A Rational Way to Control the Triplet State Wave Function Confinement of Organic Chromophores: Effect of the Connection Sites and Spin Density Distribution-Guided Molecular Structure Design Principles in Bodipy Dimers

Abstract

To study the intersystem crossing (ISC) and the spatial confinement of the triplet excited states of organic chromophores, we prepared a series of Bodipy dimers. We found that the connection position of the two units has a significant effect on the absorption and fluorescence. Singlet oxygen quantum yields of 3.8−12.4% were observed for the dimers, which are independent of solvent polarity. Nanosecond transient absorption spectra indicate the population of long-lived triplet excited states with lifetimes (τ T ) of 45−454 μs. Pulsed laser-excited time-resolved electron paramagnetic resonance (TREPR) spectra show that the T 1 triplet states are essentially delocalized, which is different from the case for the previously reported Bodipy dimers. The TREPR spectra of the triplet states imply that the delocalization over the whole dimer essentially depends on the electron density of the carbon atoms at the connection sites. This property may become a universal rule for controlling the T 1 state confinement in multichromophore organic molecules.

NIR-Absorbing Tetraphenylethene-Containing bisBODIPY Nanoplatforms Demonstrate Effective Lysosome-Targeting and Combinational Phototherapy

“…Inside the NPs, the bisBODIPYs were tightly packed, enhancing the intermolecular exciton coupling of the dimers. This combined intramolecular and intermolecular exciton coupling could help enhance the probability of intersystem crossing, which is crucial for efficient PDT. − Subsequently, the ROS production capacities of the NPs were evaluated. As shown in the Figures f,g and S16–S19, under the same molecular concentration (20 μM) and irradiation condition (755 nm, 0.10 W cm –2 ), the bisBODIPY NPs significantly improved the emission of ROS indicator 2′,7′-dichlorodihydrofluorescein (DCFH).…”

“…Known for its exceptional photophysical properties and robust photo/chemical stability, boron dipyrromethene (BODIPY) is a favored choice in designing phototherapeutic agents. − Extending the π-conjugation of the BODIPY core or constructing oligomers can yield BODIPY derivatives with NIR absorption. − However, most BODIPYs used in tumor phototherapy exhibit a single phototherapeutic effect, either photothermal or photodynamic. − In this study, to address the aforementioned issues, we designed and synthesized two α–α-linked bisBODIPYs with tetraphenylethene (TPE) moieties. The intramolecular J-type exciton coupling of the bisBODIPY not only redshifts the molecular absorption but also enhances the intersystem transition process, thereby enhancing PDT process. − Furthermore, installation of one or two bulky TPE moieties (Figure ) at the α-position of bisBODIPY further extends NIR absorption and confers aggregation-induced emission (AIE) activities, preventing ROS quenching upon aqueous aggregation. − Additionally, the combination of electron-rich TPE as the electron donor and electron-deficient BODIPY core as the electron acceptor creates a donor–acceptor (D–A) structure, likely enhancing spin–orbit coupling and reducing the energy gap between S 1 and T 1 states, thereby promoting intersystem crossing . The rotor characteristics of TPE also improve the efficiency of photothermal conversion .…”