Stereoselective synthesis of a KLM ring model of ciguatoxin: Confirmation of the C54 stereochemistry

A ciguatoxin KLM ring model and its C54 epimer were stereoselectively synthesized. Comparison of their ¹H NMR data with that on the natural toxin established the earlier stereochemical assignment at the C54 position.     

Modulation of axial coordination in N-confused porphyrin-antimony(V) dibromide complex by proton stimulus

The change of the axial bond length in the antimony(V) N-confused tetraphenylporphyrin dibromide complex was demonstrated by protonation and confirmed from X-ray single crystal analyses.     

Efficient light-harvesting antenna with a multi-porphyrin cascade

A light-harvesting antenna 1 comprising three varieties of porphyrins, each having a different number of ethynyl groups at its meso positions, was designed and synthesized. Antenna 1 exhibits intense absorption throughout the visible region up to 700 nm. Steady-state and time-resolved fluorescence studies showed that singlet-excited-state energy transfer occurs from the peripheral porphyrins to the central porphyrin with >90% efficiency and rate constants on the order of 10(10) s(-1).     

Neutral radical and singlet biradical forms of meso-free, -keto, and -diketo hexaphyrins(1.1.1.1.1.1): effects on aromaticity and photophysical properties

We have investigated the electronic structures and photophysical properties of 5,10,20,25-tetrakis(pentafluorophenyl)-substituted hexaphyrin(1.1.1.1.1.1) (1) and its meso-keto (2) and meso-diketo derivatives (3) using various spectroscopic measurements. In conjunction with theoretical calculations, these analyses revealed fundamental structure-property relationships within this series, including unusual ground-state electronic structures with neutral, monoradical, and singlet biradical character. The meso-free species 1 is a representative 26 π-electron aromatic compound and shows characteristic spectroscopic features, including a sharp Soret band, well-defined Q-like bands, and a moderately long excited state lifetime (τ = 138 ps). In contrast, the meso-keto derivative 2 displays features characteristic of a neutral monoradical species at the ground state, including the presence of lower energy absorption bands in the NIR spectral region and a relatively short excited-state lifetime (13.9 ps). The meso-diketo 3 exhibits features similar to those of 2, specifically NIR absorptions and a short excited-state lifetime (9.7 ps). Compound 3 is thus assigned as being a ground-state singlet biradicaloid. Two photon absorption (TPA) measurements revealed comparatively large σ(2) values of 600 GM for 2 and 1600 GM for 3 with excitation at λ(ex) =1600 nm as compared to that observed for 1 (σ(2): 360 GM). The enhanced nonlinear optical properties of 2 and 3 are rationalized in terms of the open-shell electronic configuration allowing a large, field-induced fluctuation in the electron density (i.e., a large polarization). This interpretation is supported by theoretical evaluations of the static second hyperpolarizabilities (γ) and γ density analyses. Furthermore, nucleus-independent chemical shift (NICS) and harmonic oscillator model of aromaticity (HOMA) values and anisotropy of the induced current density (AICD) plots revealed a clear distinction in terms of the aromatic character of 1-3. Importantly, the open-shell radicaloid 2 and singlet biradicaloid 3 can be formally regarded as 27 π-electron nonaromatic and 26 π-electron aromatic species, respectively, constrained within a dominant 28 π-electron conjugated network. On the basis of the combined experimental and theoretical evidence, it is concluded that the meso-carbonyl groups of 2 and 3 play an important role in perturbing the macrocyclic π-conjugation of the parent hexaphyrin structure 1. In particular, they lead to the imposition of intrinsic radical and biradical character on the molecule as a whole and thus easy-to-discern modifications of the overall electronic effects.     

Synthesis and properties of hybrid porphyrin tapes

Hybrid porphyrin tapes 3 and 4 , consisting of a mixture of 3,5‐di‐tert‐butylphenyl‐substituted donor‐type Zn^II–porphyrins and pentafluorophenyl‐substituted acceptor‐type Zn^II–porphyrins, were prepared by a synthetic route involving cross‐condensation reaction of a Ni^II–porphyrinyldipyrromethane and pentafluorophenyldipyrromethane with pentafluorobenzaldehyde followed by appropriate demetalation, remetalation, and oxidative ring‐closure reaction. The Ni^II‐substituted porphyrin tapes 5 (Ni‐Zn‐Ni) and 6 (Ni‐H₂‐Ni) were also prepared through similar routes. The hybrid porphyrin tapes 3 and 4 are more soluble and more stable than normal porphyrin tapes 1 and 2 consisting of only donor‐type Zn^II–porphyrins. The solid‐state and crystal packing structures of 3 , 4 , and 5 were elucidated by single‐crystal X‐ray diffraction analysis. Singly meso–meso‐linked hybrid porphyrin arrays 12 and 14 exhibit redox potentials that roughly correspond to each constituent porphyrin segments, while the redox potentials of the hybrid porphyrin tapes 3 and 4 are positively shifted as a whole. The two‐photon absorption (TPA) values of 1–6 were measured by using a wavelength‐scanning open aperture Z‐scan method and found to be 1900, 21 000, 2200, 27 000, 24 000, and 26 000 GM, respectively. These results illustrate an important effect of elongation of π‐electron conjugation for the enhancement of TPA values. The hybrid porphyrin tapes show slightly larger TPA values than the parent ones.     

The role of electronic coupling in linear porphyrin arrays probed by single-molecule fluorescence spectroscopy

Single-molecule photophysical properties of two families of linear porphyrin arrays have been investigated by single-molecule fluorescence detection techniques. Butadiyne-linked arrays (Z(N)B) with extensive π-conjugation perform as photostable one-quantum systems. This demonstration has been suggested by the long-lasting initial emissive state and subsequent discrete one-step photobleaching in the fluorescence intensity trajectories (FITs). As in the behavior of a one-quantum system, Z(N)B shows anti-bunching data in the coincidence measurements. On the other hand, in directly-linked arrays (Z(N)) with strong dipole coupling, each porphyrin moiety keeps individual character in photobleaching dynamics. The stepwise photobleachings in the FITs account for this explanation. Most of the FITs of Z(N) do not carry momentary cessation of fluorescence emission, which has been explained by the strongly bound electron-hole pair of Frenkel exciton that suppresses charge transfer between the molecule and surrounding polymers. These results give insight into the influences of interchromophorinc interactions between porphyrin moieties in the multiporphyrin arrays on their fluorescence dynamics at the single-molecule level.     

Excited-state energy relaxation dynamics of triply linked Zn(II) porphyrin arrays

In this study, we have investigated the excited-state energy deactivation dynamics of extended π-conjugated molecular systems that consist of competitive electronic and vibrational relaxation processes.