Push–Pull Macrocycles: Donor–Acceptor Compounds with Paired Linearly Conjugated or Cross-Conjugated Pathways

Abstract

Two-dimensional π-systems are of current interest in the design of functional organic molecules, exhibiting unique behavior for applications in organic electronics, single-molecule devices, and sensing. Here we describe the synthesis and characterization of "push-pull macrocycles": electron-rich and electron-poor moieties linked by a pair of (matched) conjugated bridges. We have developed a two-component macrocyclization strategy that allows these structures to be synthesized with efficiencies comparable to acyclic donor-bridge-acceptor systems. Compounds with both cross-conjugated (m-phenylene) and linearly conjugated (2,5-thiophene) bridges have been prepared. As expected, the compounds undergo excitation to locally excited states followed by fluorescence from charge-transfer states. The m-phenylene-based systems exhibit slower charge-recombination rates presumably due to reduced electronic coupling through the cross-conjugated bridges. Interestingly, pairing the linearly conjugated 2,5-thiophene bridges also slows charge recombination. DFT calculations of frontier molecular orbitals show that the direct HOMO-LUMO transition is polarized orthogonal to the axis of charge transfer for these symmetrical macrocyclic architectures, reducing the electronic coupling. We believe the push-pull macrocycle design may be useful in engineering functional frontier molecular orbital symmetries.

AIE Based Coumarin Chromophore - Evaluation and Correlation Between Solvatochromism and Solvent Polarity Parameters

“…A m o r e q u a nt i t a t i v e an a l y s i s o f t h e f l u o r e s c en c e solvatochromism can be carried out using Weller's equation which is modified version of the Lippert − Mataga equation, which allows the excited state dipole moment (μ e ) to be estimated [60,61]. In our case the molecule shows more charge transfer characteristics in the excited state hence the above described theories cannot be applied because the absorption spectrum of the charge transfer state is not known.…”

Intramolecularcharge transfer in donor‐bridge‐acceptor compounds with paired linearly conjugated or cross‐conjugated pathways

“…In fact, in chloroform, the LE state and CT state were observed simultaneously in D(mP)A. The CT state had an emission maximum of ~600 nm …”

“…The results of the study showed that D(mP) 2 A, D(mP)A, and D(Th)A fluoresced from CT states in solvents more polar than cyclohexane. In contrast, D(Th) 2 A was found to fluoresce from a nonpolar locally excited (LE) state in cyclohexane, toluene, and dioxane, and from a CT state in more polar solvents such as chloroform and dichloromethane (DCM) . UV–Vis spectra of the DBA compounds were found to have no solvatochromism in solvents of varying polarity.…”

“…Recent interest in these molecules stems from the apparent ability to control the transfer of charge through the molecules as would be necessary in molecular electronics . In some cases, cross‐conjugated molecules have been shown to be effective at transferring charge despite reduced conjugation and have also been shown to allow for effective CS with slow CR rates . In a study by Thompson et al ., which utilized linearly or cross‐conjugated phenylacetylene bridges, CS times were shown to be faster, while the CR rates were shown to be slower in molecules containing cross‐conjugated bridging groups .…”

“…The synthesis of the asymmetric acyclic molecules D(mP)A and D(Th)A, the cyclic analogs, D(mP) 2 A and D(Th) 2 A, and the symmetric cyclic analogs A(mP) 2 A, D(mP) 2 D, A(Th) 2 A, and D(Th) 2 D (Chart 1) was recently reported . In that study, preliminary characterization was conducted primarily by steady‐state UV–Vis and fluorescence spectroscopy . From that work, Lippert–Mataga plots were constructed for each DBA compound.…”