, by using SMFS, highly π-conjugated ethynylene-bridged PDIs (PEP and PEPEP) in the solid state were investigated to unveil the intrinsic photophysical properties and the molecular interactions depending on the number of PDI units in the PDI oligomers. Single-molecule fluorescence dynamics of the PDI oligomers manifest the spectral blue shift in the fluorescence spectra and a decrease of the fluorescence lifetime in the fluorescence decay profiles that are reminiscent of a decrease in π-conjugation. Butadiynylene-bridged PDI dimer (PBP) has large conformational heterogeneity due to the long butadiynylene linker. In addition, the Huang−Rhys (HR) factor S, which means a measure of the average number of phonons that accompany a particular electronic transition and can be associated with the displacement of the equilibrium positions of the nuclei upon photoexcitation of the chromophore, is obtained to evaluate the degree of electronic coupling in view of π-conjugation and overall rigidity between the PDI units in PDI oligomers.33−36 Thus, by analyzing the HR factor for each single molecule, the electronic coupling strength between the PDI units in PDI oligomers can be estimated at the single-molecule level. To the best of our knowledge, these results are the first detailed excited state photochemistry of highly π-conjugated PDIs with low band gaps at the single-molecule level, which allows for the development of a guideline for molecular design of low band gap materials in optoelectronics. The molecular structures of PEP, PBP, and PEPEP are shown in Chart 1. PDI dimer and trimer are linked at the bay region by ethynylene or butadiynylene linker as a ladder-type to extend π-conjugation along the molecular sheet. Extended π-conjugation of PEP, PBP, and PEPEP results in a bathochromic shift in the absorption spectra as shown in Supporting Information Figure S2. These observations coincide with the HOMO−LUMO gap of each molecule (2.61 eV for PDI, 2.20 eV for PEP, 2.28 eV for PBP, and 2.07 eV for PEPEP, respectively) in that PEPEP shows the largest red shift in the absorption spectra, and the absorption spectrum of PEP is shifted to a longer wavelength than that of PBP.26 Similar to the absorption spectra, the fluorescence spectra of the three PDI oligomers, which are structureless, are also red-shifted compared to that of PDI monomer. In addition, the fluorescence lifetimes of PDI oligomers are shorter than that of the monomer as shown in Supporting Information Figure S3 and Table 1. The longer fluorescence lifetime of PEPEP is a

Table 1. Photophysical Properties of PDI Monomer, PEP, PBP, and PEPEP in CH2Cl2 molecules

fluorescence λmax [nm]

τ [ns]

ΦF [%]a

PDI monomer PEP PBP PEPEP

541 620 605 652

4.6 1.5 1.1 2.0

∼100 5.6 7.9 10.2

a ΦF indicates the relative fluorescence quantum yield, referenced to cresyl violetperchlorate in EtOH (0.54), at an excitation wavelength of 488 nm.

convincing evidence of its rigid structure. PBP exhibits the shortest fluorescence lifetime among the PDI oligomers due to the augmented degree of freedom by the long butadiynylene linker. The fluorescence quantum yields are significantly reduced compared to the approximately 100% fluorescence quantum efficiency of the PDI monomer (Table 1) because the formation of exciton states from H-type interaction reduces the quantum yield in PDI oligomers.32 Also, a smaller energy gap due to the expansion of conjugation increases the overlap between S0 and S1 vibronic states to enhance nonradiative decay rates (∼0/s for PDI monomer and ∼108/s for PDI oligomers, respectively).37,38 To investigate the fluorescence dynamics of the PDI oligomers in detail, SMFS measurements were performed. The SMFS of the PDI oligomers demonstrates a disruption of π-conjugation from stepwise photobleaching behaviors in fluorescence intensity trajectory (FIT). This feature probes the molecular state of each step by spectroscopic parameters such as fluorescence intensity, fluorescence lifetime, and spectral shift. The difference in electronic couplings gives rise to distinctive single-molecule photobleaching dynamics. Additionally, the effect of the length of the linker can be revealed by deviations in the observed spectroscopic parameters. Approximately 50% of the PEP molecules emit the fluorescence of the dimer from the beginning to the end to show one-step photobleaching. As shown in Figure 1a, PEP showed the fluorescence signal during 153 s, and then it was irreversibly photobleached afterward. This PEP molecule exhibits a short fluorescence lifetime of 2.3 ns with a bathochromic shift to 610 nm, indicative of the dimer state. Concurrently, approximately half of the PEP molecules show the monomerlike fluorescence signal after one of the PDI constituent units is photobleached. As shown in the representative FIT (Figure 1b), PEP exhibits dimeric 3896



