A Waterwheel‐Shaped meso–meso‐Linked Porphyrin Pentamer

Wheels on water? A waterwheel‐shaped porphyrin pentamer has been synthesized by palladium‐catalyzed cross‐coupling reactions. The key intermediate is a boronate porphyrin, in which four boronic ester groups are directly attached to the meso‐positions.

     

β‐to‐β 2,5‐Pyrrolylene‐Linked Cyclic Porphyrin Oligomers

β‐to‐β 2,5‐Pyrrolylene linked cyclic porphyrin oligomers have been synthesized by Suzuki–Miyaura coupling of 2,5‐diborylpyrrole and 3,7‐dibromoporphyrin. The cyclic porphyrin oligomers exhibit roughly coplanar structures, strong excitonic coupling, small electrochemical HOMO–LUMO gaps, and ultrafast excitation energy transfer between the neighboring porphyrins via the pyrrolylene bridge.     

Synthesis and Photophysical Properties of Doubly β‐to‐β Bridged Cyclic ZnII Porphyrin Arrays

A series of doubly β‐to‐β bridged cyclic Zn^II porphyrin arrays were prepared by a stepwise Suzuki–Miyaura coupling reaction of borylated Zn^II porphyrin with different bridge groups. The coupling of the building block of β,β′‐diboryl Zn^II porphyrin 1 with different bridges provided the doubly β‐to‐β carbazole‐bridged Zn^II porphyrin array 3 , the fluorene‐bridged Zn^II porphyrin array 5 , the fluorenone‐bridged Zn^II porphyrin array 7 , and the three‐carbazole‐bridged Zn^II porphyrin ring 8 . The structural assignment of 3 was confirmed by the X‐ray diffraction analysis, which revealed a highly symmetrical and remarkably bent syn‐form structure. The incorporation of bridge units with different electronic effects results in different photophysical properties of the cyclic Zn^II porphyrin arrays. Comprehensive photophysical studies demonstrate that the electron‐withdrawing bridge fluorenone has the largest electronic interaction with the Zn^II porphyrin unit among the series, thus resulting in the highest two‐photon absorption cross‐section values (σ⁽²⁾) of 6570±60 GM for 7 . The present work provides a new strategy for developing porphyrin‐based optical materials.     

β,β‐Diborylated Subporphyrinato Boron(III) Complexes as Useful Synthetic Precursors

Iridium‐catalyzed borylation of B‐aryl meso‐free subporphyrinato boron(III) complexes (hereinafter referred to simply as subporphyrins) with bis(pinacolato)diboron gave 2,13‐diborylated subporphyrins regioselectively, which served as promising synthetic precursors for 2,13‐diarylated subporphyrins and doubly β‐to‐β 1,3‐butadiyne‐bridged subporphyrin dimers. 2,13‐Diarylated subporphyrins display perturbed absorption spectra, depending upon the β‐aryl substituents. Doubly 1,3‐butadiyne‐bridged syn and anti subporphyrin dimers thus prepared exhibit differently altered absorption spectra with split Soret‐like bands, which have been accounted for in terms of exciton coupling.     

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 CHCl₃ 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 ¹H 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.     

Excitonic Coupling Strength and Coherence Length in the Singlet and Triplet Excited States of meso–meso Directly Linked Zn(II)porphyrin Arrays

Excitation‐energy transport (EET) phenomena in meso–meso directly linked Zn(II )porphyrin arrays in the singlet and triplet excited states were investigated with a view to electronic coupling strength and coherence length by steady‐state and time‐resolved spectroscopic measurements. To investigate energy transfer in the triplet states, we modified the Zn(II )porphyrin arrays with bromo substituents at both ends. The coupling strength of the Soret bands of the arrays was estimated to be about 2200 cm^?1, and that of the Q bands is about 570 cm^?1. The coherence length in the S₁ state of the Zn(II )porphyrin arrays was determined to be 4–5 porphyrin units, which is comparable to that of the well‐ordered two‐dimensional circular structure B850 in the peripheral light‐harvesting antenna (LH2) in photosynthetic purple bacteria. This indicates that the Zn(II )porphyrin arrays are well suited for mimicking natural light‐harvesting antenna complexes. On the other hand, the rate of energy transfer in the triplet state is estimated to be on the order of 100 μs^?1, and the very weak coupling between the triplet states (ca. 0.003 cm^?1), indicates that the triplet excitation energy is essentially localized on a single porphyrin moiety.     

