Sung Cho

Aromatic versus Antiaromatic Effect on Photophysical Properties of Conformationally Locked trans-Vinylene-Bridged Hexaphyrins

We have investigated the electronic structures and energy relaxation dynamics of vinylene-bridged hexaphyrins using steady-state and time-resolved spectroscopies along with theoretical calculations in order to reveal their aromaticity-dependent electronic and magnetic properties. Ethynyl-TIPS-substituted planar and rectangular [28]hexaphyrin, regarded as a Huckel antiaromatic compound, tends to adopt a twisted Mobius aromatic topology via structural distortion in order to reduce the total internal energy, in contrast to aromatic [26]hexaphyrin, which maintains a planar conformation in solution. Spectacles-shaped vinylene-bridged [26]- and [28]hexaphyrins represent highly Huckel aromatic and antiaromatic natures, respectively, as revealed by NMR spectroscopy, giving rise to remarkable differences in NICS(0) and HOMA values and shapes of steady-state absorption and emission spectra. In particular, lifetime of the lowest singlet excited state of [28]hexaphyrin (8.6 ps) is 30 times shorter than that of the aromatic congener [26]hexaphyrin (282 ps), as measured by the femtosecond transient absorption technique. Both frontier molecular orbital analyses and vertical excitation energy calculations suggest that vinylene-bridged [28]hexaphyrin has an optically dark lowest singlet state in the NIR region, as observed in the absorption spectrum with a very low oscillator strength, which might act as a ladder state in the excited-state energy relaxation dynamics. Our findings provide further insight into the aromaticity-driven electronic properties of various porphyrinoids as well as of aromatic/antiaromatic hydrocarbon systems.

Ultrafast transient dynamics of Zn(II) porphyrins: Observation of vibrational coherence by controlling chirp of femtosecond pulses

Femtosecond coherence spectroscopic study on porphyrin molecules has demonstrated that the oscillatory features residing at the transient absorption and fluorescence decay profiles are strongly correlated with the lifetimes of the excited states and the displacements of the minima of the potential energy surfaces that are involved in the pump and probe laser pulses. We have attained a greater degree of control in the wave packet dynamics in the transient absorption by controlling the chirp of the ultrashort optical pulses. This feature provides a clue to the excited potential energy surface such as its curvature and displacement. For two representative porphyrin monomers, ZnII tetraphenylporphyrin and ZnII octaethylporphyrin, we were able to obtain detailed information on the excited state dynamics and subsequent structural changes based on the comparison between the frequency spectra retrieved from the oscillatory features in the transient absorption and fluorescence temporal profiles and the ground stat...

Coherence in metal-metal-to-ligand-charge-transfer excited states of a dimetallic complex investigated by ultrafast transient absorption anisotropy.

Coherence in the metal-metal-to-ligand-charge transfer (MMLCT) excited state of diplatinum molecule [Pt(ppy)(μ-(t)Bu(2)pz)](2) has been investigated through the observed oscillatory features and their corresponding frequencies as well as polarization dependence in the single-wavelength transient absorption (TA) anisotropy signals. Anticorrelated parallel and perpendicular TA signals with respect to the excitation polarization direction were captured, while minimal oscillatory features were observed in the magic angle TA signal. The combined analysis of the experimental results coupled with those previous calculated in the literature maps out a plausible excited state trajectory on the potential energy surface, suggesting that (1) the two energetically close MMLCT excited states due to the symmetry of the molecule may be electronically and coherently coupled with the charge density shifting back and forth between the two phenylpyridine (ppy) ligands, (2) the electronic coupling strength in the (1)MMLCT and (3)MMLCT states may be extracted from the oscillation frequencies of the TA signals to be 160 and 55 cm(-1), respectively, (3) a stepwise intersystem crossing cascades follows (1)MMLCT → (3)MMLCT (T(1b)) → (3)MMLCT (T(1a)), and (4) a possible electronic coherence can be modulated via the Pt-Pt σ-interactions over a picosecond and survive the first step of intersystem crossing. Future experiments are in progress to further investigate the origin of the oscillatory features. These experimental observations may have general implications in design of multimetal center complexes for photoactivated reactions where coherence in the excited states may facilitate directional charge or energy transfer along a certain direction between different parts of a molecule.

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.

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.

An indolocarbazole-bridged macrocyclic porphyrin dimer having homotropic allosterism with inhibitory control

A macrocyclic host, a pair of zinc porphyrins bridged by two anion-acceptable indolocarbazole moieties, has shown strong positive homotropic allosterism upon anionic guest bindings with an inhibitory control mechanism by DABCO addition.

Solution Phase Exciton Diffusion Dynamics of a Charge-Transfer Copolymer PTB7 and a Homopolymer P3HT

Using ultrafast polarization-controlled transient absorption (TA) measurements, dynamics of the initial exciton states were investigated on the time scale of tens of femtoseconds to about 80 ps in two different types of conjugated polymers extensively used in active layers of organic photovoltaic devices. These polymers are poly(3-fluorothienothiophenebenzodithiophene) (PTB7) and poly-3-hexylthiophene (P3HT), which are charge-transfer polymers and homopolymers, respectively. In PTB7, the initial excitons with excess vibrational energy display two observable ultrafast time constants, corresponding to coherent exciton diffusion before the vibrational relaxation, and followed by incoherent exciton diffusion processes to a neighboring local state after the vibrational relaxation. In contrast, P3HT shows only one exciton diffusion or conformational motion time constant of 34 ps, even though its exciton decay kinetics are multiexponential. Based on the experimental results, an exciton dynamics mechanism is conceived taking into account the excitation energy and structural dependence in coherent and incoherent exciton diffusion processes, as well as other possible deactivation processes including the formation of the pseudo-charge-transfer and charge separate states, as well as interchain exciton hopping or coherent diffusion.

An Unusual Stacking Transformation in Liquid‐Crystalline Columnar Assemblies of Clicked Molecular Propellers with Tunable Light Emissions

The columnar liquid-crystalline (LC) and fluorescence properties of three-dimensional molecular propellers based on tetraphenylethylene is reported. X-ray scattering studies reveal an unusual transition from a rectangular (Colrec ) to a hexagonal columnar (Colhex ) phase. In contrast to second-order intercolumnar transitions based on a common tilt mechanism, the transition is first order and involves an unprecedented zigzag stacking of aromatic propellers in the Colrec phase. A sudden change in emission color from sky blue to green occurs rapidly and reversibly at this transition, which is due to the planarization of the propeller mesogen.