Role of the Spacer in the Singlet−Singlet Energy Transfer Mechanism (Förster vs Dexter) in Cofacial Bisporphyrins

The cofacial bisporphyrins H4DPS (DPS = 4,6-bis[5-(2,8,13,17-tetraethyl-3,7,12,18-tetramethylporphyrinyl)]dibenzothiophene), H4DPO (DPO = 4,6-bis[5-(2,8,13,17-tetraethyl-3,7,12,18-tetramethylporphyrinyl)]dibenzofuran), H4DPX (DPX = 4,5-bis[5-(2,8,13,17-tetraethyl-3,7,12,18-tetramethylporphyrinyl)]-9,9-dimethylxanthene), H4DPA (DPA = 1,8-bis[5-(2,8,13,17-tetraethyl-3,7,12,18-tetramethylporphyrinyl)]anthracene), and H4DPB (DPB = 1,8-bis[5-(2,8,13,17-tetraethyl-3,7,12,18-tetramethylporphyrinyl)]biphenylene) have been monometalated by Zn(OAc)2·2H2O and by GaCl3 to explore the singlet−singlet energy transfer from the photoexcited metal porphyrin center to the linked free base porphyrin. The spectroscopic (UV−vis and fluorescence) and photophysical properties (fluorescence lifetimes, τF, and quantum yields, ΦF) have been investigated at 298 and 77 K in degassed 2-MeTHF for the donor−acceptor systems, (Zn)H2DPS, (Zn)H2DPO, (Zn)H2DPA, (Zn)H2DPX, and (Zn)H2DPB, as well as for the bis-zinc complexes, (Zn)2DPS, (Zn)2DPO, (Zn)2DPX, and (Zn)2DPB, respectively, and the monoporphyrin derivatives, H2P, (Zn)P, and (Ga−OMe)P (P2- = 5-phenyl-2,8,13,17-tetraethyl-3,7,12,18-tetramethylporphyrin-dianion). The singlet−singlet energy transfer rate constants (K ET) were obtained using K ET = (1/τF −1/τF°), where τF° is the fluorescence lifetime of the corresponding bis-zinc(II) systems (or (Zn)P and (Ga−OMe)P) where no energy transfer occurs. The τF value for three bis-zinc(II) compounds varies from 1.69 to 2.01 ns and is 1.84 (at 298 K) and 3.20 ns (at 77 K) for (Ga−OMe)P. In the donor−acceptor bismacrocycles, depending on the spacer and the temperature, the fluorescence lifetimes decrease down to 50−240 ps. The K ET values range from ∼4 to ∼21 (ns-1) and have been analyzed considering both the Förster and the Dexter mechanisms. Using the Cmeso−Cmeso distance parameters in the calculations, the Förster and Dexter mechanisms operate for DPS and DPO, and for DPA, DPX, and DPB spacer systems, respectively. The limit distance where one mechanism dominates over the other is estimated to be around 5−6 Å.