Research lines, achievements and scientific discoveries in technologically-relevant research areas
1.Phthalocyanines and related porphyrinoids: Chemistry and physical properties
A part of Prof. Torres research is focused on the preparation of molecular materials based on phthalocyanines (Pcs) that could combine the singular physical properties of these macrocycles (i.e., conducting, optical and magnetic) with features such as great synthetic versatility, nanoscopic size, processability, biocompatibility, etc. Pcs are thermally and chemically stable compounds which present an intense absorption in the red/near-infrared (IR) region of the solar spectrum, high fluorescence quantum yields, and a reach redox chemistry. The Torres group has realized a significant contribution not only in the preparation of these class of macrocycles, using organic chemistry reactions for the post-functionalization of Pcs, but also in the study of their physicochemical properties which have prompted the use of these compounds in technologically-relevant areas such as molecular photovoltaics.
Worth to mention are some reports exploring the non-linear optical properties of Pcs or the preparation of appropriately substituted subphthalocyanines (SubPcs) able to generate second harmonic with better values than those reported till then [J. Am. Chem. Soc. 1996, 118, 2746; J. Am. Chem. Soc. 1998, 120, 12808; Chem. Rev. 2004, 104, 3723].
Several other related porphyrinoids like porphyrazines, corroles, triazolophthalocyanines [J. Org. Chem., 2002, 67, 1392] - novel compounds developed in the Torres group -, subporphyrins, subporphyrazines [Chem. Eur. J. 2005, 11, 354] - prepared for the first time by our group - and hemiporphyrazines [J. Am. Chem. Soc. 2010, 132, 12991; J. Am. Chem. Soc. 2017, 139, 14129], among others, have been prepared and their properties studied.
2.Artificial photosynthesis and photoinduced electron transfer. Multicomponent systems based on phthalocyanines and carbon nanostructures (fullerenes, nanotubes, and graphenes)
During the last years, Prof. Torres group studies focused on the preparation and study of donor-acceptor, covalent and supramolecular multicomponent ensembles based on Pcs, SubPcs and related derivatives. As counterpartner of the Pcs, many photo- and electroactive subunits have been studied [J. Am. Chem. Soc. 2006, 128, 15145; Angew. Chem. Int. Ed. 2016, 55, 5560; J. Am. Chem. Soc. 2016, 138, 12963].
Thus, the Torres group have carried out a significant amount of research in the preparation and study of multicomponent systems [Chem. Rev. 2010, 110, 6768; Adv. Energy Mater. 2017, 1601700] in which Pcs, SubPcs and other porphyrinoids have been connected, covalently or through supramolecular interactions, to carbon nanostructures. In these ensembles, the macrocycle has two main roles: first of all, it acts as an antenna as it absorbs light in very efficient way; and, once photoexcited, it behaves as an electron donor in combination, for example, with electron-accepting species such as C60 fullerene, one of the most used excellent electron accepting material [Angew. Chem. Int. Ed. 2008, 47, 2026; ibíd. 2016, 55, 11020], or endohedral fullerenes [Chem. Commun. 2015, 51, 330; J. Am. Chem. Soc. 2015, 137, 12914]. Similarly to C60 fullerene, carbon nanotubes (CNTs) also present a high affinity for electrons which, once accepted, can be transported along their 1-D tubular structure. In this context, donor-acceptor conjugates based on Pcs and CNTs have also been realized within the group [J. Am. Chem. Soc. 2007, 129, 5061, Adv. Energy Mater. 2017, 1601700]. More recently, Pc-based ensembles containing graphene, one of the latest entries in the family of carbon nanostructures and one of the “rising stars” in the field of nanotechnology, have also been prepared [Angew. Chem. Int. Ed. 2012, 51, 6421; J. Am. Chem. Soc. 2014, 136, 4593; Chem. Soc. Rev. 2017, 46, 4464].
3. Nonplanar aromatic systems: Subphthalocyanines and related compounds
An important research area within the Torres group is the development of the chemistry of SubPcs [Angew. Chem. Int. Ed. 2011, 50, 3506; ibíd. 2012, 51, 11337; ibíd. 2015, 54, 2543, J. Am. Chem. Soc., 2017, 139, 5520], an area in which the group ranks first internationally. SubPcs possess a macrocyclic ring formed by three isoindole subunits that are bound together by three aza bridges, and complexing a boron(III) atom within the cavity. This unique arrangement gives rise to an aromatic 14 π-electron heteroannulene circuit with interesting optoelectronic features which may be fine-tuned by varying the nature of the axial ligand or by functionalizing the peripheral positions. Moreover, the SubPc core is robust enough for sustaining chemical modifications of its peripheral substituents. The interest in SubPcs is manifold from the purely synthetic point of view to the applied physical properties. The Torres group has also made an important contribution in the preparation of fused SubPc oligomers [Angew. Chem. Int. Ed. 2002, 41, 2561; Chem. Commun. 2005, 2113]. These structures provide a unique opportunity to explore curved extended π-surfaces with different complex topologies apart from the standard concave geometry. For a summaries of the main achievements on the SubPcs space, see: Chem. Rev. 2014, 114, 2192. Very recently, we have reported on tuning and controlling intramolecular Föster resonance energy transfer using SubPcs as panchromatic absorbers for singlet fission [Angew. Chem. Int. Ed., 2018, DOI: 10.1002/anie.201808652].
