Program 2025

The symposium will take place at lecture hall 6G in builing 26.41 at Heinrich Heine University Campus.

Please check in at the foyer right before lecture hall 6G. 

For directions to the venue please have a look on our website: https://www.modisc.hhu.de/modisc-retreats-and-symposia/symposium-2025 

The organizing doctoral researchers of ModISC, Jennifer Kremper, Jasper Guhl and Jonathan Sanchez Gonzalez, will welcome you to the symposium. 

… will be introduced by the speaker of the consortium, Prof. Dr. Thomas J. J. Müller, Institute of Organic Chemistry, Heinrich Heine University Düsseldorf. 

Femtosecond NIR Spectroscopy: Accessing S₁→S_n≥2, T₁→T_n≥2 Transitions

Dr. Bárbara Elza Nogueira de Faria, Wiebke. Haselbach, Jasmin Matthes, Simon Zimmermann, Peter Gilch,
Institute of Physical Chemistry, Femtosecond Spectroscopy, Heinrich Heine University Düsseldorf

Abstract:

To improve the performance of the emitters in OLEDs applications, such as in displays, it is essential to harvest singlet and triplet excitons. In OLEDs, excitons are generated via electron-hole recombination, and due to spin statistics 75% have triplet and 25% singlet multiplicity. Emitters based on thermally activated delayed fluorescence (TADF) [1] can achieve 100% of internal quantum efficiency via reverse intersystem crossing (rISC). Efficient pure organic TADF emitters (see Figure 1a)) require small energy gap (ΔE_ST ~ 25 meV) and as well as higher states (S_n≥2 and T_n≥2) that via spin-vibronic mixing enable ISC and rISC. [2] These states can be located energetically by time resolved NIR spectroscopy. [2,3] Copper(I) complexes are also investigated intensively with regard to OLED applications. [4,5] In these compounds (see Figure 1b)) and transition metal complexes in general, a variety of excited states (ligand-centered, metal-to-ligand charge transfer and so on) may be encountered. Assigning spectroscopic signatures to these states can be very challenging. A broader spectral coverage can be very advantageous.
Thus, within ModISC we extended our instrumentation for time resolved spectroscopy towards NIR probing (see Figure 1c)). The focus of this talk, will be on the construction of femtosecond NIR set-up and its applications in organic TADF emitters and copper(I) complexes.

Literature:
[1] M. Y. Wong, E. Zysman-Colman, Adv. Mater. 2017, 29(22), 1605444.
[2] W. Haselbach, J.M. Kaminski, L.N. Kloeters, T.J.J. Müller, O. Weingart, C.M Marian, P. Gilch, B.E.N. de Faria, Chem. Eur. J. 2023, 29(2), e202202809.
[3] C.C. Wu, Y.X. Tsai, L.K. Chu, I.C. Chen, J. Phys. Chem. Lett. 2024, 15(4), 912-918.
[4] F. Dumur, Org. Electron. 2015, 21, 27-39.
[5] O. Nolden, J. Kremper, W. Haselbach, M. Morshedi, J. Guhl, P. Schmeinck, C.M. Marian, C. Ganter, P.Gilch, ChemPhotoChem 2023, 7(4), e202200231.

Photophysical Dynamics of TADF via Multimodal Fluorescence Spectroscopy 

Mina Chalani¹, Monika Flörke², Jennifer Kremper^1,3, Suren Felekyan¹, Ralf Kühnemuth¹, Thomas J. J. Müller², Jan Meisner¹, Claus A. M. Seidel¹

¹Institute of Physical chemistry, Heinrich-Heine-University Düsseldorf, Germany,

²Institute of Organic Chemistry, Heinrich-Heine-University Düsseldorf, Germany,

³Institute of Theoretical and Computational Chemistry, Heinrich Heine University Düsseldorf, Germany

Abstract: 

