Dipolar triplet states of para-nitroaniline and N-alkyl; Derivatives with one-, two-, and three-fold symmetry

Photoexcitation of p-nitroaniline (PNA) and its N-alkyl derivs. results in the formation of triplet states with dipole moments considerably larger than that of the ground state, as monitored by time-resolved microwave cond. (TRMC). The triplet state lifetime, tT, is only 54 ns for PNA in benzene but increases to 1400 ns on N,N-di-Me substitution and to 275 ns in the more polar solvent p-dioxane. This sensitivity of tT is attributed to the close proximity of all of the lowest lying singlet and triplet np* and pp* states of PNA, which results in substantial S1 .tautm. T1 mixing. The dipole moment of the \"pure\" 3pp* state is estd. to be 11 D. Methylene-bridged arrays of PNA moieties with two- and three-fold Dn symmetry, 2-PNA and 3-PNA, are also found to have highly dipolar triplet states, indicating that symmetry breaking and exciton localization occur in T1. Flip-flop interchange between the resonant dipolar structures is estd. to take place on a time scale of tens of picoseconds. Collective interactions between the PNA moieties in the multichromophoric arrays results in a marked blue shift in the first absorption max. from 384 nm for 1-PNA to 375 nm for 2-PNA and to 354 nm for 3-PN

Charge Recombination Via Long-Distance Electron Transfer through Frozen and Molten N-Alkyl Chains in Pulse-Irradiated Mesomorphic Phthalocyanines

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Dipolar triplet states of para-nitroaniline and N-alkyl; Derivatives with one-, two-, and three-fold symmetry

Photoexcitation of p-nitroaniline (PNA) and its N-alkyl derivs. results in the formation of triplet states with dipole moments considerably larger than that of the ground state, as monitored by time-resolved microwave cond. (TRMC). The triplet state lifetime, tT, is only 54 ns for PNA in benzene but increases to 1400 ns on N,N-di-Me substitution and to 275 ns in the more polar solvent p-dioxane. This sensitivity of tT is attributed to the close proximity of all of the lowest lying singlet and triplet np* and pp* states of PNA, which results in substantial S1 .tautm. T1 mixing. The dipole moment of the \"pure\" 3pp* state is estd. to be 11 D. Methylene-bridged arrays of PNA moieties with two- and three-fold Dn symmetry, 2-PNA and 3-PNA, are also found to have highly dipolar triplet states, indicating that symmetry breaking and exciton localization occur in T1. Flip-flop interchange between the resonant dipolar structures is estd. to take place on a time scale of tens of picoseconds. Collective interactions between the PNA moieties in the multichromophoric arrays results in a marked blue shift in the first absorption max. from 384 nm for 1-PNA to 375 nm for 2-PNA and to 354 nm for 3-PN

Peralkylated coronenes via regiospecific hydrogenation of hexa-peri-hexabenzocoronenes

A remarkable, regiospecific hydrogenation zips around the 4 nm perimeter of hexa-peri-hexabenzocoronenes (HBC) adding 18 hydrogen atoms, leading to the first peralkylated coronenes, in quantitative yields in some cases. Increasing steric bulk of side chains was found to hinder the reaction, while unsubstituted HBC could be converted to a modest extent despite its vanishing solubility. The structures of the novel coronenes are unequivocally confirmed by MALDI-TOF, 1H, 13C, and heteronuclear correlation NMR, and UV-vis absorption spectroscopy. The puckered-ring periphery of these discotics does not prohibit self-assembly to columnar structures in a fashion similar to that of the planar precursors, as determined from wide-angle X-ray diffraction, but decreases the isotropization temperature by 300 °C relative to the latter. Branching in the alkyl chains frustrates nucleation from the melt, resulting in clear polymorphism depending on the thermal treatment. Nonetheless, preliminary measurements indicate high charge-carrier mobilities and lifetimes within the bulk material, on the same order as those previously found for HBCs

Rapid Rotation of Energy in the excited state of a circular Hexa-Carbazole array

Hexacarbazolylbenzene, a sym. propeller array of six carbazole units, has been synthesized. The relaxed S1 excited state has transient broken symmetry and a dipole moment of 8 debye caused by an excimer-like interaction between neighboring carbazole units. The excimeric distortion undergoes rapid rotary diffusion within the array resulting in more than 1000 'round-trips' during the course of the ca. 5 ns lifetime of the photoexcitation, thus mimicking the 'storage-ring' function of the chlorophyllic arrays in light-harvesting bacteri