6533b862fe1ef96bd12c6dbc

RESEARCH PRODUCT

Triplet-triplet energy transfer controlled by the donor-acceptor distance in rigidly held palladium-containing cofacial bisporphyrins.

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Eleven new complexes, including mono-, heterobi-, and homobimetallic cofacial bisporphyrins, (Pd)H 2 DPS, (M)H 2 DPX, (M)H 2 DPB, (PdZn)DPS, (PdZn)DPX, (Pt) 2 DPX, (M) 2 DPB (M=Pd, Pt), and (Pt)P (DPS 4 - = 4,6-bis[5-(2,8,13,17-tetraethyl-3,7,12,18-tetramethylporphyrinyl)]dibenzothiophene tetraanion, DPX 4 - = 4,5-bis[5-(2,8,13,17-tetraethyl-3,7,12,18-tetramethylporphyrinyl)]-9,9-dimethylxanthene tetraanion, DPB 4 - =1,8-bis[5-(2,8,13,17-tetraethyl-3,7,12,18-tetramethylporphyrinyl)]biphenylene tetraanion, P 2 - = 5-phenyl-2,8,13,17-tetraethyl-3,7,12,18-tetramethylporphyrin dianion) have been synthesized and characterized. The photophysical properties of the donor (M)P (M=Pd or Pt, P= porphyrin chromophore) and the acceptor (free base H 2 P or (Zn)P) depend on the C m e s o -C m e s o distance and the presence of a heavy atom such as Pd" or Pt I I . The data were compared with those for the known compounds (Pd) 2 DPS, (Pd) 2 DPX, H 4 DPS, H 4 DPX, H 4 DPB, (Pd)P, (Zn)P, and H 2 P. The rate constants for triplet-triplet energy transfer (k E T ) were measured for the heterobimetallic (PdZn) and monometallic [(M)H 2 ] derivatives (M=Pd, Pt). The fluorescence lifetimes (Δτ F ) of the acceptors decrease as a result of the heavy-atom effect, and vary as follows: (Pd)H 2 DPS << (Pd)H 2 DPX ∼ (Pd)H 2 -DPB. The k E T values calculated according to the equation k E T =(1/τ e m i -1/ τ e m i 0 ), where τ e m i 0 is the emission life-time of the homobimetallic bisporphyrins (no ET occurs), are equal to 0, 247′57 and 133′52 s - 1 for DPS, DPX, and DPB, respectively, in the (Pd)H 2 series. These measurements allowed the range of distance over which the Dexter mechanism for T 1 -T 1 energy transfer ceases to operate to be determined. This distance is somewhere between 4.3 and 6.3 A, in agreement with our recent findings on singlet-singlet energy transfer. During the course of this study, the X-ray crystal structure for (Pd)H 2 DPX was obtained; triclinic (P1), a=11.1016(1), b=14.9868(2), c=20.6786(3) A, α= 102.091(1), β=100.587(1), γ= 101.817(1)°, V=3199.19(7) A 3 , Z=2.