Thermally activated delayed fluorescence (TADF) via reverse intersystem crossing (rISC) is a promising strategy to boost light emission efficiency in next-generation organic optoelectronic devices, including OLEDs and sensors. In this work, we present a systematic study of a series of triarylamine-dicyanobenzene donor–acceptor (D–A) chromophores focusing on how molecular design specifically steric hindrance modulates their photophysical properties and TADF performance. Here, the donor–acceptor (DA) moieties are connected by a single bond, allowing rotation that influences the electronic state energies and their coupling. To control the dihedral angle and thereby minimize the singlet–triplet energy gap (ΔE_ST), we introduced sterically bulky substituents at the donor–acceptor linkage. We have developed a multimodal approach with steady-state absorption/emission spectroscopy, time-correlated single-photon counting (TCSPC) as a function of temperature to resolve the complete photochemical kinetic scheme. In this way, we could determine the number of relevant triplet states and energies together with the specific rate constants for intersystem crossing (ISC), reverse intersystem crossing (RISC), internal conversion (IC), and radiative processes. Our findings reveal that increased torsion, induced by steric design, significantly increases reverse intersystem crossing rate. By directly linking structural modification to functional performance, this study highlights a practical design strategy for optimizing TADF emitters. 

Luminescent Crystals - MR-TADF Emitter DiKTa inside of Metal-organic Framework MOF-5

Marcus N. A. Fetzer,¹ Annette Vollrath,¹ Benedikt Bendel,² Ralf Kühnemuth,³ Oliver Weingart,⁴ Markus Suta,² and Christoph Janiak¹

¹Inorganic and Structural Chemistry, Heinrich Heine University Düsseldorf, Germany; ²Inorganic Photoactive Materials, Heinrich Heine University Düsseldorf, Germany; ³Molecular Physical Chemistry, Heinrich Heine University Düsseldorf, Germany; ⁴Institute for Theoretical and Computational Chemistry, Heinrich Heine University Düsseldorf, Germany

Here we report the first attempt to synthesize a highly emissive microporous metal-organic framework (MOF) by entrapment of a multi-resonant thermally activated delayed fluorescence emitter called DiKTa (quinolino-[3,2,1-de]acridine-5,9-dione). This molecule is known for its narrowband emission, which makes it an interesting candidate for the application in organic light-emitting diodes (OLED’s). Due to its suitable pore size, we chose MOF-5 as the MOF, based on the dimensions of DiKTa molecules. The MOF syntheses as well as the entrapment were carried out by a solvothermal in situ synthesis. We focused the synthesis on the production of MOF-5 single crystals. Four DiKTa@MOF 5 composites with different DiKTa loading were successfully synthesized as single crystals. The photophysical and structural properties of these composites were comprehensively analyzed and compared with those of pure DiKTa in both solid-state and solution. The reported DiKTA@MOF-5 composites show good emissions depending on their loading and slightly broader FWHM values compared to DiKTa in solution. Lifetime measurements of the composites yielded results comparable to those observed for DiKTa in solution, with values varying depending on the concentration of DiKTa in the composite. The measured lifetimes included both prompt and delayed components, confirming the concept of a solid solution. Using fluorescence anisotropy measurements, we were able to determine the preferred orientation of individual DiKTa molecules in the MOF matrix. In addition to the decrease in the respective BET surface areas, the powder X-ray diffraction (PXRD) of each composite provided valuable insights into how different DiKTa loadings affected the structural integrity and stability of the MOF-5 matrix. This shows how dye and MOF affect each other.

Coffee break in foyer area of lecture halls 6G,H,J

Digital Chemistry at Merck: Synergies from Academia to tackle industry challenges

Jan Gerit Brandenburg, Director for Digital Chemistry, Merck

Abstract: 

Digital chemistry is impacting Merck at all scales within Healthcare, Lifescience, and Electronics. In this talk, we will explore how advanced computational methods are enhancing molecular design, predicting materials behaviors, and optimizing synthesis pathways. We'll also discuss our collaborative efforts with academia and across Merck’s diverse business units to drive innovation and solve complex industry challenges. Through selected case studies, we’ll showcase the impact of digital chemistry in scaling biotechnological solutions and outline our vision for the future of data-driven research and development.
 

