Investigation of Photorecoordination Kinetics for Complexes of Bis(aza-18-crown-6)-Containing Dienones with Alkali and Alkaline-Earth Metal Cations via Time-Resolved Absorption Spectroscopy: Structure vs. Properties

TL;DR

This study explores how metal cations in a specific dye complex rearrange after light exposure, linking structural features to reaction speeds.

Contribution

The paper introduces a new understanding of photorecoordination kinetics in crown ether-based dye-metal complexes.

Findings

Photorecoordination occurs via an axial-to-equatorial conformational change within hundreds of femtoseconds.

Photorecoordination times correlate with the stability constants of the complexes.

An energy barrier exists in the conformational process, as shown by calculated profiles.

Abstract

The analysis of time-resolved S1–Sn absorption spectra in the 0–500 ps range, together with quantum-chemical calculations, uncovered a photorecoordination reaction for the following complexes of CD6 (a bis(aza-18-crown-6)-containing dienone (ketocyanine dye) with a central cyclohexanone fragment): CD6·(Mn+)2 (M = Ba2+, Sr2+, Ca2+, K+). This process takes place over hundreds of fs and involves an “axial-to-equatorial” conformational change, with the solvation shell undergoing rearrangement as well. The characteristic photorecoordination times were found to correlate with the stability constants of the complexes. The lifetimes for the fluorescent states of CD6 and its complexes, namely CD6·(Mn+)2 (M = Ba2+, Sr2+, Ca2+, K+), are different; ergo, there is no photoejection of crowned cations into the solution. The calculated conformational profiles in the ground and excited states indicate the presence of an energy barrier in this process. A general photorelaxation pathway is suggested for CD6·(Mn+)2 metal complexes (M = Ba2+, Sr2+, Ca2+, K+). The coordination of cations via the carbonyl moiety in the dye molecule promotes photorecoordination of metal cations in the cavities of the azacrown ether fragment. Photorecoordination times were found to correlate with the degree of conjugation between the lone pairs in the N atoms of the aza-18-crown-6 ether and the π subsystem in the dye molecules (established for the CD4–CD6 metal–dye complex series, where CD4 and CD5 are related dyes with central cyclobutanone and cyclopentanone fragments, respectively).

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