Dirk Michael Guldi’s research while affiliated with Friedrich-Alexander-University Erlangen-Nürnberg and other places

Bottom‐up syntheses of carbon nanodots (CND) using solvothermal treatment of citric acid are known to afford nanometer‐sized, amorphous polycitric acid‐based materials. The addition of suitable co‐reactants in the form of in‐situ synthesized N‐hetero‐π‐conjugated chromophores facilitates hereby the overall functionalization. Our incentive was to design a CND model that features phenazine (P‐CND) – a well‐known N‐hetero‐π‐conjugated chromophore – to investigate the influence of the CND matrix on its redox chemistry as well as photochemistry. The scope of our work was to go beyond investigating the electrochemical properties of the resulting P‐CND by shedding light onto differences relative to nano‐aggregates of phenazine (PNZNA), which served as reference. In particular, chemical as well as electrochemical reduction of PNZNA initiated a reaction cascade that affords the primary reduction intermediate, that is, the reduced and protonated (PNZ‐H)•. In accordance with literature, the final product of a bimolecular disproportionation was 5,10‐dihydrophenazine (PNZ‐H2). Reducing P‐CND also resulted in the formation of (PNZ‐H)•. But, no evidence for a subsequent bimolecular disproportionation was gathered. Instead, (PNZ‐H)• as an integrative part of P‐CND was found to be actively involved in a H2 generation reaction. A more than twofold increase in efficiency compared to PNZNA under identical conditions was the consequence.

Addressing the dual challenge of meeting the world's escalating energy needs while simultaneously decreasing carbon dioxide emissions has emerged as a pivotal issue in the new millennium. The Statistical Review of World Energy's latest report by British Petroleum Company plc reveals that global power consumption was around 18.9 terawatts (TW) in 2021, with projections estimating a surge to 30 TW by 2050. In stark contrast, solar radiation delivers a 120,000 TW to the Earth's surface, a quantity that exceeds the projected energy consumption of 2050 by 4,000 times. Efficiently harnessing and storing this immense solar energy is increasingly seen as a viable route towards a sustainable energy network. A promising solution lies in carbon-based systems. Carbon dots (CDs), an emerging class of nanomaterials, show great potential in this field. These can be synthesized through scalable methods using various carbon and nitrogen precursors, including waste, making them both cost-effective and recyclable. CDs offer a range of photophysical properties, such as variable photoluminescence and the generation of spin-active states. Furthermore, CDs are biocompatible, non-toxic, and environmentally friendly. Unlike traditional binary semiconductors, their photocatalytic attributes can be finely adjusted using a variety of organic chemistry functionalization techniques. While CDs have primarily been optimized for PL properties, their application in metal-free photocatalysis remains largely untapped. Major obstacles are the large structural diversity and complex multicomponent arrangement of CDs, along with the high variability of their photoactive centers, presenting a unique challenge. The ongoing controversy over the exact CDs structure, further compounded by a limited understanding of how this structure influences their photophysical properties, significantly hinders advancements in CDs design. This challenge significantly hampers progress in the development of more effective CDs for solar-driven water-splitting, underscoring the urgent need for increased collaboration between experimentalists and theoreticians now more than ever. In this contribution, we delve into the current understanding of photocatalytically active CD-based systems. The focus is set on model systems that feature no co-catalysts. We explore the role of CDs, shed light on the structure-activity relationship and detail photon- and charge-management in the excited state. Additionally, we share our insights on the potential opportunities that emerge from a deeper comprehension of CDs.

Bottom-up strategies have allowed the synthesis of “molecular” nanographenes with full control over size, shape and functionality. In recent years, the progress on wet chemical approaches, oxidative cyclodehydrogenation amongst all, has been the foundation to the synthesis of an impressive number of soluble and well-defined molecular nanographenes. The level of control over nanographene syntheses has allowed a fine-tuning of the photophysical and electrochemical properties and, in turn, has a compelling potential in the field of material science. In this regard, understanding and harnessing the competition between electron transfer and energy transfer in nanographenic systems is of utmost importance. However, a comprehensive structure-property relationship remains still an open aspect. In the present review we describe a large variety of hexa- peri -hexabenzocoronene (HBC)-based nanographenes obtained through wet chemical strategies and linked – either covalently or non-covalently – to porphyrins, rylenes, fullerenes, etc. Particular attention was placed on the optical, electrochemical and excited-state properties.

The goal of harnessing the theoretical potential of singlet fission (SF), a process in which one singlet excited state is split into two triplet excited states, has become a central challenge in solar energy research. Covalently-linked dimers provide crucial models for understanding the role of chromophore arrangement and coupling in SF. Sensitizers can be integrated into these systems to expand the absorption bandwidth through which singlet fission can be accessed. Here, we define the role of the sensitizer-chromophore geometry in sensitized singlet fission model systems. To this end, several conjugates have been synthesized consisting of a pentacene dimer (SF motif) connected via a rigid alkynyl bridge to subphthalocyanines (the sensitizer motif). Steady-state and time-resolved photophysical measurements are used to confirm that both conjugates operate as per design, displaying near unity energy transfer efficiencies and high triplet quantum yields from SF.

