Theory Untangles Fascinating Properties of Spiro-Compounds

Theory was instrumental in rationalizing complex photophysical phenomena experimentally observed for a series of spiro-bridged heterotriangulenes in solution and their aggregates.

Continuing our interest in triangulenes (see posts 1, 2, 3) in this study we performed joined experimental and theoretical investigation of spirofluorene-bridged triphenylamine, -phosphine, and -phosphine oxide and their aggregates.

The crucial question what is the nature of electronic excitations and why all compounds had very different photophysical behavior despite the same structural motifs. For this, we needed to split the system into units shown with different colors at the top and perform analysis of the one-electron transition density matrix between excited and ground states (Felix Plasser‘s TheoDORE package was very helpful for this – highly recommended!). Our theoretical calculations showed for …

• … compound with N-central unit that the lowest-energy absorption is localized on the N-triangulene core. Emission also originates from this core.
• … compound with P-central unit that the lowest-energy absorption is localized on the spirofluorene units. This compound features interesting dual emission not observed in other compounds: one from fluorene and another (lower energy) from the P-triangulene core.
• … compound with P=O-central unit that the lowest-energy absorption is localized on spirofluorene units. Emission originates from these units too.

Another intriguing observation was that aggregation of these compounds can be induced by adding water to their solutions and during aggregation the fluorescence quenched. Theory explained the quenching in P- and P=O-containing compounds with indirect calculations on fluorene dimers (since emission originates from fluorene units in these compounds) that are much weaker fluorescent than fluorene monomers. Apparently, more contacts between fluorene units in aggregates are responsible for quenching in phosphorus-containing spirofluorene-bridged triangulenes.

We also explained the unusual configuration of phosphorus-containing spiro-compound dimers with anti-parallel dipole moments in solid state (see Figure below). Apparently, the dispersion interaction between fluorene units is so strong that it forces dimers to adopt this configuration even in gas phase, while the alternative configuration is not stable. This conclusion followed from calculations in vacuum with and without dispersion interactions included for both configurations. Interestingly, without D3-corrections included, both dimers are unstable.

Finally theory also explained the trends in NMR spectra of compounds with N, P, and P=O central units as we could quantify their inductive and mesomeric effects.

All these observations obtained with state-of-the-art theoretical analysis would be rather difficult (if possible at all) to rationalize with pure experimental techniques that highlights how important quantum chemistry is in modern science.

Raw computational data including geometries in xyz format in individual files can be downloaded from figshare.

• Marcel Krug, Maximilian Wagner, Tobias A. Schaub, Wen-Shan Zhang, Christoph M. Schüßlbauer, Johannes D. R. Ascherl, Peter M. Münich, Rasmus R. Schröder, Franziska Gröhn, Pavlo O. Dral, Mario Barbatti, Dirk M. Guldi*, Milan Kivala*, The Impact of Aggregation on the Photophysics of Spiro-bridged Heterotriangulenes. Angew. Chem. Int. Ed. 2020, accepted. DOI: 10.1002/anie.202003504.