F8BT based OLEDs emitting CPL

Annemarie Berger, Claudia Dillmann and Klaus Meerholz* 
Institute of Physical Chemistry, University of Cologne, Germany 
E-Mail: annemarie.berger@uni-koeln.de

Organic light-emitting diodes (OLEDs), that can directly emit circularly polarized light (CPL), represent a promising advancement in display technology. Conventional OLED displays rely on circular polarizers to suppress reflected ambient light, but these anti-glare filters also reduce the device efficiency by absorbing 50% of the emitted nonpolarized light. By using the appropriate handedness an enhanced brightness with reduced power consumption can be achieved. Among different methods to generate CPL, a macroscopically ordered chiral environment within the emitting layer seems to be crucial to obtain a high degree of CP emission, characterized by a large dissymmetry factor (g-factor).^[1,2]

In this study, we investigate the optical activity, CPL and electrical behavior of blends based on the achiral conjugated polymer F8BT and the chiral BINOL derivative R5011. We varied the molecular weight of F8BT, the blend ratio and the preparation method of the polymer film to evaluate their impacts on circularly polarized emission. The correlation between chiral confocal microscopy and topographical analyses by atomic force microscopy (AFM) of these polymer films provide deeper insight into the underlying mechanisms of supramolecular chirality formation in these systems. 

[1] C.F. Cheng et al., Org. Chem. Front. 2022, 9, 6441–6452.

[2] M. J. Fuchter et al., Nat. Photonics 2024, 18, 658–668.

Theoretical Insights into Spin State Interconversion in Carbazole-Containing TADF Emitters

Jonas Weiser, Institute of Physics, Computational Biology, Augsburg University

We present a comprehensive computational investigation of spin state interconversion mechanisms in carbazole-containing thermally activated delayed fluorescence (TADF) emitters, focusing on the role of spin-orbit coupling (SOC) in facilitating reverse intersystem crossing (RISC). We systematically analyzed three cyanoarene-based TADF emitters – 3CzClIPN, 4CzIPN, and 5CzBN – with different singlet-triplet energy gaps (ΔEST). Our computational approach identified points of contact between the excited state potential energy surfaces of these molecules, including minimum energy crossing points and minimum energy conical intersections, to uncover the spin state interconversion dynamics without requiring explicit excited-state molecular dynamics simulations.
We found that SOC-mediated RISC in these molecules can proceed via an intermediary higher-energy triplet state (T2) in addition to the more direct T1-to-S1 pathway. Consequently, RISC is not directly dependent on ΔEST, but instead on the activation energies of the individual channels.
These findings highlight the importance of considering higher triplet states in the design of efficient TADF materials and provide a robust framework for understanding spin dynamics in organic emitters. This work advances the theoretical understanding of spin state interconversion and offers insights for optimizing TADF-based OLEDs.

Hydrogen Bonding and Acridones: What a dream team!

Lennart Bunnemann, Institute of Organic Chemistry, Asymmetric Chemistry, Heinrich Heine University Düsseldorf

Like anthracene and other polyaromatic compounds with a carbonyl group, acridones possess the predisposition to be a blue emitter. However, it comes down to the energies of the excited states relative to each other to answer the question whether this luminescence is actually bright. Literature shows that the N-methylated acridone is dark in nonpolar solvents but emission is bright in polar solvents. Furthermore, it was shown by Mitsui and Ohshima that interaction of the carbonyl group of the acridone with water molecules in the gas phase changes the luminescence significantly.  The reason for this is the specific orbital interaction of polar – in particular protic – solvent molecules with the n-orbital at the carbonyl oxygen atom.
We present our findings that the luminescence characteristics of the acridones can be modulated by  inter-/intramolecular hydrogen bonding and/or by introducing suitable substituents at different positions of the heteroaromatic core. These changes allow for high fluorescence quantum yields regardless of the molecule´s environment as well as the possibility to employ triplet excitons for fluorescence emission (HIGHrISC/hot exciton).

The Poster Session will take place in Heinrich Heine Saal. Please follow the guides from lecture hall 6G to the room. 

The dinner will be served in the same room at ca. 18:30 p.m.

The list of all posters and poster abstracts are available below: Link to list of posters

The symposium will take place at lecture hall 6G in builing 26.41 at Heinrich Heine University Campus.

Please check in at the foyer right before lecture hall 6G. 