Generations of triplet‐excited, charge transfer (CT), and fluorescent states are appealing for optoelectronic applications. In this work, we prepared a series of electron donor‐acceptor systems based on spiro[fluorene‐9,7’‐dibenzo[c,h]acridine]‐5’‐one (SFDBAO). Our SFDBAOs consist of orthogonally positioned fluorenes and aromatic ketones. By fine‐tuning the substitution of electron‐donating pyrenes, the complex interplay among different excited‐state decay channels and the overall impact of solvents on these decay channels were uncovered. Placing pyrene, for example, at the aromatic ketones resulted in profound solvatochromism in the form of a bright CT emission spanning from yellow to red‐NIR upon going more polar. In contrast, a dark non‐emissive CT was noted upon pyrene substitution at the fluorenes. In apolar solvents, efficient triplet‐excited state generation was observed for all SFDBAOs with singlet oxygen quantum yields (ΦΔ) of around 70% albeit different mechanisms were active. Either charge transfer was concluded to mediate the intersystem crossing (ISC) in the case of pyrene substitution or El‐Sayed rule was applicable when lacking pyrene substitution as in the case of SFABAO. In polar solvents, charge separation is the sole decay upon pyrene substitution. Moreover, competition between ISC and CT lowered the triplet‐excited state generation in SFDBAO.

Triplet dynamics in singlet fission depend strongly on the strength of the electronic coupling. Covalent systems in solution offer precise control over such couplings. Nonetheless, efficient free triplet generation remains elusive in most systems, as the intermediate triplet pair 1(T1T1) is prone to triplet‐triplet annihilation due to its spatial confinement. In the solid state, entropically driven triplet diffusion assists in the spatial separation of triplets, resulting in higher yields of free triplets. Control over electronic coupling in the solid state is, however, challenging given its sensitivity to molecular packing. We have thus developed a hexameric system (HexPnc) to enable solid‐state‐like triplet diffusion at the molecular scale. This system is realized by covalently tethering three pentacene dimers to a central subphthalocyanine scaffold. Transient absorption spectroscopy, complemented by theoretical structural optimizations and steady‐state spectroscopy, reveals that triplet diffusion is indeed facilitated due to intramolecular aggregation. The yield of free triplets in HexPnc is increased by a factor of up to 14 compared to the corresponding dimeric reference (DiPnc). Thus, HexPnc establishes crucial design aspects for achieving efficient triplet dissociation in strongly coupled systems by providing avenues for diffusive separation of 1(T1T1), while, concomitantly, retaining strong interchromophore coupling without compromising rapid 1(T1T1) formation.

Subphthalocyanines (SubPcs) [1] are well-known cone-shaped chromophores consisting of three 1,3-diiminoisoindole units assembled around a boron atom. As a result of their 14 pi-electron aromatic core and their tetrahedral geometry, SubPcs exhibit outstanding physical and optoelectronic properties (e.g., strong dipole moment, excellent light absorption in the 550-650 nm, rich redox features, and excellent charge transport capabilities), that have been skilfully used in variety of applied fields, such as molecular photovoltaics, among others. SubPcs were used by us as non-fullerene acceptors in bulk heterojunctions (BHJ) solar cells. On the other hand as part of our systematic investigation in the preparation and study of novel SubPc-based D–A systems, we have used 1,1,4,4-tetracyanobuta-1,3-diene (TCBD) as partner for SubPcs. Moreover, in the case of unsymmetrically substituted SubPcs (i.e., prepared by cyclotrimerization of a phthalonitrile with no C2v symmetry), they present inherent chirality and the corresponding couple of enantiomers can be isolated [2]. Columnar aggregates based on chiral SubPcs have been also prepared, giving rise to ferroelectric self-assembled molecular materials showing both rectifying and switchable conductivity. These chromophores have been incorporated in multicomponent systems showing a panchromatic response and allowing the tuning and controlling intramolecular FÖRSTER Resonance Energy Transfer for Singlet Fission. [1] G. Lavarda, J. Labella, M. V. Martinez-Diaz, M. S. Rodriguez Morgade, A. Osuka, T. Torres, “Recent advances in subphthalocyanines and related subporphyrinoids” Chem. Soc. Rev . 2022 , 51 , 9482-9619. [2] J. Labella, G. Lavarda, L. Hernández-López, F.Aguilar-Galindo, S.Díaz-Tendero, J. Lobo-Checa, T. Torres “Preparation, Supramolecular Organization, and On-Surface Reactivity of Enantiopure Subphthalocyanines: From Bulk to 2D-Polymerization” J. Am. Chem. Soc . 2022 , 144 , 16579-16587.

