Synthesis of a tetrakis(1-pyrenylethynyl)-substituted rigid hinge-like molecule (1) is described. The intramolecular pi-stacking interaction of the pyrene units is studied by 1H NMR and fluorescence spectroscopy. Due to intramolecular pi-stacking interactions, chemical shifts of the pyrene protons in 1 are highly shielded in the NMR spectrum. Fluorescence from the static excimer state is observed due to pi-stacking interactions among the pyrene units in the ground state of 1. Based on the spectroscopic evidence, conformations and dynamics of 1, arising from the hindered rotation of the major axis, are proposed.

SummarySeveral 2-(phenylethynyl)triphenylene derivatives bearing electron donor and acceptor substituents on the phenyl rings have been synthesized. The absorption and fluorescence emission properties of these molecules have been studied in solvents of different polarity. For a given derivative, solvent polarity had minimal effect on the absorption maxima. However, for a given solvent the absorption maxima red shifted with increasing conjugation of the substituent. The fluorescence emission of these derivatives was very sensitive to solvent polarity. In the presence of strongly electron withdrawing (–CN) and strongly electron donating (–NMe2) substituents large Stokes shifts (up to 130 nm, 7828 cm−1) were observed in DMSO. In the presence of carbonyl substituents (–COMe and –COPh), the largest Stokes shift (140 nm, 8163 cm−1) was observed in ethanol. Linear correlation was observed for the Stokes shifts in a Lippert–Mataga plot. Linear correlation of Stokes shift was also observed with E T(30) scale for protic and aprotic solvents but with different slopes. These results indicate that the fluorescence emission arises from excited state intramolecular charge transfer in these molecules where the triphenylene chromophore acts either as a donor or as an acceptor depending upon the nature of the substituent on the phenyl ring. HOMO–LUMO energy gaps have been estimated from the electrochemical and spectral data for these derivatives. The HOMO and LUMO surfaces were obtained from DFT calculations.

A diacetylenic molecular hinge bearing two ethynyltriphenylene units (1) has been synthesized. Evidence from (1)H NMR and variable temperature NMR (VT-NMR) of 1 in comparison to model compounds bearing only one triphenylene unit suggests that there is an equilibrium between the open conformer and the intramolecularly pi-pi interacting closed conformer in solution (equilibrium constant K = 6.5 at 298 K in CDCl(3)) arising from the rotation of the diacetylenic hinge. Unusual fluorescence emission observed from 1 has been assigned to excimer formation arising from intramolecularly pi-pi interacting triphenylene units in the excited state. Steady state and picosecond time resolved fluorescence spectra of 1 were nearly identical and corresponded to intramolecular excimer emission.

Acetylene-bridged N-confused porphyrin (NCP) dimers were synthesized by Stille coupling reactions. Moderate electronic interaction between the NCP chromophores was observed in the inner-3H tautomers while strong interaction was observed in the inner-2H tautomers.Cross-coupling reactions are one of the most powerful methods to construct large ³-conjugated molecules.1 Among such molecules, porphyrin dimers as well as oligomers are a particularly important class of compounds, which often show fascinating electronic properties and functions, such as twophoton absorption, light-harvesting, and electron transport. One intrinsic characteristic of NCP is NH-tautomerization, which allows change of the electronic state significantly (Figure 1).5 Thus, the inner-3H tautomer (NCP3H) has an [18]annulenic substructure and is strongly aromatic, while the inner-2H tautomer (NCP2H) loses the [18]annulenic substructure and is moderately aromatic.6 Accordingly, NCP3H is thermodynamically more stable than NCP2H and much difference is found in their photophysical properties.7 Previously, we have demonstrated that intramolecular energy transfer could be controlled by NH-tautomerization in the porphyrinNCP dyad system. 8 Additionally, substitution effect at the 3-position would be much different between NCP3H and NCP2H.9 Along the lines of these findings, we can expect interesting electronic structures in 3,3¤-acetylene-bridged NCP dimers. In this paper, the synthesis and properties of 3,3¤-acetylene-bridged NCP dimers by the Stille coupling are reported.First, the reactions of 3-bromo-NCPs with tributyl(phenylethynyl)tin were studied to prepare 3-alkynyl-NCPs (Scheme 1). The alkynylated NCPs were previously prepared by the Sonogashira coupling 9 or inversion of alkynylated N-fused porphyrins. 10 The reaction of N-confused 3-bromotetraphenylporphyrinatosilver(III) complex 1 proceeded smoothly with Pd(OAc) 2 /AsPh 3 catalyst to give Ag III (NCP3H)CCPh 2 in 63% yield. Transmetalated product, Pd II (NCP2H)CCPh 3, was also obtained in 19% yield. This unique reductive transmetalation from Ag(III) to Pd(II) inside the NCP core was also observed in the Sonogashira coupling reaction. 9 The reaction of N-confused 3-bromotetrakis[4-(trifluoromethyl)phenyl]porphyrinatocopper(II) complex 4 proceeded in a similar manner to give Cu II (NCP2H)CCPh 5 in 48% yield. Transmetalated product 6 was also isolated in 10% yield. The AsPh 3 ligand so far afforded the better yields than the phosphine ligands such as PPh 3 and diphenylphosphinoethane.Then, syntheses of acetylene-bridged NCP dimers were examined by the reactions with bis(tributylstannyl)acetylene (Scheme 2). The Pd-catalyzed reaction of 1 successfully gave Ag III (NCP3H)CCAg III (NCP3H) 7 in 20% yield together with Ag III (NCP3H)CCPd II (NCP2H) 8 in 7% yield. In the 1 H NMR spectrum of 7 in CDCl 3 , the signals due to ¢-pyrrolic protons (12H) were observed in the region of ¤ 8.58.9, suggesting its strong aromatic character. Additionally, the number of proton signals indicated the symmetric stru...