For directions to the venue please have a look on our website: https://www.modisc.hhu.de/modisc-retreats-and-symposia/symposium-2025 

How Multiple RISC Pathways Shape Efficiency and Stability in TADF OLEDs

Paloma Lays dos Santos, School of Electrical and Electronic Engineering, University of Sheffield, UK

Abstract: 

Thermally-activated delayed fluorescent (TADF) based OLEDs have already proven to be capable of achieving high external quantum efficiencies (EQE). However, a problem that these devices suffer and prevent their use in industrial-level applications is the decrease in EQE with the increase of current density, a phenomenon known as efficiency roll-off. This has been revealed to be just as (or even more) important than maximum EQE itself. Yet, understanding of emitter design rules that help suppressing this effect have not yet been thoroughly explored. In this talk, we will look into the detailed photophysical characterisation of a multi-donor-acceptor (D-A) family of TADF emitters to find correlations with device performance [1].
Increasing the number of closely packed Ds around the A core leads to changes in dihedral angles between Ds and A, affecting the HOMO/LUMO separation and impacting the singlet-triplet energy gaps. Moreover, D-A dihedral angles change molecular conjugation affecting the spread of charge-transfer state energies as well as the energy of D local triplet states. The coupling between these triplet states and the dispersion in CT states lead to the appearance of multiple rISC channels, a phenomenon that is host-dependant, i.e., hosts with different rigidities twist the dihedral angles differently. We show that different subsets of rISC rates directly impact device performance, where faster rISC leads to external quantum efficiencies above 20% while slower rISC rates act as parasitic traps, severely affecting device roll-off. This explains why emitters with excellent peak external quantum efficiencies can also present very poor roll-off.

Literature:

[1] Paloma dos Santos et al. Influence of Multiple rISC Channels on the Maximum Efficiency and Roll-Off of TADF OLEDs; The Journal of Physical Chemistry C, 16308-16319, 2024
 

Structural Modulation of Charge Transfer in Donor–Acceptor Systems - Effects of Donor Type and Connectivity

Christopher Wallerius,¹ Robert Herzhoff,¹ Soyoung Boo,¹ Andreas Mischok,¹ Dirk Hertel,¹ Daniele Fazzi,² Malte C. Gather ¹ and Klaus Meerholz*¹

¹ Institute of Light and Matter, University of Cologne, Greinstr. 4-6, 50939 Cologne, Germany

Email: c.wallerius(at)uni-koeln.de 

² Universita di Bologna, Dipartimento di Chimica “Giacomo Ciamician”, Via P. Gobetti 85, 40129 Bologna, Italy

Efforts in designing efficient luminescent materials for organic light-emitting diodes (OLEDs) and understanding their photophysical properties remain pivotal areas of research. Organic donor-acceptor molecules have demonstrated promising characteristics that render them particularly suitable as emissive components, given their ability to harvest triplet states with up to 100% efficiency via thermally activated delayed fluorescence (TADF).[1] 

This work studies the design, synthesis and characterization of four novel emitters, all featuring a N-phenyl phthalimide acceptor.[2] Two of these molecules were developed with direct linkage between the donor (either carbazole or spiro[acridine-9,9'-fluorene]) and the acceptor moiety (D-A), while the two analogs incorporate a bridging phenyl moiety between donor and acceptor (D-B-A).[3] Our main goal is the understanding of structure-photophysics relationships resulting from these variations.[4]

We investigated the photophysical properties of the four compounds using a range of spectroscopic techniques. Variation in donor moiety enabled tuning of the singlet–triplet energy gap (ΔE_ST)​, with the spiro-based systems exhibiting particularly small gaps (as low as 14 meV), supporting efficient TADF. In contrast, incorporation of a phenyl bridge modulated conjugation and spatial separation, leading to a hypsochromic shift of emission. These structural differences result in distinct emission colors ranging from blue to green and provide deep insights on structure–property correlations in DA emitters. 

[1] H. Nakanotani, K. Masui, J. Nishide, T. Shibata, C. Adachi, “Promising stability of high-efficiency organic light-emitting diodes based on thermally activated delayed fluorescence” Sci Rep 3, 2127 (2013). 

[2] Y. Qin, G. Li, T. Qi, H. Huang, “Aromatic imide/amide-based organic small-molecule emitters for organic light-emitting diodes” Mater. Chem. Front. 4, 1554-1568 (2020).

[3] T. Matulaitis, P. Imbrasas, N. A. Kukhta, P. Baronas, T. Bučiu̅nas, D. Banevičius, K. Kazlauskas, J. V. Gražulevičius, S. Juršėnas, “Impact of Donor Substitution Pattern on the TADF Properties in the Carbazolyl-Substituted Triazine Derivatives” J. Phys. Chem. C 121, 42, 23618-23625 (2017). 

