A Facile Molecular Machine: Optically Triggered Counterion Migration by Charge Transfer of Linear Donor‐π‐Acceptor Phosphonium Fluorophores. Issue 38 (9th August 2019)

• Record Type: Journal Article
• Authors: Belyaev, Andrey; Cheng, Yu‐Hsuan; Liu, Zong‐Ying; Karttunen, Antti J.; Chou, Pi‐Tai; Koshevoy, Igor O.
• Is Part Of: Angewandte Chemie international edition. Volume 58:Issue 38(2019)
• Page Start: 13456
• Page End: 13465
• Publication Date: 2019-08-09
• Subjects: charge transfer -- donor–acceptor systems -- dual emission -- fluorescence -- ion migration; Chemistry -- Periodicals; 540
• DOI: 10.1002/anie.201906929
• Languages: English
• ISSNs: 1433-7851

Abstract

Abstract: The D‐π‐A type phosphonium salts in which electron acceptor (A=‐ + PR3 ) and donor (D=‐NPh2 ) groups are linked by polarizable π‐conjugated spacers show intense fluorescence that is classically ascribed to excited‐state intramolecular charge transfer (ICT). Unexpectedly, salts with π=‐(C6 H4 ) n ‐ and ‐(C10 H6 C6 H4 )‐ exhibit an unusual dual emission (F1 and F2 bands) in weakly polar or nonpolar solvents. Time‐resolved fluorescence studies show a successive temporal evolution from the F1 to F2 emission, which can be rationalized by an ICT‐driven counterion migration. Upon optically induced ICT, the counterions move from ‐ + PR3 to ‐NPh2 and back in the ground state, thus achieving an ion‐transfer cycle. Increasing the solvent polarity makes the solvent stabilization dominant, and virtually stops the ion migration. Providing that either D or A has ionic character (by static ion‐pair stabilization), the ICT‐induced counterion migration should not be uncommon in weakly polar to nonpolar media, thereby providing a facile avenue for mimicking a photoinduced molecular machine‐like motion. Abstract : Pair at work : Optically driven charge transfer in nondissociated donor–acceptor ion pairs induces a counterion migration, reminiscent of a molecular machine. This motion leads to relocation of the cation and anion, and results in unusual dual fluorescence through the previously unrecognized properties of ionic D‐π‐A systems in nonpolar media.