Solvent-Induced Symmetry Breaking of the Photoinduced Charge Transfer Dynamics in the Bridged Perylene Dimer

The charge transfer dynamics of the bridged perylene dimer were investigated with the recently developed solvent coupling Redfield time propagation model. The results are compared with previous experimental findings to showcase the significance and applicability of the model. The charge transfer dynamics in vacuum showed that no breaking of the charge transfer direction symmetry occurred upon optical excitation, in perfect agreement with the experiment. Meanwhile, attractive solute-solvent interactions facilitated by the dipole moments of the polar solvents were observed to break the charge transfer direction symmetry. The conformational isomerism effect on the transfer dynamics manifested itself by promoting different transport channels upon solvation. Consequently, the solvent coupling Redfield time propagation model was indeed found to be able to quantitatively describe the charge transfer dynamics including exotic phenomena such as symmetry breaking of charge transfer direction.

A Symmetry-Broken Charge-Separated State in the Marcus Inverted Region

We report a long-lived charge-separated state in a chromophoric pair (DC-PDI₂ ) that uniquely integrates the advantages of fundamental processes of photosynthetic reaction centers: i) Symmetry-breaking charge-separation (SB-CS) and ii) Marcus-inverted-region dependence. The near-orthogonal bichromophoric DC-PDI₂ manifests an ultrafast evolution of the SB-CS state with a time constant of τ S B - C S ${{\tau }_{{\rm S}{\rm B}-{\rm C}{\rm S}}}$ =0.35±0.02 ps and a slow charge recombination (CR) kinetics with τ C R ${{\tau }_{{\rm C}{\rm R}}}$ =4.09±0.01 ns in ACN. The rate constant of CR of DC-PDI₂ is 11 686 times slower than SB-CS in ACN, as the CR of the PDI radical ion-pair occurs in the deep inverted region of the Marcus parabola ( - Δ G C R ${{-{\rm \Delta }G}_{{\rm C}{\rm R}}}$ >λ). In contrast, an analogous benzyloxy (BnO)-substituted DC-BPDI₂ showcases a ≈10-fold accelerated CR kinetics with τ C R / τ S B - C S ${{\tau }_{{\rm C}{\rm R}}/{\tau }_{{\rm S}{\rm B}-{\rm C}{\rm S}}}$ lowering to ≈1536 in ACN, by virtue of a decreased CR driving force. The present investigation demonstrates a control of molecular engineering to tune the energetics and kinetics of the SB-CS material, which is essential for next-generation optoelectronic devices.

Merging of a Perylene Moiety Enables a RuII Photosensitizer with Long-Lived Excited States and the Efficient Production of Singlet Oxygen

Multichromophoric systems based on a RuII polypyridine moiety containing an additional organic chromophore are of increasing interest with respect to different light-driven applications. Here, we present the synthesis and detailed characterization of a novel RuII photosensitizer, namely [(tbbpy)2 Ru((2-(perylen-3-yl)-1H-imidazo[4,5-f][1,10]-phenanthrolline))](PF6 )2 RuipPer, that includes a merged perylene dye in the back of the ip ligand. This complex features two emissive excited states as well as a long-lived (8 μs) dark state in acetonitrile solution. Compared to prototype [(bpy)3 Ru]2+ -like complexes, a strongly altered absorption (ϵ=50.3×103  M-1  cm-1 at 467 nm) and emission behavior caused by the introduction of the perylene unit is found. A combination of spectro-electrochemistry and time-resolved spectroscopy was used to elucidate the nature of the excited states. Finally, this photosensitizer was successfully used for the efficient formation of reactive singlet oxygen.

Excited State Charge Separation in an Azobenzene-Bridged Perylenediimide Dimer - Effect of Photochemical Trans-Cis Isomerization

Photoinduced charge transfer and separation events in a newly synthesized azobenzene-bridged perylenediimide-dimer (PDI-dimer) are demonstrated. Trans-to-cis conversion (∼50 % efficiency) from the initial trans PDI-dimer by 355 nm pulsed laser light, and its reversal, cis-to-trans, process by 435 nm laser light irradiation has been possible to accomplish. Efficient fluorescence quenching in the PDI-dimer, more so for the cis isomer was witnessed, and such quenching increased with increasing solvent polarity. DFT-calculated geometry and electronic structures helped in visualizing the charge transfer in the PDI-dimer in both isomeric forms, and also revealed certain degree of participation of the azobenzene entity in the charge transfer events. Femtosecond transient absorption spectral studies confirmed occurrence of both charge transfer followed by charge separation in the studied PDI-dimer in both trans and cis forms in polar solvents, and the evaluated time constants from Global target analysis revealed accelerated events in the cis PDI-dimer due to proximity effects. The present study offers key insights on the role of the azobenzene bridge, and the dimer geometry in governing the excited state charge transfer and separation in symmetrically linked PDI dimer.

Symmetry-Breaking Charge-Transfer Chromophore Interactions Supported by Carbon Nanodots

Carbon dots (CDs) and their derivatives are useful platforms for studying electron-donor/acceptor interactions and dynamics therein. Herein, we couple amorphous CDs with phthalocyanines (Pcs) that act as electron donors with a large extended π-surface and intense absorption across the visible range of the solar spectrum. Investigations of the intercomponent interactions by means of steady-state and pump-probe transient absorption spectroscopy reveal symmetry-breaking charge transfer/separation and recombination dynamics within pairs of phthalocyanines. The CDs facilitate the electronic interactions between the phthalocyanines. Thus, our findings suggest that CDs could be used to support electronic couplings in multichromophoric systems and further increase their applicability in organic electronics, photonics, and artificial photosynthesis.

Change of Quadrupole Moment upon Excitation and Symmetry Breaking in Multibranched Donor-Acceptor Dyes

Upon photoexcitation, a majority of quadrupolar dyes, developed for large two-photon absorption, undergo excited-state symmetry breaking (ES-SB) and behave as dipolar molecules. We investigate how the change of quadrupole moment upon S₁ ←S₀ excitation, ΔQ, influences the propensity of a dye to undergo ES-SB using a series of molecules with a A-π-D-π-A motif where D is the exceptionally electron-rich pyrrolo[3,2-b]pyrrole and A are accepting groups. Tuning of ΔQ is achieved by appending a secondary acceptor group, A', on both sides of the D core and ES-SB is monitored using a combination of time-resolved IR and broadband fluorescence spectroscopy. The results reveal a clear correlation between ΔQ and the tendency to undergo ES-SB. When A is a stronger acceptor than A', ES-SB occurs already in non-dipolar but quadrupolar solvents. When A and A' are identical, ES-SB is only partial even in highly dipolar solvents. When A is a weaker acceptor than A', the orientation of ΔQ changes, ES-SB is observed in dipolar solvents only and involves major redistribution of the excitation over the D-π-A and D-A' branches of the dye.