A strategy for the design of molecules with large two-photon absorption cross sections, delta, was developed, on the basis of the concept that symmetric charge transfer, from the ends of a conjugated system to the middle, or vice versa, upon excitation is correlated to enhanced values of delta. Synthesized bis(styryl)benzene derivatives with donor-pi-donor, donor-acceptor-donor, and acceptor-donor-acceptor structural motifs exhibit exceptionally large values of delta, up to about 400 times that of trans-stilbene. Quantum chemical calculations performed on these molecules indicate that substantial symmetric charge redistribution occurs upon excitation and provide delta values in good agreement with experimental values. The combination of large delta and high fluorescence quantum yield or triplet yield exhibited by molecules developed here offers potential for unprecedented brightness in two-photon fluorescent imaging or enhanced photosensitivity in two-photon sensitization, respectively.

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The one- and two-photon spectroscopic properties of four symmetrically substituted donor−acceptor−donor distyrylbenzenes with either di-n-butyl- or diphenylamino donor groups and cyano acceptor groups are reported. It has been found that the position of the substitution of the electron-withdrawing cyano groups on the central phenylene ring as compared to the vinylene bond strongly affects the observed properties. In particular, the molecules with cyano substitution on the α-carbon of the vinylene linkage are characterized by weak fluorescence, short fluorescence lifetimes, and two-photon cross sections (δ) that are comparable to analogous molecules with no acceptor groups. In contrast, the molecules with acceptor substitution on the central phenylene ring are strongly fluorescent and have δ values roughly twice those of the vinyl-substituted molecules. These results are discussed in terms of the larger deviation of the conjugated backbone from planarity and the smaller distance between the donors and acceptors when the cyano groups are substituted on the vinylene carbon rather than the central phenylene ring.

Rapid growth in the field of supramolecular chemistry 1 has resulted in a new generation of conformationally rigid polyhedral structures of controlled shape and size which can be built from the spontaneous assembly of complementary subunits. The structural and functional features of the self-assembled product may be entirely encoded in its components without the use of templates. 2 The resulting ensemble possesses a central void that makes it potentially useful for the development of molecular sensors, new types of catalysts, and various other materials with useful properties. 3 Here we report the preparation of dodecahedra with outer dimensions of ∼5 and ∼8 Å. The actual process of self-assembly of a single dodecahedron molecule involves formation of 60 metal-ligand bonds and participation of 50 individual molecules. As the size of these self-assembled metallocyclic structures increases, the rigidity of the individual building blocks becomes essential for transmission of directing effects over larger distances. Flexible linear connectors cause the formation of oligomers instead of discrete molecules with defined shape presumably due to the formation of defects which cannot selfcorrect.The most complex of the five Platonic polyhedra, the dodecahedron, contains 12 fused five-membered rings that comprise the highest symmetry group I h . Its 12 pentagonal faces are formed from 20 vertexes and 30 edges; hence it can be prepared via edgedirected assembly from 20 tridentate angular subunits with approximately 108°directing angles combined with 30 bidentate linear subunits (Figure 1). A suitable angular subunit, tri(4′-pyridyl)methanol (1), was prepared from the known di(4-pyridyl)-ketone 4 in good yield. X-ray crystallographic analysis established the structure of compound 1 (Figure 2), 5 demonstrating that the tetrahedral directing angles in 1 are near the optimal 108°. The linear bidentate units bis[4,4′-(trans-Pt(PEt 3 ) 2 OTf)]benzene (2), bis[4,4′-(trans-Pt(PPh 3 ) 2 OTf)]biphenyl (3), and bis[4,4′-(transPt(PEt 3 ) 2 OTf)]biphenyl (4) were prepared according to procedures previously described. 6 Bis[1,4-(trans-Pt(PPh 3 ) 2 OTf)]ethynylbenzene (5) was obtained via a base-catalyzed coupling of 1,4-diethynylbenzene with trans-diiodobis(triethylphosphine)platinum(II) and subsequent treatment with AgOTf.When a solution of the angular component 1 in acetone was added to the linear unit 2 in dichloromethane, under careful monitoring of the stoichiometry by 1 H and 31 P NMR, a selfassembled dodecahedron 6 was formed in 99% isolated yield. The observed 1 H, 31 P, 13 C NMR spectra 5 are all consistent with the formation of a single, highly symmetrical molecule. This molecule has 60 positive charges and contains 60 triflate counterions. Its estimated diameter along the 3-fold axis is about 5.5 nm and it has a molecular weight of 41656.0 Da, making it comparable in size and molecular weight to small proteins. Despite the large size and molecular weight, it is remarkably soluble in polar organic solvents, presumably due to ext...

Self-assembly of predesigned angular and linear dipyridylporphyrin modules with bisphosphine-coordinated Pd(II) and Pt(II) angular and linear modules leads to cyclic porphyrin arrays containing two or four units and ranging in size from 15 to 39 Å. Multinuclear NMR spectra indicate high symmetry for these macrocycles. Restriction in rotation of trans-DPyDPP groups around the axis defined by the terminal metal−nitrogen bonds distorts the symmetry of the tetramers, but the rotation is unrestricted at elevated temperatures. Chiral metal triflates containing R(+)- or S(−)-BINAP phosphines promote formation of enantiomeric macrocycles with a puckered geometry. CD spectra of the chiral macrocycles reveal a strong exciton coupling between the porphyrin chromophores in the tetramers. Emission spectra reveal moderate fluorescence quenching of the dipyridylporphyrin fluorophores upon treatment with metal triflates and concomitant incorporation into the macrocycles.

Exhaustive direct fluorination of dimethyl bicyclo[1.1.1]pentane-1,3-dicarboxylate leads to dimethyl pentafluorobicyclo[1.1.1]pentane-1,3-dicarboxylate (2) and hexafluorobicyclo[1.1.1]pentane-1,3-dicarboxylate (3). The latter was hydrolyzed to the diacid (4) and converted to the 1,3-dibromo and 1,3-diiodo analogues (5 and 6) by the Hunsdieker reaction followed by treatment with SmI2. Na/NH3 reduction of the disodium salt 10 causes cage C−C bond cleavage. Single-crystal X-ray diffraction analysis of 3 revealed very short nonbonded F−F separations of 2.41 Å and an interbridgehead distance of 1.979 Å, long compared with 1.875 Å in 1,3-diacetylbicyclo[1.1.1]pentane [19; cf. 1.954 Å calculated (MP2/6-31G*) for 2,2,4,4,5,5-hexafluorobicyclo[1.1.1]pentane (13)]. Calculation suggests a strain energy of 101 kcal/mol (MP2/6-31G*) for the hexafluorinated cage, compared with 68 kcal/mol for the parent bicyclo[1.1.1]pentane (20). The remarkably low pK a values of 4 [0.73 and 1.34; cf. 3.22 and 4.26 for the parent diacid 24] originate in a direct field effect of fluorine atoms, combined with an increased s character of the exocyclic hybrid orbital on the bridgehead carbon in 4 (calculated 34% in 13) relative to 24 (calculated 30% in 20). Analysis of the strongly coupled nuclear spin systems of 2 and 3, based on a combination of two-dimensional NMR, spectral simulations, and GIAO-HF/6-31G* calculations of chemical shifts, revealed large and stereospecific long-range 1H−13C, 1H−19F, 13C−19F, and 19F−19F spin−spin coupling constants.

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