Shanmugam Easwaramoorthi

Synthesis of Arylated Perylene Bisimides through C−H Bond Cleavage under Ruthenium Catalysis

Treatment of perylene bisimide (PBI) with various arylboronates in the presence of a ruthenium catalyst provides tetraarylated PBIs at 2,5,8,11-positions in good yields with perfect regioselectivity. The electronic nature of the introduced aryl substituents has a significant impact on their optical and electronic properties. This protocol has been applied to the synthesis of a water-soluble emissive PBI derivative.

Benzoselenadiazole Fluorescent Probes – Near-IR Optical and Ratiometric Fluorescence Sensor for Fluoride Ion

A highly selective and sensitive near-IR optical sensor, benzoselenadiazole based diarylamine (TBS-HN), for fluoride (F(-)) has been designed and synthesized. TBS-HN also shows turn-on ratiometric fluorescence signaling in the presence of F(-) by inhibiting the excited state intramolecular proton transfer (ESIPT) processes.

meso-(4-(N,N-dialkylamino)phenyl)-substituted subporphyrins: remarkably perturbed absorption spectra and enhanced fluorescence by intramolecular charge transfer interactions.

A series of meso-(4-(N,N-dibenzylamino)phenyl)-substituted subporphyrins was synthesized by means of Buchwald-Hartwig amination protocol. Substitution of the amino group at the 4-position of the meso-phenyl substituent resulted in a remarkable red shift in the absorption spectra and drastic enhancement of fluorescence intensity probably as a consequence of intramolecular CT interaction. These characteristics have been utilized to construct a cation-sensing system by appending a 1-aza-15-crown-5 unit to subporphyrin that displays large spectral changes upon cation binding.

Synthesis and photophysical properties of N-fused tetraphenylporphyrin derivatives: near-infrared organic dye of [18]annulenic compounds.

A variety of N-fused porphyrin derivatives were prepared and their photophysical properties were investigated. Although intact N-fused tetraarylporphyrins showed almost no emission, introduction of electron-withdrawing groups such as a nitro group and a cyano group on the macrocycles caused significant refinements in their emission efficiency. Long emission wavelengths (900-1000 nm) as well as fairly large Stokes shifts (∼1200 cm(-1)) are exceptionally unique photophysical properties among [18]annulenic compounds, which could be rationalized by the excited state intramolecular proton transfer (ESIPT) process. Relatively weak emission quantum yields (∼5.0 × 10(-4)) and unusually short S(1) state lifetimes (∼13.5 ps) are in good agreement with the ESIPT process. The solvent and substituent effects on the photophysical properties are also discussed in conjunction with the theoretical studies, where the mesityl groups at the meso-positions play a unique role.

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.

Meso‐Trialkyl‐Substituted Subporphyrins

In recent years, subporphyrins, which are ring-contracted porphyrins, have emerged as promising functional molecules because of their attractive features, including bowl-shaped structures, 14p-electronic aromatic systems, porphyrinlike spectral characteristics, intense fluorescence, and supramolecular chemistry based on axial chelation of the boron atom. Notably, the free rotation of meso-aryl substituents of subporphyrins leads to the large electronic effects that give rise to highly perturbed optical properties, as demonstrated by oligo-1,4-phenyleneethynylene and 4-aminophenyl substituted subporphyrins. Despite this progress, the chemistry of subporphyrins still remains in its infancy and an effective synthetic entry into novel subporphyrins is highly desirable. Considering the important roles that meso-alkyl-substituted porphyrins have played in the development of porphyrin chemistry since the first porphyrin synthesis in 1935, it is desirable to study the chemistry of meso-trialkyl-substituted subporphyrins, but the synthesis of this class of molecules has not been reported to date. Herein, we report the first synthesis of meso-trialkyl-substituted subporphyrins. We initially applied our synthetic protocol for mesotriaryl-substituted subporphyrin to the synthesis of mesotrialkyl-substituted subporphyrins. Pyridine-tri-N-pyrrolylborane was condensed with 1-pentanal under various reaction conditions by changing reaction parameters such as solvent, temperature, molar ratio, and any possible additives. Unfortunately, the desired meso-tributyl subporphyrin was not detected. These negative results led us to explore an indirect route that involved the synthesis of meso-thienyl subporphyrins, and subsequent reductive desulfurization with Raney nickel. Thus, subporphyrins 1a and 1b were prepared by using our protocol in 1.7 and 3.7% yield, respectively (Scheme 1).

