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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.

Phthalocyanines (Pc) are well-known light-harvesting compounds. However, despite the tremendous efforts on phthalocyanine synthesis, the achieved energy conversion efficiencies for Pc-based dye-sensitized solar cells are moderate. To cast light on the factors reducing the conversion efficiency, we have undertaken a time-resolved spectroscopy study of the primary photoinduced reactions at a semiconductor-Pc interface. ZnO nanorods were chosen as a model semiconductor substrate with enhanced specific surface area. The use of a nanostructured oxide surface allows to extend the semiconductor-dye interface with a hole transporting layer (spiro-MeOTAD) in a controlled way, making the studied system closer to a solid-state dye-sensitized solar cell. Four zinc phthalocyanines are compared in this study. The compounds are equipped with bulky peripheral groups designed to reduce the self-aggregation of the Pcs. Almost no signs of aggregation can be observed from the absorption spectra of the Pcs assembled on a ZnO ...

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.

Most macrocycles are made from a simple repeat unit, resulting in high symmetry. Breaking this symmetry allows fine-tuning of the circumference, providing better control of the host–guest behavior and electronic structure. Here, we present the template-directed synthesis of two unsymmetrical cyclic porphyrin hexamers with both ethyne (C2) and butadiyne (C4) links, and we compare these nanorings with the symmetrical analogues with six ethyne or six butadiyne links. Inserting two extra carbon atoms into the smaller nanoring causes a spectacular change in binding behavior: the template affinity increases by a factor of 3 × 109, to a value of ca. 1038M–1, and the mean effective molarity is ca. 830 M. In contrast, removing two carbon atoms from the largest nanoring results in almost no change in its template-affinity. The strain in these nanorings is 90–130 kJ mol–1, as estimated both from DFT calculation of homodesmotic reactions and from comparing template affinities of linear and cyclic oligomers. Breaking the symmetry has little effect on the absorption and fluorescence behavior of the nanorings: the low radiative rates that are characteristic of a circular delocalized S1excited state are preserved in the low-symmetry macrocycles.

Cumulenes are sometimes described as “metallic” because an infinitely long cumulene would have the band structure of a metal. Herein, we report the single-molecule conductance of a series of cumulenes and cumulene analogues, where the number of consecutive C=C bonds in the core is n = 1, 2, 3, and 5. The [n]cumulenes with n = 3 and 5 have almost the same conductance, and they are both more conductive than the alkene (n = 1). This is remarkable because molecular conductance normally falls exponentially with length. The conductance of the allene (n = 2) is much lower, because of its twisted geometry. Computational simulations predict a similar trend to the experimental results and indicate that the low conductance of the allene is a general feature of [n] cumulenes where n is even. The lack of length dependence in the conductance of [3] and [5]cumulenes is attributed to the strong decrease in the HOMO–LUMO gap with increasing length, We thank the EPSRC (grants EP/M016110/1, EP/M014452/1, EP/M014169/1, and EP/M029522/1) and the ERC (grant 320969) for support, and the EPSRC UK National Mass Spectrometry Facility at Swansea University for MALDI spectra. H.S. and S.S. acknowledge the Leverhulme Trust for Leverhulme Early Career Fellowships no. ECF‐2017‐186 and ECF‐2018‐375. H.S. acknowledges the UKRI Future Leaders Fellowship no. MR/S015329/1. IMDEA Nanociencia acknowledges support from the “Severo Ochoa” Programme for Centres of Excellence in R&D (MINECO, Grant SEV‐2016‐0686). N.A. and C.J.L. acknowledge EC H2020 FET Open project 767187 “QuIET”. N.A. and G.R.‐B. were funded by Spanish MINECO (grants MAT2014‐57915‐R, MAT2017‐88693‐R, and MDM‐2014‐0377) and Comunidad de Madrid (grant NANOFRONTMAG‐CM, S2013/MIT‐2850)

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 link between allosteric cooperativity and template-directed synthesis has been investigated by studying complexes in which two oligopyridine ligands bind inside a zinc porphyrin nanoring in a stacked arrangement. The binding of a 6-porphyrin nanoring to two tridentate ligands (with s-triazine or benzene cores) occurs with high negative allosteric cooperativity (α ≈ 10-3-10-4). Formation constants for 1:1 and 1:2 complexes were determined by UV-vis-NIR denaturation titration, using pyridine as a competing ligand, and cooperativity factors were confirmed by NMR spectroscopy. The rate constants for formation of the 1:1 and 1:2 complexes are approximately equal, and the negative cooperativity can be attributed to faster dissociation of the 1:2 complex. These tridentate ligands are not effective templates for directing the synthesis of the 6-porphyrin nanoring, in keeping with their negative cooperativity of binding. In contrast, the binding of a 12-porphyrin nanoring to two hexadentate ligands occurs with high positive allosteric cooperativity (α > 40), and the ligand is an effective Vernier template for directing the synthesis of the 12-porphyrin nanoring. This stacked Vernier template approach creates the product in an open circular conformation, which is advantageous for preparing macrocycles that do not easily adopt a figure-of-eight geometry.

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.

This is the peer reviewed version of the following article:Chemistry - A European Journal 22.13 (2016): 4369-4373, which has been published in final form at http://dx.doi.org/10.1002/chem.201600166. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving, Phthalocyanines (Pcs) are used as sensitizers in dye-sensitized solar cells (DSSCs) because of their stability and intense absorption in the red and near-IR regions. Impressive progress has been made in photovoltaic efficiencies by introduction of bulky peripheral substituents to help suppress macrocycle aggregation. To reach benchmark efficiencies reported for other related dyes, new designs need to be explored. Single carboxy-ZnPc regioisomers substituted at the non-peripheral positions by rigid aryl groups have now been studied, which has shed light on the influence of steric hindrance and/or orientation of the substituent around the anchoring group on the photovoltaic response. The regioisomer bearing the aryl group far away from the anchoring group produces a more effective sensitization of the TiO2 films and higher short-circuit photocurrent density (Jsc). Taking advantage of the good photovoltaic performance in the near-IR region of this ZnPc, it was combined with another appropriate dye for panchromatic sensitization of the mesoporous photoelectrode and an increase of the overall device efficiency, Financial support from the European Union (FP7-ENERGY-2012- 1 framework, GLOBASOL project, Proposal No 309194-2), from the Spanish MINECO (CTQ2014-52869-P), Comunidad de Madrid (FOTOCARBON S2013/MIT-2841), and MECD (F.P.U. fellowship to L.T.) is gratefully acknowledged