Insight into the Drastically Different Triplet Lifetimes of Bodipys by Optical/Magnetic Spectroscopies and Theoretical Computations

The triplet state lifetimes of organic chromophores are crucial for fundamental photochemistry studies as well as applications as photosensitizers in photocatalysis, photovoltaics, photodynamic therapy and photon upconversion. It is noteworthy that the triplet state lifetime of a chromophore can vary significantly for its analogues, while the exact reason was rarely studied. Herein with few exemplars of typical Bodipy derivatives, which show triplet lifetime varying up to 110- fold polarization (1.4 us - 160 us), weof found the time-resolved that for these electron derivatives paramagnetic with shortresonance triplet state (TREPR) lifetimes spectra (ca. 1 - 3 us), triplet the state electron inverted spin at longer delay time after laser pulse, as a consequence of a strong anisotropy in the decay rates of zero-field states sublevel of the triplet state. For the derivatives showing longer triplet state lifetimes (> 50 us), no such ESP inversion was observed. The observed fast decaying of one sublevel is responsible for the short triplet state lifetime; theoretical computation indicate that it is due to a strong coupling between the Tz sublevel and the ground state mediated by the spin-orbit interaction. Another finding is that the heavy atom effect on shortening of the triplet state lifetime is more significant for the T1 states with lower energy. To the best of our knowledge, this is the first systematic study to rationalize the short triplet state lifetime of visible-light-harvesting organic chromophores. Our results are useful for fundamental photochemistry and the design of long-lived triplet photosensitizers.