Jaesung Yang

Regioselective Ru-catalyzed direct 2,5,8,11-alkylation of perylene bisimides.

Perylene tetracarboxylic acid bisimides (PBIs) are an important class of dyes and pigments for widespread practical use, which have been extensively investigated for a long time both in academia and industry. Recently, they have also received much attention as n-type semiconducting materials. Furthermore, owing to high fluorescence quantum efficiency and photostability, they have been a popular motif for single-molecule spectroscopy. Chemical modifications of PBIs are quite important to gain desirable photophysical and electronic properties as well as solubility. In spite of their rich material chemistry, functionalization of the perylene core of PBIs relies on halogenation of the bay area (1,6,7,12-positions) and subsequent transformations (Scheme 1). Selective functionalization at 2,5,8,11-positions remains unavailable to date. Here we wish to disclose the first selective synthesis of 2,5,8,11-substituted PBIs. We have envisioned the potential of direct functionalization of PBIs by organometallic and catalytic means: ruthenium-catalyzed C H bond activation and addition strategy, namely, the Murai–Chatani–Kakiuchi protocol (Scheme 2). This reaction can introduce alkyl substituents to the proximal position of the directing groups. Successful installation of alkyl chains at the 2,5,8,11-positions of PBIs would allow greater modification of properties in the solid state or condensed phase. The reaction procedure is quite simple. A mixture of bis(N-ethylpropyl)PBI 1a and trimethylvinylsilane was heated in mesitylene at 165 8C for 60 h in the presence of [a] Prof. Dr. H. Shinokubo Department of Applied Chemistry Graduate School of Engineering, Nagoya University Chikusa-ku, Nagoya 464-8603 (Japan) E-mail : hshino@apchem.nagoya-u.ac.jp [b] S. Nakazono, Y. Imazaki, Prof. Dr. A. Osuka Department of Chemistry, Graduate School of Science Kyoto University, Sakyo-ku, Kyoto 606-8502 (Japan) E-mail : osuka@kuchem.kyoto-u.ac.jp [c] H. Yoo, J. Yang, Prof. Dr. D. Kim Department of Chemistry, Yonsei University Seoul 120-749 (Korea) E-mail : dongho@yonsei.ac.kr [d] Dr. T. Sasamori, Prof. Dr. N. Tokitoh Institute for Chemical Research Kyoto University, Kyoto 611-0011 (Japan) [e] T. C dric, Prof. Dr. H. Kageyama Department of Chemistry, Graduate School of Science Kyoto University, Sakyo-ku, Kyoto 606-8502 (Japan) E-mail : kage@kuchem.kyoto-u.ac.jp Supporting information for this article is available on the WWW under http ://dx.doi.org/10.1002/chem.200901318. Scheme 1. Functionalization of perylene bisimides.

Excitation energy migration processes in cyclic porphyrin arrays probed by single molecule spectroscopy.

By using single molecule fluorescence spectroscopy we have investigated the excitation energy migration processes occurring in a series of cyclic porphyrin arrays bearing a close proximity in overall architectures to the LH2 complexes in purple bacterial photosynthetic systems. We have revealed that the conformational heterogeneity induced by the structural flexibility in large cyclic porphyrin arrays, which provides the nonradiative deactivation channels as an energy sink or trap, reduces significantly the energy migration efficiency. Our study provides detailed information on the energy migration efficiency of the artificial light-harvesting arrays at the single molecule level, which will be a guideline for future applications in single molecular photonic devices in the solid state.

Structure-Property Relationship of Perylene Bisimide Macrocycles Probed by Atomic Force Microscopy and Single-Molecule Fluorescence Spectroscopy

Properties of a series of acetylene-linked perylene bisimide (PBI) macrocycles with different ring size composed of three to six PBI dyes were investigated by atomic force microscopy (AFM) and single-molecule fluorescence spectroscopy in a condensed phase. It was demonstrated that the structures of PBI cyclic arrays (CNs, N = 3, 4, 5, and 6) become distorted with increasing the ring size through molecular dynamics (MD) simulations (PM6-DH2 method) and AFM height images of CNs on highly ordered pyrolytic graphite (HOPG) surface. The MD simulations showed that only C5 and C6 rings are highly flexible molecules whose planarization goes along with a significant energetic penalty. Accordingly, both molecules did not show ordered adlayers on a HOPG surface. In contrast, C3 and C4 are far more rigid molecules leading to well-ordered hexagonal (C3) and rectangular (C4) 2D lattices. At the single-molecule level, we showed that the fluorescence properties of single CNs are affected by the structural changes. The fluorescence lifetimes of CNs became shorter and their distributions became broader due to the structural distortions with increasing the ring size. Furthermore, the CNs of smaller ring size exhibit a higher photostability and an efficient excitation energy transfer (EET) due to the more well-defined and planar structures compared to the larger CNs. Consequently, these observations provide evidence that not only PBI macrocycles are promising candidates for artificial light-harvesting systems, but also the photophysical properties of CNs are strongly related to the structural rigidity of CNs.

Synthesis, Ensemble, and Single Molecule Characterization of a Diphenyl-Acetylene Linked Perylenediimide Trimer

The synthesis and the photophysical characterization at the ensemble and single molecule level of a terrylenediimide (TDI) dimer are reported. The spectroscopic experimental data are compared with those obtained for the corresponding model compound TDI. Steady-state and ps time-correlated single photon counting have shown that both chromophores in the TDI dimer are in the weak coupling regime allowing their interaction by Förster resonance energy transfer. Femtosecond transient absorption experiments showed an excitation power dependence of the fluorescence decay, which could indicate the occurrence of singlet-singlet annihilation. Single molecule intensity traces were investigated for the TDI dimer and suggested two intensity levels. For both intensity levels several parameters among which emission maximum, fluorescence decay times, antibunching, blinking off-times and rate of dark state formation were compared. The blinking analysis revealed that the yield of dark state formation is an order of magnitude higher when the two chromophores are still active compared to the case where one is photobleached. The off-times remain however similar.

