Symmetry breaking charge transfer leading to charge separation in a far-red absorbing bisstyryl-BODIPY dimer.

Symmetry breaking charge transfer is one of the important photo-events occurring in photosynthetic reaction centers that is responsible for initiating electron transfer leading to a long-lived charge-separated state and has been successfully employed in light-to-electricity converting optoelectronic devices. In the present study, we report a newly synthesized, far-red absorbing and emitting BODIPY-dimer to undergo symmetry-breaking charge transfer leading to charge-separated states of appreciable lifetimes in polar solvents. Compared to its monomer analog, both steady-state and time-resolved fluorescence originating from the S1 state of the dimer revealed quenching which increased with an increase in solvent polarity. The electrostatic potential map from DFT and the time-dependent DFT calculations suggested the existence of a quadrupolar type charge transfer state in polar solvents, and the singlet excited state to be involved in the charge separation process. The electrochemically determined redox gap being smaller than the energy of the S1 state supported the thermodynamic feasibility of the envisioned symmetry-breaking charge transfer and separation. The spectrum of the charge-separated state arrived from spectroelectrochemical studies, revealing diagnostic peaks helpful for transient spectral interpretation. Finally, ultrafast transient pump-probe spectroscopy provided conclusive evidence of diabatic charge separation in polar solvents by far-red pulsed laser light irradiation. The measured lifetime of the final charge-separated states was found to be 165 ps in dichlorobenzene, 140 ps in benzonitrile, and 43 ps in dimethyl sulfoxide, revealing their significance in light energy harvesting, especially from the less-explored far-red region.

Excited-State Charge Transfer in Push-Pull Platinum(II) π-Extended Porphyrins Fused with Pentacenequinone.

Platinum(II) π-extended porphyrins fused with pentacenequinone and dihydropentacene have been successfully synthesized. These porphyrins were investigated using various techniques including absorption, steady-state, and time-resolved phosphorescence spectroscopy and differential pulse voltammetry. UV-vis absorption spectra of pentacenequinone-fused porphyrins (SW-Pt1 and SW-Pt2) showed unusually broad and nontypical absorption patterns. Phosphorescence spectra of SW-Pt1, SW-Pt2, and SW-Pt3 displayed similar emissions in the 704-706 nm region indicating electronic transitions of similar origin; however, the triplet lifetimes were found to be quenched in the case of both SW-Pt1 and SW-Pt2, suggesting the occurrence of excited-state events. Facile reductions were obtained for both the pentacene-quinone-fused monomer (SW-Pt2) and dimer (SW-Pt1) and were identified to be located at the pentacenequinone components. The observed orbital segregations for SW-Pt2 and SW-Pt1 from DFT calculations supported the possibility of charge transfer in these push-pull systems. Interestingly, the established energy level diagram revealed that the charge transfer from the triplet excited Pt porphyrin is thermodynamically an uphill process. Systematic studies involving both femtosecond and nanosecond transient absorption techniques revealed that the singlet excited Pt porphyrins undergo an intermediate charge transfer state prior to populating the triplet state, providing a plausible explanation for phosphorescence quenching. The lifetime of the intermediate charge transfer states was found to be 25.9 and 5.68 ps, respectively, for SW-Pt1 and SW-Pt2.

Intramolecular Photoinduced Energy and Electron Transfer Reactions in Phenanthroimidazole-Boron Dipyromethane Donor-Acceptor Dyads.

Donor-acceptor systems in which a donor phenanthroimidazole (PhI) is directly connected to a BODIPY acceptor (Dyad1) and separated by an ethynyl bridge between PhI and BODIPY (Dyad2) have been designed, synthesized, and characterized by various spectroscopic and electrochemical techniques. Optical absorption and 1H NMR characteristics of both dyads with those of constituent individuals suggest that there exists a minimum π-π interaction between phenanthroimidazole and BODIPY. Quenched emission of both the dyads was observed when excited either at phenthaoimidazole absorption maxima or at BODIPY absorption maxima in all three investigated solvents. The detailed spectral analysis provided evidence for an intramolecular photoinduced excitation energy transfer (PEnT) from the singlet excited state of phenanthroimidazole to BODIPY and photoinduced electron transfer (PET) from the ground state of phenanthroimidazole to BODIPY. Transient absorption studies suggest that charge-separated species (PhI•+ - BODIPY•-) are generated at a rate constant of (1.16 ± 0.01) × 108 s-1 for the dyads Dyad1 and (5.15 ± 0.03) × 108 s-1 and for Dyad2 whereas energy transfer rate constants were much higher and were on the order of (1.1 ± 0.02) × 1010 s-1 and (1.6 ± 0.02) × 1010 s-1 for Dyad1 and Dyad2, respectively, signifying their usefulness in light energy harvesting applications.

π-Extended Pyrazinepyrene-Fused Zinc Phthalocyanines: Synthesis and Excited-State Charge Separation Involving Coordinated C60.

A series of pyrazinepyrene-fused zinc phthalocyanines (ZnPc-Pyrn) have been newly synthesized by reacting quinoxaline and the corresponding diamino-functionalized phthalocyanines as a new class of π-extended phthalocyanine systems. Bathochromically shifted absorption as a function of the number of pyrazinepyrene entities due to extended π-conjugation and quenched fluorescence due to the presence of fused pyrazinepyrene were witnessed. The electronic structures of these phthalocyanines were probed by systematic computational and electrochemical studies, while the excited-state properties were examined by pump-probe spectroscopies operating at the femto- and nanosecond time scales. Similar to the excited singlet lifetimes, the excited triplet states also revealed diminished lifetimes with an increased number of pyrazinepyrene entities. Further, the coordinatively unsaturated zinc in these molecules was coordinated with phenyl imidazole-functionalized fullerene, ImC60, to form a new series of donor-acceptor conjugates. Upon full characterization of these conjugates, the occurrence of excited-state charge separation was established by transient pump-probe spectroscopy, covering wide temporal and spatial regions. The lifetime of the final charge-separated states was ∼2 ns and decreased with an increase in the number of fused pyrazinepyrene units.