Broad-Band-Capturing Tetracyanobutadiene-Incorporated Phenothiazine-<i>aza</i>BODIPY Push–Pull Systems: Excited State Charge Separation and Relaxation Dynamics
Ajyal Z. Alsaleh, Dilip Pinjari, Somnath Das, Rajneesh Misra, Francis D’Souza
Abstract
Far-red/near-infrared (NIR) absorbing donor–acceptor (D-A) based π-conjugated organic sensitizers are at the forefront of energy research owing to their novel charge transfer (CT) processes for technological advancements, particularly in solar energy conversion and related photonic applications. Though low-energy CT compared to π→π* transitions was known to be quite common in organic D–A chromophores, herein, a series of phenothiazine (PTZ) terminated aza -boron-dipyrromethene (azaBODIPY) D–A dyes bridged with tetracyanobutadienes (TCBDs) 1 – 3 were designed and synthesized via Pd-catalyzed Sonogashira cross-coupling and [2 + 2] cycloaddition–retroelectrocyclization reactions, and we photoelectrochemically elucidated their unique high-energy CT characteristics in detail. A broad absorption extending even in the far-red region was observed along with a distinct high-energy CT band without much perturbation of the azaBODIPY ground state, thanks to the different spectral origin of CT events comprising particularly the PTZ δ+ -TCBD δ− entity. While electrochemical studies enabled the feasibility of CT in 1 – 3 upon photoexcitation, theoretical data helped to identify the associated molecular orbitals in this process and combined them with the proposed plausible excited state photoevents. Though conversion of CT into a complete charge separated (CS) state was found to be the likely mode for deactivation of photogenerated charge carriers in polar solvent; however, this was strongly suppressed in nonpolar media. Additionally, the spectral and dynamical characteristics of the photoevents were further investigated through femtosecond transient absorption studies coupled with global target analysis of the data, demonstrating efficient and prolonged CS state formation upon incorporating TCBD entities for both the CT and azaBODIPY-centered excitation. The broad panchromatic absorption and facile CT with subsequent formation of fairly long-lived charge separation under variable excitations pave the way for utilization of these new multimodular NIR-sensitizers in photovoltaics and beyond.