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

Abstract

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.

Dimethylaniline-Tetracyanobutadiene and Dimethylaniline-Extended-Tetracyanobutadiene Functionalized BODIPYs Witnessing Ultrafast Charge Transfer

“…π-Conjugated push–pull molecular systems have gained considerable interest in the scientific community due to their tunable optoelectronic properties and wide range of applications such as in organic field-effect transistors, nonlinear optics, photovoltaic devices, energy harvesting, bioimaging, sensors, and organic light-emitting diodes. − The optoelectronic properties and HOMO–LUMO gap of the π-conjugated chromophores can be altered by modifying the donor (D) or acceptor (A) unit or the π-linker between the D and A units. , 4,4-Difluoro-4-bora-3a,4a-diaza- s -indacene dyes, frequently known as BODIPY, have attracted a lot of attention in the last few decades because of their excellent chemical and photophysical properties including strong absorption, high fluorescence quantum yields and molar absorption coefficients, low toxicity, long fluorescence lifetime, as well as good photochemical stability. − The optoelectronic characteristics of the BODIPYs can be tuned by expanding the conjugation and incorporation of appropriate D/A groups at the meso and pyrrolic locations (α and β positions). − As a result, the BODIPY-based π-conjugated dyes have been extensively used in building fluorescent imaging agents, molecular switches, chemosensors, laser dyes, light-emitting devices, photosensitizers, and optoelectronic materials. − Noticeably, depending upon the peripheral substituents, the BODIPY unit can act as an electron donor or acceptor. − …”

Excited Charge Separation in a π-Interacting Phenothiazine–Zinc Porphyrin–Fullerene Donor–Acceptor Conjugate

“…The photodynamics of D–A systems revealing charge-separated species of appreciable lifetimes is not only useful for understanding the initial events of photosynthesis but also useful in the design of efficient materials for optoelectronic applications. , Quite a few molecular or supramolecular systems, such as dyads, triads, tetrads, etc., have been elegantly designed and studied to witness long-lived charge separation mimicking “antenna-reaction center” events of photosynthesis. − Of the various scaffolds used for building D–A systems, porphyrins, structurally close to the natural photosynthetic chlorophyll pigment and due to well-established synthetic protocols, have conquered this area of research. Many groups, including ours, have reported multimodular systems by introducing secondary donors/acceptors on the porphyrin macrocycle to achieve sequential electron and hole transfer, resulting in the generation of long-lived charge-separated species. ,− For instance, Tian and coworkers have developed zinc porphyrin–viologen (ZnP–V) nanoparticles in an aqueous solution that showed a charge separation (CS) state lifetime of up to 4.3 ms .…”

“…Many groups, including ours, have reported multimodular systems by introducing secondary donors/acceptors on the porphyrin macrocycle to achieve sequential electron and hole transfer, resulting in the generation of long-lived charge-separated species. ,− For instance, Tian and coworkers have developed zinc porphyrin–viologen (ZnP–V) nanoparticles in an aqueous solution that showed a charge separation (CS) state lifetime of up to 4.3 ms . This can be recognized as charge hopping induced by aggregation or distance modification between the donor and acceptor induced by electronic interaction . D’Souza and coworkers have developed bis-benzimidazole-fused porphyrin hetero and homo dimers in which the axial position of zinc porphyrin is coordinated by phenyl imidazole-functionalized C 60 .…”