Aromaticity and Antiaromaticity in the Excited States of Porphyrin Nanorings.

Aromaticity can be a useful concept for predicting the behavior of excited states. Here we show that π-conjugated porphyrin nanorings exhibit size-dependent excited-state global aromaticity and antiaromaticity for rings containing up to eight porphyrin subunits, although they have no significant global aromaticity in their neutral singlet ground states. Applying Baird's rule, even rings ([4 n] π-electrons) are aromatic in their lowest excited states, whereas the lowest excited states of odd rings ([4 n + 2] π-electrons) are antiaromatic. These predictions are borne out by density functional theory (DFT) studies of the nucleus-independent chemical shift (NICS) in the T1 triplet state of each ring, which reveal the critical importance of the triplet delocalization to the emergence of excited-state aromaticity. The singlet excited states (S1) are explored by measurements of the radiative rate and fluorescence peak wavelength, revealing a subtle odd-even alternation as a function of ring size, consistent with symmetry breaking in antiaromatic excited states.

Template-Directed Synthesis of a Conjugated Zinc Porphyrin Nanoball.

We report the template-directed synthesis of a π-conjugated 14-porphyrin nanoball. This structure consists of two intersecting nanorings containing six and 10 porphyrin units. Fluorescence upconversion spectroscopy experiments demonstrate that electronic excitation delocalizes over the whole three-dimensional π system in less than 0.3 ps if the nanoball is bound to its templates or over 2 ps if the nanoball is empty.

Towards higher electron mobility in modulation doped GaAs/AlGaAs core shell nanowires.

Precise control over the electrical conductivity of semiconductor nanowires is a crucial prerequisite for implementation of these nanostructures into novel electronic and optoelectronic devices. Advances in our understanding of doping mechanisms in nanowires and their influence on electron mobility and radiative efficiency are urgently required. Here, we investigate the electronic properties of n-type modulation doped GaAs/AlGaAs nanowires via optical pump terahertz (THz) probe spectroscopy and photoluminescence spectroscopy over the temperature range 5 K-300 K. We directly determine an ionization energy of 6.7 ± 0.5 meV (T = 52 K) for the Si donors within the AlGaAs shell that create the modulation doping structure. We further elucidate the temperature dependence of the electron mobility, photoconductivity lifetime and radiative efficiency, and determine the charge-carrier scattering mechanisms that limit electron mobility. We show that below the donor ionization temperature, charge scattering is limited by interactions with interfaces, leading to an excellent electron mobility of 4360 ± 380 cm2 V-1 s-1 at 5 K. Above the ionization temperature, polar scattering via longitudinal optical (LO) phonons dominates, leading to a room temperature mobility of 2220 ± 130 cm2 V-1 s-1. In addition, we show that the Si donors effectively passivate interfacial trap states in the nanowires, leading to prolonged photoconductivity lifetimes with increasing temperature, accompanied by an enhanced radiative efficiency that exceeds 10% at room temperature.

Synthesis of Five-Porphyrin Nanorings by Using Ferrocene and Corannulene Templates.

The smallest and most strained member of a family of π-conjugated cyclic porphyrin oligomers was synthesized by using pentapyridyl templates based on ferrocene and corannulene. Both templates are effective for directing the synthesis of the butadiyne-linked cyclic pentamer, despite the fact that the radii of their N5 donor sets are too small by 0.5 Šand 0.9 Å, respectively (from DFT calculations). The five-porphyrin nanoring exhibits a structured absorption spectrum and its fluorescence extends to 1200 nm, reflecting strong π conjugation and Herzberg-Teller vibronic coupling.

Breaking the Symmetry in Molecular Nanorings.

Because of their unique electronic properties, cyclic molecular structures ranging from benzene to natural light-harvesting complexes have received much attention. Rigid π-conjugated templated porphyrin nanorings serve as excellent model systems here because they possess well-defined structures that can readily be controlled and because they support highly delocalized excitations. In this study, we have deliberately modified a series of six-porphyrin nanorings to examine the impact of lowering the rotational symmetry on their photophysical properties. We reveal that as symmetry distortions increase in severity along the series of structures, spectral changes and an enhancement of radiative emission strength occur, which derive from a transfer of oscillator strength into the lowest (k = 0) state. We find that concomitantly, the degeneracy of the dipole-allowed first excited (k = ±1) state is lifted, leading to an ultrafast polarization switching effect in the emission from strongly symmetry-broken nanorings.

Six-Coordinate Zinc Porphyrins for Template-Directed Synthesis of Spiro-Fused Nanorings.

