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

Abstract

+ + ]T hese authors contributed equally to this work.Supportinginformation (including synthetic, other experimental, and computational details) and the ORCID identification numbers for some of the authors of this article can be found under: https://doi.

Counterion Migration Driven by Light-Induced Intramolecular Charge Transfer

“…Qualitatively, the result may correlate with the size of the anion, which is in the order of BPhCl – (6.22 Å) > OTf – (2.48 Å) ∼ PF 6 – (2.42 Å). 35 Similar results were observed for C1 + , and the relaxation rate was in the order of C1[BPhCl] (86 ps) −1 < C1[OTf] (53 ps) −1 < C1[PF 6 ] (40 ps) −1 ( Table S7 and Figure S66 ). The correlation of the excited state dynamics only with the size of the anion is undoubtedly an oversimplified approach as the ion pairing behavior might depend on other factors such as the interaction of anion with the nearest solvent molecules, anion shape, the distribution of electrostatic potential, and mutual anion–cation disposition.…”

“…This, along with the continuous relaxation pattern and the strongly counterion-dependent property, allows us to generalize the mechanism of charge-transfer induced anion migration to the target ionic D−π– A + system. 35 ESICT leads to the distribution of the electron density toward the ionic acceptor A + site and leaves a positive charge, in part, at the D site. This, along with the neutralization of A + (which weakens the A + [ X ] − ion pair), renders the driving force to execute the anion [ X ] − migration for stabilization ( Scheme 2 b).…”

“…The cationic moiety is charge-balanced by a counteranion. 35 Upon electronic excitation, these D−π– A + systems undergo the expected excited-state intramolecular charge transfer (ESICT) from the diphenyl amino group to the phosphonium moiety via the π-bridge, producing regular emission solvatochromism in medium and strong polar solvents. However, in weak polar solvents such as toluene, exceedingly broad and even dual emission bands were observed.…”

“…The lack of solvent stabilization in toluene led us to tentatively propose an ESICT that triggers the counteranion migration toward the D δ+ site (the diphenyl amino group) for further stabilization. 35 The trajectory of photoinduced anion migration has been scarcely reported previously. 40 , 41 In addition, a number of supramolecular and receptor systems exhibiting pH-, redox-, or photoinduced metal cation translocation with similar mechanisms have been developed over the past decades.…”

“…In addition, different counteranions ( Scheme 1 ) are incorporated into this system to fine-tune the dynamics of anion motion, if any. 35 Importantly, the neutral form of the acceptor, namely, the pyridine ring (A = −Py) with D = −NPh 2 forms a D−π– A system, which was synthesized and further underwent the ESICT process ( vide infra ). 48 Thus, a fair comparison can be made between NPh 2 –π–PyMe + and NPh 2 –π–Py systems to highlight the difference of the former regarding the counterion migration.…”