Fluorescence Dynamics of Directly Meso−Meso Linked Porphyrin Rings Probed by Single Molecule Spectroscopy

Porphyrin rings CZ4, CZ6, and CZ8 that respectively comprise four, six, and eight porphyrins, immobilized in a thin PMMA film, have been investigated using single molecule fluorescence spectroscopy with a focus on the influences of the overall structural rigidity as the ring size of porphyrin array increases. Neighboring porphyrin moieties were linked directly to enhance through-bond electronic interactions and, as a consequence, efficient excitation energy migration processes like the natural LH2 complex. Unlike the ensemble study, the single molecule study using confocal microscopy could eliminate the averaging effect, and consequently provide detailed information on individual molecular behaviors. Indeed, in solution, as a dihedral angle between neighboring porphyrins decreases in the order of CZ6 > CZ8 > CZ4, red-shifted Q-absorption bands and faster excitation energy hopping rates were observed. However, at the single molecule level, we found that they show longer survival times, less frequent on-off behaviors, narrower fluorescence lifetime distributions, and high relative single molecular brightness in the order of CZ8 > CZ6 > CZ4, by recording fluorescence intensity trajectories. Especially, CZ4 reveals high photostability with its rigid structure, and about 3 porphyrin units among the 4 chromophores-constituted molecule behave as a collective coherent domain. Thus, our results single out CZ4 as a potential and promising candidate for application in artificial light harvesting solid-state devices.

Control of Molecular Structures and Photophysical Properties of Zinc(II) Porphyrin Dendrimers Using Bidentate Guests: Utilization of Flexible Dendrimer Structures as a Controllable Mold

We have prepared supramolecular assemblies of hexaaryl-anchored polyester zinc(II) porphyrin dendrimers (6P(Zn)W, 12P(Zn)W, and 24P(Zn)W) with various bipyridyl guests (C(n)Py2; n = 1, 2, 4, 6, and 8) through self-assembled coordination to control the structures and photophysical properties. We comparatively investigated the photophysical properties of porphyrin dendrimers with and without guest binding by using ensemble and single-molecule spectroscopy. The spectrophotometric titration data of dendrimers with guest molecules provide a strong indication of the selective intercalation of bipyridyl guests into porphyrin dendrimers. The representative dendrimer assembly 12P(Zn)W [symbol: see text] C6Py2 exhibits increased fluorescence quantum yield and lifetime in ensemble measurements, as well as higher initial photon count rates with stepwise photobleaching behavior in the single-molecule fluorescence intensity trajectories (FITs) compared to 12P(Zn)W. At the single-molecule level, the higher photostability of 12P(Zn)W [symbol: see text] C6Py2 can be deduced from the long durations of the first emissive levels in the FITs. We attribute the change in photophysical properties of the dendrimer assemblies to their structural changes upon intercalation of guest molecules between porphyrin units. These results provide new insight into the control of porphyrin dendritic structures using appropriate bidentate guests in poor environmental conditions.

Excitonic coupling in linear and trefoil trimer perylenediimide molecules probed by single-molecule spectroscopy.

Perylenediimide (PDI) molecules are promising building blocks for photophysical studies of electronic interactions within multichromophore arrays. Such PDI arrays are important materials for fabrication of molecular nanodevices such as organic light-emitting diodes, organic semiconductors, and biosensors because of their high photostability, chemical and physical inertness, electron affinity, and high tinctorial strength over the entire visible spectrum. In this work, PDIs have been organized into linear (L3) and trefoil (T3) trimer molecules and investigated by single-molecule fluorescence microscopy to probe the relationship between molecular structures and interchromophoric electronic interactions. We found a broad distribution of coupling strengths in both L3 and T3 and hence strong/weak coupling between PDI units by monitoring spectral peak shifts in single-molecule fluorescence spectra upon sequential photobleaching of each constituent chromophore. In addition, we used a wide-field defocused imaging technique to resolve heterogeneities in molecular structures of L3 and T3 embedded in a PMMA polymer matrix. A systematic comparison between the two sets of experimental results allowed us to infer the correlation between intermolecular interactions and molecular structures. Our results show control of the PDI intermolecular interactions using suitable multichromophoric structures.

Determination of the Superradiance Coherence Length of Directly Linked Linear Porphyrin Arrays at the Single‐Molecule Level

Down to the wire: Photobleaching dynamics show the exciton delocalization length of directly linked porphyrin arrays (see picture) to be about four or five porphyrin units at the single-molecule level. This result provides a better understanding of how light-signal transmission occurs in the solid state and gives a perspective for the porphyrin arrays to be used as single-molecule photonic wires.

Reconstruction of the Molecular Structure of a Multichromophoric System Using Single-Molecule Defocused Wide-Field Imaging

Single-molecule defocused wide-field imaging (DWFI) has been demonstrated to be useful to determine molecular structure parameters, such as the orientations of transition dipole moments and the angular relationships between chromophores in multichromophoric molecular systems. For a series of acetylene-linked perylene bisimide (PBI) macrocycles with different ring size comprising three to six PBI dyes, we reconstructed the molecular structure of the multichromophoric system using DWFI method. Furthermore, we revealed that the structural heterogeneities and distortions depend on the ring size. Our findings illustrate the use of DWFI to gain deeper insight into the structure-property relationships of artificial light-harvesting molecular systems.

Inhomogeneity in the excited-state torsional disorder of a conjugated macrocycle.

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.