Synthesis, Characterization, and Photophysical Properties of a Free Base and a Biszinc(II) Complex of 1,3-Bisporphyrincalix[4]arene:  Evidence for “Tunable Intramolecular Open and Closed Conformations”

Abstract

The bismacrocycle 5,17-bis[5-(2,8,13,17-tetraethyl-3,7,12,18-tetramethylporphyrinyl)]-25,26,27,28-tetrapropoxycalix[4]arene (4) was synthesized in three steps from the corresponding bisaldehyde 5,17-diformyl-25,26,27,28-tetrapropoxycalix[4]arene. The biszinc(II) complex (5) was prepared as well, and the photophysical properties were measured using 2-MeTHF as solvent at 298 and 77 K. While computer modeling for 5 predicts that both pinched cone conformers, closed (porphyrins near each other) and open (porphyrins away from each other), may exist in the "gas phase", the experimental data indicate clearly that no zinc porphyrin...zinc porphyrin interactions are present in solution at 298 K, favoring the open conformer, where the two macrocycles are placed away from each other. On the other hand, clear evidence for a closed conformer is observed at 77 K. Variable-temperature (1)H NMR experiments show that 5 is fluxional between 298 and 183 K, while 4 keeps the open conformation for the whole temperature range. This behavior is unprecedented for calix[4]areneporphyrin compounds, and the relative porphyrin ring rigidity is postulated to explain this difference.

Triplet–Triplet Energy Transfer Controlled by the Donor–Acceptor Distance in Rigidly Held Palladium‐Containing Cofacial Bisporphyrins

“…We are currently developing flexible spacers such as calix [4]arenes [69][70][71][72][73][74][75][76] that would respond to stimuli either bringing the donor and the acceptor closer together leading to a quenching of the donor emission, or separating them rendering the molecular device luminescent. In this way, an "onoff" molecular device can be designed to take advantage of the energy-transfer processes either in the singlet states for lighter-element systems or in the triplet states for heavieratom-containing molecules.…”

Comments on the through-space singlet energy transfers and energy migration (exciton) in the light harvesting systems

“…Over the past 7 years, our research groups investigated a large series of cofacial bisporphyrin dyads along with the corresponding homo-bimetallic models [23][24][25][26][27][28][29][30][31][32][33][34][35]. Most of these compounds were especially designed for through space singlet-singlet (S 1 ) and triplet-triplet (T 1 ) energy transfer (ET) investigations (Scheme 1) [23][24][25][26][27][28][29][30][31][32][33][34][35].…”

Evidence for Dual Pathway in Through‐Space Singlet Energy Transfers in Flexible Cofacial Bisporphyrin Dyads

“…So far, species containing identical chromophores have been used for redox induced tuning of the porphyrins spacing in the pacman (cofacial) arrangement, [10,11] or for thermal geometry tuning in a slipped dimer arrangement. [12] As suggested by recent reports using resorcinarenes, [14] or calix [4]arenes, [15] there is much to expect Abstract: Flexible "pacman" scaffolds built upon a calix [4]arene platform bearing a [18]crown-6 ether and either two OH functions or two OPr groups at the lower rim have been used to generate donor-acceptor (D-A) dyads incorporating a zinc-porphyrin donor and a free-base porphyrin acceptor. Through-space singlet energy transfer (SET) in the D-A dyads was studied by time-resolved fluorescence spectroscopy.…”

“…Such behaviour is known for cofacial free-base bisporphyrins similar to 4 H 4 -8 H 4 ( Figure 6). [3,4,12] The short lifetime of the ZnP donor in the dyad 1 ZnH 2 is remarkable, because it suggests that a very efficient SET still occurs in a medium that favours the formation of hydrogen bonds and forces an open cone conformation with a C meso-meso distance of about 10 or more. This unexpectedly fast SET triggered a more detailed comparison with a set of reported dyads.…”