Azobenzene‐Bridged Porphyrin Nanorings: Syntheses,Structures, and Photophysical Properties

Azobenzene‐bridged β‐to‐β and meso‐to‐meso porphyrin nanorings were successfully synthesized by a palladium‐catalyzed Suzuki–Miyaura coupling reaction in a logical synthesis. The dimeric structure was confirmed by XRD analysis. The azo linkages in di‐ and tetramers are in the all‐trans conformation, whereas in the trimers one azo linkage can be interconverted between cis and trans under external stimulation. When trimeric isomers are heated to 333 K or higher, the azo linkages will be in the all‐trans configurations: the pure all‐trans trimer can be kept in the dark for several months. Fluorescence anisotropy and pump‐power‐dependent decay results revealed excitation energy transfer for azobenzene‐bridged zinc–porphyrin nanorings. The distances between porphyrin units of these azobenzene‐bridged porphyrin arrays are almost the same, but the exciton energy hopping (EEH) times for each wheel are markedly different. The dimer and meso‐to‐meso tetramer possess relatively short excitation energy transfer (EET) times (1.28 and 2.48 ps, respectively) due to their good planarity and rigidity. In contrast, the EET time for the trimeric zinc(II)–porphyrin array (6.9 ps) is relatively long due to its nonradiative decay pathway (i.e., cis/trans isomerization of azobenzene). Both di‐ and tetramers exhibit relatively high fluorescence quantum yields, whereas the trimers show weak emission because of structural differences.     

Synthesis of Extremely π‐Extended Porphyrin Tapes from Hybrid meso‐meso Linked Porphyrin Arrays: An Approach Towards the Conjugation Length

Directly meso‐meso, β‐β, β‐β triply linked porphyrin arrays are exceptional π‐conjugated molecules exhibiting remarkably red‐shifted absorption bands extending deeply in the IR region. In order to determine the effective conjugated length (ECL), we embarked on the synthesis of the porphyrin tapes far beyond the 12‐mer, which is the longest we have prepared so far. In this study, to find the compromise between the feasibility of the meso‐meso coupling reaction up to longer arrays and the sufficient solubility and chemical stability of the resultant porphyrin tapes, we prepared hybrid meso‐meso linked porphyrin arrays BOn up to 24‐mer, which have two different aryl groups, a 2,4,6‐tris(3,5‐di‐tert‐butylphenoxy) phenyl group (Ar¹) and a 3,5‐dioctyloxy phenyl group (Ar²). All these arrays were effectively converted into the corresponding triply linked porphyrin tapes TBOn by oxidation with DDQ‐Sc(OTf)₃. Importantly, the low energy Q‐band‐like absorption bands of TBOn are progressively red‐shifted with an increase in the number of porphyrins n until 16 but the red‐shift is saturated at n=16, indicating the ECL of the porphyrin tape to be around 14–16. The regularly introduced meso‐aryl bulky substituents impose facial encumbrance, hence leading to the effective suppression of π–π interactions as well as improvement of the chemical stabilities of TBOn .     

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.

     

Enhanced Electron‐Transfer Properties of Cofacial Porphyrin Dimers through π–π Interactions

π–π assisted : Photoinduced electron transfer from cofacial porphyrin dimers to electron acceptors is prominently accelerated, whereas the back electron transfer is decelerated, relative to the corresponding porphyrin monomer (see figure).

     

α‐Arylation/Heteroarylation of Chiral α‐Aminomethyltrifluoroborates by Synergistic Iridium Photoredox/Nickel Cross‐Coupling Catalysis

Direct access to complex, enantiopure benzylamine architectures using a synergistic iridium photoredox/nickel cross‐coupling dual catalysis strategy has been developed. New C(sp³)? C(sp²) bonds are forged starting from abundant and inexpensive natural amino acids.