Excited-State Symmetry Breaking in Cofacial and Linear Dimers of a Green Perylenediimide Chlorophyll Analogue Leading to Ultrafast Charge Separation

Abstract

Photoexcitation of chromophoric dimers constrained to a symmetric pi-stacked geometry by their molecular structure usually produces excimers independent of solvent polarity, while dimers with edge-to-edge perpendicular pi systems undergo excited-state symmetry breaking in highly polar solvents leading to intradimer charge separation. We present direct evidence for symmetry breaking in the lowest excited singlet state of a symmetric cofacial dimer of 1,7-bis(pyrrolidin-1'-yl)-perylene-3,4:9,10-bis(dicarboximide) (5PDI) in the low polarity solvent toluene to produce a radical ion pair quantitatively. This dimer, cof-5PDI2, was synthesized by attaching two 5PDI chromophores via imide groups to a xanthene spacer. For comparison, a linear symmetric dimer, lin-5PDI2, was prepared in which the 5PDI chromophores are linked end-to-end via a N-N single bond between their imides. The edge-to-edge pi systems of the 5PDI chromophores within lin-5PDI2 are perpendicular to one another. Ground-state absorption spectra of both 5PDI dimers show exciton coupling, which is consistent with the orientation of the 5PDI chromophores relative to one another. Ultrafast transient absorption spectroscopy following excitation of the dimers with 700 nm, 100 fs laser pulses shows that quantitative intradimer electron transfer occurs in cof-5PDI2 in toluene with tau = 0.17 ps followed by charge recombination to the ground state with tau = 222 ps. Similar measurements on lin-5PDI2 reveal that photoinduced electron transfer does not occur in toluene, but occurs in more polar solvents such as 2-methyltetrahydrofuran, wherein tau = 55 ps for charge separation and tau = 99 ps for charge recombination. Excited-state symmetry breaking in 5PDI dimers provides new routes to biomimetic charge separation and storage assemblies that can be more easily prepared and modified than those based on multiple tetrapyrrole macrocycles.

Approaches for biological and biomimetic energy conversion

“…Although some of the materials are self-organized through chemical interactions to produce larger scale arrays, others are organized on surfaces by using techniques such as Langmuir-Blodgett films or synthesized by assembling encapsulated materials into or onto host-guest inorganic matrices. Wasielewski et al (47,84) recently reported that xanthene-linked green chromophores tetrafunctionalized with perylene-bis(dicarboximide) were highly efficient at both light harvesting and ultrafast charge separation. The researchers attributed this efficiency to the molecules' ability to self-assemble into stacked arrays with defined orientations between adjacent planar aromatic groups (47,84).…”

“…Wasielewski et al (47,84) recently reported that xanthene-linked green chromophores tetrafunctionalized with perylene-bis(dicarboximide) were highly efficient at both light harvesting and ultrafast charge separation. The researchers attributed this efficiency to the molecules' ability to self-assemble into stacked arrays with defined orientations between adjacent planar aromatic groups (47,84). Self-assembled monolayers of carotenoids and chlorophylls on titania surfaces showed long-lived photoinduced charge separation due to a reductive quenching of the pheophytin molecules upon pheopytin excitation at 670 nm (81).…”

Characterization of New Materials for Photovoltaic Thin Films: Aggregation Phenomena in Self-Assembled Perylene-Based Diimides

“…The fact that the behavior observed is identical for the two geometries shown above indicates that effect is not likely due to field-induced charge carrier generation and separation as is often described in similar experiments. 4 To further explore the origins of this effect, we have also performed power-dependent and field-dependent studies. Figure 11 shows the relative fluorescence signal (field on relative to field off) to be independent of incident power, indicating these effects do not arise from any nonlinear photoinduced processes.…”

“…tension of the air-water interface from 71.0 mN/m to 65. 4 X-ray scattering data obtained in studies of the C 12 -mN/m at a concentration of 100/uM, as determined by the PDI+/PA-composite thin films are shown in Figure 2. capillary rise method.…”

“…In the past, thin film deposition has usually involved energy/equipment intensive methods like vapor deposition, '4'36 or solution-based methods such as spin casting or drop casting,' 1 " 7 ' 18 self-assembly,1 9 22 Langmuir-Blodgett techniques, 23 or electrochemical methods. 4 The infrastructure required for vapor deposition precludes use of such methods for preparing large-area photovoltaics. Solvent casting is better in this respect,…”