Pyosang Kim

Direct observation of ultrafast coherent exciton dynamics in helical π-stacks of self-assembled perylene bisimides

Ever since the discovery of dye self-assemblies in nature, there have been tremendous efforts to exploit biomimetic supramolecular assemblies for tailored artificial photon processing materials. This feature necessarily has resulted in an increasing demand for understanding exciton dynamics in the dye self-assemblies. In a sharp contrast with J-type aggregates, however, the detailed observation of exciton dynamics in H-type aggregates has remained challenging. In this study, as we succeed in measuring transient fluorescence from Frenkel state of π-stacked perylene tetracarboxylic acid bisimide dimer and oligomer aggregates, we present an experimental demonstration on Frenkel exciton dynamics of archetypal columnar π–π stacks of dyes. The analysis of the vibronic peak ratio of the transient fluorescence spectra reveals that unlike the simple π-stacked dimer, the photoexcitation energy in the columnar π-stacked oligomer aggregates is initially delocalized over at least three molecular units and moves coherently along the chain in tens of femtoseconds, preceding excimer formation process.

Benzimidazole-embedded N-fused aza-indacenes: synthesis and deprotonation-assisted optical detection of carbon dioxide.

N-fused aza-indacene-based fluorophores 1 and 2 were prepared via a three-component condensation involving benzimidazole-carbinol, trifluoroacetic acid, and either pyrrole or indole, respectively. The N-fused aza-indacenes act as optical-based chemosensors for dissolved carbon dioxide gas following fluoride anion-mediated deprotonation.

Solvent- and temperature-dependent conformational changes between Hückel antiaromatic and Möbius aromatic species in meso-trifluoromethyl substituted [28]hexaphyrins.

We investigated the photophysical properties of figure-eight-like meso-hexakis(trifluoromethyl) [26]- and [28]hexaphyrins(1.1.1.1.1.1) denoted as TFM26H and TFM28H, respectively, using steady-state and time-resolved spectroscopy along with theoretical calculations to explore their electronic and magnetic natures depending on their molecular aromaticity. TFM26H exhibited a well-resolved absorption feature and intense fluorescence, both of which were neither solvent- nor temperature-dependent. These optical properties were in agreement with its Hückels [4n + 2] aromaticity as observed in typical aromatic porphyrinoids. The S(1)-state lifetime of ~50 ps for TFM26H in solution was shorter than those in planar aromatic hexaphyrins (>100 ps) presumably due to nonplanar figure-eight geometry of TFM26H. However, TFM28H exhibited remarkable changes in solvent- and temperature-dependent absorption spectra as well as excited-state lifetimes indicating that a dynamic equilibrium occurs between the two conformational species. With the help of quantum mechanical geometry optimization and vertical excitation energy calculations, we found that the figure-eight double-sided conformer observed in the solid-state and single-sided distorted one could be the best candidates for the two conformers, which should be Hückel antiaromatic and Möbius aromatic species, respectively, based on their optical characteristics, molecular orbital structures, and excited-state lifetimes. Conformational dynamics between these two conformers of TFM28H was scrutinized in detail by temperature-dependent (1)H NMR spectra in various solvents, which showed that the conformational equilibrium was quite sensitive to solvents and that a conformational change faster than the NMR time-scale occurs even at 173 K.

Large porphyrin squares from the self-assembly of meso-Triazole-Appended L-Shaped meso-meso-Linked ZnII-Triporphyrins: synthesis and efficient energy transfer

meso-Triazolyl-appended Zn(II)-porphyrins were readily prepared by Cu(I)-catalyzed 1,3-dipolar cycloaddition of benzyl azide to meso-ethynylated Zn(II)-porphyrin (click chemistry). In noncoordinating CHCl(3) solvent, spontaneous assembly occurred to form tetrameric array (3)(2) from meso-meso-linked diporphyrins 3, and dodecameric porphyrin squares (4)(4) and (5)(4) from the L-shaped meso-meso-linked triporphyrins 4 and 5. The structures of these assemblies were examined by (1)H NMR spectra, absorption spectra, and their gel permeation chromatography (GPC) retention time. Furthermore, the structures of the dodecameric porphyrin squares (4)(4) and (5)(4) were probed by small- and wide-angle X-ray scattering (SAXS/WAXS) measurements in solution using a synchrotron source. Excitation-energy migration processes in these assemblies were also investigated in detail by using both steady-state and time-resolved spectroscopic methods, which revealed efficient excited-energy transfer (EET) between the meso-meso-linked Zn(II)-porphyrin units that occurred with time constants of 1.5 ps(-1) for (3)(2) and 8.8 ps(-1) for (5)(4).

Versatile Photophysical Properties of meso‐Aryl‐Substituted Subporphyrins: Dipolar and Octupolar Charge‐Transfer Interactions

