We present the synthesis, photophysical properties, and biological application of nontoxic 3-azo-conjugated BODIPY dyes as masked fluorescent biosensors of hypoxia-like conditions. The synthetic methodology is based on an operationally simple NN bond-forming protocol, followed by a Suzuki coupling, that allows for a direct access to simple and underexplored 3-azo-substituted BODIPY. These dyes can turn on their emission properties under both chemical and biological reductive conditions, including bacterial and human azoreductases, which trigger the azo bond cleavage, leading to fluorescent 3-amino-BODIPY. We have also developed a practical enzymatic protocol, using an immobilized bacterial azoreductase that allows for the evaluation of these azo-based probes and can be used as a model for the less accessible and expensive human reductase NQO1. Quantum mechanical calculations uncover the restructuration of the topography of the S 1 potential energy surface following the reduction of the azo moiety and rationalize the fluorescent quenching event through the mapping of an unprecedented pathway. Fluorescent microscopy experiments show that these azos can be used to visualize hypoxia-like conditions within living cells.

Paper published as part of the special topic on Special Collection in Honor of Women in Chemical Physics and Physical Chemistry ARTICLES YOU MAY BE INTERESTED INModeling solvation effects on absorption and fluorescence spectra of indole in aqueous solution

This work investigates the photophysics of barbituric acid at different pH conditions using ab initio methods. Our calculations ascribe the most intense bands at ca. 260 nm at neutral pH...

The intriguing and rich photophysical properties of three curved nanographenes (CNG 6, 7, and 8) are investigated by time‐resolved and temperature‐dependent photoluminescence (PL) spectroscopy. CNG 7 and 8 exhibit dual fluorescence, as well as dual phosphorescence at low temperature in the main PL bands. In addition, hot bands are detected in fluorescence as well as phosphorescence, and, in the narrow temperature range of 100–140 K, thermally activated delayed fluorescence (TADF) with lifetimes on the millisecond time‐scale is observed. These findings are rationalized by quantum‐chemical simulations, which predict a single minimum of the S1 potential of CNG 6, but two S1 minima for CNG 7 and CNG 8, with considerable geometric reorganization between them, in agreement with the experimental findings. Additionally, a higher‐lying S2 minimum close to S1 is optimized for the three CNG, from where emission is also possible due to thermal activation and, hence, non‐Kasha behavior. The presence of higher‐lying dark triplet states close to the S1 minima provides mechanistic evidence for the TADF phenomena observed. Non‐radiative decay of the T1 state appears to be thermally activated with activation energies of roughly 100 meV and leads to disappearance of phosphorescence and TADF at T > 140 K.

The design of novel chromophores showing specific photophysical traits is nowadays crucial for the development of new dyes and optical devices. Azobenzene derivatives comprise the largest fraction of the industrial dyes and remain one of the main functions to be employed in the design of general-purpose photoactivated switches. In this context, we have investigated the optical and photoisomerization properties of two push-push azo derivatives, from a purely theoretical perspective, to outline a mechanistic landscape that can contribute to setting the grounds for the future goal of formulating dyes with particular photophysical properties. Both derivatives show downhill excited-state potential energy surfaces, hinting at fast photoactivated isomerization. In addition, both the trans and cis forms show nearly complementary absorption spectra leading to discriminatory color and differential excitation possibilities. Additionally, the reverse cis to trans ground state thermal isomerization reactions show higher energy barriers than the parent azobenzene molecule, supporting their potential use in different technological applications.