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Exploiting supramolecular interactions in phthalocyanine chemistry

Periodic Reporting for period 1 - PhthaloSupra (Exploiting supramolecular interactions in phthalocyanine chemistry)

Reporting period: 2017-10-01 to 2019-09-30

"Objective: During the MSCA fellowship carried out in the Department of Organic Chemistry at the Universidad Autónoma de Madrid (UAM), Spain, the Fellow was actively engaged in the synthesis, characterization and study of novel phthalocyanine (Pc) and Pc-cyclopenta[hi]aceantrylene (CPA)-based systems. The present project aims at the formation of stable Pc- and Pc-CPA-based supramolecular polymers through the combination of π-stacking and hydrogen bonding interactions “simultaneously” working in a controlled fashion. During PhthaloSupra, the researcher successfully synthesized and characterized the structures initially proposed, and, in most of the cases, the evaluation of their photophysical properties and aggregation capability studied. We encountered several difficulties during the synthetic work, problems that, in most of the cases, we successfully overcome. As part of PhthaloSupra, we also extended the idea of supramolecular systems solely based on CPA dimers, where both CPA units were linked by either one (1TB), two (2TB) or four (4TB) triple bonds. These CPA dimers were expected to form highly conjugated systems due to the acetylenic spacer(s) connecting the two units. The synthesis of these dimers was challenging due to the reduced solubility of some of the CPA precursors and mainly involved Sonogashira coupling reactions.