Photon energy conversion can be accomplished in two opposing manners, down conversion (i.e., singlet fission, SF), and up-conversion (often referred to as triplet-triplet annihilation up-conversion, TTA-UC), both of which have the potential to help overcome the Shockley-Queisser limit of single-junction solar cells. Inter-chromophore electronic coupling plays opposing roles in these two processes. The energies of the lowest singlet excited state and twice the triplet excited state are comparable in tetracene dimers, which exhibit both up- and down-conversion. Here, we have designed meta -phenylene- and 1,3-diethynyladamantyl-linked tetracene dimers with different electronic coupling to probe the interplay between intramolecular SF (intra-SF) and intramolecular TTA-UC (intra-TTA-UC) viasteady-state and time-resolved absorption and fluorescence spectroscopy. In addition, temperature-dependent measurements are carried out to shed light on the thermal effects on intra-SF and intra-TTA-UC. We have also used a Pd-phthalocyanine as photosensitizer to enable intra-TTA-UC in the two dimers via indirect photo-excitation in the near-infrared. These results not only reveal the interplay between intra-SF and intra-TTA-UC, but also provide guidelines and strategies for the design of photon down- and up-converting materials.

Graphene has captured the imagination of researchers around the world due to its groundbreaking chemical and physical properties. Opening a band gap in graphene must be achieved without, however, compromising its exceptional properties as they are of paramount importance for its use in electronic devices. Notable is the fact that the band gap design in graphene is typically carried out by either chemical or physical methodologies. Chemical modification of graphene is mostly centered around “top-down” or “bottom-up” approaches. The earlier alters, nevertheless, the graphene lattice and, as a consequence, poorly defined structures emerge. The latter by means of, for example, organic synthesis offers a wide palette of tools to control sizes as well as geometries of the resulting “molecular” nanographenes with atomic precision. It allows the fabrication of uniform and well-defined molecular structures. Such “molecular” nanographenes are compelling choices for “on demand” molecular electronics, photovoltaic applications, hydrogen storage, and sensing. In recent years, two main strategies have been developed to fabricate “molecular” nanographenes of defined chemical structures. It is, on one hand, oxidative cyclodehydrogenation of custom-made polycyclic aromatic hydrocarbons (PAHs) and, on the other hand, on-surface cyclodehydrogenation, which enabled the preparation of atomically precise “molecular” nanographenes. To this end, the 13 fused-benzene rings of hexa- peri -hexabenzocoronene (HBC), which are arranged in a 2D disk-shaped fashion, render HBCs the smallest “molecular” nanographenes.

As result of their synthetic versatility and unique physical features, porphyrinoids have been widely utilized as building blocks for the preparation of molecular and supramolecular electron donor-acceptor (D-A) ensembles in which photoinduced energy/electron transfer events can be triggered/modulated as a function of the electronic character of the macrocycles’ counterpart(s). In this talk, after a general introduction about porphyrinoids and their use in optoelectronic applications, I will present some examples from our group on the preparation and study of phthalocyanine-,[1] subphthalocyanine-,[2] and subporphyrin-[3] based D-A systems in which the porphyrinoid has been connected to tetracyanobuta-1,3-diene, a molecular fragment with unique structural and electronic features (Figure 1a-c). References Sekita, M.; Ballesteros, B.; Diederich, F.; Guldi, D. M.; Bottari, G.; Torres, T. Angew. Chem. Int. Ed. 2016 , 55 , 5560. a) Winterfeld, K. A.; Lavarda, G.; Guilleme, J.; Sekita, M.; Guldi, D. M.; Torres, T.; Bottari, G. J. Am. Chem. Soc. 2017 , 139 , 5520; b) Winterfeld, K. A.; Lavarda, G.; Guilleme, J.; Guldi, D. M.; Torres, T.; Bottari, G. Chem. Sci. , 2019 , 10 , 10997; c) Lavarda, G.; Bhattacharjee, N.; Brancato, G.; Torres, T.; Bottari, G. Angew. Chem. Int. Ed. , 2020 , 59 , 21224; d) Esteso, V.; Caliò, L.; Espinós, H.; Lavarda, G.; Torres, T.; Feist, J.; García-Vidal, F. J.; Bottari, G.; Míguez, H. Solar RRL , 2021 , 5 , 2100308. Winterfeld, K. A.; Lavarda, G.; Yoshida, K.; Bayerlein, M. J.; Kise, K.; Tanaka, T.; Osuka, A.; Guldi, D. M.; Torres, T.; Bottari, G. J. Am. Chem. Soc. 2020 , 142 , 7920. Figure 1