Charge‐Transfer in Panchromatic Porphyrin‐Tetracyanobuta‐1,3‐Diene‐Donor Conjugates: Switching the Role of Porphyrin in the Charge Separation Process

Abstract

Using a combination of cycloaddition-retroelectrocyclization reaction, free-base and zinc porphyrins (H 2 P and ZnP) are decorated at their β-pyrrole positions with strong charge transfer complexes, viz., tetracyanobuta-1,3-diene (TCBD)-phenothiazine (3 and 4) or TCBD-aniline (7 and 8), novel class of push-pull systems. The physico-chemical properties of these compounds (MP-Donor and MP-TCBD-Donor) have been investigated using a range of electrochemical, spectroelectrochemical, DFT as well as steady-state and time-resolved spectroscopic techniques. Ground-state charge transfer interactions between the porphyrin and the electron-withdrawing TCBD directly attached to the porphyrin π-system extended the absorption features well into the near-infrared region. To visualize the photo-events, energy level diagrams with the help of free-energy calculations have been established. Switching the role of porphyrin from the initial electron acceptor to electron donor was possible to envision. Occurrence of photoinduced charge separation has been established by complementary transient absorption spectral studies followed by global and target data analyses. Better charge stabilization in H 2 P derived over ZnP derived conjugates, and in phenothiazine derived over aniline derived conjugates has been possible to establish. These findings highlight the importance of the nature of porphyrins and second electron donor in governing the ground and excited state charge transfer events in closely positioned donoracceptor conjugates.

Excitation Wavelength-Dependent Charge Stabilization in Highly Interacting Phenothiazine Sulfone-Derived Donor–Acceptor Constructs

“…It may be mentioned here that the spectra associated with the charge-separated state appear slightly different due to different excited-state species produced at different wavelengths. These lifetime values are comparable to the lifetime of other donor-TCBD and donor-exTCBD systems (donor = electron-rich photosensitizer), reported earlier, − all originating from their respective singlet excited states.…”

“…The TCBD and exTCBD-based directly connected DA push–pull systems , show strong ICT covering absorption in the visible and near IR regions, that is, exhibiting optical properties of black absorbers. Photosensitizers such as porphyrins, phthalocyanines, subphthalocyanines, BODIPYs, azaBODIPYs, triphenylamine, phenothiazine, and diketopyrrolopyrroles have been used in these constructions to extend the optical coverage into the visible and near-IR regions. − Sadly, owing to close proximity and high exergonicity, the excited state CS and CR in these systems occurred within a few picoseconds. − Extending the lifetime of the CSS in these systems thus far has been a challenge. In the present study, we have overcome this issue by designing multimodular DA systems derived from a bis-phenothiazine-phenothiazine sulfone (PTZ-PTZSO 2 -PTZ, PTS2 in Figure ) scaffold.…”

Star-Shaped Triphenylamine–Tetracyanobutadiene–Phenothiazine Push–Pull Systems: Role of Terminal Phenothiazine in Improving Charge Transfer

“…The strong push–pull effects result in electron polarization (also often termed as ground-state charge transfer), resulting in a new low-energy optical transition extending the optical coverage. Making use of the facile reaction and optical properties, they have extensively explored building TCBD-substituted push–pull systems − and polymers as promising materials for photovoltaic applications. − Our groups have explored a wide variety of donor-functionalized TCBD-based molecular systems exhibiting novel optical and excited-state properties including ultrafast charge transfer and intervalence charge transfer properties. − …”

β-Tetracyanobutadiene-Appended Porphyrins: Facile Synthesis, Spectral and Electrochemical Redox Properties, and Their Utilization as Excellent Optical Limiters

“…29,30 Indeed, Misra and coworkers have reported the synthesis of β-functionalized push− pull porphyrins possessing charge-transfer interaction (TCBDphenothiazine or TCBD-aniline) and studied the role of porphyrin in charge-separation processes. 31 Our design encompasses placing TCBD units at the β-position adjacent to phenylethynyl groups and the nitro moiety at the antipodal β-position, which provides extended π-conjugation.…”

“…In general, tetracyanoethylene (TCNE) acts as a powerful dienophile for electron-rich alkenes and alkynes and form donor–acceptor (D–A)-appended π-conjugated chromophores, which possess desirable properties for electrical conductivity and NLO, and electron transfer salts of TCNE exhibit magnetic properties like ferromagnetism. − Given that the porphyrin core is electron-rich and the 1,1,4,4-tetracyano-buta-1,3-diene (TCBD) unit is electron-deficient, it seems that linking them should lead to molecules possessing desirable NLO properties. In 2012, Gryko and co-workers have reported the reaction of TCNE at the meso-position of the porphyrin macrocycle, and the NLO properties and the self-assembly behavior of meso -TCNE-appended porphyrins are explored by Gao and co-workers. , Indeed, Misra and co-workers have reported the synthesis of β-functionalized push–pull porphyrins possessing charge-transfer interaction (TCBD-phenothiazine or TCBD-aniline) and studied the role of porphyrin in charge-separation processes . Our design encompasses placing TCBD units at the β-position adjacent to phenylethynyl groups and the nitro moiety at the antipodal β-position, which provides extended π-conjugation.…”