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Figure 1. Representative fluorescence intensity trajectories and corresponding fluorescence decay profiles and spectra of PEP. The y axis of the FIT indicates the number of photon counts per 20 ms. The fluorescence lifetimes are fitted with photons detected for 2 s (a) or 1 s (b) and are exhibited as points. The fluorescence decay profiles have been obtained from the first group of photons for 1 s at each molecular state. Each spectrum is averaged from a multiple single-molecule spectra at the same intensity level that are measured for 2 s. The FIT in (b) is indicated by different colors with corresponding fluorescence spectra and decay profiles. The fitted fluorescence lifetime of each decay profile and the maximum peak position values of each fluorescence spectrum are as follows; 2.3 ns and 610 nm in (a) and 2.3, 6.7 ns and 614, 574 nm in (b), respectively.

Figure 2. Representative FITs and corresponding fluorescence decay profiles and spectra of PBP. The data processing is the same as that of Figure 1. The fitted fluorescence lifetimes and maximum peak position values are as follows: 1.5, 1.2, 3.0 ns and 610, 615, 587 nm. Three fluorescence decay profiles and spectra correspond to each part, which is distinguished by background patterns.

fluorescence following a momentary drop in fluorescence intensity to the background level at 22 s. After 15 s, one PDI unit emits its own monomerlike fluorescence for 3 s. When one of the PDI units is photobleached, the molecular state is changed from the dimer to the monomerlike state. Simultaneously, the fluorescence intensity jumps by approximately 7 times, and as a

consequence, the fluorescence lifetime increases from 2.3 to 6.7 ns. Furthermore, the fluorescence spectrum shows vibronic structures along with a blue shift (from 614 to 574 nm). The fluorescence intensity of the monomerlike state is much greater than that of the dimeric state because the fluorescence quantum yield of the PEP dimer is approximately 18 times less than that of the PDI monomer. The fluorescence lifetimes and spectral 3897



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Figure 3. Representative FITs and corresponding fluorescence decay profiles and spectra of PEPEP. The data processing is the same as that of Figure 1. The fitted fluorescence lifetimes and the maximum peak position values are as follows; 3.0, 2.3, 5.5 ns and 624, 588, 566 nm from top to bottom panels.

shifts were well matched with the ensemble experimental results of the PDI monomer and PEP. These remarkable changes from the dimeric state to the monomerlike state are induced by a disruption in the degree of π-conjugation in the PEP molecule. Because an increase in linker length is accompanied by an augmentation in the structural flexibility, a single-molecule study was performed for PBP. Similar to PEP molecules, the PBP molecule exhibits one-step (Supporting Information Figure S4a) and two-step photobleaching behaviors (Supporting Information Figure S4b). These features are similar to the experimental results of PEP molecules. However, the PBP molecule with dimeric fluorescence shows relatively severe fluctuations in the fluorescence intensity and lifetime traces compared to the PEP molecule (Figure 2). Though the FIT of PBP exhibits a stepwise behavior, short fluorescence lifetimes (1.5, 1.2, and 3.0 ns for each step, respectively) and bathochromic shifts in the fluorescence spectra (610, 615, and 587 nm for each step, respectively) indicate that all steps represent the dimer state of PBP despite fluctuations in the FITs. The PBP molecule does exhibit flexible motions between PDI monomers due to the longer butadiynylene linker, which results in a conformational change of the molecule giving rise to fluctuations in the fluorescence signal. Additionally, PEPEP molecules show one- and two-step photobleaching behaviors that are described in Supporting Information Figure S5. In addition, approximately 8.3% of the PEPEP molecules show the FITs, as shown in Figure 3, which manifests trimeric, dimerlike, and monomerlike states with its characteristic fluorescence lifetime (3.0, 2.3, and 5.5 ns for trimer, dimerlike, and monomerlike state, respectively) and an intensity change (38, 27, and 70 counts/20 ms of the averaged fluorescence intensity for trimer, dimerlike, and monomerlike state, respectively) of each state. Additionally, the fluorescence spectrum is gradually blue-shifted (from 624 to 588 and 566 nm in order from trimer to dimerlike and to monomerlike state), which corresponds to the decreased π-conjugation as photobleaching occurs stepwise in PEPEP. A few PEPEP molecules exhibit multistep photobleaching behaviors in FITs. These molecules are assumed to have disruption in the onequantum system and, thus, a decrease in electronic coupling.