[4] C. Wallerius, R. Herzhoff, S. Boo, A. Mischok, D. Hertel, D. Fazzi, M. C. Gather, K. Meerholz, Manuscript in preparation.

No ISC? Design of efficient NIR-luminescent complexes

Jasper Guhl^a, Paul Ruer^b, Philipp Ralle^b, Andreas Steffen^b and Christel M. Marian^a

^aInstitute of Theoretical and Computational Chemistry, Heinrich Heine University Düsseldorf

^bFaculty of Chemistry and Chemical Biology, Technical University of Dortmund

At the doorstep to the near infrared region (NIR), the energy gap law greets. ^[1] It states that the nonradiative decay rate k_nr increases exponentially with decreasing energy difference. Unfortunately, the radiative rate shrinks cubically with the energy gap. Additionally, non-adiabatic coupling and direct spin—orbit coupling (SOC) can affect k_nr. Efficient luminescence requires thus a) weak direct coupling to the ground state S₀, but b) sizeable transition dipole moments to achieve sufficient radiative rates.

A reliable prediction of NIR luminescent molecules requires a quantitative description of the electronic state landscape, for which we use the DFT/MRCI method. ^[2] It combines the description of dynamic correlation via density functional theory (DFT) and static correlation via the multireference configuration interaction (MRCI) component. SOC is described in SOCI, which treats spin-free and spin-dependant operators on the same footing. ^[3]

Figure 1: Different conformers of linear bis-carbene Pd-complexes.

Counterintuitively, metal to ligand charge transfer (MLCT) states of neutral d¹⁰-complexes may fulfil all listed requirements, provided that the underlying complex is designed well. Linear bis-carbene complexes can possess low direct coupling to S₀ and sizeable radiation rates, but at first sight, their linearity conflicts with low-lying carbene orbitals and NIR luminescent states.^[4] Careful construction of the employed carbenes can overcome the challenges and enable efficient IR-luminescence. Literature and own results support the underlying theoretical model for the design guide lines. ^[5,6] 

[1] R. Englman, J. Jortner, Mol Phys. 1970, 18, 145-164

[2] C. M. Marian, A. Heil, M. Kleinschmidt, WIRE: Comp. Mol. Sci. 2018, 9, 1-31

[3] M. Kleinschmidt, J. Tatchen, C. M. Marian, J. Chem. Phys.2006, 124, 124101

[4] L. P. Wolters, F. M. Bickelhaupt, ChemistryOpen 2013, 2, 106-114

[5] A. F. Henwood et al., Chem. Sci. 2015, 6, 3248-3261

[6] T. Tsubomura et al., Inorg. Chem. 2008, 47, 2, 481–486

Coffee break in foyer area of lecture halls 6G,H,J

Golden Age of Coinage Metal Emitters and Host Materials for Energy Efficient and Stable OLEDs

Alexander S. Romanov, Department of Chemistry, University of Manchester, UK

Abstract:

Two-coordinate coinage metal complexes with linear geometry L–Metal–X (L = carbene and X = anionic ligand, Metal = copper, silver or gold) have recently emerged as a new class of strongly photoemissive materials – Carbene Metal Amides (CMA) [1]. We will discuss the advances in CMA molecular design to cover the full visible spectrum, UV-A (down to 380 nm) and near-IR regions (up to 850 nm). Advanced deep-blue CMA emitters will be demonstrated to have radiative rates approaching 10⁷ s^-1 [2]. UV-A, deep-blue, red and near-IR CMA OLEDs with significantly improved device lifetimes (LT50 from several minutes to hours) will be presented to demonstrate the promising molecular design concepts and applied potential of the new CMA materials. New classes of the Carbene-Metal-based emitters with acetylides, carboranyls and aryls will be presented [3–6]. Comprehensive comparison across various classes of emitters will be based on the bond dissociation energies of the Metal–Ligand bonds to disclose the molecular designs leading to the best photophysical and OLED performance. Various complexes will be demonstrated to show fluorescence, phosphorescence, TADF, two-photon triggered luminescence and energy transfer OLEDs [3–6]. We will discuss our very recent development of the advanced NON-host materials showing high stability, wide energy gap, and high triplet energy level over 3.3 eV. NON-host materials unlock a true potential of the above mentioned luminophores by enabling highly energy-efficient OLED devices with EQE over 21% while producing a blue-shifted and narrow (60 nm) electroluminescence profile [7]. 
Acknowledgment: for the Royal Society (grant nos. URF\R1\180288, RGF\EA\181008, URF\R\231014).

