Research

As a key part of the Cells in Motion Interfaculty Centre (CiMIC), the professorship is focused on the design, synthesis and implementation of functional coordination compounds for biomedical application, optoelectronics and photocatalysis (see below).

The Strassert Lab has established coordination compounds that are used for the optical manipulation of structural features and dynamic processes in living systems[7,9] and as dual probes for high-resolution multimodal imaging, acting simultaneously as contrast agents for electron microscopy and as phosphorescent markers for spatially and temporally resolved luminescence microscopy.[1] Another fundamental aspect under current investigation with tailored NIR (near infrared) absorbers is the optoacoustic labelling[2] and the targeted ROS (reactive oxygen species) response of antibiotic-resistant pathogens and neoplastic cells.[8] The research efforts in the field of photosensitizing nanoarchitectures and self-assembling triplet emitters are also oriented towards the construction of printable devices employing soft self-organized nanoarchitectures and solution-processable phosphors by unravelling their coupling with (semi)conductive surfaces for (oxygen) sensing and (electro)luminescence.[3,5,6,10] Besides the relevance for optoelectronics, the fundamental understanding of photoinduced charge separation processes in light-harvesting units is also an aspect that is explored for applications in photo(redox)catalysis and photovoltaics.

© C. Strassert

In particular, the Strassert Lab has synthesized and characterized in terms of excited state properties a series of tailored phosphorescent transition metal complexes that have been implemented in electroluminescent devices.[3,6,10] The fundamental understanding was gained by means of scanning tunnelling microscopy and spectroscopy with the support of (TD)DFT (time-dependent density functional theory) methods. Regarding molecular bioimaging, the Strassert Lab has been devoted to the development of NIR-absorbing luminescent and photoacoustic labels bridging the gap between high-resolution luminescence-based visualization and deep tissue imaging.[2,8] In this sense, fine-tuning of photofunctional coordination compounds with exchangeable cationic centres enabled the realization of photosensitizers, fluorescent or photoacoustic reporters able to label and to photoinactivate antibiotic resistant bacteria. Moreover, phosphorescent Pt(II) complexes were used to site-specifically decorate biomacromolecules[4] and polymers[5] while providing contrast for high resolution electron microscopy.[1] The resulting orthogonal readouts arising from one single probe were used to track their uptake in living cells via luminescence microscopy, and their localization was refined employing TEM.

Besides the pursuit of new concepts towards functional coordination compounds, the photophysical investigation plays a fundamental role towards an in-depth understanding of luminescent excited state properties at sub- and supramolecular level. State of the art spectrally and temporally resolved luminescence analysis techniques are combined with the high spatial and temporal resolution of confocal optical microscopy with multiphoton excitation capabilities (including luminescence lifetime imaging microscopy). We characterize discrete luminescent entities or ensembles thereof in fluid solutions, homogeneous matrices, solids and films. This includes monomeric molecules or aggregates thereof confined at inert interfaces, in solvents or glassy matrices, crystals, fibres, nano- or microparticles, bacteria, biofilms, organelles, cells and tissues. Specific samples are investigated at variable temperatures (down to 4 K) to suppress roto-vibrational relaxation and thermal equilibration of triplet sub-states to assess spin-orbit-coupling-mediated zero-field splitting and thermally activated delayed fluorescence processes.

Since 2001, Prof. Strassert has co-authored more than 95 publications in peer-reviewed journals. Between 2015 and 2019, the Strassert Lab has published more than 60 original research articles devoting substantial efforts to the synthesis and photophysical investigation of molecular and nanostructured arrays employing spectrally and spatiotemporally resolved methods:

