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The effect of acceptor strength on excited state charge‐transfer (CT) and charge separation (CS) in central phenothiazine (PTZ) derived symmetric 1 (PTZ-(TCBD-TPA)2) and asymmetric, 2 (PTZ-(TCBD/DCNQ-TPA)2) push-pull conjugates, in which triphenylamine (TPA) act as end capping and 1,1,4,4–tetracyanobuta–1,3–diene (TCBD) and cyclohexa–2,5–diene–1,4–ylidene–expanded TCBD (DCNQ) act as electron acceptor units is reported. Due to strong push-pull effects, intramolecular charge transfer (ICT) was observed in the ground state extending the absorption into the near-IR region. Electrochemical, spectroelectrochemical and computational studies coupled with energy level calculations predicted both 1 and 2 to be efficient candidates for ultrafast charge transfer. Subsequent femtosecond transient absorption studies along with global target analysis, performed in both polar and nonpolar solvents, confirmed such processes in which the CS was efficient in asymmetric 2 having both TCBD and DCNQ acceptors in polar benzonitrile while in toluene, only charge transfer was witnessed. This work highlights significance of number and strength of electron acceptor entities and the role of solvent polarity in multi-modular push-push systems to achieve ultrafast CS.
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Far‐Red Excitation Induced Electron Transfer in Bis Donor‐AzaBODIPY Push‐Pull Systems; Role of Nitrogenous Donors in Promoting Charge Separation
Abstract A far‐red absorbing sensitizer, BF2‐chelated azadipyrromethane (azaBODIPY) has been employed as an electron acceptor to synthesize a series of push‐pull systems linked with different nitrogenous electron donors, viz.,N,N‐dimethylaniline (NND), triphenylamine (TPA), and phenothiazine (PTZ) via an acetylene linker. The structural integrity of the newly synthesized push‐pull systems was established by spectroscopic, electrochemical, spectroelectrochemical, and DFT computational methods. Cyclic and differential pulse voltammetry studies revealed different redox states and helped in the estimation of the energies of the charge‐separated states. Further, spectroelectrochemical studies performed in a thin‐layer optical cell revealed diagnostic peaks of azaBODIPY⋅−in the visible and near‐IR regions. Free‐energy calculations revealed the charge separation from one of the covalently linked donors to the1azaBODIPY* to yield Donor⋅+‐azaBODIPY⋅−to be energetically favorable in a polar solvent, benzonitrile, and the frontier orbitals generated on the optimized structures helped in assessing such a conclusion. Consequently, the steady‐state emission studies revealed quenching of the azaBODIPY fluorescence in all of the investigated push‐pull systems in benzonitrile and to a lesser extent in mildly polar dichlorobenzene, and nonpolar toluene. The femtosecond pump‐probe studies revealed the occurrence of excited charge transfer (CT) in nonpolar toluene while a complete charge separation (CS) for all three push‐pull systems in polar benzonitrile. The CT/CS products populated the low‐lying3azaBODIPY* prior to returning to the ground state. Global target (GloTarAn) analysis of the transient data revealed the lifetime of the final charge‐separated states (CSS) to be 195 ps for NND‐derived, 50 ps for TPA‐derived, and 85 ps for PTZ‐derived push‐pull systems in benzonitrile.
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- Award ID(s):
- 2000988
- PAR ID:
- 10444113
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Chemistry – A European Journal
- Volume:
- 29
- Issue:
- 53
- ISSN:
- 0947-6539
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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