Literature:

(1) Di, D.; Romanov, A. S.; et al. Science. 2017, 356, 159
(2) Brannan, A.C. Romanov A.S. et al., Adv. Mater. 2024, 2404357
(3) Brannan, A.C. Romanov A.S. et al., Adv. Mater. 2024, 2306249
(4) Brannan, A.C. Romanov A.S. et al., J. Mater. Chem C, 2024, 12, 13545
(5) Powley, S.; Romanov, A.S., Chem. Comm., 2023, 59, 12035.
(6) Gu. Q, Romanov A.S., et al., Adv. Mater. 2024, 2402790
(7) Romanov, A.S.2024, Comm.Chem. 2024, 7, article number: 298.

A universal non-radiative decay mechanism for [M(ppy)]-derived phosphors

Leon Geerkens and Cristian A. Strassert
Institute for Inorganic and Analytical Chemistry, University of Münster, Germany
Email: l.g(at)uni-muenster.de 

The radiative decay and the accompanying rate constant has been studied in great detail for many heavy d-block metal-based phosphorescent dyes. Its dark counterparts, the non-radiative pathways, often involve intersystem crossing from the valence triplet state manifold into the singlet ground state. The performance of an emitter is defined by the balance between the rates of all of these pathways, and unravelling the non-radiative decay mechanisms is crucial to the design of new phosphors.

The ligand field splitting (LFS) energy introduced by d-block metal ions commonly used in phosphorescent dyes like Ir(III) and Pt(II), but also lighter metals like Pd(II) is too high to allow for thermal population of pure metal-centered states at rt. For phenylpyridine (ppy) based ligands, experimental and theoretical results hint towards a ligand-centered mechanism comprised of two elemental steps: structural (nuclear) reorganization and electronic spin flip, i.e., intersystem crossing from T₁ to S₀. Even large modifications on the complex such as an exchange of the metal center, the coordination geometry, the coligands, or the introduction of substituents and coordinative extensions, seemingly have little effect on the nuclear distortion pathway, which is highly localized within the ppy moiety. 

We were able to derive the LC-character of these processes using temperature dependent (6 K to rt) time-resolved photoluminescence spectroscopy on complexes with varying LFS. Supported by quantum chemical calculations, we propose a mechanism for non-radiative decay. We further designed and tested a set of new complexes aiming to manipulate the non-radiative rate constant selectively, ideally keeping the radiative process unchanged. 
 

The lunch break and Poster Session will take place in Heinrich Heine Saal. Please follow the guides from lecture hall 6G to the room. 

The list of all posters is available here after the registration deadline has passed: Link to list of posters

From Twist to Triplet: Red-Light Photoswitching a π-extended Thioindigo-Dye

C. Müller, Erlangen/GER, M. Hartinger, Erlangen/GER, M. Herm, Erlangen/GER, H. Dube Erlangen/GER, D. Guldi, Erlangen/GER

Computer-Chemistry-Center, Friedrich-Alexander-University Erlangen-Nürnberg 

Abstract:
Controlling molecular motion with light is fundamental to the development of future technologies in materials science, biology, and nanotechnology. A major challenge remains to design efficient photoswitches that operate under red and near-infrared light. π-Extended thioindigo-derived photoswitches, such as peri-anthracenethioindigo (PAT) [1, 2], have recently emerged as promising candidates, but their underlying isomerization mechanisms have remained unexplored. Earlier studies on thioindigoids suggested that photoisomerization proceeds via different mechanisms depending on the switching direction: E-to-Z isomerization occurs predominantly through a triplet pathway [3, 4], whereas Z-to-E switching involves a more complex interplay of singlet and triplet states [5, 6].
In this talk, I will present a combined theoretical and experimental study that reveals how E-to-Z and Z-to-E photoisomerization occurs in the PAT photoswitch, and how these pathways differ. Consistent with earlier thioindigo findings, we observe that both processes in PAT crucially involve triplet states. However, for Z-to-E isomerization, a competing channel in the singlet manifold exists that does not lead to productive switching. Our results demonstrate that π-extension and peri-connectivity in PAT preserve the core photoswitching mechanism of the thioindigo motif, while enhancing its performance and enabling efficient, red-light-driven molecular switching. These insights open up new strategies for designing advanced photoswitches optimized for biological and technological applications.

