A New Photocage Derived from Fluorene

• Published in: Chemistry : a European journal 2018-09, Vol.24 (49), p.13026-13035
• Main Authors: Reinfelds, Matiss, von Cosel, Jan, Falahati, Konstantin, Hamerla, Carsten, Slanina, Tomáš, Burghardt, Irene, Heckel, Alexander
• Format: Article
• Language: English
• Subjects: Absorptivity; Aromatic compounds; Biphenyl; Chemistry; conformation analysis; Conjugation; donor–acceptor systems; Fluorene; fluorenes; photochemistry; Properties (attributes); Protecting groups; Rotational spectra; substituent effects; Time dependence
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201802390

Photolabile protecting groups (PPGs or photocages) are increasingly subject to molecular design to meet requirements such as absorbance in the visible spectral range, high molar absorption coefficients, and high quantum yields of leaving group release. Improvements in these properties for the promising 3‐diethylaminobenzyl (DEAMb) photocage, the photoactivity of which is based on the Zimmerman meta effect, are reported. Expansion of the aromatic system with a second aromatic ring resulted in improved spectral properties. A systematic trend relating the electronic (π‐donor or acceptor) properties of the new aryl substituent and its position in the DEAMb ring to changes in the spectral properties could be observed. Conclusions from the experimental results were supported by computations obtained by using time‐dependent DFT. A second generation of DEAMb‐based photocages was designed. A rigid linker was introduced to ensure more efficient conjugation of the aromatic ring π systems by limiting rotational freedom. The resulting fluorenol (9‐hydroxyfluorene)‐based photocages had superior spectral properties to those of simple biphenyl systems. The best uncaging cross section achieved was 5320 m−1 cm−1 (ϵΦ365). Fixed in place: 3‐Diethylaminobenzyl (DEAMb) photocages, upon excitation, undergo electron density changes in the molecule, which result in the release of a leaving group (LG; the Zimmerman meta effect). The spectral properties of DEAMb can be improved without loss in photoactivity by installing a second aromatic ring (see scheme). The best improvements are achieved if a rigid fluorene system is formed by conformational locking.