Tailoring the optical and dynamic properties of iminothioindoxyl photoswitches through acidochromism.

Miroslav Medved',Mark W H Hoorens,Wiktor Szymanski,Wybren Jan Buma,Maria Taddei,Ben L Feringa,Michiel Hilbers,Adèle D Laurent,Mariangela Di Donato,Jiayun Fan

doi:10.1039/d0sc07000a

Abstract

Multi-responsive functional molecules are key for obtaining user-defined control of the properties and functions of chemical and biological systems. In this respect, pH-responsive photochromes, whose switching can be directed with light and acid–base equilibria, have emerged as highly attractive molecular units. The challenge in their design comes from the need to accommodate application-defined boundary conditions for both light- and protonation-responsivity. Here we combine time-resolved spectroscopic studies, on time scales ranging from femtoseconds to seconds, with density functional theory (DFT) calculations to elucidate and apply the acidochromism of a recently designed iminothioindoxyl (ITI) photoswitch. We show that protonation of the thermally stable Z isomer leads to a strong batochromically-shifted absorption band, allowing for fast isomerization to the metastable E isomer with light in the 500–600 nm region. Theoretical studies of the reaction mechanism reveal the crucial role of the acid–base equilibrium which controls the populations of the protonated and neutral forms of the E isomer. Since the former is thermally stable, while the latter re-isomerizes on a millisecond time scale, we are able to modulate the half-life of ITIs over three orders of magnitude by shifting this equilibrium. Finally, stable bidirectional switching of protonated ITI with green and red light is demonstrated with a half-life in the range of tens of seconds. Altogether, we designed a new type of multi-responsive molecular switch in which protonation red-shifts the activation wavelength by over 100 nm and enables efficient tuning of the half-life in the millisecond–second range.

Highlights

Due to the outstanding spatiotemporal resolution offered by light delivery, molecular photoswitches[1,2] are widely used to precisely control the function of molecules and systems in chemistry,[3,4] biology[5,6,7] and materials science.[8,9] Yet, an even higher level of molecular control can be obtained by combining light with a second external stimulus such as the binding of a proton to the molecular photoswitch
Neutral ITIs 8a–d feature two absorption bands in their UV spectra: a strong one at l $400–450 nm associated with the S0 / S2 electronic transition, and a weak one at l $350 nm which is attributed to the S0 / S3 transition.[27]
Spectroscopic investigations revealed that protonation of ZITI leads to the emergence of a strong absorption band which can extend to over 600 nm, and the calculations attributed this band to the S0–S1 transition that becomes bright in protonated Z-ITIs

Summary

Introduction

Due to the outstanding spatiotemporal resolution offered by light delivery, molecular photoswitches[1,2] are widely used to precisely control the function of molecules and systems in chemistry,[3,4] biology[5,6,7] and materials science.[8,9] Yet, an even higher level of molecular control can be obtained by combining light with a second external stimulus such as the binding of a proton to the molecular photoswitch. The even stronger electron-donating character of the methoxy group in compound 8d results in a pronounced absorbance of the red-shi ed band, while in the UV region of the spectrum no increase is observed.

Results

Conclusion