These features enhance conformational heterogeneities of molecules that result in complicated FITs. Based on these measurements, a statistical analysis was performed by constructing the histograms for the fluorescence spectral peak and lifetime distributions of PEP, PBP, and PEPEP (Supporting Information Figure S6). The fluorescence spectral peak and lifetime distributions were fitted by Gaussian functions, in which the fluorescence lifetimes, the spectral peak positions, and fwhm values are listed in Table 2. The spectral Table 2. Fluorescence Lifetime and Spectral Peak Position Values Based on the Gaussian Function Fitting and Their FWHM in Supporting Information Figure S6 molecules PEP PBP PEPEP

state

τ [ns]

fwhm

λ [nm]

fwhm

dimer monomerlike dimer monomerlike trimer dimerlike monomerlike

2.6 4.8 4.0 5.3 3.1 4.0 5.4

1.5 1.8 4.3 1.8 1.5 3.0 2.3

600 549 581 545 639 594 565

29 46 38 31 33 28 40

peak distributions of the dimeric and trimeric states of PEP (600 nm), PBP (581 nm), and PEPEP (639 nm) were redshifted depending on the degree of π-conjugation of the molecule compared to the PDI monomer (549, 545, and 565 nm for monomerlike state of PEP, PBP, and PEPEP, respectively). As shown in the histograms, PEPEP shows the strongest electronic coupling because of the largest red-shifted spectral position. The lifetime distributions of the dimeric and trimeric states of PEP (2.6 ns), PBP (4.0 ns), and PEPEP (3.1 ns) are shorter than their corresponding monomerlike states (4.8, 5.3, and 5.4 ns for monomerlike states of PEP, PBP, and PEPEP, respectively). Such statistical results are congruent with the aforementioned ensemble results as shown in Supporting Information Figures S2 and S3. The dimeric state of PBP shows a broader distribution (fwhm: 4.3 ns) in the fluorescence lifetime histogram than other oligomeric PDIs (1.5 ns for fwhm of both PEP and PEPEP). This feature implies that various tilting angles between the PDI constituent units cause more severe 3898



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conformational heterogeneities in PBP. Alternatively, the distribution in the fluorescence lifetime histogram of PEPEP is rather narrow (fwhm: 1.5 ns). The increasing number of subunits will increase the degree of freedom of the molecular system leading to conformational heterogeneity. However, the electronic coupling of PEPEP is strong enough to reduce the conformational heterogeneity. In addition, the one-step photobleaching is a major FIT behavior in PDI oligomers; 50% of PEP, 52% of PBP, and 66% of PEPEP molecules show a one-step photobleaching behavior. The more the molecule shows one-step photobleaching behavior, the more likely the molecule behaves as a onequantum system. Thus, this feature is a convincing evidence of strong interactions between the PDI constituents in a series of PDI oligomers.39,40 This observation is consistent with the absorption and fluorescence spectra of PEPEP to exhibit the largest red-shifted spectra, and the lowest HOMO−LUMO gap, indicating stronger interactions between the PDI constituents by extended π-conjugation. Alternatively, PEP and PBP show a similar ratio of one-step photobleaching behavior unlike differences in the red shift of the absorption and fluorescence spectra between PEP and PBP. In this regard, the HR factor was investigated to examine the electronic coupling in more detail. In previous studies, the conjugated thiophenes, [N]phenylenes, acenes, phenyl, and enyne oligomer series having a planar rigid structure have been investigated to reveal the relationship between conjugation effect and HR factor.35,36 In these studies, as the number of constituent units in the conjugated molecules increases, the HR factors decrease. The HR factor is proportional to the square of displacement of the potential energy minima between the ground and excited states as a function of the configuration coordinate.33−36,41 Moreover, the HR factor is related to the vibrational relaxation energy and thus provides a measure of the strength of electron−phonon coupling.33−35 As the number of units in planar and rigid molecular systems like the above conjugated systems increases, the number of the vibrational modes participating in vibrational relaxation process increases. The increased number of modes adds to the capacity of the molecule to distribute the vibrational relaxation energy, and then the vibrational relaxation energy per mode and, thus, the nuclear redistribution are diminished, reducing the HR factors.35,36,41 In the same manner, the PDI oligomers show a planar and rigid structure acting as onequantum system based on the high ratio of one-step photobleaching behaviors in FITs. Accordingly, as the number of PDI constituent units in PDI oligomers increase, the HR factor becomes smaller, indicating that PDI oligomers become rigid to expand π-conjugation leading to strong electronic couplings between the PDI constituent units exp( −Sj) =

I0 − 0 I0 − 0 + I j

Figure 4. Histograms (a, b, and c) of the Huang−Rhys (HR) factors, S, for PEP, PBP, and PEPEP indicated by orange, green, and red bars, respectively, from top to bottom. The histograms consist of all the HR factor values that have been calculated based on ref 42 using eq 1 by measuring I0−0 and I1−0 of each molecule.42,43 I0−0 and I1−0 were evaluated by integration of Gaussian fitting curves in the spectrum.44 The lines correspond to the fitted Gaussian distribution functions with 0.85, 0.87, and 0.62 for HR factors of PEP, PBP, and PEPEP, respectively, with fwhm values of 0.41, 0.66, and 0.35.

