Insights into the Efficient Intersystem Crossing of Bodipy-Anthracene Compact Dyads with Steady-State and Time-Resolved Optical/Magnetic Spectroscopies and Observation of the Delayed Fluorescence

Abstract

Spin-orbit charge transfer-induced intersystem crossing (SOCT-ISC) is of particular interest for preparation of heavy atom-free triplet photosensitizers. Up to now, examples for SOCT-ISC dyads are limited and electron donor/acceptor SOCT-ISC dyads showing strong visible-light harvesting are rare. Herein, we studied the photophysics of a series of Bodipy-anthracene (BDP-An) compact dyads, especially the triplet state electron spin dynamics with the time-resolved electron paramagnetic resonance (TREPR) spectroscopy. The electronic coupling matrix elements (V DA *) between the 1CT (charge transfer) state and 1LE (locally excited) state are in the range 773–1545 cm–1. For one dyad, we observed three triplet states simultaneously with TREPR, that is triplet states confined on the anthracene (3An) and the Bodipy (3BDP) moieties as well as a 3CT state. Based on the electron spin polarization of these three triplet states and the optical experiments, the SOCT-ISC mechanism is confirmed and the radical pair-ISC mechanism as the main ISC channel was excluded. Triplet–triplet annihilation-induced delayed fluorescence was observed for the dyads, which is rare for Bodipy fluorophores.

Bodipy Derivatives as Triplet Photosensitizers and the Related Intersystem Crossing Mechanisms

“…The purpose of this kind of study was to study the ISC mechanisms, e.g., to discriminate the radical pair ISC and the SOCT-ISC mechanisms. We observed an ESP of (e, e, e, a, a, a) for 33 and 34, which was similar to that of 2,6-diiodoBodipy (Wang et al, 2019b). This finding was different from the previous reports that the ESP of the triplet state accessed with SOCT-ISC should be always different from the SO ISC (Dance et al, 2008).…”

“…We prepared a series of Bodipy-anthryl dyads, in which the anthryl and the Bodipy units adopted orthogonal geometry, but the dipole moments of the two subunits were either in parallel or perpendicular (Figure 10; Wang et al, 2019b), which was different from the previous reports of the anthryl-Bodipy dyads (Filatov et al, 2017).…”

“…It is worth mentioning that these apparent triplet-state lifetimes were shorter than the intrinsic triplet-state lifetimes as a result of the triplet-triplet-annihilation (TTA) self-quenching effect. Later, Zhao and Dick reported a kinetic model with the TTA effect that was considered to determine the intrinsic triplet-state lifetimes, and the triplet-state lifetime of the diiodoBodipy was up to 276 µs (Lou et al, 2018;Wang et al, 2019b). To the best of our knowledge, this was the first report of the intrinsic triplet-state lifetime of Bodipy derivatives.…”

“…The singlet oxygen photosensitizing of 38 was weaker (24.6%). Note for the Bodipyanthryl dyads, high singlet oxygen photosensitizing was observed in acetonitrile (Wang et al, 2019b). An apparent triplet-state lifetime of 116 µs was observed with nanosecond transient absorption spectroscopy.…”

Efficient Intersystem Crossing in the Tröger's Base Derived From 4‐Amino‐1,8‐naphthalimide and Application as a Potent Photodynamic Therapy Reagent

“…Thus the ESP of the TREPR spectrum of the triplet states give information on the triplet‐state formation mechanism. On the other hand, the ZFS parameters D and E obtained from the simulation of the TREPR spectra are useful for discrimination of the 3 CT and 3 LE states, that is, confinement of the triplet‐state wave functions.…”