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Enhanced thermodynamic stability is a fundamental characteristic of aromatic molecules, yet most previous studies of aromatic stabilization energy (ASE) have been limited to small rings with up to 18 π-electrons. Here we demonstrate that ASE can be detected experimentally in π-conjugated porphyrin nanorings with Hückel circuits of 76-108 π-electrons. This conclusion is supported by analyzing redox potentials to calculate the energy change for isodesmic reactions that convert an aromatic ring to an antiaromatic ring or vice versa. It is also supported by analyzing the energy barriers to conformational equilibria that disrupt aromaticity in the transition state. Both types of experiment indicate that cationic porphyrin nanorings display ASEs of 1-5 kJ mol-1. Density functional theory calculations reproduce the results for both types of experiment and predict ASEs in the range of 1-16 kJ mol-1. The experimental ASEs in porphyrin nanorings are compared with an experimental ASE of [18]annulene of ∼11 kJ mol-1, deduced from analysis of the energy barriers to conformational equilibria in [16], [18], and [20]annulene. Calculated energies of isodesmic reactions give an ASE of ∼37 kJ mol-1 in [18]annulene. This work contributes to a fundamental understanding of aromaticity in large macrocycles.

Aromaticity is an important concept for predicting electronic delocalisation in molecules, particularly for designing organic semiconductors and single-molecule electronic devices. It is most simply defined by the ability of a cyclic molecule to sustain a ring current when placed in a magnetic field. Hückel’s rule states that if a ring has [4n+2] π-electrons, it will be aromatic with an induced magnetisation that opposes the external field inside the ring, whereas if it has 4n π-electrons, it will be antiaromatic with the opposite magnetisation. This rule reliably predicts the behaviour of small molecules, typically with circuits of less than about 22 π-electrons (n = 5). It is not clear whether aromaticity has a size limit and whether Hückel’s rule is valid in much larger macrocycles. Here, we present evidence for global aromaticity in a wide variety of porphyrin nanorings, with circuits of up to 162 π-electrons (n = 40; diameter 5 nm). We show that aromaticity can be controlled by changing the molecular structure, oxidation state and three-dimensional conformation. Whenever a global ring current is observed, its direction is correctly predicted by Hückel’s rule. The magnitude of the current is maximised when the average oxidation state of the porphyrin units is around 0.5–0.7, when the system starts to resemble a conductor with a partially filled valence band. Our results show that aromaticity can arise in large macrocycles, bridging the size gap between ring currents in molecular and mesoscopic rings.

To date β-aryloxy-substituted designs have led to the best results in phthalocyanine-sensitized solar cells (Pc-SCs) because of their low aggregation properties. By incorporating the bulky semiflexible 2,6-diphenylphenoxy group at three α-positions of the Pc, different regioisomers were separated by column chromatography and their photovoltaic performance was thoroughly studied. Efficiencies in the range of 1.9–4.1 % were found, thus demonstrating the importance of the steric interaction between the substituents and the semiconductor surface, also in the case of bulky semiflexible substituents. It was discovered that regioisomers which presented greater steric hindrance around the anchoring group, had lower adsorption densities, and, consequently, lower short-circuit photocurrents (Jsc) and efficiencies.

A new carboxy-Zn(II)phthalocyanine bearing rigid 2,6-diarylphenyl peripheral substituents linked to the macrocycle through alkynyl spacers has been prepared by a convergent approach, which implies a triple Sonogashira coupling between the formyltriiodoZnPc and the corresponding terminal alkyne. Indeed, the introduction of this type of [Formula: see text]-conjugated peripheral substituents accounts for a remarkable red shift of the phthalocyanine Q-band until ca. 700 nm. However, aggregation phenomena could not be supressed and may explain the moderate overall efficiencies achieved with these devices.

This document is the unedited author's version of a Submitted Work that was subsequently accepted for publication in Organic Letters, copyright © American Chemical Society after peer review. To access the final edited and published work, see http://pubs.acs.org/doi/abs/10.1021/ol503557c, A new design of non-aggregated zinc(II) carboxyphthalocyanines with potential application in dye-sensitized solar cells has been developed. It is based on the introduction of bulky and rigid aryl groups at three α positions of the macrocycle. The synthesis has been carried out following a convergent route in which the bulky aryl groups are introduced by Suzuki-Miyaura cross-coupling reaction on a preformed triiodophthalocyanine derivative. Two regioisomers of this α-triaryl-β-car-boxyphthalocyanine could be isolated by column chromatography, Financial support is acknowledged from European Union within the FP7-ENERGY-2012-1 framework, GLOBALSOL project, Proposal No 309194-2 and from the Spanish MEC (CTQ2011- 24187/BQU) and MICINN (PRI-PIBUS-2011-1128)

The synthesis of ethyne-linked porphyrin nanorings has been achieved by template-directed Sonogashira coupling. The cyclic hexamer and octamer are predicted by density functional theory to adopt low symmetry conformations, due to dihedral twists between neighboring porphyrin units, but their symmetries are effectively D6h and D8h, respectively, in solution by 1H NMR. The fluorescence spectra indicate that the singlet excited states of these nanorings are highly delocalized.