Deciphering the Intersystem Crossing in a Helical BODIPY for Recorded Low Dose Photodynamic Therapy.

Abstract
Intersystem crossing (ISC) of triplet photosensitizers is a vital process for fundamental photochemistry and photodynamic therapy (PDT). However, the currently effective long wavelength operating photosensitizers have to rely on heavy atoms to enhance ISC. Unfortunately, the heavy atoms, besides their toxicity, can also cause significant reduction in the triplet lifetime of the photosensitizers, which is fundamentally detrimental to their applications. To overcome this challenge, herein we report the co-existence of efficient ISC and long triplet excited lifetime in a heavy atom-free Bodipy helicene molecule. Via advanced theoretical computation and time-resolved electron paramagnetic resonance spectroscopy, we confirmed the ISC of the Bodipy is resulted from its twisted molecular structure and reduced symmetry. The twisted Bodipy shows exceptionally intense long wavelength absorption (ε = 1.76 × 10 5 M -1 cm -1 at 630 nm), satisfactory triplet quantum yield (Φ T = 52%) and long-lived triplet state (τ T = 492 μs), leading to unprecedented performance as a triplet photosensitizer for PDT. Moreover, nanoparticles constructed with such helical Bodipy show surprisingly efficient PDT mediated antitumor immunity amplification with ultra-low dose (0.25 μg/kg body weight), which is several hundred times lower than that of the existing PDT reagents. Our study paves the way to design the next generation of triplet photosensitizers.

Citation
Wang, Z., Huang, L., Yan, Y., El-Zohry, A. M., Toffoletti, A., Zhao, J., … Han, G. (2020). Deciphering the Intersystem Crossing in a Helical BODIPY for Recorded Low Dose Photodynamic Therapy. Angewandte Chemie International Edition. doi:10.1002/anie.202005269

Acknowledgements
J. Zhao thank the NSFC (21673031, 21761142005, 21911530095 and 21421005), the State Key Laboratory of Fine Chemicals (ZYTS201901), the Fundamental Research Funds for the Central Universities (DUT19TD28), Dipartimento di Scienze Chimiche, Università degli Studi di Padova (Visiting Scientist) for support. B. Dick thanks Dalian University of Technology for the Haitian Professorship support. L. Huang and G. Han are supported by the startup funding of University of Massachusetts.

Publisher
Wiley

Journal
Angewandte Chemie

DOI
10.1002/anie.202005269

PubMed ID
32449273

Additional Links
https://onlinelibrary.wiley.com/doi/abs/10.1002/anie.202005269

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