4. Molecular photovoltaics: Organic, hybrid and perovskite sensitized solar cells
Since the middle of the previous decade, the Torres group has broadened the use of Pcs with their incorporation as active components into solar cells, achieving important breakthroughs in the applications of this family of compounds in organic solar cells. Within the area of the hybrid solar cells (Grätzel cells), significant advances are represented by i) the discovery of the phenomenon of catalysis of recombination that takes place on the Pc surface during the electronic injection process [J. Am. Chem. Soc. 2008, 130, 2906], ii) the preparation of Pcs with the maximum efficiencies described till then [Angew. Chem. Int. Ed. 2007, 46, 8358; ibíd. 2012, 51, 4375], iii) the use of energy relay dyes based on Pcs to increase light harvesting [Nat. Photonics, 2009, 3, 406; Nano Lett. 2010, 10, 3077], and iv) the development of inverted solar cells [Angew. Chem. Int. Ed. 2015, 54, 7688]. Currently, he is one of the world’s leading experts in the application of Pcs to molecular photovoltaics. During the last decade, the Torres lab has produced a series of compounds (the so-called TT´s) especially tailored for their integration into hybrid solar cells. Particularly, compound TT1, first reported in 2007, represents a breakthrough in the performance of this kind of devices. Significant progress has also been made in the use of SubPcs as acceptors instead of fullerenes, in small molecule solar cells for all-organic highly efficient evaporated bilayer devices [Adv. Energy Mater. 2011, 1, 565; ibíd. 2014, 4, 1301413; J. Am. Chem. Soc. 2015, 137, 8991]. These findings have been protected as an international patent [PCT/EP2012/057684] that has been transferred to IMEC (Belgium). Torres group has also reported the use of SubPcs as acceptors in bulk heterojunction solar cells (BHJSC) with the highest values obtained so far [Angew. Chem. Int. Ed. 2017, 56, 148]. More recently, the Torres group has described the use of Pcs as hole transporting materials (HTM) in perovskites sensitized solar cells with a record efficiency of 17.5% [Adv. Energy Mater. 2017, 1601733].
5. Porphyrinoids for nanoscience and nanotechnology
A more recent research topic within the Torres group is the use of porphyrinoids in new nanotechnological spaces, a process triggered by the integration of Prof. Torres and some of his group members in the IMDEA Nanoscience Institute. For example, in SubPcs functionalized with strong dipolar side groups it has been demonstrated that supramolecular organization provides long-range polar order that supports collective ferroelectric behavior of the side groups as well as charge transport through the stacked semiconducting cores. It was found that the ferroelectric polarization in these supramolecular polymers couple to the charge transport leading to a bulk conductivity that is both switchable and rectifying [Sci. Adv. 2017, 3, e1701017]. In another example, it has been demonstrated that an expanded porphyrinoid is able to organize on Au(111) through a unique growth mechanism based on long-range orientational self-assembly. Furthermore, a spatially controlled “writing” protocol on such self-assembled architecture has been achieved based on the STM tip-induced deprotonation of the inner protons of individual macrocycles. Finally, the capability of these surface-confined macrocycles to host lanthanide elements is assessed, introducing a novel off-centered coordination motif [J. Am. Chem. Soc. 2017, 139, 14129].
6. Biomedicine: Photodynamic therapy of cancer and atherosclerosis and bacteria photoinactivation
Recently, the Torres group has started a new research line in the area of photodynamic therapy (PDT) aiming at investigating the use of Pcs as photosensitizers for singlet oxygen generation as part of a research work financed by two European projects, namely CosmoPhos and SO2S. Moreover, the group is benefiting from a major EU-funded project entitled "Noνel nanotechnology-enabled system for endoνascular near-infrared targeted photodynamic therapy of atherosclerοtic heart disease'', in which the Torres group is playing a crucial role in the preparation of photosensitizers linked to nanoparticles. The results obtained so far using Pc-dendrimer conjugates for the treatment of atherosclerosis in animals are extremely promising and the next step is the clinical phase I. Two international patents have been issued (PCT/EP 16168476.6, 2016 and PCT/ EP16177001.1, 2016), and other two will be issued soon.
Significant progress has also been made in the encapsulation of Pcs in viral capsid-nanoparticles [Nano Lett. 2015, 15, 1245], their organization with protein cages in photoactive crystals [ACS Nano 2016, 10, 1565], and the use of SubPcs in PDT of cancer [Adv. Funct. Mater. 2018, 28, 1705938]. Very recently the group has published a comprehensive review on the topic of Porphyrinoid biohybrid materials as an emerging toolbox for biomedical light management [Chem. Soc. Rev., 2018, DOI:10.1039/C7CS00554G].