On the Performance of DFT/MRCI for Singlet—Triplet Gaps and Emission Energies of Thermally Activated Delayed Fluorescence Molecules 

Mike Pauls¹, Thomas Froitzheim², J.-M. Mewes^2,3, S. Grimme², C. Bannwarth^1
1 Institute of Physical Chemistry, RWTH Aachen University, 52074 Aachen, Germany.
² Mulliken Center for Theoretical Chemistry, University of Bonn, 53115 Bonn, Germany.
³ beeOLED GmbH, Niedersedlitzer Str. 75c, 01257 Dresden, Germany.
E-mail: pauls(at)pc.rwth-aachen.de 

In the present contribution, we investigate the performance of the density functional theory multireference configuration interaction (DFT/MRCI)^[1,2] method for donor—acceptor and multiresonance thermally activated delayed fluorescence (TADF) emitters of the recent STGABS27 benchmark set.^[3] Using experimental S1—T1 energy gaps ΔE_ST and fluorescence energies E_EM as reference, DFT/MRCI yields mean absolute deviations (MADs) of 0.06 eV and 0.21 eV, which is close to other excellently performing methods. We explore method-specific settings of DFT/MRCI like the available parametrizations and evaluate how a good cost-to-accuracy-ratio is achieved. We also investigate how different theory levels used to describe ground and excited-state geometries interplay with DFT/MRCI for the predicted ΔE_ST and E_EM. The role of different implicit solvation models for the mostly charge-transfer (CT)-type lowest excited-states is investigated. An approximate state-specific solvation treatment is achieved employing reaction fields from restricted-open shell Kohn-Sham (ROKS) calculations specific to the targeted excited singlet or triplet state. We show that, while being physically motivated, accounting for (state-specific) solvation conditions does not lead to systematic improvements in the calculated ΔE_ST and E_EM. The quantities determined with DFT/MRCI based on a gas phase anchor configuration in the vertical approximation perform better, which we partially attribute to error compensation encompassed in the parametrization of the Hamiltonian.

A comparison to other methods like orbital-optimized or Tamm-Dancoff-approximated (TDA)-DFT, which have been used in earlier studies concerning the STGABS27 set, is drawn to further understand the quality of DFT/MRCI for studying these molecules. While the state-specific nature of orbitaloptimized DFT remains superior to the accuracy achievable with DFT/MRCI to describe ΔE_ST and E_EM of the studied TADF emitters, we expect the latter to be particularly suited for exploratory work due to its rather “black-box” nature. Compared to TDA-DFT, DFT/MRCI performs much more robust in in the vertical approximation already, and neither an implicit solvation treatment nor laborious tuning of the exchange-correlation functional is required. Our results suggest that DFT/MRCI is feasible to generate first information about the excited-states at hand, before moving to state-specific theories may be aspired.

References:  

^[1] S. Grimme and M. Waletzke, J. Chem. Phys. 1999, 111, 5645—5655.
^[2] C. M. Marian, A. Heil and M. Kleinschmidt, WIREs Comput. Mol. Sci. 2019, 9, e1394.
^[3] L. Kunze, A. Hansen, S. Grimme and J.-M. Mewes, J. Phys. Chem. Lett. 2021, 12, 8470–8480.
 

Mechanistic Insights into the Addition of Photoexcited Nitroarenes to Alkenes from Femtosecond Spectroscopy

David Klaverkamp¹, Friederike Schneider ², Lara Denninger³, Lukas Rach¹, Jens Bredenbeck³, Andreas Dreuw², Peter Gilch¹

¹ Institute of Physical Chemistry, Heinrich-Heine-University Düsseldorf, Universitätsstr. 1, 40225 Düsseldorf, Germany
² IWR, Heidelberg University, Im Neuenheimer Feld 205, 69120 Heidelberg, Germany
³ Institute of Biophysics, Goethe University Frankfurt, Max-von-Laue-Str. 1, 60438 Frankfurt/Main, Germany