Structure-Property Relationship for Two-Photon Absorbing Multiporphyrins: Supramolecular Assembly of Highly-Conjugated Multiporphyrinic Ladders and Prisms

Two-photon absorption (TPA) phenomena of a series of single-strand as well as supramolecular self-assembled ladders and prisms of highly conjugated ethyne bridged multiporphyrin dimer, trimer, and star shaped pentamer have been investigated. The ligand mediated self-assembled supramolecular structures were characterized by UV-visible spectroscopy and small- and wide-angle X-ray scattering (SAXS/WAXS) analysis. The TPA cross section values of multiporphyrins increase nonlinearly from approximately 100 to approximately 18000 GM with an increased number of porphyrin units and elongated pi-conjugation length by virtue of charge transfer and excited-state cumulenic configurations. The observed opposite TPA behavior between their supramolecular ladder and prism configurations necessitates the importance of interstrand interactions between the multiporphyrinic units and the overall shape of the assembly. Furthermore, the diminished TPA cross section of the pentamer, despite the increased pi-conjugation resulting from duplex formation suggests that destabilizing the essential functional configurations at the cost of elongation of pi-delocalization pathway must cause unfavorable effects. We have also shown that one- and two-photon allowed energy-levels of linear multiporphyrins are nearly isoenergetic and the latter transition originates exclusively from the extent of pi-delocalization within the molecule. The identical TPA maximum position of the trimer and pentamer indicates that the TPA of the pentamer arises only from its basic trimer unit in spite of its extended two-dimensional pi-conjugation pathway involving five porphyrinic units.

Peripheral Hexabromination, Hexaphenylation, and Hexaethynylation of meso-Aryl-Substituted Subporphyrins

Effective peripheral fabrication methods of meso-aryl-substituted subporphyrins were explored for the first time. Hexabrominated subporphyrins 2 were prepared quantitatively from the bromination of subporphyrins 1 with bromine. Hexaphenylated subporphyrins 3 and hexaethynylated subporphyrins 4 and 5 were synthesized by Suzuki-Miyaura coupling and Stille coupling, respectively, in good yields. X-ray crystal structures of 2 b, 3 b, 4 b, and 5 a revealed preservation of the bowl-shaped bent structures with bowl depths similar to that of 1. Hexaethynylated subporphyrins exhibit large two-photon-absorption cross-sections due to effective delocalization of the conjugated network to the ethynyl substituents.

Meso-beta doubly linked Zn(II) porphyrin trimers: distinct anti-versus-syn effects on their photophysical properties.

Meso-beta doubly linked Zn(II) porphyrin dimer 1 and anti- and syn-trimers 2 and 3 were synthesized by DDQ-Sc(OTf)(3) oxidation of the corresponding meso-beta singly linked porphyrin precursors. The absorption and fluorescence spectra and TPA cross-section values of 2 and 3 are distinctly different, reflecting their molecular shapes.

Excess polarizability reveals exciton localization/delocalization controlled by linking positions on porphyrin rings in butadiyne-bridged porphyrin dimers.

We report direct measurements of the excess polarizability volumes of butadiyne-bridged zinc porphyrin dimers at singly beta-to-beta (1Zn) and doubly beta-to-beta (2Zn) positions using the transient dc photoconductivity (TDCP) technique. The excess polarizability volumes of the singlet exciton for 1Zn and 2Zn are 110 and 270 A(3), respectively, while those of the triplet exciton are approximately 100 A(3) for both dimers. Our measurements suggest that the singlet exciton is mainly localized on one porphyrin subunit for 1Zn, similar to the case for the porphyrin monomer. While the exciton is fully delocalized on two porphyrin subunits in the case of meso-to-meso linked dimer (3Zn), the extent of exciton localization/delocalization for doubly beta-to-beta linked dimer lies between those of singly beta-to-beta and meso-to-meso linked dimers. Electronic structure calculations show that the dramatically different extents of exciton localization/delocalization are the results of frontier orbital coefficients being small at beta positions but large at meso positions. Two butadiyne linkages between the porphyrins at beta positions (2Zn) clearly facilitate electronic communication between the two porphyrin subunits by virtue of stabilization of cumulenic charge resonance structures through enforced planarity.