Five-coordinate geometry is the standard binding mode of zinc porphyrins with pyridine ligands. Here we show that pseudo-octahedral six-coordinate zinc porphyrin complexes can also be formed in solution, by taking advantage of the chelate effect. UV-vis-NIR titrations indicate that the strength of this second coordination is ca. 6-8 kJ mol(-1). We have used the formation of six-coordinate zinc porphyrin complexes to achieve the template-directed synthesis of a 3D π-conjugated spiro-fused array of 11 porphyrin units, covalently connected in a nontrivial topology. Time-resolved fluorescence anisotropy experiments show that electronic excitation delocalizes between the two perpendicular nanorings of this spiro-system within the experimental time-resolution of 270 fs.

Self-Assembly of Russian Doll Concentric Porphyrin Nanorings.

Electronic communication between concentric macrocycles with wave functions that extend around their circumferences can lead to remarkable behavior, as illustrated by multiwalled carbon nanotubes and photosynthetic chlorophyll arrays. However, it is difficult to hold one π-conjugated molecular ring inside another. Here, we show that ring-in-ring complexes, consisting of a 6-porphyrin ring locked inside a 12-porphyrin ring, can be assembled by placing different metals in the two rings (zinc and aluminum). A bridging ligand with carboxylate and imidazole binding sites forms spokes between the two rings, resulting in a highly cooperative supramolecular self-assembly process. Excitation is transferred from the inner 6-ring to the outer 12-ring of this Russian doll complex within 40 ps. These complexes lead to a form of template-directed synthesis in which one nanoring promotes formation of a larger concentric homologous ring; here, the effective template is an eight-component noncovalent assembly. Russian doll templating provides a new approach to amplifying the size of a covalent nanostructure.

A Molecular Nanotube with Three-Dimensional π-Conjugation.

A π-conjugated twelve-porphyrin tube is synthesized in 32% yield by a template-directed coupling reaction that joins together six porphyrin dimers, forming twelve new C-C bonds. The nanotube has two bound templates, enclosing an internal volume of approximately 4.5 nm(3). Its UV/Vis/NIR absorption and fluorescence spectra resemble those of a previously reported six-porphyrin ring, but are red-shifted by approximately 300 cm(-1), reflecting increased conjugation. Ultrafast fluorescence spectroscopy demonstrates extensive excited-state delocalization. Transfer of electronic excitation from an initially formed state polarized in the direction of the nanotube axis (z axis) to an excited state polarized in the xy plane occurs within 200 fs, resulting in a negative fluorescence anisotropy on excitation at 742 nm.

Structure-Directed Exciton Dynamics in Templated Molecular Nanorings.

Conjugated polymers with cyclic structures are interesting because their symmetry leads to unique electronic properties. Recent advances in Vernier templating now allow large shape-persistent fully conjugated porphyrin nanorings to be synthesized, exhibiting unique electronic properties. We examine the impact of different conformations on exciton delocalization and emission depolarization in a range of different porphyrin nanoring topologies with comparable spatial extent. Low photoluminescence anisotropy values are found to occur within the first few hundred femtoseconds after pulsed excitation, suggesting ultrafast delocalization of excitons across the nanoring structures. Molecular dynamics simulations show that further polarization memory loss is caused by out-of-plane distortions associated with twisting and bending of the templated nanoring topologies.

A Molecular Nanotube with Three-Dimensional π-Conjugation.

A π-conjugated twelve-porphyrin tube is synthesized in 32 % yield by a template-directed coupling reaction that joins together six porphyrin dimers, forming twelve new C-C bonds. The nanotube has two bound templates, enclosing an internal volume of approximately 4.5 nm3. Its UV/Vis/NIR absorption and fluorescence spectra resemble those of a previously reported six-porphyrin ring, but are red-shifted by approximately 300 cm-1, reflecting increased conjugation. Ultrafast fluorescence spectroscopy demonstrates extensive excited-state delocalization. Transfer of electronic excitation from an initially formed state polarized in the direction of the nanotube axis (z axis) to an excited state polarized in the xy plane occurs within 200 fs, resulting in a negative fluorescence anisotropy on excitation at 742 nm.

Chromophores in Molecular Nanorings: When Is a Ring a Ring?

The topology of a conjugated molecule plays a significant role in controlling both the electronic properties and the conformational manifold that the molecule may explore. Fully π-conjugated molecular nanorings are of particular interest, as their lowest electronic transition may be strongly suppressed as a result of symmetry constraints. In contrast, the simple Kasha model predicts an enhancement in the radiative rate for corresponding linear oligomers. Here we investigate such effects in linear and cyclic conjugated molecules containing between 6 and 42 butadiyne-linked porphyrin units (corresponding to 600 C-C bonds) as pure monodisperse oligomers. We demonstrate that as the diameter of the nanorings increases beyond ∼10 nm, its electronic properties tend toward those of a similarly sized linear molecule as a result of excitation localization on a subsegment of the ring. However, significant differences persist in the nature of the emitting dipole polarization even beyond this limit, arising from variations in molecular curvature and conformation.