E-mail: David.Klaverkamp(at)hhu.de 

The photoaddition of nitroarenes to alkenes has recently gathered significant attention in organic synthesis. It was initially utilized for the oxidative cleavage of alkenes ^[1,2] as an alternative to ozonolysis but has also been used to access a plethora of other, valuable compound classes.^[3-5] Mechanistic details on the initial steps are however unknown until now. Here, femtosecond UV/VIS and IR absorption spectroscopy together with quantum chemical computations are used to investigate the kinetics and intermediates for an exemplary nitroarene (4-cyanonitrobenzene (4CNNB)) and alkene (cyclooctene (cyO)) pair. The triplet state of 4CNNB is shown to be quenched by cyO with a bimolecular rate constant of 6∙10⁹ M^-1 s^-1. Quenching goes along with the formation of a transient species persisting for ca. 1 ns. Femtosecond IR spectroscopy gives strong evidence for the triplet biradical character of this intermediate (for structure see Fig. 1).^[1,2] It is shown, that only 10% of the quenching events result in product formation while the rest reform the starting material, thus severely diminishing the energetic efficiency. We attribute the loss channel either to triplet energy transfer to the alkene or -more likely- to an unproductive decay of the biradical intermediate.

Figure 1. Femtosecond UV/VIS absorption and kinetic scheme of the processes and intermediates involved in the addition of photoexcited 4CNNB to cyclooctene in MeCN.

References: 

[1] D. E. Wise, E. S. Gogarnoiu, A. D. Duke, J. M. Paolillo, T. L. Vacala, W. A. Hussain, M. Parasram, J. Am. Chem. Soc. 2022, 144 (34), 15437-15442.

[2] A. Ruffoni, C. Hampton, M. Simonetti, D. Leonori, Nature 2022, 610, 81–86.

[3] Hampton, C.; Simonetti, M.; Leonori, D., Angew. Chem. Int. Ed. 2023, 62 (8), e202214508.

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Coffee break in foyer area of lecture halls 6G,H,J

Molecular Design to Bridge the Luminescence Efficiency Gap Between Fluorescent and Triplet Harvesting Organics in the Near-Infrared

Dr. Daniel Congrave, Department of Chemistry, University of Oxford

Abstract:

Organics that efficiently emit and absorb light in the near-infrared (NIR) are highly desirable for the next generation of wearable and smartphone sensors, and integral to multiple strategies that have been proposed to enhance commercial solar panels – NIR organic electronics effectively hold the potential to benefit the efficiency of both the consumption and generation of clean energy in one fell swoop. However, NIR organics have so far been unable to fulfil this potential. In even the most efficient examples, the vast majority of energy is trapped into energy sinks known as ‘triplet states’, converting energy to unwanted heat, rather than the desired light or electricity.
Prevalent non-radiative decay via coupling to aromatic C–C stretching modes has limited organics that display complete NIR emission (> 700 nm) with appreciable (> 10%) photoluminescence quantum efficiency (PLQY) to select fluorescent dyes such as cyanines, squaraines and benzo(bis)thiadiazoles. Such materials feature dark (non-emissive) low energy triplet states. Considering that electron-hole recombination generates triplets with 75% probability, fluorescent NIR organics have a fundamental efficiency limitation for optoelectronic applications that must be overcome.
Thermally activated delayed fluorescent (TADF) molecules narrow the singlet–triplet energy gap, affording a promising approach to turn triplets bright. However, the development of complete NIR TADF molecules has been extremely challenging. A 2 order-of-magnitude PLQY gap exists between fluorescent dyes and triplet harvesting TADF materials capable of complete NIR emission. This is exemplified by our previously reported molecule CAT-1, which displays an emission onset of ca. 750 nm in neat-film, but a PLQY of merely 0.18%.
Molecular design rules for highly luminescent complete NIR TADF molecules are urgently needed. In this talk I will discuss how we have been developing the understanding required to engineer a highly luminescent complete NIR TADF molecule with an emission onset ca. 750 nm and ca. 30% PLQY, bridging the gap between fluorescent and TADF organics. The work combines synthetic organic and physical chemistry and photophysics.

Literature:

CAT-1 reference: J. Am. Chem. Soc. 2019, 141, 46, 18390–18394 
 

The poster abstracts are available as a PDF here: Abstractbooklet