Moreover, the smallest HR factor of PEPEP corroborates the highest proportion of one-step photobleaching behavior, which corresponds to the strongest interactions between the PDI constituents in the PDI oligomers in view of π-conjugation and overall structural rigidity and, as a consequence, the largest red shift in the absorption and fluorescence spectra. The result is well matched with the increasing portion of one-step photobleaching behaviors with increasing the number of PDI units in the PDI oligomers. Though the red shift in the absorption and fluorescence spectra for PEP is larger than PBP, in PEP, the ratio of one-step photobleaching behavior and HR factor values of PEP (50%, S: 0.85) and PBP (52%, S: 0.87) are similar. When we consider that the HR factor is dependent on the number of atoms (S = a exp(−n2/b) where a and b are arbitrary constants and n is the number of atoms in the molecular system),36 a small difference in the HR factors between PEP and PBP seems to be reasonable. Instead, it is noticeable that the distribution of the HR factor of PBP (fwhm: 0.66) is broader than those of PEP (fwhm: 0.41) and PEPEP (fwhm: 0.35), implying more severe conformational heterogeneities in PBP. PEPEP exhibits the narrowest distribution of HR factor, which implies the relatively rigid structure of

(1)

Equation 1 was used to evaluate the HR factor for each spectrum of the PDI oligomer using j = 1 to consider only the first phonon-sideband (PSB) because other PSBs are negligible in intensity.42,43 Figure 4 exhibits the histograms of all the HR factors of PEP, PBP, and PEPEP. The mean HR factors of PEP (S: 0.85) and PEPEP (S: 0.62) based on the Gaussian function fitting manifest the same trend as observed in the molecule showing significant conjugation effect with a small HR factor.35,36,41 3899



