Reversible Symmetry-Breaking Charge Separation in a Series of Perylenediimide Cyclophanes

Abstract

Excited-state symmetry-breaking charge separation (SB-CS) can offer an efficient pathway to solar energy capture and conversion. We synthesized a series of 1,6,7,12-tetrakis­(4-t-butylphenoxy)­perylene­(3,4:9,10)­bis­(dicarboximide) cyclophane dimers with m-xylylene, p-xylylene, and 4,4′-diyldimethane-1,1′-biphenyl spacers and studied them with steady-state and time-resolved optical spectroscopies. Photoinduced SB-CS occurs in all three cyclophanes in CH2Cl2, with the SB-CS rate decreasing as the interchromophore distance is increased. Time-resolved emission spectroscopy and kinetic modeling reveal that the charge-separated state exists in pseudoequilibrium with the excited state prior to decay. Notably, the meta-spaced cyclophane also undergoes SB-CS in toluene within ∼100 ps, despite the lack of charge stabilization by the low dielectric constant solvent. These results demonstrate that SB-CS can occur across long distances and in weakly polar environments, which offers the possibility of harnessing SB-CS for solar energy capture and conversion.

Linker dependent symmetry breaking charge separation in 9,10-bis(phenylethynyl)anthracene dimers

“…CS state formation involving molecular distortions can accelerate the nonradiative decay of the singlet state and lead to a short-lived CS state. As expected, in many cases, 12,13,18,20,40 the CS state is not emissive and shows a short lifetime. Furthermore, the recombination rate of the CS state becomes faster and faster with the increase in the polarity of the solvent.…”

“…It has been found that the SB-CS rate and the recombination rate is strongly sensitive to the electronic coupling between the adjacent molecular units that is determined by the relative inter-/intra-molecular distance and orientation. 13,18–21 For example, Wasielewski group investigated the SB-CS process in cofacial and linear perylene diimide (PDI) dimers and found that ultrafast SB-CS (1/0.52 ps −1 ) could be conducted in cofacial PDI dimer even in weakly polar toluene but with a fast recombination rate (1/222 ps −1 ). 22 Instead, SB-CS cannot proceed in linear PDI dimer in toluene but only in polar solvents.…”

“…Recently, the same group studied the SB-CS process in a series of cofacial PDI dimers with different intramolecular distances and found that the SB-CS rate is decreased from 1/140 ps −1 to 1/827 ps −1 as the inter-chromophore distance is increased. 20 Additionally, Guo et al synthesized two isomeric N -annulated perylene diimide dimers linked via a phenyl ring at the para - ( p -BDNP) and meta - ( m -BDNP) substitutions and studied their SB-CS process comparatively. They discovered that the SB-CS rate in p -BDNP is an order of magnitude faster than that in m -BDNP due to the stronger electronic coupling in p -BDNP.…”

Perspectives in Dye Chemistry: A Rational Approach toward Functional Materials by Understanding the Aggregate State

“…121 Such SBCS relaxation processes have indeed been unambiguously proven by the detection of both the PBI radical anion and cation species in transient absorption spectra for the examples shown in Figure 10, with the spacers depicted in green. 122,123 These examples show that a direct π−π contact is not necessarily required and maybe even not favorable because closely π-stacked photoexcited PBI dyes have an increased tendency to relax into excimer states. 118 Because the latter process is typically accompanied by structural relaxation, the SBCS process appears to be most efficient in rigid structures such as cyclophanes that prohibit the deep trapping of the excimer species by structural relaxation.…”

Porous shape-persistent rylene imine cages with tunable optoelectronic properties and delayed fluorescence

“…Since oxidation of 1c or 3c is not reversible, we could not measure the absorption spectrum of the corresponding PDI˙ + . Nevertheless, PDI˙ + generated from related PDI derivatives 33,34 possess an absorption that matches the observed spectroscopic signature of the generated transient. Hong et al 34 a attributed a similar transient feature at 615 nm to the ME state.…”