Modulation of the Singlet−Singlet Through-Space Energy Transfer Rates in Cofacial Bisporphyrin and Porphyrin−Corrole Dyads

A new series of relatively flexible cofacial donor−acceptor dyads for singlet−singlet energy transfer with the corrole
or etio-porphyrin free base and zinc porphyrin as the acceptor and donor, respectively, were synthesized and
characterized (represented as (PMes2COx)ZnH3 (13), (PMes2CO)ZnH3 (14), and (PMes2CX)ZnH3 (15)) where (PMes2COx = [2-[5-(5,15-dimesitylcorrol-10-yl)-diphenylether-2‘-yl]-13,17-diethyl-2,3,7,8,12,18-hexamethylporphyrin]), (PMes2CO = [5-[5-(5,15-dimesitylcorrol-10-yl)-dibenzofuran-4-yl]-13,17-diethyl-2,3,7,8,12,18-hexamethylporphyrin]), and
(PMes2CX = [5-[5-(5,15-dimesitylcorrol-10-yl)-9,9-dimethylxanthen-4-yl)]-13,17-diethyl-2,3,7,8,12,18-hexamethylporphyrin]), respectively) along with the homobismacrocycles (DPOx)ZnH2 (17) and (DPOx)Zn2 (18) (where (DPOx =
2,2‘-bis[5-(2,8,13,17-tetraethyl-3,7,12,18-tetramethylporphyrinyl)]diphenylether) as comparison standards. The rate
for energy transfer (kET) extracted by the measurements of fluorescence lifetimes are of the same order of magnitude
as those recently reported for the rigidly held face-to-face dyads ((DPB)ZnH2 (1), (DPX)ZnH2 (2), (DPA)ZnH2 (3),
(DPO)ZnH2 (4), and (DPS)ZnH2 (5) where (DPB = 1,8-bis[5-(2,8,13,17-tetraethyl-3,7,12,18-tetramethylporphyrinyl)]biphenylene), (DPX = 4,5-bis[5-(2,8,13,17-tetraethyl-3,7,12,18-tetramethylporphyrinyl)]-9,9-dimethylxanthene), (DPA
= 1,8-bis[5-(2,8,13,17-tetraethyl-3,7,12,18-tetramethylporphyrinyl)]anthracene), (DPO = 4,6-bis[5-(2,8,13,17-tetraethyl-3,7,12,18-tetramethylporphyrinyl)]dibenzofuran), and (DPS = 4,6-bis[5-(2,8,13,17-tetraethyl-3,7,12,18-tetramethylporphyrinyl)]dibenzothiophene), respectively), but for the first time, it is shown that the presence of a bulky group
located between the acceptor and the donor moiety influences the transfer rate. The presence of perpendicular
mesityl groups on the acceptor macrocycle prevents the two macrorings from interacting strongly; therefore, kET is
slower. On the other hand, by rendering the rigid spacer flexible (i.e., changing the dibenzofuran rigid spacer to the
flexible diphenylether assembling fragment), kET increases due to stronger intermacrocycle interactions. This study
is complemented by DFT computations (B3LYP/3-21G*) as a molecular modeling tool where subtle structural features
explain the changes in kET. During the course of this study, X-ray structures of 17 and 18 were investigated and
exhibit a linear stacking of the bismacrocycles where intermolecular porphyrin−porphyrin interactions are observed
(dinter(Zn···Zn) = 4.66 and 4.57 Å, for 17 and 18, respectively).