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

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, E0,0 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 E0,0 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.