Mechanistic Insights into the Triplet Sensitized Photochromism of Diarylethenes.

Sebastian Fredrich,Tobias Morack,Michel Sliwa,Stefan Hecht

doi:10.1002/chem.202000877

Abstract

Operating photoswitchable molecules repetitively and reliably is crucial for most of their applications, in particular in (opto)electronic devices, and related to reversibility and fatigue resistance, which both critically depend on the photoisomerization mechanism defined by the substitution pattern. Two diarylethene photoswitches bearing biacetyl triplet sensitizers either at the periphery or at the core were investigated using both stationary as well as transient UV/Vis absorption spectroscopy ranging from the femtosecond to the microsecond time scale. The diarylethene with two biacetyl moieties at the periphery is switching predominantly from the triplet excited state, giving rise to an enhanced fatigue resistance. In contrast, the diarylethene bearing one diketone at the photoreactive inner carbon atom cyclizes from the singlet excited state and shows significantly higher quantum yields for both cyclization and cycloreversion.

Highlights

In order to be incorporated into devices, photoactive materials need to exhibit a precise and typically repeatable response to light stimuli
The “ideal” DAE should combine several properties to exhibit optimal switching behavior: (i) The absorption of the open isomer should be shifted from the UV to the visible range to utilize excitation with lower energies to affect ring closure; (ii) ring closure should result from the triplet excited state to avoid potential side reactions originating from the singlet excited state; (iii) the absorption of the closed isomer should be in the red region of the visible spectrum to allow for its selective excitation and ring opening at low excitation energies; and (iv) ring opening should occur with reasonable cycloreversion quantum yield, which requires sufficient population of the singlet excited state and limited intersystem crossing (ISC)
Low energy, visible light excitation of the DAE should lead to increased population of the singlet excited state due to less efficient ISC and enhanced ring opening when compared to 1 c

Summary

Introduction

In order to be incorporated into devices, photoactive materials need to exhibit a precise and typically repeatable response to (localized) light stimuli In this context, photoswitches have been heavily explored over the past decades and among them diarylethenes (DAEs)[1] have proven exceptionally useful due to their thermal bistability and ability to modulate fluorescence[2] and electronic properties.[3] This has led to the development of optical transistors[4] as well as storage media[5] based on optical writing and erasing of DAE-based materials. This lowest energy transition of 2 o is slightly blue-shifted and less intense when compared to 1 o, which can be explained by the interaction between the thiophene donor and diketone acceptor units that extends over a longer p-system in 1 o including the intermediate phenyl moiety

Results and Discussion

Conclusions

Conflict of interest