Through space singlet-singlet and triplet-triplet energy transfers in cofacial bisporphyrins held by the carbazoyl spacer.

The through space singlet-singlet and triplet-triplet energy transfers in cofacial bis(etio-porphyrins) rigidly held by the carbazoyl spacer were investigated. The studies on singlet-singlet transfer, which operates via a Förster mechanism, were performed using the zinc porphyrin and free base chromophores as energy donor and acceptor, respectively, while the investigation on triplet-triplet processes was performed using the palladium porphyrin, and the zinc porphyrin and free base chromophores as donor and acceptors, respectively. The rate for singlet-singlet transfer (kET(singlet)) is unexpectedly slower than that reported for other similar, rigidly held bisporphyrins such as H2(DPO)Zn(DPO = 4,6-bis[5-(2,8,13,17-tetraethyl-3,7,12,18-tetramethylporphyrinyl)]dibenzofuran) and H2(DPS)Zn(DPS = 4,6-bis[5-(2,8,13,17-tetraethyl-3,7,12,18-tetramethyl-porphyrinyl)]dibenzothiophene)). This slower rate is interpreted by the presence of the H-atom exactly located between the two meso-carbons in the dyads. The rates for triplet-triplet transfer are also slow but not too different from that recently reported for H2(DPX)Pd (DPX = 4,5-bis[5-(2,8,13,17-tetraethyl-3,7,12,18-tetramethylporphyrinyl)]-9,9-dimethylxanthene) and H2(DPB)Pd (DPB = 1,8-bis[5-(2,8,13,17-tetraethyl-3,7,12,18-tetramethylporphyrinyl)]biphenylene) at 77 K. In such cases, the slow rate is interpreted by a through space energy transfer pathway which operates according to a (small range) Dexter mechanism (since the Förster mechanism is inoperative for triplet-triplet processes). The kET(triplet) increases at 298 K which is tentatively interpreted by favorable excited state distortions in the triplet state and fluxion processes which ease intramolecular transfers for these dyads in fluid solution in comparison with former glassy matrices.