Donor-acceptor systems based on subporphyrins with nitro and amino substituents at meta and para positions of the meso-phenyl groups were synthesized and their photophysical properties have been systematically investigated. These molecules show two types of charge-transfer interactions, that is, from center to periphery and periphery to center depending on the peripheral substitution, in which the subporphyrin moiety plays a dual role as both donor and acceptor. Based on the solvent-polarity-dependent photophysical properties, we have shown that the fluorescence emission of para isomers originates from the solvatochromic, dipolar, symmetry-broken, and relaxed excited states, whereas the non-solvatochromic fluorescence of meta isomers is of the octupolar type with false symmetry breaking. The restricted meso-(4-aminophenyl) rotation at low temperature prevents the intramolecular charge-transfer (ICT)-forming process. The two-photon absorption (TPA) cross-section values were determined by photoexcitation at 800 nm in nonpolar toluene and polar acetonitrile solvents to see the effect of ICT on the TPA processes. The large enhancement in the TPA cross-section value of approximately 3200 GM (1 GM = 10(-50) cm(4) s photon(-1)) with donor-acceptor substitution has been attributed to the octupolar effect and ICT interactions. A correlation was found between the electron-donating/-withdrawing abilities of the peripheral groups and the TPA cross-section values, that is, p-aminophenyl > m-aminophenyl > nitrophenyl. The increased stability of octupolar ICT interactions in highly polar solvents enhances the TPA cross-section value by a factor of approximately 2 and 4, respectively, for p-amino- and m-nitrophenyl-substituted subporphyrins. On the other hand, the stabilization of the symmetry-broken, dipolar ICT state gives rise to a negligible impact on the TPA processes.

Structural Dependence on Excitation Energy Migration Processes in Artificial Light Harvesting Cyclic Zinc(II) Porphyrin Arrays

A series of covalently linked cyclic porphyrin arrays CNZ that consist of N/2 of meso-meso directly linked zinc(II) porphyrin dimer subunits Z2 bridged by 1,3-phenylene spacers have been prepared by Ag(I)-promoted oxidative coupling reaction. We have investigated the excitation energy migration processes of CNZ in toluene by using femtosecond transient absorption anisotropy decay measurements by taking 2Z2 composed of two Z2 units linked by 1,3-phenylene as a reference molecule. On the basis of the excitation energy transfer rate determined for 2Z2, we have revealed the excitation energy hopping rates in the cyclic arrays CNZ by using a regular polygon model. The number of excitation energy hopping sites N(flat) calculated by using a regular polygon model is close to the observed N(expt) value obtained from the transient absorption anisotropy decays for C12Z-C18Z with circular and well-ordered structures. On the other hand, a large discrepancy between N(flat) and N(expt) was found for smaller or larger arrays (C10Z, C24Z, and C32Z). In the case of C10Z, m-phenylene linked 2Z2 motif with the interchromophoric angle of 120 degrees is not well suited to make a cyclic pentagonal array C10Z based on planar pentagonal structure. This geometrical factor inevitably causes a structural distortion in C10Z, leading to a discrepancy between N(expt) and N(flat) values. On the contrary, the presence of highly distorted conformers such as figure-eight structures reduces the number of effective hopping sites N(expt) in large cyclic arrays C24Z and C32Z. Thus, our study demonstrates that not only the large number of porphyrin chromophores in the cyclic arrays CNZ but the overall rigidity and three-dimensional orientation in molecular architectures is a key factor to be considered in the preparation of artificial light harvesting porphyrin arrays.

Excitation energy migration in covalently linked perylene bisimide macrocycles

A series of acetylene-linked perylene bisimide (PBI) macrocycles 3a–d with various ring sizes from trimer to hexamer have been synthesised by a palladium-catalysed homocoupling reaction of perylene bisimide 1. Photophysical properties of these PBI macrocycles have been examined by steady-state absorption, fluorescence, fluorescence lifetime, fluorescence anisotropy decay, and transient absorption measurements. Both pump-power dependence on the femtosecond transient absorption and the transient absorption anisotropy decay profiles are directly related with the excitation energy migration processes within PBI macrocycles where the exciton-exciton annihilation time and the polarization anisotropy decay time are well described in terms of the Forster-type incoherent energy hopping model. Consequently, the excitation energy hopping times of macrocycles become slower and then saturated as the ring size increases. Nevertheless, the intrinsically large transition dipole moment of PBI leads to fast energy transfer processes as compared to other artificial light-harvesting complexes such as those constructed from porphyrin building blocks.

Relationship between Dynamic Planarization Processes and Exciton Delocalization in Cyclic Oligothiophenes

In cyclic molecular structures, while the effect of conformational disorder on exciton delocalization is well understood, the impact of dynamic planarization processes remains unclear due to a lack of detailed investigation on the associated exciton dynamics. Thus, we have investigated the exciton delocalization of π-conjugated linear and cyclic oligothiophenes in the course of dynamic planarization processes by time-resolved fluorescence spectra measurements and theoretical calculations. Especially, through a comparative analysis of linear and cyclic oligothiophenes, we found that the evolution of 0-0 and 0-1 vibronic bands is strongly related to the conformations of cyclic molecular systems, reflecting the extent of exciton delocalization. Collectively, we believe that our findings are applicable to various π-conjugated organic materials and will provide new insights into the relationship between exciton delocalization and cyclic molecular conformation.

Subporphyrins with an Axial BC Bond

Axial fabrications of subporphyrins have been conveniently accomplished by the reaction of B(methoxo)triphenylsubporphyrin with Grignard reagents such as aryl-, heteroaryl-, ferrocenyl-, β-styryl-, phenylethynyl-, and ethylmagnesium bromides. The axial groups thus introduced are not conjugated with the subporphyrin core. This situation leads to effective fluorescence quenching of subporphyrins when the axial group is strongly electron donating such as 4-dimethylaminophenyl and ferrocenyl groups.

Synthesis and Properties of Boron(III)-Coordinated Subbacteriochlorins

Subbacteriochlorins, which were prepared via hydrogenation of subporphyrins with Raney nickel, are modestly aromatic due to 14π-diazaannulenic circuit and exhibit characteristic blue-shifted Soret-like bands, intensified fluorescence spectra, and high oxidation potentials.