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(4) Ahrens, M. J.; Fuller, M. J.; Wasielewski, M. R. Cyanated Perylene-3,4-dicarboximides and Pery le ne-3,4:9,10-bis(dicarboximide): Facile Chromophoric Oxidants for Organic Photonics and Electronics. Chem. Mater. 2003, 15, 2684−2686. (5) Zhao, Y.; Wasielewski, M. R. 3,4:9,10-Perylenebis(dicarboximide) Chromophores That Function as Both Electron Donors and Acceptors. Tetrahedron Lett. 1999, 40, 7047−7050. (6) Keerthi, A.; Valiyaveettil, S. Regioisomers of Perylenediimide: Synthesis, Photophysical, and Electrochemical Properties. J. Phys. Chem. B 2012, 116, 4603−4614. (7) Ego, C.; Marsitzky, D.; Becker, S.; Zhang, J.; Grimsdale, A. C.; Müllen, K.; MacKenzie, J. D.; Silva, C.; Friend, R. H. Attaching Perylene Dyes to Polyfluorene: Three Simple, Efficient Methods for Facile Color Tuning of Light-Emitting Polymers. J. Am. Chem. Soc. 2003, 125, 437−443. (8) Würthner, F.; Thalacker, C.; Diele, S.; Tschierske, C. Fluorescent J-Type Aggregates and Thermotropic Columnar Mesophases of Perylene Bisimide Dyes. Chem.Eur. J. 2001, 7, 2245−2253. (9) Céspedes-Guirao, F. J.; García-Santamaría, S.; Fernández-Lázaro, F.; Sastre-Santos, A.; Bolink, H. J. Efficient Electroluminescence from a Perylenediimide Fluorophore Obtained from a Simple Solution Processed OLED. J. Phys. D: Appl. Phys. 2009, 42, 105106. (10) Chen, Z.; Debije, M. G.; Debaerdemaeker, T.; Osswald, P.; Würthner, F. Tetrachloro-Substituted Perylene Bisimide Dyes as Promising n-Type Organic Semiconductors: Studies on Structural, Electrochemical and Charge Transport Properties. ChemPhysChem 2004, 5, 137−140. (11) Zhang, Z.; Lei, T.; Yan, Q.; Pei, J.; Zhao, D. ElectronTransporting PAHs with Dual Perylenediimides: Syntheses and Semiconductive Characterizations. Chem. Commun. 2013, 49, 2882− 2884. (12) Park, S. K.; Kim, J. H.; Yoon, S.-J.; Kwon, O. K.; An, B.-K.; Park, S. Y. High-Performance n-Type Organic Transistor with a SolutionProcessed and Exfoliation-Transferred Two-Dimensional Crystalline Layered Film. Chem. Mater. 2012, 24, 3263−3268. (13) Jeong, Y. J.; Jang, J.; Nam, S.; Kim, K.; Kim, L. H.; Park, S.; An, T. K.; Park, C. E. High-Performance Organic Complementary Inverters Using Monolayer Graphene Electrodes. ACS Appl. Mater. Interfaces 2014, 6, 6816−6824. (14) Yan, Q.; Zhou, Y.; Zheng, Y.-Q.; Pei, J.; Zhao, D. Towards Rational Design of Organic Electron Acceptor for Photovoltaics: A Study Based on Perylenediimide Derivatives. Chem. Sci. 2013, 4, 4389−4394. (15) Zhou, Y.; Yan, Q.; Zheng, Y.-Q.; Wang, J.-Y.; Zhao, D.; Pei, J. New Polymer Acceptors for Organic Solar Cells: The Effect of RegioRegularity and Device Configuration. J. Mater. Chem. A 2013, 1, 6609−6613. (16) Planells, M.; Céspedes-Guirao, F. J.; Forneli, A.; Sastre-Santos, Á .; Fernández-Lázaro, F.; Palomares, E. Interfacial Photo-Induced Charge Transfer Reactions in Perylene Imide Dye Sensitised Solar Cells. J. Mater. Chem. 2008, 18, 5802−5808. (17) Wilson, T. M.; Tauber, M. J.; Wasielewski, M. R. Toward an nType Molecular Wire: Electron Hopping within Linearly Linked Perylenediimide Oligomers. J. Am. Chem. Soc. 2009, 131, 8952−8957. (18) Bhang, H. W.; Yoon, M.-C.; Lee, J.-E.; Murase, Y.; Yoneda, T.; Shinokubo, H.; Osuka, A.; Kim, D. Ensemble and Single-Molecule Spectroscopic Study on Excitation Energy Transfer Processes in 1,3Phenylene-Linked Perylene Bisimide Oligomers. J. Phys. Chem. B 2012, 116, 1244−1255. (19) Schlosser, F.; Sung, J.; Kim, P.; Kim, D.; Würthner, F. Excitation Energy Migration in Covalently Linked Perylene Bisimide Macrocycles. Chem. Sci. 2012, 3, 2778−2785. (20) Lee, J.-E.; Stepanenko, V.; Yang, J.; Yoo, H.; Schlosser, F.; Bellinger, D.; Engels, B.; Scheblykin, I. G.; Würthner, F.; Kim, D. Structure−Property Relationship of Perylene Bisimide Macrocycles Probed by Atomic Force Microscopy and Single-Molecule Fluorescence Spectroscopy. ACS Nano 2013, 7, 5064−5076.

PEPEP, leading to sustaining strong interactions between the PDI units. In summary, the single-molecule fluorescence spectroscopy of PEP, PBP, and PEPEP reveals the effect of linkers in multichromophores on the molecular interactions. The extended π-conjugation through ethynylene and butadiynylene linkers shows that the molecular fluorescence spectrum is shifted to longer wavelength, shorter fluorescence lifetime, and an intensity decrease. The simultaneous measurement of the single-molecule fluorescence spectrum, intensity, and lifetime clearly exhibits the sequential decrease of the π-conjugation length during sequential photobleaching in the PDI oligomers. The distributions of the spectroscopic parameters for each molecule reveal relative conformational heterogeneity. The ratio of the one-step photobleaching behavior and the HR factor represent the interactions between the PDI constituent units in PDI oligomers. Moreover, the distribution of the HR factor for each molecule reveals subtle differences between the molecules, such as PEP and PBP, having the same HR factor value though the distribution broadness is different. The expansion of π-conjugation enhances the interactions between the PDI constituent units and suggests that structural factors, such as the linkage length and overall structural rigidity, have a strong influence on the photophysical properties of the molecular systems. Therefore, the first direct observation of various emission states and structures of PDI oligomers through a single-molecule fluorescence study envisions the excited state dynamics to understand the molecular properties for further applications.

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ASSOCIATED CONTENT

S Supporting Information *

Supporting figures, data analysis, and the experimental procedures. 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] *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research at Yonsei University was financially supported by the Midcarrier Researcher Program (2010-0029668) of a National Research Foundation grant funded by MEST of Korea. The work at Peking was financially supported by the National Natural Science Foundation of China (No. 21174004 and 21222403).

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