Impact on society: PhthaloSupra was set on an ambitious goal involving the interdisciplinary fields of molecular materials and supramolecular chemistry, encompassing the preparation and study of novel Pc-based functional materials using noncovalent interactions. During my stay in Prof. Torres group, I enjoyed a good relationship with the supervisor and a good research environment, which helps in fulfilment of contractual and legal obligations. I got a chance to supervise a graduate student and an interaction on daily basis with doctoral students and post-doctoral colleagues within the Torres’ group, collaborating on common projects. I also got a chance to attend several workshops organised by IMDEA Nanociencia, Madrid. The output of the research gave rise to short-term benefits in terms of a deeper understanding of the relationship between structure, self-organization, morphology and physical (i.e. light-harvesting, electrochemical, charge separation and transport) properties of the molecular and supramolecular systems prepared. Medium- and long-term benefits will stem from the collaboration studies that are still under progress. The study and application of these new materials in the field of photovoltaic technologies such as DSSC, OPV etc, are under progress, hopefully, contributing to advance in one of the most importantly societal challenge such as the production of “Secure, clean and efficient energy”.
The progress and the results of the studies carried out in PhthaloSupra generated a pile of interesting results and soon it will be disseminated to the scientific community through publications in high-level international science. As a further channel of dissemination, I attended two international conferences (12th International Symposium on Supramolecular and Macrocyclic Chemistry (ISMSC), June 2019, Lecce, Italy; and 3rd International Symposium on ''the Synthesis and Application of Curved Organic π-Molecules & Materials (CURO-π³)"", 5–7 September 2018, University of Oxford, United Kingdom) where I got the chance to communicate and discuss the results obtained to some of the best researchers in the fields of Pc and supramolecular chemistry. I also participated in the European Research Night (2018), Madrid, a MSCA action, under the Horizon 2020 specific programme. We strongly believe that such outreach activities are extremely important to fill the gap that sometimes exists between science and technology, and the non-specialized public."
In PhthaloSupra, we aimed to synthesize novel “stable Pc-based supramolecular polymers through the combination of π-stacking, and hydrogen bonding interactions”. In this respect, Pc, CPA, and Pc-CPA fragments peripherally functionalized with isophthalic acid were prepared aiming at forming hexameric structures in solutions (Fig. 1,2), through π-stacking interactions between the peripheral Pc, CPA, or Pc-CPA fragments giving rise to elongated supramolecular polymers. Their aggregation studies were performed in different solvents (Fig. 6), which suggested a strong aggregation and formation of supramolecular aggregates in non-polar solvents like methylcyclohexane (MCH), which was further confirmed from 1H-NMR studies of all compounds in CHCl3, vs THF-d8 (fig. 3-5). Further studies not finished due to time constraint and their dissemination are under progress.
While carrying out the main objectives of Phthalosupra, we decided to explore a novel research line, that is the preparation of CPA dimers, where both CPA units were linked by either one (1TB), two (2TB) or four (4TB) triple bonds. These CPA dimers were expected to form highly conjugated systems due to the acetylenic spacer(s) connecting the two units (Fig. 8-10). A combination of photophysical studies at variable temperature (i.e. UV-Vis absorption, fluorescence, and Raman), and spectroscopic experiments (i.e. 1H-NMR) of these systems in MCH suggesting a transition from a polyyne configuration to a cumulenic one in MCH upon decreasing the temperature (Fig. 11-13). Interestingly, this “cumulenization” process is accompanied by the aggregation of the CPA dimers in MCH as observed by variable-temperature UV-vis spectroscopy. Analysis of the polymerization process showed that it occurs following a cooperative process.
During the synthesis of CPA-based dimeric compounds, an unusual trimeric structure was isolated while trying to synthesize 1TB. The Sonogashira cross-coupling reaction between 15 and 16 expected to form 1TB via insitu deprotection of 16, shows single spot reaction. However, the mass spectroscopy studies show the presence of three alkoxyl group suggesting a trimeric product, while proton NMR was highly aggregated in CHCl3. In order to optimize, several reactions and different conditions were tested, and a careful study is still under progress to determine its exact structure. The excellent property of trimeric product is evident as its 1H NMR shows a high degree of aggregation due to π‒π stacking of CPA units. Another milestone was achieved through the synthesis of 4TB derivative, due to the excellent photovoltaic properties of 1TB and 2TB. The synthesis of 4TB was extremely challenging and was successfully achieved even within the time constraint of MSCA duration. The new derivative 4TB was expected to show an unprecedented extended form of cumulene, like 1TB and 2TB, an array of 9 double bonds, which has been not reported so far and will be a new milestone for a cumulene chemistry. We also tried to functionalize the A4 isomer of Pc, via reaction at one of the nitrogen atoms of Pcs, a totally new chemistry in the field of phthalocyanines chemistry. These ground-breaking results will soon be disseminated to the scientific community through publication in high-level international science. These studies a greater impact on the phthalocyanines chemistry, its functionalization, photovoltaic properties and a deeper understanding of sonogashira coupling reactions with terminal alkynes.
Raman Spectra and Reversible and temperature-induced ‘polyyne-to-cumulene’ transition for 1TB in MCH
X Ray Crystal srtucture and proposed self-assembly
Synthetic route for the preparation of CPA based dimers 1TB, 2TB, and 4TB.
Synthesis of compounds 3 and 4.
1H-NMR spectrum of 1 in CDCl3 and THF-d8
1H-NMR spectrum in THF-d8 and MALDI-TOF mass spectrum (in DCTB matrix) of chiral Pc 3
UV-Vis absorption spectra of 1 and 6a in different solvents
Variable temperature UV-Vis spectra in MCH of the CPA dimer 1TB
1H-NMR spectrum in CDCl3 and MALDI-TOF mass spectrum (in DCTB matrix) of 4TB.
1H-NMR spectrum in CDCl3 and MALDI-TOF mass spectrum (in DCTB matrix) of 1TB.
1H-NMR spectrum (CDCl3) and MALDI-TOF mass spectrum (in DCTB matrix) of 2TB.
1H-NMR spectra of 2 in a) CDCl3 and b) THF-d8.
Molecular structures of CPA derivatives and its reduction chemistry
Molecular structure of isophthalic acid-functionalized derivatives 1-4.
Synthesis of compounds 1 and 2.