Photoswitched singlet energy transfer in a porphyrin-spiropyran dyad

A photochromic nitrospiropyran moiety (Sp) has been covalently linked to a zinc (P_Zn) and to a free-base (P_H2) porphyrin. In the resulting dyads (P_Zn-Sp_c and P_H2-Sp_c), the porphyrin first excited singlet states are unperturbed by the closed form of the attached spiropyran. Excitation of the spiropyran moiety of either dyad in the near-UV region results in ring opening to a merocyanine form (P-Sp_o) that absorbs at 600 nm. The open form re-closes thermally in 2-methyltetrahydrofuran with a time constant of 20 s, or following irradiation into the 600 nm band. Excitation of the zinc porphyrin moiety in the merocyanine form of the dyad yields ¹P_Zn-Sp_o. The lifetime of the zinc porphyrin excited state is reduced from its usual value of 1.8 ns to 130 ps by singlet-singlet energy transfer to the merocyanine moiety to give P_Zn-¹Sp_o. The quantum yield of energy transfer is 0.93. Quenching is also observed in the free base dyad, where ¹P_H2-Sp_o and P_H2-¹Sp_o exchange singlet excitation energy. This photoswitchable quenching phenomenon provides light-activated control of the porphyrin excited states, and consequently control of any subsequent energy or electron-transfer processes that might be initiated by these excited states in more complex molecular photonic or optoelectronic devices.