A Facile Molecular Machine: Optically Triggered Counterion Migration by Charge Transfer of Linear Donor‐π‐Acceptor Phosphonium Fluorophores

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=‐(C6H4)n‐ and ‐(C10H6C6H4)‐ 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.

A Facile Molecular Machine: Optically Triggered Counterion Migration by Charge Transfer of Linear Donor‐π‐Acceptor Phosphonium Fluorophores

“…Thermal ellipsoids are shown at the 50 %probability level. [43] Angewandte Chemie Forschungsartikel ures 2, 3, S3, and S4). Thep ertinent photophysical data are tabulated in Table S3.…”

Solvatochromic dual luminescence of Eu–Au dyads decorated with chromophore phosphines

“…This process can be conventionally activated in the bipolar phosphine ligands upon the oxidation of the phosphorus atom or its bonding to the Au(I) center. 27 For this purpose, we have prepared compounds L1 28 -L3 in good yields from the bromo-π-spacer-NPh 2 (π-spacer = biphenyl, 29 naphthaleneethynylphenyl 30 ) and ethynylanthracenyl-ethynyl-diphenylaniline precursors (Scheme S1 †). Metalloligands Au1-Au3 were obtained by the depolymerisation of [Au(epbpy)] n (epbpy = 5-(4-ethynylphenyl)-2,2′-bipyridine) 31 with phosphines L1-L3 under oxygen-free conditions, which is a common synthetic approach to alkynyl gold(I)-phosphine complexes.…”

Cationic Organophosphorus Chromophores: A Diamond in the Rough among Ionic Dyes

“… Schematic structures of the D ‐π‐ A + ( 16 , 18 ) [30c, 36] and A + ‐π‐ A + ( 17 ) [34] linear λ 4 σ 4 phosphonium chromophores (the photo shows dichloromethane solutions of 18 a – d under UV light). Adapted from reference [36] with the permission of Wiley‐VCH GmbH (2020).…”

“…Nevertheless, the combination of the phosphonium group with a strong electron donor gives D ‐π‐ A + dyes with wide tunability of the optical band gap (compounds 18 , Figure 8). [36] For example, the oligophenylene‐based salts 18 a – d are brightly fluorescent in dichloromethane solutions ( Φ em =0.71–0.95) with emission wavelength changing from blue (487 nm) to orange (619 nm) upon increasing the number of the phenylene rings in the π‐spacer. Remarkably, the pyridinium donor–acceptor dyes, including the direct congeners of the phosphonium salts 18 a – c , exhibit much lower quantum efficiencies (typically less than 0.01) [37] .…”