Does Location of BF₂-Chelated Dipyrromethene (BODIPY) Ring Functionalization Affect Spectral and Electron Transfer Properties? Studies on α-, β-, and Meso-Functionalized BODIPY-Derived Donor–Acceptor Dyads and Triads

Abstract

The effect of positioning an electron donor, ferrocene (Fc), and a charge transfer complex, Fc–tetracyanobutadine (TCBD), at different locations of the BF2-chelated dipyrromethene (BODIPY) ring on governing excited-state charge separation is reported. For this, BODIPY was functionalized at the meso, α-, or β-pyrrole positions with an acetylene spacer carrying either an Fc or an Fc–TBCD charge transfer complex. Among the meso-, α-, or β-pyrrole-derivatized BODIPYs, E 0,0 and the Stokes shift were found to depend upon the position of BODIPY ring functionalization, independent of polarity. The Stokes shift followed the order β > meso > α-substitution for a given series of BODIPY derivatives, while E 0,0 followed the order meso > α- > β-substitution. Using a combination of Pd-catalyzed Sonogashira cross-coupling reaction and [2 + 2] cycloaddition–retroelectrocyclization reaction, involving tetracyano ethylene to introduce TCBD between the BODIPY and Fc entities was proposed and followed. From the newly established energy level diagram using spectral, computational, and electrochemical results, formation of BODIPY•‑–Fc+ in the case of dyads and (BODIPY–TCBD)•‑–Fc+ in the case of triads from 1BODIPY* was possible to arrive. Femtosecond transient absorption studies followed by data analysis through the target analysis confirmed this to be the case. Importantly, in the case of BODIPY–Fc dyads, α-pyrrole-functionalized derivatives performed better in terms of stabilizing the charge-separated state, while in the case of BODIPY–TCBD–Fc triads, stabilization of (BODIPY–TCBD)•‑–Fc+ for β-pyrrole-functionalized derivatives was better. The present findings on spectral and photochemical properties in differently functionalized BODIPYs are important not only for light energy harvesting but also in designing the next generation of BODIPY-based fluorescence probes and sensors.

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

“…From population kinetics data, the average lifetimes for T 1 and CS were found to be 11.55 and 3.10 ns. Lifetime for the CS state was generally higher than that reported for other donor-TCBD systems by us and others − wherein singlet excited-state species were involved in promoting CS events. A similar trend representing S 1 → T 1 → CS processes was observed in the case of PTS4 (see Figure b), wherein the SAS spectra closely resembled that expected for the given species.…”

“…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. − …”

Synthesis and Photophysical Properties of 1,1,4,4‐Tetracyanobutadienes Derived from Ynamides Bearing Fluorophores**

“…[14][15][16][17][18] In contrast, the luminescence properties of TCBD derivatives have barely been explored as they are generally considered non-emissive due to intrinsically rapid nonradiative deactivations. [19] However, some examples of luminescent TCBDs in solution or in the solid state have recently emerged, [20][21][22][23][24][25][26][27][28][29][30][31][32][33][34] although often with very low quantum yields at room temperature that hardly exceed 1 %, except for a work from Jayamurugan et al [27] where the authors claimed quantum yields up to 4.3 % at room temperature in acetonitrile. We also reported TCBDs bearing triphenylamine and fluorenyl units that showed quantum yields comprised between 2 and 6 % in rigid media at room temperature.…”