[1]Phosphorescent Pt(II) complexes spatially arrayed in micellar polymeric nanoparticles providing dual readout for multimodal imaging”, M. T. Proetto, J. Sanning, M. Peterlechner, M. Thunemann, L. Stegemann, S. Sadegh, A. Devor, N. C. Gianneschi, C. A. Strassert, Chem. Commun. 2019, 55, 501. [doi: 10.1039/c8cc06347h]
[2]Towards Optimized Naphthalocyanines as Sonochromes for Photoacoustic Imaging in vivo”, M. J. Duffy, O. Planas Marques, A. Faust, T. Vogl, S. Hermann, M. Schäfers, S. Nonell Marrugat, C. A. Strassert, Photoacoustics 2018, 9, 49. [doi: 10.1016/j.pacs.2017.12.001]
[3]Toward Tunable Electroluminescent Devices by Correlating Function and Submolecular Structure in 3D Crystals, 2D-Confined Monolayers, and Dimers”, S. Wilde, D. Ma, T. Koch, A. Bakker, D. Gonzalez-Abradelo, L. Stegemann, C. G. Daniliuc, H. Fuchs, H. Gao, N. L. Doltsinis, L. Duan, C. A. Strassert, ACS Appl. Mater. Interfaces 2018, 10, 22460. [doi: 10.1021/acsami.8b03528]
[4]Oxygen-Insensitive Aggregates of Pt(II) Complexes as Phosphorescent Labels of Proteins with Luminescence Lifetime-Based Readouts”, P. Delcanale, A. Galstyan, C. G. Daniliuc, H. E. Grecco, S. Abbruzzetti, A. Faust, C. Viappiani, C. A. Strassert, ACS Appl. Mater. Interfaces 2018, 10, 24361. [doi: 10.1021/acsami.8b02709]
[5]Oxygen-insensitive phosphorescence in water from a Pt-doped supramolecular array”, L. Straub, D. González-Abradelo, C. A. Strassert, Chem. Commun. 2017, 53, 11806. [doi: 10.1039/c7cc05435a]
[6]Color-tunable asymmetric cyclometalated Pt(II) complexes and STM-assisted stability assessment of ancillary ligands for OLED”, J. Sanning, L. Stegemann, P. R. Ewen, C. Schwermann, C. G. Daniliuc, D. Zhang, N. Lin, L. Duan, D. Wegner, N. L. Doltsinis, C. A. Strassert, J. Mater. Chem. C 2016, 4, 2560. [doi: 10.1039/C6TC00093B]
[7]Spatiotemporally Resolved Tracking of Bacterial Responses to ROS-Mediated Damage at the Single-Cell Level with Quantitative Functional Microscopy”, A. Barroso Peña, M. C. Grüner, T. Forbes, C. Denz, C. A. Strassert, ACS Appl. Mater. Interfaces 2016, 8, 15046. [doi: 10.1021/acsami.6b02605]
[8]Labeling and Selective Inactivation of Gram-Positive Bacteria Employing Bimodal Photoprobes with Dual Readouts”, A. Galstyan, D. Block, S. Niemann, M. C. Grüner, S. Abbruzzetti, M. Oneto, C. G. Daniliuc, S. Hermann, C. Viappiani, M. Schäfers, B. Löffler, C. A. Strassert, A. Faust, Chem. Eur. J. 2016, 22, 5243. [doi: 10.1002/chem.201504935]
[9]Photofunctional surfaces for quantitative fluorescence microscopy: Monitoring the effects of photogenerated reactive oxygen species at single cell level with spatiotemporal resolution”, L. Stegemann, K. C. Schürmann, C. A. Strassert, H. E. Grecco, ACS Appl Mater Interfaces 2015, 7, 5944. [doi: 10.1021/acsami.5b00130]
[10]Scanning-tunneling-spectroscopy-directed design of tailored deep-blue emitters”, J. Sanning, P. R. Ewen, L. Stegemann, J. Schmidt, C. G. Daniliuc, T. Koch, N. L. Doltsinis, D. Wegner, C. A. Strassert, Angew. Chem. Int. Ed. 2015, 54, 786. [doi: 10.1002/anie.201407439]