The synthesis, photophysical characterization, and modeling of a new library of halogen-free photosensitizers (PS) based on orthogonal boron dipyrromethene (BODIPY) dimers are reported. Herein we establish key structural factors in order to enhance singlet oxygen generation by judiciously choosing the substitution patterns according to key electronic effects and synthetic accessibility factors. The photosensitization mechanism of orthogonal BODIPY dimers is demonstrated to be strongly related to their intrinsic intramolecular charge transfer (ICT) character through the spin-orbit charge-transfer intersystem crossing (SOCT-ISC) mechanism. Thus, singlet oxygen generation can be effectively modulated through the solvent polarity and the presence of electron-donating or withdrawing groups in one of the BODIPY units. The photodynamic therapy (PDT) activity is demonstrated by in vitro experiments, showing that selected photosensitizers are efficiently internalized into HeLa cells, exhibiting low dark toxicity and high phototoxicity, even at low PS concentration (0.05-5×10 m).

A series of mono‐ to hexachlorinated BODIPY dyes have been prepared in good to excellent yields through the use of N‐chlorosuccinimide as an inexpensive halogenating reagent. This library of chlorinated dyes allowed analysis in detail, from the experimental and theoretical points of view, of the dependency of the photophysical and optical properties of the dyes on the number and positions of the chlorine substituents on their BODIPY cores. Quantum mechanical calculations predict the regioselectivity of the halogenation reaction and explain why some positions are less prone to chlorination. The new chlorinated BODIPYs exhibit enhanced laser action with respect to their non‐halogenated analogues, both in liquid solution and in the solid phase. In addition, chlorination is a facile and essentially costless protocol for overcoming important shortcomings exhibited by commercially available BODIPYs, which should favor their practical applications in optical and sensing fields.

The development of highly efficient and stable blue-emitting dyes to overcome some of the most important shortcomings of available chromophores is of great technological importance for modern optical, analytical, electronic, and biological applications. Here, we report the design, synthesis and characterization of new tailor-made BODIPY dyes with efficient absorption and emission in the blue spectral region. The major challenge is the effective management of the electron-donor strength of the substitution pattern, in order to modulate the emission of these novel dyes over a wide spectral range (430-500 nm). A direct relationship between the electron-donor character of the substituent and the extension of the spectral hypsochromic shift is seen through the energy increase of the LUMO state. However, when the electron-donor character of the substituent is high enough, an intramolecular charge-transfer process appears to decrease the fluorescence ability of these dyes, especially in polar media. Some of the reported novel BODIPY dyes provide very high fluorescence quantum yields, close to unity, and large Stokes shifts, leading to highly efficient tunable dye lasers in the blue part of the spectrum; this so far remains an unexploited region with BODIPYs. In fact, under demanding transversal pumping conditions, the new dyes lase with unexpectedly high lasing efficiencies of up to 63 %, and also show high photostabilities, outperforming the laser action of other dyes considered as benchmarks in the same spectral region. Considering the easy synthetic protocol and the wide variety of possible substituents, we are confident that this strategy could be successfully extended for the development of efficient blue-edge emitting materials and devices, impelling biophotonic and optoelectronic applications.

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