Switching of the triplet excited state of the C60-dimethylaminostyryl BODIPY dyads/triads

Abstract

In order to switch the triplet excited states in organic compounds, dimethylaminostyryl BODIPY-C₆₀ dyads and triads were prepared. The triplet excited states of the compounds were switched with acid/base, and the mechanism was studied with nanosecond time-resolved transient difference absorption spectroscopy. The visible light-harvesting BODIPY antennas are the electron or singlet energy donor, whereas C₆₀ moiety is the electron/singlet energy acceptor, as well as the spin converter to produce triplet excited states. Our strategy of triplet state switching is to control either the photoinduced electron transfer (PET) or the singlet state energy transfer (EnT) from the antenna to C₆₀ moiety by protonation of the dimethylaminostyryl BODIPY unit. Population of the triplet state was observed for the dyads with mono(4-dimethylaminostyryl) substituents on BODIPY antenna in nonpolar solvent such as toluene (τ_T = 168.6 μs). Formation of the charge transfer state (CTS) in polar solvent quenches the triplet excited state (τ_T < 10 ns). In the presence of acid, the dimethylaminostyryl BODIPY moiety is protonated, thus the electron transfer (ET) was inhibited. The cascade acid-activated EnT and the intersystem crossing (ISC) of C₆₀ produce the triplet excited state. For the dyad and the triads with bis(4-dimethylaminostyryl) substituents on BODIPY antenna, the antenna S₁ state energy level is lower than the S₁ state energy level of C₆₀; thus, no EnT to C₆₀ exists, and no triplet state was produced upon selective excitation into the BODIPY moiety. With protonation of the amino styryl substituents, the S₁ state energy level of the antenna is promoted to be higher than S₁ state of C₆₀ moiety, and as a result EnT is activated and triplet state is produced. In all the compounds, the triplet excited state is localized on the dimethylaminostyryl BODIPY moiety and not on the C₆₀ moiety. The triplet state switching was conveyed to the singlet oxygen (¹O₂) photosensitizing ability of the compounds, and the variation of the singlet oxygen quantum yield, Φ_Δ, is from 1.9% to 73%.