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

Eleven new complexes, including mono-, heterobi-, and homobimetallic cofacial bisporphyrins, (Pd)H2DPS, (M)H2DPX, (M)H2DPB, (PdZn)DPS, (PdZn)DPX, (Pt)(2)DPX, (M)(2)DPB (M=Pd, Pt), and (Pt)P (DPS4- = 4,6-bis[5-(2,8,13,17-tetraethyl-3,7,12,18-tetramethylporphyrinyl)]di- benzothiophene tetraanion, DPX4-= 4,5-bis[5-(2,8,13,17-tetraethyl-3,7,12,18-tetramethylporph\lrinvl)]-9,9-dimethyl-xanthene tetraanion, DPB4- = 1,8-bis[5(2,8,13,17-tetraethyl-3,7,12,18-tetrame- thylporphyrinyl)]biphenylene tetraanion, P2-=5-phenyl-2,8,13,17-tetraethvl-3,7,12,18-tetramethylporphyin 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 H2P or (Zn)P) depend on the C-meso - C-meso distance and the presence of a heavy atom such as Pd-II or Pt-II. The data were compared with those for the known compounds (Pd)(2)DPS, (Pd)(2)DPX, H4DPS, H4DPX, H4DPB, (Pd)P, (Zn)P, and H2P The rate constants for triplet-triplet energy transfer (k(ET)) were measured for the heterobimetallic (PdZn) and monometallic [(M)H-2] derivatives (M=Pd, Pt). The fluorescence lifetimes (Delta tau(F)) of the acceptors decrease as a result of the heavy-atom effect, and vary as follows: (Pd)H2DPS < (Pd)H2DPX-(Pd)H2DPB. The k(ET) values calculated according to the equation k(ET) = (1/tau(emi) - 1/tau(0)(emi)), where tau(0)(emi) is the emission lifetime 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,DPX was obtained; triclinic (P1), a = 11.1016(1), b = 14.9868(2), c = 20.6786(3) A, alpha = 102.091(1), beta =100.587(1), gamma 101.817(1)degrees, V = 3199.19(7) A(3), Z = 2.