Role of Polar Protic Solvents in the Dissociation and Reactivity of Photogenerated Radical Ion Pairs

Abstract

The UV photolysis of bimolecular charge transfer complexes is employed to yield reactive radical ions in their solvent-equilibrated electronic ground state. In polar protic media, noncovalent complexes of 1,2,4,5tetracyanobenzene and toluene undergo efficient, ultrafast dissociation to ion pairs and equilibrate with their solvent environment before the resulting radical ions engage in electron transfer and proton abstraction on subnanosecond time scales. Solvent molecules play a critical role in these reactive pathways and in the dissociation and relaxation processes that precede them. We report a clear separation of time scales for these relaxation and reactive processes, which implies that solvent−solute interactions can be used as a tool for tuning the reaction pathways of equilibrated radical ions in solution.

Selectively Altering the Reactivity of Transient Organic Radical Ions via Their Solvation Environment

“…The direct photoexcitation of the charge transfer band of MeBz:TCNB electron donor–acceptor complexes in solution leads to the ultrafast formation of radical ion pairs. In polar media, ion pair dissociation is favored and photoexcited complexes quickly equilibrate into solvent-separated radical ion pairs. − These solvent-relaxed radical ions then engage in reactive mechanisms involving solvent and/or cosolute species. The presence of transient radical ions results in photoinduced changes in light absorption (Figure S1), which can be spectrally resolved: the excited state absorption from TCNB •– is a narrow feature located in the blue region of the visible spectrum, while the signal from MeBz •+ is broader and extends into the infrared.…”

“…Ultrafast transient absorption spectroscopy was employed to monitor the dynamics of photogenerated radical ions. Instrumentation and data analysis have been previously described . Samples were measured in several independent trials, and each acquisition included a minimum of 20 transient absorption traces at 470 nm (to detect TCNB) and 40 traces at 700 nm (to detect MeBz •+ ).…”

“…Using ultrafast transient absorption spectroscopy, we probe the dynamics of the photogenerated radical ions following the photolysis of the charge-transfer complex formed between toluene (MeBz) and 1,2,4,5-tetracyanobenzene (TCNB). The MeBz:TCNB charge transfer complex is a good model system because its behavior in protic media has been linked to a reactive mechanism that proceeds from an equilibrated, solvent-separated radical ion pair and involves solvent molecules − and because there is a substantial body of knowledge on their nonreactive dynamics. − Moreover, the optical absorption features of the neutral and radical ion species, as well as the charge transfer band of the electron donor–acceptor complex, are spectrally distinct, which greatly simplifies the measurement of their ultrafast dynamics with independent spectroscopic signals. A systematic comparison of the ultrafast dynamics of MeBz:TCNB complexes in eight alkyl alcohols revealed that the rates for their solvent-mediated reactions are strongly determined by bulk solvent properties.…”

“…Instrumentation and data analysis have been previously described. 31 Samples were measured in several independent Materials. 1,2,4,5-Tetracyanobenzene was dissolved in toluene ([TCNB] = 5 mM) via ultrasonication.…”

Binding of dsRNA byD. melanogasterDicer-2 is substrate-dependent and regulated by Loquacious-PD

“…Low-background fluorescence measurements of chromophore-labeled dsRNA constructs were performed using a polarization-resolved ultrafast fluorescence instrument with few modifications from a previously described setup. (35) Instrumental and data analysis details in the Supporting Information.…”